Radioactive production and diffusional loss of radiogenic ⁴⁰Ar in clays in relation to its flux to the atmosphere

This paper addresses the release mechanisms of radiogenic ⁴⁰Ar from clays, for the purpose of estimating the ⁴⁰Ar flux from the sedimentary crust to the atmosphere. Clays from sedimentary sequences of Cambrian to Neogene age show discrepancies between their depositional stratigraphic ages and the K-Ar apparent ages or dates of the different size fractions. The possible geochemical mechanisms that may be responsible for the K-Ar dates being either higher or lower than the stratigraphic age are discussed. In general, the known observation of a decrease in the K-Ar apparent age of clays with decreasing particle size is interpreted as due to a faster escape of radiogenic ⁴⁰Ar from the smaller particles. In the younger, Neogene clays, the mass fractions of ⁴⁰Ar lost are in a range from 13 to 34% in the fine-size particles that are smaller by a factor of 10 in their linear dimension than the bigger particles. In the older formation samples, Cambrian to Triassic, the ⁴⁰Ar loss is smaller, from 7 to 19%, for the same particle-size range. This smaller decrease in the ⁴⁰Ar/⁴⁰K ratio in the stratigraphically older samples may be, at least in part, accounted for by the closure of the mineral system after the process of ⁴⁰Ar escape went on for some time. In all sediments, a lowering of the K-Ar apparent age of the different size fractions, mainly in the smaller, is possibly due to authigenesis of mixed-layered illite-smectite and illite as a new generation of particles that result from, and are therefore better adapted to, the new diagenetic environment. In a Neogene basin sediments of depositional age < 18 Ma, the amount of the authigenic illite fraction increases with an increase of its crystallization age from about 22 to 38%. If the rate of new crystallization is linear with time, this change corresponds to about 1.5%/Ma of authigenic particles added to the smaller-size fractions. As an overall model, we present a mechanism that explains the lowering of the ⁴⁰Ar/⁴⁰K ratio and K-Ar apparent age in fine-clay size fractions based on the production of ⁴⁰Ar in a closed system for some period of time, followed subsequently by the diffusional escape of ⁴⁰Ar with its continuing production from ⁴⁰K in the particles. This analysis gives ⁴⁰Ar diffusion coefficients consistently in a range from 10^{- 28} to 10^{- 27} cm²/s. From the results in this paper, the flux of radiogenic ⁴⁰Ar from the sedimentary crust to the atmosphere is estimated as 5 to 13 × 10⁶ mol/year, which is about 15 to 40% of the flux from the whole continental crust, as reported by other investigators.