Imprint of chondrule formation on the K and Rb isotopic compositions of carbonaceous meteorites.

Chondrites display isotopic variations for moderately volatile elements, the origin of which is uncertain and could have involved evaporation/condensation processes in the protoplanetary disk, incomplete mixing of the products of stellar nucleosynthesis, or aqueous alteration on parent bodies. Here, we report high-precision K and Rb isotopic data of carbonaceous chondrites, providing new insights into the cause of these isotopic variations. We find that the K and Rb isotopic compositions of carbonaceous chondrites correlate with their abundance depletions, the fractions of matrix material, and previously measured Te and Zn isotopic compositions. These correlations are best explained by the variable contribution of chondrules that experienced incomplete condensation from a supersaturated medium. From the data, we calculate an average chondrule cooling rate of ~560 ± 180 K/hour, which agrees with values constrained from chondrule textures and could be produced in shocks induced by nebular gravitational instability or motion of large planetesimals through the nebula.

Fast and precise boron isotopic analysis of carbonates and seawater using Nu Plasma II multi-collector inductively coupled plasma mass spectrometry and a simple sample introduction system.

RATIONALE: The boron (B) isotopic composition in marine carbonates provides important insights into paleoclimate reconstruction and biomineralization. However, precise and accurate measurements of B isotopes using plasma-based mass spectrometry is difficult due to the volatile nature of B, which typically requires complex and more specialized sample introduction systems. Existing analytical protocols have mostly been based on Thermo Scientific Neptune Plus multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) instruments, whereas methods based on Nu Plasma mass spectrometers are scarce. METHODS: We have developed a simplified analytical protocol using Nu Plasma II MC-ICPMS with standard glass sample introduction systems. Boron extraction and purification were conducted using a two-stage column chemistry with cation-exchange and Amberlite IRA 743 B-specific resin. A sample drying step was avoided, which allows for direct isotopic analysis after column chemistry. A wet plasma mode with a standard glass cyclonic spray chamber and a glass nebulizer was used instead of a more specialized perfluoroalkoxy (PFA) sample introduction system. Low residual B signals were achieved with a relatively short period of wash-out with 0.5 N HNO3 . RESULTS: The external precision is better than 0.30‰ (2SD) calculated from the long-term bracketing standard, NIST SRM 951a. The overall robustness of the method was demonstrated by measurements of the international carbonate standard JCp-1 (δ11 B = +24.49 ± 0.36‰, 2SD) and seawater (δ11 B = +39.98 ± 0.35‰), which are consistent with reported values. CONCLUSIONS: Our method provides an alternative approach for B isotope analysis using a routine wet plasma MC-ICPMS setup that can facilitate geochemical and environmental application of B isotopes.