Granulite facies amphibole and biotite equilibria,and calculated peak-metamorphic water activities |
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| Authors: | Wimam M Lamb John W Valley |
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| Institution: | (1) Department of Geology, Texas A & M University, College Station, 77843, TX, USA;(2) Department of Geology and Geophysics, University of Wisconsin, 53706 Madison, WI, USA |
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| Abstract: | Samples located near the Oregon Dome anorthosite massif in the south-central Adirondack Mountains, New York contain the fluid-buffering mineral assemblages: amphibole + clinopyroxene + orthopyroxene + quartz or biotite + quartz + orthopyroxene + K-feldspar. These rocks were metamorphosed under granulite-facies conditions (T=725°–750°C, P=7.5 kbar) during the Grenville orogeny. The Mg-rich nature of amphiboles, micas, and pyroxenes allow accurate calculation of water activities because corrections for the effects of solid solution are relatively small. The activity of water was low during the peak of granulite-facies metamorphism, with  H2O 0.15±0.14. Wollastonite occurrences indicate that the CO2 was low (<0.3) in nearby rocks, demonstrating that large quantities of CO2 did not infiltrate in a pervasive manner. The combination of low H2O with low CO2 is consistent with the hypothesis that magmatic processes were dominant, generating dry, fluid-absent conditions.Abbreviations
fi
Fugacity of species i in a fluid
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Xi
mole fraction of component i in a phase
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T
temperature
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P
lithostatic pressure
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P
F
fluid pressure
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i
x
activity of component i phase X |
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