The main objective of this thesis was to elucidate the authigenesis of carbonate minerals in modern and Devonian ocean-floor volcanic rocks and to demonstrate that Late Devonian (Frasnian) pillow basalts from the Saxothuringian zone once harbored microbial life. The ultramafic-hosted Logatchev hydrothermal field (LHF) at the Mid-Atlantic Ridge, the Arctic Gakkel Ridge (GR) and the Late Devonian Frankenwald feature carbonate precipitates (aragonite, calcite, dolomite) in voids and fractures of different types of rocks. Carbonate veins cut the rock texture, postdating the emplacement and serpentinization of the upper mantle rocks. Petrographic and stable isotope (13C, 18O) patterns were compared in an attempt to understand the genesis of carbonate minerals in these settings. Specifically, were the carbonate sample from the modern seafloor settings and the Devonian analogue of hydrothermal origin, low-temperature abiogenic or biogenic origin? Aragonite is the most abundant carbonate mineral in serpentnites from the LHF and GR and occurs within massive sulfides of the LHF. 18O values of aragonite hosted in serpentinites and sulfides are consistent with precipitation from cold seawater. Most of the corresponding 13C values indicate a marine carbon source, while 13C values of sulfide-hosted aragonite as high as 3.6 may reflect residual carbon dioxide in the zone of methanogenesis. Calcite veins from LHF, by contrast, have low 18O values (as low as 20.0) and 13C values (as low as 5.8) indicative of precipitation from hydrothermal solutions dominated by magmatic carbon dioxide. To gain deeper insights in the formation of carbonates in hydrothermal environments, chemical and strontium isotopic composition of these different carbonates were analyzed to examine the conditions that led to their formation. Seawater-like 87Sr/86Sr ratios of aragonite in serpentinites from LHF are similar to those of aragonite from the GR, indicating aragonite formation from seawater at ambient conditions at both sites. Aragonite veins in sulfides from LHF also have seawater-like 87Sr/86Sr ratios, however, the rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of small (<1%) proportions of hydrothermal fluids. In contrast to aragonite, dolomite and calcite from the LHF precipitated at much higher temperatures, indicative of formation from evolved hydrothermal fluids. A simple mixing model based on strontium mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Aragonite samples corroborate the stable isotope patterns and formed at low temperatures form pure seawater. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that indicate conductive heating, probably of seawater prior to mixing. Hydrothermal serpentinite-hosted calcite formed from a mixture of 67% hydrothermal fluid and 33% seawater due to conductive cooling of hydrothermal fluid effusing a fault. REE patterns corroborate the results of the mixing model; since the calcite that formed from waters with the greatest hydrothermal contribution have REE that closely resemble those of vent fluids from the LHF. δ13C values of Late Devonian (Frasnian) calcite hosted in basalts indicate precipitation from seawater, while δ13C values of serpentinite-hosted calcite agree with mantle-derived carbon dioxide with a contribution of amagmatic carbon (for values low as −8.6), presumably methane. Low 18O values reflect diagenetic and metamorphic overprinting. Furthermore, Late Devonian pillow basalts from the Saxothuringian zone were found to contain abundant putative biogenic filaments, indicating that the volcanic rocks once harbored microbial life. The mineralized filaments are found in carbonate-filled vesicles, where they start to form on internal surfaces after seawater ingress. A biogenic origin of filaments is indicated by their size and morphology resembling modern microorganisms, their independence of crystal faces and cleavage plans, complex branching patterns, and internal segmentation. They became preserved upon microbial clay authigenesis similar to the encrustation of modern prokaryotes in iron-rich environments. The results presented in this thesis give rise to a new and better understanding of carbonate formation in the ocean crust, especially in ultramafic-hosted hydrothermal environments. Further, the results highlight the important of mixing processes in the subseafloor of hydrothermal systems that led to the formation of carbonates and highlights that seafloor basalt may thus represent a common, perhaps universal niche for life in the oceanic crust
