T147 The Changing Planet I: A Special Tribute Session Celebrating the Contributions of Fred T. Mackenzie

Tuesday, 18 October 2005
8:00 AM - 12:00 PM

POST-DEPOSITIONAL LOSSES OF RADIOGENIC 40AR IN CLAYS

Paper number: T147-15
Program: Technical Sessions
Time: 11:40 AM – 12:00 PM

Authors

Abstract

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.