*5.5. Implications for CO2 Mineral Sequestration*

The storage of CO2 through carbonate precipitation appears to be one of the solutions to compensate for the anthropogenic greenhouse gas emissions [92]. Among the Earth's crust rocks, ultramafic rocks have one of the highest potentials for carbon sequestration due to their high magnesium content [6,93–99]. The results of this study provide constraints for the process of CO2 sequestration in ultramafic rocks. We show here that low-temperature carbonation on the seafloor (T < 50 ◦C) produces Ca-carbonates. The calcium source is the seawater, and high water to rock ratios are needed to induce carbonate precipitation. The high MgO content of the ultramafic rock is therefore not an asset for CO2 storage in the conditions prevailing on the seafloor. However, thermodynamic modeling indicates that high temperature carbonation (T > 100 ◦C) requires smaller water to rock ratios since magnesium can be incorporated into the carbonates. Mimicking the high temperature process thus appears to be more relevant to develop an efficient CO2 storage solution. Such a solution would also need to circumvent the potential issues associated with the modification of the porous network by carbonation. Serpentine reaction occurring at high temperature to form talc and then quartz indeed requires significant magnesium and silica transport.

#### **6. Conclusions**

We provide microtextural observations of pervasive serpentine replacement by calcite and synchronous calcite growth in veins in exhumed mantle from the Newfoundland passive margin. We interpret those calcite grain cores as the first carbonate to grow in exhumed mantle. This is shown by the crosscutting relationship between brecciation and calcite grains textures revealed by CL images. Replacive calcite maintains the serpentine mesh texture and grows as scalenohedral crystals with a characteristic Mn-compositional banding. Preciseness of SIMS allowed us to measure O and C isotopic composition of each band in a single calcite grain. Measured O isotopes highlight no systematic variation, C isotope measurements display seawater range values and O isotope thermometry reveals that carbonation is cold (<20 ◦C) since the onset of the reaction.

Our thermodynamic modeling predicts Ca-rich carbonate crystallization near-surface during seawater influx and hydrothermal fluid discharge. Si-rich phases appear in the system with carbonation front evolution through time and space (e.g., talc).

In the discharge model, this stability field limitation is associated with carbonate dissociation into HCO3 <sup>−</sup>,aq as the pH decreases with temperature. In the recharge model, the limitation of the carbonate stability field is associated with anhydrite (CaSO4) formation at temperatures above 100 ◦C. Anhydrite formation only occurs in the first box of the model at high temperature in the discharge model (250 ◦C). We summarize the petrological, geochemical and numerical modeling results of this study in Figure 11, where the differences between recharge and discharge regarding carbonate precipitation are highlighted.

**Figure 11.** Conceptual model for the relationship between carbonation, hydrothermal circulation and magmatism. Two different models are proposed without (**a**) and with (**b**) magmatic extrusions. They both include recharge and discharge zones. Low-temperature carbonation (<50 ◦C) occurs in the recharge zones leading to Ca(±Mg)-carbonate formation (**c**). High-temperature carbonation starts at 150 ◦C in the discharge zones where Mg-carbonates only precipitate (**c**). Carbonation induces more magnesium and silica transport in the discharge zones leading to serpentine replacement by talc and then quartz as the water to rock ratio increases. These elements may precipitate at the surface in hydrothermal chimney as it is observed at the Lost City Hydrothermal Field [23]. The discharge model requires large scale fluid transport in permeable zones whereas carbonation in the recharge model is widespread and occurs through pervasive fluid transport.

According to our petrological and modeling results, we wonder why there are no cold ophicarbonates sampled yet in the Alpine ophiolites, and we propose that, in the case of absence of post-rift melting, O and C isotopic composition might have been re-equilibrated during Alpine deformation.

We show that carbonation of ultramafic rocks is more efficient (lower water to rock ratio) at high temperature (>100 ◦C) since Mg-bearing carbonates can be formed. This result may guide the development of future engineering solutions for CO2 sequestration.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-163X/10/2/184/s1; Table S1: Compilation of carbon and oxygen isotope composition on calcite in serpentinized peridotites collected on the seafloor and in metamorphic environments. Table S2: Carbon and oxygen isotope composition on calcite from IODP Site 1277. Table S3: Electron microprobe analyses of serpentine and carbonate, and seawater composition used in thermodynamic modeling.

**Author Contributions:** S.P. and B.M. contributed to the data acquirement, interpretation of the data and wrote the article; L.B. contributed to the interpretation; A.-S.B. contributed to the setting of the SIMS. All authors have read and agreed to the published version of the manuscript.

**Funding:** Suzanne Picazo acknowledges Center of Excellence in Basin Analysis Grant from ExxonMobil for financial support. Benjamin Malvoisin acknowledges support from the Swiss National Science Foundation (Ambizione grant n◦PZ00P2\_168083). Lukas Baumgartner obtained funding from Swiss National Science Foundation and KIP6 PCI CASA.

**Acknowledgments:** The authors are grateful to Tiffany Barry for language corrections. We thank Susanne Seitz and Guillaume Siron for SIMS technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
