Abstract

The oxidation state of the 1991 Pinatubo dacite and many other oxidized silicic arc volcanic rocks has been overestimated. We reproduce the compositions of magnetite and ferrian ilmenite found in Pinatubo dacite by doing crystallization experiments at 780 degrees C, 2.2 kbar, and NNO + 1.7 (+ or -0.2) log units. The extra components in igneous ilmenite, notably geikielite, expand the solvus in a way that is not correctly accommodated in current formulations of the iron titanium oxide thermobarometer.

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