Experimental Limits for Melting in the Earth's Crust and Upper Mantle

Abstract

The conditions for melting in the crust and upper mantle are governed by the mineralogy (determined by bulk composition, depth, and temperature), the water content, the physical state of the water (available in pore fluid or bound in crystals), P_e_H_2O, and the temperature distribution. The average composition of the crust is andesitic and its mineralogy is dominated by feldspars and quartz. Melting curves in the presence of excess water at pressures ranging to more than 10 kb (40-km depth) have now been determined for individual feldspars, for most feldspar-quartz combinations, and for many major rock types. In the presence of an aqueous vapor phase, the granitic components of many crustal rocks combine to produce water saturated liquid of granite composition. Starting assemblages for melting in rock-water systems consist of minerals with interstitial vapor, hydrous and anhydrous minerals with no vapor, or anhydrous minerals with no vapor. Models for magma generation must consider whether the liquids produced are water saturated or water deficient under the conditions of melting. From estimates of temperatures in the crust it becomes apparent that no granitic liquids can be produced at a depth shallower than 20 km. Results from water-excess experiments and interpolated water deficient conditions indicate that the normal product of partial fusion of many crustal rocks is a water undersaturated granite liquid in a crystal mush which persists through a wide temperature range. It is not likely that liquids of intermediate composition are generated directly, because temperatures are too high, but crystal-liquid assemblages of intermediate bulk composition may move to higher levels in the crust by diapiric rise. The generation of basaltic magmas in a dry mantle requires unusually high temperatures. Thus, most basal tic magmas must be produced under conditions where the local temperature greatly exceeds that of the average geothermal gradient. However, the presence of trace amounts of water in the mantle does permit incipient melting of eclogite or peridotite with a normal geothermal gradient. The depth interval within which such melting occurs in rock-water systems coincides with the seismic low-velocity zone; this fact may explain the presence of the zone as a continuous layer in the earth's mantle. Crustal melting, localized in orogenic belts, is not likely to produce continuous layers. However, removal of pore fluids from deep seated continental basement rocks by repeated melting may have produced regions with laterally extensive uniform properties, despite their variegated compositions.