Fluid-absent melting of the fluoro-hydroxy amphibole pargasite to 35 kilobars |
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| Authors: | John R Holloway CE Ford |
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| Institution: | Division of Geochemistry, Department of Chemistry, Arizona State University, Tempe, Ariz. USA;Grant Institute of Geology, University of Edinburgh, Edinburgh Scotland |
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| Abstract: | In order to evaluate the effect of fluorine substitution on hornblende stability in basaltic melts, the upper stability of a synthetic pargasite with 43% of its OH sites replaced by fluorine was studied under fluid-absent conditions at pressures up to 35 kbars. The fluorohydroxy pargasite melts incongruently over an interval of 25–55°C depending on pressure. Liquid, amphibole, clinopyroxene, spinel, forsterite and garnet, at higher pressures, appear in the melting interval. Coordinates for the amphibole-out boundary are: 5 kbars, 1060°C; 15 kbars, 1230°C; 25 kbars, 1285°C; 30 kbars, 1290°C and 35 kbars, 1285°C.Substitution of F for OH in the amphibole structure increases both its temperature and pressure stability limits, and results in a substantial melting interval through which hornblende and melt coexist in fluid-absent situations. Partial melting events could produce fluorine-rich hornblende as a refractory, residual phase. |
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