Abstract

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 degrees C. The temperatures of phase changes observed in individual fluid inclusions are interpreted according to published, experimentally determined phase equilibria as follows: (1) Most inclusions contain nearly pure CO2 and, in some cases, a small amount (on the order of 0.05 to 0.10 mole fraction) of SO2, H2S, or COS. Only one inclusion contains a possible aqueous phase. (2) The CO2 fluid densities range from 0.34 to 1.14 gm/cm3. Assuming a temperature of entrapment of 1200 degrees C, this implies confining pressures of more than 10 kilobars at the time of entrapment of the densest inclusions. The presence of glass linings on some of the inclusion walls suggests coexistence at depth of a CO2-rich volatile phase with a melt phase at the time of entrapment of the two fluids in the 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 .

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