RADIATION-DAMAGE ENHANCED CHEMICAL ALTERATION OF ZIRCON IN LATERITE, NSIMI, CAMEROON

Zircon, as a nearly inert mineral, does not undergo chemical loss during soil formation and has been used for mass balance calculations; however, radiation damage from the alpha-decay of U and Th increases its chemical reactivity during the formation of laterite, especially in the upper organic rich zone of the profile. The relation between zircon alteration and accumulated radiation damage in a laterite was examined in three sets of samples that formed by direct alteration of the bedrock, the homogeneous soil and the organic rich horizon. Secondary electron (SE) and back-scattered electron (BSE) images of the zircons clearly revealed evidence of alteration within regions that were highly radiation damaged. The chemical compositions of zircons have a trace amount of impurity elements; Al and Fe (up to 1 wt. %).The correlation of alpha-decay dose (0.1 to 1.5 displacements per atom) and the increased concentrations of Al and Fe indicates that the extent of alteration is controlled by the extent of remnant crystallinity. EMPA of zircons near the source rock, gave discordant chemical ages (0.2-1.9 By), but the actual age (2.7 By) was preserved in several zircons. Based on TEM analysis, there is no evidence of a reprecipitation of baddeleyite (ZrO2) or incongruent dissolution of zircon. The slight Zr-enrichment inferred by electron microprobe analysis of the altered zones (~ 8 wt. %) is probably due to selective leaching of the silica-rich nanodomains over zirconium-rich nanodomains, which exist due to alpha-decay event damage. TEM observation reveals two different micro-textures at the scale of a few tenths of microns. In zircon with randomly oriented crystallites, fractures contain alteration products, i.e. Fe, Al, and Si. In pristine zircon, nano-vesicles are abundant; the vesicles do not form as a connected network and contain some chlorine (more than 5 wt. %) and to a lesser extent, Na and Ca. Moreover, Electron energy loss spectra do not indicate the presence of He, excluding the possibility that those vesicles are the result of accumulation of alpha-particles. The recovery of the crystalline structure may account for the formation of voids because the amorphous domains that are susceptible to recrystallization have a lower density. Amorphous domains probably facilitate elemental diffusion, such as non-magmatic Cl.