Rapid diffusive infiltration of sodium into partially molten peridotite

Abstract

Recent seismological, geochemical and experimental observations suggest that, as mantle peridotite melts, the resulting basaltic liquid forms an interconnected network, culminating in the rapid ascent of the basalt relative to the surrounding solid matrix. Mantle melting is therefore a polybaric process, with melts produced over a range of pressures having differing chemical characteristics. Modelling and peridotite-melting experiments designed to simulate polybaric mantle melting generally assume that there is no interaction between melts generated at greater pressures and the overlying solid mantle at lower pressures. Beneath mid-ocean ridges, melts derived from greater depth are probably channelized during ascent, so preventing direct re-equilibration with shallow peridotite, as required by geochemical observations. I show here, however, that sodium in ascending melts will quickly diffuse into the melt formed within nearby peridotite at lower pressures. This process fundamentally changes the manner by which the peridotite melts, and can account for both the creation of silica-rich glass inclusions in mantle xenoliths and the anomalous melting modes recorded by abyssal peridotites. Increased melting of lithosphere and upwelling asthenosphere could result from this process without the need to invoke higher mantle temperatures.