Experimental insight into redox transfer by iron- and sulfur-bearing serpentinite dehydration in subduction zones

Abstract

Dehydration of antigorite in subduction zones releases a large amount of aqueous fluid and volatile elements, which can potentially oxidize the mantle wedge. The redox capacity of three synthetic serpentinites with variable Fetotal, Fe3+ and S− contents is investigated using XANES spectroscopy at both, Fe and S K-edges. Experiments are performed between 450 and 900 °C, at 2 GPa and fO2∼QFM-2; conditions similar to those encountered in subduction zones.Redox reactions in the synthetic serpentinites, which involve Fe and S can be summarized as follows: 1) the reduction of (S−)-pyrite into (S2−)-pyrrhotite (∼450 °C), with ∼4.4 mg/g of the sulfur degassed most likely as H2S, 2) the consumption of magnetite that reacts with antigorite to form Fe-rich olivine (<500 °C), 3) the reduction of (Fe3+)-antigorite into (Fe2+)-antigorite (∼580 °C), occurring about 100 °C below the temperature of antigorite breakdown, 4) the main (Fe2+)-antigorite breakdown that forms olivine and enstatite (∼675 °C), and 5) the decomposition of minor amounts of (Fe2+/3+)-clinochlore (∼800 °C). The bulk Fe3+/Fetotal ratio is found to decrease with run temperature from ∼0.82–0.97 depending on the hydrous starting material, down to 0.1–0.2 in the high-temperature anhydrous assemblages.The evolution of mineral modes and Fe3+/Fetotal with temperature in our synthetic samples shows similar trends to what has been reported in serpentinite rocks collected, for example, along a metamorphic transect in the western Alps. We show that a large amount of O2-equivalent – up to 10 mol/kg of rock – can be generated at temperature around 450 °C due to the presence of oxides and sulfides such as magnetite and pyrite. Owing to the poor capacity of aqueous fluid to transfer redox conditions, we surmise that this O2-equivalent is “consumed” at the scale of the lithospheric-mantle top which is partially serpentinized and therefore bear strong redox gradients.