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Thorium (Th) is a naturally occurring radioactive element found in the environment, and recent advancements have been made in identifying and characterizing Th-bearing nanoparticles (NPs). However, the main focus is still on synthesized Th-bearing NPs and knowledge about natural Th-bearing NPs remains limited. Here, high-resolution transmission electron microscopy (HRTEM) observations of thorite from the Huayangchuan uranium ore deposit in Shaanxi Province, Central China, have revealed the nanoscale mineral characteristics of thorite. In this study, thorite NPs ranging from 5–10 nm in size were identified within the uranium ore. A combination of transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) elemental mapping and corresponding HRTEM images alongside Selected Area Electron Diffraction (SAED) and Fast Fourier Transform (FFT) patterns revealed a complex nanoscale structure in the thorite NPs, consisting of both amorphous and crystalline nanodomains with abundant defects. These nanostructures are associated with a metamictization mechanism in micrometer-sized thorite. Our findings indicate that the metamictization process can generate numerous thorite nanoparticles. Given the high penetrability and mobility of these NPs, the metamictization of thorite poses new challenges for the long-term stability of radioactive substances and the storage containers for radioactive waste. Furthermore, considering the likelihood of environmental release and the chemical toxicity, radioactivity, and nanotoxicity of natural Th-bearing NPs, increased attention should be given to the presence of natural thorite NPs in the ore deposit. [Display omitted] • Thorite comprises both originally crystalline minerals and amorphous matrix interspersed with isolated thorite nanoparticles. • Th-bearing NPs could play a significant role in the migration and potential environmental distribution of thorium. • Natural Th-bearing NPs is crucial for addressing future environmental challenges. [ABSTRACT FROM AUTHOR]

This study investigated the feasibility of Raman spectroscopy as a rapid and nondestructive chemical analysis tool for assessing radiation-induced amorphization in aggregate-forming minerals. Metachert and sandstone concrete aggregates, rich in quartz and containing additional minerals, such as albite and microcline in the case of sandstone, were exposed to various neutron fluence levels (1.2, 2.2, 7.0, and 14.3 × 1019 n/cm², E ≥ 0.01 MeV) at ∼53.3°C. The Raman spectra of both the nonirradiated and irradiated samples were collected with a specific focus on quartz characterization. Concurrently, X-ray diffraction (XRD) refinement was employed to calculate the cell volume expansions of the studied mineral. The crystal lattice defects in α-quartz induced by neutron irradiation cause changes in the Raman band attributes (band position, full width at half maximum, and relative intensity), and are also responsible for the growth of defect bands. The behavior of the most intense vibrational bands can be used to estimate the change in the cell volume of irradiated α-quartz in different rocks. The data analysis presented in this study demonstrates a good correlation between the changes in the cell volume of quartz, as measured using XRD, and the Raman band attributes. This correlation enhances our understanding of structural alterations, emphasizing the potential of Raman spectroscopy as a reliable method for investigating the structural changes induced by the irradiation of minerals and highlighting its agreement with well-established XRD analyses. [ABSTRACT FROM AUTHOR]

Accessory minerals in granitic rocks are unlikely significant radionuclide contributions to groundwater due to their remarkable durability. However, accessory minerals incorporating U and Th may suffer structural damages due to the radioactivity and become highly susceptible to alteration. This study investigates geochemistry coupled with textural analysis of the U–Th bearing accessory minerals using a field emission scanning electron microscope and an electron probe micro-analyzer. Altered zircons with numerous open structures related to the radioactive decay show higher contents of U and Th and low analytical totals. Some thorites show high contents of U and non-formula elements due to the hydrothermal alteration in the metamicted thorite. The cerianite including U occurs as micro-veinlet in fracture with trace of Fe and Mn oxides, which indicates secondary phase formation from the decomposed accessory minerals in an oxidizing environment. Some accessory minerals with the high content of U and Th have been found in Mesozoic granite terrain in South Korea, where high concentration levels of radionuclide in groundwater were also reported. The leaching of U may be more likely when the accessory minerals are highly metamicted or altered as found in our samples. The altered zircon and thorite of the study area could be major carriers of radioelement in Mesozoic granitic aquifers where the occurrence of soluble U-minerals has not been reported. [Display omitted] • High radionuclides concentration in groundwater poses a threat to human health. • Accessary minerals can enrich groundwater with radionuclides in granite aquifers. • Altered zircons contain high U and Th levels with open structures. • Thorites show elevated U levels and non-formula elements due to alteration. • Metamicted and/or altered accessory minerals increase uranium leaching potential. [ABSTRACT FROM AUTHOR]

