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

Abstract:

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 (δ¹⁸O_SMOW = 11 to 20‰, corresponding to 75-180°C for fluid δ¹⁸O_SMOW 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 δ¹³C_VPDB ranging from -2 to -5‰. Later veins (70% calcite and 30% aragonite) cut high-T veins. These veins precipitated near seawater temperatures (δ¹⁸O_SMOW ~ 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.