Tracing the Garnet Signature in MORB from Ultra-Slow Spreading Centers: A Lu-Hf Isotopic Study of the Southwest Indian Ridge

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The role of garnet remains a major unresolved question in the generation of mid-ocean ridge basalt (MORB). Trace element abundances in MORB suggest that melting occurs in the presence of garnet at most mid-ocean ridges, but if melting does begin within the garnet stability field (75 km depth) most melting models predict significantly more melt production than is needed to generate the 7 km thickness of typical oceanic crust. This paradox is even more problematic at ultra-slow spreading ridges (1 mm/y half-rate), as lavas from these ridges display the strongest garnet signatures found in MORB, yet the crust produced is significantly thinner than that generated at other mid-ocean ridges. We intend to probe the origin of the garnet signature in the ultra-slow spreading Southwest Indian Ridge (SWIR) by conducting a Lu-Hf isotopic investigation of MORB from this ridge. In particular we wish to test the hypothesis that the garnet signature may be due to the tapping of garnet pyroxenite veins in the upper mantle beneath the SWIR. These pyroxenite veins may represent ancient unhomogenized continental or oceanic lithospheric materials that were introduced into the southwestern Indian Ocean mantle during the breakup of Gondwanaland, and may be responsible for large observed Sr, Nd and Pb isotopic anomalies. The sensitivity of the Lu-Hf system to melting in the presence of garnet makes this isotope system ideal for testing the garnet pyroxenite hypothesis.