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USGS2025

Using peak geometry and shifts in the x-ray spectrum of carbon from electron probe microanalysis to determine thermal maturity of organic matter

During the burial of mudstones, the associated organic matter undergoes gradual thermal maturation, a key process that can influence the reactivity of organic matter during catagenesis, the formation of hydrocarbon deposits and the chemical weathering of mudstones. Conventional methods for assessing the thermal maturity of organic matter often fail to reflect the geochemical heterogeneity between individual organic phases in mudstone samples. Here, we report an alternative, non-destructive, surficial and micro-scale (analytical spot size of ~ 300 nm with about 4 μm diffusion depth for micrometre-size organic grains) method to evaluate the thermal maturity of organic matter in mudstones using the carbon K α X-ray spectrum measured by field emission-electron probe microanalyser (FE-EPMA). Using this method, we observed correlations between parameter values derived from FE-EPMA spectra, including the peak position, the peak area and the intra-sample heterogeneity of these measurements, and independently measured vitrinite/solid bitumen reflectance for a suite of mudstones, representing different age, geological context and burial depth. With the increased values in peak area and position, we identified an increase in the carbon mass fraction of organic matter and the mean nominal oxidation state of carbon approaching zero. These trends, which are consistent with aromatisation and graphitisation, provide the rationale for using FE-EPMA to estimate the thermal maturity of organic matter. To explore some of these trends in more detail, we employed time-of-flight secondary ionisation mass spectrometry, X-ray photoelectron spectroscopy and optical reflectance measurements on a subset of samples.

Geostandards and Geoanalytical Research↗

USGS2025

Characterizing Meteor Crater impact melts through geochemistry and textural analysis

The U.S. Geological Survey Astrogeology Science Center houses the Meteor Crater sample collection, an assemblage of over 2,500 meters of cuttings from 161 drill holes into Meteor Crater’s rim, flanks, and ejecta blanket. We have utilized this unique collection to study the composition and spatial distribution of impact-generated materials from within the ejecta blanket. Meteor Crater has historically been known to have generated only a relatively small amount of impact melt compared to other terrestrial craters of similar size. A detailed compositional and textural dataset of impact-derived melts from this impact can therefore be a useful asset in improving our understanding of crater formation, and in particular impact melt formation. We have characterized 42 impact-melt particles from Meteor Crater using a scanning electron microscope and an electron microprobe for textural and compositional analysis. We analyzed samples from six drill holes in the ejecta blanket, situated to the northwest, southeast, south, and southwest of the crater (ejecta northeast of the crater is devoid of impact melts). Impact melts were collected from drill cuttings at various depths within the ejecta blanket, ranging from a few centimeters below the surface down to ~6.5 meters. Backscattered electron (BSE) images were acquired for each analyzed impact-melt particle. To characterize the various textures and phases present in each impact melt, we also took many detailed BSE images. Our geochemical analyses include full spectral profiles using energy dispersive X-ray spectrometry and well-calibrated wavelength dispersive spectrometry for a number of phases, including minerals (olivine, pyroxene, and so on), pristine glass, and metallic inclusions. The full dataset is available in ScienceBase as a data release (Gullikson and others, 2024), accessible at https://doi.org/10.5066/P9OGAJ8P . Our goal for this Open-File Report is to provide a summary of this immense dataset, details on data collection, descriptions of the different phases observed within impact-melt particles (both geochemically and texturally), and observable trends.

Arizona↗

USGS2019

Root-driven weathering impacts on mineral-organic associations in deep soils over pedogenic time scales

