Heavy iron isotopes reveal mantle oxidation in addition to pyroxenite sources for intraplate basalts

The heavier iron isotopic composition of some oceanic island basalts (OIB) relative to primary mid-ocean ridge basalts (MORB) has commonly been interpreted to reflect the heavy bulk isotopic compositions of their sources (e.g., of a pyroxenite-dominated mantle region). Alternatively, comparable Fe isotopic signatures observed in continental intraplate basalts (CIB) may record enhanced isotope fractionation during partial melting of an oxidized source, offering valuable insights into mantle oxidation induced by subducted carbonate. To distinguish between these two independent contributions, here we present high-precision Fe isotopic data (delta Fe-57) and Fe3+/& sum;Fe ratios for a suite of Cenozoic intraplate basalts from Inner Mongolia, East Asia. These basalts are distributed along a northwest-southeast (NW-SE) profile and associated with earlier subduction of the Paleo-Asian oceanic slab. Variations in Fe/Mn ratios and zinc isotopic compositions (delta Zn-66) of these basalts reveal that altered oceanic crust has always contributed to their mantle sources but the amounts of recycled carbonate gradually vary. Together with existing data, we demonstrate that all of these basalts generally exhibit heavier delta Fe-57 (0.07 parts per thousand to 0.31 parts per thousand), higher Fe3+/& sum;Fe-melt (0.08 to 0.39) and Fe/Mn (> 65) relative to primary MORB (delta Fe-57 similar to 0.10 parts per thousand; Fe3+/& sum;Fe-melt similar to 0.10; Fe/Mn similar to 60), pointing to a common origin of heavy Fe isotopes in intraplate basalts that may reflect those of pyroxenite sources (e.g., recycled crust). However, these basalts containing larger amounts of recycled carbonate in their sources (i.e., higher delta Zn-66) have heavier delta Fe-57 at similar Fe/Mn ratios and proportions of pyroxenite melt. Such Fe isotopic signatures exceed the exclusive role of pyroxenite and are best attributed to partial melting at oxidizing conditions (i.e., high Fe3+/& sum;Fe-source) with an inferred enlarged fractionation factor. This oxidized mantle source formed through the reduction of recycled carbonate to graphite/diamond at depths greater than 150 km. A global dataset of Fe isotopes in intraplate basalts is compiled in order to examine this interpretation further. Compared with OIB typically from a pyroxenite/eclogite-dominant source, CIB with similarly heavy delta Fe-57 and formed by similar melting degrees are commonly more silica-undersaturated, indicating a widespread carbonated mantle source. Thus, these observations highlight that heavy Fe isotopes can record carbonate-induced mantle oxidation in addition to source lithological heterogeneity for intraplate basalts.