Electron Microprobe Research Articles

Microstructure Evolution and Cryogenic Impact Toughness Changes in Ce‐Containing Novel Cryogenic High‐Mn Austenitic Steel Weld Metals under Different Heat Inputs

The Ce‐containing novel cryogenic high‐Mn austenitic steel weld metals under different heat inputs are prepared by submerged‐arc welding to provide a satisfactory welding parameter scheme and further optimize its cryogenic impact toughness. The effect of heat input on microstructure, inclusion, and cryogenic impact toughness of weld metals are investigated via optical, scanning electron microscopy, electron backscatter diffraction, electronic probe microanalyzer, and cryogenic impact test. The results show that the dendrite arm spacing, segregation ratio of Mn, Si, and C elements and average effective grain size of the weld metals gradually increase, and the proportion of high‐angle grain boundaries decrease with the heat input increase from 11.91 to 20.24 kJ cm−1. The type of inclusions of weld metal with different heat inputs are all Ce2O3 and Ce2O2S. The number density of inclusions decreases monotonically from 5976 to 4912 mm−2, while the average size increases from 0.32 to 0.45 μm with the heat input increased. The increase in heat inputs consistently decreases the mean cryogenic impact toughness of weld metals from 123 to 102 J. The higher proportion of high‐angle grain boundaries and finer inclusions at lower heat input play the critical role in improving the cryogenic impact toughness.

Effects of Al Substitution for Fe in Na₅FeSi₄O₁₂ (5.1.8) Glasses: Structure and Crystallization

In this study, the effects of substituting Al for Fe in 5Na2O∙(Al2O3)x∙(Fe2O3)1-x∙8SiO2 glass, x=0 to 1, and Na5AlxFe1-xSi4O12 (5.1.8) crystal, were investigated using thermal analysis, Fe K-edge X-ray absorption, X-ray diffraction, Raman spectroscopy, and Electron Probe Microanalysis. In both glass and crystallized glass, nearly all the Fe was tetrahedrally coordinated Fe3+, as expected from the high concentration of Na2O. The substitution of Al for Fe in the glasses caused the glass transition temperature to increase as polymerization increased, as evidenced by Raman, likely due to both field strength differences of Al vs Fe and a small amount of Fe2+ network modifier present with Fe. After heat treatment at 700 °C for 24 hours, the glasses had crystallized, forming Na2SiO3 and NaAlSiO4 in compositions with high Al concentrations and the 5.1.8 crystal in compositions with high Fe concentrations. Through electron microprobe, it was determined that <0.04 formula unit Al incorporated into the 5.1.8 crystal, i.e. Na5Fe0.96Al0.04Si4O12. The 5.1.8 crystal only formed when Fe concentration was higher than Al in the starting glass.

Hoperanchite, (NH4)2(S2O3), a new mineral from an active vent in a burning bituminous shale

Abstract Hoperanchite (IMA2024-017), (NH4)2(S2O3), is a newly approved mineral species from an active vent in a burning bituminous shale at Hope Ranch, Santa Barbara County, California, U.S.A. Hoperanchite occurs as tabular crystals up to about 0.3 mm in diameter. Tablets are flattened on {001} and exhibit the forms {100}, {010}, {0-10}, {110}, and {-1-10}. The mineral is colorless and transparent with vitreous luster and white streak. The Mohs hardness is ~2½. The mineral has brittle tenacity, irregular fracture, and one good cleavage on {001}. The measured density is 1.68(2) g·cm−3. The mineral dissolves instantly in room temperature H2O. The mineral is optically biaxial (+), α = 1.602(2), β = 1.616(2), γ = 1.634(2) (white light); 2Vmeas = 84(2)°; orientation Y = b, X ^ a = 20° in obtuse β; nonpleochroic. Electron microprobe analysis provided the empirical formula N1.89H7.40S1.97O3. Hoperanchite is monoclinic, C2, a = 10.2313(5), b = 6.4998(3), c = 8.8098(6) Å, β = 94.611(7)°, V = 583.97(6) Å3, and Z = 4. The crystal structure (R1 = 0.0325 for 1176 I > 2σI reflections) is the same as that of synthetic (NH4)2(S2O3). Hoperanchite is the first thiosulfate mineral that does not contain essential Pb.

