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.