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- Title
Technical note: Studying lithium metaborate fluxes and extraction protocols with a new, fully automated in situ cosmogenic 14C processing system at PRIME Lab.
- Authors
Lifton, Nathaniel; Wilson, Jim; Koester, Allie
- Abstract
Extraction procedures for in situ cosmogenic 14 C (in situ 14 C) from quartz require quantitative isotopic yields while maintaining scrupulous isolation from atmospheric and organic 14 C. These time- and labor-intensive procedures are ripe for automation; unfortunately, our original automated in situ 14 C extraction and purification systems, reconfigured and retrofitted from our original systems at the University of Arizona, proved less reliable than hoped. We therefore installed a fully automated stainless-steel system (except for specific borosilicate glass or fused-silica components) incorporating more reliable valves and improved actuator designs, along with a more robust liquid nitrogen distribution system. As with earlier versions, the new system uses a degassed lithium metaborate (LiBO 2) flux to dissolve the quartz sample in an ultra-high-purity oxygen atmosphere, after a lower-temperature combustion step to remove atmospheric and organic 14 C. We compared single-use high-purity Al 2 O 3 against reusable 90 %Pt / 10 %Rh (Pt/Rh) sample combustion boats. The Pt/Rh boats heat more evenly than the Al 2 O 3 , reducing procedural blank levels and variability for a given LiBO 2 flux. This lower blank variability also allowed us to trace progressively increasing blanks to specific batches of fluxes from our original manufacturer. Switching to a new manufacturer returned our blanks to consistently low levels on the order of (3.4 ± 0.9) × 10 4 14 C atoms. We also analyzed the CRONUS-A intercomparison material to investigate sensitivity of extracted 14 C concentrations to the temperature and duration of the combustion and extraction steps. Results indicate that 1 h combustion steps at either 500 or 600 ∘ C yield results consistent with the consensus value of Jull et al. (2015), while 2 h at 600 ∘ C results in loss of ca. 9 % of the high-temperature 14 C inventory. Results for 3 h extractions at temperatures ranging from 1050 to 1120 ∘ C and 4.5 h at 1000 ∘ C yielded similar results that agreed with the nominal value and published results from most laboratories. On the other hand, an extraction for 3 h at 1000 ∘ C was judged to be incomplete due to a significantly lower measured concentration. Based on these results, our preferred technique is now combustion for 1 h at 500 ∘ C followed by a 3 h extraction at 1050 ∘ C. Initial analyses of the CoQtz-N intercomparison material at our lab yielded concentrations ca. 60 % lower than those of CRONUS-A, but more analyses of this material from this and other labs are clearly needed to establish a consensus value.
- Subjects
UNIVERSITY of Arizona; LITHIUM; CHLOROGENIC acid; ALUMINUM oxide; QUARTZ; ATMOSPHERIC oxygen; BOROSILICATES
- Publication
Geochronology, 2023, Vol 5, Issue 2, p361
- ISSN
2628-3697
- Publication type
Article
- DOI
10.5194/gchron-5-361-2023