EFFECTS OF PRESSURE AND WATER ON THE MELTING OF BASALT

Abstract

Melting of natural olivine basalt was measured at pressures up to 35 kilo bars by using the differential thermal analysis as well as the quenching method. Melting relations of basalt were classified into four cases according to the water contents in the samples : those were (1) the total pressure was equal to the water pressure, (2) 1.60 wt% of water was contained in the samples, (3) 0.98 wt% of water was in the samples, and (4) no water in the samples. Melting temperatures are found to be the highest in the case (4) and the lowest in case (1). In the case (4) the melting temperatures increase with pressure but in the case (1) decrease with pressure, and the cases (2) and (3) correspond to the melting relations when the water pressures are less than the total pressures. These melting relations were compared with the temperature distribution in the upper mantle. The results suggest the existence of the molten zone in the upper mantle, assuming the presence of basaltic composition and water (even to the extent of water of crystallization).