Determination of carbon isotopes in carbonates (calcite, dolomite, magnesite, and siderite) by femtosecond laser ablation multi-collector ICP-MS

Abstract

This study reports a method for in situ determination of stable carbon (δ¹³C‰) isotope compositions for calcite, dolomite, magnesite and siderite by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS). The chemical and bulk C isotope compositions of various carbonates have been characterized by LA-ICP-MS and isotope ratio mass spectrometry (IRMS), respectively. Magnesite shows limited chemical variation, whereas the major element compositions of dolomite and siderite vary greatly among samples and individual grains. The spectral interference of doubly charged ions (²⁴Mg²⁺ for ¹²C⁺ and ²⁶Mg²⁺ for ¹³C⁺) and polyatomic species (¹²CH⁺ for ¹³C⁺) was evaluated. The associated internal precision of in situ C isotope analysis correlates with intensities and is better than 0.20‰ (2SE) when the measured ¹²C ion signal is >12.5 volts. Carbonate samples DOL-8, MGS-1 and SD-5 show relatively uniform bulk and in situ carbon isotope compositions with precisions better than 0.24‰ and 0.45‰ (2SD) and are adopted as in-house standards. The laser ablation analysis results of various calcite, dolomite, magnesite and siderite samples with δ¹³C in the range of −6.28‰ to 0.32‰ agree excellently with IRMS determined values using standard-sample bracketing (SSB) calibration, with a precision of 0.37–0.68‰ (2SD). Matrix effects have been investigated and determined to be insignificant for the same carbonate matrix with variable chemical compositions when adopting a femtosecond laser. Instrumental carbon isotope fractionation observed among different carbonate matrices is non-negligible (e.g., up to 4.29‰ between calcite and magnesite) using fs-LA-MC-ICP-MS, and an external multi-reference calibration can be applied for non-matrix matched analyses.