Evaluation of Uncertainties in Dating of Monazite by Electron Microprobe

Over the last decades, determination of ages in monazite using Electron Probe Microanalysis (EPMA) has progressed to the point that it is now considered as a routine procedure within several laboratories worldwide [1,2]. Nevertheless, the typical age errors of individual electron probe spots reported in the literature are an order of magnitude larger than attainable by Secondary Ion Mass Spectrometry (SIMS) and two orders of magnitude larger than attainable by Thermal Ionization Mass Spectrometry (TIMS). The age errors frequently range from 40 Ma to 120 Ma, although after statistical treatment of a set of homogeneous ages, a precision of approximately 20 Ma can be obtained. Despite the low accuracy, the numerous studies have shown that the Th-U-totalPb method using the EPMA is a reliable method, easy to use, for detecting age variations on a very small scale. The effective limitation of the EPMA monazite ages determination concerns its relatively poor accuracy regarding mainly Pb determination. Similarly the relative uncertainties for U and Th are generally larger than 2%, which can lead to large errors. The error on the age for each analysed point depends on the U, Th and Pb concentration. As the Pb concentration is directly related to the age of the monazite and to the U and Th concentration, older is the monazite, better is the relative error. Since the first monazite dating study by EPMA [1] there has been improvements in data age processing, but only few evaluations of uncertainties in the Pb, U, Th determinations. As the error calculated on the age for each analysed point is the whole inaccuracies on Pb, U, Th determinations, this method requires high accuracies determinations of trace elements [3] as well as of major elements.