High-resolution Elemental Mapping Research Articles

Naturally occurring radioactive materials in offshore infrastructure: Understanding formation and characteristics of baryte scale during decommissioning planning

Contaminants, including naturally occurring radioactive material (NORM) of the 238-uranium and 232-thorium decay series, have been recognized as a global research priority to inform offshore petroleum infrastructure decommissioning decisions. This study aimed to characterize pipeline scale retrieved from a decommissioned subsea well tubular pipe through high-resolution elemental mapping and isotopic analysis. This was achieved by utilizing transmission electron microscopy, Synchrotron x-ray fluorescence, photostimulated luminescence autoradiography and Isotope Ratio Mass Spectrometry. The scale was identified as baryte (BaSO4) forming a dense crystalline matrix, with heterogenous texture and elongated crystals. The changing chemical and physical microenvironment within the pipe influenced the gradual growth rate of baryte over the production life of this infrastructure. A distinct compositional banding of baryte and celestine (SrSO4) bands was observed. Radioactivity attributed by the presence of radionuclides (226Ra, 228Ra) throughout the scale was strongly correlated with baryte. From the detailed scale characterization, we can infer the baryte scale gradually formed within the internals of the tubular well pipe along the duration of production (i.e., 17 years). This new knowledge and insight into the characteristics and formation of petroleum waste products will assist with decommissioning planning to mitigate potential radiological risks to marine ecosystems.

Submicroscopic iron-rich grains throughout impact glasses in Chang'E-5 regolith

In lunar regolith returned by Chang'E-5, Apollo and Luna missions, submicron-sized iron-rich grains that are dominated by reduced iron and with sizes of about 10 s to 100 s nanometers are widespread on surfaces of impact glasses, such as glass spherules and glass that welds agglutinates. With flat or mound-like tops, such submicroscopic iron-rich grains typically occur in chains, nets and/or mists, and they were interpreted to be surface adherents that are mainly composed of native iron. Processes such as cooling of iron melt veneer, condensation of iron vapor, and in situ thermal reduction of regolith particles have been separately invoked to explain their origin. Meanwhile, some iron-rich grains have lower hemispheres being embedded in their host glasses, and spherical iron-rich grains with similar sizes, compositions and spatial distributions are also widespread in interiors of their host glasses. It is possible that both the surface and interior iron-rich grains were formed simultaneously during cooling of the host impact melt. However, the precise composition and crystallography of the surface and interior iron-rich grains have not been adequately constrained. In this study, we prepared focused ion beam (FIB) sections for chains of iron-rich mounds that occur on surfaces of impact glasses in the Chang'E-5 lunar regolith. Several FIB sections also exposed interior iron-rich grains that exhibit similar sizes and regular distribution patterns. High-resolution elemental mapping shows that both the surface and interior iron-rich grains are mainly composed of iron and sulfur, and crystallographic inspections reveal that the grains are similarly composed of α-Fe and troilite with varying volume proportions. As the glass spherules and agglutinate glasses are mainly melt products of local regolith materials, both the surface and interior iron-rich grains are most consistent with being formed due to liquid immiscibility between FeS and silicate melt. Troilite preferentially occurs along margins of the iron-rich grains due to heterogeneous distribution of S in the iron-rich melt and the relatively greater compatibility between troilite and silicate melt. The varying morphology of the surface iron-rich grains (e.g., flat tops, mounds) is likely caused by different surface tensions of FeS melt droplets and different wettability between the solidifying Fe-dominating melt and FeS melt with silicate melt. This interpretation predicts that iron-rich grains formed during cooling of impact melt may be dispersed in melt strips and even detached from the melt body during ejecta motion, and they could be subsequently adhered to surfaces of other lunar regolith particles. For reflectance spectra of lunar regolith at visible to near-infrared wavelength, the long-noticed darkening effect caused by submicron-sized iron may be caused by such two-phase iron-rich grains (i.e., α-Fe plus troilite).

