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Abstract: Chemical (Sr, Mg) and isotopic (δ 18O, 87Sr/86Sr) compositions of calcium carbonate veins (CCV) in the oceanic basement were determined to reconstruct changes in Sr/Ca and Mg/Ca of seawater in the Cenozoic. We examined CCV from 10 basement drill sites in the Atlantic and Pacific, ranging in age between 165 and 2.3Ma. Six of these sites are from cold ridge flanks in basement <46Ma, which provide direct information about seawater composition. CCV of these young sites were dated, using the Sr isotopic evolution of seawater. For the other sites, temperature-corrections were applied to correct for seawater–basement exchange processes. The combined data show that a period of constant/low Sr/Ca (4.46–6.22mmol/mol) and Mg/Ca (1.12–2.03mol/mol) between 165 and 30Ma was followed by a steady increase in Mg/Ca ratios by a factor of three to modern ocean composition. Mg/Ca–Sr/Ca relations suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. However, additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate. [Copyright &y& Elsevier]

Abstract: Carbonate veins hosted in ultramafic basement drilled at two sites in the Mid Atlantic Ridge 15°N area record two different stages of fluid-basement interaction. A first generation of carbonate veins consists of calcite and dolomite that formed syn- to postkinematically in tremolite–chlorite schists and serpentine schists that represent gently dipping large-offset faults. These veins formed at temperatures between 90 and 170°C (oxygen isotope thermometry) and from fluids that show intense exchange of Sr and Li with the basement (87Sr/86Sr=0.70387 to 0.70641, δ7LiL-SVEC =+3.3 to +8.6‰). Carbon isotopic compositions range to high δ13CPDB values (+8.7‰), indicating that methanogenesis took place at depth. The Sr–Li–C isotopic composition suggests temperatures of fluid-rock interaction that are much higher (T>350–400°C) than the temperatures of vein mineral precipitation inferred from oxygen isotopes. A possible explanation for this discrepancy is that fluids cooled conductively during upflow within the presumed detachment fault. Aragonite veins were formed during the last 130kyrs at low-temperatures within the uplifted serpentinized peridotites. Chemical and isotopic data suggest that the aragonites precipitated from cold seawater, which underwent overall little exchange with the basement. Oxygen isotope compositions indicate an increase in formation temperature of the veins by 8–12°C within the uppermost ~80m of the subseafloor. This increase corresponds to a high regional geothermal gradient of 100–150°C/km, characteristic of young lithosphere undergoing rapid uplift. [Copyright &y& Elsevier]