Chlorine in the Elba, Monti Livornesi and Muro serpentines; evidence for sea-water interaction

A study of the chemical and mineralogical compositions of these three Italian serpentinites shows them to contain appreciable amounts of Cl; bulk analyses indicate 182-950 ppm and contents of Cl as high as 0.6 wt.% have been determined within the serpentine pseudomorphs. The chlorine is not present as a specific phase but is isomorphously replacing hydroxyl groups. Although Cl is widespread, it is not entirely homogeneously distributed, increasing from mesh rims to mesh cores, bastites and chrysotile veins. This matches the two-stage serpentinization process based on thermal fracturing of the peridotite (formation of the mesh rim) and massive water penetration into the weakened rock (formation of the mesh core). Geochemical balance of the Cl content and the textural Cl distribution are in agreement with a sea-water origin for the Cl during serpentinization.