In-situ U-Pb analyses of highly altered zircon from sediments overlying the Bangombé natural fission reactor, Gabon

摘要

In-situ isotopic analyses of Pb and U in the highly altered zircons found in the clay and black shale layers above the Bangombé natural reactor, Republic of Gabon, were analyzed using a sensitive high resolution ion microprobe (SHRIMP) to understand the geological history in this area. The analyzed zircons widely vary in the U contents ranging from <100 to 59000 ppm. To confirm the problem on insufficient Pb/U calibration of high-U zircon in the SHRIMP technique, 206Pb/238U isotopic ratios from the SHRIMP analysis were compared with those from the analytical combination of U/Pb elemental ratios by electron probe micro analyzer (EPMA) with Pb isotopic ratios by SHRIMP. A significant deviation was found in the 206Pb/238U data of high-U zircons between two analytical techniques. Then, we concluded that the analytical data with U concentrations less than 2500 ppm can be used as reliably calibrated SHRIMP U-Pb data and those with high U concentration greater than 2500 ppm were determined by the analytical combination by EPMA and SHRIMP. The U-Pb data of zircons provide chronological information on the igneous activity associated with the basement rock formation at ca. 2.8 Ga. Moreover, nearly constant 207Pb/206Pb data corresponding to the mean age of 1959 ± 12 Ma and wide variation of 238U/206Pb data suggest that U and Pb were originated from 2.05-Ga old uraninite and migrated into zircon grains independently under the recent severe weathering.