The Nature of the 40Ar/39Ar Plateau-like Spectra Corresponding to Apparent Ages: a Combination of
Stepwise Crushing and Stepwise Heating Methods as a Basis for the Problem Solution

Yu. D. Pushkarev VSEGEI, Sredny 74, St.-Petersburg, 199026, Russia

There appeared a number of publications in which some cases of 40Ar/39Ar plateau-like spectrum corresponding to an apparent age, were observed. This phenomenon raises doubts on the geological sense of any age determined by a single plateau spectrum, because any of them can be supposed to show an apparent age.

The main object of this work is to introduce a stepwise crushing method which is very useful in combination with the usual stepwise heating technique and in this way to explain the nature of the phenomenon. To solve the problem, it is necessary to answer two questions in what way the phenomenon of any unreal age plateau arises and where
(in nature or in experiment) it occurs.

Three types of experiments make the basis for the work: 1) stepwise crushing of minerals (after their irradiation), having step age spectra; 2) heating of mica under the condition when part of radiogenic argon and dehydrated water penetrate the three-dimension trap-defects and are preserved there; then hydration of that mineral in the autoclave under pressure of water vapour traced by deuterium to follow the water and Ar returning from three-dimension trap-defects into the crystalline lattice when Ar comes again into solid-solution state; 3) studying kinetics of Ar, H and D release by the 'tempering' method. The experimental results obtained led us to the following.

With partial losses of radiogenic 40Ar, whether in nature or in experiment, the minerals show 40Ar/39Ar step age spectra; in this case relic 40Ar can be preserved in two forms: 1) in three-dimension trap-defects and 2) as monoatomic inclusions. Radiogenic 40Ar (formed after some previously accumulated Ar had been removed from the mineral) always appears as monoatomic inclusions. Stepwise crushing of minerals showing step age spectra (obtained by the stepwise heating technique) results finally in giving the latter a plateau-like shape.

If located in three-dimension trap-defects, relic Ar is removed when the defects are completely destroyed. As a result, only 40Ar accumulated after heating remains as monoatomic inclusions. Afterwards, the 40Ar/39Ar spectrum takes a plateau-like shape, corresponding to the age of the process, that causes Ar losses.

If relic Ar is in the monoatomic form, sample crushing cannot change the 40Ar/39Ar ratio. Therefore, in this case the plateau-like spectrum will correspond to the apparent Ar-Ar age which the mineral sample at whole had before crushing.

Experimental mica hydration after its preliminary heating suggests another way for plateau-like spectra to appear in natural samples. Relic Ar located in three-dimension
trap-defects can once again take the monoatomic form, which is the same as the solid solution one. This takes place when Ar- and water-filled three-dimension trap-defects resulted from mica dehydration are taken apart by vacancies when the mineral is heated under water vapour pressure. This is just the case when the step age spectrum became plateau-like.

So, a plateau-like age spectrum corresponding to an apparent age can be obtained both in nature-altered minerals (tectonites-hosted and polymetamorphozed minerals) and in experimentally changed ones (minerals extracted after the sample overcrushing).

Understanding the nature of the phenomenon under discussion enables us to offer two ways for its identification: 1) determination of the whole mineral 40Ar/39Ar ratio in the same sample, but properly crushed, 2) comparison of ages corresponding to plateau-like spectra obtained on different minerals.