Amphibole stability in H 2O-undersaturated calc-alkaline melts

Abstract

The upper stability limit of amphibole in silicate melts has previously been shown to increase in temperature when the mole fraction of H 2O in the fluid phase in equilibrium with the melt was decreased from 1.0 to 0.5. New experiments on amphibole stability in melts of andesite composition show that the amphibole stability limit also decreases in temperature when mole fraction H 2O in fluid is less than 0.5; these experiments imply that there is an isobaric temperature maximum of stability. At 5.5 kb this maximum occurs at a melt composition of 4.5 percent H 2O for andesite composition. The maximum at 5.5 kb may be explained by a change in amphibole breakdown from an incongruent melting reaction to a dehydration reaction. Calculations in the pressure range 0–10 kb indicate that the isobaric temperature maximum of amphibole stability occurs at a mole fraction of H 2O in the fluid phase ranging from 1.0 to 0.4, depending on total pressure. The maximum may represent either a melting or a dehydration reaction. Calculations also suggest that above 8 kb amphibole is as stable in silicate melt when the mole fraction of H 2O in the fluid phase is 0.3 as when the mole fraction is 1.0. Such conditions affirm the possibility of hydrous partial melting of amphibole-bearing basalt in the earth's mantle to form the calc-alkaline series.