High-resolution electron backscattered diffraction (EBSD) and correlated in situ secondary ion mass spectrometry (SIMS) U–Pb geochronology have been conducted on impact-metamorphosed zircon, titanite, and apatite grains from the 453 Ma Lac La Moinerie impact structure of Québec, Canada. Building on previous work, our new U–Pb results highlight the challenges associated with SIMS dating of low U, common Pb-rich accessory phases (apatite and titanite) within terrestrially shocked impact lithologies. Unlike zircon, microstructurally controlled Pb loss is limited in apatite and titanite, providing evidence that the mechanisms governing impact resetting in these phases are distinct from high U, low common Pb phases (zircon, monazite, baddeleyite). EBSD mapping reveals internal complexity within all three phases, providing new insights into their microstructural evolution during impact-related metamorphism. Correlated EBSD and Raman analysis of Former Reidite In Granular Neoblastic (FRIGN) zircon provides the first documented case of post-recrystallization radiation damage of individual zircon neoblasts. Two modes of recrystallization are recorded in apatite: systematic (Type I), wherein strain-free domains are preferentially oriented along specific crystallographic planes, and non-systematic (Type II), where strain-free domains exhibit no preferred orientation. A new shock twin is observed in titanite that has a disorientation relationship of 74°/ < 103 > , interpreted to be the result of strain-induced misorientation of pre-existing 1 1 ¯ 1 shock twins. We document the first case of impact-recrystallized titanite, which is intergrown with granular rutile and partially preserves former 1 1 ¯ 1 shock twins along boundaries of strain-free subdomains. Based on our EBSD observations, we provide a microstructural framework outlining the sequential evolution of zircon, titanite, and apatite during shock and thermal metamorphism. [ABSTRACT FROM AUTHOR]

Cheongwon is well-known in South Korea for the abnormally high aqueous concentrations of U and Rn in the underlying Jurassic granite aquifer. This study reports on the chemistry and texture of a new U-host mineral to indicate the effective source of U in the granite aquifer, and discusses the implications of the occurrence of U near groundwater monitoring wells. To investigate the internal fabric and homogeneity, the mineral chemistry and texture were examined using field-emission scanning electron microscope and electron probe micro-analyzer, respectively. Uranopolycrase, a mineral of the euxenite group, occurred as a large subhedral grains in a pegmatite related to a Jurassic granite pluton. A significant U content indicated metamictization, which resulted in an optically isotropic body and generate cracks and voids. Elemental distribution maps revealed that uranopolycrase had relatively low in U and Y contents, but high in Ti and Pb contents near a micro-veinlet and void. These findings suggest that U could be preferentially leached out as a result of alteration and the loss of A-site cations in metamict uranopolycrase. The alteration of metamict uranopolycrase is an effective process for releasing radionuclides into the environment. Based on the U- and Ti-remobilized alteration texture, uranopolycrase is considered to be the most important host mineral of the interpreting the availability of U- and Rn-supplies in the granite aquifer of the study area in Cheongwon. [ABSTRACT FROM AUTHOR]

[Display omitted] • We identified W-rich zircons from the granite in the Hukeng deposit. • These W-rich zircons record the activity of W-rich ore-forming fluids. • W/Gd ratios of zircons can be used as an indicator to estimate mineralization potential. South China is endowed with copious hydrothermal tungsten ore deposits, which are generally suggested to be associated with highly fractionated granites. However, the mechanism of tungsten enrichment and mineralization during magmatic-hydrothermal processes is not clear. Here, we clearly identified W-rich zircons (up to several thousand ppm) from the granite in the Hukeng tungsten deposit in South China. W-rich zircons are generally dark in cathodoluminescence images with irregular cracks, cavities and blurry oscillatory zones. They also show high contents of U, Nb, REEs, and high W/Gd ratios, which are not possible to be produced only by magma differentiation. Raman spectra of zircons indicate that these W-rich zircons have experienced different degrees of metamictization. Thus, these W-rich zircons should be crystallized from the late magma and subsequently altered by W-rich magmatic-hydrothermal fluids. W-rich zircons record the activity of W-rich ore-forming fluids which were exsolved from the granitic magma. We suggested that the W/Gd ratios of > 0.2 of zircons can be used as an indicator to estimate the W mineralization potential of a granite. This study provides a new pathway for using zircon to record tungsten mineralization processes and to guide tungsten ore prospecting. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is a convenient, efficient, and non‐destructive technique that could estimate the metamictization degree of zircon, particularly for gem‐class or historical samples. The full width at half maximum value of the ν3(SiO4) band around 1000 cm−1 can classify the structural state of zircon as crystallized (the full width at half maximum value is less than 5 cm−1), intermediate (the full width at half maximum value ranges from 5 to 15 cm−1), and metamict (the full width at half maximum value is more than 15 cm−1). This study focuses on the zircon samples from Sri Lanka in the gemological collection of Abraham Gottlob Werner. These samples show red to green, according to the Munsell color system and the CIEL*C*h* color system, and their density is from 4.07 to 4.87 g/cm3. Comparing the color and metamictization degree data of zircon samples, crystallized zircon displays a more red tone with yellow. The green tone in zircon occurs with metamictization. The density of zircon could be an indicator of the metamictization degree as crystallized zircon is 4.57 to 4.70 g/cm3 in a 95% confidence interval, which matches the theoretical density (4.61–4.76 g/cm3) better, whereas metamict zircons present a density of 4.37–4.59 g/cm3, and the density of zircon decreases with metamictization. The metamictization has a significant influence on the gemological properties of zircon, whereas the typical color tone and low density can be a sign of the metamictization. [ABSTRACT FROM AUTHOR]

