Simple models for trace element fractionation during decompression melting of a two-lithology mantle

Abstract

Batch melting, fractional melting, continuous melting and two-porosity melting models have been used widely in geochemical studies of trace element fractionation during mantle melting. These simple melting models were developed for melting an homogeneous mantle source. Here we revisit and further develop these melting models in the context of decompression melting of a two-lithology mantle. Each lithology has its own source composition and melting parameters. During decompression melting, melt and solid flow vertically in the melting column. Part of the melt produced in one lithology is transferred to the other lithology at a prescribed rate. We use a set of conservation equations to solve for melt and solid mass fluxes, extent of melting and concentrations of a trace element in interstitial melt and aggregated melt in each lithology and mixed-column melt between the two lithologies. We uncover conditions under which batch melting, fractional melting, continuous melting and two-porosity melting models are realized during decompression melting through four case studies. We show that porosity in the continuous melting model varies along the melting column during decompression melting, contrary to what was assumed in its original development. We unify the batch melting, fractional melting, continuous melting and two-porosity melting models through a two-lithology melting model for decompression melting in a two-lithology mantle column. We discuss basic features of the two-lithology melting model through worked examples. We show that it is possible to produce partial and well-mixed melts with a range of REE patterns, from LREE depleted to LREE enriched, similar to those observed in mid-ocean ridge basalts by decompression melting of a two-lithology mantle.