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

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.