Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2 |
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| Authors: | A Ebadi W Johannes |
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| Institution: | 1. Institut für Mineralogie, Universit?t Hannover, Welfengarten 1, D-3000, Hannover 1, Germany
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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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