Amphiboles in andesite and basalt: I. Stability as a function of P-T-f_o2 Available to Purchase

The stabilities of amphiboles have been determined in an andesite, three basalts, and an olivine nephelinite in the presence of H₂O vapor at values of oxygen fugacity approximating those of Fe₃O₄–Fe₂O₃, Ni-NiO, and Fe₃O₄–FeO. The thermal stability decreases with increasing activity of silica in the parent rocks. Maximum thermal stability of amphibole occurs at 1090°C at 13 kbar in the Hualalai alkali basalt. Maximum pressure stability occurs at 31.5 kbar at a temperature of 1030°C in the olivine nephelinite. Amphibole-bearing assemblages convert to garnet-bearing assemblages at pressures above 18 to 30 kbar. The amphiboles straddle the calciferous-subcalciferous boundary, and all of them fall in the pargasite-tschermakite-tschermakitic hornblende category. No orthopyroxene was found in any of the basaltic compositions under any of the conditions investigated, although it did occur in the andesite. This restricts the potential P-T-X conditions under which fractional crystallization of orthopyroxene is an important mechanism in the derivation of SiO₂-undersaturated magmas.

Our results are consistent with those hypotheses in which some andesitic magmas derive from basaltic compositions through amphibole-liquid equilibria. Fractionation of alkalis by amphiboles may contribute to the proposed gradient in K₂O across subduction zones at island arcs and continental margins. The shallow dips and depths of subducting slabs beneath orogenic-zone volcanoes is in concert with this model.