Traces of life in Río Tinto (Spain): Decoupling of morphological and geochemical biosignatures

The Río Tinto system located in southern Spain is a 100-km long acidic (pH ≈ 2.3) river rich in dissolved iron, sulfate and heavy metals. The red-tinted river, formed as a product of natural acid rock drainage that has been exacerbated over the years by anthropogenic mining activities, hosts a variety of extremophilic microorganisms that are potential analogues to those found on early Earth and ancient Mars. Here we detailed the investigation of potential biosignatures in the forms of microbialites and sediments deposited along the river. The 20-50 cm thick microbialites (estimated age at most 100 years) are composed of layered structures of Fe(III) minerals with alternating porosities. High resolution imaging and elemental mapping coupled to mineralogical analysis suggest the transformation of K-jarosite (KFe3(SO4)2(OH)6; Vm = 153 cm3/mol) to goethite ( α -FeOOH; Vm = 21 cm3/mol), generating porosities in the process due to a reduction in the molar volume Vm. Meanwhile, riverbed sediments are composed of mixtures of schwertmannite (Fe 8 O8(OH)6(SO4) · nH2O), jarosite, goethite and other minor minerals (ferrihydrite, lepidocrocite, hematite). Culture experiments indicate that crystalline jarosite is formed via transformation from poorly-crystalline schwertmannite (Fe 8 O8(OH)6(SO4) · nH2O), but that the presence of Acidithiobacillus thiooxidans actually inhibits this transformation, possibly due to organic matter stabilization of schwertmannite. Fe isotopic analyses show no clear variations between layers in the microbialites (δ56Fe = -0.85 ± 0.10 ‰), while the riverbed sediments indicate progressively more negative values downstream over a 5-km distance (δ56Fe = +0.4 to -0.8‰). The spatial isotopic pattern of the riverbed sediments is consistent with rapid microbial oxidation of Fe2+ and inconsistent with the much slower abiotic Fe2+ oxidation rate. Taken together, our results show that while the microbialites’ structure is suggestive of microbially-induced precipitation, its geochemical features are not. Meanwhile, the inconspicuous-looking riverbed sediments provide clear evidence of biogenicity from their isotopic compositions. This highlights the challenge in detecting life especially when decoupling of morphological and geochemical biosignatures is to be expected.

From divergent to convergent plate boundary: A ca. 200 Ma Wilson cycle recorded by ultrahigh-pressure eclogites in the Dora-Maira Massif, Western Alps

Sparse eclogite exposure in accretionary-to-collisional orogens cannot only reveal the sites of ancient subduction zones and plate boundaries, but also elucidate the multi-stage tectonic evolution in the Wilson cycle. The southern part of the Dora-Maira Massif in the Western Alps is well known for its ultrahigh-pressure (UHP) rocks and contains small occurrences of petrographically distinct types of eclogite. Due to the lack of detailed geochronological and geochemical data on these rocks, the nature of their protolith and related geodynamic setting has remained unknown. In the current study, 33 samples from 13 localities were studied by whole-rock major- and trace-element analysis, and a selection of the samples were studied using Sr-Nd-Hf isotopes, as well as high-resolution elemental mapping and U-Pb geochronology of zircon. According to macroscopic appearance, petrography, chemical composition, and principal-component analysis of multi-elements, two major types are distinguished. Light-colored eclogite is phengite-rich, commonly foliated, bears kyanite and quartz/coesite, shows relatively high MgO, K2O, SiO2, εNd(t) (−2.2 to +1.4), and εHf(t) (+3.5 to +7), with a protolith geochemically similar to an enriched-type mid-oceanic-ridge basalt (E-MORB). Dark eclogite generally is massive, rutile-rich, shows higher values of Ti, Fe, P, Nb, and Zr, εNd(t) values between −2.8 and −0.8, εHf(t) values between −6.1 and −3.2, with an oceanic island basalt (OIB)-type protolith composition. Magmatic zircon cores, which are characterized by steep heavy rare earth element (HREE) patterns and negative Eu anomalies, yield consistent protolith ages of ca. 253−252 Ma in both eclogite types. Metamorphic zircon domains with flat HREE patterns and insignificant Eu anomalies yield a younger mean age of ca. 34 Ma, which is the age commonly assigned to eclogite-facies UHP metamorphism. Considering the various geochemical signatures of E-MORB- to OIB-like eclogites and the regional tectonic evolution, their protolith is best explained as rocks, which crystallized from rift-related basaltic magma, associated with the break-up of the Pangea Supercontinent that eventually resulted in the birth of the Piemonte-Liguria Ocean (Alpine Tethys Ocean) between the future Eurasia and Adria plates. Therefore, the UHP eclogites in the Dora-Maira Massif likely fingerprint a multi-stage tectonic evolution from divergent (continental extension, rifting) to convergent (subduction zone) plate boundary, corresponding to the beginning and end of a Wilson cycle. Incidentally, they reveal that mafic rocks in the Alpine basement units may not be polymetamorphic, but may actually consist of post-Variscan products that underwent only Alpine metamorphism.

