9:30 AM - 9:45 AM
[MTT28-03] A new high-precision method for determining stable chlorine isotopes using UV-LA-MC-ICP-MS
Keywords:stable chlorine isotopes, UV-LA-MC-ICP-MS
δ37Cl = [(37Cl/ 35Cl)sample / (37Cl/ 35Cl)SMOC – 1] × 1000. (Eggenkamp 2014)
Their large relative mass difference suggests that these isotopes should fractionate during reactions between aqueous chloride solutions and solid phases in which Cl is at least partly covalently bound. Taken together, the δ37Cl also may provide a good tracer of subducted materials into the mantle. Measurement of chlorine isotope ratios has been performed by the gas source isotope ratio mass spectrometry (IRMS) (Kaufmann et al., 1984), the positive and negative thermal ionization mass spectrometry (P-TIMS / N-TIMS) (Xiao and Zhang 1992; Desaulniers et al., 1986) and the inductively coupled plasma mass spectrometry (ICPMS) (Fietzke et al., 2008).
We report a new rapid high-precision determination method of chlorine isotope ratios in halite and AgCl pellet formed from seawater and igneous rock samples. Use of a 266 nm ultra violet–femtosecond laser ablation (UV-FsLA) provided quantitative sampling of halite and AgCl, and enabled precise determination of 37Cl/35Cl isotope ratios (δ37Cl) coupled with a multiple Faraday collector–inductively coupled plasma mass spectrometer (MFC-ICPMS). We used 36Ar+/38Ar+ as an external standard for the mass bias corrections between 39K+–41K+, 36Ar1H+–38Ar1H+–40Ar1H+, and 35Cl+–37Cl+ with isobaric overlap corrections between K+, ArH+, and Cl+ ions. Sulfur (36S+) isobar on 36Ar+ was indirectly monitored and corrected by a baseline modelling using 36Ar+/38Ar+ measurement. Those combined to accomplish an accurate and high-precision measurement of 37Cl+/35Cl+ ratios. Using the new analytical protocol, δ37Cl in the natural halite samples were analyzed by direct laser ablation. The δ37Cl in igneous rocks were analyzed by AgCl powder pellets produced after pyrohydrolysis separation and co-precipitation of the separated Cl with silver. The external reproducibility of δ37Cl measurement was ±0.2 ‰ 2SD (2-standard deviation) for halite and ±0.3 ‰ 2SD for AgCl rivaling to the gas source isotope ratio mass spectrometry. The new analytical protocol enabled a precise and rapid δ37Cl analysis of igneous rock samples from AgCl with as small as 4 µg chlorine. This is also the first report analyzing a high-precision δ37Cl in situ from halite of the same amount of chlorine.