Garnet and pyroxenes in the mantle: a test of the majorite fractionation hypothesis

Experimental data are reported for the compositions of garnets and pyroxenes that coexist with liquid in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) at 80-160 kbar. All experiments were run using the uniaxial split sphere apparatus. The data have permitted a refinement of Gasparik's (1990a) thermodynamic analysis of garnets and pyroxenes that occur at and below the solidus. An examination is also made of the composition of garnet that crystallizes as a liquidus phase; it is demonstrated to be a function of both composition and pressure. The results support the hypothesis that extensive or possibly total melting occurred during the growth of the Earth by accretion of planetesimals and that majorite fractionation could have occurred during this time. Removal of majorite to the lower mantle during the crystallization of a magma ocean is a successful way of explaining differences in MgO/SiO₂ and CaO/Al₂O₃ between mantle peridotite and chondrite. -from Authors