Experimental determination of liquidus H2O contents of simple granites at deep crustal conditions

Abstract:

The liquidus water content of a granitic melt at high pressure (P) and temperature (T) is important because it constrains the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (≤0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700-1000˚C, 0.6-1.5 GPa). To improve the high-P constraints on water concentration at the granite liquidus, we preformed experiments in piston-cylinder apparatus at 1 GPa using a range of granite compositions. In each experiment, granite glass + H₂O was homogenized well above the liquidus T, then T was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T, then raised the T until we found that the crystalline phases were resorbed into the liquid. Four different bulk compositions were studied (ab-ksp-qz, in wt%): 40-40-20, 37.5-37.5-25, 36.25-36.25-27.5, 35-35-30. Quenched charges were analyzed by petrographic microscope, SEM and electron microprobe. Microprobe analysis of all-glass charges reproduced our intended starting compositions. The minimum temperature of the 1 GPa liquidus at a given water content occurs at 27.5±0.3 wt% quartz, regardless of H₂O content. The reversed liquidus temperatures at 3.0, 4.1, 5.85, 7.95 wt% water are respectively 935-985, 850-915, 775-825 and 700-750˚C. Our results conform closely to the extrapolation of Holtz 2001, which give significantly higher water contents than used by most dehydration melting models. This presents a challenge for producing voluminous amounts of metaluminous granites from lower crustal biotite-amphibole gneisses by dehydration melting. For example, a deep-crustal tonalitic gneiss with 0.6-0.8 wt% H₂O would yield less than 20 vol% granitic liquid for complete dehydration and perfect extractability, neither of which are likely to be realized in deep crustal melting.