Journal of Conference Abstracts

Compositional and Textural Variations in Oceanic Gabbros from the Mid-Atlantic Ridge, 23°N

L. A. Coogan¹ (lac8@le.ac.uk), A. D. Saunders¹ & P. D. Kempton²

¹ Leicester University, Leicester LE1 7RH.

² Nerc Isotope Geoscience Laboratories, Keyworth.

Textural and mineral chemical studies of closely spaced gabbroic samples from 55m of ODP drill core (Hole 923A, Leg 153) from the Mid-Atlantic Ridge (23°N) reveal significant variations in mode and texture. Two main rock types predominate, orthocumulate olivine gabbros and grain size layered meso-adcumulate gabbros. Orthocumulates poikilitic olivine gabbros are plagioclase (zoned) +/- olivine & clinopyroxene cumulates. Mode and grain size are very variable (doleritic troctolites/coarse gabbros). Intercumulus clinopyroxene shows strong zoning from green chromian (Cr₂O₃ up to 1.5wt %) and magnesian (Mg# up to 90, which is more magnesian than predicted by experiments on nearby MORB) rich cores, to brown titanium-and iron-rich rims. Rims of clinopyroxenes are often associated with late hornblende and iron-titanium oxides. Paradoxically the clinopyroxenes with the highest TiO₂ contents are also the most magnesian. This suggests that either (i) iron-titanium oxide (intercumulus ?) crystallisation lead to overall titanium depletion or (ii) these rocks trapped a larger percentage of evolved melt. Adcumulate gabbros and olivine gabbros (low in NiO in olivine and Cr₂O₃ in cpx), with common grain size layering are the other main rock type. Small evolved magnetite-and illmenite-rich deformed gabbronorites occur throughout the core and probably reflect filter pressed interstitial melts. Compatible elements (e.g. Nickel and forsterite in olivine, Anorthite in plagioclase, etc.) define a progressive evolution with height in the core with two spikes to more primative compositions, possibly reflecting replenishment events.

These rocks probably reflect the crystallisation products of a small mid-ocean ridge chamber in which intercumulus crystallisation processes were important.

Rare-Earth and High-Field-Strength Element Mobility Associated with the North Qôroq Nepheline Syenite Centre, Gardar Province, South Greenland: a Geochemical, Mineralogical and Isotopic Study

Ian M. Coulson¹ (i.m.coulson@bham.ac.uk), Andrew D. Chambers¹ (a.d.chambers@bham.ac.uk) & Melanie J. Leng² (mjl@wpo.nerc.ac.uk)

¹ School of Earth Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.

² NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK.

Investigation of the syenites and country-rocks of the North Qôroq centre has demonstrated the variable nature of REE and HFSE chemistry and the minerals containing these elements.

Extensive fractional crystallisation produced rocks ranging from augite syenite to peralkaline lujavrite. This compositional range is reflected in the varied mineralogy and chemistry. Coupled with this is an increase in incompatible elements resulting, for the most evolved rocks, in the crystallisation of eudialyte.

Extensive metasomatism accompanied the emplacement of the centre. Early fluid activity was dominantly Na(K)-Cl rich and pervasive, whereas later fluids, often seen to overprint this alkali-metasomatism, were dominated by Ca-CO₃^2--F^--PO₄^3-.

Evidence suggests that the Ca-CO₃^2--F^--PO₄^3- rich fluid modified the pre-existing HFSE and REE chemistry and mineralogy. This often resulted in enrichment in the country-rocks and metasomatised syenites, but locally caused depletion. In response to these changes the new metasomatic mineralogy is dominated by phases such as apatite, zircon and rare-earth fluorcarbonates. It seems likely that these elements were transported in the form of complexes of F^- and CO₃^2- and possibly PO₄^3-.

The source of this fluid activity is not well constrained but it is likely that as the syenites evolved, a Cl-rich aqueous fluid separated at an early stage, producing the extensive albitisation. Later, as the syenites fractionated, incompatible element enrichment occurred in the residual F^--rich magmas. Fluids emanating from this were responsible for the element redistribution seen in the metasomatised rocks. The source of the CO₃^2- and PO₄^3- is problematical. Isotopic studies are inconclusive but suggest it is magmatic and is related to the late-stage evolution of the syenites, or possibly the carbonatitic activity seen in the Igaliko area.