The structural and chemical properties of zircon inclusions in garnet megablasts from the Dora Maira Massif (Western Alps, Italy) were characterized in detail using charge contrast imaging, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The aim of this work is to determine to what extent the degree of metamictization, metamorphic recrystallization, inherent structural heterogeneity, chemical composition, and zoning, along with the elastic stress imposed by the host mineral, can influence the Raman peak position of the zircon inclusion and hence, the residual pressure estimated via Raman geo-thermobarometry. We show and confirm that metamictization and inherent structural heterogeneity have a major influence in the Raman spectra of zircon in terms of peak position and peak width. We suggest that, for spectral resolution of 2 cm–1, the peak width of the B1g mode near 1008 cm–1 of reliable grains must be smaller than 5 cm–1. The method can be applied to both inherited igneous and newly formed Alpine metamorphic crystals. By coupling structural and chemical information, we demonstrate that there are no significant diferences between the Raman spectra of zircon with oscillatory-zoned texture, formed during magmatic crystallization, and those formed by fluid-induced Alpine (re)crystallization. The discrimination between magmatic and metamorphic zircon based only on micro-textural constraints is not robust. Finally, our results allow establishing a protocol devoted to the selection of reliable buried zircon inclusions, relying only on Raman spectroscopic measurements, to use for elastic thermobarometry applications. [ABSTRACT FROM AUTHOR]

In situ U-Pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry (LA-ICP-MS) on shocked and thermally metamorphosed apatite, titanite, and zircon grains from the Steen River impact structure, Canada. The dated relict mineral phases occur within impact melt-bearing breccias that underwent post-deposition sintering at 450 °C > T < 800 °C. Apatite yields a refined lower intercept age of 141 ± 4 Ma, which we interpret as the best estimate age for the Steen River impact event. Titanite was only partially reset; yielding a lower intercept age of 113 ± 41 Ma. Zircon yields a lower intercept impact age of 120 ± 14 Ma, which is considered a minimum best-estimate impact age. The most reset zircon analyses that control this lower intercept are complicated by combinations of common-Pb incorporation and recent Pb loss associated with granularized and radiation-damaged (metamict) domains. All three phases preserve Paleoproterozoic crystalline basement ages, with a single concordant 206Pb/238U age of 1914 ± 39 Ma from apatite, an upper intercept age of 1882 ± 11 Ma from zircon, and an upper intercept age of 1842 ± 9 Ma from titanite. For apatite, the degree of isotopic resetting is largely thermally controlled, with the extent of reset closely associated with textural setting (degree of grain armouring, melt proximity and sample temperature) and, to a lesser extent, by shock/thermally generated microstructures (i.e., planar fractures, micro-vesicles, and recrystallized domains). While titanite records an impact age that falls within error of apatite and zircon, the majority of grains experienced only partial isotopic resetting, which we attribute to incomplete Pb loss associated with rapid cooling and post-depositional sintering of the breccia matrix below the titanite closure temperature (∼800 °C). In zircon, ancient (impact) Pb loss was facilitated along defect-related, fast-diffusion pathways within pre-impact metamict domains, shock defects, and via recrystallization. These same domains were also subject to recent (post-impact) Pb loss and common Pb contamination, significantly compromising the reliability of zircon ages. The distribution of U-Pb ratios in apatite and titanite is unlike those obtained in crystalline targets, a feature we interpret to be characteristic of impact structures developed in mixed (sedimentary – crystalline) targets, such as Steen River. In this case, disaggregated melt systems create thermal regimes distinct from those derived from predominantly igneous/metamorphic targets. With an age of 141 ± 4 Ma, Steen River joins the Dellen (Sweden), Gosses Bluff (Australia), Mjølnir (Barents Sea), and Morokweng (South Africa) impact structures in being formed at, or close to, the Jurassic-Cretaceous boundary. [ABSTRACT FROM AUTHOR]

