The Southwest Indian Ridge, between 61° and 68°E, is one of the deepest, slowest, and coldest sections of the global mid-ocean ridge system, and the composition of the basaltic crust suggests an extremely low degree of melting (Meyzen et al., 2003). In contrast to normal expectations, the subaxial mantle contains peridotites highly depleted in clinopyroxene (<2%), the lowest-melting mineral phase, but anomalously enriched in orthopyroxene (modal olivine/orthopyroxene ≈ 2). Furthermore, orthopyroxene grains host mineral inclusions enriched in OH, Na, K, P, S, and light rare earth elements never previously reported in residual mineral assemblages of abyssal peridotites; these inclusions are primarily Na-, Cr-rich diopside variably associated with apatite, amphibole, mica, albite, and sulfides. Surprisingly, these metasomatic minerals do not occur within the other mineral phases (olivine, spinel) or as interstitial phases or veinlets. We conclude that the metasomatic mineral inclusions represent traces of a fertile mantle component that locally escaped extraction during decompression beneath the ridge. Our observations (1) imply interactions of the suboceanic asthenosphere with incompatible element–rich melts, and (2) provide evidence for refractory mantle blobs in the suboceanic mantle that have compositional similarities to continental lithosphere.

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