Evolution of the upper mantle viewed from chemical composition of spinel and olivine.

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The following points are clarified in this research.1. Olivine-spinel mantle array, which is a mineralogical trend in the upper mantle and which is characterized by positive but nonlinear correlation between Mg/(Mg+Fe) (Mg#) of olivine and Cr/(Cr+Al) (Cr#) of spinel, can be explained by either batch or fractional melting with starting primitive mantle composoition of olivine Mg#=0.89 and spinel Cr#=0.1.2. Mantle olivine array, which is a mineralogical trend in the upper mantle and which is characterized by positive and near linear correlation between Mg# and NiO content of olivine, should be divided into Mg-rich and Fe-rich portion. The former trend can be produced by either batch or fractional melting from primitive mantle with livine Mg#=0.89 and NiO=0.38 wt. %, whereas the latter trend is due to interaction between peridotites on the mantle olivine array and silicate melt in equilibrium with olivine having smaller Mg# and NiO content than olivine on the array. Common occurrence of the Mg-poor peridotites on the mantle olivine array indicates that such reaction is quite general in the upper mantle.3. Parameters of open-system melting processes are inverted from observed REE and trace element abundances in clinopyroxene and bulk peridotite. Estimated parameters are : extent of melting, melt fraction during melting, trace element composition of influxed melt, melt influx rate relative to melting rate, stoichiometric coefficients for the melting reaction.The final goal of this project is to construct petrogenetic grid, which is nearly completed for Mg# and NiO content of olivine. We have not yet complete to construct the grid for spinel vs. olivine compositions, but two extreme cases of meling (batch and fractiona) have been throughly investigated. After completing the latter grid and examining each parameter in the model, we may be able to do inversion of melting opocesses in the upper mantl on the basis of such petrogenetic grid in near future.