Calcium Isotopes (δ44/40Ca) in MPI-DING Reference Glasses, USGS Rock Powders and Various Rocks: Evidence for Ca Isotope Fractionation in Terrestrial Silicates

We report delta Ca-44/40((SRM 915a)) values for eight fused MPI-DING glasses and the respective original powders, six USGS igneous rock reference materials, the U-Th disequilibria reference material TML, IAEA-CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM-GEOMAR and Saskatchewan Isotope Laboratory) using Ca-43/Ca-48- and Ca-42/Ca-43-double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2 parts per thousand at the 95% confidence level. The impact of different preparation methods on the delta Ca-44/40((SRM 915a)) values was found to be negligible. Except for ML3-B, the original powders and the respective MPI-DING glasses showed identical delta Ca-44/40((SRM 915a)) values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of delta Ca-44/40((SRM 915a)) values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS-1 (1.49 +/- 0.06 parts per thousand) and the peridotite PCC-1 (1.14 +/- 0.07 parts per thousand) are enriched in Ca-44 relative to volcanic rocks (0.8 +/- 0.1 parts per thousand). The Carrara marble (1.32 +/- 0.06 parts per thousand) was also found to be enriched in Ca-44 relative to the values of assumed precursor carbonates (< 0.8 parts per thousand). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.