ELECTRON MICROPROBE DATING AS A TOOL FOR UNDERSTANDING CLOSURE OF THE U-TH-PB SYSTEM IN MONAZITE: EXAMPLES FROM MIGMATITE

Formerly, monazite used for dating was considered as a rare but concordant mineral that had no known inherited cores and a relatively low closure temperature of 600-650 °C. Its scarcity led to only few conventional U-Pb TIMS data, added to the fact that monazite is less common than zircon and difficult to identify in thin section.

However, it is more common than earlier thought, especially in migmatite and granite (heavy fraction). With an electron microprobe, only few monazite grains are needed to calculate precise and accurate ages, and the resolution of the electron beam (spot ~1 to 2 µm) has shown common age zoning in migmatitic monazite. Samples from France, French Guiana and Ivory Coast, of different ages and origins, show inherited cores to be common in monazite close to concordant domains that give a migmatization age. TIMS and SHRIMP zircon geochronology data confirm these ages.

Our work shows the so-called “closure temperature” of the U-Th-Pb system in monazite to be quite high. We suggest using the expression “opening temperature” when studying the temperature of completely resetting the monazite isotopic system, as this temperature is much higher than the real closure temperature of magma cooling. Actually, the U-Th-Pb system in monazite can remain closed at higher temperatures than the closure temperature of coexisting zircons (700-800 °C), especially if no high-temperature fluid percolates through the system. Electron microprobe data (Th/Pb vs U/Pb diagram: Cocherie and Albarède, 2001) show that monazite from migmatite has a wide variation in U content; the commonly slightly younger age for the highest U/Th ratio is interpreted as the result of late U-enriched fluids percolating during monazite crystallization in migmatite.

Ref.: Cocherie A. and Albarède F., 2001- An improved U-Th-Pb age calculation for electron-probe dating of monazite. Geochim. Cosmochim. Acta., 65, 4509-4522.