Fission Products as Isotopic Tracers for Weathering Processes at the Natural Reactor Site at Bangombé, Gabon

Abstract

The unique natural fisson process which happened 2 Ga ago at Bangomb6, Gabon, allows study of migration due to weathering by using the isotope composition of fisson products as tracers. The weathering effects are expected to be detectable since this is the only reactor core which is located close to the surface at 10 m depth (Gauthier-Lafaye, 1995). Two main processes of a fission reaction lead to change in isotope ratios of natural elements: a) accumulation of nonradioactive isotopes which are endmembers of a decay chain, b) depletion of isotopes due to neutron capture depending on different cross section and subsequent decay The isotope compositon of light REE are greatly affected by the fission reaction and therefore suitable as tracers for weathering effects. It is well known that migration of elements occurs in fractures of the host rock preferently. A fracture located in the FA formation (sandstones) 4 m under the reactor core of the Bangomb6 site has been chosen for a closer investigation. One sample of the fracture itself and three samples of host rock beneath of it have been investigated by ICP-MS after a chromatographic clean up. A schematic scetch is given in Fig. 1. The isotope ratios 143Nd/146Nd and the 1478rn/149Sm have been chosen for a first evaluation and are shown in Table 1. The 149Sm/147Sm ratio is depleted close to 0 in the reactor core whereas the natural ratio is 0,92. All investigated samples are slightly depleted concerning this ratio. The 143Nd/146Nd ratio is enriched in the reactor core whereas the natural ratio is 0,707. Surprisingly one sample investigated is slightly depleted concerning this ratio. With some respect to the error of measurement (estimated to be 1 % after correction for mass discrimination) these results do not prove a migration of rare earth elements in or perpendicular to this fracture 4 m below the reactor core due to weathering. In contrary a small depletion of 143Nd/lg6Nd and l~7Sm/149Sm could be a result of a neutron flux. The cross section of 143Nd (336 barns) is 300 times larger than 146Nd (1,3 barns) and the cross section of 149Sin (41000 barns) is larger than 147Sm (52 barns). This would be sufficient to explain the relative depletion of the isotopes 143Nd and 149Snl.