How do we get ages and what do we date?

Working method

Naturally, some marvellous field weeks are unescapable to find GOOD samples... On this point, I invite you to visit my pictures galleries to give you some idee of the Central Alps charme! Some other regions are also available...

Figure on the right: Diagram presenting the essential of my working method employed in the frame of this thesis.

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Ages from literature

The age of the alpine metamorphism peak is relatively well defined in the central and southern part of the Central Alps. However, more to the North. These old basement have suffered two metamorphisms, one during the Variscian (more than 300 Ma) and an other more recent, during the Alpine (circa 50 to 20 Ma). The datations present in these regions are so intermediate ages bend to a partial reopening of the isotopic system, in particular with Rb-Sr ages (not represented on the map below).

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But what are we dating?

In order to give a sense to those datations, we have to join them to a precise geological phenomenon or point. Up to now, a lot of geochronologists have based their study on structural considerations and on rules of closure of an isotopic system at a given temperature and for a given mineral (cooling age). As the temperature is not the only factor, this last concept has to be changed, so that fluid and deformation can be taken in account.

In the framework of my study, we will attempt to join those datations to a point of the pressure-temperature (PT) path underwent by these rocks. Our work hypothesis is that if chemical equilibrium is achieved (good retentivity of PT conditions), then the isotopic equilibrium is also achieved and its related datation reflect the age of the determined PT condition.

Figure on the left: Age of metamorphism in the North of Central Alps (in million years [Ma]). Data from Armstrong et al. (1966), Arnold & Jäger (1965), Deutsch & Steiger (1985), Hunziker et al. (1986), Jäger et al. (1961, 1967), Jäger (1962), Köppel & Grünenfelder (1975), Muralt (1986), Purdy & Jäger (1976), Siegenthaler (1984).

Pressure-Temperature (PT) conditions

The PT conditions can be calculated by the use of thermodynamic: an equilibrium between several minerals in a P-T-X space (X = chemical composition of minerals or rock) is characterised by exchange and mineral phases appearance/disappearance reactions. It is so possible to write in PT space chemical reactions between various minerals, such as for example the iron-magnesium exchange between garnet and biotite (GARB themometer) or the reaction leading to the appearance of anorthitic plagioclase by reaction of grossular (calcic garnet), aluminosilicate and quartz (GASP geobarometer).

A precious tool... TWQ

The program of thermobarometric calculations TWQ from Berman (1991) is used (see my link page for more informations on this program). This program allow to describe the whole mineralogical equilibria possible between the various mineral phases (poly-equilibrium) with the help of a coherent basis of thermodynamic models. The examination of many equilibria for a given paragenesis (i.e. mineral supposed in equilibrium in rock) should cut in a precise point of the PT diagram, significative of a chemical equilibrium achieved by mineralogical assemblage choosed.

To resume, the ages we will determine will be joined to a point of the PT path of rocks and will permit a best understanding of the metamorphism and perhaps of the geodynamic in the northern part of Central Alps. Moreover, we hope a best comprehension on behaviour of argon-argon geochronometer and other factors contributory of these restrictions (see page on argon theory).