Unmasking pipefish otolith using synchrotron-based scanning X-ray fluorescence

Scientists use otoliths to trace fish life history, especially fish migrations. Otoliths incorporate signatures of individual growth and environmental use. For many species, distinct increment patterns in the otolith are difficult to discern; thus, questions remain about crucial life history information. To unravel the history of such species, we use synchrotron-based scanning X-ray fluorescence. It allows the mapping of elements on the entire otolith at a high spatial resolution. It gives access to precise fish migration history by tagging landmark signature for environmental transition and it also characterises localised growth processes at a mineral level. Freshwater pipefish, which are of conservation concern, have otoliths that are small and fragile. Growth increments are impossible to identify and count; therefore, there is a major lack of knowledge about their life history. We confirm for the first time, by mapping strontium that the two tropical pipefish species studied are diadromous (transition freshwater/marine/freshwater). Mapping of other elements uncovered the existence of different migratory routes during the marine phase. Another major breakthrough is that we can chemically count growth increments solely based on sulphur signal as it is implicated in biomineralization processes. This novel method circumvents reader bias issues and enables age estimation even for otoliths with seemingly untraceable increments. The high spatial resolution elemental mapping methods push back limits of studies on life traits or stock characterisation.

Ubiquitous dendritic olivine constructs initial crystal framework of mafic magma chamber

Layered intrusions are fossilized mafic magma chambers in the Earth's crust. The pathways that led to crystallization and solidification of layered intrusions have been hotly debated as the growth model of primocrysts (the earliest-formed crystals) in mafic magma chambers remains enigmatic. In this study, we carried out high-resolution elemental mapping of mm-scale olivine primocrysts from the Sept Iles layered intrusion (Canada), the third largest one in the world, with a focus on phosphorus (P) zoning of olivine. Our results reveal that complex P zoning of olivine with intense dissolution textures is ubiquitous in the ∼4.7 km-thick Layered Series of the intrusion. The P-rich zones of olivine are featured with dendritic, hopper and sector-zoned patterns, which are attributed to significant magma undercooling. Thermal modeling based on a 1-D conductive cooling model suggests that initially hot parental magma intruding into cold country rocks would result in high degrees of undercooling (-ΔT >60 °C) in the margins (i.e., floor, roof and sidewalls) of magma chamber, facilitating rapid growth of dendritic olivine, which may be then spread within the magma chamber by dynamic convection and crucial to construct initial crystal framework of a solidifying magma chamber. Additionally, diffusion modeling based on the P gradients in olivine suggests a minimum cooling rate of 2.7 to 3.3×10−3 °C/year in the center of the intrusion, similar to the averaged cooling rate of other layered intrusions (e.g., Bushveld, Stillwater and Skaergaard) reported in previous studies. This indicates that rapid cooling (ca. 10−2 to 10−3 °C/year) at high temperature (>800 °C) may be predominant regardless of the size of magma chambers. Our study demonstrates that P zoning of olivine is powerful in decoding crystallization and thermal histories of mafic-ultramafic intrusions.

Mobilization and fractionation of Ti-Nb-Ta during exhumation of deeply subducted continental crust

