Analysis of δ 13C and δ 18O in calcite, dolomite, rhodochrosite and siderite using a laser extraction system

Abstract

δ ¹³C- and δ ¹⁸O-values of calcite, dolomite, rhodochrosite and siderite have been determined in situ using a laser extraction system, in which a focused Nd: YAG laser beam excites a sample surface producing a high-energy plasma cloud which forms CO and CO₂. Laser ionization of calcite, dolomite, rhodochrosite and siderite produces different yields of CO₂ and CO and different CO₂/CO ratios for each mineral. Isotopic analyses of combined CO and CO₂ are reproducible to ± 1–2‰ for δ ¹⁸O and ± 2–3‰ for δ ¹³C and are distinctly different from the isotopic compositions determined using standard acid dissolution techniques. CO₂ produced from laser ionization is consistently enriched in ¹³C and ¹⁸O compared to CO. A two-stage model is proposed to explain variations observed in the experimental results. In the first stage, the energy is absorbed by the mineral but the quantity and isotopic composition of gases depends on the chemical composition of the sample, especially the quantity of transition metals which have electronic transitions commensurate with the wavelength of the Nd: YAG. In the second stage, the relative isotopic compositions of CO and CO₂ are affected by reactions in the cooling plasma. This model should be applicable to all laser ionization systems.