Lu-Hf isotope systematics of garnet pyroxenites from Beni Bousera, Morocco: Implications for basalt origin

Six garnet pyroxenites from Beni Bousera. Morocco, yield a mean Lutetium-ha fnium age of 25 +/- 1 million years ago and show a wide range in hafnium is otope compositions (epsilon(Hf) = -9 to +42 25 million years ago), which ex ceeds that of known basalts (0 to +25), Therefore, primary melts of garnet pyroxenites cannot be the source of basalts. The upper mantle may be an agg regate of pyroxenites that were Left by the melting of oceanic crust at sub duction zones and peridotites that were contaminated by the percolation of melts from these pyroxenites. As a consequence, the concept of geochemical heterogeneities as passive tracers is inadequate, Measured Lutetium-hafnium partitioning of natural minerals requires a reassessment of some experimen tal work relevant to mantle melting in the presence of garnet.