Evolution of δ 56Fe in serpentinites during subduction: example in the Western Alps.

During subduction, prograde metamorphism leads to the disappearance of Fe³⁺-rich phases in serpentinites (mostly magnetite and lizardite). This redox reaction is accompanied with a decrease of Fe³⁺/Fe_Tot ratio in serpentinites and a release of fluid mobile elements (e.g. B, S) but little is known about the iron mobility in fluids during subduction. We investigate this problem with an isotopic study of Fe in serpentinites from Western Alps ophiolites. These samples record different metamorphic conditions modelling a subduction gradient that allow deciphering the impact of prograde metamorphism on the Fe isotopic composition of serpentinites during subduction.

At mid-oceanic ridges, ultramafic rocks are serpentinized by interaction with seawater derived fluids. This process leads to the replacement and oxidation of ferromagnesian minerals (olivine and orthopyroxene) to Fe³⁺-rich lizardite and magnetite (Liz-serpentinite). Mantle peridotites commonly display δ⁵⁶Fe between -0.1 and +0.1 ‰ while Liz-serpentinites display a δ⁵⁶Fe ranging from -0.09 to +0.04 (± 0.03)‰, which could potentially reflect interactions with low-δ⁵⁶Fe hydrothermal fluids. During subduction, from greenschist to blueschist facies, the transition lizardite to antigorite leads to a progressive disappearance of magnetite and a reduction of Fe in serpentine. This redox reaction is accompanied with an increase of δ⁵⁶Fe from +0.03 to +0.13‰. At eclogite facies, fully recrystallized Atg-serpentinites display a δ⁵⁶Fe ranging from +0.14 to +0.20‰ while partly dehydrated serpentinites composed of antigorite, secondary olivine and chlorite display a lower δ⁵⁶Fe ranging from -0.03 to +0.03‰.

Our results show that, during the first 70 km of subduction, the transition lizardite to antigorite conduct to the formation of a serpentinite with a high δ⁵⁶Fe value that can be accompanied with the loss of a low δ⁵⁶Fe fluid. The increase of δ⁵⁶Fe in serpentinites is correlated with a decrease of Fe³⁺/Fe_Tot ratio suggesting that the main factor controlling Fe isotopic fractionation during the transition lizardite to antigorite is the oxidation state. At greater depth, antigorite breakdown is accompanied with a reduction of iron in serpentinites and a decrease of δ⁵⁶Fe suggesting that release fluids could have a high δ⁵⁶Fe value.