<p>•This study reports widespread mineral carbonation of mantle rocks in an oceanic transform fueled by magmatic degassing of CO<sub>2</sub>. </p>
<p>•The findings describe a previously unknown part of the geological carbon cycle in transform faults that represent one of the three principal plate boundaries on Earth. </p>
<p>•The confluence of tectonically exhumed mantle rocks and CO<sub>2</sub>-rich alkaline basalt formed through limited extents of melting characteristic of the St. Paul’s transform faults may be a pervasive feature at oceanic transform faults in general. Because transform faults have not been accounted for in previous estimates of global geological CO<sub>2</sub> fluxes, the mass transfer of magmatic CO<sub>2</sub> to the altered oceanic mantle and seawater may be larger than previously thought. </p>
<p>•The amount of CO<sub>2</sub> emitted at the transform faults is negligible compared with the amount of anthropogenic CO<sub>2</sub>. However, on geological timescales and before humans emitted so much CO<sub>2</sub>, geological emissions were a major driving force of Earth’s climate. </p>