The zircon U-Pb system is one of the most robust geochronometers, but during an impact event individual crystals can be affected differently by the passage of the shock wave and impact generated heat. Unraveling the potentially complex thermal history recorded by zircon crystals that experienced variable levels of shock and heating, as well as additioanl pre- and post-impact thermal events, has been difficult using classical geochronological methods. The existing high-precision 40Ar/39Ar age constraints for the K-Pg Chicxulub event, and the previous U-Pb dating of the basement rocks from the impact site, make Chicxulub an ideal location to study impact-induced effects on the zircon U-Pb systematics and to evaluate potential 'memory effects' of pre-impact U-Pb signatures preserved within those individual zircon crystals. Recent IODP-ICDP drilling of the Chicxulub impact structure recovered 580 m of uplifted shocked granitoid and 130 m of melt and suevite, providing an unprecedented opportunity to study zircon crystals subjected to a range of shock pressures, thermal, and deformational histories. Zircon morphologies were classified using scanning electron microscopy (SEM) imaging and then samples were depth profiled using laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) to document the range of preserved age domains from rim-to-center within individual crystals. The results show U-Pb ages range from 66 to 472 Ma, which are consistent with both inherited Carboniferous and Late Paleozoic basement ages as well as Pb loss ages in response to the K-Pg impact event. While the bulk of the zircon grains preserve Paleozoic ages, high U (metamict) zones within fractured zircon crystals exhibited an age within uncertainty (66 ± 6.2 Ma) of the impact age (66.038 ± 0.049 Ma), indicating that inherited intragrain U-Pb kinetics and/or hydrothermal fluid flow may have controlled age resetting those zircon crystals rather than impact-induced shock and heating alone. Moreover, the calculated α-decay doses suggest that the zircon crystals experienced Stage 1 or early Stage 2 radiation damage accumulation. Therefore, we suggest that the lowered crystal annealing temperature in crystals that previoulsy experienced radiation damage make the zircon U-Pb clock either more susceptible to the relatively short heat pulse of the impact event, the moderate pressure and temperature conditions in the peak ring, and/or to hot-fluid flow in the long-lasting post impact hydrothermal system. • Shocked zircon crystals extracted from Chicxulub's peak ring rocks are analyzed via U-Pb LA-ICP-MS depth profiling. • U-Pb LA-ICP-MS depth profiling and the subsequent data export in 1 s increments allows to spatially resolve age trends. • Zircon U-Pb clock partially and possibly fully reset due to the impact event. • Younger U-Pb ages are preserved in fractured zircon with a high U [ppm] concent. or in zircon displaying granular textures. [ABSTRACT FROM AUTHOR]

Recent studies have shown that α-damage in titanite influences He diffusivity and thus the closure temperature of the (U-Th)/He system in titanite. We compare different methods for measuring the α‑dose in titanite by Raman spectroscopy. Raman spectra of randomly oriented titanite fragments from the Archean Karelian domain in eastern Finland along with some well-studied young titanites and U-Pb standard reference materials were analyzed and related to the concentration of α-emitting elements (U and Th) that generated damage in the respective grains. Automated curve-fitting was performed by the IFORS software and different curve-fitting protocols were tested and compared. The Raman bands at 424 and 465 cm-1 show a good correlation of full-width at half maximum (FWHM) and position with the α-dose. However, these bands are not always present because titanite is highly anisotropic implying that Raman spectra are sensitive to orientation. The intensity-weighted mean FWHM (iw-FWHM) of all Raman bands of a spectrum proves to be the most robust measure of the α-dose. A simplified fitting approach considering 15 peaks is sufficient to describe the accumulated α-dose. For α-doses below 5 × 1016α/g the iw-FWHM is independent of α-dose and ranges from 25 to 50 cm-1. Above this value the iw-FWHM increases linearly with increasing α-dose up to 3 × 1018α/g. The linear correlation can be described as iw-FWHM[cm-1] ≈ 39(±1.2)[cm-1] + 3.84(+0.61,–0.26) × 10-17[cm-1/(α/g)] × α-dose[α/g]. The approach provides a pre-selection method to optimize the range of α-doses of titanite crystals to be dated by (U-Th)/He thermochronology. [ABSTRACT FROM AUTHOR]

Abstract The enriched basaltic (martian) shergottite Ksar Ghilane (KG) 002, discovered in 2010, is exceptionally rich in coexisting but discrete apatite and merrillite crystals. It has been selected to better constrain the formation conditions and post-crystallization processes, and thus the evolution of martian rocks based on Ca-phosphates. A petrological, chemical, chronological and microstructural approach using a series of high-spatial resolution techniques including Raman spectroscopy, electron microscopy (SEM, EPMA, CL-imaging) and secondary ion mass spectrometry (SIMS) analysis has been applied to a representative number of Ca-phosphate grains. Analytical results for apatite and merrillite reveal: (i) zoning in F, Cl, Br and I concentrations, (ii) elevated Cl concentrations in the range of ∼11,900–35,300 µg/g and halogen ratios, i.e., Cl/Br and Cl/I, as well as stable chlorine isotope composition, reported as δ37Cl values rel. to Standard Mean Ocean Chloride (SMOC, defined as 0‰) with a value of +0.67 ± 0.14‰ (1σ), distinguishing KG 002 phosphates from that of other enriched and depleted shergottites. The halogen and heavier δ37Cl record indicate a slightly higher degree of ∼3.5% assimilation of Cl-rich and isotopically heavier crustal reservoir on Mars when compared to other enriched shergottites. (iii) Structural investigations together with the chemical and petrological context of the grains confirm the occurrence of hydroxyl-poor merrillite, indicate weak if any alteration effects induced by metamictization, only minor structural modifications due to shock metamorphism, and absence of replacement reactions. Therefore, igneous crystallization of Ca-phosphates from a fractionated, hydrous and ferrous mantle source, rich in volatiles including the halogens and Na and lithophile rare earth-elements, and absence of interaction with crustal fluids/brines of the sample is deduced. (iv) The Pb isotopic composition of six apatite and three merrillite grains is highly unradiogenic and the 238U-206Pb record yields a phosphate crystallization time at 395 ± 240 Ma (2σ), which is similar to those of other enriched shergottites. [ABSTRACT FROM AUTHOR]

