SIMS determination of trace element partition coefficients between garnet,clinopyroxene and hydrous basaltic liquids at 2-7.5 GPa and 1080-1200 degrees C

Trace element partition coefficients (D's) for up to 13 REE, No, Ta, Zr, Hf Sr and Y have been determined by SIMS analysis of seven garnets, four clin opyroxenes, one orthopyroxene and one phlogopite crystallized from an undop ed basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. E xperiments were conducted at 2-7.5 GPa, achieving near-liquidus crystalliza tion at relatively low temperatures of 1080-1200 degrees C under strongly h ydrous conditions (5-27 wt.% added water). Garnet and pyroxene D-REE show a parabolic pattern when plotted against ionic radius, and conform closely t o the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 19 94. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452-454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-foe partition coefficient (D-0) for the lar ge cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garn et, the effect of lower temperature, which serves to increase D-0, and high er water content, which serves to decrease D-0, counteract each other to th e extent that water has little effect on garnet-melt D-0 values. In contras t, the effect of water on clinopyroxene-melt D-0 overwhelms the effect of t emperature, such that D-0 is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiment s tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments. Three sets of near-liquidus clinopyroxene-garnet two-mineral D values incre ase the range of published experimental determinations, but show significan t differences from natural two-mineral D's determined for subsolidus minera l pairs. Similar behaviour is observed for the first experimental data for orthopyroxene-clinopyroxene two-mineral D's when compared with natural data . These differences are in large part of a consequence of the subsolidus eq uilibration temperatures and compositions of natural mineral pairs. Gnreat care should therefore be taken when using natural mineral-mineral partition coefficients to interpret magmatic processes. The new data for strongly hydrous compositions suggest that fractionation o f Zr-Hf-Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserv e source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths w here both garnet and clinopyroxene will cause Zr-Hf-Sm fractionation, Also, at shallower depths, clinopyroxene-dominated fractionation may produce a p ositive Sr spike in melts from spinel Iherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts wit h negative Sr spikes may crystallize from magmas without anomalous Sr conte nts when plotted on mantle compatibility diagrams. Because the characterist ics of strongly hydrous silicate melt and solute-rich aqueous fluid converg e at high pressure, the hydrous data presented hen are particularly pertine nt to modelling processes in subduction when aqueous fluids may have an imp ortant metasomatic role. (C) 2000 Elsevier Science B.V. All rights reserved .