Melting of hydrous upper mantle and possible generation of andesitic magma: An approach from synthetic systems |
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Authors: | I Kushiro |
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Institution: | Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. USA |
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Abstract: | Phase equilibria in a portion of the system forsterite-plagioclase (An50Ab50 by weight)-silica-H2O have been determined at 15 kbar pressure under H2O-saturated conditions. The composition of the liquid pertinent to the piercing point forsterite + enstatite solid solution + amphibole + liquid + vapor is similar to that of calc-alkaline andesite. The electron microprobe analysis of the glass coexisting with the above three crystalline phases is very close to that of the piercing point determined by phase assemblage observations; however, the glass near (< 8 μm) forsterite crystals is significantly depleted in the normative forsterite component. With the addition of 10 wt.% KAlSi3O8, the composition of this piercing point becomes even closer to the compositions of calc-alkaline andesites. It is also shown that the liquid coexisting with forsterite and enstatite solid solution remains silica-rich (60–62 wt.%) over a wide (~ 100°C) temperature range. The present experimental studies support the view that liquids similar in composition to calc-alkaline andesites can be generated by direct partial melting of hydrous upper mantle at least at or near 15 kbar. |
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