Journal of Conference Abstracts

Session E01:2A

E01 : 2A/01 : F1

Surface Dynamic Topography and Lithospheric Extensional Stresses Induced by Mantle Plumes

Cinzia G. Farnetani (cinzia@ipgp.jussieu.fr)

IPGP Boite 89, 4, Place Jussieu, 75252, Paris, France

One important question in geodynamics is the relation between flood volcanism and continental rifting. To address this problem it is necessary to estimate the stress field induced in the lithosphere by a large plume head rising in the mantle and impinging upon the base of the lithosphere. In this study I focus on two issues: 1) the amount and timing of dynamic uplift/subsidence generated at the Earth's surface by a plume head, and 2) the induced horizontal deviatoric stresses that may cause extension of the lithosphere. I use Paul Tackley's three-dimensional Cartesian code to model the dynamics of solid state convective flow (for a detailed description of the numerical model see for example Tackley, 1998 and references therein). The code solves for the conservation of mass, momentum and energy equations and provides the velocity components, the dynamic pressure and temperature. My numerical experiments are designed to simulate a plume with characteristics appropriate to explain major flood volcanism. A thermal plume is modelled from its formation in the lower mantle to its spreading beneath the lithosphere. The dynamic model allows me to calculate simultaneusly the thermal and the mechanical response of the lithosphere to the arrival of a large plume head. I explore the effects of a number of model variables, including the thickness and temperature of the lithosphere, the mantle viscosity structure, the effect of strain weakening rheology, and the pre-existing far-field stress regime in the lithosphere. I also calculate the isostatic uplift component due to the hot plume head, in order to compare the predicted uplift/subsidence history to recent geological observations, for example in West and East Greenland.

Tackley PJ, Earth Planet. Sci. Lett, 157, 9-22, (1998).

E01 : 2A/02 : F1

A Dynamical Model of Extension Above Plumes

Robert Bourne Newman (newman@esc.cam.ac.uk)

Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge, U.K.

When extension of continental lithosphere occurs above a mantle plume, the elevation of the lithosphere and the higher than normal mantle temperature associated with the plume affect the dynamics of the extension. The importance of these factors is hard to assess. Even when there is no involvement of a plume, there are still many uncertainties about how the strain rate varies during extension, about the driving forces, and about the rheology of the lithosphere. Recently, attempts have been made to deduce detailed extensional histories for several sedimentary basins using their subsidence histories. For each basin, it is found that a characteristic relationship exists between the strain rate during extension and the final stretching factor reached. This relationship strongly suggests that it is cooling and strengthening of the lithospheric mantle that limits extension rather than removal of the driving force (Newman & White, 1997). Using a two-dimensional model, the variation of strain rate with time has been calculated for a range of possible lithospheric rheologies. The agreement between the numerical results and the strain rate data from basins is best when the lithospheric mantle has a power-law creep rheology similar to that found for olivine in laboratory experiments.

Having established a lithospheric rheology and a basic mechanism for limiting extension, the model can be extended to include the effects of a plume. Since a small variation in the temperature of the lithospheric mantle can lead to a large variation in its strength, it is possible that the temperature increase caused by a plume can help to start extension or can prolong an episode of extension. If a large amount of melt is generated by the plume and is emplaced at the base of the crust, that melt can also help to keep the Moho hot and the strain rate high. The model predicts that in order to reach a given stretching factor, the strain rate will vary in a particular way with time. This reduces the volume of melt one would expect compared with instantaneous or constant strain rate extension.

The uplift of the lithosphere caused by the plume generates additional extensional stress in the lithosphere. Depending on the rheology, this stress can: (i) itself initiate extension; (ii) reactivate an area of earlier extension; (iii) affect the lateral distribution of strain; (iv) affect the vertical distribution of strain. In addition, the uplift will affect any sedimentation. There isevidence that in some case such uplift is pulsed, and whether that is because of pulsing of the plume, or because of pulsed intrusion of melt, the effect on strain rates can be estimated.

Newman R & White N, Nature, 385, 621-624, (1997).

E01 : 2A/03 : F1

Toward Plume Tectonics Models Including Compression

Daniel Mège (dmege@lgs.jussieu.fr)

Département de Géotectonique, case 129, Université Paris VI, 4 Place Jussieu, 75252 Paris cedex 05, France

A considerable debate exists aiming at understanding the origin of extension above plumes. Compression at plumes has been scarcely discussed. Undoubtedly, compressive structures form rarely there - however, possible additional reasons may be that compressive structures in plume regions are more readily attributed to other events, or merely they are not sought-after. The Tharsis province on Mars provides exceptional insight into the influence of stress induced by plumes in an environment where it is not overprinted by stress resulting from plate movements, and suggests that even in extensional settings, plume impingement on the lithosphere may cause compressive structures to form.

Tharsis shares a number of well-known volcanic-tectonic characteristics with terrestrial hotspots (Mège and Masson, 1996, Mège, 1999) such as regional uplift, a magma centre, flood basalt and radiating dyke swarm emplacement, and tectonic extension (including rifting?) parallel to the dyke trends. In addition, compressive structures are observed in the form of (1) wrinkle ridges (Watters, 1988) perpendicular to the dykes, and (2) a concentric peripheral compressive belt bounding the southern part of the bulge (Schultz and Tanaka, 1994).

The wrinkle ridges formed solely in flood basalts. Tectonic analysis combined with stratigraphic data, and comparison to terrestrial plume tectonics models and mechanisms of wrinkle ridge formation on the Moon, suggest that the wrinkle ridges may have formed early in the Tharsis history in response to rapid subsidence of that part of the crust loaded by lava flows, possibly enhanced in the uplifted areas by thermal subsidence following the waning of the early strong positive thermal anomaly. The wrinkle ridges likely formed during dyke swarm emplacement, which requires temporary stress permutation.

Comparison with experiments of scaled analog modeling of volcanic spreading under controlled boundary conditions performed by Merle and Borgia (1996) suggests that the peripheral belt may result from gravitational spreading of the excess topography created by the plume in a brittle upper crust underlain by a weak layer of greater thickness.

Therefore, even in extensional settings and provided that lithospheric layering is appropriate, mantle plumes may modify the ambient stress field in such a way that formation of concentric contractional structures is favoured in volcanic traps during their emplacement, and at the periphery of the uplift until the excess topography has relaxed. On Earth, such a modification of stress is expected to be usually diluted within the complex stress patterns resulting from plate motions, then passing unnoticed. However, possible compressive structures akin to the Tharsis wrinkle ridges and peripheral belt may include the Yakima ridges on the Columbia Plateau (Watters, 1988), attributed to the early Yellowstone hotspot, and a peripheral compressive belt formed during amalgamation of the Yilgarn craton in Australia (Passchier, 1995). Other possible examples are still to be found.

Mège D & Masson P, Planet Space Sci, 44, 1499-1546, (1996).

Mège D, Geol. Soc. Am. Sp. Pap, in press, (1999).

Merle O & Borgia A, J. Geophys. Res., 101, 13805-13817, (1996).

Passchier C, Geol. Mijnbouw, 74, 141-150, (1995).

Schultz RA & Tanaka KL, J. Geophys. Res, 99, 8371-8385, (1994).

Watters TR, J. Geophys. Res, 93, 10236-10254, (1988).2744

E01 : 2A/04 : F1

Lithospheric Stress and Hotspot Distribution

Bernhard Steinberger (steinber@geophysik.uni-frankfurt.de)

Inst. f. Meteorologie u. Geophysik, Feldbergstr. 47, 60323 Frankfurt am Main, Germany

The distribution of hotspots shows concentrations in the Pacific region and around Africa. It has been surmised that this is partly due to the state of stress of the lithosphere: There may also be plumes in other regions, but they may not be able to penetrate a lithosphere under compression. To analyse this hypothesis, lithospheric stresses caused by mantle flow are calculated: mantle flow exerts tangential stresses on the base of the lithosphere and also causes vertical deflections (dynamic topography) of the lithosphere; both cause stresses inside the lithosphere. Mantle flow is calculated with the method of Hager and O'Connell. Density heterogeneities inside the mantle that drive the flow are inferred from seismic tomography. It is in fact found that hotspots are located almost entirely in regions of extensive, or marginally compressive, stress: Calculated stresses are extenive in Africa and most oceanic region, while they are compressive in most other continental regions. Good agreement of calculated directions of maximum compression with the World stress map is achieved in most regions on a large scale. At sufficiently long wavelength, calculated dynamic topography resembles actual topography in regions with no recent tectonic activity, such as Africa. Lithospheric shear stresses calculated with this method may help to better assess earthquake risk in intraplate settings.

E01 : 2A/05 : F1

Double-Diffusive Convection in an Heterogeneous Mantle: Hotspots and Superswells

Anne Davaille (davaille@ipgp.jussieu.fr)¹ &

Annette Lee (Annette@hess.geology.yale.edu)²

¹ Laboratoire de Dyanmique des Systemes Geologiques, IPG, 4 Place Jussieu, PARIS cedex 05, FRANCE

² Dept of Geology & Geophysics, Yale University, POB 208109, New Haven, CT 06520-8109, USA

The detailed dynamics and origin of upwards motions in themantle, like those creating hospots and superswells, are still not well imaged nor understood. However, laboratory experiments have recently shown that narrow, hot and stable plumes could result from the interaction of thermal convection with a density heterogeneity, when the chemical heterogeneity is stronger than the thermal density heterogeneity. We present here further laboratory experiments investigating the regime where the amplitudes of the two heterogeneities are comparable, i.e the regime of oscillatory double-diffusive convection. Rayleigh numbers between 200 and 5.10⁷ were obtained for viscosity ratios between 1 and 6.10⁴ and layer depth ratios between 1 and 30. The initial density stratification was either sharp or gradual with contrasts ranging from 0.10 to 1%. In all cases, hot domes are produced, moving up and down in the mantle. Depending on the viscosity contrast, they are crowned by one or several thin tubular plumes. Quantitative measurements of motions wavelength, thermal anomalies, heat and mass fluxes were made and scaling laws derived, which allow to extrapolate those results back to the Earth.

E01 : 2A/06 : F1

A Computer Animation of the Creation of the Indian Ocean: Lessons in Continental Breakup

Colin V. Reeves (reeves@itc.nl)¹ &

Roy A. Livermore (RAL@pcmail.nerc-bas.ac.uk)²

¹ International Institute for Aerospace Survey and Earth Sciences (ITC), Kanaalweg 3, 2628 EB Delft, The Netherlands

² British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, England

Topographic and topologic constraints provided by satellite altimetry data (Smith and Sandwell, 1997) and timing constraints provided by ocean-floor magnetic anomalies have been used to reverse the creation of the Indian Ocean as an accurate computer animation around which this presentation is built. Digital geoinformation has been created and processed in a GIS and then imported into the Cambridge 'Atlas' and 'Timetrek' PC software.

Reversing the creation of the Indian Ocean is undertaken as four distinct regimes of activity, each lasting about 40 m yr. Within each regime, drifting takes place predictably and uniformly with instantaneous Euler rotation poles lying close to interval poles for the entire duration of the regime. By contrast, regimes begin and end abruptly with ridge jumps and the migration of other plate boundaries, often over large distances and without warning. Mantle plumes or hotspots can be invoked to trigger some of these changes, perhaps anchoring plates at points that may become new rotation poles or ensuring that plate boundaries (coincidentally?) lying astride plumes become active spreading axes, such as the Tristan plume in the embryo South Atlantic. The history of dyke injection in southern Africa during the Mesozoic, also shown in the animation, can be linked to some of these possibilities. The production of other seafloor features, such as the Ninety-East Ridge and the Laccadive-Maldive-Reunion chain, as oceanic plates track over hotspots is clearly visualised.

Rifting, often prolonged, pre-dates the drifting apart of continental fragments. Reversing the effects of such rifting can reduce the area of the non-oceanic crust quite significantly and leads logically to a tight and geometrically (and geologically) satisfactory re-assembly of the Precambrian fragments that surround the Indian Ocean once they are divested of their younger margins. In this way we animate the tight reunion of the central regions of the Gondwana supercontinent. In forward time, after about 200 m yr of Paleozoic supercontinent stability, widespread Karoo (Africa) and Gondwana (India) rifting (without drifting) affected the landmass of central Gondwana for 100 m yr before continental dispersal at modern rates started at about 160 Ma. Prior to this, Karoo tectonism culminated in a major magmatic pulse at about 182 Ma that produced extensive extrusions over large areas of southern Africa and the Ferrar igneous province in Antarctica. This we relate timewise to the rapid motion of the Falkland Islands and Ellsworth-Whitmore blocks in Antarctica, predating seafloor spreading in the Weddell Sea and the northward migration of initial spreading between East and West Gondwana. A Karoo-Dronning Maud plume could have provided a driving force for such motions, predating the initiation of Gondwana dispersal in time but coincident in space, as can also be seen from the animation.

Smith WHF & Sandwell D, Measured and estimated seafloor topography (version 4. 2), world Data Center A for Marine Geology and Geophysics research publication RP-1, poster, 34" x 53", (1997).

E01 : 2A/09 : F1

The Eifel Plume Project

Joachim Ritter

(ritter@carl-f.uni-geophys.gwdg.de)

Institut für Geophysik, Herzberger Landstr. 180, 37075 Göttingen, Germany &

The Eifel Plume Team

Recent models of the upper mantle underneath Europe propose several possible small mantle plumes based on geophysical and geochemical evidence (Granet et al., 1995). One such plume is expected under the Rhenish Massif in Central Europe which contains the Quaternary volcanic fields in the Eifel region. Previous research found evidence for an anomaly in the upper mantle which can be correlated with volcanism and uplift. From Nov. 1997 to June 1998 about 150 mobile seismic stations were operated in the Eifel and surrounding regions by the Eifel Plume Project. Together with data from about 100 permanent stations these seismic recordings will be used to conduct several seismological studies including tomography of the crust and upper mantle, broadband array seismology and local seismo-tectonics. During the observation period approx. 200 teleseisms, 50 local events and 30 quarry blasts were recorded. The data will be analysed at the involved institutions in Belgium (ROB), France (Strasbourg), Germany (Bochum, Goettingen, Koeln, Krefeld, Potsdam) and Luxembourg (ECGS). Besides seismology the project also involves electro-magnetic deep sounding and geodynamic modelling

http://www.geo.physik.uni-goettingen.de/~eifel

Granet M, Wilson M & Achauer U, EPSL, 136, 281-296, (1995).

