Experimental Study of Element Partitioning Between Majorite, Olivine, Merwinite, Diopside and Silicate Melts At 16 Gpa and 2,000°C

Abstract

We have determined experimentally the partition coefficients of elements between silicate minerals and coexisting melts at 16 GPa and 2,000°C in chondritic and Ca0-rich silicate systems. Majorite garnet + melt ± olivine forms in the MgO-rich chondritic system, while in the CaO-rich system, merwinite coexists with melt ±clinopyroxene. The partition coefficients (D-values) of La, Ce, Sm, Yb, Sc, Zr, Hf and other minor or major elements for majorite garnet, merwinite, and clinopyroxene/melt pairs have been determined through EPMA analysis. D-values of a limited number of elements were also determined for the olivine/melt pair with EPMA. The D(HREE and Sc) for majorite/melt are close to unity. These values are much lower than reported values for pyrope/melt pairs at 2–3 GPa and 1,200-1,500 °C and for pyrope megacryst/host rock matrix pairs. The pressure-induced coordination change in Al3+at higher pressures above 10 GPa is responsible for the enormous difference in the D values as well as the contrasting temperature conditions: The coupled substitution of (vmM2+, lvSi4+) =(vlllHREE3+, iva13+)?important for accommodating HREE3+into garnets, is severely limited because iva13+is unlikely to exist in the majorite/melt sysiem. The mechanism to reduce D(REE and Sc) with in-creasing pressure may also be operative between the clinopyroxene/melt pair. Systematic comparison of Onuma diagrams for vmM3+and VIUM3+ in almandine, pyrope, and majorite garnet /melt pairs clearly indicates that high enrichments of HREE in almandine and pyrope garnets are related to charge-ba.lanc-ed substitution and the presence of more or less polymerizing silicate melts with iva13+. These On.uma diagrams also suggest that the partitioning behaviors of Fe2+and Co2+between garnet and melt are anomalous. This may reflect the tendency for Fez+and Co2+to prefer six coordinated sites in melts to cubic sites in garnets due to the crystal field effect. The D(REE and Sc) value pattern for the mer- melt pair has been found to be quite similar to those for CaTi03/melt and CaSi03(perovskite)/melt pairs. This is interesting in view of the substructure of merwlnite that mixed Ca2+and O2-layers comprise a pseudo-hexagonal dense packing analogous to the perovskite structure. © 1992, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.