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Abstract: Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (204Pb/206Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (204Pb/206Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a 207Pb/206Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons. [ABSTRACT FROM AUTHOR]

The crystal structure of a Nb-rich zirconolite-3T from the Laach Lake volcano, Eifel, Germany, was studied by single-crystal XRD (R=0.0295). The mineral is trigonal, P3121; unit-cell dimensions are: a=7.3095(2), c=16.9604(5) Å, V=784.78(4) Å3. The empirical formula based on 14 O atoms (Z=3) is Ca1.28Ce0.31La0.14Nd0.12Pr0.06Th0.16 Zr1.94Hf0.04Ti1.74Nb1.22Fe0.72Mn0.28O14. The crystal-chemical formula based upon the structural model is: VIII(Ca0.70 Ce0.30)VIII(Ca0.58Ce0.32Th0.10)VII(Zr0.88Th0.04Hf0.02□0.06)2VII(□0.94 Zr0.06)2VI(Ti0.72Nb0.26Zr0.02)VI(Ti0.50Nb0.47Zr0.03)2IV(Fe0.355 Mn0.145)2O14 (Z=3). [ABSTRACT FROM AUTHOR]

Broad variability in intrasample effective U concentration (eU) in zircon crystals is required to characterize zircon (U-Th)/He (zircon He) date-eU relationships that resolve thermal histories in ancient rocks. We describe a new protocol for selecting grains for zircon He thermochronometry that leverages visual crystal metamictization, a proxy for radiation damage accumulation, to generate a range in intrasample eU and zircon He date-eU/metamictization trends. Plane polarized light images, eU concentration, and zircon He data from 59 individual grains from six Proterozoic and three Archean North American granitoids reveal a progressive increase in crystal opacity and discoloration that correspond to increasing eU in each sample. Zircon eU ranges from 89 to 1885 ppm across the dataset and individual samples have eU variations of 538 ppm to 1579 ppm. Three Proterozoic samples yield distinct negative zircon He date-eU patterns with individual dates of ~908 to 20 Ma. Archean grains collectively define a negative date-eU trend from ~661 to 4 Ma. In contrast, three other Proterozoic samples yield uniform ~25 Ma zircon He dates over an 1800 ppm eU range, despite increasing eU with increasing metamictization. Simple thermal history models that account for coevolution of zircon radiation damage and He diffusion demonstrate visible metamictization and diffusivity are either coupled or decoupled depending on a sample's thermal history. Zircon He date-eU/metamictization pediments develop when variable but high eU grains with variable visual damage and closure temperature close with respect to He at the same time. Uniform dates with progressive increases in eU and visual damage, reflect decoupled He diffusion and visual damage, are associated with rapid cooling, and/or imply some forms of damage in zircon may not fully anneal. [ABSTRACT FROM AUTHOR]

The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO 2 ), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study demonstrates the importance of the careful examination of compositional, microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals. [ABSTRACT FROM AUTHOR]

Hydro-chemical processes significantly alter the original composition of metamict minerals. In the work presented here, an attempt was made to reconstruct the chemical composition of betafite in the earlier stages of its geological history. The time scale is determined by the leaching rate of the isotope 238U, a process that takes its course in line with first-order kinetics, or something close to it. The leaching rate constant of this uranium isotope was assessed. Based on available data in the literature, the hydro-chemical behavior of various atoms in group A of betafite was analyzed. The chemical composition of the mineral was calculated taking into account the total charge of the cations that the betafite had at the time it was formed, or possibly the last time it was completely reformed as a result of diverse endogenic processes. [ABSTRACT FROM AUTHOR]

Two baddeleyite megacrysts were evaluated as potential reference materials (RMs) for SIMS oxygen isotope analysis, and utilized to understand and calibrate instrumental mass fractionation (IMF). A baddeleyite crystal (S0045) from the Phalaborwa carbonatite, South Africa has a mean δ 18 O VSMOW  = +4.6 ± 0.3‰ (range 0.75‰) measured using laser fluorination gas source mass spectrometry (LF-GMS) and one (S0069) from the Mogok metamorphic belt, Myanmar has δ 18 O VSMOW  = +22.2 ± 0.4‰ (range 0.89‰). SIMS standardization utilizing these inherently heterogeneous RMs is possible by analyzing a number of crystal fragments and utilizing one of them lying at the median of the range. Metamictization, lattice orientation, and chemical composition do not appear to be significant (<0.5‰) variables in matrix matching of RMs and unknowns. Propagation of errors while utilizing the imperfect RMs results in 10 μm diameter spot uncertainties of about ±0.3‰ (2σ). SIMS oxygen isotope analysis of co-crystalline zircon and baddeleyite from the 2.2 Ga Duck Lake sill (DLS) in the Northwest Territories, Canada, yield predominant δ 18 O VSMOW modes of +6.0‰ and +3.2‰, respectively. This difference is consistent with preserving high-temperature isotopic equilibrium between zircon and baddeleyite. DLS baddeleyite δ 18 O data as a whole are negatively skewed (to 0.0‰), and interpreted to reflect low temperature, open-system behaviour. Zircon δ 18 O are less affected, but also show hints of the same influences of secondary alteration and oxygen isotope exchange. [ABSTRACT FROM AUTHOR]