Titanite as an indicator of granite fertility and gold mineralization in the Xiaoqinling gold province, North China Craton

The Xiaoqinling gold province, located in the southern margin of the North China Craton (NCC), is the second largest gold-enriched region in China. In this region, the Mesozoic Huashan (HS) and Wenyu (WY) plutons are the major magmatic intrusions coeval with gold mineralization, although they show contrasting characteristics in the distribution of gold. In this study, we use geochemical features of titanite determined by LA-ICP-MS and EPMA analyses and elemental mapping to decipher the mechanisms that led to the difference in gold enrichment related to the two plutons. Titanite from the Wenyu granitic pluton exhibits significantly higher (La/Sm)N, (La/Yb)N, ΣLREE/ΣHREE ratios, and ΣREE concentration and slightly higher (Gd/Yb)N values than those of the Huashan Pluton, suggesting that the Wenyu pluton might have experienced more complex magmatic evolution, widespread hydrothermal alteration, and higher silica activity in the melt than the Huashan pluton. The titanite grains from the Huashan pluton show higher (Nb/Ta)N and (Lu/Hf)N values and significantly lower Zr concentration than those of the Wenyu pluton. The titanite grains from the Wenyu pluton show higher vanadium and gallium concentrations and Fe/Al ratio than those of the Huashan pluton, indicating comparatively higher fo2. Furthermore, the titanite grains from Wenyu pluton indicate higher water content in the magma. In addition, magma mingling and magmatic hydrothermal fluids derived from the crust/mantle are critical sources for ore-forming materials. These results suggest that the Wenyu pluton is more conducive to gold migration and enrichment than the Huashan pluton.

A novel pillar-layered MOF with urea linkers as a capable catalyst for synthesis of new 1,8-naphthyridines via the anomeric-based oxidation.

Metal-based catalysts play an essential role in organic chemistry and the chemical industry. This research designed and successfully synthesized a pillar-layered metal-organic framework (MOF) with the urea linkers, namely Basu-HDI, as a novel and efficient heterogeneous catalyst. Various techniques such as FT-IR, EDX, elemental mapping, SEM, XRD, BET, and TGA/DTA studied its structure and morphology. Then, we investigated the synthesis of new 1,8-naphthyridines utilizing Basu-HDI in mild conditions via a one-pot, three-component tandem Knoevenagel/Michael/ cyclization/anomeric-based oxidation reaction. Final products were achieved by anomeric-based oxidation without employing an oxidation agent. Remarkably, this tandem process gave a good range of new 1,8-naphthyridines with high yields in a short reaction time. The pure products were confirmed by FT-IR, 1H NMR, 13C NMR, and mass spectrometry techniques. Moreover, the introduced catalyst showed good efficiency and stability and can be reused four times without significantly reducing efficiency.

Core-shell upconversion nanoparticles with suitable surface modification to overcome endothelial barrier.

Upconversion nanoparticles (UCNPs), capable of converting near-infrared (NIR) light into high-energy emission, hold significant promise for bioimaging applications. However, the presence of tissue barriers poses a challenge to the effective delivery of nanoparticles (NPs) to target organs. In this study, we demonstrate the core-shell UCNPs modified with cationic biopolymer, i.e., N, N-trimethyl chitosan (TMC), can overcome endothelial barriers. The core-shell UCNP is composed of NaGdF4: Yb3+,Tm3+ (16.7 ± 2.7 nm) as core materials and silica (SiO2) shell. The average particle size of UCNPs@SiO2 is estimated at 26.1 ± 3.7 nm. X-ray diffraction (XRD), transmission electron microscopy (TEM) and element mapping shows the formation of hexagonal crystal structure of β-NaGdF4 and elements doping. The surface of UCNPs@SiO2 has been modified with poly(ethylene glycol) (PEG) to enhance water dispersibility and colloidal stability, and further modified with TMC with the zeta potential increasing from -2.1 ± 0.96 mV to 26.9 ± 12.6 mV. No significant toxic effect is imposed to HUVECs when the cells are treated with core-shell UCNPs with surface modification up to 250 µg/mL. The transport ability of the core-shell UCNPs has been evaluated by using the in vitro endothelial barrier model. Transepithelial electrical resistance (TEER) and immunofluorescence staining of tight junction proteins have been employed to verify the integrity of the in vitro endothelial barrier model. The results indicate that the transport percentage of the UCNPs@SiO2 with PEG and TMC through the model is up to 4.56%, which is twice higher than that of the UCNPs@SiO2 with PEG but without TMC and six times that of the UCNPs@SiO2.

