Volume 110, Issue 8

Hematite, commonly found in endogenetic deposits, plays a crucial role in monitoring temperature variations during cooling history and can be used for (U-Th)/He dating. Hematite thermochronometry, when combined with apatite and zircon (U-Th)/He dating, also provides the clearest guide to the timing of the latest thermal evolution path. In this study, we established a new method of hematite (U-Th)/He thermochronometry based on an optimized analytical protocol to reveal the evolution of the latest uplift and cooling history of the Dexing porphyry copper deposit (DPCD) in South China. Two hematite samples from the Tongchang and Fujiawu mines in the DPCD yielded age distributions of 92–11.2 and 112–24 Ma, respectively, and gave closure temperatures between 180 and 215 °C based on their relations to hematite particle sizes (88–199 μm). Combined with the published (U-Th)/He thermochronological data on zircon and apatite, we propose that the DPCD may have experienced a prolonged thermal history characterized by a temperature decrease. The initial exhumation and uplift happened at 112 Ma, and then a rapid uplift occurred from 11.2 to 8.0 Ma. This study demonstrates that hematite (U-Th)/He dating can be effectively applied to reveal the uplift and preservation history of porphyry copper deposits.

In the context of evaluating lunar construction options, this study focuses on characterizing the viscosities and glass transition properties of lunar regolith simulants to support the development of additive manufacturing processes using molten regolith. Employing the modular TUBS lunar regolith simulant system, we measured the viscosities of different simulants through high-temperature experiments conducted between 1051 and 1490 °C using concentric cylinder viscometry in air. Additionally, differential scanning calorimetry (DSC) was utilized to evaluate the glass transition temperatures, which were in the range between 689 and 815 °C. The measured viscosity data were parameterized by the Vogel-Fulcher-Tammann (VFT) equation, which is adept at describing the viscosities and related properties of silicate liquids. The measured viscosities were compared with the predicted values of six viscosity models. The model by Sehlke and Whittington (2016) best predicts the viscosities of the tested lunar regolith simulants at superliquidus temperatures, and no model adequately predicts viscosities at the glass transition temperature, indicating a need for further research in this area. We infer that 3D printing technologies based on molten lunar regolith are, viscosity-wise, best constrained to highland regions. The reduced environment on the Moon influences the 3D printing process in a positive manner.

The occurrence and mechanisms behind the supernormal enrichment of dispersed elements in sphalerite remain controversial. In this study, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and scanning transmission electron microscopy (STEM) were employed to investigate the occurrence state and enrichment mechanism of cadmium (Cd) in Cd-rich sphalerite from the Jinding deposit. In regions with relatively low Cd concentrations (∼4000 to 10 000 ppm), Cd was observed to substitute for Zn within the sphalerite lattice. However, in regions with high Cd concentrations (exceeding 30 000 ppm), a novel nanoscale occurrence state of Cd was identified, specifically as a nano-CdS solid solution. This finding provides new nanoscale mineralogical evidence to refine the understanding of the supernormal enrichment process of Cd in sphalerite. The enrichment of Cd is driven by mechanisms such as reactivation and dynamic recrystallization induced by plastic deformation in sphalerite, as well as solid solution precipitation during cooling. A coherent interface relationship between CdS and ZnS plays a critical role in the formation and stabilization of the nano-CdS solid solution. The formation of such nano solid solutions may represent a potential mechanism for the supernormal enrichment of dispersed elements, offering new insights into the complex distribution characteristics of these elements at the micrometer scale.

It is generally accepted that carbon from Earth’s surface can be recycled into the mantle during oceanic subduction, thereby playing an important role in the global carbon cycle. However, evidence as to whether the mantle contains recycled carbonates, and of which types, remains ambiguous. Here, we present Mo-Mg isotopic data for two distinct mafic rock types in the Lhasa terrane, southern Tibet: the Taruocuo gabbros and Zhongba basalts. These units were formed in response to Neo-Tethys oceanic subduction at the sub-arc and post-arc depths of the mantle, respectively. Our results, combined with existing data, show that the Late Cretaceous (ca. 90 Ma) Taruocuo gabbros with arc-like geochemical characteristics (e.g., Nb-Ta depletions) have high δ 98/95 Mo (0.19 to 0.61‰) and mantle-like δ 26 Mg (−0.29 to −0.22‰) values. In contrast, the Early Cenozoic (ca. 52 Ma) Zhongba basalts with ocean island basalt-like geochemical signatures (e.g., Nb-Ta enrichments) have low δ 98/95 Mo (−0.66 to −0.24‰) and δ 26 Mg (−0.38 to −0.30‰) values. Neither shallow processes (e.g., alteration, crustal contamination, and fractional crystallization) nor partial melting can explain the Mo-Mg isotopic features of the Taruocuo gabbros and Zhongba basalts. Thus, we infer that those features may well reflect their source characteristics. We argue that the Taruocuo gabbros were derived from a mantle source that contained recycled Ca-rich carbonates at the sub-arc depths. In contrast, the Zhongba basalts appear to have been derived from a mantle source containing recycled Mg-rich carbonates and some oceanic crust in the form of eclogite at the post-arc depths. Our findings suggest that recycled carbonate sediments at the sub-arc depths of the mantle may increase their δ 98/95 Mo values without significantly affecting δ 26 Mg values, whereas recycled carbonate sediments at the post-arc depths of the mantle would decrease both δ 98/95 Mo and δ 26 Mg values. Therefore, the Mo-Mg isotopic pair may be a unique tool for determining whether the mantle contains recycled carbonates and for identifying the types of carbonates involved during oceanic subduction.

