Electron-microprobe dating of monazite: The story

Abstract

This paper is a testimony of how electron microprobe dating of monazite was developed in the early days of this technique, in the 90's. Five contributions have been collected from Nagoya, Clermont-Ferrand, Orléans, Salzburg, and Amherst. The technique was designed in Nagoya by Professor Suzuki, using an old JXA-5A JEOL electron microprobe. The first results were published in 1991, with great difficulty. Professor Suzuki also created the isochron-like Pb-Th* diagram to present the data. Progressive improvement of the method finally allowed the Nagoya team to date Cenozoic monazite. The technique was re-discovered in Clermont-Ferrand using a Cameca Camebax microprobe. This team demonstrated the accuracy of the method, and proposed statistical tools to improve the precision and to handle multimodal populations. The first results were published in 1993. In Orléans, the French Geological Survey (BRGM) developed the method with a Cameca SX50 to support mapping projects. They developed a new method for calculating the age, and showed that this technique was very efficient for dating monazite in migmatites. In Salzburg, the first results were obtained in 1996, using a Jeol JX 8600. It proved to be very efficient in the Alps to distinguish Cadomian, Ordovician, Variscan and Alpine events. It allowed this team to directly date a monazite-allanite reaction they described for the first time. Finally, the Amherst team investigated in detail the technical issues of electron microprobe monazite dating. They showed that the best analytical procedures must include at least: detailed chemical mapping, several interference corrections and special care for background measurements. With the help of Cameca they designed a specific microprobe for dating, named “Ultrachron”. With this machine, high precision ages can be obtained on well characterized individual grains in thin section, giving birth to a new era in geochronology of complex metamorphic rocks.