For anyone else working with LA-MC-ICP-MS to measure radiogenic strontium ratios (87/86), this might be useful. The NanoSr microanalytical calcium carbonate reference material is available on request from Michael Weber, the first author of the paper here: https://doi.org/10.1111/ggr.12296
I have used this pressed nanopowder for the last couple of years for sample-standard bracketing and found it comparable in effectiveness to other nanopowders like JCp-1NP (https://www.my-standards.com/en/products/carbonates/jcp-1-np-sold-out/).
The .json file I have attached uses the bulk solution ICP-MS and MC-ICP-MS values from Weber's paper, not the laser ablation values. It includes the 87/86Sr ratio and concentrations for Rb, Sr, Ba, Dy, Er, and Yb.