Ionic diffusivity and electrical conductivity are two important transport properties of Earth’s silicate materials (melts and minerals) and are widely used to constrain many chemical and physical aspects of Earth’s interior. Both transport properties are related to the mobility of ionic species, and a genetic link between them has long been assumed, which implies similarity or comparability between their relevant transport nature and parameters. I examine the activation energy, anisotropy behaviors, and diffusivity and conductivity values of three well-documented ionic species (Na, F, and H) in a series of Earth’s typical silicate materials and demonstrate that there are profound differences between the two transport properties. I have evaluated the theory that is commonly used to link the two transport properties of relatively simple and ideal materials in physics and materials science. It turns out that the theoretical model may not always be applicable to silicate materials in the Earth’s interior, because their compositions and the bonding and interactions between their constitutive species are both much more complicated. A further assessment of the experimental studies on the diffusivity and conductivity of silicate materials reveals critical differences in the samples and analytical methods. In diffusion experiments, the sample is heterogeneous in composition (for producing the measurable diffusion profile): chemical or isotopic potential gradients are exclusively present, and during the runs, there is a net transfer of the charged species. In conduction experiments, the sample is usually homogeneous in composition: no potential gradients are involved, and there is no net transfer of the charged species (e.g., for analyses using impedance spectroscopy, which is necessary for Earth’s silicate materials). The contrasting patterns of transport behaviors and parameters between the diffusivity and electrical conductivity of Earth’s silicate materials, as experimentally observed, are more likely caused by discrepancies in the theoretical basis and laboratory work. The ionic mobility of a charged species between diffusion and conduction experiments on a silicate sample is, in fact, strikingly different. I suggest that the experimentally yielded diffusivity and conductivity of Earth’s silicate materials cannot be simply compared or correlated with each other. The possible circumstances where a link between the two transport properties might be established are also briefly discussed.

The cement components in deep geological disposal facilities (DGFs) for spent nuclear fuel can increase groundwater pH, potentially altering minerals within natural barriers. Mineral dissolution (biotite, quartz, plagioclase, chlorite, and K-feldspar) and secondary-phase precipitation were investigated to provide a visually integrated understanding of the multifaceted processes. This study was based on the morphological features of granitic rock thin sections exposed to alkaline aqueous solutions (initial pH: pHo 9 and 13) using atomic force microscopy (AFM), micro-X-ray fluorescence, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. Batch kinetic-alteration tests were conducted for durations from 4 h to 20 daysays using solutions with different initial pH values (pHo) The minerals exhibited more pronounced changes in surface roughness and Si release at pHo 13 than at pHo 9. Furthermore, precipitates were more abundant on the mineral surfaces at pHo 9 than at pHo 13. Fe (oxy)hydroxides and Al (oxy)hydroxides prevailed as precipitates at pHo 9, whereas Ca (oxy)hydroxides dominated at pHo 13 (pH ≥ 12.8). These findings indicate that aqueous solutions were significantly involved in the formation of the secondary-phase precipitates. Interestingly, secondary phases precipitated not only on the surface of the mineral (i.e., biotite), providing constituent ions, but also on the surfaces of adjacent minerals (i.e., quartz and plagioclase). Moreover, the possibility of a multistep process involving Al precursors for nucleation of gibbsite precipitates on the surface of K-feldspar at pHo 9 and colloidal particle formation through surface modification, often overlooked in mineral research, was identified via AFM image analysis. This methodological approach using rock thin sections can provide new visual insights regarding the dissolution–precipitation processes, including nucleation reactions, under conditions closely resembling the expected environmental settings within DGFs.

