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

Petrographic and stable-isotope (δ¹³C, δ¹⁸O) 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 δ¹⁸O_PDB as high as +5.3‰ for serpentinite-hosted aragonite and as high as +4.2‰ for sulfide-hosted aragonite are consistent with precipitation from cold seawater. Most of the corresponding δ¹³C values indicate a marine carbon source, whereas δ¹³C values for sulfide-hosted aragonite as high as +3.6‰ may reflect residual carbon dioxide in the zone of methanogenesis. Calcite veins from the LHF, by contrast, have low δ¹⁸O_PDB (−20.0‰ to −16.1‰) and δ¹³C values (−5.8‰ to −4.5‰), indicative of precipitation from hydrothermal solutions (∼129°–186°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 δ¹⁸O values, reflecting diagenetic and metamorphic overprinting. Values of δ¹³C around 0‰ for basalt-hosted calcite indicate seawater-derived inorganic carbon, whereas δ¹³C values for serpentinite-hosted calcite agree with mantle-derived CO₂ (for values as low as −6‰) with a contribution of amagmatic carbon (for values as low as −8.6‰), 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.