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 lherzolite in the upper mantle. Starting material was a high-Na, Al basalt glass previously shown to be a very low degree (~1%) partial melt of spinel lherzolite at 1.5 GPa, 1269°C [Robinson et al., Earth Planet. Sci. Lett., in press]. The experiment was run with a spinel seed under sub-liquids conditions (1255°C) to ensure clinopyroxene crystallisation. The experimental clinopyroxene composition is consistent with equilibrium close to the solidus of fertile mantle lherzolite, most notably in its high contents of Ca-Tschermak's (CaTs) molecule (~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.