Plant roots are critical weathering agents in deep soils, yet the impact of resulting mineral transformations on the vast deep soil carbon (C) reservoir are largely unknown. Root-driven weathering of primary minerals may cause the formation of reactive secondary minerals, which protect mineral-organic associations (MOAs) for centuries or millennia. Conversely, root-driven weathering may also transform secondary minerals, potentially enhancing the bioavailability of C previously protected in MOAs. Here we examined the impact of root-driven weathering on MOAs and their capacity to store C over pedogenic time scales. To accomplish this, we examined deep horizons (100-160 cm) that experienced root-driven weathering in four soils of increasing ages (65-226 kyr) of the Santa Cruz Marine Terrace chronosequence. Specifically, we compared discrete rhizosphere zones subject to root-driven weathering, with adjacent zones that experienced no root growth. Using a combination of radiocarbon, mass spectrometry, 57Fe Mössbauer spectroscopy, high-resolution mass spectrometry, and X-ray spectromicroscopy approaches, we characterized transformations of MOAs in relation to changes in C content, Δ14C values, and chemistry across the chronosequence. We found that the onset of root-driven weathering (65-90kyr) increased the amount of C associated with poorly crystalline iron (Fe) and aluminum (Al) phases, particularly highly disordered nano-particulate goethite (np-goethite). This increase coincided with greater C concentrations, lower Δ14C values, and greater abundance of what is likely microbially-derived C. Continued root-driven weathering (137-226kyr) did not significantly change the amount of C associated with crystalline Fe and Al phases, but resulted in a decline in the amount of C associated with poorly crystalline Fe and Al phases. This decline coincided with a decrease in C concentrations, an increase in 14C values, and a shift toward plant-derived C. In contrast, soil not affected by root-driven weathering showed comparatively low amounts of C bound to poorly crystalline Fe and Al phases regardless of soil age and, correspondingly, lower C concentrations. Our results demonstrate that root-driven formation and disruption of MOAs are direct controls on both C accrual and loss in deep soil. This finding suggests that root impacts on soil C storage are dependent on soil weathering stage, a consideration that is critical for future predictions of the vulnerability of deep soil C to global change.

California↗

USGS2024

Cathodoluminescence imaging and spectrometry of a jadeite microbeam reference crystal: Detection of Ce3+

Options for selecting a high Na concentration mineral for instrument calibration that are suitably stable under the electron beam are limited [ 1 ]. NaCl (approximately a mass fraction of 39 % Na) is not practical for use alongside other embedded and polished materials in a mounted block of standards. While albite (NaAlSi 3 O 8 ; approximately a mass fraction of 8 % Na) represents a typical choice for some microanalysts, it may be subject to time dependent signal intensity issues, such as those observed for Na and other cations during analysis of hydrous glasses [ 2 ]. Near endmember jadeite pyroxene (NaAlSi 2 O 6 ; approximately mass fraction of 11 % Na) represents a desirable option for calibration given its relatively large concentration of Na and insensitivity to electron beam radiation with respect to X-ray output. Despite these desirable characteristics, jadeites have been shown to contain complex microstructures as the result of intracrystalline zoning [ 3 , 4 ], and the impact of chemical inhomogeneity within mineral standards and natural glasses has been documented [ 5-7 ]. In this study, a commercially obtained jadeite crystal (source location “China”) was studied to assess its microscale uniformity using a suite of microbeam methods, including dispersive red-green-blue multispectral cathodoluminescence imaging (CL RGB ), hyperspectral CL imaging spectrometry, energy dispersive X-ray spectrometry (EDS), SEM-based micro-X-ray fluorescence (μXRF), and wavelength dispersive X-ray spectrometry (WDS).

Microscopy and Microanalysis↗

USGS2019

Variability in results from mineralogical and organic geochemical interlaboratory testing of U. S. Geological Survey shale reference materials

The expansion of unconventional petroleum resource exploration and production in the United States has led to an increase in source rock characterization efforts, particularly related to bulk organic and mineralogical properties. To support the analytical and research needs of industry and academia, as well as internal work, the U.S. Geological Survey (USGS) has collected and prepared shale geochemical reference materials (GRMs) from several major shale petroleum systems in the U.S. The sources of these materials are the Late Cretaceous Boquillas (lower Eagle Ford-equivalent) Formation (roadcut near Del Rio, TX), Late Cretaceous Mancos Shale (outcrop near Delta, CO), Devonian–Mississippian Woodford Shale (outcrop near Ardmore, OK), Late Cretaceous Niobrara Formation (quarry near Lyons, CO), Middle Devonian Marcellus Shale (creek bed in LeRoy, NY), and Eocene Mahogany zone oil shale of the Green River Formation (oil shale mine near Rifle, CO). Of particular interest in the development of these GRMs has been the examination of variability between laboratories and specific methods or instruments in commonly made measurements, including major- and trace-element concentrations, X-ray diffraction (XRD) mineralogy, total organic carbon (TOC) content, and programmed pyrolysis (PP) parameters. For the component concentrations and parameters we measured, the techniques and instrument types included: (1) elemental analysis by X-ray fluorescence, inductively coupled plasma mass spectrometry, and instrumental neutron activation analysis; (2) XRD mineralogy with various preparatory methods (spray drying or micronizing with or without internal standard); (3) TOC by combustion with infrared detection after carbonate removal or the PP approach; (4) PP by Rock-Eval 2 or more recently developed instruments (Rock-Eval 6, Source Rock Analyzer or SRA, and Hydrocarbon Analyzer With Kinetics or HAWK). Overall, the results showed that the selected shales cover a wide range of source rock organic and mineralogical properties. Major- and trace-element chemistry results showed low heterogeneity consistent with other USGS GRMs. Comparison of TOC results showed coefficients of variation (COV) of around 5% and the most consistent organic geochemical results between different laboratories and methods. Arguably the most relevant PP measurement, S2 or kerogen hydrocarbon-generating potential (mg-HC/g-rock), showed a somewhat wider range of variability than TOC (COV ~10%), but was consistent between the three modern instruments and the industry-standard Rock-Eval 2. Major phase mineralogy (mineral concentrations ≥10 wt. %, organic-free basis) were comparable between laboratories, but variability in minor phase identification and quantification was observed. Utilization of these shale GRMs as quality control samples and testing materials is expected to help support analytical and experimental efforts in the continued development of unconventional petroleum resources.

