Compositions of Glasses from St. Paul's Peridotite Partially Melted at 20 Kilobars

Abstract

St. Paul's peridotite with 5.7% H_₂O (considered to be derived from the upper mantle) was partially-melted at 20 kb in Ag_(75)Pd_(25) and Pt capsules with run durations up to 17 hours. Run products were analyzed for Fe, Mg, Ca, and Al using an electron microprobe. FeO loss from sample to capsule wall increased with temperature and run duration and was as much as 90% in Pt capsules at 1,250°C and 4 hours, and 73% at 1,150°C and 17 hours in Ag_(75)Pd_(25) capsules. Iron loss was lower in larger Pt capsules with a higher sample/platinum ratio. The amount of liquid produced in runs increased as a function of run duration, confirming the nonequilibrium condition. It was difficult to measure the composition of glass in narrow seams and wedges between mineral boundaries, and the problem was compounded by the presence of numerous minute olivine grains from the mylonite starting material and of quench amphibole in the glass. Glass compositions were estimated from analyses of glassy areas and quench amphiboles, and the compositional trend between 1,100°C and 1,250°C was plotted on variation diagrams. Expressed in terms of Al_₂O_₃ and CaO only, the glass analyses are similar to andesite and tholeiite. Our estimated glass compositions are compared with published data for measured glasses from partially melted peridotites of similar composition. In terms of the four elements that we analyzed the compositions of glasses vary considerably from one set of experimental conditions to another. We conclude that this is due in part to the experimental problems outlined above. The best results that can be achieved represent a compromise between runs long enough to ensure reaction and short enough to minimize the changing composition of the reacting system through iron loss.

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