POST-DEPOSITIONAL LOSSES OF RADIOGENIC 40AR IN CLAYS

The known observation that smaller size fractions of K-bearing silicates in sedimentary rocks yield younger K-Ar apparent ages than the larger particles of the same stratigraphic age is commonly attributed to diffusional loss of 40Ar and(or) diagenetic addition of elemental K. The ⁴⁰Ar/⁴⁰K atomic ratio was analyzed from the Lower Permian, Upper Triassic, and Neogene formations. In some sections, the K-Ar ages of mineral particles of smaller size ranges are significantly older than the stratigraphic age, indicating a long history prior to the particle deposition. However, the ⁴⁰Ar loss from the fine clay fractions (diameter <0.25 μm) relative to the larger particles is greater in the stratigraphically younger units. The loss in the Neogene clays is about 50%, in the Late Triassic about 35%, and in the Late Paleozoic samples 20%. This trend suggests that escape of ⁴⁰Ar from the smaller particles may be a phenomenon of geologically relatively short duration: the release of ⁴⁰Ar that started at some time might have stopped later if the sediment became a closed system; subsequent production of ⁴⁰Ar by ⁴⁰K decay decreases the relative difference between the smaller and larger particles. The apparent higher ⁴⁰Ar losses in some of the younger Tertiary sediments also indicate that addition of K to the finest clay particles may be a significant process.

There are three main uncertainties in the interpretation of the ⁴⁰Ar escape rates based on the ⁴⁰Ar/⁴⁰K apparent ages: (i) particle shape, (ii) starting time of the escape, and (iii) ⁴⁰Ar diffusivity in clay particles. From a model of ⁴⁰Ar production and diffusional escape in spherical particles, with the escape starting times in a range from 20 to 180 Ma ago, the ⁴⁰Ar diffusion coefficient is bracketed by 10^-28 to 10^-27 cm²/s.

In several 10² to 10³ m-thick sections, authigenic illite of <0.1 to <2 μm size fractions yields young K-Ar apparent ages, often below the stratigraphic age. ⁴⁰Ar production and first-order escape during diagenesis give an escape rate parameter ε = (0.2 to 4)×10^-8 yr^-1, corresponding to a diffusion coefficient of an order of 10^-27 to 10^-26 cm²/s, which is reasonably close to the preceding model estimates. Within the uncertainties of the clay-size K-minerals' abundance in sediments, the ⁴⁰Ar flux out of continental sediments is comparable to that from the continental crust.