Redetermination of the Ne-21 relative abundance of the atmosphere, using a high resolution, multi-collector noble gas mass spectrometer (HELIX-MC Plus)

摘要

Analyses of noble gas isotopes by high-resolution, multi-collector mass spectrometry have the potential to revolutionise applications in the cosmo-geo-sciences. The HELIX-MC Plus noble gas mass spectrometer installed at the Australian National University (ANU) is uniquely equipped with three high mass resolution collectors, which permits complete separation of Ne-20 from doubly charged interfering Ar-40, (HF)-H-1-F-19, (H2O)-H-1-O-18 and partial separation of the Ne-21 peak from interfering (NeH)-Ne-20-H-1. Because of the high mass resolving power, Ne-21 can be measured, essentially without interference from Ne-20(1) H. This capability provides an important opportunity to re-evaluate the relative Ne-21 abundance in the atmosphere. Our analyses demonstrate that (NeH)-Ne-20-H-1 contributes approximately 2% to previously determined atmospheric Ne-21 relative abundance values. We calculate a new atmospheric Ne-21/Ne-20 ratio of 0.002905 +/- 0.000003 relative to an atmospheric Ne-22/Ne-20 ratio of 0.102; this new value is distinctly lower than the current IUPAC recommended Ne-21/Ne-20 value of 0.00298 +/- 0.00011. There are several significant implications ensuing from the newly determined value. For example, in the Earth sciences, a critical issue relates to cosmogenic Ne-21 surface exposure ages, which involve the calculation of Ne-21 concentrations from excess Ne-21, relative to the atmospheric Ne-21/Ne-20 ratio. For young samples, where cosmogenic Ne-21 contents are small and the Ne-21/Ne-20 ratio is close to the atmospheric value, the revised value could increase cosmogenic Ne-21 ages significantly. (C) 2015 Elsevier B.V. All rights reserved.