Late seafloor carbonate precipitation in serpentinites from the Rainbow and Saldanha sites (Mid-Atlantic Ridge)

Serpentinized ultramafic rocks recovered during several recent oceanographic missions (1997-2002) on the Rainbow hydrothermal field and on the Saldanha seamount (36 degrees 14'N and 36 degrees 34'N, Mid-Atlantic Ridge) often exhibit late-stage carbonatization associated to secondary oxidation effects. These carbonate occurrences, mostly vein-filling aragonite, occasionally form dense webs almost completely engulfing and replacing the serpentinite itself. The Sr-87/Sr-86 (approx. 0.709) and stable isotope signatures (delta C-13(PDB) = 0.2-3.3 parts per thousand; delta O-18(v-SMOW) = 32.2-35.2 parts per thousand) of the carbonate fraction in these serpentinites indicate carbonate precipitation from unmodified seawater, under abiotic conditions, and very low temperatures, close to bottom-water temperatures measured at the sampling stations. These analytical data imply that, unlike the serpentinite-hosted carbonate chimneys in the Lost City hydrothermal field (Ludwig et al., 2006), the vein-filling aragonite in the Rainbow and Saldanha serpentinites has a non-hydrothermal, low-temperature seawater origin.