Subduction of oceanic lithosphere involves substantial chemical and mass transfer from the Earth’s surface to the mantle, playing a key role in the global water and carbon cycles, and exerting a fundamental control on arc magmatism. During subduction water and other volatiles are released due to the breakdown of hydrous minerals, including lawsonite, chlorite, chloritoid, phengite and serpentine (e.g. Schmidt & Poli, 1998; Magni et al., 2014). In the oceanic crust, lawsonite, CaAl2Si2O7(OH)2·H2O, is a common hydrous mineral containing ~12 wt% water, making the metamorphic reaction (breakdown) of lawsonite an important fluid-producing reaction within subduction zones. Over recent years, it has been suggested that the liberation of oxidizing slab-derived fluids to the overlying sub-arc mantle may, at least in part, account for the oxidized and volatile-rich signatures seen in arc magmas (e.g. Kelley & Cottrell, 2009). In parallel, it has been suggested that the release of oxidizing sulfur species, such as sulfate (SOx) species, occurs during the breakdown of hydrous phases such as amphibole and lawsonite (e.g. Walters et al., 2020). Recent work combining mineral oxybarometry and Fe isotope measurements on garnet from metabasalts from Sifnos (Greece) indicates that that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions (Gerrits et al., 2019). These garnets grew during lawsonite dehydration consistent with the hypothesis that release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones.
The Western Alps are a classic subduction-related collisional orogen with well-preserved, deeply subducted remnants of oceanic lithosphere. Amongst these, the Zermatt-Saas ophiolite forms a coherent 30-km-long tectonic body that experienced P-T conditions of 550–600°C and 25–30 kbar. Zermatt-Saas ‘classical’ metabasalts (with typical N-MORB geochemical signatures) contain relict eclogite facies assemblages with mm- to cm-large garnet porphyroblasts hosting inclusions of other high-pressure phases (mainly epidote, omphacite, paragonite and quartz). Whereas, ‘Magnesian’ metabasalts, mainly found in the southern Aosta valley, preserve unusual high-pressure lithologies, including garnet glaucophanites (see Angiboust et al., 2009).
This project aims to determine the timing of garnet growth accompanying dehydration (using high precision Sm-Nd chronometry) and the redox conditions accompanying garnet growth (using mineral oxybarometry and Fe stable isotopes). With the overall aims of (i) documenting the redox nature of the fluids released during dehydration and garnet growth, and (ii) determining the timing of that release relative to the timing of high-pressure metamorphism and subduction.