Amphiboles in andesite and basalt: II. Stability as a function of P–T–fH₂O–fO₂ Available to Purchase

The stabilities of amphiboles at high pressures in an andesite and a basalt have been determined in the presence of H₂O–CO₂ vapors for values of mole fraction of H₂O in the vapor (X^vH₂O) of 1.0, 0.75, 0.5, and 0.25. The maximum thermal stability of amphibole in the andesite is about 970°C in the range of 10 to 20 kbar and X^vH₂O of ∼0.75. Comparable data for the basalt are 1050°C from 10 to 15 kbar and X^vH₂O of 0.25. The maximum pressure stability of amphibole is 21.5 kbar at X^vH₂O ∼1.0 in the andesite and 20.5 kbar at X^vH₂O ∼1.0 in the basalt. Electron microprobe analyses are presented for orthopyroxenes, clinopy-roxenes, amphiboles, garnets, and glasses synthesized over a range of pressures, temperatures, X^vH₂O, and bulk composition. Most of the amphiboles are nepheline-normative, calciferous, and tschermakitic.

These data on the chemistry of amphiboles and the temperature-pressure conditions over which they are stable are consistent with our hypothesis in which andesites of the circum-Pacific zone are derived by amphibole–liquid equilibria from basaltic magma.