HEAVY REE ARE COMPATIBLE IN CLINOPYROXENE ON THE SPINEL LHERZOLITE SOLIDUS

Trace element partitioning between clinopyroxene and melt was investig ated experimentally under conditions appropriate to near-solidus melti ng of spinel Iherzolite in the upper mantle. Starting material was a h igh-Na, Al basalt glass previously shown to be a very low degree (simi lar to 1%) partial melt of spinel Iherzolite at 1.5 GPa, 1269 degrees C [Robinson et al., Earth Planet. Sci. Lett., in press]. The experimen t was run with a spinel seed under sub-liquidus conditions (1255 degre es C) to ensure clinopyroxene crystallisation. The experimental clinop yroxene composition is consistent with equilibrium close to the solidu s of fertile mantle Iherzolite, most notably in its high contents of C a-Tschermak's (CaTs) molecule (similar to 22 mol%) and Na2O (1.4 wt%). Clinopyroxene-melt partition coefficients (D) for a wide range of tra ce elements, determined by SIMS analysis of run products, differ marke dly from those reported in other studies under conditions less appropr iate to mantle melting. In particular partition coefficients for the h eavy rare earth elements (Gd-Lu) are greater than unity (e.g. D-Lu = 1 .45), and the critical partitioning parameter, (D-Sm/D-Nd) X (D-Hf/D-L u), is 0.68. These features, which arise due to the high CaTs content of the clinopyroxene, dramatically reduce the required involvement of garnet in the melting region beneath mid-ocean ridges. (C) 1998 Elsevi er Science B.V. All rights reserved.