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Earthquake Research Institute, University of Tokyo, Yayoi, Tokyo 113, Japan
Analyses of the compositions of liquids from various melting experiments indicate that the liquids coexisting with magnesian olivine, orthopyroxene, clinopyroxene and spinel at 10 kbar occupy a large volume in the normative projection OL-CPX-PL-SIL and that the application of the invariant melting concept analogous to the simple system CMAS is not warranted in the natural peridotite system. The compositions of magnesian mid-ocean ridge basalts (MORBs) are similar to those of liquids coexisting with olivine, orthopyroxene, clinopyroxene and spinel at 10 kbar. The relationships between Cr/(Cr+Al) in spinel and Mg/(Mg+Fe) in olivine or orthopyroxene in both abyssal peridotites and the solid phases of the melting experiments of peridotites indicate that abyssal peridotites may represent the residual materials after the extraction of liquids formed at pressures around 10 kbar. Reconstructed modal compositions of abyssal peridotites indicate that most of them included calcium-rich clinopyroxene at their solidus temperatures. It is suggested that many magnesian MORBs represent primary magmas formed at pressures around 10 kbar, leaving residual materials composed of olivine, orthopyroxene, calcium-rich clinopyroxene and spinel.
