Carbonate formation in the ocean crust as a proxy for water-rock interactions

• Main Author: Eickmann, Benjamin
• Other Authors: Peckmann, Jörn, Bach, Wolfgang
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Bremen 2009
• Subjects: carbonate formation; serpentinization; hydrothermal systems; ocean crust; water-rock interactions; 550; 550 Earth sciences and geology; ddc:550; Arctic; Gakkel Ridge; Mid-Atlantic Ridge
• Online Access: https://media.suub.uni-bremen.de/handle/elib/2668; https://nbn-resolving.org/urn:nbn:de:gbv:46-diss000114636

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 ...