The behavior of Ti-Nb-Ta is crucial to reveal the genesis of island arc magmatism. However, mobilization and fractionation of Ti-Nb-Ta in subduction zone settings remain poorly understood. The discovery of felsic veins rich in coarse-grained rutile within retrograde eclogite of the North Qaidam UHP metamorphic belt provides a unique and novel opportunity to study age variation during rutile formation and alteration, as well as Ti-Nb-Ta mobility and fractionation during fluid/melt-rock interaction. Rutile high-resolution elemental mapping, and U-Pb bulk grain (ID-TIMS), and in-situ U-Pb geochronology have been utilized to focus on the properties of rutile in both, felsic vein and retrograde eclogite host to gain insight into possible similarities and differences. Three groups of rutile were distinguished according to its host rock, trace elements signature, and genetical connection to ilmenite: eclogite-hosted rutile (Rt-1), felsic vein-hosted rutile not associated with ilmenite (Rt-2a), and associated with ilmenite (Rt-2b). Field evidence and rutile trace elements characteristics document the source of vein-hosted rutile to be mainly derived from the eclogite during fluid/melt-rock interaction. Principal Component Analysis reveals that Nb, Ta, Sn, and W are more enriched in Rt-2a compared to Rt-1; Rt-2b has higher Nb, U, and Hf than Rt-2a. High-resolution mapping across large rutile grains shows the enrichment of high field strength elements (HFSEs) in rutile near to ilmenite, which indicates a HFSEs back diffusion from the rutile–ilmenite boundary during the replacement of rutile by ilmenite. The Nb/Ta ratios of Rt-2a are lower than those of Rt-1, which result from different partition coefficients of Nb and Ta during fluid/melt-rock interaction. The diffusion-influenced rutile exhibits suprachondritic Nb/Ta ratios and demonstrates that diffusion of Nb in rutile is higher than that of Ta under identical P-T conditions. Rutiles Rt-1 and Rt-2a yield consistent 206Pb/238U ages of 426–423 Ma, which is similar to the 433 ± 3 Ma determined by ID-TIMS results of bulk rutile grains. This indicates that Ti-Nb-Ta must have been mobilized during the exhumation of deeply subducted continental crust. However, the diffusion-influenced rutile shows a large variation of ages compared to the rutile not associated with ilmenite, demonstrating that the back-diffusion may affect the U-Pb system in rutile. Therefore, when rutile is partially altered into ilmenite or titanite, its dating should be used with caution. Thus, this study demonstrates volume diffusion is a very important geological process to result in extreme HFSEs fractionation and age variation of rutile on the mineral scale. The rutile aggerates that occur in the felsic veins in 3–5 m distance to the adjacent retrograde eclogite suggest that Ti-Nb-Ta-rich melts/fluids were transported over a distance of at least several meters and that rutile does not represent a residual phase of the Na-Si-Al-, F- and CH4-bearing fluid/melt environment that formed during anatexis of the subducted continental crust. The formation of rutile-rich aggregates during the generation, transport, and crystallization of subducted continental crust-derived melts/fluids in the deep roots of orogenic belts may be a critical trigger for the depletion of HFSEs in arc magmatic rocks during the formation of the continental crust.

Petrographic and geochemical implications for ore genesis and mineralogical composition studies of the Tarat formation hosted sandstone uranium deposit in the Tamari Prospect from Arlit, Niger

The ongoing uranium mining activities coupled with mines development in SOMAIR open pit mine at Arlit led to the discovery of a new prospect called Tamari prospect endowed with a potential uranium ore deposit and have been subjected to a very few scientific studies but their characteristics still need to be examined in further detail investigation in order to determine the mineralogical composition, ore genesis and geochemical characteristics of the deposit. Based on detailed studies using optical microscopy, backscattered electron imaging, electron-microprobe and high resolution elemental mapping, the authors have established that detrital, authigenic and uranium ore-stage minerlization are associated with the deposit. Previous studies on the deposits documented two stages of uranium deposits one close to 190 Ma and the other close to 150 Ma. The dominant uranium ore minerals occur as uraninite. Pyrite and galena are the most dominant sulfide minerals associated with the uranium mineralization and therefore, chemical composition analysis shows that pyrite contains an average U concentration up to of 7.62 wt% and galena has an average of 27.16 wt%. The association U–Zr–Pb present in the zircon and monazite indicates that the geochemical environment responsible for this correlation should be that with a large association with lithophile elements which occur in monazite and zircon minerals that could possibly be the potential source of uranium. The impregnation of organic matter with U–Ti oxide indicates a synsedimentary or early source of uranium. Uraninite contains a high concentration of Zr (av. 5.77 wt%) suggesting a probable succession of fluid circulation that would allow either the deposition of two generations of uraninites or mineralization leaching. The excellent positive correlation of U, Fe, Ca and Mn in the mineralized pole of U–Ti oxides is related to the presence of high concentrations of complexing ligands such as carbonate, oxalate, hydroxide, fulvic and humic acids in the deposits. Uranium mineralization occurs as Iriginite (U–Mo) and the powellite (Pb–MoO4) cemented the quartz grains and kaolinite highlights a late fluid circulation.

