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 CaO-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 (^VIIIM²⁺, ^IVSi⁴⁺) = (^VIIIHREE³⁺, ^IVAl³⁺), important for accommodating HREE³⁺ into garnets, is severely limited because ^IVAl³⁺ is unlikely to exist in the majorite/melt system. The mechanism to reduce D(REE and Sc) with increasing pressure may also be operative between the clinopyroxene/melt pair. Systematic comparison of Onuma diagrams for ^VIIIM²⁺ and ^VIIIM³⁺ in almandine, pyrope, and majorite garnet/melt pairs clearly indicates that high enrichments of HREE in almandine and pyrope garnets are related to charge-balanced substitution and the presence of more or less polymerizing silicate melts with ^IVAl³⁺. These Onuma diagrams also suggest that the partitioning behaviors of Fe²⁺ and Co²⁺ between garnet and melt are anomalous. This may reflect the tendency for Fe²⁺ 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 merwinite/Ca-rich melt pair has been found to be quite similar to those for CaTiO₃/melt and CaSiO₃ (perovskite)/melt pairs. This is interesting in view of the substructure of merwinite that mixed Ca²⁺ and O^2- layers comprise a pseudo-hexagonal dense packing analogous to the perovskite structure.