Redox-variability and controls in subduction zones from an iron-isotope perspective

Abstract

An ongoing controversy in mantle geochemistry concerns the oxidation state of the sources of island arc lavas (IAL). Three key factors control oxidation–reduction (redox) of IAL sources: (i) metasomatism of the mantle wedge by fluids and/or melts, liberated from the underlying subducted slab; (ii) the oxidation state of the wedge prior to melting and metasomatism; and (iii) the loss of melt from IAL sources. Subsequently, magmatic differentiation by fractional crystallisation, possible crustal contamination and degassing of melts en route to and at the surface can further modify the redox states of IAL. The remote nature of sub-arc processes and the complex interplay between them render direct investigations difficult. However, a possible gauge for redox-controlled, high-temperature pre-eruptive differentiation conditions is variations in stable Fe isotope compositions (expressed here as δ57Fe) in erupting IAL because Fe isotopes can preserve a record of sub-surface mass transfer reactions involving the major element Fe. Here we report Fe isotope compositions of bulk IAL along the active Banda arc, Indonesia, which is well known for a prominent subducted sediment input. In conjunction with other arc rocks, δ57Fe in erupted Banda IAL indicates that fractional crystallisation and possibly crustal contamination primarily control their Fe isotope signatures. When corrected for fractional crystallisation and filtered for contamination, arc magmas that had variable sediment melt contributions in their sources show no resolvable co-variation of δ57Fe with radiogenic isotope tracers. This indicates that crustal recycling in the form of subducted sediment does not alter the Fe isotope character of arc lavas, in agreement with mass balance estimates. Primitive sources of IAL, however, are clearly isotopically lighter than those sourced beneath mid-ocean ridges, indicating either preferential Fe3+-depletion in the mantle wedge by prior, δ57Fe-heavy melt extraction, and/or addition of an isotopically-light slab-derived agent. Based on our findings and previous models of arc redox conditions, we propose a three-stage process to explain the Fe isotope composition of IAL: (i) prior melt depletion lowers Fe3+/ΣFe (Fe3+ over total Fe) in the residues, leaving refractory, δ57Fe-light and possibly reduced mantle wedge protoliths beneath arcs. The oxygen fugacity (fO2) of these refractory protoliths may be up to −2 log10 units reduced relative to the fayalite–magnetite–quartz synthetic oxygen buffer (ΔFMQ); (ii) oxidised, slab-derived fluids, Fe-poor but possibly rich in sulphate (i.e., S6+), trigger melting of depleted protoliths with minimal effect on δ57Fe. Melts derived from this fluid-modified wedge source have high Fe3+/ΣFe, oxidised by the reduction of S6+, but importantly retain the light δ57Fe from their mantle wedge source; (iii) after melt liberation from the mantle wedge, arc magmas initially become progressively oxidised and isotopically heavier in Fe through fractional crystallisation of ferromagnesian silicates. In summary, reduction consequent to Fe3+-rich melt extraction and subsequent oxidation, likely by S6+-rich fluids, results in a “redox yo-yo” in IAL sources. Fractional crystallisation will further oxidise and elevate δ57Fe in erupting IAL. Iron isotope signatures in IAL record a complex magmatic history with no simple relation between δ57Fe and calculated fO2 in erupted lavas. Records of higher fO2 in subduction zones compared to MORB sources are inherited from the subduction component.