E01 : 2A/10 : F1

Distinct Magma Batches in Tertiary Basalts from the Vogelsberg, Central Germany

Paul Johannes Felix Bogaard (pbogaar@gwdg.de)¹,

Gerhard Wörner (gwoerne@gwdg.de)¹ &

Friedhelm Henjes-Kunst (henjes-kunst@bgr.de)²

¹ Geochemisches Institut, Goldschmidtstraße 1, 37077, Göttingen, Germany

² Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655, Hannover, Germany

Recent studies on Tertiary and Quaternary basaltic volcanism in Europe led to the definition of a common deep mantle component, the European Asthenospheric Reservoir (EAR) (Wilson et al. 1991; Cebria et al. 1995; Hoernle et al. 1995). These studies concentrated on the Massif Central, the Eifel and the Canary Islands. Relatively little work was done on one of the largest Tertiary volcanoes in Central Europe, the Vogelsberg (Wittenbecher 1992; Jung et al. 1996). A new drillcore in the central Vogelsberg unearthed a 656,50 m long section through 45 lavaflows and numerous pyroclastic deposits. In the Vogelsberg, exposure and topography are limited and dateable minerals are scarce. Therefore it is not possible to define long stratigraphic sections in the field. In such an environment, a drillcore provides a unique opportunity to study the detailed volcanological and geochemical variation in time.

All flows in the core were sampled and analysed for major and trace elements. Representative samples were analysed for Sr and Nd isotopic compositions. Pb and O isotopes will be measured in the near future. Amphibole separates from two samples were dated by Ar/Ar at 17.5 ± 0.2 and 16.8 ± 0.3 Ma. Further samples for geochronology are in preparation.

Major and trace element chemistry, isotope geochemistry, magneto-stratigraphy and rock characteristics all support a subdivision of the core section in 3 units:

0-100 m: Upper Core; Many thin flows, sparse thin pyroclastic deposits. Primitive lavas (basanite, melanephelinite, picro-basalt) and basalt. Little geochemical variation. Strong enrichment in LREE and strongly incompatible elements, slightly lower HFSE. ⁸⁷Sr/⁸⁶Sr: 0.7032-0.7034; ¹⁴³Nd/¹⁴⁴Nd: 0.51283-0.51293.

100-275 m: Middle Core; Thicker flows, pyroclastics (thick section 100-145 m). Basalt and basaltic andesite. These rocks show clear, ongoing trends in most variation diagrams. Trace element concentrations vary strongly. Incompatible element concentrations gradually decrease with increasing SiO₂. 87Sr/⁸⁶Sr: 0.7032-0.7036; ¹⁴³Nd/¹⁴⁴Nd: 0.51275-0.51285.

275-656 m: Lower Core; Thick flows and pyroclastics. Mostly evolved rocks (hawaiites - trachytes). Ti-rich basalts lower in the section do not show relative HFSE depletion. ⁸⁷Sr/⁸⁶Sr: 0.7033-0.7036; ¹⁴³Nd/¹⁴⁴Nd: 0.51280-0.51282.

This subdivision corresponds with magnetic reversals, and thus indicates temporal variation in the processes of magma generation and transport. The magnetic reversals suggest that periods of magma production and extrusion alternated with quiet periods. Such behaviour may be explained with successive magma batches or pulses. Changes in mantle (-plume?) temperature and composition, and in crustal conditions, may then explain the change in character of lavas.

Results of further analyses and geodynamical interpretations will be presented at the conference.

Cebria JM & Wilson M, Terra Abstracts, 7, 162, (1995).

Hoernle K, Zhang YS & Graham D, Nature, 374, 34-39, (1995).

Jung S & Masberg P, J Conf Abs, 1, 298, (1996).

Wilson M & Downes H, J Petrology, 32, 811-849, (1991).

Wittenbecher M, Geol Abh Hessen, 97, 52pp, (1992).

E01 : 2A/11 : F1

Is There a Hot Spot beneath the French Massif Central? Some Petrological Constraints and New Propositions

Emmanuel T. Berger (berger@geophy.ensmp.fr),

Gilles Chazot (chazot@opgc.univ-bpclermont),

Olivier Merle (merle@opgc.univ-bpclermont.fr) &

Laurent Michon

(michon@opgc.univ-bpclermont)

Centre de Geophysique, Ecole des Mines de Paris, 35 Rue Saint-Honore, France

Since 1974 the volcanism of the French Massif Central has been considered as being related to upper mantle diapirism. Recent studies lead to an up-to-date modelling that takes into account: 1) A large sheet-like upwelling region in the upper mantle which extends from the eastern Atlantic Ocean to central Europe, 2) The ascent of a thermally anomalous mantle plume beneath the Massif Central during Tertiary times. According to seismic tomographic and geochemical models the central and southern part of the Massif Central is underlain by a hot body with a potential temperature that is "100-200°C higher than the average potential temperature of the upper mantle".

A. It appears that some petrological data are not consistent with the above model: 1) The Ca-Olivine geobaromer is not an appropriate tool for most samples, 2) It is impossible to explain the presence of amphiboles in equilibrium with the lithospheric upper mantle at temperatures as high as those proposed the in literature, 3) The coexistence of amphibole and phlogopite in some ultramafic nodules reveals the presence of hydrated lithosphere beneath the French Massif Central. Then the estimated temperatures (from teleseismic tomography) are too high and corrections for both anharmonicity as anelasticity should be reconsidered.

B. The existing data may be explained in the following way: 1) Hydration and metasomatism of upper mantle beneath the MCF could be related to geological events prior to Tertiary volcanism (i. e., Hercynian orogeny) and/or fluids associated with Tertiary volcanism. 2) Instead of a mantle plume, partial melting of the upper mantle may be induced by the coeval events of the nearby alpine mountain chain.

E01 : 2A/12 : F1

Eger Graben: Earthquake Swarms Induced by Magma and Fluid Intrusions

Ales Spicak (als@ig.cas.cz)¹,

Josef Horalek (jhr@ig.cas.cz)¹,

Alena Bouskova (ab@ig.cas.cz)¹,

Jiri Vanìk (hanus@ig.cas.cz)¹ &

Cestmir Tomek (tomek@edvz.sbg.ac.at)²

¹ Geophysical Institute, Bocni II/1401, 141 31 Prague 4, Czech Republic

² Univ. Salzburg, Hellbrunner str. 34, 50 20 Salzburg, Austria

Eger Graben in the western part of the Bohemian Massif belongs to the Cenozoic rift system of the Alpine foreland. Recent manifestations of geodynamic activity in this region together with our knowledge on an uplift of the Moho-Discontinuity (Bucha et al., 1994) and elevated lithosphere-asthenosphere boundary (Babuska et al., 1987) has often been connected with the idea of a mantle plume beneath the Bohemian Massif. This idea seems to be supported by our recent interpretation of earthquake swarm origin in the West Bohemia seismoactive region. We are submitting a hypothesis that earthquake swarms in this relatively stable continental environment are generated by the current magmatic activity transported to the upper crustal layers. The origin time of swarms, their duration and intensity probably depend on the dynamics of the respective magma injection and related fluid and gas release. The injection of magma and/or fluids, and gases at depths of about 10 km causes hydraulic fracturing that manifests itself as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence, coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the detailed analysis of local seismological data and the information from neighbouring KTB deep drilling project and from the 9HR seismic reflection profile. 1) Recent manifestations of geodynamic activity include Quaternary volcanism (age 0.85 Ma), rich CO₂ emissions, high mantle-derived ³He content, mineral springs, moffets, etc. 2) The local broadband seismological network WEBNET provides high quality data that enable precise localisation of seismic events. The individual earthquakes of a swarm are confined to an extremely narrow volume at depths between 9-10 km. Seismograms reveal pronounced reflections of P- and S-waves corresponding to a reflective boundary immediately beneath hypocentres with the S-wave velocity approaching 0 km/s - the S-wave velocity in a liquid or molten medium. Advanced source mechanism studies have disclosed a considerable non-double couple component in many events. 3) Some reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by tabular intrusions of basaltic magma in the upper mantle and lower crust (Tomek et al., 1997) The mechanism of intraplate earthquake swarms generated by magma intrusions is similar to that of induced seismicity. As the recent tectonic processes and manifestations of geodynamic activity are similar in European areas with repeated earthquake swarm occurrence (Bohemian Massif, French Massif Central, Rhine Graben), we assume that magma intrusions and related fluid and gas release at depths of about 10 km are the universal mechanism of intraplate earthquake swarm generation.

Babuska V & Plomerova J, Earthquake Swarm 1985/86 in Western Bohemia. Geophys. Inst., Prague, 30-33, (1987).

Bucha V & Blizkovsky M, Crustal Structure of the Bohemian Massif and West Carpathians. Academia, Prague, 1-355, (1994).

Tomek C, Dvorakova V & Vrana S, J. Geol. Sci, 47, 43-50, (1997).

E01 : 2A/13 : F1

Majorite Destabilisation on Decompression: Constrains from Natural Samples on Plume Velocity

Violaine Sautter (Sautter@mnhn.fr)¹,

Ben Harte (H.HARTE@glg.ed.ac.uk)² &

Jeff Harris (J.Harris@geology.gla.ac.uk)³

¹ laboratoire Minéralogie, M.N.H.N., 61 rue Buffon 75005 Paris, France

² Dept. of geology and geophysics, grant Institute of Geology, University of Edinburgh, EH9 3JW Scotland UK

³ Dept. of Geology and applied Geology, University of Glasgow, G128QQ, Scotland UK

The presence of natural majorite garnet in xenolithes from Jagersfontein kimberlite (Sautter et al. 1991) and as syngenetic inclusions in diamonds from Monastary (Moore and Gurney 1985) and Sao Luis (Wilding et al. 1989) bear witness to sampling depths within the Transition Zone (from 400 to 670 km).

In this abstract we use the varying degrees of majorite destabilisation on decompression as well as the striking textural differences (whether destabilisation occurs in xenolith or in diamond) to infer the exhumation history of these ultradeep samples. In xenoliths, that reaction has gone to completion and is best described by a continuous exsolution of pyroxene rods in the garnet host. On the other hand, in diamond, the reaction is incomplete due to internal confining pressure exerted from diamond on the inclusion and proceeds via a discontinuous eutectoid-like process where pyroxene nucleates at the majorite grain boundary.

All our analysed ultradeep samples were carried up at the head of a rising plume at an ascent rate in the range 10 to 100 cm/y.

Sautter V, Haggerty SE & Field S, Science, 252, 827-830, (1991).

Moore R & Gurney J, Nature, 318, 553-555, (1985).

Wilding MC, Harte B & Harris JW, 28th Int. Geol. Congr. Washington DC, 3, 359, (1989).

Session E01:2B

E01 : 2B/21 : F1

Crustal Structure of Central and Northeastern Iceland Constrained by Receiver Function Modelling

Fiona A. Darbyshire (fiona@esc.cam.ac.uk)¹,

Keith F. Priestley (keith@esc.cam.ac.uk)¹,

Robert S. White (rwhite@esc.cam.ac.uk)¹,

Gunnar Gudmundsson (gg@vedur.is)²,

Steinunn Jakobsdottir (ssj@vedur.is)² &

Ragnar Stefansson (ragnar@vedur.is)²

¹ Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, U.K.

² Iceland Meteorological Office, Bustadavegur 9, 150 Reykjavik, Iceland

Iceland is created by the interaction of the Mid-Atlantic Ridge with the Iceland mantle plume. The presence of the plume gives rise to enhanced melt production, resulting in anomalously thickened crust. The plume centre is currently located below central Iceland, and the Mid-Atlantic Ridge is expressed on land as a set of three volcanic rift zones.

Seismic refraction surveys across cenral and northeastern Iceland (Staples et al. 1997; Darbyshire et al. 1998; Menke et al. 1998) have been used to model crustal thickness variations in the region. The Moho depth ranges from 19 km below the northern part of the Northern Volcanic Zone to 40 km below central Iceland.

Data from large teleseismic earthquakes, recorded by broadband seismic stations in central and northeastern Iceland, are processed to produce receiver functions from which models of the one-dimensional shear velocity structure of the crust can be obtained. Despite rather noisy data and indications of lateral heterogeneity in the crust, the receiver functions can be used to constrain successfully the main features of the velocity structure.

The crustal thicknesses obtained by receiver function modelling show a good agreement to those from refraction profiling results. Across the Northern Volcanic Zone in northeastern Iceland, the Moho lies at 20-27 km depth, with the thinnest crust in the centre of the rift zone and the thickest to the east of the eastern flank. The central Iceland receiver functions give a crustal thickness of 34-42 km.

In the central and western part of the Northern Volcanic Zone, the receiver functions are best modelled by the inclusion of a low-velocity zone (LVZ) in the mid-crust. Close to the Krafla central volcano in the centre of the rift zone, the LVZ lies at a depth of 10-14 km, and has a minimum shear wave velocity of 2 km/s. On the western edge of the rift zone, the LVZ is less pronounced, with a minimum shear wave velocity of 3 km/s. It lies between 13 and 17 km depth in the crust.

Data from magnetotelluric measurements in northeastern Iceland in the late 1970s were interpreted to sow a low-resistivity layer at 10-15 km depth (Beblo & Bjornsson 1978, 1980). These results were orignally interpreted as a layer of partial melt at the base of a thin (10-15 km) crust. Since then, more recent crustal models have taken the crustal thickness to be at least 20 km. The LVZ in the receiver function models lies at a similar depth to the low-resistivity layer, and the implications of these results for melt distribution in the Icelandic crust are discussed.

Beblo M & Bjornsson A, J. Geophys., 45, 1-16, (1978).

Beblo M & Bjornsson A, J. Geophys., 47, 184-190, (1980).

