Calcium Isotopic Compositions of Sixteen <scp>USGS</scp> Reference Materials

Abstract

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 42 Ca‐ 48 Ca 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 (&gt; 96%) and efficient separation of Ca from the sample matrix. The intermediate measurement precision was evaluated at 0.14‰ (2 s ) for δ 44/40 Ca 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‰ at the 2 s 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.