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

Stern, C. R. and Wyllie, P. J. (1975) Effect of iron absorption by noble-metal capsules on phase boundaries in rock-melting experiments at 30 kilobars. American Mineralogist, 60 (7-8). pp. 681-689. ISSN 0003-004X.

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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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Additional Information:© 1975 Mineralogical Society of America. Manuscript received, September 30, 1974; accepted for publication, February 21, 1975. This work was supported by the Earth Sciences Section, National Science Foundation, NSF Grant DES 73-00191 A01. We wish to acknowledge also the general support of the Materials Research Laboratory by the National Science Foundation. The analyzed tonalite was kindly supplied by P. C. Bateman and F. C. Dodge. W.-L. Huang and R. B. Merrill provided invaluable assistance with apparatus and manuscript review.
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NSFDES 73-00191 A01
Issue or Number:7-8
Record Number:CaltechAUTHORS:20160203-084320072
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64181
Deposited By: Tony Diaz
Deposited On:03 Feb 2016 23:22
Last Modified:03 Oct 2019 09:35

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