Subducting serpentinites release reduced, not oxidized, aqueous fluids

Abstract

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

“…Opaque phases such as sulphides occur as inclusions in olivine 1 and are absent in the mosaic domain (Fig. 5 c, for sulphides see Additional file 2 : Figure S2, see Piccoli et al ( 2019 )).…”

“…The produced SO 2 would then be able to leave the system with the water-rich fluid. However, a recent study has shown, that the fO 2 for the antigorite + brucite to olivine reaction is well below QFM and that SO 2 is only present in trace amounts in the fluid at the corresponding P–T conditions (Piccoli et al 2019 ). (3) While the FeO component of magnetite is incorporated into olivine, some MgO of olivine is incorporated into magnetite and the ferric iron (or Cr in the case of Cr-magnetite) might be accommodated by existing or newly formed phases.…”

“…(1) They partially re-equilibrated where their iron component (ferrous) partitioned into olivine while their Ni-content passively increased in sulphides, thereby decreasing the volume of sulphides present. (2) At the low oxygen fugacity, sulphur might be lost as H 2 S (Piccoli et al 2019 ) decreasing the amount of sulphides and making Ni and Fe available for olivine. Considering thermodynamic equilibrium, it is unlikely that the sulphides remained inert during the olivine forming reaction.…”

“…The lower NiO content is due to the presence of Ni-sulphides (pentlandite e.g. Piccoli et al 2019 ), the large amount of magnetite and the former presence of FeNi-alloys (taenite-awaruite). The elevated MnO in olivine points to a co-existence with antigorite that has MnO contents below 0.05 wt%.…”

Reduced methane-bearing fluids as a source for diamond

“…We note that this process can generate diamond at essentially constant temperature, pressure and ƒO 2 , even at sub-solidus conditions. The relevance of such a process in nature can be found in the interaction of subducted graphite 51,52 with CH 4 and H 2 -bearing reduced fluids that may be generated by high-pressure metamorphism of ophiocarbonates (carbonate-bearing ultramafic rocks) in the subducting slab 52,53 . Subducted components and lithologies have frequently been implicated in diamond formation 1,54 .…”