Effect of iron absorption by noble-metal capsules on phase boundaries in rock-melting experiments at 30 kilobars

Abstract

Andesite-H_2O and basalt-H_2O mixtures encapsulated in Pt, Ag_(30)Pd_(70), and Ag_(75)Pd_(25) with high ratios of metal/sample were run in half-inch diameter piston-cylinder apparatus at 30 kbar at temperatures where the mixtures were molten or partly crystallized. For andesite liquid at l 100°C the rate of iron absorption by the capsules is greatest in the first hour, 70 percent of initial FeO being lost with Pt capsules. After 3 hours, percentages of initial FeO lost are 85 percent for Pt, 35 percent for Ag_(30)Pd_(70), and 25 percent for Ag_(75)Pd_(25). For the phase assemblage garnet(Ga) + clinopyroxene(Cpx) + liquid(L) with 80 percent L at 1100°C in Ag_(75)Pd_(25) capsules, clinopyroxene and liquid lose about 30 percent FeO in the 4-hour interval between 1 and 5 hour runs, but garnet does not change composition. Distribution coefficients for Fe and Mg between liquid and minerals therefore vary as a function of run duration. At 1000°C and below, iron loss is not a serious problem in Ag_(30)Pd_(70) for runs of 10 hours duration, but attempts to reverse the Cpx-out phase boundary in andesite-10% H_2O near 950°C demonstrate kinetic and nucleation problems; the reversal bracket is wide, 890°-975°C. Consistency between one-stage and two-stage low-to-high temperature runs for andesite and basalt with at least 10 percent H_2O between 1050°C and 1200°C suggests a close approach to equilibrium for garnet and clinopyroxene after 1-hour runs. For runs with basalt-H_2O in Pt capsules, longer runs cause the recorded positions of the Cpx-out and Ga-out phase boundaries to increase and decrease in temperature, respectively, with discrepancies of 25-50°C between runs of 1 hour and 12 hours. This makes it difficult to demonstrate reversible equilibrium of a high-temperature phase boundary in a rock-H_2O system; reversibility in a system of changing bulk composition is not necessarily an equilibrium condition.

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