MELTING EXPERIMENTS ON ANHYDROUS PERIDOTITE KLB-1 FROM 5.0 TO 22.5 GPA

Melting experiments have been performed on anhydrous mantle peridotite KLB-1 at 5 to 22.5 GPa using the multianvil press, with special atten tion paid to precision and accuracy of temperature and pressure measur ement, oxygen fugacity, equilibrium, temperature gradient, and the eff ects of temperature gradient on the phase diagrams. The new phase diag ram reveals complexities in the liquidus and solidus phase relations t hat were not reported by Takahashi (1986). At no pressure do the liqui dus and solidus converge to a common temperature or to a narrow range of temperatures, a result that refutes the conjecture that mantle peri dotite formed on the solidus as a partial melt (Herzberg and O'Hara, 1 985) or a residual liquid. However, mantle peridotite could have forme d as a cotectic liquid because KLB-1 exhibits nearly cotectic liquidus crystallization behavior in garnet plus magnesiowustite [L + Gt + Mw] from 18 to 22 GPa. The implication is that mantle peridotite may be t he product of a large-scale differentiation event and that MgO/SiO2 fo r mantle peridotite may have been phase equilibrium controlled by cote ctic crystallization in a magma ocean. Fractionation of magnesiowustit e or majorite garnet is inferred at 18 to 22 GPa for a bulk Earth MgO/ SiO2 that is higher or lower than mantle peridotite, respectively. Alt ernatively, if the Earth formed from materials that were the same in M gO/SiO2 as mantle peridotite, then the liquidus phase equilibria repor ted here may be irrelevant or incidental in understanding Earth struct ure.