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Science 26 February 1999:
Vol. 283. no. 5406, pp. 1303 - 1306
DOI: 10.1126/science.283.5406.1303
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Reports

Lu-Hf Isotope Systematics of Garnet Pyroxenites from Beni Bousera, Morocco: Implications for Basalt Origin

Janne Blichert-Toft, 1* Francis Albarède, 1 Jacques Kornprobst 2

Six garnet pyroxenites from Beni Bousera, Morocco, yield a mean lutetium-hafnium age of 25 ± 1 million years ago and show a wide range in hafnium isotope compositions (epsilon Hf = -9 to +42 25 million years ago), which exceeds 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 aggregate of pyroxenites that were left by the melting of oceanic crust at subduction 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 experimental work relevant to mantle melting in the presence of garnet.

1 Ecole Normale Supérieure de Lyon and CNRS, 46 Allée d'Italie, 69364 Lyon Cedex 7, France.
2 L'Unité Mixte de Recherche CNRS "Magmas et Volcans," Observatoire de Physique du Globe de Clermont-Ferrand, 5 rue Kessler, 63038 Clermont-Ferrand, France.
*   To whom correspondence should be addressed.

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