GEOCHRONOLOGY OF POLYGENETIC MONAZITES CONSTRAINED BY IN-SITU ELECTRON-MICROPROBE TH-U TOTAL LEAD DETERMINATION - IMPLICATIONS FOR LEAD BEHAVIOR IN MONAZITE

Monazite grains are generally concordant and, with an electron micropr obe, give Th-U-total ph isochron ages in agreement with conventional U -Pb data. Monazites from four samples of post-Archean migmatite from t he 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 elec tron microprobe determination has also shown a third event that system atically 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 b y the Th-U-total Pb system in monazite were each obtained on independe nt homogeneous parts of a single grain with no sign of Pb diffusion. T hus the oldest monazite remained closed during a high-grade resetting (migmatization) at a temperature estimated at around 700 degrees C, an d also remained closed during the growth of secondary monazite. Conven tional U-Pb data (isotope dilution) on monazite from the same rocks di d not allow precise age determinations because the representative poin ts were highly (up to 83%) discordant and did not fit well along a cho rd in the Concordia diagram. The U-Pb isotope dilution method gave a l ower intercept age of 2029 +/- 25 Ma in agreement with the electron mi croprobe age, although with an unusually high uncertainty due to a poo r fit of the data. The Ph-evaporation method gave an old Archean age a t 2830 +/- 7 Ma and Proterozoic ages ranging from 2417 +/- 10 to 2074 +/- 7 Ma. Thus, although the oldest Archean age obtained with the Pb-e vaporation and electron microprobe methods is similar within the analy tical error, the Proterozoic ages obtained by the ph-evaporation metho d are significantly older than the Th-U-total Pb Proterozoic age obtai ned with the electron microprobe. This discrepancy is presumed to be d ue 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 det ermined by other methods. The Th-U-total Ph method appears to be an in expensive alternative method for dating simple monazites with a precis ion close to 20 Ma, even if the U-Pb isotopic dilution technique remai ns the reference method. Moreover, owing to the very high spatial reso lution (1 mu m) of the electron microprobe, it appears to have no comp etitor for dating complex polygenetic monazites, especially when more than two events are recorded. Homogeneous zones of different ages coex ist in a single grain with no major indication of lead diffusion. A pr ocedure for age and error calculations is defined. Copyright (C) 1998 Elsevier Science Ltd.