Home – Home – Jornals – Russian Geology and Geophysics 2006 number 2

A semi-empirical model is presented, which describes the processes of formation, migration, and accumulation of radiogenic lead (Pb_Rn) in opals deposited in open cavities. In contrast to lead that forms in situ through radioactive decay of uranium trapped by opal, Pb_Rn is produced from uranium disseminated in the rock enclosing the cavities. Its incorporation into the opal is described by the following chain of processes: decay of parental uranium to yield radon - emanation and diffusion migration of radon from the rock into the cavity - decay of radon to yield lead - diffusion migration of lead in the cavity - adsorption of lead on colloidal silica particles - coagulation and settling of the colloidal particles - formation of opal. Besides the colloidal adsorption, Pb_Rn can also be incorporated into the growing opal through direct diffusion flow onto its surface. The latter mechanism is also relevant to minerals crystallizing from ionic solutions; it is less efficient than the mechanism of colloidal adsorption.
Distribution of Pb_Rn isotopes throughout a cavity depends on the cavity geometry and the half-life of the parental Rn isotope. In cavity filled with stagnant water, the concentration profiles of Pb_Rn show maxima at some distance from the cavity wall. The movement of water through a cavity leads to a more complex distribution of Pb_Rn isotopes. The model describes accumulation of Pb_Rn on silica micelles during their growth until the critical size of coagulation (5-10 nm) is reached and during the subsequent coagulation. For micelles 5-50 nm in size, the calculated concentrations of Pb_Rn agree with those in young (Miocene and younger) natural opals.
Opals formed in open cavities in acidic volcanic rocks (with U = 4-5 ppm) can trap significant amounts of Pb_Rn. Applying standard U-Pb dating equations, which ignore Pb_Rn, will yield an overestimated age of such opals. This is true for young opals (younger than ≈30 Ma). As the age of opal increases, the share of radiogenic Pb formed in situ also grows, and the PbRn correction becomes negligible.