Brief theorical overview about Ar-Ar dating method

40Ar/39Ar method: basic principles

The main datation method used in this thesis is the so-called argon-argon method (Ar-Ar) on micas (biotites and white micas). As all essential geochronological methods, the principle of this method is the disintegration of a radioactive element in a stable one, called radiogenic. The ratio between these two elements is proportionnal to the age of the system closure temperature. For K-Ar (potassium-argon) method, it is the disintegration of potassium 40 [40K] radioactive to stable radiogenic argon 40 [40*Ar]. This exponential disintegration is effective through millions of years (Ma). Time-depedency can be calculate by knowing the 40*Ar/40K ratio.

However, those two isotopes have to be measured separately, by two distinct methods. This often induce large errors as two distinct fractions are used for each analysis. For this reason, the argon-argon (Ar-Ar) method of dating has been finalised: a part of 39K is transform into 39Ar, with the help of controlled irradiatio. The ratio 40*Ar/39Ar is equivalent of 40*Ar/40K ratio as the isotopic ratio 39K/40K is constant in the nature and that contents of 39Ar is dependant of this of 39K. A standard for which is age is well defined is simultaneously irradiated and its ratio 40*Ar/39Ar measured to recalculate age of sample.

Closure of isotopic system

One of the major problem is to know at which moment isotopic system is closed. Since Seventies, notably with the team of Emilie Jäger of Bern (Purdy & Jäger 1976), numerous geochronologists have supposed that each isotopic system of each mineral has a specific temperature of closure. at which radiogenic isotops cannot be taken out of cristal network. Nevertheless, deformation (engendrering a partial recrystallization and changing in average grain size) just as passage of hydrothermal fluids can lead to reopening of system at a different temperature than the theorical temperature of the mineral for a certain isotope.

Irradiation of samples

As we already say it, for Ar-Ar method, samples have to be irradiated in a nuclear reactor, so that a part of potassium is transformed to argon. This irradiation is controled in the sense that we know time of irradiation and that irradiation power can be recalculated by irradiating simulateously a sample of known age. Unfortunatly, potassium is not the only one who interact during irradiation (in fact, all atomes are sensitive to interaction). In case of argon, some calcium and chlorine atoms are transformed in argon (see opposite picture). Moreover, potassium produce not only 40Ar, but also 39 and 38Ar. Calcium is mainly source of 37Ar, with some 39, 38 and 36. Regarding chlorine, it produces 38Ar In natural rock sample, atmorspheric argon is also present (36/40Ar ratio known and constant through time). Radiogenic argon (the famous 40*Ar needed in ratio) has so to be corrected. All these interferences have to be corrected after measurements so that interessant ratio can be determinated, notably 40*Ar/39Ar, Ca/K and Cl/K, without forgetting the quantity of potassium interacting to produce argon.

Stepwise heating technique

A first method extract the gas in a oven which allow to heat step by step the sample (generally from 400°C up to 1400°C), destroying so progressively the mineral which release gas progressively (see visit of laboratory). This method allow to obtain a specter of Ar-Ar ratio in function of temperature steps. If the sample present an excellent isotopic homogeneity, then the specter is flat and the deduced age is generally accurate.

However this is often not the case, notably when the minerals have been alterated by a partially retromorphosis (chloritisation of biotites for example), a partial loss of radiogenic argon (late reopenig of system),... In such cases, resulting ages are difficult to interpret. Other problems arrive when mineral separate is not pure, In such case, suppositions on interpretation age spectrum can be sent out with the help of Ca/K and Cl/K ratio [obtained from respective measurement of 37 and 38Ar]. However a second analysis of a pure separate is more safe.

Laser ablation technique

An other method of evaporation in situ allow to measure a precise point of a mineral by the help of laser ablation method (infrared or ultraviolet). Actually, this method always suffered of the disadvantage to liberate only a low quantity of gas (some hundreds of µm3), particularly for "young" samples (less than 50 Ma), even if this is better from day to day. Moreover if the sample has suffered a late isotopic system reopening, this cannot be clearly define with this method, unless of a great precision in results (old sample and numerous point of analysis to interpret presumed inhomogeneities).

Treatment and measurement of gas

In all case, the exiting gas has to be purified, in order that no other element annoy the measurement of various argon isotopes in the mass spectrometer. For example, presence of CO2 in gas may increase uncontrollable the background of spectrometer, exactly where are situated argon peaks. All argon isotopes (36, 37, 38, 39 and 40Ar) are measured, in order to be able to give corrections on the 40*Ar/39Ar ratio.

And after ???

In this thesis work, other geochronological method are also imaginable, notably those joined to the disintegration of uranium to thorium and then led (U-Th-Pb) in the allanites (epidote rich in rare earth element) and eventually the monazites (this last one should appear in the studied region). Garnet can also be dated by samarium-neodynium or luthetium-hafnium methods. Measurement of fission tracks in apatite and zircon can also give more informations about cooling history of the samples.

To be brief, it should be interessant to be able to obtain several ages and to compare them, in order to obtain a greater certitude. Finally when these ages will be well determined, we will be able to join them to the pressure and temperature estimations (and thus to the structural position of the rock at the dated stage, see preceding page) in the same way as the literature and already existing geological studies, in order to give a signification which, I hope, will be... as hard as rock ;-)

For more informations on these geochronological method, some links are available in my links page.