Effects of Oxygen Plasma Ashing treatment on Carbonate Clumped Isotopes.

Abstract

For clumped isotopes analysis (Δ ), hydrocarbon and organic molecules present an important contaminant that cannot always be removed by CO purification via a poropak trap. Treatment by low-temperature oxygen plasma ashing (OPA) is a quick and easy approach; however, the impact of this treatment on the original carbonate clumped isotopes values has never been fully assessed.We tested the isotopic impact of OPA using three natural samples selected for their large range in initial Δ 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 46mL/min and a power of 100W at a vacuum of 0.2 mbar for 10, 20, 30 and 60 minutes prior to clumped isotope analysis using two MAT 253 isotope ratio mass spectrometers modified to measure masses 44-49.OPA treatment for 30 minutes 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 Δ 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 should shorten the runs to 10 to 20 minutes, or to do successive runs of 10 minutes followed by sample stirring, as this procedure showed no alteration of the initial Δ value. Our results thus validate the use of OPA for clumped isotope applications and will allow future research using clumped isotopes in challenging samples, such as oil-stained carbonates, bituminous shales, or host-rocks with very high organic carbon content.