Metamictization and chemical durability of detrital zircon

We have investigated the effect that metamictization has on the weathering of zircon in detrital continental sediments and tropical soils of the Amazo n basin. Brazil. The degree of radiation damage in the near-surface region of the zircon grains was determined by Raman microprobe. In each of the fou r series investigated (i.e., sediment, podzol, topsoil, and subsoil horizon s of lateritic soil), the degree of radiation damage ranges from less than 10(14) to similar to3.5 x 10(15) alpha -decay/mg. The maximum degree of rad iation damage coincides with the first percolation threshold of the metamic tization process at similar to3.5 x 10(15) alpha -decay/mg. Below this thre shold, amorphous volumes in the structure of damaged zircon are not connect ed to each other. The ranges of U, Th, and Ph contents (in ppm) measured by proton induced X-ray emission (PIXE) microanalysis are 100 < U < 7000, 100 < Th < 18000, and 100 < Pb < 1300. Chemical ages, assessed from U, Th, and total-Pb, range between 0.15 Ga and 2.8 Ga. This range is roughly consiste nt with the ages reported for the Precambrian shields of the Amazon basin ( 0.45-3.5 Ga). Corresponding radiation doses range between <2 x 10(15) and 3 x 10(16), <alpha>-decay/mg. Comparison of calculated doses with the degree of structural damage indicates that most of the zircon grains have experie nced significant annealing. However, the degree of annealing differs from o ne grain to another. Thus, the acute maximum limit observed for the degree of radiation damage of the whole zircon series is better explained by low-t emperature alteration or weathering processes than by thermal resetting. Fo llowing this interpretation, our results provide evidence for a dramatic de crease in the chemical durability of zircon in natural weathering environme nts when the radiation dose exceeds 3.5 x 10(15) alpha -decay/mg. Below the first percolation threshold., the zircon population survives the soil form ation intact, but more damaged zircons are dissolved during weathering/ alt eration processes.