Darbyshire FA, Bjarnasson ITh, White RS & Flovenz OG, Geophys. J. Int., 135, (1998).

Menke W, West M, Brandsdottir B & Sparks D, Bull. Seism. Soc. Am, (1998).

Staples RK, White RS, Brandsdottir B, Menke W, Maguire PKH & McBride JH, J. Geophys. Res, 102, 7849-7866, (1997).

E01 : 2B/22 : F1

The Origin of Iceland Plume Mantle: Constraints from High-Precision Oxygen and Pb Isotope Analyses of Reykjanes Ridge Lavas

Matthew Thirlwall (matthewt@gl.rhbnc.ac.uk)¹,

Rex Taylor (rex@soc.soton.ac.uk)²,

David Mattey (d.mattey@gl.rhbnc.ac.uk)¹,

Mary Gee (m.gee@rhbnc.ac.uk)¹ &

Bram Murton (Bramley.J.Murton@soc.soton.ac.uk)³

¹ Dept. of Geology, Royal Holloway Univ. London, Egham, Surrey, UK

² School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton SO14 3ZH, UK

³ Southampton Oceanography Centre, Southampton SO14 3ZH, UK

Tholeiites erupted in the active rift zones of Iceland possess some of the lowest  ¹⁸O values of recent basalts worldwide, ca. +3 to +4.2‰ for olivine. Many workers from 1985-1993 attributed this to large-scale interaction between mantle-derived magmas with  ¹⁸O_ol of +5.1 to +5.2‰ and hydrothermally altered Icelandic crust, with  ¹⁸O of 0 to -5‰. Recently, however, Harmon & Hoefs (1995) and Chauvel &Hémond (1998) have proposed that  ¹⁸O values as low as +3‰ may characterize the low - ¹⁴³Nd/¹⁴⁴Nd incompatible element enriched component of the Iceland plume. They inferred from this that the plume was, like many other ocean island mantle sources, derived from hydrothermally altered ocean crust that had undergone deep recycling into the mantle.

The very low  ¹⁸O values observed in Icelandic crustal xenoliths and deep drillhole samples are a consequence of alteration in hydrothermal systems dominated by high-latitude meteoric water, and cannot therefore be significant in the ocean ridges adjacent to Iceland. It has long been recognised that Reykjanes Ridge lavas north of ca. 61° contain a substantial component from Icelandic plume mantle: using new Pb data obtained with the double-spike technique (Thirlwall et al., 1998) we can show that the Icelandic component in these Reykjanes Ridge lavas is identical to that in incompatible-element enriched lavas from the Reykjanes Peninsula on Iceland, with ²⁰⁶Pb/²⁰⁴Pb > 18.7 and ¹⁴³Nd/¹⁴⁴Nd < 0.51305. Consequently, if the enriched component of the Iceland plume has low  ¹⁸O, so too should Reykjanes Ridge lavas north of 61°.

We have analysed  ¹⁸O in glass, olivine and plagioclase from 10 Reykjanes Ridge lavas using laser fluorination techniques. Mean  ¹⁸O are +5.60+/0.31 (2sd, N=5), +5.12+/-0.17 (N=7) and +5.61+/-0.17 (N=4)‰ respectively. The olivine mean and spread are closely similar to those of mantle xenolith olivine, indicating little trace of an abnormal ¹⁸O-depleted oxygen signature. There is a slight indication of decreasing  ¹⁸O with decreasing ¹⁴³Nd/¹⁴⁴Nd, with the lowest values (ca. +5.00) in samples with ¹⁴³Nd/¹⁴⁴Nd = 0.51306. Linear extrapolation to ¹⁴³Nd/¹⁴⁴Nd = 0.51295, the lowest values observed in Iceland, suggests that  ¹⁸O in Iceland plume mantle olivine cannot be less than +4.8‰, similar to lowest values observed in HIMU OIB. Hyperbolic extrapolation can yield slightly lower  ¹⁸O, but the minimum  ¹⁸O_ol is constrained to > ca. 4.6 by plausible Nd contents of the enriched plume mantle. We conclude that  ¹⁸O_ol values < ca. 4.8 in Icelandic lavas do reflect crustal interactions, but that the slightly low mantle  ¹⁸O_ol indicated is consistent with a similar origin for the plume to the extreme HIMU plumes, as also suggested by Pb isotopes.

Chauvel C & Hémond C, Mineral Mag, 62A, 308-309, (1998).

Harmon RS & Hoefs J, Contrib. Mineral Petrol, 120, 95-114, (1995).

Thirlwall MF, Gee MAM, Taylor RN & Murton BJ, Mineral Mag, 62A, 1507-1508, (1998).

E01 : 2B/23 : F1

Generation of the Tertiary East Greenland Flood Basalts and Implications for the Tertiary Iceland Plume

Charles E. Lesher (lesher@topaz.ucdavis.edu)¹,

Christian Tegner (christian.tegner@geo.aau.dk)²,

Lotte M. Larsen (melchior@geus.dk)³ &

Stuart W. Watt (watt@geus.dk)³

¹ Dep. of Geology, University of California, Davis, CA 95616, USA

² Dep. of Earth Sciences, University of Aarhus, DK-8000 Aarhus C, Denmark

³ Geological Survey of Greenland and Denmark, Thoravej 8, DK-2400 København NV, Denmark

Six thousand meters of flood basalt erupted onto the East Greenland volcanic rifted margin close to the proposed track of the Tertiary Iceland plume at the time of continental breakup in the Northeast Atlantic (~55 Ma). A representative profile through this voluminous lava succession includes more than 330 flow units for which we have measured the major element compositions and the rare-earth-element (REE) compositions of a subset (150 units). This high-resolution geochemical record reveals remarkable temporal changes in magma composition related to the tectonics of continental breakup and establishment of seafloor spreading in the Northeast Atlantic, and to the conditions for sublithospheric mantle melting. The most striking association is trace element enriched, FeTi basalt (1.63-6.18 wt% TiO₂; La/Sm_N = 1.09-2.11 and Dy/Yb_N = 1.18-1.62), typical of continental flood basalt provinces elsewhere, and trace element depleted, low-Ti basalt (0.82-1.96 wt% TiO₂; La/Sm_N = 0.42-0.94; Dy/Yb_N = 0.99-1.14), resembling normal MOR basalt. MOR-like basalts are intercalated with FeTi basalt only within the middle portion of the succession. For both suites there are systematic up-section changes in REE fractionation unrelated to crustal contamination and decoupled from major elements variations related to crystal fractionation. For example, in the lower portion of the succession, totalling 193 flow units, the La/Sm_N of FeTi basalt increases from 1.2 to 1.6, while the Dy/Yb_N decreases from 1.6 to 1.3. Over this same stratigraphic interval La/Sm_N of the MOR-like basalts increases from 0.4 to 0.8, while Dy/Yb_N ratios are close to unity. Conversely, in the upper portion of the succession MOR-like basalts are absent, and La/Sm_N and Dy/Yb_N for FeTi basalts correlate positively. These regular changes in REE fractionations can be related to fundamental changes in temperature and composition of the melting source, and melt segregation dynamics. Forward modelling of the REEs shows that changes in REE fractionations reflect a temporal decline in mean degree (F) and pressure of melting, interpreted to reflect shoaling of the melting column due to falling mantle potential temperature of ~80°C. Concurrent eruption of FeTi and MOR-like basalts shows that flood volcanism was linked to the establishment of seafloor spreading in the region, and that the mantle was heterogenous. The high rate of melt productivity for the East Greenland flood basalts is in conflict with the geochemical evidence for modest and declining degrees of melting during emplacement of the lava succession. These observations are difficult to reconcile with models of a starting or incubating plume centered beneath the central East Greenland rifted margin prior to continental breakup. We suggest that the inconsistency between melt productivity and F can be explained by active (enhanced) upwelling of mantle along the rifting continental margin.

E01 : 2B/24 : F1

Implications for the Source of Basalts from Jan Mayen, Atlantic Ocean from Trace Elements and He, Sr, Nd and Pb Isotopes

Finlay M. Stuart (f.stuart@surrc.gla.ac.uk)¹,

Robert M. Ellam (r.ellam@surrc.gla.a) &

Godfrey Fitton (gfitton@glg.ed.ac.uk)²

¹ Isotope Geoscience Unit, Scottish Universities Research and Reactor Centre, East Kilbride, UK

² Dept. Geology and Geophysics, University of Edinburgh, Edinburgh, UK

Jan Mayen is situated on a 300 km long ridge south of the West Jan Mayen Fracture Zone between the Mohns and Kolbeinsey Ridges in the North Atlantic Ocean. Seismic profiles suggest that the southern end of the Jan Mayen Platform is composed of a continental fragment stripped from the Greenland margin during the northward propagation of the Kolbiensey Ridge c.43 Ma ago. We have made detailed geochemical analysis of a suite of alkali basalts with 7 to 15% MgO which span the full stratigraphic range of sub-aerial volcanism on Jan Mayen. Major and trace element variations are consistent with low pressure differentiation, incompatible element abundances are higher than local MORB for similar MgO testifying to an enriched source region beneath the ridge. All samples have positive  Nb values, thought to be indicative of Iceland plume material, and Ce/Pb (19-60) and Ba/La (13-30) that are higher than typical oceanic basalts. High Ce/Yb are consistent with melting in the garnet stability field (>60 km) perhaps due to the lowering of mantle temperatures near the fracture zone. The basalts are isotopically homogeneous (⁸⁷Sr/⁸⁶Sr = 0.7034-0.7036; ¹⁴³Nd/¹⁴⁴Nd = 0.51284-0.51291; ²⁰⁸Pb/²⁰⁴Pb = 38.19-38.5) and are distinct from Mohns and Kolbeinsey Ridge MORB. Sr isotope compositions are the most radiogenic, and Nd the least radiogenic, in the North Atlantic and represent an end-member composition which is only overlapped by the most extreme values measured in Iceland and the Vesteris seamount basalts. The isotopic homogeneity, the lack of a correlation between isotopic composition and stratigraphy or magmatic differentiation, and the high Ce/Pb rule out the incorporation of any of the crustal fragment. Collectively the isotope and trace element data have characteristics of metasomatised mantle peridotite nodules rather than mantle contaminated by subducted sediments or oceanic crust, and we tentatively explain the chemical enrichment by ancient metasomatism of lithosphere.The ³He/ ⁴He of coexisting clinpyroxene and olivine (5.1-6.3 Ra) are lower than typical MORB and indistinguishable from each other, providing no evidence that initially higher-than-MORB ³He/⁴He have been lowered by magmatic ingrowth or crustal assimilation. Despite the similarity of Pb isotopes and  Nb with Icelandic basalts, the low ³He/⁴He demonstrates that the heat source responsible for melting beneath Jan Mayen is unrelated to the thermal anomaly beneath Iceland, or the proto-Iceland plume responsible for the Tertiary volcanism in E. Greenland, where ³He/ ⁴He are higher than MORB. The evidence tends to confirm geophysical and bathymetric evidence for the absence of a deep mantle plume, such as a linear track of volcanic islands, thermal anomaly and plume swell.

E01 : 2B/25 : F1

Generation and Evolution of West and East Greenland Picrites: Evidences from Melt Inclusions

Charles K. Brooks (kentb@geo.geol.ku.dk)¹,

Lia N. Kogarko (kogarko@geokhi.msk.su)²,

Troels F. D. Nielsen (nielsent@dlc.ku.dk)³,

Igor D. Ryabchikov⁴,

Irina P. Solovova⁴ &

Victor A. Turkov (elkor@geokhi.msk.su)²

¹ Geological Institute University of Copenhagen, Oester Voldgade, 10, Copenhagen, 1350, Denmark

² Vernadsky Institute, Kosygin str., 19, Moscow, 117975, Russia

³ Danish Lithosphere Centre, Oester Voldgade, 10L, Copenhagen, 1350, Denmark

⁴ IGEM, Staromonetny, 39, Moscow, Russia

An attempt was made to estimate composition of primary magmas, mantle temperatures and source compositions for picritic rocks from Greenland through the investigation of melt inclusion compositions and thermodynamic modelling of these data. The compositions exhibit wide ranges in MgO (5-20%) and FeO (3-18%) contents. This could be due partly to the mixing of melts formed at diverse levels of the melting column and partly to the fractionation of olivine in intermediate magma chambers and/or the involvement of several sources. Trapped melts in olivines and clinopyroxenes from East Greenland are characterised by higher TiO₂/Al₂O₃ and in many cases higher CaO/Al₂O₃ ratios by comparison with melt inclusions from West Greenland. The concentrations of trace elementsanalysed in homogenised melt inclusions by ion microprobe reveal significantly higher incompatible element levels in trapped melts from East Greenland picrites and exhibit relative enrichment in LREE. Melt inclusions from West Greenland in the majority of cases are characterized by Ca/Al ratios identical to pyrolite, and, therefore, they are likely to be derived by olivine fractionation from primary magmas, produced by high degrees of partial melting of material identical to primitive mantle with respect to the major elements (possibly normal asthenosphere). In contrast, Ca/Al ratios in East Greenland inclusions exhibit considerable scatter and in the majority of cases they are higher by comparison with pyrolite. This could have been explained by parent magmas being the low fraction melts of pyrolite at very high pressures (deep inside the garnet stability field), but large variations of FeO contents at MgO>10% would require that the primary melts were produced in the wide range of pressures or alternatively that several sources were tapped. The elevated Ca/Al ratios appear to reflect the composition of source material, which may be represented by certain varieties of "enriched harzburgites". Enriched harzburgite, from which E. Greenland picritic melts were produced, could represent a reaction in a heterogeneous plume with entrained former lithospheric mantle. The estimated fO₂ values (olivine+chrome spinel equilibrium, Ballhaus et al., 1990) are higher for East Greenland picrites than for West Greenland picrites, which is also consistent with an enriched source for the former.

