Activity disequilibrium of Th, U, and U in old stilbite: Effects of young U mobility and α-recoil

Stilbite from Malmberget and Svappavara is part of hydrothermal mineral assemblages occupying regionally occurring open Palaeoproterozoic fractures in northern Sweden. At these locations, stilbite is characterized by Pb_rad excess relative to U and by activity ratios of ²³⁴U]/²³⁸U] > 1 and ²³⁰Th]/²³⁸U] > 1. The activity disequilibrium requires a disturbance of the U-Th systematics within the last one million years. Leaching and infiltration experiments on Malmberget stilbite demonstrate: (i) preferential leaching in the order Pb >U >Th and uptake in the order Pb > U, and (ii) isotopic fractionation of U by preferential mobilization of ²³⁸U and ²³⁵U relative to ²³⁴U. Stepwise-leaching further indicates that the bulk of U is hosted in the channel sites of stilbite. The Th-U disequilibrium systematics observed in untreated Malmberget and Svappavara stilbite can be explained by: (1) addition of U with ²³⁴U]/²³⁸U] > 1 from a fluid, or alternatively (2) loss of U from a two-component system, consisting of a component that is “open” or accessible and a component that is “closed” or inaccessible to mobilization. U addition requires a multistage history involving multiple gain or loss of U and/or Pb. In contrast, U loss does not necessarily require multistage processes but can also be explained by preferential removal of ²³⁸U (and ²³⁵U) relative to recoiled daughter isotopes such as ²³⁴U, ²³⁰Th, and ²⁰⁶Pb (and ²⁰⁷Pb) during a single event. Such a behavior could be obtained if the recoiled daughter isotopes of channel-sited uranium are implanted into the crystal lattice and, in such a way, become less mobile than their parent isotopes. This case implies an open-system behavior for ions in the channel sites and a closed-system behavior for ions in the silicate framework of stilbite. Each α-recoil directly or indirectly, i.e., through its recoil cascade, damages the silicate framework. Subsequent (continuous) low-temperature annealing of the damaged stilbite lattice could trap the recoiled daughter isotopes in the repaired crystal lattice or sealed-off channels. Such immobile recoiled material can, in part, represent the “closed” component of the system. This model can account for all observations regarding the Th-U-Pb systematics, including the Th-U disequilibrium systematics, the similarity in Th/U as deduced from Th-U disequilibrium and Pb isotope data, and the excess of radiogenic Pb (²⁰⁸Pb-parents also had been multiply recoiled). These two contrasting explanations involve either multistage or multicomponent systems. They do not permit the derivation of an accurate age.