Trace Element Partitioning between Silicate Minerals and Carbonatite and Silicate Melts at 18kb to 46kb Pressure

Abstract

The equilibration of mantle mineralogies with silicate or carbonatite melts may fractionate trace elements in the mantle in distinctively different ways. To address this question in this study, we present new experimental data to quantify the manner in which selected trace elements (Rb, Ba, Na, K, Nb, Ta, Sr, Zr, Ti, Y, Lu, Cr) may distribute between garnet, clinopyroxene and phlogopite and silicateand carbonatite-melts. New data are also presented for describing the partitioning of these elements between olivine and orthopyroxene and a carbonatite melt. In terms of major elements, three bulk systems are considered (1) carbonate dominated, (2) silicate (carbonate-absent) and alkali-rich and (3) silicate (carbonate-absent) and alkali-poor. The composition and phase relations of the carbonate bulk system are given in Sweeney (1994). The alkali-rich silicate bulk system has a MARID (principally comprising mica-amphibole-diopside) bulk composition (AJE137, Sweeney et al., 1993) and the alkali-poor silicate bulk composition is the quartz tholeiite of Kingwood and Green (1966). In terms of their major element composition, the liquids produced in these three bulk systems are (1) alkali-rich dolomitic melts, (2) K-rich silicaundersaturated basaltic melts, and (3) K-poor silica-saturated basaltic melts. It is suggested that these melt compositions cover the likely spectrum of low degree melt compositions in the Earth's upper mantle. Melts (1) and (2) may be produced in a mantle peridotite at low melting degrees and (3) by the partial fusion of a subducting slab. It is these low degree melts which may carry substantial budgets of incompatible elements and thus we consider the bulk systems studied to be relevant to the quantification of mantle metasomatism. Experimental technique