Plastic Deformation Experiments Under Mantle Conditions of MgSiO₃ High-Pressure Polymorphs: Akimotoite and Perovskite

Patrick Cordier (patrick.cordier@univ-lille1.fr)

LSPES, ESA 8008. Université des Sciences et Technologies de Lille, France and Bayerisches Geoinstitut. Universität Bayreuth, Germany.

Pyroxene with the composition (Mg, Fe)SiO₃ is one of the main constituent of the Earth's crust and upper mantle. At pressures corresponding to the transition zone, it transforms to majorite garnet at high temperature and to the sequence wadsleyite + stishovite, ringwoodite + stishovite, and akimotoite (ilmenite structure) at low temperature. Finally (Mg, Fe)SiO₃ transforms to the perovskite structure at pressures corresponding to the uppermost lower mantle. These minerals are likely to be important constituents of the Earth's mantle. It is thus necessary to determine their deformation mechanism in order to constrain the rheology of the mantle.

We present here an experimental investigation of the plastic deformation of two high pressure phases with the composition MgSiO₃: akimotoite and perovskite. The specimens have been synthesised in a multianvil apparatus with MgSiO₃ glass as a starting material. After the run, the samples were recovered and the nature of the phases were checked by Raman spectroscopy. The samples were then placed in a second high-pressure assembly designed to induce plastic deformation. To reach the stability pressures of these phases, it has been necessary to adapt the deformation set-up to the 10/4 assembly (10 millimetre edge length octahedron compressed between WC cubes with 4 millimetres truncations).

Akimotoite has been deformed at 21 GPa and 1400°C. Preliminary transmission electron microscopy investigation shows that basal a glide is the dominant slip system. Secondary slip in pyramidal planes {10-12} is also observed.

In case of perovskite, growth stage resulted in a coarse grain structure (several 100 micrometers). The samples have been deformed at 25 GPa and at temperatures ranging from 1000°C to 1800°C. The samples are plastically deformed as shown by optical examination. At 1400°C, for instance, one observes numerous deformation bands which shear the twins.

Microlite Crystallisation ­ An Experimental Study

Susan Couch (susan.couch@bristol.ac.uk), Mike Carroll (carroll@campus.unicam.it) & Steve Sparks (steve.sparks@bristol.ac.uk)

Department of Earth Science, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK

Close observation of the recent eruptions at Soufriere Hills, Montserrat has lead to the proposal that degassing rather than cooling controls solidification in dome forming eruptions. Thus as magma ascends and loses gas, the solidus is raised and microlite crystallisation from the undercooled magma results in solidification. This study is concerned with determining how quickly this crystallisation takes place, both in terms of nucleation and growth rate, as it is presently poorly understood. Experiments to characterise microlite growth kinetics are being carried out using an analogue of the Montserrat groundmass composition, and a haplogranitic liquid, at pressures of 25-250 MPa and 750-925°C. Undercooling experiments involving growth of plagioclase and clinopyroxene microlites are presently running and these will be compared with the equilibrium experiments to determine growth rates of the microlites. The kinetic data obtained from the experimental work will also be applied to the interpretation of textural variations observed in natural samples.

Experimental Determination of the Rates and Mechanisms of Thermochemical Sulphate Reduction (TSR)

Martin M. Cross (mcross@fs1.ge.man.ac.uk)¹, David A. C. Manning (dmanning@fs1.ge.man.ac.uk)¹, Simon H. Bottrell (s.bottrell@earth.leeds.ac.uk)² & Richard H. Worden (r.worden@queens-belfast.ac.uk)³

¹ Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom

² Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom

³ School of Geosciences, The Queen's University, Belfast, BT7 1NN, United Kingdom

Thermochemical Sulphate Reduction (TSR) reactions have been postulated as the cause of high H₂S concentrations in petroleum gas and oil fields (e.g. Krouse et al., 1988; Orr, 1974; Worden et al., 1995). Laboratory simulations of TSR, at formation water pH, have enabled simultaneous determination of reaction kinetics and mechanisms.

TSR experiments have been performed in the sodium sulphate - acetic acid - sodium acetate - elemental sulphur system, at temperatures between 100 and 350°C. Fluid-sampling hydrothermal pressure vessels were used (Seyfried et al., 1987), and the reactants were held within a gold-titanium reaction cell (Seyfried et al., 1979). Fluid samples were taken periodically to monitor reaction progress without quenching the reaction vessel to room conditions.

Kinetic data suggest that TSR occurs rapidly on a geological time-scale at relatively low temperatures (150°C). Sulphate half-lives are 15 days at 300°C, and 1650 years at 150°C. The activation energy of TSR is 142 kJ/mol. TSR is a first order reaction with respect to sodium sulphate, acetic acid and hydrogen ion activity. Fluid pH increases by 1/2-1 unit during these reactions.

TSR is initiated by a low valence sulphur species. Elemental sulphur undergoes hydrolysis to form hydrogen sulphide and sulphate. At in situ pH (5.23-6.05), a low equilibrium concentration of fully protonated sulphate is present which undergoes reaction with hydrogen sulphide to form a thiosulphate reactive intermediate and water. This reaction is reversible and decomposition of thiosulphate allows for sulphur isotope exchange between sulphate and hydrogen sulphide, and exchange of oxygen between sulphate and water. Sulphur isotopic fractionation factors of 1.019-1.020 have been determined for TSR in this system. Equilibrium oxygen isotopic exchange is attained in approximately 1 hour at 300°C.

In acid conditions, thiosulphate decomposes to form sulphite and nascent (monatomic) sulphur that undergoes further reaction with organic species to form H₂S and CO₂.

Krouse HR, Viau CA, Eliuk LS, Ueda A, & Halas S, Nature, 333, 415-419, (1988).

Orr WL, AAPG Bull, 50, 2295-2318, (1974).

Seyfried WE Jr, Gordon PC, & Dickson FW, Amer. Mineral, 64, 646-649, (1979).

Seyfried WE Jr, Janecky DR, & Berndt ME, Hydrothermal Experimental Techniques (eds GC Ulmer and HL Barnes), Wiley, Chicester, 216-239, (1987).

Worden RH, Smalley PC, & Oxtoby NH, AAPG Bull, 79(6), 854-863, (1995).