Liquidus of SiO2 in the Li2O‐SiO2, Na2O‐SiO2, and K2O‐SiO2 systems

AbstractThe phase equilibria in the SiO2‐rich region of the Li2O‐, Na2O‐, and K2O‐SiO2 systems at 1550°C to 1650°C were experimentally studied to determine accurate liquidus of SiO2 in the systems. A classical equilibration/quenching method was used, and the equilibrated samples were analyzed by X‐ray diffraction (XRD) and electron probe microanalysis (EPMA). Samples were contained in sealed Pt crucibles and held at the target temperatures for 7–21 days to reach equilibrium condition. Sealed Pt crucibles were employed to avoid the volatile loss of alkali oxides during high temperature phase equilibration and hydration upon quenching. Accurate liquidus of SiO2 in alkali‐silicate systems was determined for the first time, and the results were analyzed using the limiting slope rule in order to understand the dissolution behavior of alkali oxides in SiO2 melt. It was confirmed that alkali oxide dissolves in SiO2 melt by producing Li+, Na+, and K+ cationic species rather than Li22+, Na22+ and K22+ species.

Microscopic mineralogy of zoned pyroxene in NWA 12522: Implications for the crystallization histories of the shergottites

AbstractBasaltic shergottites are the most abundant rock type of Martian meteorites, and pyroxene grains within shergottites commonly show a zoned structure. Here, the detailed microscopic mineralogical characteristics of patchy zoned pyroxene in basaltic shergottite NWA 12522 were investigated by a combination of scanning electron microscopy, electron microprobe, Raman spectroscopy, and transmission electron microscopy. The results show that the cores of zoned pyroxene in NWA 12522 have a homogeneous Mg# value and consist mainly of augite and pigeonite. By contrast, the rim of zoned pyroxene is extremely ferroan and can be further divided into two regions based on quite distinct mineralogy and textures (i.e., far‐core and near‐core pyroxene rims). The near‐core rim shows narrow exsolution lamellae (~35 nm) that were cross‐cut by thin pigeonite veinlets and contain abundant nano‐sized particles of metastable pyroxferroite and pigeonite. Only relatively coarse exsolution lamellae (~80 nm) were observed in the far‐core pyroxene rim regions. The distinct mineralogical characteristics of the pyroxene rims and cores in NWA 12522 imply different crystallization conditions, and the homogeneous Mg‐rich pyroxene cores should have slowly crystallized from magma within a deep‐seated chamber, followed by an overgrown evolved melt on these pyroxene cores during their ascent to the Martian surface, and disequilibrium crystallization of nano‐sized metastable phase (pyroxferroite) occurred in the near‐core region. The abnormally low ΣREE contents and steep REE pattern (high Yb/La ratio) of the pyroxene rims in NWA 12522 imply that merrillite should have crystallized prior to the pyroxene rims, making the residual melt become REE‐depleted and HREE‐enriched.

A semiconducting supramolecular Co(II)-metallogel based resistive random access memory (RRAM) design with good endurance capabilities.