Colorado, Texas, Oklahoma↗

USGS2021

Organo-facies and mineral effects on sorption capacity of low-maturity Permian Barakar shales from the Auranga Basin, Jharkhand, India

Shales associated with the Lower Permian (Barakar Formation) sediments of the Auranga Coalfield, India, occur in the immature–early mature stage. The sorption capacity of Barakar shale samples has been studied through high-pressure methane (CH 4 ) adsorption and low-pressure N 2 gas adsorption (LPN 2 GA) methods, supported with proximate analyses, programmed pyrolysis, optical petrography, and with energy-dispersive spectroscopy, X-ray diffraction, and inductively coupled plasma mass spectrometry. The sorption capacity is a function of the organic and inorganic constituents present in the shale samples. The methane sorption capacity (MSC) and Langmuir volume of the shale samples vary from 0.217 to 0.314 and 0.315 to 0.429 mmol/g rock, respectively. The BET-calculated surface area of the studied shales varies from 8.12 to 30.36 m 2 /g. The sorption capacities show the importance of the total organic content (TOC) through weak but positive correlations with MSC ( r 2 = 0.45) and S 1 values (mg hydrocarbons/g rock from programmed pyrolysis; r 2 = 0.40). Moreover, apparent inverse relationships were observed between MSC and clay mineral abundances, suggesting that individual clay mineral types may influence MSC, although more work is needed. The TOC-normalized MSC (MSC*) of shale samples shows a positive trend with quartz plus clay mineral content and ash yield of r 2 = 0.64 for both. In addition, MSC* shows a negative logarithmic relationship with S 1 + S 2 ( r 2 = 0.63) and a positive linear relationship with TOC-normalized total organic matter (TOM*) ( r 2 = 0.88, when 5 low TOM* samples are excluded) indicating complex relationships possibly including bitumen retention in the sample pore spaces. The micropore study of the samples through LPN 2 GA, applying Dubinin–Radushkevich, Dubinin–Astakhov, and density functional theory models, shows the dominance of micro-mesopore concentrations in the shale matrix of ∼2 nm pore diameter. However, these pores might be present as blind or closed pores. The presence of thorium and zirconium is reflective of terrigenous detrital matter, i.e., moderately to strongly recycled sediments. The fluviatile facies of deposited shales in the Auranga Coalfield are noted by the significant presence of kaolinite (32.5–78.3%), which suggests the importance of its effect on the sorption capacity of proximal terrigenous shales.

Jharkhand↗

USGS2019

Magmato-tectonic links: Ignimbrite calderas, regional dike swarms, and the transition from arc to rift in the Southern Rocky Mountains

