**Suzanne Picazo 1, Benjamin Malvoisin 1,2,\*, Lukas Baumgartner <sup>1</sup> and Anne-Sophie Bouvier <sup>1</sup>**


Received: 22 January 2020; Accepted: 11 February 2020; Published: 18 February 2020

**Abstract:** Serpentinite replacement by carbonates in the seafloor is one of the main carbonation processes in nature providing insights into the mechanisms of CO2 sequestration; however, the onset of this process and the conditions for the reaction to occur are not yet fully understood. Preserved serpentine rim with pseudomorphs of carbonate after serpentine and lobate-shaped carbonate grains are key structural features for replacement of serpentinite by carbonates. Cathodoluminescence microscopy reveals that Ca-rich carbonate precipitation in serpentinite is associated with a sequential assimilation of Mn. Homogeneous δ18O values at the μm-scale within grains and host sample indicate low formation temperature (<20 ◦C) from carbonation initiation, with a high fluid to rock ratio. <sup>δ</sup>13C (1–3 <sup>±</sup> 1%) sit within the measured values for hydrothermal systems (−3–3%), with no systematic correlation with the Mn content. δ13C values reflect the inorganic carbon dominance and the seawater source of CO2 for carbonate. Thermodynamic modeling of fluid/rock interaction during seawater transport in serpentine predicts Ca-rich carbonate production, at the expense of serpentine, only at temperatures below 50 ◦C during seawater influx. Mg-rich carbonates can also be produced when using a model of fluid discharge, but at significantly higher temperatures (150 ◦C). This has major implications for the setting of carbonation in present-day and in fossil margins.

**Keywords:** carbonation; CO2 sequestration; replacement process; low temperature carbonate precipitation; Secondary Ion Mass Spectrometer; seawater influx; hydrothermal circulation; ophicalcite
