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

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.