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Doctoral thesis
French

Essai d’application de la méthode des traces de fission à la datation de quelques massifs éruptifs du sud des Alpes

ContributorsDeferne, Jacques
DirectorsVuagnat, Marc
Number of pages57
Imprimatur date1972-05-02
Abstract

Spontaneous fission of U238 nuclei dispersed in rock minerals leaves tracks which can be observed with an ordinary microscope after an appropriate chemical attack. These tracks are straight, isotropically oriented in space and fairly easy to distinguish from usual mineral imperfections.

The density of these tracks per unit of surface is proportionnal to the amount of uranium in the mineral and to the time elapsed since latter's formation. Hence they constitute a means of measuring rock ages. Approximately 2000 tracks per cm2 are formed per million years and per 1 ppm of uranium.

Using this method, we have measured a certain number of ages on several young granitic massivs of the southern Alps : Traversella, Biella, Bergell and Adamello massivs. Their ages are known to range between 30 and 40 m.y.

Apatite and titanite were extracted by the usual crushing and separating methods. One fraction was mounted on a thick dise of epoxy resin, polished to expose the mineral's internal surfaces and finally chemically attacked to enlarge the tracks. The track density was measured under 500 manification with an ordinary microscope equiped with a graticule eyepiece. When possible several hundred tracks were measured for each sample.

The quantity of uranium contained in the mineral was evaluated by an indirect method. Another fraction of the initial separation was heated to 700° C for 12 hours to anneal fossil spontaneous tracks. The samples were then irradiated by thermal neutrons in the heavy water canal of "Melusine" Reactor at Centre de recherches nucléaires in Grenoble.

This irradiation induces fission in a certain number of U235 nuclei. Whith the value of the flux, the cross section of U235 and the isotopie ratio of U238 /U235, it is possible to calculate ages by an equation in which age is a function of spontaneous and induced fission track density and neutron flux.

Ages measured whith titanite are always greater than those with apatite. This difference can be explained by the fact that during initial cooling of rocks, titanite starts to conserve fission tracks at about 450° compared to 150° for apatite. Their age differences can be increased by metamorphism which affect apatite well before titanite. Hence these age discrepencies can furnish precious information about their thermal history.

The mean ages in million years found respectively for apatite and titanite for these massivs are Biella 29 and 37, Traversella 31 and 40, Bergell 20 and 37, Adamello 26 and 48. These results confirm the tarditectonic apparition of these massivs. The tonalite facies appears slightly older. This method of dating by fission tracks is useful however it does not replace the other more precise classical radiometric methods. Nevertheless it can be used when other methods are unapplicable. Moreover it contributes interesting information on rock's thermal evolutions.

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Citation (ISO format)
DEFERNE, Jacques. Essai d’application de la méthode des traces de fission à la datation de quelques massifs éruptifs du sud des Alpes. 1972. doi: 10.13097/archive-ouverte/unige:172756
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