Melting phase relations of natural peridotite + CO2 as a function of degree of partial melting at 15 and 30 kbar

Phase relations of peridotite * CO, have been determined at 15 and 30 kbar using rtiW to monitor the degree of melting. At 15 kbar the initial <5 percent melting interval is affected by the presence of HrO formed by the reaction of some H, (diffused through the capsule wall) w'ith CO, to form H2O + CO. This effect becomes insignfficant with>5 percent melt (-1400'C), where the melting (to-20 percent melt) is nearly invariant. In this latter melting interval, the melt coexisting with olivine, orthopyroxene, clinopyroxene, and vapor is tho-leiitic. Melting coillmences more than 200oC lower at 30 kbar than at 15 kbar for both CO2-satu-rated and COr-undersaturated conditions. Again, the exact nature of the melting interval from the solidus to about 5 percent melting is uncertain because of the presence of small amounts of HrO in the experimental charges. Dolomite is a solidus phase together with oli-vine, orthopyroxene, and garnet, and the initial melt is carbonatitic, as evidenced by quench-ing of the melt to carbonate. With increasing degree of partial melting the melt compositions become richer in silicate components, as evidenced by quenching of the melt to mixtures of silicate glass and carbonate minerals. These results indicate that the principal phase relations i1 simple systems such as CaO-MgO-SiOr-CO, apply to melting of natural peridotite. Published experimental results in this system show a great depression ofthe solidus temperature (-150"C) as the pressure is raised above that of the carbonation reaction. D i + F o + V C E n * D o l At the same time, the liquid composition changes from haplobasalt to haplocarbonatite and therefore serves as a suitable model for liquids produced in the system peridotite + CO2