Steady-state Mantle–Melt Interactions in One Dimension: I. Equilibrium Transport and Melt Focusing

Abstract

Mantle–melt interaction during melt transport is explored in one dimension and steady state. We reconsider the equivalence between one-dimensional steady equilibrium transport and batch melting. In the absence of diffusion and radioactivity, conservation of mass flux requires that the major and trace element compositions of melt and solid at each point are the same as is generated by batch melting the source composition at the same pressure and temperature. Energy conservation requires that temperature and extent of melting are independent of melt migration except for irreversible source terms related to viscous compaction and gravitational energy release. The equivalence of phase compositions at each pressure between steady-state equilibrium transport and batch melting simplifies melt transport calculations. We examine the effects of increasing the melt flux to simulate melt focusing by channeling or by two-dimensional flow with converging melt streamlines. Melt focusing modifies the mineralogy of both residual matrix and erupted melt. We use MELTS calculations to model the formation of dunite by this mechanism and quantify the melt flux required to exhaust orthopyroxene from the residue as a function of pressure. The model dunites are found to be similar to natural dunites observed in the mantle section of ophiolite sequences.