Abstract
Carbon storage by the alteration of the oceanic crust (resulting in carbonate veining) may act as a sink for degassed CO2 at mid-ocean ridges and may be an important attributor to atmospheric CO2 by subduction processes. However, the carbon storage potential of the lower oceanic crust is not fully revealed
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yet. Therefore, a case study on precipitated carbonates within gabbroic rocks at site U1473A positioned at the Atlantis Bank, SW Indian Ridge, were investigated in this research. At this ultra-slow spreading ridge, the lower crust and upper mantle are uplifted along a detachment fault forming the Atlantis Bank. The style of carbonate formation was characterized by the use of microstructural investigation tools. Also the carbon, oxygen and clumped isotope signatures were measured to investigate the carbonate precipitation conditions. By accounting for all carbonate veins recovered from hole U1473A an annual storage rate of 4.59x1012 mol CO2 yr-1 was calculated. The identified fault veins, fractured veins and hydrothermal breccias of calcite composition are associated with late stage fracturing. No clear faulting associated or depth dependence correlation is found for the style of the carbonate formation. The carbonates are likely not formed by the in situ conversion of silicate minerals into carbonate due to the lack of vein-wall rock interactions. The isotopic signature depth profile shows very homogenous δ18O values indicating that the carbonates grew from a homogeneous reservoir. Therefore, constant formation temperatures in the range from 18.9 to 41.0 °C are assumed for the total depth section. Additionally, the homogeneous δ18O values indicate an in equilibrium system and therefore with no or little kinetic fractionation. This is in contrast to the identified brecciated hydrothermal injection veins that are associated with relative fast, out of equilibrium precipitation conditions. The brecciated hydrothermal injection veins are possibly precipitated under equilibrium conditions elsewhere, broken up and transported into the injection veins forming the breccia. Based on the initial results, carbonate precipitation potentially occurred as a result of hydrothermal mixing with infiltrating seawater at any depth down to ~2 km, during or prior to the uplift of the Atlantis Bank. The hydrothermal signature is supported by the obtained enriched δ18Osw values implying the interaction of hydrothermal fluids with the gabbroic crust. Complementary research on the carbonates is required to get a better understanding of the actual timing and various carbonate vein generations.
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