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Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time

Wolf, Michael B. and Wyllie, Peter J. (1994) Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time. Contributions to Mineralogy and Petrology, 115 (4). pp. 369-383. ISSN 0010-7999. doi:10.1007/BF00320972.

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We have simulated the dehydration-melting of a natural, low-K, calcic amphibolite (67.4% hornblende, 32.5% anorthite) in piston-cylinder experiments at 10 kbar and 750–1000°C, for 1–9 days. The solidus temperature is lower than 750°C; garnet appears at 850°C. The overall reaction is: Hb + P → L + Cpx + Al-Hb + Ca – Hb + Ga + Opx. Three stages of reaction are: (1) melting dominated by the growth of clinopyroxene and garnet, with little change in composition of liquid or garnet, (2) a reversal of this reaction between 875°C and 900°C, with decreases in the amounts of liquid and garnet, and (3) a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase. Garnet is enriched in pyrope and zoned from Fe-cores to Mg-edges (range ∼3 mol % pyrope); liquid composition is enriched first in An (to ∼950°C) and then in Ab. The liquids are more calcic and aluminous than natural tonalites, which is attributed to the plagioclase composition (An90). The formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite — not H2O-saturated conditions. The results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet-absent field (900–1000°C), but with a solidus backbend at ∼7–9 kbar, coincident with the garnet-in boundary. Hornblende breakdown due to garnet formation in a closed system must make H_2O available for H_2O-undersaturated melting right down to the H_2O-saturated solidus, below 700°C, which defines a large low-temperature PT area where hydrous granitoid melts can be generated with residual garnet and hornblende.

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Additional Information:© 1994 Springer-Verlag. Received January 8, 1993; Accepted August 25, 1993. This research was supported by the Earth Sciences section of the United States National Science Foundation, grant EAR 89-04375. Caltech Division of Geological and Planetary Sciences Contribution 5313. We thank Mark Fahnestock for help with the EMP-ARP phase discrimination program and George Morgan IV and Dave London for help with the BSE modal imaging. In-depth reviews by Jim Beard, Bob Rapp and Tim Grove greatly improved the manuscript.
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NSFEAR 89-04375
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Caltech Division of Geological and Planetary Sciences5313
Issue or Number:4
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ID Code:64958
Deposited By: Tony Diaz
Deposited On:02 Mar 2016 18:10
Last Modified:10 Nov 2021 23:38

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