## Abstract

The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P_{total} and {Mathematical expression}. The values of P_{total} and {Mathematical expression} depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases {Mathematical expression} ≈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 {Mathematical expression} are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that {Mathematical expression}>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 Mg_{7}Si_{8}O_{22}(OH)_{2}.

Original language | English |
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Pages (from-to) | 149-158 |

Number of pages | 10 |

Journal | Contributions to Mineralogy and Petrology |

Volume | 40 |

Issue number | 2 |

DOIs | |

Publication status | Published - Jun 1973 |