The Southeast Indian Ridge: Scale of Source Heterogeneity and Origin of the DUPAL Anomaly

Results Pb and Hf isotope ratios from 124 basalt glasses sampled at < 10 km intervals along 2000 km of the Southeast Indian Ridge (SEIR) between 86°E and 110°E show bimodal distributions. The bimodality in both Pb and Hf isotope ratios confirms the presence of ancient compositional streaks in the Indian Ocean upper mantle [1]. The density of streaks is well described by a Poisson distribution having a characteristic thickness of ~25 km. Implications Pb isotopes in SEIR basalts all carry a DUPAL [3] isotope signature. Two possible origins for the bimodality and DUPAL signature are: (1) ancient melting that involved garnet fractionation, with subsequent pollution of the upper mantle by continental material during Gondwana breakup; (2) inherited heterogeneity from the early Earth. The later explanation is difficult to reconcile as it requires an input of a non-chondritic Pb composition. Pollution of the asthenosphere by material resulting from interaction of deep mantle hotspot/plumes (e.g. Karoo, Reunion) with the shallow continental lithosphere during the breakup of the Gondwana supercontinent and formation of the Indian Ocean basin adequately accounts for the DUPAL anomaly. [1] Graham et al. (2006) Nature 440, 199-202. [2] Mahoney et al. (2002) J. Petrology 43, 1155-1176. [3] Hart (1984) Nature 309, 753-757.; Histogram for Hf isotopes and bimodal distribution in the ɛ_Hf versus ^208*Pb/^206*Pb binary projection for 197 SEIR glasses [this work; 1, 2]. The 2 populations overlap. Although the ɛ_Hf means differ by only 1.5 ɛ units, a t-test shows that the probability the means are equal is infinitesimal (t=18). The statistics derived from this analysis are fully consistent with the discriminant value ɛ_Hf = 11.8 [1].