Isotopic (δ13C, δ18O) Analysis of Small Amounts of Carbonate in Silicate Rocks by the Continuous Flow Isotope Ratio Mass Spectrometry Method

An experimental study of the main factors affecting the accuracy of oxygen and carbon isotopic analysis in carbonates dispersed in silicate matrix is carried out. Artificial 1, 2, 5, and 10% mixtures of quartz with carbonates with different isotopic parameters (KH-2, Ko, MCA-8) were analyzed by continuous flow isotope ratio mass spectrometry (CF IRMS). It is established that, in addition to the influence of the instrumental nonlinearity, the results are affected by two factors: trace amounts of CO₂, constantly present in the system (the blank effect) and the presence of chemically neutral silicate particles (the matrix effect). The blank effect depends on the isotopic parameters of the sample and has very little influence on the estimated carbonate content in the rock. The matrix effect, on the contrary, strongly affects the estimated carbonate content, and produces the isotopic shift towards underestimated contents of heavy ¹³C and ¹⁸O isotopes. It is shown that this effect is related to the processes occurring near the CO₂–acid–quartz interface, which are accompanied by kinetic fractionation of carbon and oxygen isotopes. Both effects are dependent on the amount of silicate matrix in the system and most clearly manifested during analysis of carbonate-poor rocks. When the carbonate content in the rock is about 1–2%, deviations from the true δ¹³C and δ¹⁸O values can reach the first ppm, while carbonate content obtained by chromatographic peak calibration can be underestimated by 20–40%.