A sample of plutonium tetrafluoride that was separated prior to 1966 at the Hanford Site in Washington State was analyzed at the Pacific Northwest National Laboratory (PNNL) in 2015 and 2016. The plutonium tetrafluoride, as received, was an unusual color and considering the age of the plutonium, there were questions about the condition of the material. These questions had to be answered in order to determine the suitability of the material for future use or long-term storage. Therefore, thermogravimetric/differential thermal analysis and X-ray diffraction evaluations were conducted to determine the plutonium's crystal structure, oxide content, and moisture content; these analyses reported that the plutonium was predominately amorphous and tetrafluoride, with an oxide content near ten percent. Freshly fluorinated plutonium tetrafluoride is known to be monoclinic. During the initial thermogravimetric/differential thermal analyses, it was discovered that an exothermic event occurred within the material near 414 °C. X-ray diffraction analyses were conducted on the annealed tetrafluoride. The X-ray diffraction analyses indicated that some degree of recrystallization occurred in conjunction with the 414 °C event. The following commentary describes the series of thermogravimetric/differential thermal and X-ray diffraction analyses that were conducted as part of this investigation at PNNL. [ABSTRACT FROM AUTHOR]

A rare gemstone form of "ekanite" from Okkampitiya, eastern Sri Lanka, has a chemical composition of Ca2Th0.9U0.1Si8O20, corresponding to that of the tetragonal mineral ekanite. The Okkampitiya material, however, has undergone amorphization and is found in a fully metamict state, which is ascribed to the long-term accumulation of high levels of self-irradiation damage (1.38 x 1020 alpha decay events per gram). The "ekanite" nevertheless does not show any post-growth chemical alteration, and the radiogenic Pb has been retained. The Okkampitiya "ekanite" yielded a Neoproterozoic 207Pb/206Pb age of 562.1 ± 0.8 Ma (uncertainty quoted at the 95% confidence level) that is concordant within the uncertainties of decay constants. The remarkable Pb-retention performance of "ekanite" contradicts the common hypothesis that metamictization in minerals results in U-Pb discordance. The exceptional chemical durability of the heavily radiation-damaged geological material described here has also implications for materials-science research. High radiation resistance, a key criterion in the search for advanced nuclear waste forms, may not be a prerequisite for high aqueous durability of a solid. [ABSTRACT FROM AUTHOR]

In ceramic industry, zircon sand is widely used in different applications because zirconia plays a role as common opacifying constituent. In particular, it is used as a basic component of glazes applied to ceramic tiles and sanitary ware as well as an opacifier in unglazed bulk porcelain stoneware. Natural zircon sands are the major source of zirconium minerals for industrial applications. In this paper, long, medium, and short range studies were conducted on zirconium minerals originated from Australia, South Africa, and United States of America using conventional and less conventional techniques (i.e., X-Ray Diffraction (XRD), Positron Annihilation Lifetime Spectroscopy (PALS), and Perturbed Angular Correlations (PAC)) in order to reveal the type and the extension of the regions that constitute the metamict state of zircon sands and the modifications therein produced as a consequence of the industrial milling process and the thermal treatment in the production line. Additionally, HPGe gamma-ray spectroscopy confirms the occurrence of significant levels of natural radioactivity responsible for metamictization in the investigated zircon samples. Results from XRD, PALS, and PAC analysis confirm that the metamict state of zircon is a dispersion of submicron disordered domains in a crystalline matrix of zircon. [ABSTRACT FROM AUTHOR]

We present a microstructural and U Pb systematics study comparing pristine, thermally annealed (TA) and chemically abraded (CA) ~ 3500 Ma zircon from a quartz-dioritic gneiss, with the aim to improve pre-analytical workflows for more accurate and precise LA-ICP-MS U Pb dating of ancient zircon. Four zircon domains are identified: i) low- to medium-U concentric-oscillatory zoned cores, ii-iii) two porous high-U outer alteration domains, and iv) low-U narrow inward growing recrystallization rims. Raman spectroscopy on the pristine zircon reveals a positive correlation between increasing structural damage and U content. The porous high-U outer domains show a drastic increase in non-formula Ca above estimated amorphous fractions of ~ 0.8, which we ascribe to hydrothermal alteration of high-damage zircon characterized by percolating networks of amorphous areas. Upon treatment (TA, CA), hyper-spectral CL and Raman spectroscopy suggest structural recovery of point defects, but not full repair of high-damage amorphous areas. Calculated Raman radiation damage ages suggest that natural annealing affected all domains at ~ 500 Ma, consistent with negligible laser ablation matrix effects comparing pristine and treated cores. We show that reverse discordance in pristine and TA cores is not an analytical artifact. High-U alteration domains are normal discordant and were partially reset at ca. 3410 Ma. Upon CA, U Pb discordance and scatter are reduced in the cores, yielding intercepts of 3503 ± 14 Ma and 560 ± 550 Ma (MSWD = 1.0). The lower intercept matches the timing of Pb-loss recorded in the alteration domains and the Raman radiation damage age. We argue that short distance Pb redistribution within the cores, which led to reverse discordance, was controlled by U Pb systematics recording both the crystallization age and the timing of Pb redistribution. Some excess Pb was redistributed from the partially re-set alteration domains into the cores, leading to mixed U Pb systematics and further reverse discordance. Removal of the latter excess Pb upon CA suggests that radiogenic Pb from the alteration domains accumulated within distinct damage sites accessible for CA leaching. We conclude that standardization of pre-analytical treatment is not recommended; we propose to investigate the structural state of unknown specimen by means of Raman spectroscopy, in combination with commonly applied CL imaging. During LA-ICP-MS analyses, co-measurement of non-formula and selected trace elements is deemed highly valuable in detecting ablations for use in reliable U Pb age determination. [ABSTRACT FROM AUTHOR]

