Conditions fluid-rock interactions and related chemical mobilities in magma-poor rifted margins: insights from rodingites and serpentinites

Recent studies have demonstrated that tectonic, magmatism and hydration are key features that control oceanization processes from the rifting to the establishment of the first oceanic crust. Therefore, studying Ocean-Continent Transitions (OCTs) provides a unique opportunity to investigate interactions between mantle rocks, melts and fluids and assess their feedback mechanisms and enhancers. The Upper Penninic Platta nappe (Southeastern Swiss Alps) is one of the best example of fossil OCT documented so far: it consists of a well-preserved exhumed mantle section of the Liguro-Piemont margin that was only weakly affected by alpine metamorphism (prehnite-pumpellyite facies). Peridotites are crosscut by numerous magmatic dikes, the overall been strongly metasomatized: peridotites are completely serpentinized and dikes are transformed into rodingites. Based on comprehensive mineralogical (XRD, μ-XRF mapping, Raman spectroscopy) and geochemical (major and trace elements, Sr, Nd and B isotopes) investigations of contacts between serpentinites and rodingites, this study aims at determining the conditions of fluid-rock interactions (temperature, pH) and the associated chemical mobility during their exhumation to seafloor.

Mineralogical investigations show that rodingites consist of an assemblage of chlorite, hydrogrossular and several generations of clinopyroxene, while serpentinites display a typical mesh texture mainly composed of serpentines (lizardite ± chrysotile) and rarely relics of primary minerals (olivine + pyroxenes). At the contact with dikes, serpentinites are converted into a chlorite reaction zone (blackwall) where clinochlore has almost completely replaced serpentine. In addition, trace element patterns of blackwalls are similar to those of rodingites, suggesting that elements, including those usually considered as highly immobile (i.e. Al, Zr), may be transferred from the dikes to the surrounding mantle during the hydrothermal alteration of the magmatic rocks. Finally, based on radiogenic (Sr, Nd) and stable (B) isotopes, we show that fluid-melt interactions occurred at a wide range of temperature, from ~200 to 60°C. This suggests that the rodingitization process recorded fluid-rock interactions at depth, and that this process may remain active at very shallow depths.