P
total,P_{H_2 O} and the occurrence of cummingtonite in volcanic rocks |
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| Authors: | B J Wood I S E Carmichael |
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| Institution: | (1) Department of Geology and Geophysics, University of California, 94720 Berkeley, California |
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| Abstract: | The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P
total and
. The values of P
total and
depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases
 P
total.The rhyolite magma contained over 9 per cent water (by weight) when the cummingtonite phenocrysts precipitated, and possibly as much as 12 per cent, so that it is surprising that one of these rhyolites is a coherent lava. The calculated values of P
total and
are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that
>0.7–0.8 P
total for cummingtonite to precipitate in rhyolites, and that iron-rich olivine and cummingtonite could only exist in rhyolites over a small temperature range at a pressure near 5 kilobars. Hornblende phenocrysts co-existing with fayalitic olivine in rhyolites accordingly have a very low activity of Mg7Si8O22(OH)2. |
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