Heavy REE are compatible in clinopyroxene on the spinel lherzolite solidus

Trace element partitioning between clinopyroxene and melt was investigated experimentally under conditions appropriate to near-solidus melting of spinel Iherzolite in the upper mantle. Starting material was a high-Na, Al basalt glass previously shown to be a very low degree (similar to 1%) partial melt of spinel Iherzolite at 1.5 GPa, 1269 degrees C [Robinson et al., Earth Planet. Sci. Lett., in press]. The experiment was run with a spinel seed under sub-liquidus conditions (1255 degrees C) to ensure clinopyroxene crystallisation. The experimental clinopyroxene composition is consistent with equilibrium close to the solidus of fertile mantle Iherzolite, most notably in its high contents of Ca-Tschermak's (CaTs) molecule (similar to 22 mol%) and Na2O (1.4 wt%). Clinopyroxene-melt partition coefficients (D) for a wide range of trace elements, determined by SIMS analysis of run products, differ markedly from those reported in other studies under conditions less appropriate to mantle melting. In particular partition coefficients for the heavy 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-Lu), 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 Elsevier Science B.V. All rights reserved.