Technical note: Studying lithium metaborate fluxes and extraction protocols with a new, fully automated in situ cosmogenic 14C processing system at PRIME Lab

Abstract

Extraction procedures for in situ cosmogenic 14C (in situ 14C) from quartz require quantitative isotopic yields while maintaining scrupulous isolation from atmospheric and organic 14C. These time- and labor-intensive procedures are ripe for automation; unfortunately, our original automated in situ 14C 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 (LiBO2) flux to dissolve the quartz sample in an ultra-high-purity oxygen atmosphere, after a lower-temperature combustion step to remove atmospheric and organic 14C. We compared single-use high-purity Al2O3 against reusable 90 %Pt = 10 %Rh (Pt=Rh) sample combustion boats. The Pt=Rh boats heat more evenly than the Al2O3, reducing procedural blank levels and variability for a given LiBO2 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)×104 14C atoms. We also analyzed the CRONUS-A intercomparison material to investigate sensitivity of extracted 14C 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 14C 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.