Calcium Isotopic Fractionation and Compositions of Geochemical Reference Materials

High-precision calcium isotopic compositions of a set of geological reference materials from the IAG (OU-6), ANRT (UB-N), MPI-DING, USGS and GSJ, relative to NIST SRM 915a, are reported here. Measurements were performed by thermal ionisation mass spectrometry (Triton instrument) using a Ca-42-Ca-43 double spike. Ca-44/40 values of selected reference materials, mainly felsic rocks, are reported for the first time. Felsic rock values of Ca-44/40 ranged from 0.13 parts per thousand to 1.17 parts per thousand, probably implying Ca isotopic fractionation could occur during magma evolution. Ca-44/40 values of ultramafic rocks, ranging from 0.74 parts per thousand to 1.51 parts per thousand, were positively correlated with MgO and negatively with CaO contents, possibly owing to Ca isotopic fractionation during partial melting. Ca-44/40 of intermediate-mafic rocks were around 0.78 parts per thousand and displayed limited variation, suggesting Ca isotopic fractionation is insignificant during magma evolution processes. As expected, Ca-44/40 of sedimentary and metamorphic rocks varied widely due to complex geological processes.