³⁹K(n,p)-Produced ³⁹Ar in Chondrites: New Data and Their Interpretation in Terms of Size, Exposure Age and Orbital Elements

Abstract

The ³⁹Ar in stone meteorites consists of two components: one produced by high-energy cosmic-ray particles (E≥200 MeV) via spallation reactions, mainly on iron and nickel, and one produced by MeV neutrons via the ³⁹K(n,p) reaction. As the total ³⁹Ar in different chondrites varies by about a factor of 2, and it has been shown that this is not (or only to a minor extent) due to a variation of the spallation-produced component or a variation of the potassium content, the fluxes of fast neutrons in different meteoroids must have been different by at least a factor of ten. To study in a systematic way which of the possible reasons is responsible for the different fluxes, about 20 chondrites have been analysed for their ³⁹Ar content.

• (1) For both H- and L-group chondrites there is no correlation between 39Ar and recovered mass.

• (2) The 39Ar in H-group chondrites is definitely correlated with the cosmic ray exposure age: Meteorites with short exposure ages are high in 39Ar. This correlation is entirely due to the 39K(n, p)- produced component.

• (3) In H-group chondrites the (n, p)-produced 39Ar is also correlated with the 3He/21Ne ratios: A low ratio corresponds to a high activity.

• (4) For L-group chondrites none of these correlations have been found so far.

Tentative explanations for the various correlations, as well as for their absence, are proposed. It is argued that the (n, p) production of ³⁹Ar is mainly due to secondary neutrons from solar protons. Hence, the variations in this component of the ³⁹Ar reflect differences in the orbital elements of the meteoroids.