High Water Content in Primitive Mid-Ocean Ridge Basalt from Southwest Indian Ridge（51.56oE）：Implications for Recycled Hydrous Component in the Mantle

The Southwest Indian Ridge（SWIR） is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed（ITF） and Gallieni（GTF） transform faults in the SWIR, however, displays extremely magmatic accretion with an unusual thick crust（up to 9.5 km）. Although H_2O is present in trace amounts in the mantle, it has a strong influence on mantle melting and magmatism in the shallow crust. The mid-ocean ridge basalts（MORB） worldwide show strong variation in H_2O contents, but with a nearly uniform H_2O/Ce ratio. Regionally distinctive H_2O contents and H_2O/Ce ratios are inferred to be related to the H_2O variation in the source and can be used to constrain the mantle heterogenity. In this study, we measured the H_2O and trace elements of clinopyroxene phenocrysts from one basalt dredged from the ITF-GTF segment, SWIR（51.56 o E）. The estimated H_2O content（1.3 wt.%± 0.3 wt.%） in the primitive ITF-GTF basaltic melt is much higher than that in typical MORB samples, but similar to oceanic island basalts（OIB） and back-arc basalts（BABB）. In addition, the calculated H_2O/Ce ratio（1 672–4 990） are extremely high, bearing ＂arc-like＂ signature. This study provides evidence that arc-related hydrous components are involved in the mantle source beneath the ITF-GTF ridge segment. It further lends support to the hypothesis that the mantle beneath the central SWIR may have experienced an ancient hydrous melting event in an arc terrain prior to or during the closure of the Mozambique Ocean in the Neoproterozoic.