Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Lerman, A. Articles by Clauer, N. Search for Related Content GeoRef GeoRef Citation

LOSSES OF RADIOGENIC ⁴⁰Ar IN THE FINE-CLAY SIZE FRACTIONS OF SEDIMENTS

¹ Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208, USA
² Centre de Géochimie de la Surface (CNRS/ULP) 1, rue Blessig, 67084 Strasbourg, France

^* E-mail address of corresponding author: alerman{at}northwestern.edu

The common observation that smaller particle-size fractions of sedimentary rocks yield younger K-Ar apparent ages than the larger particle-size fractions of the same stratigraphic age was analyzed with the aid of the ⁴⁰Ar/⁴⁰K ratio from 14 stratigraphically and regionally different sections. Estimation of the loss of radiogenic ⁴⁰Ar from varied clay-rich size fractions was based on two models: a relationship between particle size and the ⁴⁰Ar/⁴⁰K ratio, and a theoretical diffusional loss from spherical particles. The differences between the two models and reconciliation of their results are discussed. For the smallest fractions (up to <0.5 µm), percent-wise losses of ⁴⁰Ar from the spherical particles model increase from Upper Carboniferous and Permian (38±10%), to Late Triassic (47±10%), and to Miocene and Late Neogene (65±8%). This trend suggests that escape of ⁴⁰Ar from the smaller particles in older sediments decreased or even stopped after deposition of the sedimentary sections.

The large ⁴⁰Ar losses derived from small ⁴⁰Ar/⁴⁰K ratios in the younger Tertiary sediments, indicate that addition of K to the small fractions is, at least in part, responsible for the young K-Ar apparent ages in geologically different settings. In several 10²–10³ m thick sections, authigenic illite in the <0.1 to <2 µm fractions yields young K-Ar apparent ages resulting from simultaneous ⁴⁰Ar production and release during clay authigenesis. In a production and loss model, a first-order escape-rate parameter () was estimated at 0.2x10^–8 to 4x10^–8 y^–1, depending on the K-Ar apparent age of the size fractions and the stratigraphic age of the section. The limitations and uncertainties of the methods of evaluating diagenetic ⁴⁰Ar losses from fine clay particles are discussed.

Key Words: ⁴⁰Ar Loss • Age • Carboniferous • Clay Fraction • Diffusion • ⁴⁰K • Miocene • Neogene • Permian • Triassic