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Exposure of ultramafic rocks on the ocean floor often leads to serpentinization and carbonate precipitation, which plays an important role in the global marine bio-geochemical cycles. The Lost City Hydrothermal Field (LCHF) is as yet the only known example of an active, low temperature system driven by serpentinization processes. Its serpentinite basement, cut by a network of calcite veins, is remarkably similar to ophicalcites found in ophiolite sequences on continents. Ophicalcites can therefore be considered ancient analogues of peridotite-hosted hydrothermal systems like LCHF. Here we present a carbon geochemical study that compares Lost City with ancient systems preserved in drill cores of the Iberian margin (ODP Leg 149) and ophicalcites from Liguria (northern Apennines, Italy). Petrographic studies and analyses of C and O isotopes, as well as C contents have been conducted on serpentinized peridotites and calcite veins to better understand physical and chemical conditions during serpentinization and to determine the origin of C and possible links to microbial activity in these systems.
Samples from Leg 149 show distinct changes with depth: total carbon contents (TC) are dominated by carbonate at the top of the serpentinite unit and strongly decrease with depth. This change corresponds to a shift from more positive ä13CTCvalues to strongly negative values downhole. This trend, together with ä13C of total inorganic carbon (TIC) indicates a decrease in seawater penetration and marine carbonate precipitation with depth. However, O isotope temperature calculations show no distinct trends with depth, with relatively constant carbonate precipitation temperatures of <20°C. Interestingly, carbonates in the Ligurian ophicalcites show similar marine C isotope values but have ä18O values that vary according to vein generation and vein type, and which record temperatures of carbonate precipitation similar to serpentinization temperatures (150°C) at the Iberian margin. Total organic carbon (TOC), calculated from TC and TIC, are up to 4000 ppm in the Iberian Margin serpentinites and are considerably lower in the Ligurian ophicalcites (up to 250 ppm). Values of ä13CTOC from both localities generally lie within a narrow range of -28 to -24‰, suggesting the presence of organic matter. The range in carbon compositions and a dominance of depleted C isotope compositions is similar to trends in serpentinites from Lost City [1] and indicates that organic carbon is an important component of the carbon budget in marine serpentinites, regardless of the tectonic setting.
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