Argon-40, radiogenic

Knowing the total amounts of potassium-40 and of radiogenic argon-40 in a sample enables one to calculate first, their relative weight amounts and then, from these values, the age of the sample. The reliability of a potassium-argon date is generally expressed as a plus or minus value after the calculated date. 

Table 3.6. Iterative calculation of the z value corresponding to a lost fraction of radiogenic argon...

Let us assume that a sphere with radius a is immersed in a liquid of finite volume, e.g., a mineral in a hydrothermal fluid. Diffusion in liquids is normally fast compared to diffusion in solids, so that the liquid can be thought of as homogeneous. Similar conditions would apply to a sphere degassing into a finite enclosure, e.g., for radiogenic argon loss in a closed pore space. Given the diffusion equation with radial flux and constant diffusion coefficient... 

Figure 11,21 Age spectra of hornblendes of a Paleozoic gabbro (367 Ma) intruded by a granitic body during the Cretaceous (114 Ma). Samples underwent permeation of " Ar from lower crustal portions and differential losses of radiogenic argon (32, 57, and 78% respectively), proportional to distance from contact (0.3, 1, and 2.5 km). Reprinted from T. M. Harrison and I. McDougall, Geochimica et Cosmochimica Acta, 44, 2005-2020, copyright 1980, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Kaula W. M. (1999) Constraints on Venus evolution from radiogenic argon. Icarus 139, 32-39. 

Robbins G. A. (1972) Radiogenic argon diffusion in muscorite under hydrothermal conditions. M. S., Brown University. 

Aronson J. L. and Hower J. (1976) Mechanism of burial metmorphism of argillaceous sediment 2. Radiogenic argon evidence. Geol. Soc. Am. Bull. 87, 738-744. 

Voronov, A.N., Prasolov, E.M. and Tikhomirov, V.V., 1974. The relation between radiogenic argon and helium Isotopes in gas pools. Geochem. Intemat., II 1311-1322. 

This reaction involves the loss of part of the clay phase, yielding only 0.8 mol of illite per mol of smectite, and presumably requires solution and reprecipitation. However, the Ar/K studies of Aronson and Hower (1976) indicate that inherited radiogenic argon from illite interlayers in original detrital smectite is retained, so that the original clay mineral structure must have persisted. 

The origin of the radiogenic argon is more problematic as both mantle and crustal reservoirs are characterized by high " Ar/ Ar ratios. Kennedy et al. (1991) attributed radiogenic " °Ar in Honduran geothermal fluids to a crustal source based upon measured... 

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