Predicting radioactive accessory mineral dissolution during chemical weathering: The radiation dose at the solubility threshold for epidote-group detrital grains from the Yangtze River delta, China

Abstract

The influence of heavy-ion irradiation on the solubility of natural epidote-group minerals has been investigated. The epidote-group are calc-silicate minerals that, if soluble during terrestrial chemical weathering, are capable of consuming atmospheric CO2 over geologic time. The experimental design was to calculate the α-particle dose for actinide-rich epidote-group grains from the deltas of rivers draining large watersheds. Large watersheds should yield a suite of epidote-group grains spanning the range of compositions and radiation damage commonly observed in nature. These grains would be resistant to dissolution during chemical weathering and survive into fluvial sediments. Therefore, the highest α-particle dose calculated for the suite of grains represents a minimum solubility threshold for the epidote-group minerals during chemical weathering. The α-particle dose is calculated from the 232Th, 238U, and 235U content of a grain, and the grain’s date. Actinide-rich epidote-group grains were isolated from Yangtze River and River Nile delta sediments, although only the grains from the Yangtze delta had 232Th/208Pb ratios sufficiently high to yield meaningful dates.A total of 28 Th-rich epidote-group grains were isolated from the Yangtze delta sediment, with 24 being classified as allanite and four being classified as REE-rich epidote. Isotopic ratio data were measured by laser ablation microprobe-inductively coupled plasma-mass spectrometry. Dates and REE patterns revealed at least 17 different sources of grains, with ThO2 contents ranging from 0.057 to 4.37 wt. %, and dates ranging from 32±2 to 3788±263Ma. The calculated maximum α-particle dose of the suite of Yangtze delta epidote-group mineral grains, and hence the minimum solubility threshold, is 7.1 × 1015 α-decay mg−1. Therefore, to predict the likelihood of an epidote-group mineral dissolving during chemical weathering at a specific study site the α-particle dose may be calculated from the date of the rock, even if estimated, and the radioactinide concentrations of the mineral. A calculated α-particle dose below ~7.1 × 1015 α-decay mg−1 likely reflects epidote-group mineral stability during chemical weathering.Calculation of the α-particle dose may be superior to microscopic identification of accessory (<2% by volume) mineral dissolution during chemical weathering. This is because accessory minerals occur in relatively low abundances, may occur as relatively small grains, be highly soluble, and/or be heterogeneously distributed in the bedrock and regolith. Furthermore, the solubility of radioactively damaged accessory minerals cannot be readily predicted or quantified by geochemical thermodynamic and/or kinetic principles.The α-particle dose at the solubility threshold of ~7.1 × 1015 α-decay mg−1 value is approximately double that reported for zircon. Therefore, relative to zircon, epidote-group minerals can withstand more radiation prior to becoming soluble. This finding is surprising considering the chemical durability of zircon. This substantial variance in solubility threshold radiation doses between the two minerals cannot be explained by the bond strengths of non-tetrahedral cations and structural oxygen which are lower in allanite than zircon.