Chlorine in the Elba, Monti Livornesi and Murlo serpentines: evidence for sea-water interaction

The nature of the water source for serpentinization has been previously add ressed mostly using oxygen isotopes, that document interaction with sea-wat er and point out that lizardite and chrysotile originated within the oceani c 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 evi dence for the origin of serpentinization fluids. Based on chemical and mineralogical compositions, we conclude that the serp entinites from Elba, Monti Livornesi and Murlo contain important amounts of chlorine. In particular, bulk analyses indicate amounts ranging from 182 t o 950 ppm; contents as high as 0.6 wt.% have been observed within the serpe ntine pseudomorphs. Chlorine is not present as a specific phase, even in na nometer-size domains; instead, chlorine isomorphically replaces hydroxyl gr oups. Even if widespread, chlorine is not completely homogeneously distribu ted, but increases from mesh rims to mesh cores, bastites and chrysotile ve ins. The heterogeneous chlorine distribution matches the previously reported two -stage serpentinization process, based upon thermal fracturing of the perid otite first (formation of the mesh rim) and massive water penetration into the weakened peridotite (formation of the mesh core). Geochemical balance o f the chlorine content and the textural chlorine distribution are in agreem ent with sea-water origin of chlorine during serpentinization.