A highly efficient approach for synthesizing a supramolecular metallogel of Co(II) ions, denoted as CoA-TA, has been established under room temperature and atmospheric pressure conditions. This method employs the metal-coordinating organic ligand benzene-1,3,5-tricarboxylic acid as a low molecular weight gelator (LMWG) in DMF solvent. A comprehensive analysis of the mechanical properties of the resulting supramolecular Co(II)-metallogel was conducted through rheological investigation, considering angular frequency and thixotropic study. The hierarchical rocky network structure of the supramolecular Co(II)-metallogel was unveiled using field emission scanning electron microscopy (FESEM). Transmission electron microscopic (TEM) analysis showed rod-shaped structures via low-magnification high angle annular dark field (HAADF) bright field scanning transmission electron microscopic (STEM) imaging, while energy dispersive X-ray (EDX) elemental mapping confirmed its primary chemical constituents. The formation mechanism of the metallogel was examined via fourier transform infrared spectroscopy (FTIR) spectroscopy. The nature of the synthesized CoA-TA metallogel was affirmed through powder X-ray diffraction (PXRD) analysis. Furthermore, this study involved fabrication of Schottky diode structures in a metal-semiconductor-metal geometry based on cobalt(II) metallogel (CoA-TA), enabling observation of charge transport behavior. Remarkably, a resistive random access memory (RRAM) device utilizing cobalt(II) metallohydrogel (CoA-TA) demonstrated bipolar resistive switching at room temperature and under ambient conditions. The switching mechanism was investigated, revealing the formation and rupture of conductive filaments between metal electrodes that govern the resistive switching behavior. This RRAM device exhibited an impressive ON/OFF ratio (~ 414) and exceptional endurance over 5000 switching cycles. These structures offer great potential for diverse applications such as non-volatile memory design, neuromorphic computing, flexible electronics and optoelectronics. Their advantages lie in their fabrication process, reliable resistive switching behavior and overall performance stability.

Structural, magnetic and dielectric properties in double perovskite La2-xScxCoMnO6 (x = 0.0, 0.1, and 0.2)

The effect of Sc doping on structural, magnetic, and dielectric properties of double perovskite La2CoMnO6 synthesized by solid-state method was investigated. Crystal structure of La2-xScxCoMnO6 (x = 0.0, 0.1, and 0.2) compounds confirmed through X-ray diffraction patterns (XRD). Rietveld refinement of the XRD revealed the monoclinic phase with P21/n space group and the structural properties such as lattice parameter, bond lengths, and bond angles were obtained. Field emission scanning electron microscopy images revealed a uniform grain distribution, and the grain size was increased with increasing Sc content. The elemental mapping confirms the stoichiometry of the compounds. The room temperature Raman spectra confirm the monoclinic crystal structure and reveal two broad peaks viz., Ag, and Bg which were attributed to specific vibrational modes within the (Co/Mn)O6 octahedra. Magnetic measurements show that Curie temperature (Tc1) decreases with Sc substitution of 206 K for x = 0.0–193 for x = 0.2 due to the Co2+-Mn4+ FM coupling and it follows Curie–Weiss law in the paramagnetic region. Impedance spectroscopy studies show that the conductivity in these materials is predominantly due to the intrinsic bulk grains.

Phase equilibria investigations and thermodynamic modeling of the Eu2O3 - La2O3 and Eu2O3 - La2O3 - ZrO2 systems

The phase relationships of the ternary system Eu2O3 - La2O3 - ZrO2 and binary Eu2O3 - La2O3 subsystem were experimentally and thermodynamically studied. Six three-phase fields and nine two-phase ones in the Eu2O3 - La2O3 - ZrO2 system were investigated by X-ray diffraction, scanning electron microscopy and electron probe microanalysis with wavelength dispersive spectrometer at 1873 K and 1673 K. Nevertheless, there only existed a two-phase region in the Eu2O3 - La2O3 system in the temperature range from 1873 K to 1673 K. After that, the ternary mixing parameters for the system Eu2O3 - La2O3 - ZrO2 including the binary interaction parameters in the Eu2O3 - La2O3 subsystem are optimized with the CALPHAD (CALculation of PHAse Diagrams) method, based on obtained experimental results. The high-temperature phase relationships and the special type of continuous solid solutions (La2-xEuxZr2O7) investigated in present work will contribute to exploring new materials with specific properties such as optical transparency.

