The stability of amphibole in andesite and basalt at high pressures

Reinvestigation of earlier work on Mt. Hood andesite and Kilauea olivine tholeiite under N-NO conditions and with silver capsules. The revised curve for the appearance of garnet is significantly lower in pressure; no orthopyroxene appeared in the run products, in contrast to the results of the earlier work. The high-temperature segment of the amphibole-out curve for the tholeiite is at least as high as 1040 degrees C at 13 kbar and 1050 degrees C at 16 kbar, and the high-pressure part of this curve is at about 27 kbars, about 6 kbar higher than in the earlier work. Amphibole is the sole silicate phase on the vapor-saturated liquidus in the andesite at pressures below the garnet-in curve (>15.5 kbar). Revised phase relationships are consistent with the derivation of andesite by amphibole-liquid equilibria in basaltic magma, and reveal stability to depths of nearly 100 km. The great depths to which amphiboles are stable make it probable that they are a source of H 2 O for martial melting in deeply subducted oceanic crust.--Modified journal abstract.