Phase equilibria in fluid inclusions in ultramafic xenoliths

Over 200 fluid inclusions in five dunite, peridotite, and pyroxenite xenoliths associated with basaltic rocks from Arizona, Hawaii, and Germany were examined using a petrographic microscope equipped for cooling to - 140.C. The temperatures of phase changes observed in individual fluid inclusions are interpreted according to published, experimentally determined phase equilibria as follows: (l) Most inclusions contain nearly pure co, and, in some cases, a small amount (on the order of 0.05 to 0.10 mole fraction) of Sor, HrS, or cos. only one inclusion contains a possible aqueous phase. (2) The CO, fluid densities range from 0.34 to l.l4 gm/cms. Assuming a temperature of entrapment of 1200C, this implies confining pressures of more than l0 kilobars at the time of entrapment of the densest inclusions. The presence ofglass linings on some ofthe inclusion walls suggests coexistence at depth of a COr-rich volatile phase with a melt phase at the time of entrapment of the two fluids in ihe host minerals. Compositions of the glass linings of two samples, Dreiser Wehier, Germany, and Red Hill, Arizona, most closely match high-alumina andesite and high-alumina basalt, respectively.