Radial and linear dike swarms in the eroded roots of volcanoes and along rift zones are sensitive structural indicators of conduit and eruption geometry that can record regional paleostress orientations. Compositionally diverse dikes and larger intrusions that radiate westward from the polycyclic Platoro caldera complex in the Southern Rocky Mountain volcanic field (southwestern United States) merge in structural trend, composition, and age with the enormous but little-studied Dulce swarm of trachybasaltic dikes that continue southwest and south for ∼125 km along the eastern margin of the Colorado Plateau from southern Colorado into northern New Mexico. Some Dulce dikes, though only 1–2 m thick, are traceable for 20 km. More than 200 dikes of the Platoro-Dulce swarm are depicted on regional maps, but only a few compositions and ages have been published previously, and relations to Platoro caldera have not been evaluated. Despite complications from deuteric alteration, bulk compositions of Platoro-Dulce dikes (105 new X-ray fluorescence and inductively coupled plasma mass spectrometry analyses) become more mafic and alkalic with distance from the caldera. Fifty-eight (58) new 40 Ar/ 39 Ar ages provide insight into the timing of dike emplacement in relation to evolution of Platoro caldera (source of six regional ignimbrites between 30.3 and 28.8 Ma). The majority of Dulce dikes were emplaced during a brief period (26.5–25.0 Ma) of postcaldera magmatism. Some northeast-trending dikes yield ages as old as 27.5 Ma, and the northernmost north-trending dikes have younger ages (20.1–18.6 Ma). In contrast to high-K lamprophyres farther west on the Colorado Plateau, the Dulce dikes are trachybasalts that contain only anhydrous phenocrysts (clinopyroxene, olivine). Dikes radial to Platoro caldera range from pyroxene- and hornblende-bearing andesite to sanidine dacite, mostly more silicic than trachybasalts of the Dulce swarm. Some distal andesite dikes have ages (31.2–30.4 Ma) similar to those of late precaldera lavas; ages of other proximal dikes (29.2–27.5 Ma) are akin to those of caldera-filling lavas and the oldest Dulce dikes. The largest radial dikes are dacites that have yet younger sanidine 40 Ar/ 39 Ar ages (26.5–26.4 Ma), similar to those of the main Dulce swarm.

Colorado↗

USGS2021

Demethylation of methylmercury in bird, fish, and earthworm

Toxicity of methylmercury (MeHg) to wildlife and humans results from its binding to cysteine residues of proteins, forming MeHg-cysteinate (MeHgCys) complexes that hinder biological functions. MeHgCys complexes can be detoxified in vivo, yet how this occurs is unknown. We report that MeHgCys complexes are transformed into selenocysteinate (Hg(Sec)4) complexes in multiple animals from two phyla (a waterbird, freshwater fish, and earthworms) sampled in different geographical areas and contaminated by different Hg sources. In addition, high energy-resolution X-ray absorption spectroscopy and chromatography-ICP mass spectrometry of the waterbird liver support the binding of Hg(Sec)4 to selenoprotein P and biomineralization of Hg(Sec)4 to chemically inert nanoparticulate mercury selenide (HgSe). The results provide a foundation for understanding mercury detoxification in higher organisms, and suggest that the identified MeHgCys to Hg(Sec)4 demethylation pathway is common in nature.

Environmental Science & Technology↗

USGS2017

Spatial dependence of reduced sulfur in Everglades dissolved organic matter controlled by sulfate enrichment

Sulfate inputs to the Florida Everglades stimulate sulfidic conditions in freshwater wetland sediments that affect ecological and biogeochemical processes. An unexplored implication of sulfate enrichment is alteration of the content and speciation of sulfur in dissolved organic matter (DOM), which influences the reactivity of DOM with trace metals. Here, we describe the vertical and lateral spatial dependence of sulfur chemistry in the hydrophobic organic acid fraction of DOM from unimpacted and sulfate-impacted Everglades wetlands using X-ray absorption spectroscopy and ultrahigh-resolution mass spectrometry. Spatial variation in DOM sulfur content and speciation reflects the degree of sulfate enrichment and resulting sulfide concentrations in sediment pore waters. Sulfur is incorporated into DOM predominantly as highly reduced species in sulfidic pore waters. Sulfur-enriched DOM in sediment pore waters exchanges with overlying surface waters and the sulfur likely undergoes oxidative transformations in the water column. Across all wetland sites and depths, the total sulfur content of DOM correlated with the relative abundance of highly reduced sulfur functionality. The results identify sulfate input as a primary determinant on DOM sulfur chemistry to be considered in the context of wetland restoration and sulfur and trace metal cycling.

