Basalt-andesite-rhyolite-H_2O: Crystallization intervals with excess H_2O and H_2O-undersaturated liquidus surfaces to 35 kolbras, with implications for magma genesis

Abstract

Three rocks representing the calc-alkaline rock series gabbro-tonalite-granite or basalt-andesite-rhyolite were reacted with varying percentages of water in sealed capsules between 600 and 1300°C and pressures to 36 kbars, corresponding to depths of more than 120 km within the earth. For each rock we present complete P-T diagrams with excess water, and the water-undersaturated liquids surface projected from P-T-X_(H_2O) space mapped with contours for constant H_2O contents and with the fields for near-liquidus minerals. All changes in liquidus and solidus slopes can be correlated with changes in mineralogy from less dense to more dense, or with expansion of crystallization fields, without appeal to changes in molar volume of H_2O in liquid and vapor phases. The results indicate that tholeiites and andesites of the calc-alkaline series with compositions similar to the rocks studied are not primary magmas from mantle peridotite at depths greater than about 50 km. Primary andesitic magmas from shallower levels would require very high water contents and we do not believe such magmas could normally reach the surface. The liquids results are consistent with the derivation of andesites with little dissolved water as primary magmas from subducted ocean crust (quartz eclogite), but multi-stage models are preferred. Temperatures required for the generation of andesites by fusion of continental crust are higher than considered reasonable. The evidence precludes the generation of primary rhyolites or granites from the mantle of subducted oceanic crust at mantle depths. Primary rhyolite or granite magmas with moderate water contents (saturated or undersaturated) can be generated in the crust at reasonable temperatures, and could reach near-surface levels before vesiculation. Water-undersaturated granite liquid with residual crustal minerals could constitute plutonic magmas of intermediate composition.

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