Evolution of Fe redox state in serpentine during subduction

Abstract

Serpentinites are an important component of the oceanic lithosphere that formed at slow to ultra-slow spreading centers. Serpentine could thus be one of the most abundant hydrous minerals recycled into the mantle in subduction zones. Prograde metamorphism in subducted serpentinites is characterized by the destabilization of lizardite into antigorite, and then into secondary olivine. The nature of the fluid released during these phase transitions is controlled by redox reactions and can be inferred from oxidation state of Fe in serpentine minerals. We used bulk rock analyses, magnetic measurements, SEM observations and μXANES spectroscopy to establish the evolution of Fe2O3Tot(BR) and magnetite content in serpentinite and Fe oxidation state in serpentine minerals from ridge to subduction settings.At mid-ocean ridges, during the alteration of peridotite into serpentinite, iron is mainly redistributed between magnetite and oceanic serpentine (usually lizardite). The Fe3+/FeTotal ratio in lizardite and the modal percentage of magnetite progressively increase with the degree of local serpentinization to maxima of about 0.8 and 7 wt%, respectively, in fully serpentinized peridotites. During subduction, the Fe2O3Tot(BR) of serpentinite remains constant (∼7–10 wt%, depending on the initial Fe content of the peridotite) while the modal percentage of magnetite decreases to less than 2% in eclogite facies rocks. The Fe3+/FeTotal ratio in serpentine also decreases down to 0.2–0.4 in antigorite at eclogite facies.Our results show that, in the first 70 km of subduction, the transition from lizardite to antigorite is accompanied by a reduction of Fe in bulk rock samples and in serpentine minerals. This redox reaction might be coupled with the oxidation of reduced oceanic phases such as sulfides, and the formation of oxidized fluids (e.g. SOX, H2O, COX). At greater depths, the beginning of antigorite dehydration leads to an increase of Fe3+/FeTotal in relict antigorite, in agreement with the preferential partitioning of ferric iron into serpentine rather than into olivine.