Comparative study of ²²²Rn, ⁴⁰Ar, ³⁹Ar and ³⁷Ar leakage from rocks and minerals: implications for the role of nanopores in gas transport through natural silicates

Abstract

We have measured the leakage of radiogenic ²²²Rn and ⁴⁰Ar and reactor-produced ³⁹Ar and ³⁷Ar in samples of granites, plagioclase, orthoclase, cleavelandite and hornblende to assess the role of nanopores in gas transport through them. The ³⁹Ar and ³⁷Ar were produced by fast neutron irradiation of the samples in vacuum sealed ampoules. The leakage of all three Ar isotopes was barely measurable. In contrast, the leakage of ²²²Rn ranged between 1.45-18.1% orders of magnitude higher than that of the Ar isotopes. The extremely low Ar isotope leakage suggests that the samples studied are not permeated by extensive nanopore networks which intersect grain surfaces. The relatively high ²²²Rn loss from these samples, therefore, has to be attributed to preferential enrichment of ²²⁶Ra (238U) on grain surfaces or to the existence of U-Th accessory minerals in or around grain boundaries from which substantial quantities of ²²²Rn escape.