Abstract
Isotopic dating of laterites remains problematic and controversial. Comparison of the available data shows contradictory ages. The Rb-Sr, K-Ar and 4~ chronometers provide ages of several tens of millions years ([1], [2], [3]), while (234U/238U) disequilibria ([4], [5]) and l~ analyses ([6]) lead to recent ages. In fact, the different chronometers were not applied strictly to the same soil fractions (whole rock, clay fraction, pure mineral phase), which could partly explain the contradictory results. Actually, any analysed fraction may consist of inherited, altered and weathering mineral phases. This study was thought to analyse the same soil fractions with two different chronometers, the Rb-Sr and the (234U/238U) disequilibria methods. They were applied on < 0.2 gm fraction from the Goyoum laterite (Cameroon) consisting of three petrologic units: a saprolite at the base, a intermediate nodular zone, and a soft topsoil. The < 0.2 gm fraction was extracted from two profiles, it contains essentially kaolinite and Feoxyhydroxides. The Rb and Sr concentrations and the Sr and U isotope ratios were determined by thermoionisation mass spectrometry. The data reveal complex and wide variations of the 87Sr/86Sr and 87Rb/86Sr ratios with depth, with different patterns in each profile. No isochron line can be drawn in an 878r/86Sr v s S7Rb/s6sr diagram. The data scatters in the mixing and in the isochron diagrams suggest a Sr isotopic heterogeneity which could represent either an initial Sr isotopic variability of the analysed fractions, or a secondary process disturbing a previously homogeneous system. The (234U/238U) activity ratio varies from 0.85 to 1.20 in each profile (see Fig. 1). No correlation appears with the 87Sr/86Sr ratio. The variations of the (234U/238U) activity ratio with depth are similar in both profiles. An important disequilibrium is visible at the base of the saprolite with a (234U/238U) ratio significantly below unity. Above, the values tend progressively towards equilibrium. A second important (234U/23Su) disequilibrium occurs at the base of the nodular zone with values below and above unity. Like in the saprolite, the values tend towards equilibrium in the nodular zone. A third similar sequence could possibly exist in the topsoil. The fact that the (234U/238U) variations are similar in both studied profiles inclines us to believe that these variations are characteristic of the whole lateritic profile. The lateritic formation would then contain two horizons of U fractionation with progressive return to secular equilibrium above them. The lower horizon at the base of the saprolite, shows an enrichment of 23Su which could result from leaching of the bedrock initially in secular equilibrium, inducing preferential loss of 234U and crystallisation of residual minerals depleted in 234U. The progressive trend to secular equilibrium suggests no further fractionation in the saprolite The intermediate horizon at the base of the nodular zone could record perturbation of the kaolinite + Feoxyhydroxides mixture that could be either a recrystallisation of the pre-existing minerals or an addition of one or several new mineral phases. The fact that (234U/238U) activity ratios are again below unity suggest another episode of mineral leaching with formation of 234U-enriched fluids. However, migration of these fluids over only short distances could explain crystallisation of minerals enriched in 234U. Return to the secular equilibrium of the mineral fractions suggests a confinement of the process in the transition zone. For each fractionation front, a migration rate of 6 to 8 m/Ma may be calculated. The occurrence of (234U/238U) disequilibria in the Goyoum laterite emphasises that this laterite is presently affected by geochemical fractionations. The location of the fractionation zones at the lower parts of the petrographical units, allows to consider that the fractionation of U mirrors the pedological history of the laterite. It may also be considered that the
Published Version
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