Oxygen isotopes in ophicalcites: an ever-lasting controversy?
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ORIGINAL PAPER
Oxygen isotopes in ophicalcites: an ever‑lasting controversy? Daniel Bernoulli1 · Helmut Weissert1 Received: 26 May 2020 / Accepted: 11 September 2020 © The Author(s) 2020
Abstract Tectono-sedimentary breccias, known as ophicalcites, overlie serpentinised peridotites at a Jurassic ocean–continent transition along the Penninic-Austroalpine transition in the Eastern Alps of Switzerland. Deformation of the exhumed mantle rocks and breccia formation occurred under decreasing temperatures and along low-angle detachment faults exposing the mantle rocks at the sea floor and was coupled with hydrothermal activity and carbonation of the serpentinites at shallow depth and/or at the sea floor. Carbon isotopes in the ophicalcites persistently show marine values; however, the interpretation of oxygen-isotope values remained controversial: are they related to Jurassic hydrothermal activity or do they reflect Alpine metamorphic overprint? Here we discuss recent interpretations that relate oxygen isotope values measured in ophicalcites exclusively to Jurassic hydrothermal activity; to this end we use data that we earlier obtained along a north–south profile across Graubünden (eastern Switzerland). We revisited the sites of controversial interpretation along a north–south profile in eastern Switzerland. Along this profile, oxygen isotope values in ophicalcites and overlying pelagic sediments, up to 25 my younger than the ophicalcites, show identical values and become systematically lower with increasing Alpine metamorphism; they strongly deviate from values in ophicalcites and pelagic sediments measured along the Mid-Atlantic Ridge or ancient Atlantic ocean-continent transitions as e.g. in the Iberia–Newfoundland transect. The oxygen-isotope values measured in Alpine ophicarbonates thus reflect isotopic resetting during the Alpine orogeny, related to fluid-rock interaction during regional metamorphism. Hydrothermal processes that accompanied the formation of ophicalcites are not disputed; however, they cannot be traced by oxygen isotope geochemistry. Keywords Ophicalcite · Oxygen-isotopes · Ocean–continent transition · Jurassic · Alps · Graubünden
Introduction Ever since the pioneering work of Bärtschi (1957), a large amount of data on the stable isotope composition of Alpine carbonate rocks and minerals has been assembled. Isotope data of sedimentary, igneous or metamorphic rocks serve as proxies of past environmental and geochemical conditions, the evolution of the carbon cycle through time, the reconstruction of palaeotemperatures in marine sediments and the identification of the source of magmatic rocks or the composition and origin of fluids acting during diagenesis, and regional and contact metamorphism. * Helmut Weissert [email protected] Daniel Bernoulli [email protected] 1
Department of Earth Sciences, Swiss Federal Institute of Technology (ETH), 8092 Zürich, Switzerland
Several early oxygen isotope studies focused on carbonates derived from Jurassic tectono-sedimentary ophiolit
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