Oxygen isotopes in ophicalcites: an ever-lasting controversy?

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.

Equilibrium Fractionation of Oxygen Isotopes in the U3O8–Atmospheric Oxygen System

“…The fractionation of oxygen isotopes ( 16 O, 17 O, and 18 O) among phases is an important measure applied in many scientific fields, such as geochemistry, climate reconstruction, , and high-temperature magmatic and metamorphic processes, − to trace chemical reactions and environmental conditions. In the last 2 decades, several studies have investigated the use of oxygen isotopic composition in various uranium oxides as a signature for nuclear forensics applications. − Triuranium octoxide (U 3 O 8 ), the most stable form of uranium oxide, is a key compound in the uranium nuclear fuel cycle.…”

Successive phases of serpentinization and carbonation recorded in the Sivas ophiolite (Turkey), from oceanic crust accretion to post-obduction alteration

“…After applying Equation [2], the carbonation phases in the studied samples likely occurred at temperatures ranging from 25°C to 100°C. This temperature range suggests that no isotopic resetting occurred, otherwise all temperature values would depend directly on the degree of metamorphism as interpreted by Bernoulli and Weissert (2021). Moreover, no zonation in δ 18 O was observed in calcite grains.…”

Fossil rifted margins in the Alps

“…However, to what extent Alpine metamorphism has overprinted pre-Alpine events is a matter of ongoing debate. For instance, while Bernoulli and Weissert (2021) argue that the metamorphic facies record only the Alpine orogeny, Coltat et al (2021) claim that the rocks of the Platta Nappe preserve the record of pre-Alpine, rift-related thermal events. The Tasna OCT has been largely used as a field analogue for the most distal part of magma-poor rifted margins (e.g., Hölker et al, 2002;Manatschal, 2004;Reston and McDermott, 2011).…”