Olivine Carbonation and Multiple Episodes of Carbonate Veining during Basement Rock Denudation in an Oceanic Core Complex, ODP Site 1275, Mid-Atlantic Ridge: A Natural Analog for Engineered Carbon Sequestration

Tuesday, 16 December 2014
Timothy J Schroeder1, Wolfgang Bach2, Niels Joens3, Svenja Rausch2, Patrick Monien2 and Andreas Kluegel2, (1)Bennington College, Bennington, VT, United States, (2)University of Bremen, Bremen, Germany, (3)Ruhr University Bochum, Geology, Bochum, Germany
Carbonate precipitation in oceanic olivine-bearing rocks provides a natural case to study engineered CO2-sequestration via olivine carbonation. ODP Site 1275 recovered tectonically denuded gabbro and troctolite on the west flank of the Mid-Atlantic Ridge (15°45’N). Carbonate veins comprise ~0.15 vol.% of the recovered core, corresponding to CO2 uptake of only 0.075 wt.%. However, troctolite cores are comprised of 0.8 vol.% calcite veins, and individual troctolite samples contain up to 15 vol.% calcite that directly replaced olivine in association with talc.

Carbonate precipitated from multiple types of fluids during and following denudation. Calcite replaced olivine concurrently with the formation of high-temperature (δ18OSMOW = 11 to 20‰, corresponding to 75-180°C for fluid δ18OSMOW of 0‰) calcite veins in active shear zones. These veins are enriched in LREEs (2-10x chondrite) with strong positive Eu anomalies (Eu/Eu* = 5 to 500), and have 87/86Sr ratios of ~0.704 and δ13CVPDB ranging from -2 to -5‰. Later veins (70% calcite and 30% aragonite) cut high-T veins. These veins precipitated near seawater temperatures (δ18OSMOW ~ 35.1), and have seawater C and Sr isotopic signatures. Both late calcite and aragonite veins have lower REE contents than high-T veins and have little or no Eu anomalies; aragonite veins have negative Ce anomalies. These low-T veins precipitated in both gabbro and troctolite from seawater with varying degrees of rock interaction. High-T veins and replacive calcite precipitated from a mixture of seawater and high-T hydrothermal fluids, and are common in troctolite but rare in gabbro. Geochemical reaction path modeling suggests that the replacive calcite-talc assemblage in troctolite forms only at high fluid-to-rock ratios. The calcite’s low 87/86Sr ratios, however, indicate that the seawater-derived fluids had exchanged Sr with large volumes of rock. The olivine carbonation may hence result from upflow of deeply rooted fluids during basement denudation.