E01 : 2B/26 : F1

The Distribution of the Platinum Group Elements in a Basaltic Sequence ­ Application to Petrogenetic Processes

Harald Philipp

(harald.philpp@bio-geo.uni-karlsruhe.de),

Joerg-Detlef Eckhardt (detlef.eckhardt@bio-geo.uni-karlsruhe.de) &

Harald Puchelt

Institute of Petrology and Geochemistry, University of Karlsruhe, Karlsruhe, Germany

Platinum Group Elements (PGE) systematic provides information that can be used to reveal various petrological aspects, such as magma evolution, sulphide segregation or mantle source characteristics. However, many aspects of the PGE geochemistry are poorly understood.We present PGE data for a entire compositional range of the basaltic sequence from the North Atlantic Volcanic Province (NAVP), sampled during ODP-Legs 152 and 163 along the Volcanic Rifted Margin south-east of Greenland.Trace element distribution and REE-pattern of the sampled rocks clarify the history of the basaltic sequence. The oldest rocks formed before the break-up of the North Atlantic are contaminated by continental crust (Site 917 LS and MS). The uppermost rocks of Site 917 are primitive basalts (picrites). These rocks are not contaminated and mark the change from continental to oceanic magmatism. Based on isotopic and trace element data, samples of younger basalts are characterised as depleted Iceland Plume magmas, despite the REE-pattern shows a typical N-MORB signature with a depletion of the light REE (Sites 918, 990). Samples from the northern drill site are closer to the ancient Iceland plume track and show an increasing influence of plume-type magma with enrichment in light REE and higher concentration of incompatible elements (Site 988).The PGE concentrations of the basalts are relatively high compared to basalts from other locations. The behaviour of PGE can be shown by plotting the PGE against MgO as an indicator for magma evolution. Not only Ir and Ru but also Rh show a strong positive correlation with MgO. This indicates that Ir, Ru and Rh behave compatible during magma evolution.In contrast, for Pd and Pt, a curve with an inflection shows, that these elements are controlled by the sulphur content in the melt. In the primitive high Mg-basalts, Pt- and Pd concentrations increase with magma evolution, indicating that these melts are sulphur-undersaturated. At MgO about 9%, the shape of the curve reaches a turning-point resulting in a steep, positive correlation of Pt, Pd with MgO. At this value, precipitation of sulphide-minerals begins, because the melt becomes saturated with sulphur, resulting in a rapid decrease of Pt- and Pd-concentrations with continuous magmatic differentiation.

E01 : 2B/29 : F1

Evidence for High-MgO Basaltic Kerguelen Plume Activity at Bathymetric Highs between the Kerguelen Archipelago and Heard Island, Southern Indian Ocean

Dominique Weis (dweis@ulb.ac.be)¹,

Dimitri Damasceno (ddamasce@ulb.ac.be)¹,

James S. Scoates (jscoates@ulb.ac.be)¹,

Marc Schaming (mschaming@eost.u-strasbg.fr)²,

Raymond Montigny²,

Roland Schlich²,

Kirsten Nicolaysen (knic@mit.edu)³ &

Frederick A. Frey (fafrey@mit.edu)³

¹ Département des Sciences de la Terre et de l'Environnement, ULB CP160/02, B-1050 Brussels, Belgium

² EOST-ULP, CNRS, 67084 Strasbourg, France

³ Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge MA02139, USA

The Kerguelen Plateau, one of the largest submarine oceanic plateaus (2x10⁶ km²), is divided into several distinct domains. The northern sector contains the only subaerial expressions of the plateau (Kerguelen, Heard and Macdonald Islands) and is characterized by shallow water depths (<1000 m) and recent island volcanism. The recent scientific mission to the Northern Kerguelen Plateau (NKP) by the French research vessel Marion Dufresne 2 (Kerimis cruise) provided new multichannel seismic data for ODP Leg 183 drilling sites (KIP 2E & 3F, Dec 98-Jan 99) and recovered >1 ton of basaltic rocks from four dredge sites.

Three dredge sites (DR4, DR5, DR6), located on one of several gravity and bathymetric highs between the Kerguelen Archipelago and Heard Island, recovered in situ alkali basalts and picritic pillow basalts. All samples are exceptionally fresh as reflected by their Ba/Rb of ~12, typical of fresh oceanic basalt. The picritic basalts from DR6 (K-Ar:14-19 Ma) consist primarily of 20-25% large, euhedral, undeformed olivine phenocrysts (Fo_78-87) with chromite inclusions and 3-8% augite phenocrysts (En₅₁Fs₇Wo₄₂). Alkali basalts from DR5 (K-Ar: 16.5-17.7 Ma) contain numerous small skeletal olivine phenocrysts (Fo_78-84), whereas a single alkali basalt from DR4 is highly vesicular and contains abundant phenocrysts of zoned plagioclase (An_44-64) with lesser amounts of euhedral olivine (Fo_59-85) and clinopyroxene (En₄₈Fs₁₆Wo₃₆).

The alkali basalts from DR4 (5.8 wt.% MgO) and DR5 (7.1-7.5 wt.% MgO) are geochemically similar to lavas of the Lower and Upper Miocene series from the Southeast Province of the Kerguelen Archipelago. Although the olivine-rich DR6 lavas have much higher MgO contents (18.5-20 wt.%) than lavas erupted on the archipelago (4-5 wt.%), they have incompatible element abundance ratios similar to the flood basalts forming most of the archipelago. The sample from DR4 has a low, MORB-like ⁸⁷Sr/⁸⁶Sr isotopic composition of 0.7031, but with a ²⁰⁶Pb/²⁰⁴Pb > 19.6. However, all other dredged lavas have ⁸⁷Sr/⁸⁶Sr ratios from 0.705625 to 0.705694 that are consistent with the upper range of the Kerguelen Plume. An olivine separate from sample 88 from DR6 yields ³He/⁴He of 8.8±0.5 relative to air. This value overlaps with the low to intermediate range of ³He/⁴He ratios measured on Kerguelen Archipelago and Heard Island basalts The Kerimis samples constitute the first evidence for MgO-rich magmas with a Kerguelen Plume isotopic signature. We conclude that Miocene to Recent alkalic volcanism in this region of the Kerguelen Plateau has been spatially diffuse with coeval volcanism at the Kerguelen Archipelago, on Heard Island, and at submarine structures, probably seamounts, between these islands.

E01 : 2B/30 : F1

Plagioclase-Clinopyroxene Phenocryst Compositions and Magma Conduit Evolution in the Plume-Derived Alkali Basalts from Mont Crozier, Kerguelen Archipelago

Dimitri Damasceno (ddamasce@ulb.ac.be)¹,

James S. Scoates¹,

Dominique Weis¹,

Kirsten Nicolaysen (knic@mit.edu)²,

Frederick A. Frey (fafrey@mit.edu)² &

André Giret (agiret@univ-st-etienne.fr)³

¹ Earth and Environmental Sciences (DSTE), Université Libre de Bruxelles, Bruxelles, B-1050, Belgium

² Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 2139, USA

³ Dépt. Géologie-Pétrologie-Géochimie, Université Jean Monnet, 42023 Saint-Etienne, France

Basaltic magmatism in the southeast Indian Ocean associated with the Kerguelen mantle plume since ~115 Ma has produced a dissected Large Igneous Province (Kerguelen Plateau - Broken Ridge), the 5000 km long Ninetyeast Ridge and hotspot subaerial volcanism such as exposed on the 6500 km² Kerguelen Archipelago. Crustal thicknesses for the combined plateau and the overlying archipelago are estimated to be in the range of 20-25 km, thus magma conduit processes including polybaric fractionation, mixing, and crustal assimilation can be expected to play an important role in magma evolution. We are investigating phenocryst-groundmass compositional relations within the 978 m thick volcanic section of alkali basalts at Mont Crozier in the eastern Kerguelen Archipelago to better constrain these processes. The Mont Crozier basalts have remarkably homogeneous Sr-Nd isotopic ratios (0.70501-0.70535; 0.51251-0.51261), reflecting those of the Kerguelen plume itself, which effectively eliminates source variations as an important parameter.

The majority of the Mont Crozier alkali basalts are phenocrystic (5-20 vol%). At the scale of the stratigraphic section and within single thin sections, plagioclase phenocrysts (0-12 vol%) show an extreme compositional range (calcic cores: An₈₅Ab₁₅Or₀₀; ternary feldspar rims and microlites: An₀₅Ab₄₅Or₅₀). Detailed zoning profiles reveal the presence of small calcic cores (An_75-85) followed by an abrupt decrease in composition ( An=12), a relatively thick zone of oscillatory zonation, a small compositional increase near the margin ( An=5), perhaps related to decompression crystallization, and a rapid drop in the rim ( An=15) to the equilibrium microlite composition. The small amounts (0-5 vol%) of clinopyroxene phenocrysts (En₅₅Fs₁₂Wo₃₃ to En₄₀Fs₂₀Wo₄₀) are commonly zoned and Al-rich (Al₂O₃=2.3-8.1 wt%) with nearly all the Al present as AlVI, possibly reflecting pressure variations. Equilibrium-composition olivine (Fo_85-65) is a minor phenocrystic phase (0-3 vol%).

The geochemistry of the Mont Crozier and SE Archipelago alkali basalts is distinct from the transitional basalts of the archipelago by its diffuse major element trends and its apparent increasing Al₂O₃ with decreasing MgO. Preliminary results from modelling with MELTS are consistent with a polybaric, continuously replenished magmatic system and reproduce the observed phenocryst compositions and major element trends. They suggest initial crystallization of aluminous clinopyroxene followed by plagioclase (cpx/plag=2/1) at pressures of ~7 kb, corresponding to the base of the underlying Kerguelen oceanic plateau. Relatively rapid magma ascent to a subvolcanic chamber resulted in saturation in plagioclase only, followed by co-crystallization of plagioclase, clinopyroxene (much poorer in Al than at high pressure) and olivine. The relatively thick crust beneath the Kerguelen Archipelago thus plays an important role in the fractionation history of Kerguelen plume-related lavas.

E01 : 2B/31 : F1

Hafnium Isotope Systematics of Kerguelen Archipelago Lavas

Nadine Mattielli (nmattiel@ulb.ac.be)¹,

Janne Blichert-toft (jblicher@geologie.ens-lyon.fr)²,

Dominique Weis (dweis@ulb.ac.be)¹,

Dimitri Damasceno (ddamasce@ulb.ac.be)¹,

James S. Scoates (jscoates@ulb.ac.be)¹,

Fred A. Frey (fafrey@mit.edu)³,

Francis Albarede

(albarede@geologie.ens-lyon.fr)² &

André Giret (agiret@univ-st-etienne.fr)⁴

¹ DSTE, CP 160/02, Université Libre de Bruxelles, 50, Av. F.-D. Roosevelt, 1050 Brussels, Belgium

² Ecole Normale Supérieure de Lyon, 69364 Lyon, Cedex 7, France

³ EAPS, MIT, Cambridge, MA 02139, USA

⁴ Université Jean Monnet, 42023 St. Etienne, Cedex 2, France

We report Hf isotopic compositions obtained by plasma-source mass spectrometry (Plasma 54) for Kerguelen Archipelago lavas. The primary goal is to better characterize the Kerguelen Hf-isotopic endmember which, together with the Walvis Ridge, largely determines the slope and the shape of the ocean island array on the Hf-Nd isotope correlation diagram. We selected samples from basaltic stratigraphic sections that span the range of chemical compositions (tholeiitic to highly-alkaline), Sr-Nd-Pb isotopic compositions (Weis et al., 1998) and ages (0.1-29 Ma; Nicolaysen et al., 1996) of the archipelago lavas.

The Hf isotopic ratios are variable, spanning ¹⁷⁶Hf/¹⁷⁷Hf from 0.283070±6 to 0.282827±7 (2<sigma>_m) and are positively correlated with ¹⁴³Nd/¹⁴⁴Nd which ranges from 0.51288 to 0.51258. The age corrections for Hf isotopic ratios are small, a maximum of 10 ppm for the oldest lavas (29 Ma). The slightly less radiogenic age-corrected ¹⁷⁶Hf/¹⁷⁷Hf do not modify the isotopic data distribution nor the interpretation of the results. In the north-central part of the archipelago, the volcanic sections show both less (<0.28285) and more (>0.28307) radiogenic ¹⁷⁶Hf/¹⁷⁷Hf, which correlates respectively with the P(plume) and D(depleted) groups of Yang et al. (1998). For the Group D lavas, representing < 10% of the samples, the Sr-Nd and Pb-Pb isotopic trends imply the contribution of a depleted MORB-type component added to the Kerguelen Plume; our Hf data from a single Group D sample is consistent with this interpretation. From the central (Tourmente) to the east part (Crozier) of the archipelago, the Hf isotopic compositions become less radiogenic (from 0.28291 to 0.28286). This is consistent with the west-to-east progression in the age of the archipelago lavas (Nicolaysen et al., 1996), which correlates with a change from tholeiitic to midly alkaline compositions, an inferred decrease in extent of melting and a presumed increase in the depth of crystallization (Damasceno et al., 1997).

The majority of lavas have low Hf isotopic ratios (<0.28291) that correspond to the characteristics of the Kerguelen Plume, in agreement with Sr-Nd-Pb isotopic systematics (Weis et al., 1998). The Hf isotopic compositions encompass the signatures of EMI and EMII endmembers, although they may also require the contribution of an Indian MORB component. The predominance of enriched isotopic compositions, and the large isotopic variations in Hf, in conjunction with variations in Sr, Nd, and especially Pb isotopes, indicate that the dominant component generating the Kerguelen Archipelago lavas is the Kerguelen Plume, and also suggest a relatively inefficient mixing in the mantle beneath the Kerguelen Plateau.

Damasceno D, Nicolaysen K, Weis D, Scoates J, Frey FA, Yang HJ & Giret A, EOS, 78, 78, (1997).

Nicolaysen K, Frey FA, Hodges K, Weis D, Giret A & Leyrit H, EOS, 77, F824, (1996).

Weis D, Damasceno D, Frey FA, Nicolaysen K & Giret A, Min Mag, 62A, 1643-1644, (1998).

Yang YJ, Frey FA, Weis D, Giret A, Pyle D & Michon G, J Petrol, 39, 711-748, (1998).