Role of metal impurities in multicrystalline silicon solar cell degradation

Controlling light- and elevated temperature-induced degradation (LeTID) is one of the great challenges in silicon photovoltaics industry. Here, we performed systematic high-resolution elemental mapping and spectroscopic electrical measurements in multi-crystalline silicon wafers that underwent standard solar cell processing during LeTID. The prominent correlations between the metal precipitate formation/dissociation steps and the appearance of band-like or localized electronic states during LeTID evolution were observed. Of note, the re-precipitation of iron was found to co-localize with copper atom agglomerations. Finally, we discuss the potential mechanisms that enhance metal redistributions in mc-Si during LeTID in the context of the existing literature.

Synchrotron-Based Imaging Reveals the Fate of Selenium in Striped Marsh Frog Tadpoles.

Synchrotron-based X-ray fluorescence microscopy (XFM) coupled with X-ray absorption near-edge structure (XANES) imaging was used to study selenium (Se) biodistribution and speciation in Limnodynastes peronii tadpoles. Tadpoles were exposed to dissolved Se (30 μg/L) as selenite (SeIV) or selenate (SeVI) for 7 days followed by 3 days of depuration. High-resolution elemental maps revealed that Se partitioned primarily in the eyes (specifically the eye lens, iris, and retinal pigmented epithelium), digestive and excretory organs of SeIV-exposed tadpoles. Speciation analysis confirmed that the majority of accumulated Se was converted to organo-Se. Multielement analyses provided new information on Se colocalization and its impact on trace element homeostasis. New insights into the fate of Se on a whole organism scale contribute to our understanding of the mechanisms and risks associated with Se pollution.

Penetration of foliar-applied Zn and its impact on apple plant nutrition status: in vivo evaluation by synchrotron-based X-ray fluorescence microscopy

The absorption of foliar fertilizer is a complex process and is poorly understood. The ability to visualize and quantify the pathway that elements take following their application to leaf surfaces is critical for understanding the science and for practical applications of foliar fertilizers. By the use of synchrotron-based X-ray fluorescence to analyze the in vivo localization of elements, our study aimed to investigate the penetration of foliar-applied Zn absorbed by apple (Malus domestica Borkh.) leaves with different physiological surface properties, as well as the possible interactions between foliar Zn level and the mineral nutrient status of treated leaves. The results indicate that the absorption of foliar-applied Zn was largely dependent on plant leaf surface characteristics. High-resolution elemental maps revealed that the high binding capacity of the cell wall for Zn contributed to the observed limitation of Zn penetration across epidermal cells. Trichome density and stomatal aperture had opposite effects on Zn fertilizer penetration: a relatively high density of trichomes increased the hydrophobicity of leaves, whereas the presence of stomata facilitated foliar Zn penetration. Low levels of Zn promoted the accumulation of other mineral elements in treated leaves, and the complexation of Zn with phytic acid potentially occurred owing to exposure to high-Zn conditions. The present study provides direct visual evidence for the Zn penetration process across the leaf surface, which is important for the development of strategies for Zn biofortification in crop species.

In-situ pyrolytic processed zinc stannate incorporated graphitic carbon nitride nanocomposite for selective and sensitive electrochemical determination of nitrobenzene

In this study describes the in-situ pyrolytic synthesis of zinc stannate-graphitic carbon nitride (ZSO-gCN) nanocomposite material and its potential application in electrochemical sensing of nitrobenzene. The ZSO-gCN nanocomposite was characterized by X-ray diffraction, fourier transform infra-red, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and elemental mapping to determine the crystalline, morphology, and functionalities of the resultant nanocomposite material. The ZSO-gCN was used to modify the glassy carbon electrode (GCE) surface, and investigated for electrochemical determination of nitrobenzene. The ZSO-gCN modified GCE showed good electrocatalytic activity towards nitrobenzene than other modified electrodes. The ZSO-gCN/GCE having the electron transfer rate constant (ks = 0.746 s−1) and charge transfer coefficient (α = 1.171) towards electroanalysis of nitrobenzene. Linear sweep voltammetric (LSV) technique exhibits cathodic peak current increases in linearly with increasing nitrobenzene content in the range of 30–100 μM. The lowest detection limit (LoD) and sensitivity were found to be 2.2 μM (S/N = 3) and 0.05857 μA μM−1 cm−2, respectively. The inorganic species and organic substances such as Ca2+, K+ and 4-nitrophenol, 1-chloro,2,4-dinitrobenzene, 1-bromo,2-nitrobenzene, 1-iodo,2-nitrobenzene were showed negligible interference effect with nitrobenzene detection and good sensitivity for monitoring nitrobenzene content in real samples were acquired. This recently produced ZSO-gCN nanocomposite would be a prospective material for electrochemical based sensor applications.

