Insights into triple oxygen isotope variations in Precambrian carbonates from CO2 – O2 equilibration

Small mass-dependent variations in 17O/16O and 18O/16O, expressed as Δ‘17O (=ln(δ17O+1) – ln(δ18O+1)*λref), in carbonates provide insight into temperature dependent carbonate-water equilibration. Uncertainties between 10-15 per meg are required to distinguish equilibrium conditions and post-depositional alteration of carbonates. Measuring 17O/16O with conventional methods is hampered by isobaric interferences of the higher abundant 13C16O2. A new methodical approach based on equilibration of sample CO2 and O2 over hot platinum was developed, demonstrating external reproducibility of Δ‘17OCO2 smaller than 10 per meg [1]. Here we present a modified experimental setup of this method. We developed an automatized procedure to minimize external error sources and reproduce experimental conditions. We show that Δ‘17O of the equilibrated O2 is affected by a mass independent fractionation (MIF) during isotope exchange. The MIF is induced by a strong thermal gradient in the reactor and the order of magnitude is coupled to the equilibration temperature. We analyzed a set of platform carbonates from the Proterozoic-Archean transition from the Transvaal Supergroup, South Africa to examine the origin of the secular shift of δ18O in carbonates towards higher values from the Archean to today. The carbonates plot below the curve defined by calcite in equilibrium with seawater, which neither supports hot oceans nor lower δ18O of Precambrian seawater. Modelling diagenetic alteration suggests, that the pristine oxygen isotope composition of carbonates may have been modified after deposition or that the carbonates may last re-equilibrated with (sea)water with slightly higher δ18O and lower Δ‘17O.

[1] Mahata et al. (2013), Anal. Chem., 85, 6894-6901