Tracing the origin of mantle heterogeneities with the Fe isotopic composition of Southwest Indian Ridge Basalts

• Fe isotopes and REE have been analyzed on glassy samples from the Southwest Indian Ridge between 49° and 69°E longitude. • The positive correlations between Fe isotopes, sodium and light REE enrichment suggest mixing between partial melts derived from a peridotitic and a pyroxenitic lithology. • The linear negative correlation between Fe and Pb isotopes rules out an origin of the pyroxenitic component from low-F melt metasomatism or LCC-derived materials. Instead, it likely reflects recycling of sulfide-and garnet-bearing pyroxenite The peculiar Sr–Nd–Pb–Hf isotopic compositions of Southwest Indian Ridge (SWIR) basalts, referred to as the DUPAL anomaly, have been attributed to the recycling of Lower Continental Crust (LCC) mafic lithologies. Iron stable isotope represents a promising proxy for providing further insight into the lithological nature of the DUPAL anomaly. We report Fe isotope and Rare Earth Element (REE) data for 27 basaltic glasses along the SWIR axis between 49° and 69°E longitude. The δ 56 Fe values (deviation in 56 Fe/ 54 Fe from the IRMM-014 standard, expressed in parts per thousand) of SWIR basalts range from +0.06‰ to +0.16‰, falling within the typical range of Mid-Oceanic Ridge basalts (MORB). Variations in δ 56 Fe can only be partially explained by fractional crystallization. In particular, the easternmost SWIR basalts (Zone A) display the full range of δ 56 Fe variation at nearly constant MgO. When corrected for the effects of fractional crystallization, Zone A basalts exhibit positive correlations among δ 56 Fe, Na 2 O, and Light REE enrichment. These trends can be accounted for by mixing of melts derived from depleted peridotite and enriched pyroxenite sources. Furthermore, the linear and negative Fe–Pb isotopic array defined by Zone A basalts provides key constraints on the origin of the heavy δ 56 Fe pyroxenitic component. In particular, the involvement of an ancient low degree (Low-F) melt metasomatic source or recycled DUPAL-like LCC can be excluded. We infer that the heavy δ 56 Fe and unradiogenic Pb signatures of SWIR basalts are consistent with a contribution from ancient, recycled, sulfide-bearing pyroxenites.