Suite Of Grains Research Articles

Connecting visual metamictization to radiation damage to expand applications of zircon (U-Th)/He thermochronometry

Zircon (U-Th)/He (ZHe) thermochronometry quantifies the timing and tempo of low-temperature processes and is used to deconvolve tectonic and erosional histories. Accumulation and annealing of radiation damage impacts He diffusion in zircon and resulting ZHe dates. Resolving complex histories requires building relationships between ZHe date, effective U (eU), and radiation damage in grains sharing a common thermal history. Prior work demonstrated that purposefully selecting grains with a spectrum of visual metamictization yields a broad range of intrasample eU values (Ault et al., 2018), but it remained unclear if visual metamictization tracks effective radiation damage. Here we evaluate relationships between visual metamictization, effective damage calculated from Raman spectroscopy, and ZHe dates from a new suite of grains from some of the same Precambrian samples investigated in Ault et al. (2018) and Phanerozoic detrital grains from Armstrong et al. (2022). New ZHe analyses (n = 21) confirm increasing visual metamictization corresponds with increasing eU concentration and dates fall along previously reported ZHe date-eU trends for each sample, despite grain selection by different analysts in different sessions. Raman-based alpha dose calculations from multiple spot analyses from transects across the surface and interior of each grain (n = 480 total analyses) range from 3.19 × 1016 to 1.52 × 1019 α/g across the whole dataset. Alpha dose increases in samples characterized by a clear increase in visual metamictization from different types of ZHe date-eU patterns, supporting that visual metamictization reflects effective radiation damage in these samples. Complexities in visual metamictization-damage trends are due to damage zonation observed in cathodoluminescence, comparative internal and external spot analyses, and 2-D Raman maps, as well as limited spread in intrasample visual metamictization and user grain selection bias. Overall, visual metamictization provides a qualitative estimate of effective damage and should be leveraged when selecting grains for traditional ZHe analyses to build ZHe date-eU-damage patterns.

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

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.

187Os–186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon

More than 100 Os–Ir–Ru alloy grains from southwestern Oregon, presumably originating in the Mesozoic Josephine ophiolite, were examined for major and minor element compositions and 187Os/188Os isotopic compositions. Sixteen grains spanning the range of 187Os/188Os ratios present in the larger suite were analyzed for high-precision 186Os/188Os ratios. The ranges of major and minor element abundances of individual grains were, in general, similar to compositions previously reported for such grains worldwide. Within-suite compositional variations were quite large. Most grains were likely disaggregated from larger intergrown clusters, and were subdivided based on texture into matrix and lamellae. Composition variations within individual matrix grains were limited, whereas variations within individual lamellae grains were considerably larger.The 187Os/188Os ratios of individual grains were isotopically homogeneous at the ±0.1% level of resolution. The 187Os/188Os ratios of the entire suite of grains ranged from 0.1173 to 0.1468. The average ratio for all grains combined was 0.1241, similar to the composition of three chromitites sampled from the Josephine Creek, and consistent with formation in the Mesozoic upper mantle. The average 186Os/188Os ratio of the subset of 16 grains was 0.119838. All grains but one had isotopic ratios within ±20 ppm of that ratio.Results indicate that 187Os/188Os variations on a scale of mm3 may be quite large within the upper mantle, mimicking heterogeneities observed in both hand-and outcrop-scale samples of upper mantle materials. Highly depleted compositions relative to ambient upper mantle with chondritic 187Os/188Os require that some of the heterogeneities must be >1 Ga. The homogeneity of the 186Os/188Os ratios lend no support to the contention that isotopic heterogeneities within the upper mantle may explain coupled 186Os–187Os enrichments that have been observed in putative plume derived lavas.