Effects of oxygen plasma ashing treatment on carbonate clumped isotopes

For clumped isotope analysis (Δ₄₇ ), hydrocarbon and organic molecules present an important contaminant that cannot always be removed by CO₂ purification through a Porapak-Q trap. Low-temperature oxygen plasma ashing (OPA) is a quick and easy approach for treatment; however, the impact of this treatment on the original carbonate clumped isotope values has never been fully studied.

We tested the isotopic impact of OPA using three natural samples with a large range of initial Δ₄₇ values. Crushed and sieved (125 μm mesh) samples were placed into a Henniker Plasma HPT-100 plasma system and treated at a flow rate of 46 mL/min and a power of 100 W at a vacuum of 0.2 mbar for 10, 20, 30 and 60 min before clumped isotope analysis using two MAT 253 isotope ratio mass spectrometers modified to measure masses 44-49.

OPA treatment for 30 min or more on calcite powder samples has the potential to alter the clumped isotopic composition of the samples beyond analytical error. A systematic positive offset is observed in all samples. The magnitude of this alteration translates to a temperature offset from known values ranging from 4°C to 13°C. We postulate that the observed positive offset in Δ₄₇ occurs because the bonds within lighter isotopologues are preferentially broken by plasma treatment, leading to an artificial increase in the 'clumping' value of the sample.

We recommend that any laboratory performing OPA treatments should reduce the runs to 10-20 min or carry out successive runs of 10 min followed by sample stirring, as this procedure showed no alteration in the initial Δ₄₇ values. Our results validate the use of OPA for clumped isotope applications and will allow future research to use clumped isotopes for challenging samples such as oil-stained carbonates, bituminous shales or host rocks with very high organic carbon content.