Iron and zinc isotopes reveal redox reactions associated with fluid flow in subduction zones

Abstract

Subduction zones are areas of significant mass transfer between Earth’s crust and mantle. The dehydration of water-rich minerals such as serpentinite and lawsonite introduces water and volatiles into the subduction interface, that then travel to the mantle wedge above. The chemical composition, speciation, and redox effect of these fluids carry important implications for arc volcanism and the mobility of economically significant elements. This study uses Fe and Zn isotopic variation in eclogite-breccias from the Monviso ophiolite, combined with Sm-Nd garnet geochronology, to study the composition and redox effects of fluids that were produced during eclogite and blueschist facies metamorphism, and the timescales over which the brecciation and fluid flux events happened. Fe and Zn isotopic measurements were made on a series of four breccia matrix generations (M1-M4), generated during the progressive brecciation of the original Fe-Ti gabbros and the influx of both internally and externally derived fluids. The ∂56Fe and ∂66Zn data display a bi-modal distribution, with early matrix crystallization (M1-3) imparting progressively lighter ∂66Zn values while the ∂56Fe remains relatively unchanged. The last stage of metasomatic rind formation (M4) is associated with a decrease in both Fe and Zn isotopic values and a particularly significant shift in the Fe isotopes. This distribution suggests that early brecciation (M1-3) resulted from small-scale internal fluid flow that did not have a measurable effect on the isotopic composition and redox state of the system. By contrast, late metasomatic rind formation (M4) was facilitated by the flow of large amounts of external fluids with a strongly negative Fe and Zn isotope signature that affected the redox state of the mafic slab and may be responsible for transferring oxidized material into the mantle wedge. Dating of the M4 matrix generation yielded an age of 41.31± 0.60 Ma. A compilation of age data from Monviso suggests peak metamorphism and initial brecciation (M1 formation) likely occurred at ~45 Ma, the formation of the M4 matrix representing the end of eclogite-facies retrogression and brecciation at ~41 Ma, and final blueschist and greenschist retrogression at ~38-35Ma, yielding timescale of ~4Ma for the entire history of brecciation and fluid flux associated with the Monviso eclogite breccias.