Calcium Isotopes (δ^44/40Ca) in MPI‐DING Reference Glasses, USGS Rock Powders and Various Rocks: Evidence for Ca Isotope Fractionation in Terrestrial Silicates

Abstract

We report δ44/40Ca(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 43Ca/48Ca‐ and 42Ca/43Ca‐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‰ at the 95% confidence level. The impact of different preparation methods on the δ44/40Ca(SRM 915a) values was found to be negligible. Except for ML3‐B, the original powders and the respective MPI‐DING glasses showed identical δ44/40Ca(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 δ44/40Ca(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‰) and the peridotite PCC‐1 (1.14 ± 0.07‰) are enriched in 44Ca relative to volcanic rocks (0.8 ± 0.1‰). The Carrara marble (1.32 ± 0.06‰) was also found to be enriched in 44Ca relative to the values of assumed precursor carbonates (< 0.8‰). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.

Calcium isotopic ratios and rare earth element abundances in refractory inclusions from the Allende CV3 chondrite

“…They advocated that the Ca isotopic effects be reported relative to the bulk silicate Earth (BSE). However, several recent studies show that at a 2r level of 0.5 e units, NIST SRM 915a and seawater do not show any measurable radiogenic 40 Ca excess compared to the Earth's mantle (e.g., Amini et al, 2009;Caro et al, 2010; Table 1). Therefore, in this study NIST SRM 915a is used as the standard sample for reporting both mass-dependent and mass-independent Ca isotopic effects (Eqs.…”

δ44/40Ca variability in shallow water carbonates and the impact of submarine groundwater discharge on Ca-cycling in marine environments

“…The latter exhibit such a narrow range of d 44/40 Ca values, however, that it must be the carbonate rocks that control secular changes in d T w . For example, seven analyses of rocks of basaltic to andesitic composition, reported by Amini et al (2009), exhibit a total range of variation of just 0.16& at the 95% level of confidence, and an average d 44/40 Ca value of À1.06&.…”

Calcium isotope ratios in animal and human bone

“…It has been suggested that other dietary calcium inputs may come through the use of soft carbonate millstones for the preparation of flour from grains (Heaney, 2002); however, this again would provide a calcium input with a high 44 Ca/ 42 Ca isotope ratio (Tipper et al, 2008;Amini et al, 2009), so does not provide an explanation for low human d 44/42 Ca values.…”

“…This means that the calcium isotope ratios within a restricted area only should be compared to each other in assessing other effects such as the trophic level. We account for this difference by the small variability in calcium isotope ratios between rock types (Amini et al, 2009;Holmden, 2009), biogeochemical cycling in different ecosystems (see for example Holmden and Belanger, 2010), and differences in rock weathering (Moore et al, 2009).…”

“…These range from studies of rocks, rivers, soils, and vegetation in terrestrial samples (Skulan et al, 1997;Schmitt et al, 2003;Chu et al, 2006;Tipper et al, 2006;Tipper et al, 2008;Jacobson and Holmden, 2008;Ewing et al, 2008;Page et al, 2008;Amini et al, 2009;Cenki-Tok et al, 2009) to marine biogenic and inorganic precipitates (Zhu and Macdougall, 1998;Nägler et al, 2000;Marriott et al, 2004;DePaolo, 2005, 2007;Sime et al, 2005Sime et al, , 2007Farkaš et al, 2007a,b;Kasemann et al, 2008;Griffith et al, 2008;Teichert et al, 2009), and other biological materials such as animal soft tissues (Skulan and DePaolo, 1999;Skulan et al, 2007;Hirata et al, 2008) and bone apatite (Skulan et al, 1997;Skulan and DePaolo, 1999;Clementz et al, 2003;Chu et al, 2006;Reynard et al, 2008a;Heuser and Eisenhauer, 2008;Heuser et al, 2009).…”