In situ calcium isotope analysis of Sr-rich carbonates using laser ablation multi-collector inductively coupled plasma mass spectrometry

Abstract

The in situ Ca isotopic composition of carbonates serves as a fundamental tool for tracing geological and biological processes. However, doubly charged Sr ions pose significant interference challenges in Ca isotope measurement using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). This study reports a method established for high-precision Ca isotope microanalysis in Sr-rich carbonates. Instrumental parameters including gas flow, torch position and laser settings were optimized to minimize the yield of doubly charged Sr ions. The two key factors involved in the correction strategy for Sr²⁺ interference are the true ⁸⁷Sr/⁸⁶Sr ratio and the mass fractionation coefficient of Sr²⁺. The accuracy required for the true Sr isotope ratios of carbonates for interference correction depends on the Sr/Ca ratio, e.g., a variation in ⁸⁷Sr/⁸⁶Sr of 0.005 (SD) can lead to a deviation in δ⁴⁴/⁴²Ca_915a of approximately 0.1‰ for samples with ⁸⁷Sr²⁺/⁴⁴Ca⁺ of 10⁻³. The fractionation coefficients for Sr²⁺ and Sr⁺ were found to differ, and adopting f⁺_Sr in the correction results in a deviation of δ^44/42Ca_915a up to 0.42‰ for calcite with a Sr/Ca ratio of 0.057. Utilizing the iteratively calculated Ca⁺ fractionation coefficient improved the accuracy and precision of Ca isotope microanalysis. The resulting in situ Ca isotopic compositions of dolomite and calcite with Sr/Ca ratios up to 0.057 were consistent with those obtained via SN-MC-ICP-MS, with precisions of δ⁴⁴/⁴²Ca_915a and δ⁴³/⁴²Ca_915a in the ranges of 0.10–0.19‰ and 0.09–0.12‰ (2SD). The method was further validated through microanalysis of calcite from the Miaoya carbonatite-associated REE-Nb deposit, revealing distinct isotopic signatures indicative of magmatic-hydrothermal evolution.