Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total lead determination: implications for lead behaviour in monazite

Abstract

Monazite grains are generally concordant and, with an electron microprobe, give Th-U-total Pb isochron ages in agreement with conventional U-Pb data. Monazites from four samples of post-Archean migmatite from the Ivory Coast record two main ages: an Archean age at 2.80 Ga partly overprinted by a late migmatization event at 2.03 Ga. The in situ electron microprobe determination has also shown a third event that systematically appears at 2.72 Ga for all the monazites from two of the four studied rocks. The three events at 2.80, 2.72, and 2.03 Ga recorded by the Th-U-total Pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of Pb diffusion. Thus the oldest monazite remained closed during a high-grade resetting (migmatization) at a temperature estimated at around 700°C, and also remained closed during the growth of secondary monazite. Conventional U-Pb data (isotope dilution) on monazite from the same rocks did not allow precise age determinations because the representative points were highly (up to 83%) discordant and did not fit well along a chord in the Concordia diagram. The U-Pb isotope dilution method gave a lower intercept age of 2029 ± 25 Ma in agreement with the electron microprobe age, although with an unusually high uncertainty due to a poor fit of the data. The Pb-evaporation method gave an old Archean age at 2830 ± 7 Ma and Proterozoic ages ranging from 2417 ± 10 to 2074 ± 7 Ma. Thus, although the oldest Archean age obtained with the Pb-evaporation and electron microprobe methods is similar within the analytical error, the Proterozoic ages obtained by the Pb-evaporation method are significantly older than the Th-U-total Pb Proterozoic age obtained with the electron microprobe. This discrepancy is presumed to be due to the influence, in the Pb-evaporation method, of radiogenic lead contained within Archean zones of the monazite grains; in other words, it means that migmatization took place later than 2074 ± 7 Ma, and probably around 2030 Ma. The second Archean event at 2.72 Ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods. The Th-U-total Pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 Ma, even if the U-Pb isotopic dilution technique remains the reference method. Moreover, owing to the very high spatial resolution (1 μm) of the electron microprobe, it appears to have no competitor for dating complex polygenetic monazites, especially when more than two events are recorded. Homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion. A procedure for age and error calculations is defined.