E01 : 2B/32 : F1

Recycling of Subcontinental Lithospheric Mantle: Evidence from Os- and Nd- Isotope Compositions of Volcanic Rocks from the Caribbean Oceanic Plateau

Gerhard Brügmann

(bruegman@mpch-mainz.mpg.de)¹,

Folkmar Haff² &

Richard Walker³

¹ Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

² GEOMAR, Wischhofstr. 1-3, D-24148 Kiel, Germany

³ Dept. of Geology, University of Maryland, College Park MD 20742, USA

The Caribbean Large Igneous Province (CLIP) represents a tectonic remnant of an oceanic plateau that formed above a Pacific plume about 90 Ma ago. The Os isotopic composition of ultramafic and mafic volcanic rocks from Gorgona Island, Curacao Island and oceanic complexes of Costa Rica (Nicoya, Tortugal) have been determined to fingerprint the composition of the plume material.

The large variation of the Os isotopic composition of the volcanic rocks from the CLIP, <gamma>_Os ranges from -4 to +12, requires derivation of Os from at least 2 distinct reservoirs. However, in the <epsilon>_Nd-<gamma>_Os plane most of the samples follow 2 different, almost parallel trends, which imply the presence of a third component. The komatiitic suite of Gorgona defines a "high-<epsilon>_Nd" trend where <epsilon>_Nd tends to increase from about +9 to +11 as <gamma>_Os increases from -1 to +12. The remaining picrites and basalts follow a "low-<epsilon>_Nd" trend where <epsilon>_Nd increases from +4 to +7 as <gamma>_Os increases from -2 to +10. The coherent increase in <epsilon>_Nd and <gamma>_Os in each group can be explained by mixing of a reservoir with a generally long-term chondritic Re/Os, with a component with long-term Re enrichment, such as subducted oceanic crust. Mixing models require that if the Re-enriched component is recycled oceanic crust, the component is very old (>2 Ga) and comprises a large proportion of the source of the ¹⁸⁷Os-enriched melts (>35%). In contrast, picritic rocks from Tortugal, which represent the endmember of the trend with low <epsilon>_Nd (<5) and <gamma>_Os (<0) require a source component with a long-term subchondritic Re/Os but low Sm/Nd relative to the DMM. The only known reservoir with low <gamma>_Os (<0) and <epsilon>_Nd (<0) is the subcontinental lithospheric mantle (SCLM). It is suggested that old subducted SCLM was also recycled into this plume.

The observed ranges of ⁸⁷Sr/⁸⁶Sr (0.7028- 0.7038) and <epsilon>_Nd (+4 to +11) overlap with those observed for the Galapagos Islands and support earlier suggestions that the Galapagos plume is the source of the CLIP. For Galapagos, this heterogeneity has been explained by entrainment of asthenospheric mantle during the rise of an isotopically heterogeneous plume (White et al. 1993). The current results suggest that this heterogeneity is caused by the incorporation of both SCLM and an ancient Re-enriched component, perhaps recycled oceanic crust. This hypothesis can ultimately be tested by determining the Os isotopic composition of recent volcanic rocks from the Galapagos Islands.

E01 : 2B/33 : F1

Petrology and Geochemistry of Gabbros and Peridotites from Gorgona Island (Colombia): Plumbing System of an Oceanic Plateau

Sidonie Revillon

(Sidonie.revillon@univ-rennes1.fr)¹,

Nicholas, T. Arndt²,

Catherine Chauvel¹ &

Erwan Hallot¹

¹ Géosciences Rennes, Université de Rennes I, 35042 Rennes, France

² Université Joseph Fourier, 15 rue M. Gignoux, 38031 Grenoble, France

To study the magmatic evolution of an oceanic plateau we studied the volcanic and plutonic rocks of Gorgona island, which is part of the Cretaceous-Tertiary Caribbean oceanic plateau. Known as the site of the only Phanerozoic komatiites, the Gorgona sequence also includes basalts, picrites, gabbros and peridotites. Except for some enriched basalts, all the volcanics are depleted in LREE; La_N/Yb_N is 0.43-0.55 in komatiitic basalts, 0.14-0.31 in komatiites and 0.04-0.08 in picrites. The depleted volcanics have highly positive <epsilon>_Nd (+8.4 to +13.7). The extreme variation in REE ratios associated with relatively uniform <epsilon>_Nd values has been attributed to fractional melting of their mantle source.

The plutonic rocks in the interior of the island were studied as possible representatives of the magma chambers that fed the volcanic series. These rocks range from ophitic plag-cpx gabbros, through olivine gabbro, to wehrlite and dunite. Two textural types are observed in the olivine gabbros and peridotites: some only contain large euhedral olivines; others contain abundant, small, rounded olivines, either isolated or enclosed in anhedral clinopyroxene. Olivine compositions range from Fo₈₈ to Fo₈₀ and show no obvious correlation with olivine habit. In the gabbros, REE patterns range from almost flat (La_N/Yb_N 0.91-1.17) to moderately depleted (La_N/Yb_N 0.31-0.41); in the peridotites, the depletion is more pronounced (La_N/Yb_N 0.15-0.19). Sr and Nd isotope compositions of plutonic rocks overlap those of the volcanics (⁸⁷Sr/⁸⁶Sr_o 0.70425 to 0.70301; <epsilon>_Nd +9.6 to +11.2).

The similar range of REE ratios and isotope compositions of the volcanic and plutonic rocks suggests that the two are comagmatic. However, the exact relationship between the extrusive and intrusive rocks is problematic. Two features of the peridotites suggest that they come from ultramafic parental liquids: (1) the abundant, small uniformly distributed olivine phenocrysts texture suggest in-situ nucleation and crystallisation; (2) La_N/Yb_N values in peridotites are systematically lower than those of erupted basalts but similar to those of komatiites. On the other hand, if olivine compositions are used to estimate the MgO content of the host liquid, values of 12-13% MgO are obtained. This suggests that the liquid from which the peridotites crystallized was a magnesian basalt and that the higher MgO values in the whole rocks (27.6-35.4%) resulted from the accumulation of 5 to 55% excess olivine.

The plumbing system that fed the Gorgona plateau apparently was complex. Enriched and depleted basaltic liquids formed through mantle melting and erupted as lava flows or solidified as plag-cpx gabbros. Ultramafic liquids produced by advanced fractional melting of the same source either travelled directly to the surface to erupt as komatiite flows, or they were trapped in the crust where they evolved to basaltic liquids that differentiated to give gabbro and peridotite.

E01 : 2B/34 : F1

The Cache Creek Terrane (CCT) - Canadian Cordillera: Remnant of a Permo-Triassic Oceanic Plateau

Marc Tardy (tardy@univ-savoie.fr)¹ &

Henriette Lapierre (lapierre@ujf-grenoble.fr)²

¹ LGCA-UPRESA 5025, Univ. Savoie, 76376 Le Bourget/lac, France

² LGCA-UPRESA 5025, Univ Joseph Fourier, 38031 Grenoble, France

West of Prince Georges, British Columbia, the CCT divides the Stikinia and Quesnellia Paleozoic-Triassic arc-terranes which accreted to North America during the Late Jurassic. The Pinche fault bounds the CCT to the west. CCT is composed of mid-Permian and Upper Triassic basalts and diabases interbedded with pelagic sediments and greywackes. Slices of undated basalts, dolerites, gabbros and peridotites are exposed within the Pinche Fault System (PFS).

Permian tholeiitic basalts are slightly enriched in LREE relative to HREE [1.3 < (La/Yb)cn < 1.5]. Relative to Primitive Mantle (PM), they are enriched in Th, Ta and Nb and their <epsilon>Nd(T=260 Ma) = -1.5. Among the Triassic rocks, three types (tholeiitic Types 1&2; alkalic Type 3) have been distinguished. Type 1 is characterized by high MgO contents (> 6.5%), slightly enriched LREE relative to HREE [1.1 < (La/Yb)cn< 1.4; 0.8 < (La/Sm)cn < 1.05], and Nb, Ta, Th enrichments relative to PM. Type 2 is distinct compared with Type 1 (lower Th and convex-upwards REE patterns [(La/Sm)cn= 0.6]). Finally, Type 3 has the highest Zr, Nb and Ta abundances and the greatest LREE enrichment relative to HREE [7.6 < (La/Yb)cn < 10.3; (La/Sm)cn ~ 2). Types 1 and 3 have similar <epsilon>Nd(T=210 Ma) that ranges between +1.99 to +5.31; Type 2 has higher <epsilon>Nd_i = +7.4 to 9.1.

Major and trace element features of the PFS gabbros, basalts and dolerites are consistent with those of normal oceanic crust. However, these rocks differ from typical N-MORB with low <epsilon>Nd_i (assuming a Triassic age), similar to those of Type 2 CCT basalts.

The CCT appears to comprise Permian basalts emplaced in an intra-oceanic island environment and Upper Triassic magmatic rocks which likely represent different levels of an oceanic plateau crust. The diversity of the Triassic magmas compositions probably reflects a combination of differences in partial melting processs and heterogeneity of the plume source.

Session E01:3A

E01 : 3A/01 : F1

One Plume or Two beneath the African Rift: An Isotopic Perspective

Nick Rogers (n.w.rogers@open.ac.uk)¹,

Ray Macdonald (r.macdonald@lancaster.ac.uk)²,

Rhiannon George (r.m.m.george@open.ac.uk)¹ &

Martin Smith³

¹ Dept. of Earth Sciences, The Open University, Milton Keynes, UK

² School of Environmental Sciences, Lancaster University, Lancaster, UK

³ British Geological Survey, Edinburgh, UK

There is current debate concerning the number of mantle plumes influencing magmatism and tectonics beneath East Africa. Gravity and topographic anomalies reveal two broadly circular, dynamically supported plateaus, implying two distinct mantle plumes. However, a recent suggestion invokes impact of the Afar mantle plume beneath southern Ethiopia at 45 Ma, followed by rapid lateral flow of plume material guided by the topography of the base of the lithosphere to feed magmatism through thin-spots generated by Mesozoic or early Tertiary extension. Here we use Nd and Sr isotope analyses of Tertiary and younger basalts from throughout East Africa to distinguish between these two alternatives.

The East African Rift cuts across the East African plateau through the boundary between the Panafrican mobile belt and the Tanzanian craton of late Archaean/early Proterozoic age. This contrast in the age of the basement is reflected in the radiogenic isotope ratios in Tertiary to recent basalts associated with the Rift. Those erupted through the craton or its remobilised margins have ¹⁴³Nd/¹⁴⁴Nd ratios < 0.51275 and ⁸⁷Sr/⁸⁶Sr > 0.7035 and define two flat arrays within which ¹⁴³Nd/¹⁴⁴Nd varies little but ⁸⁷Sr/⁸⁶Sr increases to values >0.705. By contrast, basalts erupted through the mobile belt have ¹⁴³Nd/¹⁴⁴Nd >0.51275 and ⁸⁷Sr/⁸⁶Sr ~ 0.7035, defining an unusually steep negative array. These two arrays have a common end member which probably reflects the composition of the East African mantle plume (¹⁴³Nd/¹⁴⁴Nd ~ 0.51275, ⁸⁷Sr/⁸⁶Sr ~ 0.7035).

The isotope systematics of the Afar and Ethiopian flood basalts differ markedly from those of the Kenyan Rift. In the plume-derived HT2 group of the Ethiopian traps, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.5129 - 0.5130 while ⁸⁷Sr/⁸⁶Sr > 0.704. The majority of basalts from Afar also have similarly high ¹⁴³Nd/¹⁴⁴Nd although ⁸⁷Sr/⁸⁶Sr extends to lower values of 0.7035. These isotopic differences between the northern Ethiopian/Afar and Kenyan basalts indicate two distinct source regions and therefore two mantle plumes.

Basalts from southern Ethiopia have isotope sytematics more comparable with the Kenyan Rift basalts and thus support the conclusion that this earliest episode of Tertiary basaltic volcanism in east Africa was related to the East African mantle plume rather than the Afar plume. Northward movement of the African plate subsequently induced the southward migration of rifting and magmatism during the past 45 My, whereas the first magmatic product of the Afar plume was the Ethiopian flood basalts at 30 Ma.

E01 : 3A/02 : F1

Evolution in Space and Time of Afar Depression Volcanism in the Last 3 Ma

Pierre Lahitte (lahitt@geophy.geol.u-psud.fr)¹,

Tesfaye Kidane (tesfaye@ipgp.jussieu.fr)²,

Pierre-Yves Gillot (gillot@geophy.geol.u-psud.fr)¹ &

Vincent Courtillot (courtil@ipgp.jussieu.fr)²

¹ Labo. de Géochronologie, Bât 504 uni. Paris-Sud, 91405 Orsay, FRANCE

² Laboratoire de Paléomagnétisme, IPGP, place Jussieu, 75252 Paris Cedex 05, FRANCE

Located between the Ethiopian plateau and the Danakil micro-plate, the Afar depression is a triple junction characterized by thinned continental crust, where three rift systems merge (Red Sea, Gulf of Aden and East African Rift). For the last 3 million years, intense fissural volcanic activity caused the emplacement of the so-called, trap-like "stratoid series", which now covers most of the depression, except the presently active rift segments. More than 100 age determinations have been obtained using the Cassignol-Gillot K/Ar technique performed on both mineral separates and groundmass. This technique was developed for dating sub-historic rocks with an uncertainty of only a few centuries and is here particularly suitable for the highly contaminated Afar samples. Our newer results agree with earlier K/Ar ages. However, the relatively low uncertainties associated with the new determinations allow us to better constrain the mechanism of volcanic emissions. From 3 Ma to the Present, volcanic activity appears to have been more or less continuous, with less marked quiescent periods than had been suggested. Stratoid series activity decreased between 1.0±0.1 Ma and 0.6±0.1 Ma, when magmatic activity became confined to the Asal, Manda Inakir and Manda Harraro axial rifts, where an inverse relationship between extension and volcanic activity has been observed, and to some central volcanoes located around (e.g. Moussa Ali and Dama'Ali). The chrono-stratigraphy of Afar volcanics displays interesting systematic patterns: the earliest occurence of stratoid volcanism in central Afar migrated from the WSW to the ENE margin, between 3 and 1 Ma, corresponding to fissural activity along the rift segments. The age distribution suggests that during emplacement of the stratoid series, the active rift was mainly located at the eastern end of the depression and propagated from SE to NW. These data constrain models of rift propagation, opening of the depression and separation of the Danakil horst.

