CHLORINE CHEMISTRY OF SERPENTINITES FROM ELBA, ITALY AS AN INDICATOR OF FLUID SOURCE

Chlorine contents and δ³⁷Cl, δD, and δ¹⁸O values were measured from serpentinites in different tectonic positions on Elba island in order to evaluate potential geochemical-tectonic relationships. Elba consists of Ligurian and Tuscan thrust sheets that were intruded by Miocene plutons and dismembered by normal faults. Lower grade serpentinites from the Ophiolite unit contain 0.01-0.1 wt% Cl with δ³⁷Cl values from ~ –1.0 to +1.5‰ (±0.1‰), δD values from ~ –70 to –100‰, and δ¹⁸O values from ~ 0 to +3.0‰. The isotopic variations are interpreted to be primary signatures reflecting different sources of serpentinizing fluids. Samples with positive δ³⁷Cl values occur beneath gabbro and basalt and were serpentinized by interaction with seawater. Samples with negative δ³⁷Cl values are only found in samples that are in primary contact with mudstones and cherts. These samples were presumably juxtaposed against the sediments along low-angle normal oceanic faults prior to serpentinization. Serpentinization resulted from interaction with sedimentary pore fluids (pore fluids have negative δ³⁷Cl values). Samples from the Ophiolite unit <100 m from the plutons and all samples from the structurally lower Acquadolce unit (δD: ~ –55 to –95‰; δ¹⁸O values: ~ 0 to +5.0‰) lost their original Cl during metamorphism. Obducted serpentinites can thus preserve a record of seafloor tectonic processes if they are not affected by subsequent metamorphism.