Melting of plagioclase-quartz assemblages at 2 kbar water pressure |
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Authors: | W Johannes |
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Institution: | (1) Institut für Mineralogie, Weifengarten 1, D-3000 Hannover 1, Germany |
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Abstract: | The melting of plagioclase and quartz has been investigated at P
H2O =2kb. A single crystal of plagioclase was surrounded by quartz powder and water. A reaction rim consisting of glass and of An-rich plagioclase developed around unchanged starting plagioclase. Microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase. For example at 850° C the approximate X
An of melt is 0.3 and that of plagioclase is 0.8. The temperature interval between beginning of melting and complete melting of cotectic compositions is 100° C or more for quartz-plagioclase mixtures with plagioclases between An 40 and An 75. In comparison to the system Ab-An-H2O the plagioclase melting loop is somewhat wider in the investigated system Qz-Ab-An-H2O but the temperature interval is much smaller (100° C) than in the system Ab-An-H2O (200° C). The solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments. The results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi-component rocks than in the pure plagioclase system. This finding is important for the development of albite rich rocks from more basic compositions. |
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