E01 : 3A/03 : F1

Os Isotopic Data from Ethiopian and Djiboutian Basalts

Laurie Reisberg (reisberg@crpg.cnrs-nancy.fr)¹,

Bernard Marty (bmarty@crpg.cnrs-nancy.fr)¹,

Catherine Deniel

(deniel@opgcf5.univ-bpclermont.fr)²,

Raphael Pik (rpik@crpg.cnrs-nancy.fr)¹,

Dereje Ayalew (dereje@crpg.cnrs-nancy.fr)¹ &

Catherine Spatz (cspatz@crpg.cnrs-nancy.fr)¹

¹ CRPG/CNRS, BP 20, 54501 Vandoeuvre-les-Nancy, France

² Lab. de Geologie, CNRS UMR 6524, 5, rue Kessler, 63038 Clermont Ferrand Cedex, France

One major question concerning the generation of flood and early rift basalts is the extent to which their chemical compositions are influenced by crustal assimilation. Os isotopes provide an excellent means of addressing this issue, due to the extremely large difference in Os isotopic composition between the mantle (¹⁸⁷Os/¹⁸⁸Os ~ 0.13) and the continental crust (¹⁸⁷Os/¹⁸⁸Os ~ 1 to 2). For this reason, we have undertaken a Re-Os study of modern rift and Oligocene flood basalts from Ethiopia and Djibouti. Data from the rift basalts define a rough positive correlation between ¹⁸⁷Os/¹⁸⁸Os and 1/Os, with ¹⁸⁷Os/¹⁸⁸Os ranging up to 0.47 for a sample with 2 ppt Os. This value far exceeds the upper limit of the OIB range. High Ti flood basalts define an almost parallel correlation, but shifted to lower ¹⁸⁷Os/¹⁸⁸Os ratios. These latter basalts have OIB-like isotopic and trace element characteristics (Pik, 1997) and high ³He/⁴He ratios suggestive of deep mantle derivation (Marty et al., 1996). The ¹⁸⁷Os/¹⁸⁸Os vs 1/Os correlations observed in these suites strongly suggest crustal contamination. Such trends could be produced by either AFC processes or crystal fractionation followed by contamination. He isotopes are also highly sensitive to crustal assimilation. Direct comparison of He (Marty et al., 1996) and Os isotopes yields no relationship. However, if the proportion of crustal assimilation required to explain the Os ratios is plotted against the ³He/⁴He ratio a negative correlation is observed. (Decreasing ³He/⁴He implies increasing assimilation.) The wide range of He isotopic variation observed suggests that contamination was accompanied by degassing. Assuming reasonable Os compositions for the crustal endmember, including that measured for a local crustal sample, it can be shown that crustal contamination proportions were usually considerably lower than 5%. Thus contamination had little effect on the Sr, Nd and Pb characteristics of these rocks. The ¹⁸⁷Os/¹⁸⁸Os of samples with high Os concentrations most probably reflects their mantle sources. The highest concentration rift sample ([Os] = 700 ppt) yields a ¹⁸⁷Os/¹⁸⁸Os ratio of 0.140, which is near the high end of the OIB range. In contrast, the most Os-rich ([Os] = 147 ppt) high Ti flood basalt has an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.127. This sample has a high ³He/⁴He ratio of 12.6 (Marty et al., 1996) suggestive of deep mantle derivation. Nevertheless its Os isotopic ratio falls at the bottom of the OIB range. If this sample is representative of the plume source, then this source was not influenced by either recycled crust or the outer core, both of which are thought to have radiogenic Os compositions.

Marty B, Pik R & Gezahegn Y, Earth Planet Sci. Lett, 144, 223-237, (1996).

Pik R, PhD thesis, Universite d'Aix-Marseille III, (1997).

E01 : 3A/04 : F1

New Evidence for Oceanic Crust Recycling in the Hawaiian Plume: Trace Elements and Isotopes of Melt Inclusions in Olivines from Mauna Loa Lavas

Alexander V. Sobolev (asobolev@glas.apc.org)¹,

Albrecht W. Hofmann (hofmann@mpch-mainz.mpg.de)²,

Igor K. Nikogosian (niki@geo.vu.nl)¹,

Nobumichi Shimizu (nshimizu@whoi.edu)³ &

Marc Chaussidon (chocho@crpg.cnrs-nancy.fr)⁴

¹ Dept. of Petrology, Vrije Universiteit De Boelelaan 1085, 1081 HV, Amsterdam, Netherlands

² Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, D-55020 Mainz, Germany

³ Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, USA

⁴ CRPG-CNRS, BP 20, 54501, Vandoeuvre-les-Nancy Cedex, France

It is widely accepted that the geochemical characteristics of Hawaiian magmas require involvement of several source reservoirs, including at least one similar to oceanic crust (e.g. Hofmann & Jochum, 1996; Eiler et al, 1996; Hauri, 1996). This crust-related component (CRC) is inferred to originate from oceanic gabbro rich in plagioclase and thus might be responsible for the relative enrichment of Hawaiian tholeiites in Sr and Ba, and depletion in Th and U (Hofmann & Jochum, 1996), low ¹⁸O (Eiler et al., 1996), and unradiogenic Pb (Hauri, 1996). However, it has not been clear whether the magmas acquire CRC signatures by recycling such crust through the plume source, or by contamination of the plume magmas with present-day lower oceanic crust (e.g. Hofmann & Jochum, 1996; Eiler et al, 1996). Here we report the discovery of melts that display extremes of most of the above-mentioned features but at the same time are not in equilibrium with plagioclase-bearing crustal materials.

Melts extremely enriched in Sr (Sr/Ce norm=2.9-5.6) were found as melt inclusions in olivine phenocysts from Mauna Loa picrites covering the last 50 000 years of volcano activity. These melts represent less than 5% of the melt inclusions studied so far (> 140) which are on average almost identical in composition to Mauna Loa parental magmas. Sr-rich melts also contain significant negative Th, B and Zr anomalies, positive Ba anomalies and depletion in LREE, K and P. All these characteristics are typical for ophiolitic gabbros enriched in cumulus plagioclase. However, reconstructed original major element compositions of Sr-rich melts closely resemble typical primitive Mauna Loa picrites, all of which are severely undersaturated in plagioclase. This rules out an origin of the "plagioclase signatures" in Sr-rich melts by reaction with plagioclase bearing rocks. Instead, the observations are consistent with melting of a composite mantle source affected by partial melts from (originally gabbroic) CRC in eclogite facies. The compositions of normal melt inclusions and of Mauna Loa lavas can be explained by similar processes but require lower contributions from cumulus plagioclase. Pb isotopes of melt inclusions were studied by Ims 1270 ion probe at WHOI. Two normal inclusions were found to be identical to Mauna Loa lavas, while all three Sr-rich inclusions show less radiogenic Pb. Given the large analytical errors of SIMS data, statistically significant differences (>95% confidence level) were found only for one Sr-rich inclusion. This result suggests low time-integrated U/Pb ratios in CRC, consistent with (originally) high plagioclase contents. Data on O, B and Li isotopes in both normal and Sr-melts obtained on Ims-1270 ion probe at CRPG-CNRS will be discussed as well.

Hofmann AW & Jochum KP, J. Geophys. Res, 101, 11831-11839, (1996).

Eiler JM et al., Earth. Planet. Sci. Lett, 144, 453-468, (1996).

Hauri EH, Nature, 382, 415-419, (1996).

E01 : 3A/05 : F1

Lithospheric Versus Plume Contributions to Ocean Island Magmatism: Constraints from Ilha da Trindade, South Atlantic

Joanna C. Greenwood (jcg24@esc.cam.ac.uk)¹,

Sally A. Gibson (sally@esc.cam.ac.uk)¹ &

Robert N. Thompson (r.n.thompson@durham.ac.uk)²

¹ Dept. Earth Sciences, Downing Street, Cambridge, UK

² Dept. Geological Sciences, South Road, Durham, UK

The island of Trindade lies at the western extremity of the Vitória seamount chain. This narrow bathymetric high, extending 1,140 km from the Brazilian coast, is believed to represent the <40 Ma track of the fossil Trindade mantle plume. Predictions place the 85 Ma starting-plume head impact beneath central Brazil (Gibson et al., 1995). This impact led to the formation of numerous small igneous provinces, the majority of which erupted only alkaline magmas. Basaltic volcanism is confined to local rift zones where the lithosphere was thinned relative to that forming the surrounding cratons and mobile belts. Geochemical and isotopic studies of magmatism associated with this plume throughout its 85 Ma history may lead to important conclusions about the secular and geochemical variations in plumes.

Here we present new trace element analyses and isotopic ratios for highly silica-undersaturated alkaline igneous rocks from the island of Trindade in the South Atlantic (20º30'S, 29º19'W). Melt generation beneath the island appears to have occurred in two stages: an early (3.5-2 Ma) phase that included the emplacement of phonolitic ignimbrites and domes, together with minor nephelinitic and lamprophyric dykes and flows; and a late (<0.27 Ma) phase predominantly consisting of olivine-melanephelinitic flows and pyroclastics (Greenwood, 1998). All of the mafic igneous rocks are characterised by low normalised abundances of K (e.g. [La/K]_n= 0.8-5.9) and moderate enrichments in light- relative to heavy-rare earth elements (e.g. [La/Yb]_n= 17.6-29.6). This implies that the magmas were derived from an "enriched" mantle source and that phlogopite was residual during partial melting. The most recent olivine-melanephelinites (MgO= 11-12.6 wt.%) have extremely high abundances of both Fe₂O₃* (17.6 wt.%) and TiO₂ (6.2 wt.%). These concentrations are comparable to those observed for pyroxenite veins in mantle xenoliths. Pb-isotopes imply that this metasomatism represents a recent enrichment event (Halliday et al., 1992) and places one of the contributing melt source regions within the suboceanic lithospheric mantle.

Low Lu abundances (0.19-0.26 ppm) in both early- and late-phase mafic magmas (MgO= 10-13 wt.%) suggest a contribution from melts generated within the spinel-garnet transition zone (70-90 km). This corresponds with the estimated thickness of the mechanical boundary layer (MBL) of the underlying 70 Ma lithosphere and implies a melt contribution from either: (i) veined garnetiferous mantle at the base of the MBL and/or (ii) the upwelling Trindade plume. The alkaline rocks of Trindade were formed by small degrees of partial melting relative to those which generated larger oceanic islands such as Hawaii (Watson and M^cKenzie, 1991). As the lithosphere beneath these islands is predicted to be of approximately the same thickness, this suggests that the present day Trindade mantle plume may represent a relatively small thermal anomaly.

Gibson SA, Thompson RN, Leonardos OH, Dickin AP & Mitchell JG, J. Petrology, 36, 189-229, (1995).

Greenwood JC, Min. Mag., 62, 687-696, (1998).

Halliday AN, Davies GR, Lee D-C, Tommasini S, Paslick CR, Fitton JG & James DE, Nature, 359, 623-627, (1992).

Watson S & M^cKenzie D, J. Petrology, 32, 501-537, (1991).

E01 : 3A/06 : F1

The Piton de la Fournaise Volcano (Reunion Island, Indian Ocean): Temporal Evolution from High Resolution Pb Isotopes

Delphine Bosch (bosch@dstu.univ-montp2.fr)¹,

Francis Albarede

(albarede@geologie.ens-lyon.fr)² &

Philippe Telouk²

¹ UMII, UMR-CNRS 5567, Pl. Bataillon, cc 066, 34095 Montpellier, France

² ENS Lyon, UMR CNRS 5570, Lab. Sciences de la Terre, 46, Allee d'Italie, 69364 Lyon, France

The large active Piton de la Fournaise volcano (Reunion Island, Indian Ocean) yields several thick superposed lava series for which affinity changes with time from mildly alkalic to tholeiitic. In this study, we report high precision Pb isotopic data from 90 samples collected in four distinct lava series. These series correspond to distinct eruptive cycles which have occurred between 527-290 ka (Rivière des Remparts), 180-105 ka (Nez de Boeuf) and 70-40 ka (Langevin). In addition, twenty-two samples from lavas erupted between 1700 and 1956 have been also analysed. Analyses have been carried out using the Fisons Instrument Plasma 54 at the ENS Lyon which allows high accuracy and reproducibility with external precision of ca. 100-200 ppm for the Pb/Pb ratios. This approach offers the opportunity to study the temporal evolution of lavas erupted within a single volcano, to pinpoint differences which may occur within one single series, and to identify individual component involved in magma genesis. Studied samples provide enriched lead isotopic signatures significantly higher than the range of variation commonly observed for the modern Hawaïan volcanoes. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios range from 18.7904 to 19.0100, from 15.5744 to 15.6074 and from 38.8239 to 39.0749 respectively, except for one ankaramitic sample from the Langevin series which exhibits the lowest Pb signature (18.7705, 15.5318, 38.7251). Reported on the Pb/Pb diagrams, rocks from the Rivière des Remparts have the highest Pb ratios defining a field towards plume-type component. This suggests that the plume component is most clearly evidenced during the early alkalic eruptive phases. The "Nez de Boeuf" samples exhibit a plurimodal distribution but this could result from discontinuities within this series. A first group of samples have Pb isotope characteristics similar to those of the Rempart lavas. A second group, with lower ²⁰⁶Pb/²⁰⁴Pb ratios for similar or higher ²⁰⁷Pb/²⁰⁴Pb ratios, supports participation of an enriched ²⁰⁷Pb/²⁰⁴Pb component. Alkalic samples from the Langevin lavas register the influence of a HIMU component whereas samples with tholeitic affinity have significantly lower Pb isotopic compositions and indicate contribution of a more depleted component. Finally, the compositionnaly bimodal modern samples present the largest spread wich encompasses the variations of the Langevin and Nez de Boeuf lavas series. Among the four studied sequences, the transition from the oldest lavas to the youngest is accompanied by a gradual decrease of the lead isotopic ratios which do not seem to be controlled by the alkalic degree of the lavas. We infer that this can be explained by involvement of an increasing proportion through time of depleted source material from the lithosphere on a hotspot-type material reservoir. Fluctuation of lead isotopic compositions within one single sequence supports a short residence time combined with mixing at shallow level. These results question the general belief of homogeneity of the Piton de la Fournaise volcano.

