Large Th-230-excesses in basalts produced by partial melting of spinel lherzolite

Excesses of Th-230 over U-238 in mid-ocean ridge basalts (MORB) require tha t the mantle source region preferentially retains U over Th during partial melting. Based on existing mineral-melt partitioning data, Th-230 excesses are widely cited as evidence that partial melting beneath ridges begins wit hin 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 rh-exc ess. 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 tecto nic 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 similar to 4% partial melting at pres sures of 1-2 GPa, in excellent agreement with that inferred from the plate structure. However, these rocks preserve large (6-26%) 230 Th-excesses, whi ch would conventionally be ascribed to the involvement of garnet. Instead w e 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 firs t observational evidence that significant 230 rh-excesses can be produced b y partial melting of spinel Iherzolite and challenge the perceived importan ce of garnet in MORE petrogenesis. (C) 2000 Elsevier Science B.V. All right s reserved.