An Oxygen‐Self‐Produced Nanoplatform Based on MnO2 for Relieving Hypoxia

AbstractManganese dioxide (MnO2) nanoparticles are increasingly recognized for their potential in biomedical applications, particularly in the realm of hypoxic cancer therapy, due to their unique physicochemical characteristics. This investigation introduces a novel, template‐free synthesis strategy. The reducing groups inherent in bovine serum albumin (BSA) facilitate a redox reaction with potassium permanganate (KMnO4), while the abundance of functional groups in BSA is instrumental in the formation of MnO2. The transmission electron microscopy (TEM) analysis has characterized the synthesized MnO2 nanoparticles, termed BM NPs, as spherical particles with a mean diameter of 210 nm and zeta potential of −40.1 mV measured by dynamic light scattering (DLS). The Fourier transform infrared (FT‐IR) spectrum of BM NPs exhibits the characteristic peak of MnO2 at 1113 cm−1 and 620 cm−1. Furthermore, the elemental composition of BM NPs has been ascertained through energy‐dispersive X‐ray (EDX) elemental mapping analysis. Though concentration of BM NPs up to 200 μg/mL, the survival rate of 4T1 cells remained approximately 75 %. Given that BM NPs at a concentration of 100 μg/mL significantly alleviate cellular hypoxia, the high oxygen‐generating capacity of these nanoparticles is proved, suggesting their suitability as a drug delivery system, especially in the context of hypoxic tumor microenvironments.

Supported phenylalanine on core-shell mesoporous microsphere as a catalyst for the synthesis of triazolo[1,2-a] indazole-triones and spiro triazolo[1,2-a] indazole-tetraones

In recent years, mesoporous silica materials have gained attention due to their unique properties, such as high surface area, pore volume, size, and chemical stability. These characteristics make them effective in various fields, particularly efficient supports in heterogeneous catalytic reactions. The present study developed a novel type of core-shell mesoporous microsphere material by anchoring phenylalanine on a core-shell SiO2@NiO@MS support. The catalyst support (SiO2@NiO@MS) was synthesized through homogeneous precipitation and sol-gel methods, with NiO encapsulated within the porous silica. The catalyst was characterized using various techniques, including Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), Elemental mapping analysis, N2 adsorption-desorption, Transmission electron microscopy (TEM), Vibrating sample magnetometer (VSM), and Thermogravimetric analysis (TGA). It was then employed for the synthesis of triazolo[1,2-a]indazole-trione and spiro triazolo[1,2-a]indazole-tetraones compounds from 4-phenyl urazole, dimedone, and aromatic aldehydes or isatin derivatives. This synthetic approach offers multiple advantages, including high yields, faster reaction times, low catalyst requirements, and environmentally sustainable conditions.

Formation of aluminum nitride in dross by contact-diffusion reaction during aluminum recycling

Secondary aluminum dross is a solid waste after aluminum extraction from the slag produced during aluminum recycling. It is classified as a hazardous waste due to the high content of active aluminum nitride (AlN). This work studied the thermodynamics and kinetics of AlN formation in aluminum dross. Aluminum tends to react with N2 to form AlN, when the partial pressure of nitrogen (N2) is much greater than that of oxygen (O2) (PN2≫PO2) in thermodynamics. Additionally, the reaction between aluminum and N2 is accelerated with the increasing temperature. Approximately 36.37% of aluminum was transformed into AlN at 700°C according to kinetic analysis. Aluminum recycling involves smelting, refining, and dross processing. The dross produced from these different processes has various compositions and hazardous properties. The formation mechanism of AlN from the three processes was revealed based on the thermodynamic and kinetic analyses. The reaction mechanisms in these processes were identified as surface contact reaction, internal and surface contact reaction, and rotational surface contact reaction between the aluminum melt and N2, respectively. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) analyses were used to determine the phase and composition of the aluminum dross. The nitrogen content in the aluminum dross from smelting, refining, and dross processing was found to be 3.3%, 5.2%, and 9.6%, respectively. The monthly dross production were 276.3 tons, 22.7 tons, and 318.2 tons, respectively, according to statistics. It was calculated that the nitrogen generated in the three process was 9.12 tons, 1.18 tons, and 20.25 tons, respectively. Therefore, dross processing is the primary source of AlN. A rapid dross processing method with small-scale vertical stirring was proposed to shorten the reaction between the aluminum melt and N2, thereby reducing AlN formation. This work could provide guidance for reducing hazardous AlN in aluminum dross and optimizing the aluminum recycling process.

