Petrological and geochemical study of the Indian Ocean MORB from the Rodriguez Triple Junction, Indian Ocean

Mid-ocean ridge basalt (MORB) form the Indian Ocean has long been known to have distinct isotopic characteristics compared to those from the Atlantic and Pacific Oceans (e.g., Dupre and Allegre, 1983). This has been thought to be the result of a large-scale upper-mantle heterogeneity, while the cause of the heterogeneity is still controversial (e.g., Hamelin et al., 1986). Thus, it is considered that the Indian Ocean MORB is an important key to understanding mantle processes such as convection and contamination. For this purpose, we present new results of detailed petrological and geochemical investigations for an extensive suite of fresh basalt samples dredged from around Rodriguez Triple Junction (RTJ), central part of the Indian Ocean. Studied samples were dredged from RTJ and the first segments of the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR), and Southwest Indian Ridge (SWIR). The studied region around RTJ is not influenced by plume-type influxes (Michard et al., 1986) and thus is considered to be a better place to obtain pristine signatures of the Indian upper mantle. Petrogrphycal characteristics of the dredged samples are similar within each dredge site. Most of basalts dredged around RTJ have glass rind, indicating rapid quenching when the basalts were erupted onto seafloor. Most of them are aphyric to sparsely phyric (plagioclase or plagioclase- olivine) with hyalophilitic, intersertal, trachytic, or intergraular texture. Some olivine phenocrysts showing skeletal shape also suggest that magma was quenched. Basalts around RTJ are subalkalic tholeiite and are typical MORB in terms of major element compositions. Their steep increasing Na2O contents and relatively constant CaO/Al2O3 with decreasing of MgO contents as well as higher olivine mg# (=Mg/(Mg+Fe): from 83 to 90) indicate that most of them have undergone only small amounts of crystal fractionation. In the MORB-normalized trace element pattern, our samples are characterized by a noticeable negative Nb- anomaly. Concentrations of Nb correlate well with those of K, Ba, Rb, Th, and U, whereas other incompatible elements are not correlated with Nb. Moreover, elemental ratios among these elements are slight departure from the ratios of the normal MORB. The trace element characteristics of the Indian Ocean MORB samples can be explained by contamination of small amounts of lower crustal component to a depleted MORB mantle.