*5.4. Implications for Ophicarbonates Formation Preserved in Ophiolites*

Ophicarbonates have been described first in remnants of the central Jurassic Tethys, in the Klosters, Totalp and Arosa ophiolites, in the Central-Eastern Alps [4] and in the Chenaillet ophiolite in the Western Alps [9]. Over the last 30 years, multiple studies have been conducted with a renewed interest, since they contain a key to the understanding of mantle exhumation to the seafloor, and because of their implication as an analogue for present-day passive margins. In addition, carbonation of serpentinite and other mantle rocks is a potential engineering solution to CO2 sequestration [83]. Alpine ophicarbonates are interpreted to be the result of oceanic alteration of serpentinized peridotites during mantle exhumation associated with the Jurassic hyperextension phase [13], already proposed by Weissert and Bernoulli [4] and discussed by Früh-Green et al. [84].

It is important to note that carbonates are mostly calcites, and minor aragonite [2], never dolomite. Mineralogy of carbonates encountered in the Alpine Tethys ophicarbonates is mostly calcite, and minor dolomite (e.g., in Val Ventina, Alps, [82]; and in the Chenaillet, [13] and references therein). We compiled δ18O and δ13C measured in ophicarbonates from the literature in several locations in the Alpine Tethys. The data set contains bulk measurements on carbonate veins [84], disseminated carbonates [13], as well as sedimentary ophicarbonates [4,85,86].

Regional metamorphism experienced by ophicarbonates in the Platta nappes did not exceed 200 to 250 ◦C ([87] and references therein). The δ18O values of carbonates range from 21% to 14% [85] in ophicarbonates towards the thrust plane, which separates them from serpentinites in the Platta nappe. Metamorphism and fluid flow during metamorphism seems to play in important role in the δ18O record in ophicarbonates, and hence a careful re-evaluation of these values should be attempted to ascertain the initial condition of formation of these carbonates prior to metamorphism.

This explanation is supported by the low δ18O values also recorded in sedimentary ophicarbonates (from 8.6% to 24.7% in the Western and Central Alps [4,13,86,88]). These low δ18O values correspond to temperatures from 40 to 70 ◦C (Table S1). As in the Iberian margin, near-surface sedimentary carbonation precipitation also occurs in a range of temperatures below 20 ◦C, we infer that these temperatures are too high for true sedimentary deposition.

A way to optically verify if carbonate re-equilibration by metamorphism has started is to observe the Mn-banding in carbonates by cathodoluminescence. Diffusion of Mn is approximately 10 orders of magnitude faster (e.g., 1 mm at 300 ◦C in 25 Ma) than diffusion of O in calcite (30 μm to 10−<sup>6</sup> μm for the same setting [89–91]. Many carbonates precipitating in shallow hydrothermal settings will obtain rhythmic layers of CL-active elements, as is shown in this study. Since diffusion of these elements (like Mn) is fast, they will disappear upon heating, and are obliterated by recrystallization. This means that if a carbonate preserves its banding structure acquired during carbonation on the seafloor, it should also preserve its δ18O signature. Therefore, a test for recognizing re-equilibration in ophicarbonates from ophiolites would be the presence of thin CL-banding. This criterion can only be applied if diffusion is the only process acting during re-equilibration since calcite dissolution-reprecipitation may also modify the composition of the carbonates at low temperature. Nevertheless, this will also eliminate potentially the isotopic signature.

To follow up, we estimate the δ18O expected in calcite that is supposed to be equilibrated with the surrounding serpentine during metamorphism. δ18Oserpentine in Alpine ophiolites ranges from 4.3% to 13.4% [84]. If calcite and the surrounding serpentine are in equilibrium, according to the temperature of metamorphism reached, <sup>δ</sup>18Ocalcite-δ18Oserpentine should be <sup>Δ</sup>18Occ-serp <sup>≈</sup> 5.27% to 5.98% for a metamorphism at 200–250 ◦C (e.g., Platta); and <sup>Δ</sup>18Occ-serp <sup>≈</sup> 2.82% to 3.41% for a metamorphism at 450–550 ◦C (e.g., Val Ventina; [79]). Our compilation shows that samples displaying the least degree of metamorphism have Δ18Occ-serp higher than the estimated equilibrium values; whereas samples displaying the highest degree of metamorphism have Δ18Occ-serp corresponding to the estimated calcite–serpentine fractionation.

An alternative to the onset of differential re-equilibration during Alpine metamorphism could be that instead of precipitating during seawater influx, the ophicarbonates were formed during hydrothermal fluid discharge. We want to emphasize that this explanation would explain only the high temperatures recorded by the δ18O in carbonates, as the textures and brecciation processes are similar to the present-day passive margin samples and are easily explained by the seawater influx model.

In an ophiolite where hydrothermal vents are clearly observed (e.g., Chenaillet), it should be easier to establish a relationship between carbonation and the distance between the hydrothermal fluid pathways. Magmatism may also play an important role in the high temperature recorded by ophicarbonates in distal parts of the Ocean Continent Transitions such as the Chenaillet, as late syn-rift to early post-rift magmatism with intrusions crosscutting the detachment faults are observed [28].

In the case of mostly discharge-driven carbonation, as the Alpine Tethys ophicarbonates are a good analogue for the present-day Iberia passive margin, near-surface ophicarbonates with high δ18O (25–35%) should also be reported in the remnant ophiolites, which is still not the case.
