Amphibole and chlorite in mafic and ultramafic rocks in the lower crust and upper mantle |
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| Authors: | Masaaki Obata Alan Bruce Thompson |
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| Institution: | (1) Institut für Kristallographie und Petrographie, ETH-Zentrum, CH-8092 Zürich, Switzerland;(2) Present address: Dept. of Earth Sciences, Toyama University, Gofuku, 930 Toyama, Japan |
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| Abstract: | Stabilities of amphibole and chlorite in mafic and ultramafic compositions have been considered in the model system CaO+MgO+Al2O3+SiO2+H2O. From topological arguments two petrogenetic grids have been constructed. Although available experimental data in CMASH are compatible with the grid in which amphibole reacts out before chlorite in model ultramafic compositions for the pressure range 6 to 15 kbar, naturally observed mineral assemblages and some experimental data in more complex systems favour the other grid in which chlorite reacts out before amphibole at all pressures. This discrepancy between natural and model systems is due to differential movement of the dehydration equilibria of the two hydrates reflecting the variation of activity of H2O or of components other than CMASH. |
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