Mass-Independent and Mass-Dependent Ca Isotopic Compositions of Thirteen Geological Reference Materials Measured by Thermal Ionisation Mass Spectrometry

We report mass-independent and mass-dependent Ca isotopic compositions for thirteen geological reference materials, including carbonates (NIST SRM 915a and 915b), Atlantic seawater as well as ten rock reference materials ranging from peridotite to sandstone, using traditional e and d values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and Ca-Ca double-spiked measurements using a customised Triton Plus TIMS. The mean of twelve measurement results gave eCa values within ± 1.1, except for GSP2 that had eCa = 4.04 ± 0.15 (2SE). Significant radiogenic Ca enrichment was evident in some high K/Ca samples. At an uncertainty level of ± 0.6, all reference materials had the same eCa and eCa values. We suggest the use of dCa to report mass-dependent Ca isotopic compositions. The precision under intermediate measurement conditions for dCa over eight months in our laboratory was ± 0.03‰ (with n ≥ 8 repeat measurements). Measured igneous reference materials gave dCa values ranging from 0.27‰ to 0.54‰. Significant Ca isotope fractionation may occur during magmatic and metasomatism processes. Studied reference materials with higher (Dyn/Ybn) tend to have lower dCa, implying a potential role of garnet in producing magmas with low dCa. Sandstone GBW07106 had a dCa value of 0.22‰, lower than all igneous rocks studied so far.