Quantitative characterization and mapping relationship between microstructure, composition, orientation, and mechanical properties of nickel-based superalloy Inconel 718

Abstract This study explores the relationship between the microstructure, composition, orientation, and mechanical properties of the nickel-based superalloy Inconel 718. Employing scanning electron microscopy (SEM), electron microprobe (EMP), nano-indentation, and other techniques, the study observes the structure, confirms the composition, determines orientation, and tests mechanical properties in specific micro-zones. Findings reveal a uniform grain distribution in Inconel 718, with a minor δ-phase presence at grain boundaries. There is a notable enrichment of Nb at the grain boundaries, whereas Fe and Cr levels are lower at these boundaries compared to the grain interiors. The indentation hardness and modulus at the grain boundaries are markedly higher than those within the grains. Moreover, grains with different orientations exhibit diverse microscale mechanical properties, such as hardness and elastic modulus. This research establishes a quantitative mapping relationship between the microstructure, composition, orientation, and mechanical properties of Inconel 718, providing a foundation for future multiscale (micro to macro) mechanical property investigations.

Synthesis of Sn-catalyzed Ge nanowires and Ge/Si heterostructures via a gradient method

In this study, we investigate the growth of Ge nanowires (NWs) using a gas supply gradient method during plasma-enhanced chemical vapor deposition (PECVD), focusing on the effects of GeH4 partial pressure and total chamber pressure on NWs morphology. By adjusting either the GeH4 flow rate or the total pressure, we explored a gradient method to manipulate the growth process. Scanning electron microscopy (SEM) images revealed that, even with constant GeH4 partial pressure, variations in total pressure significantly influenced NWs diameter and structure. Furthermore, elemental mapping and compositional analysis demonstrated the presence of a Ge/Si heterostructure with a Sn catalyst at the apex and an amorphous Si layer encapsulating the NW surface.

Fluorescence sensor based on Methionine-Modified silver nanoparticles located on Fe-BTC metal–organic framework (Meth-AgNPs@Fe-BTC) for trace detection of fenitrothion pesticide in aqueous samples

This research introduces a new “turn-on mode” fluorescence sensor for the detection of fenitrothion (FNT) pesticide in various samples. The sensor is constructed using a porous metal–organic framework (Fe-BTC) as a template to locate silver nanoparticles (AgNPs) and methionine amino acid (Meth). Methionine acts as a bridge, facilitating the interaction between FNT and AgNPs, which subsequently results in the release of AgNPs from the composite structure. The physicochemical properties of the synthesized Meth-AgNPs@Fe-BTC composite were analyzed by Fourier-transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDAX), Transmission electron microscopy (TEM), and elemental mapping (MAP) analysis. The sensing system is based on tracking the fluorescence of the synthetic composite in such a way that the intensity of the fluorescence of the composite increases in the presence of different concentrations of fenitrothion (FNT). The effective parameters on the sensor signal, including composite dosage, pH, sonication and reaction time were investigated and optimized. The calibration graph, under optimal conditions, exhibited linearity in the concentration range of 2–95 nM for FNT, with a limit of detection of 1.9 nM. The suggested sensor was successfully validated by analyzing FNT in several real water samples and fruit juices. This research presents a significant technical achievement in the development of a fluorescence sensor for the detection of FNT, offering a sensitive and reliable method for environmental monitoring and public health preservation.

Pressure-temperature-time paths from metapelites reveal neoproterozoic continental crust underthrusting related to the West Gondwana amalgamation

In this contribution, we employed a petrochronological approach to investigate the tectonometamorphic evolution of Al-rich garnet-staurolite and garnet-staurolite-kyanite (biotite- and plagioclase-free) metapelites of the southernmost portion of the Neoproterozoic Brasília orogen. We have reconstructed the first prograde to peak pressure-temperature-time (P-T-t) paths reported for metamorphic units of the area, by combining petrographic analyses, quantitative compositional mapping of major elements, phase equilibrium modeling, and EPMA Th–U–Pb monazite chemical dating. The metamorphic reactions involved in the prograde metamorphic evolution are discussed, including the effects of garnet fractionation, exhaustion of reactants, and re-equilibration reactions on the major-element composition of garnet porphyroblasts, and their influence on P-T condition estimates. Raman spectroscopy on carbonaceous material (RSCM) thermometry provided insights into the synkinematic retrograde path. A steep prograde path marked by two stages is recorded by the garnet porphyroblasts of garnet-staurolite-muscovite (±kyanite) schists: (i) 555–585 °C and 0.60–0.90 GPa; (ii) 590–635 °C and 1.0–1.4 GPa. Monazite crystals record the prograde to peak path from ca. 630–625 Ma to ca. 605–595 Ma. Matrix graphite crystals suggest post-peak cooling to 400–500 °C concurrent with the late stages of development of the main foliation. The reconstructed P-T-t paths indicate an intermediate dT/dP metamorphism, and a burial rate of ~0.55 km/Ma, with garnet compositional zoning suggesting that the high-pressure P-T-t path resulted from a single metamorphic event. The corresponding geothermal gradients, and in-situ ages, combined with regional evidence, suggest that peak metamorphic conditions were attained during collisional underthrusting of the continental crust related to the West Gondwana amalgamation.

