A Laser-Laser Method for In Situ C and O Isotope Measurements in Carbonate Materials

Abstract

A portable device was developed to produce CO₂ through laser-induced calcination of carbonates that can be paired with a commercial field-deployable isotope ratio mass spectrometer. This new set-up allows for direct measurements of carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios of carbonates under field conditions. The set-up was designed with a pistol-form handset and backpack for ease of handling by a single operator in the field. We demonstrate how the sampling prototype circumvents the steps of sample collection, grinding, drying and acid digestion by directly producing CO₂ from a raw rock sample in conditions that are representative of fieldwork. The CO₂ evolved using our new device was isotopically measured off-site, using an isotope-ratio infrared spectrometer and the results were compared with those from a classical preparation that used acid digestion. Different ways to achieve the necessary threshold of CO₂ concentration for subsequent isotope measurements are discussed. Overall, we demonstrate that this laser portable calcination device allows for isotope measurements after a minimal conversion of 32 mg of carbonate material with an uncertainty at 1s of 0.41 and 0.68‰ for δ¹³C and δ¹⁸O, respectively. The comparison with measurements performed after acid digestion shows that the uncertainty in the preparation technique is 0.01 and 0.03‰ for δ¹³C and δ¹⁸O, respectively. Overall, the laser-induced calcination portable device offers an avenue for fast, accurate and remote measurements of carbonate isotope ratios, but field specific challenges such as the water content of samples also need to be considered.