Melting in the Earth's Crust

Abstract

Investigation of the effect of volatile materials on the melting temperatures of rocks suggests that partial melting occurs readily within the earth's crust. Granites and shales begin to melt at 660°C in the presence of water vapor at 4000 bars pressure. If the water contains 4 weight per cent hydrogen fluoride, the melting temperature of granite is lowered to 590°C. At 1000 bars the minimum liquidus temperature in the system Ca0-CO_2-H_2O is 675°C; at 4000 bars the liquidus minimum is 640°C. Addition of MgO lowers the melting temperature at 1000 bars to 625° C. A value of 30° C/km. is reasonable for geothermal gradient in geosynclines and if this remains constant with increasing depth, a temperature of 620°C is attained at 20 km. (about 5000 bars pressure). The compositions of many geosynclinal rocks may be represented in terms of granite, shale, and carbonates, and the above experimental results indicate that such rocks will be partially melted at 20 kms. depth provided volatiles are present. The extent of melting will depend upon bulk composition and especially upon the proportion of available volatiles. Partial melting therefore plays an important role in the metamorphism and deformation of rocks in orogcnic zones.

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