The escape rates of radon gas from rocks and minerals are of great relevance to many branches of geosciences, and it is, thus, important to understand the physical and mineralogical properties that control radon emanation rates. Mechanisms of radon loss from minerals have direct bearing on the reliability of U-Pb and U-Th-He geochronology. Fourteen minerals from three different mineral groups and with localities spanning three continents were selected for this study. The radon emanation coefficients (REC ) for each mineral were measured as a function of grain size, temperature, 238U and 232Th activities, total absorbed a-dose, density, and mineral melting temperature. The measured 238U and 232Th activities ranged from 0.01 to 6487 Bq/g and from below detection limit to 776 Bq/g, respectively. The REC values for unheated, pulverized samples ranged from 0.083 to 7.0%, which is comparable to previously reported ranges (except for zircon). An inverse correlation between grain size and REC was observed. Full annealing of fission tracks resulted in an overall decrease in REC values, suggesting that nuclear tracks could possibly act as conduits for radon release. While activity, a dose, density, and melting temperatures are not strongly correlated with REC values, it was observed that minerals with high melting points (>1400 °C) have lower REC values, most likely due to inhibition of radon release by compact crystal-lattice structures. This is the first attempt, to our knowledge, to correlate REC values with melting temperature, and this study reports six minerals for which no REC values have been previously reported. [ABSTRACT FROM AUTHOR]

Zircon (U–Th)/He (ZHe) data were acquired for 23 Proterozoic basement samples from an E–W transect through the Colorado Front Range to evaluate whether metamict zircons yield sensible (U–Th)/He data patterns and useful thermal history information. The 112 ZHe dates vary from 147 to 7 Ma, define positive and negative date–eU correlations, and are younger than titanite (U–Th)/He dates that range from 976 to 614 Ma. At moderate to high alpha dose of 10 18 – 10 19 α / g , zircons from the range core yield Laramide ( 52.5 ± 9.6 Ma) dates, whereas those within ∼15 km of the range front yield Miocene ( 21.6 ± 7.7 Ma) results. The He dates for the high alpha dose zircons are reproducible within each sample suite despite their visibly metamict character. The ∼20 Ma range front ZHe dates are younger than apatite (U–Th)/He (AHe) dates ( 66.5 ± 9.6 Ma) and published apatite fission-track data (65–45 Ma) for the same and nearby samples. Thermal history simulations can reproduce the first-order range front date–eU patterns and ZHe–AHe date inversion, but the high-damage zircons are more He retentive than predicted by the zircon damage He kinetic model. The ∼20 Ma ZHe dates may be explained by reheating from hydrothermal fluids along range front faults. The results demonstrate the promise of using He data for high-damage zircons to detect low-temperature (<50 °C) events within and below the temperature sensitivity of the AHe system. [ABSTRACT FROM AUTHOR]

U-rich pyrochlore (UO up to 18.63%) occurs along the magmatic bands as well as disseminated grains within rauhaugites of the Newania complex. UO appears to be primary as it seems mostly evenly distributed all over the grains, though in one grain it does show concentration in parts. Ta is also an important element in some pyrochlore grains and its concentration reaches up to 17.15%. High U and Th are responsible for bringing metamictization in the Newania pyrochlore. Newania carbonatite has a complex emplacement history; rauhaugite seems to have been replaced at 2200 Ma years followed by emplacement of ankeritic carbonatite at around 1500 Ma. Later during tectonic event pyrochlore was involved in reaction with hydrothermal fluid and at this stage Fe and FeS were deposited on the rims of pyrochlore grains. At the same time pyrochlore was subjected to hydrothermal alteration resulting in removal of Na, Ca and F leaving large vacancy in its A-site. Very few grains have escaped such alteration. [ABSTRACT FROM AUTHOR]

