Chlorine in the Elba and Murlo serpentinites: evidences for sea-water interaction

The nature of the water source for serpentinization has been previously addressed mostly using oxygen isotopes, that document interaction with sea-water and point out that lizardite and chrysotile originated within the oceanic domain. Mesh textures and bastites characterize the Elba, Monti Livornesi and Murlo (Central Italy) retrograde serpentinites. These rocks, formed by hydration of harzburgitic peridotites, were sampled and analyzed to find further evidence for the origin of serpentinization fluids. Based on chemical and mineralogical compositions, we conclude that the serpentinites from Elba, Monti Livornesi and Murlo contain important amounts of chlorine. In particular, bulk analyses indicate amounts ranging from 182 to 950 ppm; contents as high as 0.6 wt.% have been observed within the serpentine pseudomorphs. Chlorine is not present as a specific phase, even in nanometer-size domains; instead, chlorine isomorphically replaces hydroxyl groups. Even if widespread, chlorine is not completely homogeneously distributed, but increases from mesh rims to mesh cores, bastites and chrysotile veins. The heterogeneous chlorine distribution matches the previously reported two-stage serpentinization process, based upon thermal fracturing of the peridotite first (formation of the mesh rim) and massive water penetration into the weakened peridotite (formation of the mesh core). Geochemical balance of the chlorine content and the textural chlorine distribution are in agreement with sea-water origin of chlorine during serpentinization.