Atmospheric 21Ne abundance determined by the Helix-MC Plus mass spectrometer

Abstract:

Analyses of noble gas isotopes by multi-collector, high resolution 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 equipped with unique high mass resolution collectors [mass resolution (MR): ~1,800 and mass resolving power (MRP): ~8,000], including fixed axial (Ax), adjustable high mass (H2) and adjustable low mass (L2) detectors. The high mass resolution of the L2, Ax and H2 collectors permits complete separation of ²⁰Ne (measured on L2 detector) from doubly charged interfering ⁴⁰Ar (required MR of 1,777), ¹H¹⁹F (MR = 1450), ¹H₂¹⁸O (MR = 894) and partial separation of the ²¹Ne peak (on Ax detector) from interfering ²⁰Ne¹H (MR = 3,271), and ²²Ne (on H2 detector) from interfering doubly charged CO₂ (MR = 6,231). Because of the high MRP of ~8,000, ²¹Ne can be measured, essentially without interference from ²⁰Ne¹H, by setting the magnet position on a ²⁰Ne¹H interference-free position. This capability provides an important opportunity to re-evaluate the ²¹Ne abundance in the atmosphere. Our analyses demonstrate that ²⁰Ne¹H contributes ~4% to atmospheric ²¹Ne measurements, with the corresponding production ratio of ²⁰Ne¹H to ²⁰Ne being ~1E-4. We calculate a new atmospheric ²¹Ne/²⁰Ne ratio of 0.00287 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. There are several significant implications ensuing from the newly determined atmospheric ²¹Ne abundance. For example, in the area of Earth sciences the most 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 by ~30%.