Trace element mineral/melt partitioning for basaltic and basaltic andesitic melts: An experimental and laser ICP-MS study with application to the oxidation state of mantle source regions

摘要

Understanding magmatic processes such as crystallization and melting recorded in natural samples requires calibration of mineral-melt trace element partition coefficients (D) and their dependence on temperature, pressure, oxygen fugacity (fO_2) and chemical composition. However, few experimental studies have focused on measuring trace element partition coefficients for a large number of trace elements, in the various minerals present in basaltic rocks, and under diverse conditions, particularly of variable fO_2. Twenty-seven 0.1 MPa experiments provide partition coefficients for major elements and Sc, Ti, V, Mn, Co, Ni, Ga, Sr, Y, Nb, Ba, Ce, Nd, Eu, Gd, and Yb for the mineral phases olivine, plagioclase, orthopyroxene and clinopyroxene. The experimental conditions range from 1150 to 1190°C with oxygen fugacities from QFM to NNO+2. Run products were analyzed by laser-ablation ICP-MS. The new partition coefficients, combined with previously published data, can be used to model crystallization processes at low pressure. Partitioning of multivalent cations V, Fe and Eu varies as a function of the redox conditions, consistent with previous work, and can be used to constrain oxidation states of magmatic source regions. The V/Yb ratio is shown to be a useful proxy for oxidation state. The V/Yb ratio varies during mantle melting as a function of oxidation state of the mantle source, and it is not modified during fractional crystallization of olivine ± plag ± cpx. V/Yb increases from MORB, BABB to arc lavas, suggesting a progressive increase of fO_2 from QFM to NNO+2. Apparent fO_2 of arc lavas, however, is quite variable. These results demonstrate that sub-arc mantle displays a larger range of redox conditions toward a more oxidized mantle than the MORB mantle.