E01 : 3A/09 : F1

The Foundation Hotspot-Pacific Antarctic Ridge Interaction: Complexities of the Channelling Model Revealed

Marcia Maia (marcia@univ-brest.fr),

Jerome Dyment,

Pascal Gente &

David Jouannetaud

UMR 6538 CNRS Domaines Oceaniques, IUEM, Technopole Brest-Iroise, 29280, Plouzane, France

The interaction between the Foundation Hotspot and the Pacific-Antarctic Ridge (South Pacific) is one of the two known cases where a ridge is approaching a hotspot. This result in a change in the morphology of the volcanoes along the chain as the age of the lithosphere at the moment of edifice formation diminishes progressively. Chemical composition of the basalts also changes revealing a mixing between the plume source and the ridge. This zone of mixing defines what we call the ridge-hotspot interaction zone.

The geophysical mapping of the interaction zone between the Foundation Hotspot and the Pacific-Antarctic Ridge (PAR) revealed a complex structural pattern. 400 km away from the PAR axis, volcanism is present along two sub-parallel lines, instead of the single line of seamounts observed further away from the ridge. The first volcanic line lies in the prolongation of the chain of seamounts. The second line is located roughly 100 km south of it and this distance decreases to 50 km near the PAR axis. Both lines are formed by volcanoes grouped into elongated ridges at their oldest part and by smooth and small ridges near the PAR axis. The volume of volcanism associated with the first, northern line is roughly the double of that of the southern line. Magnetic modelling of the volcanoes showed that both north and south lines were formed simultaneously. The northern line, located on the prolongation of the Foundation seamounts and displaying more important volumes of magma would be the main locus of the volcanic activity.

We interpret the pattern of volcanic distribution on the interaction zone as resulting partly from lithospheric control on the emplacement of the volcanoes. In this case, the secondary southern line can result from magmas trapped by fractures near the edge of the flexural arch associated with the northern main line. The progressive reduction of the distance between the northern and the southern lines towards the PAR axis could result from the reduction in the mechanical strength of the plate at the moment of edifice formation, since the flexural wavelength also diminishes. However, this pattern of volcanism can also partly result from complexities in the sub-lithospheric flow between the ridge and the hotspot.

E01 : 3A/10 : F1

Lower Mantle Plume Component in Devonian Ultrabasic-Alkaline- Carbonatite Complexes (Kola Peninsula) as Recorded by Noble Gas Isotope Systematics

Igor Tolstikhin (igor@inep.ksc.ru),

Igor Kamensky,

Serafim Ikorsky,

Valentine Nivin,

Elena Balaganskaya,

Valery Vetrin &

Maria Gannibal

Geological Institute, Kola Sci. Center RAS, 14 Fersman, Apatity, Russia

Several ultrabasic - alkaline - carbonatite complexes (UACC) were formed on the Kola Peninsula (the North-Eastern segment of the Baltic Shield) during Devonian pulse of alkaline magmatism. To understand sources of volatile components trapped by rocks/minerals of these complexes, a systematic studies of rare gas isotope abundances and parent elements concentrations were carried out. The following principle parameters and processes operated during formation of the complexes and later on are derived from this study.

As recorded by U-Th-He systematic, initial ratio ⁴He/³He = 30,000 in trapped helium was well below the mean MORB value, (89 ± 9) x 1000, indicating a contribution from the lower mantle reservoir to the source of UAC melts. This conclusion is supported by Ne isotope abundances in UAC samples which are quite similar to those observed in Loihi seamount, Hawaii. He isotope data themselves recorded variable concentrations of trapped He having an initially homogeneous isotope composition altered by subsequent migration and re-distribution of in-situ produced radiogenic ⁴He*. Carbonatites indicate especially great range of trapped He concentrations implying variable degassing conditions during later stages of UAC parent melt differentiation. Magnetites separated from ultrabasic rocks appears to trap and retain He better than any other minerals. Step wise heating experiments with magnetites show that the major portion of trapped ³He is being released under high temperatures, >1300 C, after decripitation of vesicles (<800°C) and ⁴He* loss (<1100°C).

In contrast to He which concentration in the atmosphere is extremely low, Ne and Ar isotope systematics recorded some atmospheric contamination most probably resulting from contribution of crustal groundwaters, major carriers of atmospheric volatiles. The least contaminated gases extracted by crushing show ²⁰Ne/²²Ne = 12, ²¹Ne/²²Ne = 0.038, and ⁴⁰Ar/³⁶Ar = 3,000.

Isotope characteristics of the trapped fluid component could be reproduced if the parent melts of UAC complexes would include approximately 98% of MORB source material, 2% of lower mantle material, and 0.06% of groundwater.

E01 : 3A/11 : F1

Fingerprints of the Samoa Mantle Plume Component in Lavas at the Northern end of the Tonga Arc and Lau Basin: Implications for Mantle Dynamics in an Active Arc/Back-Arc System

J. Immo Wendt (immo@sol.earthsciences.uq.edu.au),

Marcel Regelous (regelous@sol.earthsciences.uq.edu.au),

Tony Ewart &

Kenneth D. Collerson

University of Queensland, Dept of Earth Sciences, St. Lucia Campus, Australia

Conflicting models concerning the type of fluid flow, element transport and mantle melting beneath the Tonga island arc (Wendt et al., 1997; Turner & Hawkesworth, 1997) were tested by the combined use of new trace element data and Sr-Pb-Nd isotope analyses of young back-arc lavas in the northern Lau Basin (NLB).

Lavas from young volcanic cones and the 1946 flow on the back-arc island of Niuafo'ou in the NLB display higher La/Sm and Nb/Zr ratios and more radiogenic Pb and Sr isotope compositions relative to lavas from active spreading centers further south in the Lau Basin. Their Nb/U and Ce/Pb ratios clearly demonstrate that these lavas are unaffected by the westward subduction of the Pacific Plate that is taking place at the Tonga Trench to the east.

Pb isotopic analyses of Niuafo'ou basalts plot on mixing lines defined by Indian-type MORB mantle and a source that is characterised by low ²⁰⁶Pb/²⁰⁴Pb but high ²⁰⁷Pb/²⁰⁴Pb. This feature is similar to that observed in young, post-erosional lavas from Samoa and is quite unlike any other potential source of Pb in the region. The isotopic composition is confirmed by trace element chemistry of Niuafo'ou lavas that is also consistent with mixing between depleted upper mantle and Samoa plume mantle. We therefore propose that Samoan mantle is leaking through a tear in the subducting Pacific Plate at the northern end of the Tonga Trench, a scenario that receives additional support by recent geophysical data (Millen & Hamburger, 1998). The distinctive signature of the Samoan plume is widespread in lavas younger than about 5 Ma in age from the northern Lau Basin and is also present beneath the Tonga island arc as far south as the island of Fonualei at 18°S.

In addition to the Samoan signal the northern Tonga arc lavas also contain a Pb signal derived from the Louisville Seamount Chain (LSC) that is situated on the westward subducting Pacific Plate east of the trench. However, the geochemical signature of the LSC is restricted to the northernmost arc lavas from the islands of Tafahi and Niuatoputapu that erupted less than 2 Ma ago at latitudes north of 17°S. This indicates that the geochemical signatures of the Samoa and Louisville mantle plumes are decoupled in both space and time, thus reflecting that the two OIB signatures were introduced into the arc lavas by very different mechanisms: Pb is transported through the mantle wedge in a fluid phase by a series of hydration-dehydration reactions, and has a residence time within the mantle wedge of about 3 Ma. The Samoan signal, by contrast, is introduced into the wedge by (solid) mantle mixing.

Wendt JI, Regelous M, Collerson KD & Ewart, A, Geology, 25, 611-614, (1997).

Turner S & Hakesworth, Nature, 389, 568-573, (1997).

Millen WM & Hamburger MW, Geology, 26, 659-662, (1998).

E01 : 3A/12 : F1

Are Crustal Xenoliths in Hawaiian Lavas the Source of Excess Strontium?

Igor K. Nikogosian (niki@geo.vu.nl)¹,

Alexander V. Sobolev (asobolev@glas.apc.org)²,

Albrecht W. Hofmann

(hofmann@mpch-mainz.mpg.de)³ &

Wafa Abouchami³

¹ Dept. of Petrology, Vrije Universiteit, De Boelelaan 1085, 1081 HV, Amsterdam, Netherlands

² Vernadsky Institute of Geochemistry, Russian Academy of Science, Kosigin 19, Moscow, 117975, Russia

³ Max-Planck-Institut f. Chemie, Postfach 3060, 55020 Mainz, Germany

Hawaiian magmas show evidence for involvement of crustal material in their sources (e.g. Hofmann and Jochum, 1996). Significant and common Sr excess over elements of similar incompatibility in Hawaii lavas is one of the arguments in support of this conclusion. However, the potential alternative explanation of this feature is that crustal xenoliths have contaminated the lavas on their way to the surface (e.g. Eiler et al, 1996). We report the existence of abundant crustal xenoliths in picrite R421, flow unit 195 from the Mauna Kea section of the HSDP drill core, characterized by highest Sr excess (> 20%) of the entire section. These xenoliths in picrite are represented by 2 round dark bodies (12 and 6 mm in diameter) of gabbro-dolerite, consisting of Pl, Cpx, Pig, Il, Mt and high-SiO₂ interstitial glass (few%). These bodies differ from the host rock by grain size (coarser in bodies) and more evolved composition of minerals and thus are interpreted as "crustal" xenoliths. We have separated parts of groundmass and xenoliths from different thin sections, powdered them and prepared homogeneous glasses using the laser-melting and quenching method. Compositions of these "xenolith" and "groundmass" glasses were then analyzed for major and trace elements. Also were analyzed Sr and Pb isotopes in the xenoliths.

The groundmass for major elements composition fits the olivine control trend of Mauna Kea rocks and for trace element concentrations and Sr enrichment is similar to the host rock. The xenolith shows significantly higher concentrations of all incompatible elements compared to groundmass and host rock. The overall pattern of elements is however similar with one important exception: the xenolith shows Sr deficiency rather than Sr excess. Sr and Pb isotopes in the xenolith are similar to those of adjacent Mauna Kea rocks (Lassiter et al, 1996).

Modeling of major and trace elements concentrations demonstrates that the xenolith could have originated from the "groundmass" melt as the result of 50% fractionation of Ol, Cpx, Pl, Pig at 2-4 kb pressure. The Sr deficiency in the xenolith is thus consistent with Pl crystallization.

The present study shows that xenoliths found in Sr rich picrite from Mauna Kea are likely related to the host or similar lavas by the process of fractional crystallization at crustal level. The Sr enrichment in the investigated Mauna Kea lava is not related to the crustal xenoliths present in the host rock and most likely reflects the signature of the Mauna Kea primary melt.

Eiler JM, Valley JW & Stolper EM, J. Geophys. Res., 101, 11807, (1996).

Hofmann A W & Jochum K P, J. Geophys. Res., 101, 11831, (1996).

Lassiter J C, DePaolo D J & Tatsumoto M, J., J. Geophys. Res., 101, 11769, (1996).

E01 : 3A/13 : F1

Palaeo-Oceanic Plateau in the Southern Arabian-Nubian Shield: Implications for Neoproterozoic Crust Formation

Mengist Teklay (mengist@phys.uoa.edu.er)¹,

Alfred Kröner (kroener@mail.uni-mainz.de)² &

Klaus Mezger (klaus@nwz.uni-muenster.de)³

¹ Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany

² Institut für Geowissenschaften, Universität Mainz, 55099 Mainz, Germany

³ Institut für Mineralogie, Universität Münster, 48149 Münster, Germany

In the southern Nubian Shield, Neoproterozoic tholeiitic basic metavolcanic rocks cover a considerable area in the Hagar terrane of Eritrea and its northward continuation, the Tokar terrane in Sudan. In Eritrea, the Hagar terrane is predominantly composed of thick sequences of mafic rocks (pillow metabasalts and metagabbros) containing elliptical bodies of serpentinites; minor chlorite schists and ridges of impure carbonates (de Souza Filho, 1995; Woldehaimanot, 1995). The Tokar terrane in Sudan comprises a bimodal volcanic suite of amygdaloidal and pillowed basalts as well as rhyolite and ignimbrite with associated tuffs (Linnebacher 1989; Kröner et al., 1991).

The main geochemical characteristics of these tholeiitic volcanic rocks include low concentrations of LIL elements, slightly enriched chondrite-normalized REE patterns, and high Ti, Zr, and Nb contents relative to Neoproterozoic N-type MORB. N-MORB normalized patterns displayed by the Hagar-Tokar metabasalts do not show distinct arc geochemical signatures such as a marked negative Nb-anomaly. The rather flat N-MORB normalized patterns with elemental abundances close to one for the Hagar-Tokar metabasalts indicate similarities to N-MORB as displayed by the Neoproterozoic Gerf ophiolite occurring near the Red Sea in the border region between Sudan and Egypt (Zimmer et al., 1995). However, high concentrations of incompatible elements and low initial eNd values for the Hagar-Tokar metabasalts indicate that they are neither N-MORB type basalts nor back-arc basin basalts. This is further supported by the lack of typical ophiolite sequences, i.e. ophiolites do not form part of the Hagar-Tokar terrane.

The occurrence of pillow lavas and deep sea sediments and the absence of continent-derived sediments indicating the proximity to a continental land mass strongly suggests an oceanic environment and excludes a continental flood basalt as well as passive volcanic margin setting for the Hagar-Tokar metabasalts. Rather, these rocks display primitive mantle-normalized geochemical patterns similar to those of modern oceanic plateaux. Hence, geochemically an oceanic plateau offers the closest comparison between known tectonic environments of basalt formation and the Hagar-Tokar metabasalts. The spacial association of the Hagar-Tokar tholeiitic basic metavolcanic rocks with calc-alkaline arc rocks suggest that crustal evolution in the Arabian-Nubian Shield involved accretion of arcs and oceanic plateaux similar to the Caribbean-Colombian Cretaceous igneous province (Kerr et. al. 1997). The observation that the Hagar-Tokar terrane represents such an oceanic plateau emphasizes the importance of plume-generated magmatism for crust formation in late Proterozoic times.

de Souza Filho CR, Unpubl. Ph.D. thesis, Open University, Milton Keynes, (1995).