Florida↗

USGS2019

Integration of microfacies analysis, inorganic geochemical data, and hyperspectral imaging to unravel mudstone depositional and diagenetic processes in two cores from the Triassic Shublik Formation, Northern Alaska

The Middle – Upper Triassic Shublik Formation is an organic-rich heterogeneous carbonate-siliciclastic-phosphatic unit that generated much of the oil in the Prudhoe Bay field and other hydrocarbon accumulations in northern Alaska. A large dataset, including total organic carbon (TOC), X-ray diffraction (XRD), X-ray fluorescence (XRF) and inductively coupled plasma – mass spectrometry (ICP-MS) measurements, has been built from core and outcrop samples of the Shublik, with a focus on the organic-rich intervals. In addition, two core intervals from the Shublik were analyzed using a hyperspectral imaging system in the visible, near-infrared and shortwave-infrared range. Integration of the hyperspectral results with core descriptions, microfacies interpretations, and analytical data is being used to decipher mudstone depositional and diagenetic processes. Petrographic analysis of Upper Triassic organic-rich intervals within the Shublik suggests that the main microfacies is a laminated bioclastic wackestone/packstone that was episodically disrupted by energetic events of variable intensity. These energetic events produced transitional and sparry calcite bioclastic packstone to grainstone intervals, depending on the depth of sediment column disturbance. By using hyperspectral imaging data from the Ikpikpuk core, individual distribution maps for minerals of interest have been generated and corroborate the microfacies interpretations. These maps also illustrate small-scale vertical changes in mineralogy. The laminated bioclastic wackestone/packstone intervals contain less calcite than the adjacent sparry bioclastic packstone to grainstone intervals. The calcite in these laminated intervals is more iron rich. This interpretation suggests that lower iron concentrations should be expected in the disrupted intervals than in nearby laminated intervals. Textural features are also enhanced in the hyperspectral images relative to visual description of the cores by combining the extraction of the average reflectance in the visible part of the electromagnetic spectrum and the depth of the main carbonate-related feature belonging to calcite. Examples noted in the enhanced imagery include low-angle features, calcite grain-size, and the size, shape and orientation of phosphatic nodules. This enhancement is being used to differentiate laminated from sparry bioclastic packstone to grainstone-rich intervals and provides a more comprehensive assessment of the microfacies than is practical by thin-section analysis.

Alaska↗

USGS2013

Environmental survey in the Tuul and Orkhon River basins of north-central Mongolia, 2010: Metals and other elements in streambed sediment and floodplain soil

Streambed sediment and subsurface floodplain soil were sampled for elemental analyses from 15 locations in river basins of north-central Mongolia during August 2010. Our primary objective was to conduct a reconnaissance-level assessment of potential inputs of toxicologically important metals and metalloids to Lake Baikal, Russia, that might originate from mining and urban activities within tributaries of the Selenga River in Mongolia. Samples were collected in triplicate from all sites, then dried, and sieved to <2 mm for analysis by portable X-ray florescence spectroscopy and by inductively coupled plasma mass spectrometry after digestion with concentrated nitric and hydrochloric acids. Arsenic, copper, and mercury were greatly elevated in sediment and floodplain soil collected from tributary streams located near two major mining operations. Lead and zinc were moderately elevated in streambed sediment and in floodplain soil obtained from a small tributary in the capital city of Ulaanbaatar, but those concentrations were considerably less than probable effects benchmarks. Historical and possibly present mining activities have led to considerable metal contamination in certain tributaries of the Orkhon River in north-central Mongolia; however, metals originating from those sources did not appear to be accumulating in sediments at our downstream-most sampling sites located near the border between Mongolia and Russia.

Environmental Monitoring and Assessment↗

USGS2016

Geologic and geochemical results from boreholes drilled in Yellowstone National Park, Wyoming, 2007 and 2008

Between 2007 and 2008, seven Earthscope Plate Boundary Observatory (PBO) boreholes ranging in depth from about 200 to 800 feet deep were drilled in and adjacent to the Yellowstone caldera in Yellowstone National Park, for the purpose of installing volcano monitoring instrumentation. Five of the seven boreholes were equipped with strainmeters, downhole seismometers, and tiltmeters. Data collected during drilling included field observations of drill cuttings, stratigraphy within the boreholes, water temperature, and water and drill cuttings samples from selected depths. Six of the seven boreholes encountered rhyolite lavas and tuffs. The rhyolite lavas compose the Canyon flow, the Gardner River flow, the Gibbon River flow, the Hayden Valley flow, the Nez Perce Creek flow, and the West Thumb flow. Boreholes also penetrated a vertical sequence through the Lava Creek Tuff and the Tuff of Bluff Point. In addition, one borehole drilled through a Swan Lake Flat Basalt sequence and terminated in a rhyolite lava flow. After drilling the seven PBO boreholes, cuttings were examined and selected for preparation of grain mounts, thin sections, and geochemical analysis. Major ions and trace elements (including rare earth elements) of selected cuttings were determined by x-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS); the ICP-MS provided more precise trace-element analysis than XRF. A preliminary interpretation of the results of geochemical analyses generally shows a correlation between borehole cuttings and previously mapped geology. The geochemical data and borehole stratigraphy presented in this report provide a foundation for future petrologic, geochemical, and geophysical studies.

