PETROLOGY AND STABLE ISOTOPE STUDY OF THE OPHICALCITES OF THE CHENAILLET COMPLEX, WESTERN ALPS, FRANCE

Detailed petrologic and geochemical studies of the Chenaillet ophicalcites (western Alps) suggests that the carbonatization of serpentinites occurred in several discrete phases, following an initial serpentinitization. At least three main stages can be recognized: i) the development of a network of micrometric to centimetric calcite veins that are pre- or syn-serpentinite clast development, and probably related to local carbonation of the serpentine mesh core; ii) infilling of a fine (<300 µm), micritic carbonate matrix between the serpentinite clasts, with no evidence for fossils and with probable co-precipitation of chrysotile and sulfide; iii) late stage calcite veins crosscutting the entire ophicalcite. In addition, some dolomite formed along the serpentinite-calcite interfaces.

d¹⁸O values (VSMOW) measured so far have a range between 12 ‰ and 14.5 ‰ for stage i); those for ii) average at 11.6 ‰ and finally the carbonates of stage iii) are between 12.5 ‰ and 15.5 ‰. δ¹³C (VPDB) values have a smaller range of between -0.6 ‰ and 0.4 ‰ only for all different carbonates. This suggests a relatively high temperature for carbonate deposition for all carbonates, assuming a seawater δ¹⁸O (at least 120 °C) and/or slight differences in the seawater/rock ratios for the different stages.

The Chenaillet ophicalcite appears to record the latest stages of oceanic mantle exhumation. Shallow depth serpentinitization was followed by carbonatization of the serpentinites during cooling along detachment faults (stage i)). The development of seafloor topography related to extensional tectonic activity generated a serpentinite breccia. The micritic matrix (stage ii)) was formed by rapid crystallization in a hot hydrothermal fluid, possibly similar to the root system of white smokers (e.g., Lost City or Rainbow hydrothermal fields).