Kerr AC, Tarney J, Marriner GF, Nivia A & Saunders AD, Geophys. Monogr., 100, 123-144, (1997).

Kröner A, Linnebacher P, Stern RJ, Reischmann T, Manton W & Hussein IM, Precambrian Res., 53, 99-118, (1991).

Linnebacher P, Unpubl. Diploma thesis, University of Mainz, (1989).

Woldehaimanot B, Unpubl. Ph. D. thesis, University of Giessen, (1995).

Zimmer M, Kröner A, Jochum KP, Reischmann T & Todt W, Chemical Geol, 123, 29-51, (1995).

E01 : 3A/14 : F1

Orientation and Densitiy Distribution of Cretaceous Dikes in Central Namibia from High Resolution Aeromagnetics

Tim Vietor (tvietor@gwdg.de) &

Klaus Weber (kweber@gwdg.d)

IGDL, Goldschmidtstr.3, D-37077 Goettingen, Germany

Interpretation of high resolution aeromagnetics and correlation with existing geological data shows that the orientation and density distribution of cretaceous dikes in central Namibia associated with the fragmentation of West-Gondwana is controlled by basement structures. The magnetic dikes are confined to those parts of the crust which have undergone panafrican deformation. No dikes have been detected in the Middle Proterozoic cratons in the north and south of Namibia. The overwhelming majority of the dikes are not parallel to the coast but form angles of 60 to 70 degrees with the coast. The dikes follow the structural grain of the basement inherited from the panafrican Damara Orogeny in Namiba. The dikes form the Hentjesbay-Outjo-Dikeswarm (HOD) which is 60 km wide and 350 km long. Extension in the HOD reached a maximum of 11%. The areal distribution limits of the HOD coincide with tectonostratigraphic zone boundaries of the panafrican Damara Orogen. This indicates reactivation of panafrican structures during cretaceous lithospheric extension. Distribution analysis shows maximum densities at both ends of the dikeswarm. At its northeastern termination the trend of the HOD changes from NNE to ENE. Here the dikes follow the edge of the southernmost extension of the Middle Proterozoic Congo Craton. A subordinate dike swarm cuts the edge of the Congo Craton at right angles to the main swarm. The reorientation of the trend of the dikes is considered to be due to the reorientation of the local stress field at the edge of the Congo Craton. Thus, the harder lithosphere of the Middle Proterozoic Craton is most probably the reason for the termination of the dike swarm. Consequently lithospheric extension in the South Atlantic Rift propagating from south to north had to cut the panafrican structural trend at high angles north of Swakopmund resulting in a distinct bend of the Namibian coast. The coast and the HOD form a Y-geometry resembling a rrr-triple junction. Only basement structures seam to be responsible for the observed features of the HOD. Although geochemical evidence has proved its presence (Ewart et al., 1998) the Tristan Plume lying under the Congo Craton at the time of dike intrusion (O´Conner & Duncan, 1990) appears not to be one of the controlling factors for the distribution and orientation of the dikes in central Namibia.

Ewart A, Milner SC, Armstrong RA & Duncan AR, J Petrology, 39(2), 191-225, (1998).

O'Connor JM & Duncan RA, J Geoph. Res, 95(B11), 17475-17502, (1990).

Session E01:3P

E01 : 3P/01 : PO

A Thin Spherical Shell Model of Global Asthenosphere Flow

W. Jason Morgan (wjmorgan@geo.princeton.edu)¹,

Morgan A. Crooks¹ &

Jason Phipps Morgan (jpm@geomar.de)²

¹ Geosciences Dept., Princeton Univ., Princeton NJ 08544, USA

² Geodynamics Dept., GEOMAR, D-24148 Kiel, Germany

Asthenosphere plume-to-ridge flow has often been proposed to explain both the existence of geochemical anomalies at the mid-ocean ridge segments nearest an off-axis hotspot, and the existence of apparent geochemical 'provinces' within the global mid-ocean spreading system. We have constructed a thin-spherical-shell finite element model to explore the possible structure of global asthenosphere flow. In this model, lubrication theory approximations are used to solve for the flow profile in the vertical direction, and a ~100-km-scale mesh is used to discretize and solve for the horizontal asthenosphere flux distribution. At each mesh node, the asthenosphere thickness is set according to the age/thickness of the overlying lithosphere. There is a 'point source' of new asthenosphere at each hotspot, and 'line sinks' of asthenosphere at spreading centers and at trenches (where some asthenosphere is dragged down by the subducting lithosphere). There is also a distributed sink of asthenosphere due to its cooling and attachment to the base of the aging and thickening oceanic lithosphere. Important model boundary conditions are the plate velocities (for plate drag) and the changing thickness of the asthenosphere/lithosphere at continental margins. We also assume that the strength of all the plume (hotspot) asthenosphere sources is equal to the sum of all the asthenosphere sinks, i.e. that the asthenosphere has a present-day steady-state thickness. So far, we have concentrated on the Atlantic region, adjusting the relative strength of each plume source to match where we think mid-ocean ridge bathymetric and geochemical variations indicate the boundaries between asthenosphere provinces. We use this as an independent method to estimate the relative strengths of plume fluxes.

E01 : 3P/02 : PO

A Mantle Plume Cluster Giving Synchronous Volcanism, Rifting and Uplift on the Devonian East European Platform

N J Kuznir (sr11@liverpool.ac.uk)¹,

M Wilson (M.Wilson@earth.leeds.ac.uk)² &

M Cheadle¹

¹ Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK

² Department of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK

During the Late Devonian 4 distinct localities (Kola, Vyatka, Timan Pechora and Pripyat-Dniepr-Donets) on the Eastern European Platform separated by distances of between 1300-1700 km experienced synchronous magmatism, rifting and uplift. Analysis of well resolved syn-rift stratigraphic sub-divisions of the Pripyat-Dniepr Basin shows that rifting was rapid with 80% of extension occurring in less than 3 Ma in the Upper Devonian (Famennian), and coincided with the most intense period of volcanicity. Rifting and volcanism were also synchronous with regional transient uplift of 300 m. Major- and trace-element geochemistry and Sr-Nd isotopic studies of Dniepr-Pripyat volcanics support a mantle plume origin. Observed Late Devonian stretching factors (E = 11-15 km, ß = 1.1-1.3) within the Dniepr-Pripyat Rift are insufficient to generate volcanics by rift-related decompression melting alone. Upper Devonian (Frasnian and Famennian) volcanism, rifting and uplift also occurred in the Kola, Vyatka, Timan Pechora and Pripyat-Dniepr-Donets areas. It is proposed that synchronous rifting, uplift and volcanism in these 4 areas was generated by mantle plume activity, and that the large distances between these 4 areas requires distinct upper mantle plumes. From the spacing of the proposed 4 distinct upper mantle plumes we speculative that they formed as discrete upper mantle convective instabilities originating from a thermal boundary layer at the 660 km phase transition, heated by a single longer wavelength lower mantle plume. Finite element fluid flow modelling shows that as the lower mantle plume initially develops, viscous coupling generates regional subsidence of up to 100-200 m for the overlying lithosphere. As thermal coupling of the lower and upper mantle develops, the resulting upper and lower mantle plume flow generates dynamic uplift above the rising upper mantle plume of up to 1500 m amplitude and 400-500 km wavelength. Localised tensile horizontal force in lithosphere above the plume are predicted to exceed 3x10¹² N/m and are sufficiently large to generate failure of strong continental lithosphere.

E01 : 3P/03 : PO

Interactions between Lithospheric Extension and Mantle Plumes at Volcanic Passive Margins: Analogue Modelling Approach

Olivier Bourgeois

(olivier.bourgeois@univ-rennes1.fr) &

Olivier Dauteuil

(olivier.dauteuil@univ-rennes1.fr)

Géosciences Rennes, UPR-CNRS 4661, Campus Beaulieu, bâtiment 15, 35042 Rennes Cedex, France

Volcanic passive margins result from lithospheric extension above mantle plumes. They display prominent seaward-dipping reflector sequences (SDRS), which are now proven to be composed of stacked basaltic lava flows with interbedded sediments. In seismic profiles, the SDRS display little evidence of extensional tectonics, such as normal faults and tilted blocks. Therefore, they are classically thought to represent an isostatic flexure of the lithosphere under the weight of lava flows. However, recent seismic work has shown that volcanic passive margins display several systems of SDRS, separated from each other by areas with poor seismic reflectivity, which are interpreted either as dominantly igneous features, or as tilted blocks. The feather-edge structure of the SDRS contrasts with the pattern of continentward-tilted blocks of non-volcanic passive margins. Classical rifting model cannot be applied directly: they assume that the deformation occurs at constant volume, which is not realistic in the context of volcanic passive margins.

We performed analogue modelling taking into account material supply. Small-scale models were built allowing a supply of both brittle and viscous material. Different ratios between material supply and stretching amount have been tested. When the supply is small (figure 1), the deformed zone is symmetric and corresponds to outward tilted blocks. The model can be compared to non-volcanic passive margins. When the supply is excessive (figure 2), the deformed zone is unsteady and drastically asymmetric. The deformation is accomodated by several roll-overs operating successively. These structures are controlled by growth faults whose dips change through time, tilting the sedimented layers both outward and inward. In cross-section, the viscous material displays different flowing patterns controlling the surface deformation. These results are consistent with the main features of the volcanic passive margins. This study points out the influence of material supply on the deformation pattern during continental breakup.

E01 : 3P/04 : PO

Energy of Lithosphere Plate Deformation

Avigdor Isacson (avigdor@bgumail.bgu.ac.il)¹ &

Alexander Portnov²

¹ Avraham Braver, st. 111 Beer-Sheva, 84836, Israel

² Sorokin st. 30/9, Moscow 109052, Russia

The crust surface deformation is accepted to be sequence of lithosphere processes, which in their turn are connected with convective motion within asthenosphere. The energy needed for this lithospheric process is to be estimated only by use of the crust surface deformation and common physical properties of the lithhosphere material.

When the scale of deformation has the order of 10 - 100 km, the lithosphere may be related as a layer of very viscouse liquid with the friction force from asthenophere appled to its lower boundary. This model makes it possible to estimate size and energy of lithosphere deformation by use of only horizontal extent of the crust deformaton.

The received estimate connects the energy spectrum with the wave spectrum of the surface deformation of a mountain range or more complicated relief structure. The value of the whole earth lithosphere deformation per year calculated by this way was found to be of the same order that the energy of earthquakes, related to the same period.

E01 : 3P/05 : PO

Are Plumes Really Necessary? A New Mechanism for Intraplate Volcanism: Melting, Melt Segregation and Petrogenetic Variation

Miles F. Osmaston (miles@osmaston.demon.co.uk)

The White Cottage, Sendmarsh, Ripley, Woking, Surrey GU23 6JT, UK

Plumes of lower mantle origin have been invoked, perhaps unnecessarily, for (1) extraction of early-Earth heat production, (2) causing the major dyke-swarms of the early Proterozoic, and (3) explaining trace-element features of intraplate volcanism. I discuss each in turn.

(1) Archaean heat extraction is only a problem if one assumes a mantle as dry as the present upper mantle. The constant association of felsics with komatiites, occurrences of komatiitic tuffs, Nb anomalies and rich H₂O-borne mineralizations all suggest a much wetter mantle. That could lower mantle viscosity by 1-2 orders of magnitude, giving more than ample ability to extract the heat produced.

(2) The 2.45-2.20 Ga interval saw three major 'events': (a) a clear hiatus in orogenic granitoid production, (b) dyke swarms and extension features on every craton, (c) unparalleled BIF deposition. All appear consistent with a hiatus in mantle overturn; a heat budget crisis precipitated by accelerated advective heat loss during late Archaean TTG intrusion. The dyke swarms represent thermal shrinkage of the global lithosphere with no MORs to accommodate it. See (3) below for magmatic mechanism.

(3) Intraplate volcanism. Mantle viscosity is highly non-Newtonian, is temperature sensitive and dependent on local composition. If the base of a plate is put into extension these properties may result in rapidly concentrated upward-necking of the plate. Sub-plate material thus drawn upward will undergo pressure-relief melting and eventually endow the column with net buoyancy to extend the narrow split up to the surface. Melt segregation will occur by a log-jam filter mechanism, well-known to grouting engineers and in other fields, in which the solids invariably jam in the crack if they are bigger than 20-25% of the crack width.

This basic mechanism could provide many features present in intraplate magmatism. In our diapiric-intrusive column the jam will form when the solids grow again at shallower levels where wall cooling becomes important. The diapiric column will force melt through the jam. If further opening of the crack is offset by wall accretion, a succession of jams will form at depths that can vary with time. I show that this leads directly to the OIB characteristic sequence (low-volume-alkaline to high-volume-tholeiitic to low-volume-alkaline), based simply on a (plate-shrinkage?) crack that opens rapidly and then slows. Rupturing of jams provides a source of xenoliths. The self-generated diapiric capability of the column in the crack results in pressure around its base being sub-lithostatic, so low-melting and diffusible-gas mantle constituents will be drawn from a wider zone than the main melting. This could provide isotopic enrichments (e.g. ⁸⁷Sr, ⁴⁰Ar (both from phlogopite), ³He) hitherto interpreted in OIB as of lower mantle plume origin. Effects of plate thickness and crack opening rate could yield a full eruptive range from intermittent to flood basalts.

E01 : 3P/06 : PO

Relationship between Silicic Volcanoes and Active Rifts: Example from Manda Inakir

Pierre Lahitte (lahitt@geophy.geol.u-psud.fr) &

Pierre-Yves Gillot (gillot@geophy.geol.u-psud.fr)

Labo. de Géochronologie, Bât 504 Uni. Paris-S