Production of ²¹Ne in depth-profiled olivine from a 54 Ma basalt sequence, Eastern Highlands (37° S), Australia

Abstract

In this study we investigate the cosmogenic neon component in olivine samples from a vertical profile in order to quantify muogenic 21Ne production in this mineral. Samples were collected from an 11 m thick Eocene basalt profile in the Eastern Highlands of southeastern Australia. An eruption age of 54.15 ± 0.36 Ma (2σ) was determined from ⁴⁰Ar/³⁹Ar step-heating experiments (n = 6) on three whole-rock samples. A ³⁶Cl profile on the section indicated an apparent steady state erosion rate of 4.7 ± 0.5 m Ma⁻¹. The eruption age was used to calculate in situ produced radiogenic ⁴He and nucleogenic ³He and ²¹Ne concentrations in olivine. Olivine mineral separates (n = 4), extracted from the upper two metres of the studied profile, reveal cosmogenic ²¹Ne concentrations that attenuate exponentially with depth. However, olivine (Fo₆₈) extracted from below 2 m does not contain discernible ²¹Ne aside from magmatic and nucleogenic components, with the exception of one sample that apparently contained equal proportions of nucleogenic and muogenic neon. Modelling results suggest a muogenic neon sea-level high-latitude production rate of 0.02 ± 0.04 to 0.9 ± 1.3 atoms g⁻¹ a⁻¹(1σ), or <2.5% of spallogenic cosmogenic ²¹Ne production at Earth’s surface. These data support a key implicit assumption in the literature that accumulation of muogenic ²¹Ne in olivine in surface samples is likely to be negligible/minimal compared to spallogenic 21Ne.