Wyoming↗

USGS2018

Effect of calcium on the bioavailability of dissolved uranium(VI) in plant roots under circumneutral pH

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO 3 – solutions at circumneutral pH. The accumulation of U in plants (3.1–21.3 mg kg –1 ) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30–700 μg L –1 ) and Ca (0–240 mg L –1 ) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V 0 ) ranged from 4.4 to 62 μg g –1 h –1 in experiments with no added Ca and from 0.73 to 2.07 μg g –1 h –1 in experiments with 12 mg L –1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L –1 Ca. Ternary Ca–U–CO 3 complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy–energy-dispersive spectrometry detected U–P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Environmental Science & Technology↗

USGS2016

Notes on the origin of copromacrinite based on nitrogen functionalities and δ13C and δ15N determined on samples from the Peach Orchard coal bed, southern Magoffin County, Kentucky

This paper represents the first attempt to show, by means other than just petrographic ones, that one type of macrinite, herein designated copromacrinite, may result from macrofauna feces. For that purpose a combination of coal petrography, X-ray photoelectron spectroscopy, and elemental-analysis continuous-flow isotope ratio mass spectrometry methods were used to determine nitrogen functionalities and δ 13 C and δ 15 N compositions in 1) vitrinite-rich, 2) fusinite + semifusinite-rich, and 3) macrinite-rich (with a possible coprolitic origin) samples of the high volatile A bituminous Peach Orchard coal (Bolsovian; Middle Pennsylvanian) from Magoffin County, Kentucky. There were no significant differences between pyridinic-N and quaternary-N abundance in the three samples, however, pyrrolic-N was higher (~ 54%) in the macrinite-rich sample than in the other two samples (~ 38%). The data suggest that pyridinic-N and quaternary-N are independent of maceral group composition and that pyrrolic-N is dependent on maceral composition (fusinite + semifusinite versus macrinite). δ 13 C values obtained for bulk and demineralized coal of the vitrinite- and fusinite + semifusinite-rich samples are similar with δ 13 C values of − 24.80 ± 0.01&permil; VPDB and − 24.61 ± 0.09&permil; VPDB for bulk samples and − 24.81 ± 0.07&permil; VPDB and − 24.52 ± 0.04&permil; VPDB for demineralized samples. These values are within the expected range for vitrinite-rich samples and the slightly higher δ 13 C value of the fusinite + semifusinite-rich sample is expected as δ 13 C values for inertinite are higher than for vitrinite. However, there was a significant shift to a lower δ 13 C value (− 26.80 ± 0.01&permil; VPDB for the bulk sample value) for the macrinite-rich sample. Because the samples are basically isorank, and δ 13 C (and δ 15 N) shifts do not occur during maturation until anthracite rank, the difference may be related to the presence or composition of the macrinite within the sample which lacks heat-effect indicators, such as devolatilization vacuoles and distorted pores. δ 15 N values are also similar for bulk and demineralized coal of the vitrinite- and fusinite + semifusinite-rich samples, and the bulk values were heavier in this samples (3.07 ± 0.03&permil; Air and 2.92 ± 0.10&permil; Air, respectively), and much lighter (− 2.83 ± 0.09&permil; Air) for the macrinite-rich sample. The study of Peach Orchard coal samples using reflected-light microscopy, isotopic composition, and nitrogen-forms analyses revealed that the macrinite-rich sample contains macrinite with coprolitic features (e.g. oxidation rind, mix of undigested palynomorphs, frequent and randomly located funginite, agglutination pulp of semifusinite reflectance, internal lack of bedding fabric, and suggestion of structures resulting from intestines and stomach walls), more pyrrolic-N (~ 16%), and lower δ 13 C (~ 2&permil; VPDB) and δ 15 N (~ 4&permil; Air) values than the vitrinite and semifusinite + fusinite rich samples. These findings suggest that the maceral macrinite has multiple origins based on petrography and measurable chemical differences between the macrinite, vitrinite, and semifusinite + fusinite fractions within the coal. Assuming that copromacrinite observed is an excretion then the anomalies observed may result from the symbiotic relations between the macrofauna (e.g. cockroaches) and microbiota during the digestive processes, and the nitrogen balance mechanisms inside macrofauna body.

