Two potential natural calcite reference materials for laser in situ Sr isotope analysis†

Calcite is one of the common rock-forming minerals, and its Sr isotopic composition provides useful information for revealing various geological processes. For laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analytical techniques, accurate and reliable Sr isotope data rely on high-quality calcite reference materials (RMs). However, calcite RMs with homogeneous composition and structure are rare, especially natural solid calcite RMs, making them inadequate to meet the needs of various applications within the scientific community. Here we identified two new natural calcite reference materials, HZZ-2 and TARIM, for laser in situ Sr isotope analysis. These two RMs exhibit uniform Ca and Sr contents, as confirmed by μ-XRF component analysis. EPMA and LA-ICP-MS methods indicate that they have homogeneous major and trace element compositions, both showing exceptionally low Rb/Sr ratios (<0.00001) and relatively moderate Sr contents (HZZ-2: ∼1100 μg g⁻¹ and TARIM: ∼620 μg g⁻¹). Bulk isotope analyses by TIMS yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.70941 ± 0.00001 (2s, n = 8) for HZZ-2 and 0.71042 ± 0.00001 (2s, n = 7) for TARIM. Multiple LA-MC-ICP-MS spot Sr isotope analyses on random pieces of HZZ-2 and TARIM yielded mean ⁸⁷Sr/⁸⁶Sr ratios of 0.70940 ± 0.00003 (2s, n = 318) and 0.71042 ± 0.00002 (2s, n = 305), respectively, consistent with the TIMS results. These results suggest that the HZZ-2 and TARIM calcite samples can be used as potential reference materials for determining high or low Sr samples by laser in situ Sr isotope analysis.