Isobar-free neon isotope measurements of flux-fused potential reference minerals on a Helix-MC-Plus mass spectrometer

This work presents new analytical techniques for extraction and analysis of neon from a suite of different mineral phases, including quartz, pyroxene, hematite, apatite, zircon, topaz, and fluorite. Neon was quantitatively extracted at 1100 °C from all of these minerals using an in-vacuum lithium borate-flux fusion technique. Evolved neon was purified using a cryogenic method capable of separating Ne from He present in abundances ~8 orders of magnitude higher, typical of samples carrying nucleogenic/radiogenic noble gases. The purified neon was measured on a Helix-MC-Plus10K mass spectrometer that permits isobar-free measurement of all three neon isotopes. When operated at its highest mass resolving power (MRP) of ~10,300, the shoulder representing solely 22Ne on the low mass-side of the 22Ne-CO2+2 doublet is wide enough to permit measurement of isobar free 22Ne. Operating in this mode comes with the penalty of a 50% reduction in neon sensitivity. Coupled with a mathematical isobar-stripping method, this approach excludes 99.5% of the CO2+2 while still collecting >99% of the 22Ne beam. Routine edge-centering on the dynamic CO2+2 peak prior to introduction of a sample permits rapid and robust relocation of the desired measure point in the mass spectrum. Cosmogenic 21Ne and 22Ne concentrations obtained using these methods on the Cronus-A quartz and Cronus-P pyroxene international reference materials are in excellent agreement with previous work or expectations. Similarly, the concentration of nucleogenic 21Ne and 22Ne in Durango apatite and the CIT hematite standard agree well with previous work. Durango apatite has notable heterogeneity in neon concentrations, consistent with previous observations of heterogeneous He, U and Th concentrations in this apatite. Nucleogenic neon concentrations are also presented for previously unstudied minerals including a Sri Lanka zircon (SLC), a topaz from the Imperial Topaz mine in Brazil (ITP1), and a fluorite (W-90) from New Hampshire. Taken together this set of potential reference minerals and the associated dataset provide a starting point for intercalibration among multiple mineral phases carrying 21Ne and 22Ne of cosmogenic or nucleogenic origin