## Abstract

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 ^{40}Ar/^{40}K ratio from 14 stratigraphically and regionally different sections. Estimation of the loss of radiogenic ^{40}Ar from varied clay-rich size fractions was based on two models: a relationship between particle size and the ^{40}Ar/^{40}K 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 ^{40}Ar 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 ^{40}Ar from the smaller particles in older sediments decreased or even stopped after deposition of the sedimentary sections. The large ^{40}Ar losses derived from small ^{40}Ar/^{40}K 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 10^{2}-10^{3} in thick sections, authigenic illite in the <0.1 to <2 μm fractions yields young K-Ar apparent ages resulting from simultaneous ^{40}Ar production and release during clay authigenesis. In a production and loss model, a first-order escape-rate parameter (ε) was estimated at 0.2 × 10^{-8} to 4 × 10^{-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 ^{40}Ar losses from fine clay particles are discussed.

Original language | English (US) |
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Pages (from-to) | 234-249 |

Number of pages | 16 |

Journal | Clays and Clay Minerals |

Volume | 53 |

Issue number | 3 |

DOIs | |

State | Published - Jun 2005 |

## Keywords

- Age
- Ar loss
- Carboniferous
- Clay fraction
- Diffusion
- K
- Miocene
- Neogene
- Permian
- Triassic

## ASJC Scopus subject areas

- Water Science and Technology
- Soil Science
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)

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