Abstract
The distributions of 210Pb and 210Po, short half-life products of 238U decay, in geological and related anthropogenic materials are reviewed, with emphasis on their geochemical behaviours and likely mineral hosts. Concentrations of natural 210Pb and 210Po in igneous and related hydrothermal environments are governed by release from crustal reservoirs. 210Po may undergo volatilisation, inducing disequilibrium in magmatic systems. In sedimentary environments (marine, lacustrine, deltaic and fluvial), as in soils, concentrations of 210Pb and 210Po are commonly derived from a combination of natural and anthropogenic sources. Enhanced concentrations of both radionuclides are reported in media from a variety of industrial operations, including uranium mill tailings, waste from phosphoric acid production, oil and gas exploitation and energy production from coals, as well as in residues from the mining and smelting of uranium-bearing copper ores. Although the mineral hosts of the two radionuclides in most solid media are readily defined as those containing parent 238U and 226Ra, their distributions in some hydrothermal U-bearing ores and the products of processing those ores are much less well constrained. Much of the present understanding of these radionuclides is based on indirect data rather than direct observation and potential hosts are likely to be diverse, with deportments depending on the local geochemical environment. Some predictions can nevertheless be made based on the geochemical properties of 210Pb and 210Po and those of the intermediate products of 238U decay, including isotopes of Ra and Rn. Alongside all U-bearing minerals, the potential hosts of 210Pb and 210Po may include Pb-bearing chalcogenides such as galena, as well as a range of sulphates, carbonates, and Fe-oxides. 210Pb and 210Po are also likely to occur as nanoparticles adsorbed onto the surface of other minerals, such as clays, Fe-(hydr)oxides and possibly also carbonates. In rocks, unsupported 210Pb- and/or 210Po-bearing nanoparticles may also be present within micro-fractures in minerals and at the interfaces of mineral grains. Despite forming under very limited and special conditions, the local-scale isotopic disequilibrium they infer is highly relevant for understanding their distributions in mineralized rocks and processing products.
Highlights
Introduction and210 Po are intermediate isotopes within the 238 U decay-series (Figure 1) and occur in minute amounts in nature [1]
Considerations of the likely mineral hosts for 210 Pb and 210 Po needs to include the geochemistry of each specific radionuclide, but the geochemical behaviour of the entire 238 U decay chain in the context of half-lives that range from fractions of a seconds (214 Po) to billions of years
Roudil et al [200] explored the creation of “bubbles” in uranium minerals from production of radiogenic helium as a product of alpha decay. This He may be concentrated in the mineral grains, or trapped at grain boundaries, generating matrix damage. Their measurements of He outgassing from a uranium ore showed that less than 5% of the He produced was conserved, and that one-third of the residual He is occluded in the matrix and vacancy defects, and two-thirds occurred as bubbles observable by high-resolution transmission electron microscopy
Summary
210 Po are intermediate isotopes within the 238 U decay-series (Figure 1) and occur in minute amounts in nature [1]. 210 Pb has a half-life of 22.2 years and is generated via: (i) the decay of atmospheric 222 Rn gas (“unsupported” 210 Pb); and (ii) via the continuous production of 222 Rn from natural 226 Ra contained in crustal materials (“supported” 210 Pb). 210 Pb decays to 210 Bi by emission of a beta particle. 210 Pb is useful for determining the age of a recent sediment in that, provided that the atmospheric flux is constant, the decay profile relates directly to sedimentation rate. Bi (half-life 5.01 days) and decays to. Pb by emission of an alpha particle.
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