Single whole rock K—Ar isochrons

Abstract

Conventional K-Ar age determinations are frequently discrepant. A major factor is the failure of rocks to conform to the basic simplifying assumptions made in the total fusion method. Many rocks incorporate initial argon with an isotopic composition different from the present atmosphere and others have not remained closed systems. By careful sampling and the application of isochron geochronometry more reliable and geologically acceptable K-Ar ages can be obtained. K-Ar step heating isochrons from Kenyan Miocene volcanic sanidines are used to illustrate the accumulating evidence for the presence in ancient rocks of initial argon with an isotope ratio unlike that of the modern atmosphere. Single whole rock total fusion K-Ar isochrons from the Olduvai basalt, Tanzania and the Upper basalts of Antrim are presented to illustrate a new version of the K-Ar isochron technique. Special circumstance multiple rock K-Ar isochrons from the Belfast section of the Antrim basalts and from an ignimbrite-ash flow unit in Bed I, Olduvai Gorge, are used in a further discussion of the general applicability and usefulness of K-Ar isochrons.

40Ar/39Ar dating in paleoanthropology and archeology

“…Concordance of the K-Ar and 40 Ar/ 39 Ar results and the flat 40 Ar/ 39 Ar release spectra from the feldspar proved that the argon isotopic system of anorthoclase generally remained undisturbed in the nearsurface environment of the Koobi Fora Formation. This conclusion is in marked contrast to that of Fitch and coworkers, 12 who had maintained that anorthoclase in tuffs of the Koobi Fora region were almost universally partially reset by ''thermal overprinting. ''…”

“…In principle, all of these problems can be obviated through 40 40 Ar/ 39 Ar dating efforts related to paleonthropology, their purpose being to help resolve the ages of fossiliferous sequences in East Africa. [9][10][11][12] One of their early studies dated pumice from the KBS Tuff of the Koobi Fora Formation. The 40 Ar/ 39 Ar ages they obtained later became the center of the famed KBS Tuff dating controversy.…”

Some conventional potassium–argon and 40Ar/39Ar age studies of glauconite

“…Even if initial 40 Ar/ 38 Ar ratio discrepancy (Fitch, Miller & Hooker, 1976) were present, the effect would be minor as the atmospheric contamination percentages are small. The important feature of these results is the apparently insignificant effect of detrital material on the K/Ar ages.…”

“…Although it may be possible to achieve ages comparable with the conventional K/Ar method by monitoring the temperature in the reactor can over different irradiation times and extrapolating the ages to zero time in the core, this lengthy process would render 40 Ar/ 39 Ar glauconite dating of limited geological use, apart from 39 Ar recoil diffusion studies. At present, the single rock K-Ar isochron technique (Fitch et al 1976) is the most viable method for direct calibration of the sedimentary column, and this too suffers from the geochemical complexities of glauconite. …”

Volcanic architecture, eruption mechanism and landform evolution of a Plio/Pleistocene intracontinental basaltic polycyclic monogenetic volcano from the Bakony-Balaton Highland Volcanic Field, Hungary

“…This indicates that the samples are unsuitable for dating and the obtained ages must be interpreted with caution. An advanced method of sample preparation was introduced by Fitch et al [42], who applied the isochron method to fractions prepared by magnetic and heavy liquid separation from a single rock sample. Difficulties, and a new way of applying the isochron method have been described by Balogh et al [43].…”