Melting in the deep crust and upper mantle and the nature of the low velocity layer

Abstract

It is possible that there are minor amounts of water in the upper mantle. This water could significantly lower melting temperatures and may give rise to melts of different composition to those formed by melting under anhydrous conditions. This paper summarizes our experimental study of the influence of water on phase relations in natural gabbro and tonalite samples at pressures from 10 to 25 kb. Our results are used in conjunction with recently published data for other rock-water systems to discuss melting in the upper mantle and lower continental crust. It is suggested that the low velocity zone in the upper mantle could be a layer of peridotite containing interstitial melt, which may be overlain by a layer of peridotite containing interstitial “water”. The minimum possible temperatures for the generation of andesite and basalt liquids are discussed, and it is noted that the initial melt formed from wet peridotite may be andesitic. Finally, granulite facies metamorphism in the lower continental crust and the generation of “granites” are considered.