Intra-grain variability of hydrogen and trace elements in rutile

Rutile is a commonly used petrogenetic indicator mineral to determine metamorphic temperatures, ages, host−/source lithologies, geochemical reservoirs, and subduction conditions. However, intra grain variabilities of trace elements in rutile are rarely considered. We performed trace element and hydrogen mapping of rutile to assess zoning and diffusion in natural rutile from various lithologies. Trace element and hydrogen show distinct zoning patterns, with mostly regular zoning in rutile from pegmatites and low-T hydrothermal clefts, and typically irregular zoning in rutile from high-P veins and metamorphic rocks. Whereas no clear patterns of trace element correlations can be identified, hydrogen, tri-, tetra- and pentavalent cations can show the same zoning patterns within single rutile grains, despite different substitution mechanisms. This indicates that hydrogen and trace element incorporation is externally controlled by availability and diffusivity of hydrogen and trace elements within the rock matrix, as well as rutile growth rates. H2O mapping reveals that hydrogen is retained in rutile at temperatures of up to ~650 °C. Coupled substitution of hydrogen with divalent and trivalent cations requires coupled diffusion processes for charge balance if hydrogen is diffusively re-equilibrated. Slow diffusion rates and thus relatively high temperatures for diffusive closure in rutile lead to retention of primary hydrogen and trace element zoning. At high-T conditions of >650 °C, diffusive re-equilibration of all trace elements can be observed. Complex zoning patterns of Zr in rutile show that Zr incorporation into rutile is not purely temperature dependent. In this study, Zirconium-undersaturation can lead to inaccurate Zr-in-rutile temperatures of up to ~35 °C difference from peak formation temperatures within single rutile grains and might be a useful tool to evaluate rutile growth conditions. Niobium and Ta are highly zoned in rutile, leading to extremely variable Nb/Ta ratios within single rutile grains that cannot be reconstructed from single spot analyses. Overall, mapping offers a novel and promising tool to understanding trace element behavior in rutile.

Silician zoning of magnetite in a Fe skarn deposit: A potential low-temperature indicator in magmatic-hydrothermal systems?

Single magnetite crystals from the Cihai Fe skarn deposit in Northwest China are strongly growth zoned. Magnetite cores are in equilibrium with garnet whereas magnetite rims are enclosed by calcite. The chemical zoning in magnetite is well defined by an abrupt core-to-rim Si and Ca increase as well as Ti decrease. Electron microprobe analysis reveals magnetite rims contain from 2.24 to 5.70 wt% SiO2 (averaging 4.56 wt%), which are silician magnetite. Plots of EMPA data suggest that silician magnetite in the Cihai skarn deposit present in the form of [Si4+]IV + [Fe2+]VI ↔ [Fe3+]IV + [Fe3+]VI. The LA-ICP-MS dating results show that the garnet coexisting with the magnetite core has a U-Pb age of 282.5 ± 2.6 Ma, which is consistent with the timing of Fe mineralization in the retrograde skarn stage, indicating that silician magnetite was formed within a relatively short time interval after magnetite precipitation. Based on coexisting minerals, chemical compositions, and our previous fluid inclusion analyses, we propose that the Si zoning of magnetite is largely temperature dependent and, thus, is interpreted as a retrograde growth zoning. It is suggested that silician magnetite formed as a result of changing compatibility due to decreasing temperatures and can potentially be used to trace ore-forming temperatures in hydrothermal deposits. Our study provides independent constraints on the conditions of hydrothermal magnetite formation at Cihai, and suggests that silician magnetite may serve as a potential relatively low-temperature (<300 °C) indicator in other less-well-constrained magmatic-hydrothermal systems.