High-Precision Barium Isotope Measurements of Carbonates by MC-ICP-MS

This study presents a high-precision method to measure barium (Ba) isotope compositions of international carbonate reference materials and natural carbonates. Barium was purified using chromatographic columns filled with cation exchange resin (AG50W-X12, 200-400 mesh). Barium isotopes were measured by MC-ICP-MS, using a Ba-135-Ba-136 double-spike to correct mass-dependent fractionation during purification and instrumental measurement. The precision and accuracy were monitored by measuring Ba isotope compositions of the reference material JCp-1 (coral) and a synthetic solution obtained by mixing NIST SRM 3104a with other matrix elements. The mean Ba-137/134 values of JCp-1 and the synthetic solution relative to NIST SRM 3104a were 0.21 +/- 0.03 parts per thousand (2s, n=16) and 0.02 +/- 0.03 parts per thousand (2s, n=6), respectively. Replicate measurements of NIST SRM 915b, COQ-1, natural coral and stalagmite samples gave average Ba-137/134 values of 0.10 +/- 0.04 parts per thousand (2s, n=18), 0.08 +/- 0.04 parts per thousand (2s, n=20), 0.27 +/- 0.04 parts per thousand (2s, n=16) and 0.04 +/- 0.03 parts per thousand (2s, n=20), respectively. Barium mass fractions and Ba isotopes of subsamples drilled from one stalagmite profile were also measured. Although Ba mass fractions varied significantly along the profile, Ba isotope signatures were homogeneous, indicating that Ba isotope compositions of stalagmites could be a potential tool (inaddition to Ba mass fractions) to constrain the source of Ba in carbonate rocks and minerals.