A radiochemical analysis of the valence state of uranium isotopes in minerals involves isolation of the U(IV) and U(VI) valence forms and determination of the radioactivity of the 234 U and 238 U isotopes in each fraction. If the nuclear decay of the uranium in the minerals is accompanied by a redox process, the distribution of the 234 U isotope in the U(IV) and U(VI) forms will differ from that of the 238 U isotope: oxidation leads to enrichment of the U(VI) fraction with the radiogenic 234 U isotope; reduction facilitates enrichment of the U(IV) fraction with 234 U. If the mineral is heated to 600–700 °C before the radiochemical analysis, then an equilibrium activity ratio (AR) for both valences. The question arises: What are the processes that bring this about? The authors of this research studied changes of the AR in the U(IV) and U(VI) fractions after isochronous thermal annealing. [ABSTRACT FROM AUTHOR]

• Six commercially available zircon sands were ranked by metamictization level. • Milling efficiency was better on average with low metamict samples. • Clear trends between metamictization characteristics were difficult to define. • Heat treatment restored the hardness of zircon, worsening milling performance. • One previously calcined sample behaved distinctly differently when milled. Six commercially available zircon sands were characterised by XRD, SEM, BET and helium pycnometry to determine their degree of metamictization. These sands were categorized into pairs denoted as high, medium or low levels of metamictization. The six samples were subsequently dry milled under identical conditions and their milling behaviour was tracked by measuring the particle size distribution at regular intervals. Samples with low levels of metamictization were found to achieve a marginally smaller particle size than samples with high levels of metamictization after prolonged milling. This was attributed to two potential mechanisms during breakdown – brittle fracture in the former due to retained hardness, in comparison to a more ductile resistance to fracture in the latter due to reduced hardness. However, no clear trend between specific metamictization characteristics and milling behaviour was found. The results reveal a poorer overall milling efficiency for high metamict samples, which has implications such as increased energy costs, poor quality product and a need for further processing of samples exhibiting high levels of metamictization. This study creates a novel link between the milling behaviour of zircon and its level of metamictization. The knowledge gained holds strong importance for industrial entities wishing to carry out and maintain an efficient milling process, with feed material quality likely to become more variable in future years. [ABSTRACT FROM AUTHOR]

Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206 Pb/ 238 U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6 Myr older than the 40 Ar/ 39 Ar sanidine eruption age (34.00 ± 0.03 Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3 Myr younger. U-Pb ages do not correlate with U (~ 100–800 ppm), Th (~ 50–300 ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~ 1.6 μm for CA and non-CA Caetano, R33 and TEMORA-2 zircons, and do not indicate variations in secondary ion sputtering rates due to chemical or structural changes from the CA treatment. Our new data underscore the potential for cryptic Pb-loss to go unrecognized in other geologically young magmatic centers that do not have zircons with high U, statistically discordant isotope ratios, high common Pb, or metamict textures. [ABSTRACT FROM AUTHOR]

This study provides new insights into the relationship between radiation-dose-dependent structural damage due to natural U and Th impurities and the anisotropic mechanical properties (Poisson's ratio, elastic modulus and hardness) of zircon. Natural zircon samples from Sri Lanka (see Muarakami et al. in Am Mineral 76:1510-1532, 1991) and synthetic samples, covering a dose range of zero up to 6.8 × 10 α-decays/g, have been studied by nanoindentation. Measurements along the [100] crystallographic direction and calculations, based on elastic stiffness constants determined by Özkan (J Appl Phys 47:4772-4779, 1976), revealed a general radiation-induced decrease in stiffness (~54 %) and hardness (~48 %) and an increase in the Poisson's ratio (~54 %) with increasing dose. Additional indentations on selected samples along the [001] allowed one to follow the amorphization process to the point that the mechanical properties are isotropic. This work shows that the radiation-dose-dependent changes of the mechanical properties of zircon can be directly correlated with the amorphous fraction as determined by previous investigations with local and global probes (Ríos et al. in J Phys Condens Matter 12:2401-2412, 2000a; Farnan and Salje in J Appl Phys 89:2084-2090, 2001; Zhang and Salje in J Phys Condens Matter 13:3057-3071, 2001). The excellent agreement, revealed by the different methods, indicates a large influence of structural and even local phenomena on the macroscopic mechanical properties. Therefore, this study indicates the importance of acquiring better knowledge about the mechanical long-term stability of radiation-damaged materials. [ABSTRACT FROM AUTHOR]

Although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals, rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits. Here, we investigate the source of metals for a series of fault zone- and shear zone-hosted uranium occurrences in the Mount Isa inlier, Australia. As well as containing uranium, these deposits are enriched in Zr and rare earth elements (REEs), requiring that unusual fluids were responsible for addition of these typically immobile elements. During the Isan orogeny, highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion, which contains elevated U, Th, F, Zr, and REEs. Synchrotron X-ray fluorescence spectroscopy and high-resolution electron microprobe mineral mapping show that this radioactive element-rich characteristic caused zircon crystals to become highly metamict, allowing the elements therein to become mobile. Thus, when orogenic fluids pervasively infiltrated along shear zones ~100 m.y. after granite intrusion, their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite, providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons. [ABSTRACT FROM AUTHOR]