Timescales of Oceanic Lithosphere Hydration

Abstract

Serpentinites assume a critical role in geochemical and geophysical cycles, from recycling fluid into the sub-arc mantle to facilitating exhumation within subduction zones. Rodingite dikes can be used as a lens to investigate the hydration of the oceanic lithosphere as their development is synchronous with serpentinization, and while serpentinites lack sufficient mineral phases for geochronology, rodingite dikes are rich in andradite and grossular garnet which are potentially amenable for geochronology. This research seeks to constrain the timescales and duration of hydration of the oceanic lithosphere within the Alpine Tethys ocean basin, and associated serpentinization, by examining Apennines rodingites from the Internal Ligurides (Italy). These rodingites experienced seafloor hydrothermal alteration and were obducted onto the continental margin during Alpine orogenesis. As a result, they are ideal for studying seafloor metasomatism as they were not affected by prograde subduction zone metamorphism and dehydration. Sr isotopic and trace element profiles were constructed across two rodingite-serpentinite transects, revealing a complex, multi-stage hydration history consisting of 1) Widespread serpentinization, 2) Gabbroic intrusions, 3) Rodingitization, and 4) Localized, late-stage advective fluid flow. Serpentinizing fluids locally display strong continental crustal isotopic signatures, while rodingitization fluids are characterized by seawater-like values. U-Pb geochronology on rodingite garnets produced an age of 96.1 ± 8.9 Ma, which could represent either the main rodingitization phase or the late-stage advective alteration.