Magmatic-hydrothermal processes recorded by muscovite andcolumbite-group minerals from the Bailongshan rare-element pegmatites in the West Kunlun-Karakorum orogenic belt, NW China

The mineralization of rare-element pegmatites is attributed to complex magmatic-hydrothermal processes such as magma fractionation, fluid exsolution and fluid/melt metasomatism. However, it is still unclear how these processes affect the evolution of pegmatites and the fractionation of rare elements. Here we use the textures and major and trace element compositions of muscovite and columbite-group minerals (CGM) from the newly discovered Bailongshan Li-Cs-Ta (LCT)-type pegmatites in the West Kunlun-Karakorum orogenic belt, NW China, to decipher the magmatic-hydrothermal processes related to enrichment of rare elements. Four major regional zones are recognized in the Bailongshan area; quartz-albite-tourmaline (QAT) zone, quartz-muscovite (QM) zone, quartz-albite-spodumene (QAS) zone and quartz-spodumene (QS) zone. The QAT and QM zones are barren, whereas the QAS and QS zones host six major LiRb ore segments (I to VI) along the NWW-SEE strike. From the QM zone through the QAS to the QS zone, the K/Rb ratios of muscovite decrease gradually from 26.0 to 7.8 and CGM has Mn#s increasing from 0.28 to 0.56, consistent with the trend of progressive evolution of the pegmatites. The Ta# profiles across individual CGM grains reveal a three-stage growth process, i.e., from a Nb-rich CGM magmatic stage to a Ta-rich CGM magmatic stage to a Nb-rich CGM hydrothermal stage. High-resolution elemental mapping of primary muscovite from the QAS and QS zones shows that melt metasomatism induced local enrichment of F, Li, Cs and Ta and the precipitation of secondary lepidolite along the rims or cleavages of primary muscovite. In addition, late-stage, F-poor fluid metasomatism caused the release of Fe, Mn, B, Rb and Cs from primary muscovite, and induced complex zoning patterns and crystallization of hydroxylapatite in cleavage planes of primary muscovite. Secondary muscovite has higher Li, Be, B, Rb and Cs and lower Nb than primary muscovite, indicating that these elements may have been partially dissolved and remobilized from primary spodumene, muscovite and CGM into late-stage hydrothermal fluids, from which the secondary muscovite and Nb-rich CGM crystallized. Therefore, the complex zoning patterns and compositional variations of both muscovite and CGM from the Bailongshan pegmatites record a progressive magmatic-hydrothermal process related to the rare-metal mineralization.

Methodology for the determination of <63 µm free mica fines in sand and within the cement matrix of hardened concrete blocks using scanning electron microscopy and energy dispersive spectroscopy

We propose a method for quantification of the percentage of <63 µm muscovite fines (termed free mica) in mixed sand fines and the identification/semi-quantitative analysis of free mica in the cement matrix of hardened concrete blocks. In recent times homeowners in County Donegal, Ireland reported structural problems in their buildings and concern that it is due to high concentrations of free mica within the concrete blocks. Our method requires the generation of high-resolution backscattered electron (BSE) images using field emission scanning electron microscopy (FE-SEM), where the characteristic needle-like morphology of mica can easily be identified. Additional information on the size, shape and chemical composition of the free mica fines, is gathered using energy dispersive spectroscopy (EDS or EDX). The combination of high-magnification images, high-resolution elemental maps, and mineral liberation software allows accurate identification and quantification of free mica within the sand fines and cement matrix. Supplementary material : Information pertaining to the measurements by SEM-EDX are reported in Supplementary Tables 1 and 2, available at https://doi.org/10.6084/m9.figshare.c.4709390

Accumulation of transition metals and metalloids in sulfidized stromatolites of the 3.48 billion–year–old Dresser Formation, Pilbara Craton