A hydrous Ca-Fe-rich silicate identified as hydroandradite was observed in the “Mighei-type” carbonaceous (CM) chondrite falls, Shidian and Kolang. This is the first report of hydroandradite occurring within meteorites. Hydroandradite forms through aqueous calc-silicate alteration under specific fluid conditions. Its presence within Shidian and Kolang has implications for interpreting alteration processes within the C-complex asteroid parent bodies of the CM chondrites. To better understand its occurrence, the meteoritic hydroandradite was studied with scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, and Raman spectroscopy. It occurs in four petrographic contexts: layered, perovskite-associated, sulfide-associated, and spheroidal. Kolang has all four morphologies, while only the sulfide-associated occurs in Shidian. In Kolang, hydroandraditewas likely produced by replacement of kamacite, Ti-bearing clinopyroxene in calcium- and aluminum-rich inclusions, and secondary magnetite in three distinct alteration events. The formation temperature of meteoritic hydroandradite was estimated to be 100–245 °C, based on the mineralogy of the lithologies within which it occurs as well as on its degree of hydration relative to synthetic and terrestrial hydroandradites. Because Kolang and Shidian are the only reported meteorites with hydroandradite to date, they may be from the same parent body.

The different morphologies of diamond crystals reflect the growth conditions and provide valuable information about the processes that led to the formation of the diamond. Diamond twins only occur during growth and only through reticular merohedry, which has a significant effect on the physical properties of the diamond. However, due to the scarcity of samples, particularly samples with interpenetrant twins, few investigations have been conducted on their morphological features and related formation mechanisms. In this study, natural diamonds with interpenetrant twins from the Republic of the Congo were analyzed to investigate the crystallographic features and related growth formation mechanisms using scanning electron microscopy and micro-computed tomography. The results reveal that all the samples exhibit cubic habits with deformation and a rough appearance, accompanied by fibrous growth layers, indicating rapid crystallization under high driving force conditions. Based on the different features of the crystals’ macroscopic morphological properties, three types of theoretical twin models are established. Cathodoluminescence imaging shows that there are two patterns regarding the formation of interpenetrant twins in natural diamonds, including the origination of grains during the nucleation stage of crystals in the form of twinned positions and changes in the orientation of the growth layer arrangement during crystal growth. Moreover, a mixed type of twin structure was observed, indicating the complexity of the diamond twin growth process, involving transformation of the crystallization habit of the crystal.

Pyroxene is an important carrier of ferric iron in basalt and the upper mantle. Understanding the influence of pyroxene crystallization on the oxygen fugacity of magma relies on accurate knowledge of the oxidation state of iron, expressed as the Fe 3+ /ΣFe ratio, in pyroxene. To accurately determine the Fe 3+ /ΣFe ratio in pyroxene using electron probe microanalysis, we present nine natural pyroxene samples, including one aegirine, one hedenbergite, one diopside, and six augites, for the calibration of the flank method for pyroxene. The aegirine sample is rich in the aegirine end-member with a Fe 3+ /ΣFe ratio of 0.98 ± 0.01 (1σ), while the hedenbergite sample is rich in the hedenbergite end-member and free of ferric iron. The augite and diopside samples contain variable ferric iron, with the Fe 3+ /ΣFe ratios varying from 0.21 to 0.39. Based on the flank positions of Fe L α and Fe L β determined by natural andradite and almandine, we measured the Fe L β/ L α ratios at the flank positions for the pyroxene samples. The results demonstrate a positive linear relationship between the Fe L β/ L α ratios and the Fe 2+ content of the pyroxene samples. The Fe 2+ contents and Fe 3+ /ΣFe ratios of the pyroxene samples, as determined through a multiple linear regression equation, align closely with those obtained by Mössbauer spectroscopy. This method yields the Fe 2+ content and Fe 3+ /ΣFe ratios with an error of ±0.3 wt% and ±0.06, respectively, for calcic pyroxene containing 7 wt% total FeO. These well-characterized natural pyroxene samples can serve as reference materials for determining the Fe 3+ /ΣFe ratio in unknown calcic pyroxene.

