Effects of oxygen plasma ashing treatment on carbonate clumped isotopes

Rationale:For clumped isotope analysis (Δ47), hydrocarbon and organic moleculespresent an important contaminant that cannot always be removed by CO2purification 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 onthe original carbonate clumped isotope values has never been fully studied.Methods:We tested the isotopic impact of OPA using three natural samples with alarge range of initialΔ47values. Crushed and sieved (125μm mesh) samples wereplaced into a Henniker Plasma HPT-100 plasma system and treated at a flow rate of46 mL/min and a power of 100 W at a vacuum of 0.2 mbar for 10, 20, 30 and60 min before clumped isotope analysis using two MAT 253 isotope ratio massspectrometers modified to measure masses 44–49.Results:OPA treatment for 30 min or more on calcite powder samples has thepotential to alter the clumped isotopic composition of the samples beyond analyticalerror. A systematic positive offset is observed in all samples. The magnitude of thisalteration translates to a temperature offset from known values ranging from 4 Cto13 C. We postulate that the observed positive offset inΔ47occurs because thebonds within lighter isotopologues are preferentially broken by plasma treatment,leading to an artificial increase in the‘clumping’value of the sample.Conclusions:We recommend that any laboratory performing OPA treatments shouldreduce the runs to 10–20 min or carry out successive runs of 10 min followed bysample stirring, as this procedure showed no alteration in the initialΔ47values. Ourresults validate the use of OPA for clumped isotope applications and will allow futureresearch to use clumped isotopes for challenging samples such as oil-stainedcarbonates, bituminous shales or host rocks with very high organic carbon content