Stromatolites of the ~3.48 billion–year–old Dresser Formation (Pilbara Craton, Western Australia) provide some of the oldest convincing evidence of life on Earth. Here, we augment previous evidence with a detailed investigation of the concentrations and distributions of various transition metals (Cr, Mn, Co, Ni, Cu, Zn, Mo, Se, Ag, Sn, Au, Hg, and Pb) and metalloids (As, Sb, and Te) in unweathered samples of strongly sulfidized stromatolites from drill cores. High–resolution elemental mapping and in situ compositional analysis of sulfides (pyrite and sphalerite) show that these sedimentary and hydrothermally sourced elements are strongly concentrated in texturally distinctive, nano-porous pyrite enriched in autochthonous organic matter, which forms the major, petrogenetically earliest component of wrinkly laminated and digitate growth fabrics within the stromatolites. Repeated cyclic alternations of various transition metals and metalloids (most importantly Ni and Zn), plus the presence of disconformities and overgrowth relationships between wrinkly stromatolite laminae, suggest that these element accumulations were primarily established by depositional processes during continuous stromatolite formation. Because transition metals and metalloids generally have strong affinities for organic matter, and can play active roles in biochemical processes, we interpret these element accumulations in the Dresser Formation stromatolites to be the result of binding to organic matter of living microbial communities and/or dead biomass, and perhaps also microbial utilization. Collectively, our results show that the precise characterization of transition metal–metalloid concentrations and distributions can unveil element enrichment patterns suggestive of biological activity, even in some of Earth’s oldest stromatolites.

High-resolution elemental mapping of the root-rhizosphere-soil continuum using laser-induced breakdown spectroscopy (LIBS)

Understanding the complex chemical nature of root-soil interactions is essential for building the next generation of sustainable agricultural systems and facilitating long-term environmental remediation strategies. Techniques currently suited to investigate spatial controls on nutrient exchange in plant rhizospheres, however, are hindered by limitations in throughput, cost, analytical scope, and sample preparation needs. We describe here a method for rapid, high-resolution (∼100 μm), multi-element imaging of both organic content and inorganic constituents in root-rhizosphere-soil systems using laser-induced breakdown spectroscopy (LIBS). A switchgrass assemblage (Panicum virgatum) was grown in compact rhizotrons containing a sandy loam Alfisol from the Kellogg Biological Station (KBS), Michigan, USA. Root-soil samples were extracted using custom plastic coring devices for live root sampling and a modified drill press for sectioning frozen stabilized soil. A 266 nm, Nd:YAG laser was rastered over ∼10 mm2 sample surfaces and broadband spectra from single-pulse ablations were collected and mapped to discrete XY spatial coordinates for simultaneous imaging of 17 macronutrients, micronutrients and matrix elements. In order to rapidly process LIBS raster data and investigate chemical trends in the rhizosphere, an open-source Python module was developed, in which we used a novel calibration-free masking algorithm (based on principal components analysis (PCA) of normalized spectral intensities) to discriminate soil mineral grains, root fragments, and associated rhizosphere regions. We observed fine-scale chemical gradients within only a millimeter of switchgrass roots, consistent with rhizodeposition of organic compounds and proximal uptake of inorganic nutrients. Detection of trace element and carbon accumulations with diagnostic spectral signatures in the rhizosphere suggested the presence of residues (detritusphere) that could serve as sites of preferential microbial accumulation. These results highlight potential applications of LIBS within the growing field of spatially-resolved plant-soil analysis and extend its versatile imaging capabilities to the complex root-rhizosphere-soil network.

Altered ageing behaviour of a nanostructured Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

In this study, the ageing behaviour of a nanostructured Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy produced by solution treatment followed by high pressure torsion (HPT) was systematically investigated using hardness testing, high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping, X-ray diffraction (XRD) and XRD line broadening analysis. The HPT-deformed alloy exhibits an ageing response that produces a higher peak-aged hardness at lower temperature and shorter ageing time as compared to the same alloy aged after conventional thermomechanical processing. The HAADF-STEM and elemental mapping reveal extensive segregation of solute atoms along grain boundaries during ageing. A model is developed which shows that the main structures causing hardening for peak-aged samples are the grain boundaries and the segregation of solute atoms formed along grain boundaries. The metastable β′ phase precipitates, which form on ageing of conventionally processed Mg-Gd-Y-Zn-Zr alloy samples, do not form in the present aged samples, and instead equilibrium β-Mg5(RE,Zn) phase forms on over-ageing. This altered precipitation behaviour is attributed to the high defect density (e.g. grain boundaries, dislocations and vacancies) introduced by HPT, leading to enhanced diffusion of solutes. The present processing produces an alloy that has a hardness of ∼145 HV. A model of strengthening indicates that whilst grain boundary strengthening provides the largest contribution to strengthening, it is the additional solid solution hardening, cluster hardening, and dislocation hardening that provide the main factors that caused the hardness to surpass that of other bulk processed Mg alloys studied to date.