Boehmite and diaspore are the two economic ore minerals of karstic bauxites. Although their genesis has been studied from different perspectives, their formation mechanism is controversial. The random forest (RF) model of machine learning was employed to extract the combined characteristics of trace elements in boehmite-type bauxite (BTB) and diaspore-type bauxite (DTB). The BTB predominantly exhibits higher median concentrations of Co, Ni, V, and Cr, while the DTB exhibits a more significant enrichment in elements of U, Hf, Th, and Zr. Combining the characteristics of La/Yb and Ga/Al ratios, it is found that disparities between BTB and DTB are mirroring those between basic rocks and intermediate-felsic rocks. Furthermore, the Zr-Cr-Ga diagram and Ni/Zr vs. Cr/Zr plot were utilized to examine the correlation between karstic bauxite (BTB or DTB), lateritic bauxite, and their respective weathered parent rocks. It is found that BTB exhibits consistent characteristics with lateritic bauxite weathered from basic rock and its parent rocks. Similarly, DTB exhibits consistent characteristics with lateritic bauxite weathered from intermediate-to-felsic rocks and their parent rocks. Through studying the relationship between Ni content and Fe 3+ /Fe 2+ ratios, it has been discovered that the presence of trace elements like Ni in source materials can affect or regulate the ore-forming process, ultimately driving the transformation of gibbsite into either boehmite or diaspore. Consequently, the formation of BTB and DTB is significantly influenced, or even governed, by the composition of their source materials. Our study highlights a novel insight into the significant impact of the source material’s geochemical composition on the formation of boehmite and diaspore.

Raman spectra of Na 2 Ca 2 (CO 3 ) 3 shortite and K 2 Ca(CO 3 ) 2 bütschliite were measured to 715 and 740 °C, respectively, under vacuum. The vibrational spectra demonstrate that shortite converts to nyerereite [Na 2 Ca(CO 3 ) 2 ] and calcite at 535 °C. This assemblage remains stable up to ∼700 °C, at which point nyerereite begins to decompose. Bütschliite converts to the isochemical phase fairchildite at 570 °C, which is stable to 665 °C, where it decomposes to an assemblage of K 2 CO 3 and CaO. The variation of anharmonicity between different vibrational modes of each of the low-temperature phases is assessed, and these yield insights into inter-carbonate group couplings. Both fairchildite and nyerereite exhibit spectral features consistent with extensive disordering.

Zircon is a common accessory mineral that can survive extreme temperature conditions in the crust, although U-Th series radioactive decay is responsible for its self-destruction over long timescales. Upon exhumation at lower temperatures, metamict zircon grains are then more susceptible to isotopic and trace element disturbance. In this work, we investigated the isotopic and trace element behavior of zircon through LA-ICP-MS, coupled with Raman spectroscopy, from two Archean mylonitic samples within the São José do Campestre Massif, northeastern Brazil. U-Pb in zircon dating indicates Paleoarchean (3.3 Ga) sedimentation followed by Mesoarchean (3.0–2.9 Ga) high temperature metamorphism in the mylonitic paragneiss sample, while the mylonitic amphibolite was emplaced during the late Neoarchean (2521 ± 22 Ma) and reached high-temperature conditions during shear zone development and deformation during the Paleoproterozoic (2.0 Ga). Inherited zircon grains in the amphibolite are consistent in morphology and ages with zircon grains from the host paragneiss. However, high-temperature annealing and structure recovery upon entrainment of inherited zircon grains in the hot mafic magma led to the homogenization of trace elements and preservation of internal textures. On the other hand, zircon grains from the host paragneiss accumulated radiation damage over a longer period, leading to the self-destruction of the crystalline structure. Upon later low-temperature fluid influx, non-formula elements were incorporated in metamict grains, as evidenced by the positive correlation between trace elements and radiation damage. Raman zircon dating indicates Cambrian to Late Paleozoic exhumation for these rocks, which agrees with U-Pb in zircon lower intercept ages. Although often avoided in geochronological studies, metamict zircon can provide important constraints on low-temperature events. Collectively, these observations reveal a complex trans-crustal history of zircon recycling and exhumation from the Paleoarchean to the Paleozoic, as well as the rock-dependent behavior of zircon in response to fluid flow and deformation. The results have implications for the interpretation of trace elements in naturally annealed zircon grains, especially those inherited in mafic rocks.

A deep understanding of the formation and alteration mechanisms of titanium-bearing minerals in volcanic sedimentary sequences is crucial for a clear recognition of the Ti element cycle on Earth. Here we used micro- to nanoscale characterization techniques, including focused ion beam (FIB), high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and nano-computed tomography (Nano-CT) to investigate nano mineralogical characteristics and formation mechanisms of authigenic Ti oxides and altered Ti-Zr-O minerals (srilankite) in altered volcanic ash across a Permian-Triassic boundary. The results indicate that the growth of anatase and brookite (TiO 2 polymorphs) nanoparticles within the volcanic ash matrix under acidic conditions is regulated by Ostwald ripening with minor semi-oriented attachment and recrystallization. Meanwhile, the growth of brookite crystals in altered srilankite particles (ZrTi 2 O 6 ) is predominantly controlled by the oriented attachment mechanism. This phenomenon suggests that the growth mechanisms of TiO 2 nanoparticles are highly sensitive to the microenvironments surrounding the particles, with different growth behaviors possibly occurring even within the same layer. EELS results show that, under chemical weathering, the edges of TiO 2 crystals tend to amorphize, gradually reducing to the Ti 3+ valence state at the edge. Following the alteration of srilankite, primary brookite crystals form in situ (particle size ∼10 nm), initially growing into incomplete oriented particles through oriented attachment. Subsequently, these oriented particle fragments further grow by attaching to primary crystals in the matrix, forming large (particle size along the long axis ∼300 nm) brookite crystals with consistent crystallographic orientations. This phenomenon demonstrates that during the alteration process of unstable Ti-O minerals, Ti migration does not occur across particle scales.

