**1. Introduction**

Carbonation of peridotites occurs along rifted margins (e.g., the Western Iberia margin, [1] and references therein) and at Mid-Ocean Ridges (MORs; e.g., [2,3]). It is also commonly observed in ophiolites in the Alpine Tethys (e.g., [4]), the Pyrenees ([5] and references therein), Norway (e.g., [6]), Oman (e.g., [7]) and Québec [8]. Carbonated peridotites include ophicarbonates commonly associated to brecciated peridotites, cemented by carbonate, and created by tectonic, hydrothermal and/or sedimentary processes [9,10]. Metasomatic processes responsible for pervasive replacement of serpentine by calcite occur at slow-spreading ridges and in magma-poor passive margins, where peridotites are exposed on the seafloor during tectonic extension [11,12] and has been described in fossil margins (Chenaillet in the Alpine Tethys; [13]). Understanding carbonation in natural systems provides constraints to develop efficient engineering strategies for atmospheric CO2 sequestration [14,15].

The heat released during magma cooling triggers seawater circulation in the upper part of the lithosphere. During hydrothermal circulation, mantle rocks are progressively replaced by hydrous minerals including serpentine and brucite [16,17]. This serpentinization reaction is also a redox reaction in which the ferrous iron contained in peridotite is oxidized to form magnetite and ferric serpentine, whereas water is reduced to form hydrogen [18]. Recent studies on the venting temperature of hydrothermal fluids from the Lucky Strike hydrothermal system emphasize that hydrothermal fluid circulation is subdivided into major (km-scale) and minor (m-scale) convection cells; the major cells lead to focused venting of high temperatures at the outflow, whereas the minor convection cells show mixing with seawater (4 ◦C) at the meter-scale leading to venting of diffuse fluids at low temperatures [19,20]. The composition of the fluid formed during fluid-peridotite interaction is expected to evolve from seawater to a high pH (9–11), hydrogen- and methane-rich fluid [18]. Highly alkaline fluids are expelled at low temperature (40–75 ◦C, [21,22]) at the Lost City Hydrothermal Field (LCHF). Such fluids promote Ca-carbonate precipitation ± brucite in chimneys and in veins [23,24]. Similar mineralogy has been observed within the Iberian margin, where the mixing zone provides an environment favoring microbial development (Ocean Drilling Program (ODP) Leg 149 Site 897; [25]). Peridotite-hosted hydrothermal systems exist at MORs such as the Mid-Atlantic Ridge (MAR; Kane Fracture Zone [26]; 15◦ N [27]) and in passive margins (e.g., Western Iberia margin; [25]). Fossil systems are also found in mountain belts like the Piemonte-Liguria ophiolites (e.g., Chenaillet, [28]). Acidic (pH ~3) and high-temperature (>300 ◦C) fluids were also sampled at other peridotite-hosted hydrothermal systems such as Rainbow and Logatchev, suggesting a contribution of mafic bodies [18]. Ultimately, fluid-rock interactions in the presence of mafic rocks may lead to crystallization of Si-rich phases, like talc [10,29] or quartz [30].

The composition of carbonates foundin ophicarbonates varies betweenMg and Ca-rich end-members. Calcite, aragonite, magnesite and dolomite show variable isotopic signatures, with δ18OVSMOW ranging from 8.3% to 36.6% and <sup>δ</sup>13CVPDB from <sup>−</sup>4.5% to 4.5% (see Supplementary Table S1). As a result, temperatures of carbonate crystallization estimated using δ18O range from 1 up to ~200 ◦C [26,27], assuming seawater isotopic compositions for the fluid. This can be used to suggest precipitation under completely different conditions [27] from seawater on the seafloor [3] to high temperature hydrothermal fluids (100–150 ◦C; [13]). However, details about the different conditions are not well constrained nor how it relates to the process of carbonate formation. In hydrothermal systems, carbonates may be formed during either seawater influx or hydrothermal fluid discharge, involving different temperatures, compositions and fluid flow regimes.

Here, we investigate the process of serpentinite replacement by carbonates in a sample from the Newfoundland (NF) margin. In order to constrain the timing of calcite replacement, we first establish the brecciation event succession. Then we characterize the textures of calcite growth and their in-situ oxygen isotopic signature from core to rim. We couple our micro-textural observations and O and C isotopic measurements with a thermodynamic model to provide new constraints on the conditions of carbonation in mantle rocks exposed on the seafloor. Furthermore, we compile published O–C data from present-day and fossil distal margins and slow-spreading ridge ophicarbonates to discuss the possible re-equilibration of O–C in ophicarbonates during metamorphism.

#### **2. Geological Setting**

In the Newfoundland hyper-extended margin and conjugate Iberian margin, subcontinental lithospheric mantle was exhumed on the seafloor by detachment faulting. During the mantle exposure on the seafloor, a superposition of near-seafloor processes i.e., tectonic, hydrothermal and sedimentary brecciation occurred, resulting in tectono-sedimentary breccias [31–33]. Ophicarbonates are observed from the proximal to the distal margin of both the Newfoundland and Iberian margins. Carbonation processes overlap the brecciation, leading to calcite veins, replacement of serpentine and carbonate cement. The tectono-sedimentary breccias rework exhumed serpentinized footwall and thus preserved every stage of carbonation from its onset. Previous work from the Iberian margin has estimated that

the temperature of carbonation in calcite veins and cement was < 45 ◦C based on δ18OVSMOW values of 25% to 35% [1,3,34–36]. The carbonation was thus interpreted as a near-surface cold process [35].

Here, we study samples from the International Ocean Discovery Program (IODP) site 1277 (Figure 1; [37]). The drill core contains serpentinized peridotites and locally magmatic intrusions (e.g., alkaline sills of 124–112 Ma, followed by Mid-Ocean Ridge Basalt-type magmatism <112 Ma at the onset of oceanic spreading, [38,39] and references therein). The drill core is also composed of brecciated serpentinized peridotites over 20 m (Figure 1c); we focus on these serpentinite samples that show evidence of replacement by carbonates.

**Figure 1.** (**a**) Interpretative cross-section after Sutra and Manatschal [40] based on the seismic lines Lusigal 12 and of the TGS-NOPEC Geophysical Company for Iberia margin, and line 2 of the Study of Continental Rifting and Extension on the Eastern Canadian Shelf (SCREECH) project for Newfoundland. (**b**) Location of the Site 1277 in the Mid-Atlantic Ridge (MAR). (**c**) Sketch of the repartition of drilled mineralogies.
