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 ε and δ values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and 43 Ca‐ 48 Ca double‐spiked measurements using a customised Triton Plus TIMS . The mean of twelve measurement results gave ε 40/44 Ca values within ± 1.1, except for GSP ‐2 that had ε 40/44 Ca = 4.04 ± 0.15 (2 SE ). Significant radiogenic 40 Ca enrichment was evident in some high K/Ca samples. At an uncertainty level of ± 0.6, all reference materials had the same ε 43/44 Ca and ε 48/44 Ca values. We suggest the use of δ 44/42 Ca to report mass‐dependent Ca isotopic compositions. The precision under intermediate measurement conditions for δ 44/42 Ca over eight months in our laboratory was ± 0.03‰ (with n ≥ 8 repeat measurements). Measured igneous reference materials gave δ 44/42 Ca values ranging from 0.27‰ to 0.54‰. Significant Ca isotope fractionation may occur during magmatic and metasomatism processes. Studied reference materials with higher (Dy n /Yb n ) tend to have lower δ 44/42 Ca, implying a potential role of garnet in producing magmas with low δ 44/42 Ca. Sandstone GBW 07106 had a δ 44/42 Ca value of 0.22‰, lower than all igneous rocks studied so far.