Determination of rock ages by chemical analysis of Th, U and Pb in the mineral monazite (Ce, La, Th, REE, U) PO4 using EPMA

URRAZA IVANA ALEJANDRA; GRECCO LAURA EDITH; DELPINO SERGIO HUGO; ARRESE MONICA LILIANA

Halifax, Nova Scotia, Canada

Workshop; IRM (Institute for research in materials) Annual General Meeting; 2008

IRM Istitute for Research in Materials

Abstract: Electron Probe Microanalysis (EPMA) is an established technique for the determination of elemental concentrations, allowing major and minor element analysis in solid phases on a ca. 1 µm diameter area of a polished specimen coated with a thin conductive layer (e.g. carbon). The geochronology (age dating) of natural Monazite (Ce, La, Th, REE, U)PO4  in a rock sample  using EPMA is a new powerful tool effective in solving geological problems. The methodology is based in the ratio of U and Th (parents) to Pb (daughter), assuming that during crystallization of the mineral, the incorporation of Pb was insignificant, and that the mineral phase remained a closed system with respect to the elements in question. Then, the proportion of Pb present in the monazite crystal is a result of the radioactive decay of U and Th since the time of crystallization. The accurate determination of trace elements (U, Th, Pb and Y) allows the in situ ages of different domains in zoned monazite crystals.  The ages result to the application of the Montel Equation: Pb= (232Th/232)(eλ232t -1)*208 + (0.9928 238U/238.04)(eλ238t -1)*206 +(0.0072 235U/238.04)(eλ235t - 1)*207.   The EPMA technique was used at the Dalhousie University Microprobe Facility in the Department of Earth Sciences to obtain backscatter images, maps of domains with different chemical composition and the chemical analytical data necessary to obtain the ages of two representative samples of crystalline basement in the Aluminé Batholithic Complex, Argentina. Monazite geochronology determined two different age populations in 10 analysed grains: 240 to 250 million years for one rock unit and 350 to 360 million years in another. These data represent the first Palaeozoic radiogenic ages available in this area and have implications for the tectonic history of the northern Patagonian region.