Heavy δ57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle. (1st January 2021)

• Record Type: Journal Article
• Authors: Soderman, Caroline R.; Matthews, Simon; Shorttle, Oliver; Jackson, Matthew G.; Ruttor, Saskia; Nebel, Oliver; Turner, Simon; Beier, Christoph; Millet, Marc-Alban; Widom, Elisabeth; Humayun, Munir; Williams, Helen M.
• Is Part Of: Geochimica et cosmochimica acta. Volume 292(2021)
• Page Start: 309
• Page End: 332
• Publication Date: 2021-01-01
• Subjects: Iron isotopes -- Mantle heterogeneity -- Pyroxenite -- Mantle geochemistry -- Ocean island basalt; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.09.033
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: 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 recycled mantle components in generating heavy δ 57 Fe melts. Heavy δ 57 Fe 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 δ 57 Fe 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 δ 57 Fe 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 δ 57 Fe by ambient lower δ 57 Fe materials. No single process we consider can generate the heavy δ 57 Fe 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 δ 57 Fe melts in OIB, nor can these signatures be related to contributions from the Earth's core. Instead, the observation of heavy δ 57 Fe 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 temperature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands.