Heavy δ57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle

Lithological heterogeneity is a widely accepted feature of the Earth's mantle, with recycled crustal material accounting for a significant part of heterogeneity in ocean island basalt (OIB) geochemistry. Fe isotopes have been used to link geochemical heterogeneity in OIB sources to distinct mantle lithologies due to their mineral-specific equilibrium fractionation effects, or their composition, such as incorporation of kinetically-fractionated core liquids entrained from the core-mantle boundary. Here we present Fe isotope data for Samoan shield, and Azores volcanoes, together with a combined phase-equilibria and equilibrium mineral-melt isotope fractionation model. These OIB lavas allow us to study the roles of core-derived and recy-cled mantle components in generating heavy delta Fe-57 melts. Heavy delta Fe-57 correlates with radiogenic isotope signatures of enrichment by a crustal component and not with Fe/Mn or any indicator of core involvement. However, single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the heavy delta Fe-57 seen in Pitcairn, Azores, and rejuvenated Samoa lavas. Melts of a reaction-zone pyroxenite (commonly suggested to form part of the OIB source), derived from eclogite melts hybridised with peridotite, also fail to generate the heaviest Fe isotopic compositions seen in OIB. Instead, the generation of heavy delta Fe-57 melts in OIB requires: (1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (e.g., hydrothermal alteration, metamorphism, sediment input); (2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with heavy delta Fe-57 by ambient lower delta Fe-57 materials. No single process we consider can generate the heavy delta Fe-57 seen in the Azores, Pitcairn, and rejuvenated Samoan lavas. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of heavy delta Fe-57 melts in OIB, nor can these signatures be related to contributions from the Earth's core. Instead, the observation of heavy delta Fe-57 OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low tem-perature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study investigates the components from the mantle and crust in the sources of ocean island basalts using iron isotope data. The results indicate that heavy iron isotopes are correlated with radiogenic isotope signatures of enriched crustal material, rather than the involvement of the Earth's core. The generation of heavy delta Fe-57 melts in OIB requires various complex processes.