Geochemistry of the mantle beneath the Rodriguez Triple Junction and the South-East Indian Ridge

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Rare earth element abundances and Sr, Nd, Pb isotope compositions have been measured on zero-age dredge samples from the Rodriguez Triple Junction (RTJ) and the South-East Indian Ridge (SEIR), Along the SEIR, the geochemical ``halo'' of the St. Paul hot spot has a half-width of about 400 km and the data may be fairly well accounted for by a binary mixing between an Indian MORB-type component (87Sr/86Sr = 0.7028, 143Nd/144Nd = 0.51304, 206Pb/204Pb = 17.8) and the plume-type St. Paul component (0.7036, 0.5129, and 18.7 respectively). The alignment of the lead isotope data is particularly good with an apparent age of 1.95 +/- 0.13 Ga and Th/U source value of 3.94. One sample dredged on the ridge 60 km southeast of St. Paul bears a definite Kerguelen isotopic signature.
The RTJ has distinctive geochemical properties which contrast with those of the adjacent ridge segments. Low 206Pb/204Pb ratios which plots to the left of the geochron, rather high 208Ph/204Ph and 87Sr/86Sr ratios (17.4, 37.4, and 0.7031 respectively), a striking isotopic homogeneity, and variable LREE/HREE fractionation with (La/SM)N = 0.3-0.8 make this triple junction an anomalous site.
The geochemical properties of the Indian Ocean basalts have been examined using a three-component mantle model involving (a) a normal MORB-type source though to represent the depleted upper mantle matrix, (b) an OIB-type source of uncertain parentage (recycled oceanic crust?), and (c) a component with low μ, low Sm/Nd, high Rb/Sr (time-averaged value) which is tentatively assigned to ancient hydrothermal and abyssal sediments recycled in the mantle. The high 201Ph/204Ph and 87Sr/86Sr ratios typical of the Dupal anomaly are likely due to the widespread distribution of this latter component in the basalt source from this area, including that for MORBs.

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