Calcium Isotopic Compositions of Sixteen USGS Reference Materials

Calcium isotopic compositions of sixteen Ca-bearing USGS geological reference materials including igneous and sedimentary rocks are reported. Calcium isotopic compositions were determined in two laboratories (GPMR, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan; and CIG, Centre for Isotope Geochemistry, University of California, Berkeley) using the Ca-42-Ca-48 double-spike technique by thermal ionisation mass spectrometry. As opposed to common cation exchange resin, a micro-column filled with Ca-selective resin (DGA resin) was used in order to achieve high recovery (>96%) and efficient separation of Ca from the sample matrix. The intermediate measurement precision was evaluated at 0.14 parts per thousand (2s) for delta Ca-44/40(SRM915a) at GPMR, based on replicate measurements of pure Ca reference material NIST SRM 915a, NIST SRM 915b and seawater. Overall, the measurement uncertainties in both laboratories were better than 0.15 parts per thousand at the 2s level. Result validation was carried out for all available data sets. The Ca isotopic compositions of USGS reference materials are not only in agreement between GPMR and CIG, but also in agreement with previously published data within quoted uncertainties. The comprehensive data set reported in this study serves as a reference for both quality assurance and interlaboratory comparison of high precision Ca isotopic study.