What is it about?
We report widespread mineral carbonation of mantle rocks in an oceanic transform fault fueled by magmatic degassing of CO2. In 2017, we collected oceanic mantle rocks during an expedition with two submersibles near an archipelago called St. Paul's Rocks (Arquipélago de São Pedro e São Paulo) in the equatorial Atlantic. St. Paul's Rocks is located within an oceanic transform fault, one of three principal tectonic plate boundaries on Earth. Spectroscopic analysis of the rocks revealed that they contain carbonate minerals that formed during interaction of mantle rocks with a CO2-enriched hydrothermal fluid. Stable isotope measurements and the chemical composition of these rocks suggest that the carbon dioxide needed for carbonate formation is derived from a CO2-rich magma. Our study is the first to describe widespread carbon sequestration in mantle rocks driven by magmatic CO2 in an oceanic transform fault setting.
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Why is it important?
Our findings describe a previously unknown part of the geologic carbon cycle in an oceanic transform fault. Carbon dioxide, sourced from partial melting of the oceanic mantle and crystallization of basalt at high temperatures, dissolves in percolating hydrothermal fluids that then react with mantle rocks at lower temperatures, leading to geological carbon sequestration. The confluence of tectonically exhumed mantle rocks and CO2-rich (alkaline) basalt formed through limited extents of melting caused widespread carbon sequestration in the St. Paul's Rocks area. Transform faults, where mantle rocks are particularly common, cover vast stretches of the ocean floor, rivaling the overall length of the global mid-ocean ridge system. Hence, there is an enormous potential for naturally occurring geological carbon sequestration. Because transform faults have not been accounted for in previous estimates of global geological CO2 fluxes, the mass transfer of magmatic CO2 to the altered oceanic mantle and seawater may be larger than previously thought.
Perspectives
This study was particularly exciting to me because it confirms that carbon sequestration in mantle rocks via mineral carbonation is taking place in (sub-)seafloor environments. We had predicted that this should be happening (Klein and Garrido, 2011: Thermodynamic constraints on mineral carbonation of serpentinized peridotite, Lithos). To find carbonate-altered mantle rocks at St. Paul's Rocks was totally serendipitous as we were exploring for ongoing low-temperature hydrothermal activity at the time. While we did not find evidence for ongoing hydrothermal activity, we did find these amazing carbonate-altered rocks. As we dug deeper in the geochemical data, we realized that there is a connection between those mantle rocks and highly vesicular basalt that had been dredged from the seafloor nearby in the 1960s. It became clear that the CO2 from the emplacement of that type of basalt was the source of CO2. This study would not have been possible without the support of my wonderful family and amazing collaborators - Kudos to them!
Frieder Klein
Woods Hole Oceanographic Institution
Read the Original
This page is a summary of: Mineral carbonation of peridotite fueled by magmatic degassing and melt impregnation in an oceanic transform fault, Proceedings of the National Academy of Sciences, February 2024, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2315662121.
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