Feldspar from carbonate-rich silicate metasomatism in the shallow oceanic mantle under Kerguelen Islands (South Indian Ocean)

Abstract

Some highly depleted harzburgitic mantle xenoliths from two different localities in the Kerguelen Islands display a secondary mineral assemblage unusual for both oceanic and continental settings. Petrographic and chemical evidence indicate reaction of primary orthopyroxene and spinel with an infiltrating melt to form Na–Cr-rich clinopyroxene+olivine+Cr-rich spinel±apatite±rutile±carbonate and a Si–Al–Na-rich phase, which is either a glass and/or a Na-rich plagioclase. Major-element compositions of secondary minerals strongly indicate that their formation resulted from the infiltration of a sodium-rich carbonate–silicate melt with a low water content through the host depleted harzburgite. The trace-element signatures of metasomatic clinopyroxene and glass indicate that the metasomatic agent was enriched in Sr, P, Cl, Th, U, and LREE and low in K, Rb and HFSE (Nb, Ta, Zr, Hf, Ti). The infiltration of this metasomatic agent led to heterogeneous crystallisation and wall-rock reactions, which are recorded by the mode and the major-element variations of minerals. The trace-element composition of clinopyroxene suggests that the carbonate to silicate melt proportion in the metasomatising agent was locally variable, resulting in places in trace-element signatures typical of alkali silicate melt metasomatism. The migration of sodic, carbonate-rich silicate melt is inferred to result in feldspar-bearing metasomatic assemblages formed by reaction of the melt with the host peridotite if carbonate is not stable at shallow depth in the oceanic upper mantle. The metasomatic imprint was acquired shortly before eruption and formed areas of felsic composition, heterogeneous at the mm scale. This study demonstrates the close relationship between migrating carbonate and silicate melts in the oceanic mantle beneath Kerguelen Islands.