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

Abstract

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 20Ne from doubly charged interfering 40Ar, 1H19F, 1H218O and partial separation of the 21Ne peak from interfering 20Ne1H. Because of the high mass resolving power, 21Ne can be measured, essentially without interference from 20Ne1H. This capability provides an important opportunity to re-evaluate the relative 21Ne abundance in the atmosphere. Our analyses demonstrate that 20Ne1H contributes approximately 2% to previously determined atmospheric 21Ne relative abundance values. We calculate a new atmospheric 21Ne/20Ne ratio of 0.002905±0.000003 relative to an atmospheric 22Ne/20Ne ratio of 0.102; this new value is distinctly lower than the current IUPAC recommended 21Ne/20Ne 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 21Ne surface exposure ages, which involve the calculation of 21Ne concentrations from excess 21Ne, relative to the atmospheric 21Ne/20Ne ratio. For young samples, where cosmogenic 21Ne contents are small and the 21Ne/20Ne ratio is close to the atmospheric value, the revised value could increase cosmogenic 21Ne ages significantly.