Redistribution of REE, Pb and U by supergene weathering studied from in-situ isotopic analyses of the Bangombé natural reactor, Gabon

Abstract

The isotopic analyses of rare earth elements (REE), Pb and U in several kinds of minerals from the clay and black shale layers above the Bangombé natural reactor, Gabon, were performed using a sensitive high-resolution microprobe (SHRIMP) to investigate the migration and retardation processes of fission products released from the reactor. REE isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays, which reveals that the clays play an important role in preventing fission products from spreading.Zircon crystals in the clays heterogeneously contain high-U regions (up to 28.3 wt%) with normal 235 U/ 238 U ratios (=0.00725) and significant amounts of fissiogenic REE, which suggest the occurrence of significant chemical fractionation between REE and U during the dissolution of reactor uraninite and the recrystallization of secondary U minerals. The Pb data suggest that galena grains in the clays were also formed by the mixing of the two components during a recent alteration event, and that a significant amount of Pb was derived from 2.05-Ga-old original uraninite rather than reactor uraninite. The U-Pb systematics of zircon provide chronological information on the old igneous activity associated with the basement rock formation at 2.8 Ga and geochemical evidence of the incomplete mixing of independent Pb and U sources. This result is consistent with previous chronological results in this area.

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

“…5a and b) have the large variation of the 238 U/ 206 Pb data from 0.23 to 38.8 and the small variation of 207 Pb/ 206 Pb data (most values are near 0.11-0.12), which suggests the existence of independent U and Pb sources that have migrated into zircon grains. Judging from the 207 Pb/ 206 Pb ratios commonly observed especially in low U/Pb parts of the zircon, the migrated Pb into the zircon was probably derived from the same source as that which formed the galena above the Bangombé reactor zone (Kikuchi et al, 2007). The previous studies revealed that the U and Pb originated 2.05-Ga old uraninite have occurred a large chemical fractionation under recent severe weathering within one million years (Hidaka et al, 2005;Kikuchi et al, 2007).…”

“…Judging from the 207 Pb/ 206 Pb ratios commonly observed especially in low U/Pb parts of the zircon, the migrated Pb into the zircon was probably derived from the same source as that which formed the galena above the Bangombé reactor zone (Kikuchi et al, 2007). The previous studies revealed that the U and Pb originated 2.05-Ga old uraninite have occurred a large chemical fractionation under recent severe weathering within one million years (Hidaka et al, 2005;Kikuchi et al, 2007). Because the 207 Pb/ 206 Pb data show a small variation of ~0.12 corresponding to the mean age of 1959 ± 12 Ma selected from 23 of 46 as the best of the zircon analyses, U and Pb of these two components were originated from 2.05-Ga old uraninite and migrated into zircon grains independently under the recent severe weathering.…”

“…However, Figure 4 shows that there is no clear correlation between Δ 206 Pb/ 238 U and U content in analyzed zircons. The reason for the scattered data points especially in high-U content region (>5000 ppm U) in Figure 4 might be that the high-U zircons have incorporated uranium derived from uraninite in and around Bangombé reactor zone (Kikuchi et al, 2007;2008). Therefore, the U/ Pb correction by Williams and Hergt (2000) could not be applied for the analyzed zircons in this study.…”

“…The U− Pb data of zircons provide chronological information on the igneous activity associated with the basement rock formation at 2.8 Ga and geochemical evidence of the incomplete The diagram obtained by the analytical combination of SHRIMP and EPMA analyses (see in the text). The data points with circles (clay layer) and squares (black shale layer) are from this study, and those with diamonds (clay layer) are from previous work (Kikuchi et al, 2007). The open symbols are from SHRIMP analyses and solid symbols are from the combination of SHRIMP and EPMA analyses.…”

“…On the other hand in Figure 5b, the data derived from the analytical spots having U concentrations less than 2500 ppm were obtained by the conventional calibration technique and those more than 2500 ppm were obtained by the analytical combination of EPMA and SHRIMP. The previously analyzed U−Pb data of zircons in the clay layer cited from Kikuchi et al (2007) are also plotted in the same figures. The detailed comparison of two data sets between Figures 5a and b indicates that most of the 238 U/ 206 Pb values having U > 2500 ppm calibrated by the conventional method (Fig.…”