Constraints imposed by experimental petrology on possible and impossible magma sources and products

Abstract

Experimental petrology can be used in forward and inverse approaches. The forward approach defines the compositions of liquids generated by partial melting of possible source rocks at various depths. The inverse approach determines conditions for multiple-mineral saturation at the liquidus of primitive magmas, correlates them with residual minerals of possible source rocks, and thus provides estimates of depths and temperatures required for their derivation. Review of a selection of forward and inverse results is followed by evaluation of petrological and geophysical processes in layered mantle and in subduction zones. Physical constraints imposed by solidus curves and geotherms present problems for models that derive basalts from deep mantle reservoirs, separated from overlying convecting layers. Magmas from mantle are limited to compositions less siliceous than basaltic andesite, with rare exceptions. Granite liquids cannot be generated from normal peridotite, nor from oceanic crust at mantle pressures in subduction zones. In continental crust, hydrous granite liquid is generated at depths of less than 30 km. Basaltic andesite and picritic basalt are parental magmas for the calc-alkaline series. Andesite is not primary from subcontinental depths, and can be generated as liquid in continental crust only if temperatures exceed about 1100 degrees C. Calc-alkaline magmas may contain components from mantle peridotite, subducted oceanic crust, and continental crust.

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