MINERAL-MELT TRACE ELEMENT PARTITIONING AT 1 BAR TO 10 KBAR PRESSURES

Accurate geochemical models require improved understanding of the controls on mineral-melt partitioning. We have determined plagioclase-basalt, clinopyroxene-dacite and melilite- melt partition coefficients to constrain the processes of mantle melting, crystal fractionation and Calcium Aluminum Inclusion genesis in meteorites. Piston cylinder and 1-bar furnace experiments were analyzed by SIMS for a variety of trace elements (La, Nd, Sm, Eu, Er, Yb, Zr, Nb, Hf, Ta, Th, U, Cs, Rb, K, Ba, Sr, Li, and B, doped at ppm levels), allowing us to examine the effects of both crystal composition and melt structure.

Starting materials for plagioclase-melt experiments consist of synthetic Diopside40-Albite28-Anorthite32 (DAA) glass (~53% SiO2), a Gorda Ridge basalt (~50% SiO2), and basaltic andesite from Arenal Volcano (Costa Rica; ~56% SiO2). We ran the DAA samples in Pt capsules and the natural powders in Mo foil inside evacuated silica tubes. DAA experiments yield ~5-15% modal plagioclase (An87) as 50-100-mm width crystals coexisting with basaltic melt at 1264°C. Analytical traverses of these crystals show no major element zoning, and ion microprobe results indicate trace element homogeneity. The low measured partition coefficients for HFSEs (0.004-0.00005) indicate our results are robust. Partition coefficients for REE increase in compatibility from HREE (0.006 to 0.003) to LREE (~0.04 to 0.017). LILE partition coefficients range from ~1.2 for Sr to lower values for Cs, Rb, K and Ba (0.2-0.005). Finally, DLi and DBe range from 0.4 to 0.02 and DU and DTh vary between 0.0002 and 0.0005.

We ran 1-atm experiments in the CMAS+Ti system in Pt capsules which yielded ~24% modal melilite (0.5-1.2 mm). Melilite is euhedral and relatively homogeneous (Åk35-39) with DTi changing from 0.005 to 0.09 over this composition. Piston cylinder experiments at 1 GPa on two synthetic Fe bearing compositions produced euhedral clinopyroxene coexisting with melts containing 62 and 70 wt% SiO2. The glasses and clinopyroxenes are similar in composition to those commonly found as high silica melts within mantle xenoliths. Surprisingly, despite small amounts of tetrahedral Al within the clinopyroxene, DTi is relatively high (0.8). Ion probe results of trace element partitioning on both of these latter sets of experiments will be presented.