A regular solution model for met-aluminous silicate liquids: Applications to geothermometry,immiscibility, and the source regions of basic magmas |
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| Authors: | M S Ghiorso I S E Carmichael |
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| Institution: | (1) Lawrence Berkeley Laboratory, University of California, 94720 Berkeley, USA;(2) Dept. of Geology and Geophysics, University of California, 94720 Berkeley, USA |
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| Abstract: | Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi3O8 in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi3O8 in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica. |
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