Calcium isotope measurement by combined HR-MC-ICPMS and TIMS

Abstract

We report a novel approach for the chemical purification of Ca from silicate rocks by ion-exchange chromatography, and a highly-precise method for the isotopic analysis of Ca—including the smallest isotope ⁴⁶Ca (0.003%)—by high-resolution multiple collector inductively coupled plasma source mass spectrometry (HR-MC-ICPMS), in combination with thermal ionization mass spectrometry (TIMS). Using this approach, we measured the Ca isotope composition of a number of terrestrial rock standards and seawater. Based on these data, we show that the non-mass-dependent abundances of μ⁴³Ca, μ⁴⁶Ca, and μ⁴⁸Ca (normalized to ⁴²Ca/⁴⁴Ca) can be measured with an external reproducibility of 1.8, 45 and 12.5 ppm, respectively, when measured by HR-MC-ICPMS and μ⁴⁰Ca and μ⁴³Ca to 80 and 23 ppm, respectively, when measured by TIMS (μ notation is the per 10⁶ deviation from the reference material). Comparison with earlier studies demonstrate that it is possible to measure the mass-dependent Ca isotope composition of terrestrial materials using HR-MC-ICPMS with an external reproducibility comparable to that typically obtained with double spike TIMS techniques. The resolution of the mass-independent ⁴³Ca, ⁴⁶Ca and ⁴⁸Ca data obtained by HR-MC-ICPMS represents more than a 45-, 120-, and 18-fold improvement, respectively, relative to earlier measurements obtained by TIMS. This improvement allows for a better understanding of the mass fractionation laws responsible for the mass-dependent fractionation of Ca present in natural samples and synthetic standards. For example, the presence of an apparent excess of ∼60 ppm in the μ⁴⁸Ca composition of the SRM 915a suggests that equilibrium fractionation processes have generated the mass-dependent fractionation of this material. In contrast, the absence of residual anomalies in the mass-independent composition of seawater implies that biogenic and inorganic processes of carbonate formation fractionate Ca kinetically from seawater. Finally, we note that SRM 915b has a mass-dependent and mass-independent Ca isotope composition that is within the resolution of our method identical to that of bulk silicate Earth (BSE). This observation, together with the potential heterogeneity in the ⁴⁰Ca composition of the SRM 915a inferred from our measurements, suggests that the SRM 915b is a better reference material to study the Ca isotope composition of terrestrial and non-terrestrial materials.