Uranyl silicates are common constituents of the oxidized portions of uranium ore deposits. They are also known to form on used nuclear fuel pellets under simulated geologic repository conditions. These phases can incorporate various cations, including higher actinides and fission products, which makes them relevant for studies that probe the stability of these phases in various contexts, including irradiation, high temperatures, and high pressures. The changes in the single-crystal structure and Raman spectroscopy of the uranyl silicate mineral boltwoodite, K 0.63 Na 0.37 [(UO 2 )(SiO 3 OH)](H 2 O) 1.5 , have been investigated at 11 pressures from ambient to 16.8 GPa. The bulk modulus was determined to be 26.3(4) GPa froma third-orderBirch-Murnaghan fit of the unit-cell volume vs. pressure. No obvious phase transition was observed between ambient pressure and 16.8 GPa. The structure is most easily compressed along the a -axis, which decreases by ca. 15% throughout the investigated pressure range, leading to distortions in the bonding geometries and higher coordination numbers of the interlayer cations. The b - and c -axes parallel to the uranyl silicate sheets are each compressed by ca. 4% compared to the ambient-pressure structure at 16.8 GPa, attributed to the uranyl silicate sheets becoming more highly corrugated with increasing pressure. Previous experimental high-pressure work on uranium(VI) phases is limited due to the complexity of refining heavy atom crystal structures with disordered cations and/or water molecules, as well as the limitations imposed by the opening angle of the diamond-anvil cell used to achieve high pressures. A method for overcoming these challenges is presented here to facilitate further high-pressure studies of heavy element minerals and other phases.

We present the first textural and chemical characterization at nanometer scale of chrysocolla [(Cu 2–x Al x )H 2–xSi 2 O 5(OH) 4 ·nH 2 O], black chrysocolla (a Mn-rich variety of chrysocolla), and pseudo-malachite [Cu 5 (PO 4 ) 2 (OH) 4 ] from two distinct supergene copper deposits from Atacama Desert in northern Chile. These minerals are the most common copper minerals found in the supergene deposits associated with copper porphyries from Atacama Desert. However, the lack of nanoscale morphological information prevents a deeper understanding of their formation process. Nanoscale characterization using transmission electron microscope (TEM) imaging allows further characterization of the structural states of chrysocolla, black chrysocolla, and pseudomalachite, offering valuable insights into their genesis. Chrysocolla and black chrysocolla are not single crystals but assemblages of Cu nanoparticles embedded in an Si-rich amorphous matrix. Scanning TEM (STEM) images reveal that chrysocolla consists of rounded Cu-rich nanoparticles embedded in an amorphous matrix, while black chrysocolla consists of rounded Cu-rich nanoparticles with few needle-shaped Mn-rich particles, all embedded in an amorphous matrix. The richness in nanoparticles defines a layering that mimics the colloform texture observed in optical microscopy. In contrast, pseudomalachite is a massive polycrystalline mineral consisting of a juxtaposition of large nanocrystal grains of ∼500 nm. The STEM-electron energy loss spectrometry (EELS) spectra show that copper in chrysocolla and black chrysocolla is in a reduced state. This suggests that chrysocolla and black chrysocolla form under reducing conditions, probably just below the water table. Alternatively, it could be that water table oscillation allows for the cyclical precipitation of Cu 0 -rich nanoparticles and oxidized copper-rich silicates. Conversely, pseudomalachite crystallization requires oxidative conditions. The oxidation state variations, from chrysocolla (Cu 0 ) to pseudomalachite (Cu 2+ ), certainly occur during the episodic switch of the water table linked to tectonic events or climatic changes. The findings also have implications for the U-Pb dating of supergene copper deposits, since black chrysocolla and pseudomalachite can incorporate significant U contents. The different structural states of the three minerals may explain their different behaviors regarding U and Pb mobility and, therefore, the preservation of the U-Pb chronometric signal.