Reply to ‘Discussion of oxygen isotopes in ophicalcites: an ever‐lasting controversy?’ by R. Coltat, Ph. Boulvais, Y. Branquet, M. Poujol, P. Gautier and G. Manatschal

Our publication ‘Oxygen isotopes in ophicalcites: an ever‐ lasting controversy?’ in this journal (Bernoulli and Weissert 2020) discussed the interpretation of oxygen isotopes in Alpine ophicalcites and deep-sea sediments. Based on the remarkable correlation of the isotope values with the degree of Alpine metamorphism (Bernoulli and Weissert 2020: Fig. 4), we argued that the oxygen isotope signatures measured in ophicalcites and pelagic limestones were equilibrated during Alpine metamorphism. They do not reflect the temperatures of carbonation of the serpentine host rocks or the formation of calcite in veins and cements or of marine biogenic calcite in pelagic sediments associated with the ophicalcites. In particular, we doubted the interpretation of Coltat et al. (2019a) that the oxygen isotope values in the ophicalcites of South-Penninic Platta Nappe at Falotta faithfully recorded the temperature of carbonation and of the emplacement of calcite veins in the serpentinite host rock, estimated by Coltat et al. (2019a) at ~ 100 °C. In contrast, the near-identical oxygen isotope values in the ophicalcites and in the overlying Upper Jurassic–Lower Cretaceous pelagic limestones clearly indicate that the Middle Jurassic ophicalcites and the overlying sediments underwent together one or more thermal events resetting their oxygen isotope signatures, which in contrast were related by Coltat et al. (2019a) exclusively to a ‘single, sudden event’ of hydrothermal activity during the Middle Jurassic. We do not doubt the importance of hydrothermal fluids in the formation of ophicalcites (Früh-Green et al. 1990), nor that at Falotta carbonation of the serpentinite host rock occurred during the exhumation of mantle rocks along low-angle detachment faults (e.g. Desmurs et al. 2001), nor do we doubt deformation associated with ophicalcite formation. We only relate the present-day oxygen values observed in the ophicalcites and associated sediments to Alpine metamorphism.1 Coltat et al. (2019a, b, 2020) regard the localities Falotta and Marmorera–Cotschens as singularities where practically no Alpine fluid exchange or recrystallization occurred. However, we doubt that no Alpine recrystallization should have occurred at Falotta [see Fig. 5 in Bernoulli and Weissert (2020) for an example in the less metamorphic Totalp Nappe] and that no fluids circulated during Alpine deformation and metamorphism. That all calcite crystals in the ophicalcites at Falotta show identical δ18O-values, irrespective of their habit (Coltat et al. 2019a:185, 2020), is, in our view, much better explained by Alpine metamorphic re-equilibration than by one single hydrothermal event. Coltat et al. (2021) write that isotopic re-equilibration ‘could be partly true’ for post-rift sediments but they do not explain why this should be only partly true. We do not know of any pelagic limestones of Late Jurassic–Early Cretaceous age outside orogens showing oxygen isotope values of – 6 to – 14 ‰. These values can only be the result of metamorphic overprint. In addition, ophicalcites and pelagic limestones consistently show a parallel evolution along the entire transect from northern Graubünden to Val Malenco and all pelagic sediments show re-equilibrated values along it (Fig. 1). We are well aware that the oxygen isotope values are difficult to interpret and that different processes at different times have altered them; however, close correlation with metamorphic grade shows that re-equilibration occurred during Alpine metamorphism. Coltat et al. (2020) distinguish between ‘cooler ophicalcites’ in the South-Penninic nappes and the ophicalcites of the Platta Nappe. However, at other localities like Arosa, the