Abstract

Serpentinized ultramafic rocks recovered during several recent oceanographic missions (1997–2002) on the Rainbow hydrothermal field and on the Saldanha seamount (36°14′N and 36°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 87Sr/86Sr (approx. 0.709) and stable isotope signatures (δ13CPDB = 0.2–3.3 ‰; δ18OV–SMOW = 32.2–35.2 ‰) 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.

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