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

Highlights

• Subducting sediment has no influence on Fe isotope compositions of arc lavas.

• Fe isotopes are decoupled from mantle redox but record mantle depletion.

• Depleted arc sources are isotopically lighter than fertile mantle.

• An oxidant other than Fe is required for oxidised arc lavas.

• Fe isotopes in arc lavas are controlled by source depletion and crystal fractionation.

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 ${\log }_{10}$ 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.

Introduction

Lavas at convergent oceanic margins (hereafter referred to as island arc lavas; IAL) are chemically distinct from mid-ocean ridge basalts (MORB) particularly with respect to lithophile trace element abundances. Most IAL are sourced from the mantle by fluxed melting in the wedge (i.e., sub-arc asthenosphere) between the down-going and the overriding tectonic plates. In addition to the type of dry, decompressional melting occurring beneath MOR, a major fraction of melting in the mantle wedge is triggered by liquids sourced from the subducting plate. Fluids that cause melting in the wedge are liberated by dehydration reactions in the slab with the breakdown of hydrous phases hosted by altered oceanic crust. These fluids carry the so-called “subduction component” into the mantle wedge and upwards to the surface in arc melts (Arculus and Powell, 1986, Elliot et al., 1997, Gamble et al., 1996, Stolper and Newman, 1994, Thirlwall et al., 1996, Woodhead et al., 1998). The compositions and physical characteristics of IAL are consequent to the contribution of subduction component(s) and partial melting processes within the mantle wedge.

Element partitioning between residual minerals and the liberated slab-derived liquids at variable pressure–temperature (P–T) conditions in the downgoing slab define the compositional range of the subduction component (Pearce and Parkinson, 1993). For elements with multiple valence states, this partitioning is strongly dependent on the redox state of the system, reflected in the precisely calculable parameter of oxygen fugacity (fO₂, Canil, 2002, Lee et al., 2005, Mallmann and O'Neill, 2007, McCanta et al., 2004). Determination of fO₂ in the mantle wedge through studies of IAL have proved difficult due to weakly constrained processes of partial melting, fluid–rock interactions, and magmatic differentiation in the mantle wedge, together with compositional variations in subduction components and/or the local mantle wedge. However, despite these factors, it is well known that, on average, primitive IAL are more oxidised than MORB (see a recent review by Evans, 2012), yet the sources and locus of this oxidation process remain elusive (e.g., Lee et al., 2005).

Key to the redox history of igneous systems is the oxidation state of Fe (commonly expressed as the ratio of Fe³⁺ over total Fe, or ΣFe, i.e., Fe³⁺ + Fe²⁺), which is typically the predominant variable valence element in basaltic–andesitic igneous systems. Determinations of Fe³⁺/ΣFe in primitive IAL reveal an elevated oxidation state (expressed in ${\log }_{10}f{\mathrm{O}}_2$ units relative to equilibrium in the synthetic oxygen buffer system of fayalite–magnetite–quartz, i.e., ΔFMQ) of IAL, in the order of 1–4 ${\log }_{10}$ units above FMQ (Arculus, 1994, Carmichael, 1991, Osborn, 1959), compared to MORB with $\mathrm{\Delta }\mathrm{FMQ}\sim +0.5$ (Cottrell and Kelley, 2011).

In understanding the oxidation state of the sub-arc mantle wedge, three opposing scenarios have been suggested. Parkinson and Arculus (1999) argued for a more oxidised mantle wedge in comparison to the mantle sources of MORB. Their calculated fO₂ for the mantle wedge from olivine–orthopyroxene-spinel thermobarometry of peridotites sourced from active island arcs, correspond to $\mathrm{\Delta }\mathrm{FMQ}=+0.5$ to +1.7. Equally high estimates ($\mathrm{\Delta }\mathrm{FMQ}=+1$ to +4) are deduced from spinel compositions in primitive IAL (Evans et al., 2012). Frost and Ballhaus (1998) and Parkinson and Arculus (1999) argued H₂O (as part of the subduction component) is unlikely to be a mantle wedge oxidiser, the latter nevertheless suggesting a “solute-rich hydrous fluid” is most likely involved. By means of mass balance calculations of potential oxidising species in subduction zones, Evans (2012) suggested S⁶⁺, Fe³⁺ or CO²⁻₃ could be involved. Although the evidence from arc peridotites suggests an oxidised mantle wedge, the similarity of Fe/Zn and V/Sc between IAL and MORB, noting the higher compatibility of oxidised Fe and V in a melt phase compared with residual solids relative to Zn and Sc respectively, argues for the same oxidation state of respective IAL and MORB sources (Lee et al., 2010, Lee et al., 2005, Mallmann and O'Neill, 2009).

Recent analyses of Fe isotope compositions in IAL are potentially key in this debate. Iron isotopes are considered to fractionate as a function of Fe redox state during partitioning of Fe between liquid and crystalline residues, e.g., during partial melting or by fractional crystallisation (Polyakov and Mineev, 2000, Shahar et al., 2008, Young et al., 2015), generally with the heavier Fe isotope following Fe³⁺. However, primitive arc melts are characterised by light Fe isotope compositions (Dauphas et al., 2009, Nebel et al., 2013) and may thus, counterintuitively, indicate a Fe³⁺-depleted (by prior melt extraction), or a Fe²⁺-enriched (by metasomatism) wedge source. Hence, our present collective interpretation of the sum of geochemical proxies allows for a more oxidised, equally oxidised or reduced IAL source mantle compared with MORB.

Iron isotopic characteristics of IAL are potentially important for resolving several problems regarding primitive arc magma genesis. These are the extents of prior melt depletion in the arc source(s) compared with those of MOR; whether transfer of oxidised Fe from slab-derived subducted sediments to the wedge is a significant redox process; and the influence of processes such as assimilation–fractional crystallisation (AFC), in modifying Fe isotope characteristics during magmatic evolution. In addressing these issues, we present Fe isotope compositions for a series of Holocene IAL and subducting sediments from the active Banda arc in Indonesia. Banda is a prime example wherein subducted sediment has clearly contributed towards the trace element and isotopic characteristics of the arc magmas (Nebel et al., 2011, Vroon et al., 2001, Vroon et al., 1995). We aim to simultaneously investigate the role of Fe in subducting sediment as an oxidiser in the mantle wedge, and to test whether Fe isotope compositions of IAL are reflective of the redox-character of their mantle wedge sources.