Tremolite and H2O activity attending metamorphism of hornblende-plagioclase-garnet assemblages |
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| Authors: | Edward D Ghent |
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| Institution: | (1) Department of Geology and Geophysics, University of Calgary, T2N 1N4 Calgary, Alberta, Canada |
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| Abstract: | Estimation of the activity of tremolite component (atr) in calcic amphiboles is an important problem in igneous and metamorphic petrology because equilibria involving tremolite are used in the estimation of the activity of H2O attending crystallization of igneous and metamorphic rocks. Estimated values of atr from hornblende analyses using both ionic and coupled substitution crystalline solution models can be compared to values of atr calculated from vapor-absent mineral equilibria. In addition, these values of atr can be used in calculations of aH2O for rocks for which there is an independent estimate of aH2O. The values of atr calculated from vapor-absent equilibria are generally consistent with those estimated from the different crystalline solution models, but uncertainties in the calculations preclude choosing a preferred solution model. From computed mineral equilibria, it is clear that mineral assemblages with low values of atr can be in equilibrium with high values of aH2O. Consequently, the low values of aH2O estimated from hornblende-bearing high-grade rocks with low values of atr may be real. Rocks from Mica Creek, British Columbia, probably achieved extremely low values of aH2O by vapor-absent metamorphism. |
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