Viscosities of basalt and andesite melts at high pressures

Abstract

The viscosities of melts of Kilauea 1921 olivine tholeiite and Crater Lake calc-alkaline andesite have been determined with a falling sphere method at superliquidus temperatures from 7.5- to 30-kbar pressure. Spheres of chromian diopside and of platinum were used for the tholeiite and andesite melts, respectively, and graphite capsules 10 mm long in which the temperature gradient was less than 15° were used as containers. The viscosity of the Kilauea, Hawaii, olivine tholeiite melt decreases significantly with increasing temperature at constant pressure (e.g., by a factor of 1.5 per 25°C at 15 and 20 kbar) but decreases only slightly with increasing pressure at constant temperature. Consequently, the viscosity of the tholeiite melt decreases with increasing pressure along its anhydrous liquidus, 40, 25, and 8 P at 15, 20, and 30 kbar, respectively. The viscosity of the Crater Lake, Oregon, andesite melt also decreases with increasing pressure along its anhydrous liquidus, about 3200 and 900 P at 7.5 and 20 kbar, respectively. The viscosity of the andesite melt with 4 wt % H2O, measured in a sealed Pt capsule, is lower by a factor of about 20 than that of the anhydrous melt at the same temperature and pressure. The results of the present experiments are applicable to the problems of crystal settling at high pressures, ascent of magma in the upper mantle, and separation of magma from its source region. Settling rates of garnet and clinopyroxene crystals 2 mm in diameter in basaltic (or eclogitic) magma at 30 kbar should be approximately 3 and 2 m/h, respectively. It is estimated that magmas carrying peridotite xenoliths 10 cm in diameter would have to rise from a depth of 50 km to the surface in less than 60 hours. Because of the decrease in viscosity with depth, magmas are probably more easily separated from deeper source regions in the upper mantle. It is suggested that alkali basalt magmas are probably less viscous than tholeiitic magmas under upper mantle conditions.