Calcite TLM and LSJ07 – two natural low-Sr reference materials for microbeam Sr isotope analysis

Abstract

In situ microbeam Sr isotope analysis relies on the microanalytical homogeneity of reference materials (RMs) for calibration, method validation and data quality control. We present two new natural low-Sr (<500 μg g⁻¹) calcite RMs, TLM and LSJ07, for in situ Sr isotope analysis by LA-MC-ICP-MS. μ-XRF images show that the two RMs are roughly homogeneous with respect to Ca, Mg, and Sr, except for Sr growth zonation in LSJ07. LA-ICP-MS analyses show that both materials have low Rb/Sr ratios (<0.0002), low heavy rare earth element contents and low Sr contents (TLM = ∼100 μg g⁻¹ and LSJ07 = ∼200 μg g⁻¹). The laser parameters were investigated for Sr isotope analysis of low-Sr samples (<500 μg g⁻¹). Using modern instruments (e.g., Thermo Scientific Neptune Plus), the internal precision (2SE) for samples with Sr contents of ∼100 μg g⁻¹ can be better than 0.00010 (0.15%) with a laser spot size of 150 μm, ablation frequency of 15 Hz, and energy density of 2.0 J cm⁻². TLM and LSJ07 have homogeneous Sr isotope ratios, as shown by a large number of LA-MC-ICP-MS spot analyses of each RM. Multiple LA-MC-ICP-MS spot Sr isotope analyses of different grains yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.70969 ± 0.00023 (2SD, n = 219) and 0.71006 ± 0.00015 (2SD, n = 219) for TLM and LSJ07 respectively, which are consistent with the results measured by traditional TIMS and SN-MC-ICP-MS bulk isotope analyses, yielding a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.70970 ± 0.00003 (2SD, n = 8) for TLM and 0.71004 ± 0.00003 (2SD, n = 8) for LSJ07. These RMs are useful additions to the previously distributed calcite RMs for microbeam Sr isotope analysis (e.g., NanoSr and MNP), practically for low-Sr samples (<500 μg g⁻¹).