In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

The cumulates of the Bushveld Complex, which form the largest layered intrusion on Earth, are known to have crystallized from several isotopically distinct magma pulses. Here, we present in situ Sr isotope compositions combined with the corresponding mineral chemistry of plagioclase from all lithological zones, covering > 6 km of stratigraphy, to constrain the petrogenesis of the complex. The in situ data coupled with high-resolution elemental maps of individual plagioclase grains reveal complex zonation patterns with respect to mineral chemistry and Sr isotope composition. This suggests that interstitial plagioclase in the Bushveld Complex crystallized from multiple, isotopically distinct influxes of melt percolating through a mafic cumulate framework and displacing the resident melt. Similarly, cumulus plagioclase grains are the result of continual ingress of a distinct magma, which picked up previously formed plagioclase crystals. Sr isotope compositions across the layered sequence seem to be largely decoupled from differentiation indices, such as Mg#, anorthite content or trace element ratios. As these observations cannot be explained by bulk assimilation, we propose that the elevated Sr isotopic signature of the Bushveld cumulates may have resulted from the interaction of the parental magma with a fluid derived from the up to 2 km thick dolomitic footwall, which caused a major shift in Sr isotope composition without significantly affecting the degree of differentiation or trace element signature. The decarbonation and/or assimilation of dolomite during the intrusion of the Bushveld Complex may be of major importance, not only for the petrogenesis but also for the emplacement of the layered intrusion, because devolatilization is directly linked to space creation owing to volume loss, thus producing a lopolith.

Evolution of reduced Ti containing phase(s) in MgH2/TiO2 system and its effect on the hydrogen storage behavior of MgH2

The present study aims to consolidate information regarding the catalytic reaction mechanism of MgH2/TiO2 hydrogen storage system. Reduction of titania, resulting to evolution of a monoxide rock salt nanocrystalline phase was identified by in-situ X-ray diffraction (XRD) study of a 3 mol% TiO2 additive mixed MgH2. Existence of homogeneously distributed Ti in a crystalline rock salt matrix was confirmed by combined analysis of X – ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and elemental chemical mapping techniques. Moreover, titanium in Ti3+ and Ti2+ valance states was confirmed by X - ray photoelectron spectroscopy (XPS). Differential scanning calorimetry (DSC) study proves that the reduced Ti oxide phases promote the low temperature dehydrogenation of MgH2. The present observations reiterate that the MgO rock salt layer existing in oxide additives loaded MgH2 is not necessarily a barrier layer that blocks the diffusion of hydrogen.

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

The Lower Zone of the Bushveld Complex comprises an up to 2-km-thick package of different ultramafic rock types with an approx. 90-cm-thick, sulphide-bearing noritic interval that occurs in the western and eastern limbs. The distribution and geometry of the zone are highly variable across the Complex, showing pronounced, yet laterally discontinuous layering on different scales. Together with the ubiquitous lack of large-scale fractionation in the Mg# of orthopyroxene, variable Sr isotope compositions and erratic Pt/Pd ratios, these observations strongly suggest an emplacement of the Lower Zone as a sill complex, as these contrasting geochemical characteristics are difficult to account for in a large Bushveld magma chamber, as previously suggested. It is more likely that these sills were episodically fed from a sub-Bushveld staging chamber, and variably contaminated, while passing through the crust before their final emplacement in the Lower Zone. Detailed mineralogical and microtextural work based on high-resolution elemental mapping of a set of samples, covering the entire Lower Zone stratigraphy of the western Bushveld shows that the variations in the late crystallising interstitial mineral mode are different from what would be expect, if all phases crystallised from a fixed initial mass of interstitial liquid. The interstitial mineral mode, represented by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.