Authigenesis of Carbonate Minerals in Modern and Devonian Ocean-Floor Hard Rocks

Petrographic and stable-isotope (delta C-13, delta O-18) patterns of carbonates from the Logatchev Hydrothermal Field (LHF), the Gakkel Ridge (GR), and a Late Devonian outcrop from the Frankenwald (Germany) were compared in an attempt to understand the genesis of carbonate minerals in marine volcanic rocks. Specifically, were the carbonate samples from modern sea floor settings and the Devonian analog of hydrothermal origin, low-temperature abiogenic origin (as inferred for aragonite in serpentinites from elsewhere on the Mid-Atlantic Ridge), or biogenic origin? Aragonite is the most abundant carbonate mineral in serpentinites from the two modern spreading ridges and occurs within massive sulfides of the LHF. The precipitation and preservation of aragonite suggests high Mg2+ and sulfate concentrations in fluids. Values of delta O-18(PDB) as high as +5.3 parts per thousand for serpentinite-hosted aragonite and as high as +4.2 parts per thousand for sulfide-hosted aragonite are consistent with precipitation from cold seawater. Most of the corresponding delta C-13 values indicate a marine carbon source, whereas delta C-13 values for sulfide-hosted aragonite as high as +3.6 parts per thousand may reflect residual carbon dioxide in the zone of methanogenesis. Calcite veins from the LHF, by contrast, have low delta O-18(PDB) (-20.0 parts per thousand to -16.1 parts per thousand) and delta C-13 values (-5.8 parts per thousand to -4.5 parts per thousand), indicative of precipitation from hydrothermal solutions (similar to 129 degrees-186 degrees C) dominated by magmatic CO2. Calcite formation was probably favored by fluid rock interactions at elevated temperatures, which tend to remove solutes that inhibit calcite precipitation in seawater (Mg2+ and sulfate). Devonian Frankenwald calcites show low delta O-18 values, reflecting diagenetic and metamorphic overprinting. Values of delta C-13 around 0 parts per thousand for basalt-hosted calcite indicate seawater-derived inorganic carbon, whereas delta C-13 values for serpentinite-hosted calcite agree with mantle-derived CO2 (for values as low as -6 parts per thousand) with a contribution of amagmatic carbon (for values as low as -8.6 parts per thousand), presumably methane. Secondary mineral phases from the LHF for which a biogenic origin appears feasible include dolomite dumbbells, clotted carbonate, and a network of iron- and silica-rich filaments.