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Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2

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Abstract

Solidus temperatures have been determined for minimum melt compositions in the system Qz(SiO2)-Ab(NaAlSi3O8)-Or(KAlSi3O8) at P(fluid)=2,5 and 10 kbar and at various water activities. The dry solidus was investigated in a dry argon environment. Water activities (aH2O) between 0.0 and 1.0 were obtained by using H2O-CO2 mixtures. The Or/Ab+Or ratio of first melts increases considerably with decreasing water activity. At 10 kbar it is 0.28 in the water-saturated system and 0.56 at water activity 0.1. The Qz-content does not change with changing water activities. The Ab-content of minimum melts formed at high pressures and low aH2O may remain almost constant in ascending magmas that are cooling and crystallizing. Qz-content increases at the expense of the Or-component. Solidus temperatures decrease considerably when aH2O increases slightly from zero. At 10 kbar, the temperature difference between dry melting and the solidus for aH2O=0.1 is 120°C. The influence of pure CO2 on the solidus is very limited in the investigated P-T range. The solidus is approximatively 760°C at aH2O=0.5 between 2 and 10 kbar and approximatively 830°C at aH2O=0.3. This means that melting of quartz-feldspar assemblages may induce dehydration reactions at P-T conditions of the granulite facies.

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  1. Institut für Mineralogie, Universität Hannover, Welfengarten 1, D-3000, Hannover 1, Germany

    A. Ebadi & W. Johannes

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  1. A. Ebadi
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  2. W. Johannes
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Ebadi, A., Johannes, W. Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2 . Contr Mineral Petrol 106, 286–295 (1991). https://doi.org/10.1007/BF00324558

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