Isotopic evidence for iron mobility during subduction

Abstract

Subduction zones are one of the most important sites of chemical interchange between the Earth's surface and interior. One means of explaining the high Fe 3+ /ƩFe ratios and oxidized nature of primary arc magmas is the transfer of sulfate (SO X), carbonate (CO 3-), and/or iron (Fe 3+) bearing fluids from the slab to the overlying mantle. Iron mobility and Fe stable isotope fractionation in fluids are influenced by Fe redox state and the presence of chlorine and/or sulfur anions. Here we use Fe stable isotopes (d 56 Fe) as a tracer of iron mobility in serpentinites from Western Alps metaophiolites, which represent remnants of oceanic lithosphere that have undergone subduction-related metamorphism and devolatilization. A negative correlation (R 2 = 0.72) is observed between serpentinite bulk d 56 Fe and Fe 3+ /ƩFe that provides the first direct evidence for the release of Fe-bearing fluids during serpentinite devolatilization in subduction zones. The progressive loss of isotopically light Fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate-rich and/or hypersaline fluids, which preferentially complex isotopically light Fe in the form of Fe(II)-SO X or Fe(II)-Cl 2 species. Fe isotopes can therefore be used as a tracer of the nature of slab-derived fluids.

Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)

“…Subsequent devolatilisation reactions progressively release Cl, at the transition from oceanic serpentinites to high-pressure serpentinites, as well as under P-T conditions of antigorite breakdown (Scambelluri et al 2004). In this context, a recent study has revealed an unusual increase in δ 56 Fe in subducted serpentines (from −0.02 ± 0.15 to +0.08 ± 0.11‰; 2σ SD) with the decrease of their Fe 3+ /ΣFe ratio (from 0.7 to 0.5) at the transition from lizardite to high-pressure antigorite (Debret et al 2016). The authors conclude that isotopically light-Fe is carried as Fe(II)-SO X or Fe(II)-Cl 2 species by sulfate-rich and/or hypersaline fluids that are released in the mantle wedge (Debret et al 2016).…”

“…In this context, a recent study has revealed an unusual increase in δ 56 Fe in subducted serpentines (from −0.02 ± 0.15 to +0.08 ± 0.11‰; 2σ SD) with the decrease of their Fe 3+ /ΣFe ratio (from 0.7 to 0.5) at the transition from lizardite to high-pressure antigorite (Debret et al 2016). The authors conclude that isotopically light-Fe is carried as Fe(II)-SO X or Fe(II)-Cl 2 species by sulfate-rich and/or hypersaline fluids that are released in the mantle wedge (Debret et al 2016). While the Fe isotope database for ultrabasic and basic rocks is growing substantially, further work is required to understand Fe mobility and isotope fractionation in subduction-related basaltic rocks of blueschist and eclogite facies.…”

“…These processes in turn control the redox state of arc magmas, which are shown to be more oxidised (Fe 3+ /ΣFe = 0.3) than mid-ocean ridge basalts (MORB) (Fe 3+ /ΣFe = 0.07-0.16; Christie et al 1986;Bezos and Humler 2005;Kelley and Cottrell 2009;Cottrell and Kelley 2011), and have long-term (>1 Ga) consequences for the bulk mantle (Evans 2012). Hypersaline brines, supercritical fluids, silicate-rich C-bearing fluids, silicate melts or sulphate-bearing fluids derived from the oceanic lithosphere are recognised as effective metasomatic oxidising agents in the lithospheric mantle (Kelley and Cottrell 2009;Malaspina et al 2010;Debret et al 2016). The prograde transition from lizardite to antigorite results in a decrease of the serpentine Fe 3+ / ΣFe ratio and magnetite modal percentage under eclogite facies conditions (Debret et al 2014).…”

“…Dehydration-redox reactions happening in the subducting slab are responsible for a transfer of redox-sensitive (Fe) or volatile elements (C, S, F, Cl) from subducted sediments, altered oceanic crust and serpentinised lithospheric mantle to the mantle wedge (e.g. Malaspina et al 2010;Evans 2012;Galvez et al 2013;Debret et al 2016). These processes in turn control the redox state of arc magmas, which are shown to be more oxidised (Fe 3+ /ΣFe = 0.3) than mid-ocean ridge basalts (MORB) (Fe 3+ /ΣFe = 0.07-0.16; Christie et al 1986;Bezos and Humler 2005;Kelley and Cottrell 2009;Cottrell and Kelley 2011), and have long-term (>1 Ga) consequences for the bulk mantle (Evans 2012).…”

The Fina Nagu volcanic complex: Unusual submarine arc volcanism in the rapidly deforming southern Mariana margin

“…Serpentinization reactions on the seafloor transform olivine to magnetite, releasing H 2 -rich fluids and leaving the oceanic lithosphere with a more oxidized magnetite-bearing assemblage. If high pressure, high temperature deserpentinization reactions transform magnetite back to olivine, the fluids produced from that reaction must contain oxidized components (e.g., SO x species) [Debret et al, 2016]. Similarly, amphibole can contain Fe 31 and the breakdown reactions that produce H 2 O-rich fluids that would equilibrate with that oxidizing assemblage.…”