Large [formula omitted]-excesses in basalts produced by partial melting of spinel lherzolite

Abstract

Excesses of 230 Th over 238 U in mid-ocean ridge basalts (MORB) require that the mantle source region preferentially retains U over Th during partial melting. Based on existing mineral-melt partitioning data, 230 Th excesses are widely cited as evidence that partial melting beneath ridges begins within the garnet stability field, at pressures over 2.8 GPa. However, recent experimental and theoretical studies of U–Th partitioning show that melting in the presence of aluminous mantle clinopyroxene may also generate 230 Th -excess. In order to try to distinguish between these models we sought basalts with independent constraints on their depth and extent of partial melting. We report data from alkali basalts from the Antarctic Peninsula whose tectonic setting indicates that they formed by <6% partial melting at pressures of <2 GPa, well within the spinel stability field. Their major and trace element chemistry is best modelled by ∼4% partial melting at pressures of 1–2 GPa, in excellent agreement with that inferred from the plate structure. However, these rocks preserve large (6–26%) 230 Th -excesses, which would conventionally be ascribed to the involvement of garnet. Instead we show that the trace element signature and isotopic data can be reconciled with partial melting involving residual aluminous–clinopyroxene within the spinel stability field. These Antarctic Peninsula basalts provide the first observational evidence that significant 230 Th -excesses can be produced by partial melting of spinel lherzolite and challenge the perceived importance of garnet in MORB petrogenesis.