Pressure effect on element partitioning between minerals and silicate melt: Melting experiments on basalt up to 20GPa

摘要

High pressure melting experiments of alkali basalt were performed from 3 to 20GPa, and the partitioning of major and trace elements between silicate melt and coexisting minerals was investigated using EPMA and LA-ICP-MS. Garnet existed at around the liquidus at all pressures, while clinopyroxene (cpx) was observed between 3 and 12GPa. K-hollandite was identified at 20GPa. The calculated partition coefficients (D ~*-values) for garnet/melt and cpx/melt showed a similar pressure dependence: D _(Na) ~* and D _(K) ~* increase with pressure, while D ~*-values of rare earth elements decreased. Apparent differences were found only in the behavior of D _(Sr) ~*, which increased with pressure for garnet/melt but remained almost constant for cpx/melt. Partition coefficient profiles on PC-IR diagrams display a remarkable increase in peak position with increasing pressure for monovalent ions. However, the peak positions of divalent ions remained nearly constant and a slight decrease was found for the partition curve of trivalent ions. Under high pressure, both the crystal structure and ions are compressed. The observed pressure dependences in the peak positions of partition coefficient curves can be qualitatively explained by the difference in compressibility of monovalent and trivalent cations: the former being more compressible than the latter.