Trace element fractionation and isotope ratio variation during melting of a spatially distributed and lithologically heterogeneous mantle

Abstract

The source region of basalts in the mantle is chemically and lithologically heterogeneous. During decompression melting of a spatially distributed and lithologically heterogeneous mantle, mineral modes of distinct mantle sources vary continuously, resulting in spatial and temporal variations in the bulk partition coefficient of a trace element in different lithologies in the melting column, which in turn affects the fractionation of the trace element in partial melt and residual solid. This problem can be quantified by following the motion of solid in the melting column. This study presents a new melting model that can be used to keep track of spatial and temporal variations of mineral mode, melting reaction, bulk partition coefficient, and trace element concentration in the lithologically heterogeneous melting column. Simple analytical solutions for a time-dependent perfect fractional melting model are obtained. Essential features of the new model are elucidated through case studies of melting a two-lithology mantle that consists of blobs of orthopyroxene-rich lithology in the upwelling lherzolitic mantle; and an application to Sr-Nd-Hf isotope ratio variations in basalts from the Mid-Atlantic Ridge is presented. Fractional melting of the two-lithology mantle results in large temporal variations in incompatible trace element concentrations and Sr-Nd-Hf isotope ratios in the pooled melt. Mixing of fractional melts derived from different lithologies in the melting column produces enriched and depleted melts that form mixing loops in Sr-Nd-Hf isotope ratio correlation diagrams. These mixing loops rotate away from mixing lines defined by the binary mixing model and are a unique feature of melting a spatially heterogeneous mantle. Formation of the mixing loop can be traced to the location and spacing of the enriched lithological units in the melting column. The role of lithological heterogeneity is to change the bulk partition coefficient of a trace element from its values in the lherzolitic mantle to new values in the pyroxenitic mantle, which alters the extent of depletion of the trace element in the melting column. Changing bulk partition coefficient with time and space through lithological heterogeneity can result in greater variabilities in Sr-Nd-Hf isotope ratios and highly incompatible trace element concentrations in the pooled melt. Results from this study establish a framework for systematic studies of trace element fractionation and isotope ratio variation during decompression melting of a spatially distributed and lithologically heterogeneous mantle.