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 epsilon and delta values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and Ca-43-Ca-48 double-spiked measurements using a customised Triton PlusTIMS. The mean of twelve measurement results gave epsilon Ca-40/44 values within +/- 1.1, except for GSP-2 that had epsilon Ca-40/44 = 4.04 +/- 0.15 (2SE). Significant radiogenic Ca-40 enrichment was evident in some high K/Ca samples. At anuncertainty level of +/- 0.6, all reference materials had the same epsilon Ca-43/44 and epsilon Ca-48/44 values. We suggest the use of delta Ca-44/42 to report mass-dependent Ca isotopic compositions. The precision under intermediate measurement conditions for delta Ca-44/42 over eight months in our laboratory was +/- 0.03 parts per thousand (with n >= 8 repeat measurements). Measured igneous reference materials gave delta Ca-44/42 values ranging from 0.27 parts per thousand to 0.54 parts per thousand. Significant Ca isotope fractionation may occurduring magmatic and metasomatism processes. Studied reference materials with higher (Dy-n/Yb-n) tend to have lower delta Ca-44/42, implying a potential role of garnet in producing magmas with low delta Ca-44/42. Sandstone GBW07106 had a delta Ca-44/42 value of 0.22 parts per thousand, lower than all igneous rocks studied so far.