Stable and radiogenic isotopes (δ18O, δ13C and 87Sr/86Sr) as tracers of complex carbonation of ultramafic rocks: Evidence from three magnesite deposits

Natural carbonation of ultramafic rocks is a key process controlling the long-term carbon cycle, as exposed peridotites can directly sequester atmospheric CO₂ through carbonation associated with chemical weathering. To constrain the conditions and sources of fluids involved in past natural carbonation processes and magnesite formation, isotopic analyses (δ¹⁸O, δ¹³C and ⁸⁷Sr/⁸⁶Sr) were conducted on massive magnesite veins (n = 37) hosted within the exhumed mantle section of the ophiolitic sequence of the Central Sudetic Ophiolite (SW Poland). Samples were collected from three tectonically dismembered ultramafic units: (1) Szklary; (2) Braszowice; (3) Wiry.

Oxygen isotope compositions are most variable in Szklary (δ¹⁸O = 22.4 to 31.0‰ SMOW), show a moderately narrower range in Braszowice (22.0 to 29.6‰ SMOW), and are relatively homogeneous in Wiry (27.3 to 28.8‰ SMOW). Carbon isotope values further differentiate the units: Szklary magnesites exhibit the lightest carbon (δ¹³C = -11.8 to -17.9‰ VPDB), Braszowice samples show consistently heavier values (-10.6 to -13.9‰ VPDB), whereas Wiry displays the widest range toward heavier carbon (-5.5 to -13.8‰ VPDB). Strontium isotopes also vary systematically, with uniformly low ⁸⁷Sr/⁸⁶ ratios in Braszowice (~0.7065), more variable values in Szklary (~0.7071 - 0.7117), and the most radiogenic signatures in Wiry (~0.710 - 0.721).

Previous interpretations commonly assumed that magnesite formation was associated with weathering under tropical conditions, in which the oxygen isotopic composition of meteoric water can be approximated as δ¹⁸O = 0.0‰ (SMOW). This model is widely invoked for the formation of massive magnesite veins and is supported by evidence for intense weathering of the ultramafic host rocks, including the presence of laterites. Under this assumption, calculated crystallization temperatures range from ~46 °C in Szklary to ~100 °C in Braszowice. Carbon isotope data indicate a dominant contribution of soil-derived CO₂ in Szklary, with increasing influence of additional carbon sources in Braszowice and especially in Wiry.

For samples with low Rb/Sr ratios, variations in ⁸⁷Sr/⁸⁶ can be attributed primarily to differences in the isotopic composition of the fluids, indicating multiple Sr sources. The predominance of homogeneous, low ⁸⁷Sr/⁸⁶ values at Braszowice is consistent with a crustal fluid source, comparable to ratios reported for Variscan granitoids and nephrites hosted in ultramafic rocks ^[1]. This suggests that at least some magnesite bodies formed during the Variscan overprint of ophiolitic massifs, contemporaneously with serpentinite-related nephrite formation. This interpretation is supported by elevated ⁸⁷Sr/⁸⁶Sr ratios in samples with higher Rb/Sr from both Szklary and Braszowice, which likely reflect radiogenic ingrowth over hundreds of millions of years. In contrast, the high variability and generally elevated ⁸⁷Sr/⁸⁶ values observed in Wiry are more consistent with contemporaneous Sr isotope heterogeneity and may record Sr mobilization during Miocene tropical weathering of older crustal rocks ^[2].

^[1] Gil, G. et al., 2020. Ore Geology Reviews, 118, 103335.

^[2]  Waroszewski, J. et al., 2021. Catena, 204, 105377.

Acknowledgements: Research financially supported by NCN PRELUDIUM project 2022/45/N/ST10/00879