ISOTOPIC VARIABILITY OF THE NINETYEAST RIDGE: CONSTRAINTS ON ITS MANTLE SOURCES AND INSIGHT INTO THE DEEP INDIAN MANTLE

The Ninetyeast Ridge (NER) is a ~5500 km long, N-S oriented, submarine volcanic ridge in the eastern Indian Ocean that represents the age-progressive, ~80 to ~40 Ma hotspot track of the 120 million year old Kerguelen mantle plume. Basaltic basement samples recovered during DSDP and ODP drilling campaigns have the characteristic Dupal geochemical signatures of Indian Ocean basalts, however, debate concerning the nature and number of components in their mantle source persists. In 2007, dredging at 21 locations along ~3200 km of the ridge recovered over 2000 kg of basalt. Pb-Hf-Sr-Nd isotopic compositions were determined on a subset of 60 basaltic samples. The samples are mostly tholeiitic with transitional to alkalic compositions present at some dredge sites. The samples are enriched in highly incompatible trace elements (e.g., La/Yb_N = ~1-13.5), cover a greater range of isotopic compositions than that of basalts drilled during ODP Leg 121, and their Pb-Hf-Sr-Nd isotopic compositions are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam-St. Paul mantle plumes. These characteristics reflect melting from a compositionally heterogeneous mantle source. At least three, possibly four, distinct source components with relatively enriched and depleted signatures are required to explain the observed isotopic variability along the NER. Mixing with Indian MORB does not account for the lower ⁸⁷Sr/⁸⁶Sr and higher Nd and Hf isotopic ratios of basalts from some of the dredges (i.e., ⁸⁷Sr/⁸⁶Sr = 0.70369 – 0.70428, ¹⁴³Nd/¹⁴⁴Nd > 0.51300 and ¹⁷⁶Hf/¹⁷⁷Hf > 0.28322). These depleted signatures are instead consistent with the presence of a previously depleted, garnet-enriched component intrinsic to their deep mantle source that had been identified based on incompatible trace element abundances alone (Frey et al., 2011). Together with other Indian Ocean island basalts of typical EM-1-like compositions, the isotopic compositions of the NER basalts are consistent with provenance from a deep mantle source that has incorporated a mixture of recycled sediments and lower continental crust together with altered oceanic crust (Nobre Silva et al., 2013). This supports a deep origin for the EM-1-like Dupal signatures encountered in ocean island basalts (Weis et al., 2011; Weis and Scoates, 2014).