In situ oxygen isotope thermometry of carbonate–silicate assemblages in carbonated ultramafic rocks from the Point-Rousse Complex (Newfoundland, Canada)

Carbonated ultramafic rocks such as soapstones and listvenites provide natural evidence of extensive fluid-rock interaction between mantle-derived lithologies and CO₂-bearing fluids and serve as natural analogues for carbon sequestration. Oxygen isotope fractionation represents a powerful tool for constraining both temperature conditions and fluid sources during the carbonation process. Here, we present preliminary results from an integrated study combining in situ oxygen-isotope analyses with microscale textural observations in a carbonated ultramafic sequence from the Point-Rousse Complex (Baie Verte Ophiolite, Newfoundland, Canada). In situ oxygen isotope measurements were performed using Secondary Ion Mass Spectrometry (SIMS) targeting five mineral phases: antigorite, talc, magnesite, dolomite, and quartz. Phase-specific reference materials [1,2,3,4] were analyzed during the same analytical session to correct for matrix effects and to monitor instrumental drift.

The studied Point-Rousse Complex sequence comprises ophicarbonates (≤ 5.4 wt% CO₂), antigorite‑bearing soapstones (antigorite–talc–carbonate rocks, 12.8–17.1 wt% CO₂), quartz‑bearing soapstones (quartz–talc–carbonate rocks; 19.5–34.9 wt% CO₂), and listvenites (28.6–46.1 wt% CO₂). Ophicarbonates display non-pseudomorphic textures, with δ¹⁸O values of 4.3–5.2‰ (VSMOW) in antigorite, 7.6–8.6‰ in talc, and 11.1–12.2‰ in magnesite. Antigorite‑bearing soapstones show massive to foliated textures, with recrystallized antigorite overgrowing large magnesite grains, dolomite veins, and talc defining foliated domains. These rocks exhibit similar δ¹⁸O values in antigorite (4.2–5.2‰) and magnesite (9.9–11.7‰), but distinct values in talc (4.3–7‰) and dolomite (10.1–10.7‰). Quartz‑bearing soapstones and listvenites show more complex textures, including Fe‑rich zones in magnesite and talc–quartz coronas around dolomite. Magnesite exhibits a wide range of δ¹⁸O (10.6–17.7‰) with variable values in Mg-rich cores (Fe# = 0.01) and Fe-rich rims (Fe# = 0.16). Talc, dolomite, and quartz show relatively homogeneous δ¹⁸O values (6.1–6.9‰, 10.5–12.8‰, and 11.1–13‰, respectively).

Preliminary oxygen isotope thermometry based on texturally equilibrated serpentine-magnesite and serpentine-talc pairs yields carbonation temperatures of 244 ± 21 °C for ophicarbonates and 309 ± 43 °C for the antigorite‑bearing soapstones. Calculated apparent δ¹⁸O values of the fluid at these temperatures range between 3.7 and 5.4‰, consistent with metamorphic fluids. These results suggest a multi‑stage carbonation at moderate temperatures involving a progressively evolving fluid composition.

Funding: We acknowledge funding for doctoral fellowship FPI2022/PRE2023_IACT_059 (IDG) and Grants PID2022-136471NB-C21 & 22 (RUSTED) by MCIN/AEI/10.13039/501100011033 and FSE+. JAPN, CJG & MDM further acknowledge funding from the ERC project OZ (DOI: 10.3030/101088573).

[1] Scicchitano et al. (2021) DOI: 10.1111/ggr.12359

[2] Scicchitano et al. (2022) DOI: 10.1016/j.gca.2021.11.025

[3] Scicchitano et al. (2025) DOI: 10.1111/ggr.70031

[4] Sliwinski et al. (2018) DOI: 10.1111/ggr.12194