Kentucky↗

USGS2026

Geochemical geodatabase of sedimentary strata (coal, coal-adjacent rocks, tuffaceous oil shale, phosphate-rich rocks) and produced water in the Uinta region, Utah and Colorado

The Geochemical Geodatabase of Sedimentary Strata (Coal, Coal-adjacent Rocks, Tuffaceous Oil Shale, Phosphate-rich Rocks) and Produced Water in the Uinta Region, Utah and Colorado, consists of compiled datasets acquired as part of the Carbon Ore, Rare Earth, and Critical Mineral (CORE-CM) Uinta Region assessment funded by the U.S. Department of Energy (DEFE0032046, 2021–2024; Birgenheier et al., 2024). The CORE-CM assessment focused on providing comprehensive geological and geochemical characterization of current and prospective sedimentary-hosted resources including coal, oil shale, phosphatic limestone, and produced water from oil and gas targets present in eastern Utah and northwestern Colorado (Figure 1). This Data Series includes a geodatabase that consists of analytical geochemical data collected September 2021 through December 2024 via portable X-ray fluorescence (pXRF), and laboratory measured analyses produced by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The coal-related geochemical data are derived primarily from the Cretaceous Blackhawk Formation and Ferron Sandstone of Utah, and the Mesaverde Group of Colorado. Additional non-coal resources assessed include oil shale-bearing strata of the Eocene upper Green River Formation (Utah and Colorado), phosphate-rich limestone of the Permian Park City Formation (Utah) and produced water from oil and gas-bearing strata of the Eocene Green River and Wasatch Formations (Uinta Basin) and the Pennsylvanian Paradox Formation (Paradox Basin) (Table 1). The CORE-CM assessment included a wide range of lithologies present in the coal, oil shale, and phosphate geologic resource systems whether or not the specific lithology has current economic value. Geochemical analyses of produced water from oil and gas wells focused on current and emerging hydrocarbon targets in the central Uinta Basin and northern Paradox Basin. A total of 13,092 geochemical analyses from these geologic systems is provided in the included geodatabase. A series of coal quality data (e.g., composition and maceral analyses) is also included in the database and was digitized from archived coal samples from the Utah Geological Survey (Appendix A).

Colorado, Utah↗

USGS2019

Soil mineralogy and geochemistry along a north-south transect in Alaska and the relation to source-rock terrane

Soils collected along a predominately north-south transect in Alaska were used to evaluate regional differences in the soil mineralogy and geochemistry in the context of a geotectonic framework for Alaska. The approximately 1,395-kilometer-long transect followed the Dalton, Elliott, and Richardson Highways from near Prudhoe Bay to Valdez. Sites were selected with a site spacing of approximately 10 road-kilometers; soil was sampled by soil horizon at 175 sites. Terrane boundaries were estimated from digitized versions of the lithotectonic terrane map of Alaska (Silberling and others, 1994). Terrane assignments for each site were based on the site’s distance along the transect. We also present data for 15 minerals or mineral groups and 58 elements, as well as total, inorganic, and organic carbon. Quantitative mineralogy of the mineral-soil horizons was characterized by X-ray diffraction. Elemental contents were determined by a combination of inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) analysis following a multi-acid or sodium-sinter decomposition of the samples. Total carbon and carbonate carbon contents were determined using an automated carbon analyzer and coulometric titration, respectively; organic carbon content was obtained by calculating the difference between total and carbonate carbon. Mercury and selenium were analyzed using cold-vapor atomic absorption (CV-AA), and hydride-generation atomic absorption spectrometry (HG-AAS), respectively. The mineralogical and geochemical patterns from these soils are used to assess the relation between soil characteristics and the geology of surrounding terranes.

Alaska↗

USGS2021

A review of spatially resolved techniques and applications of organic petrography in shale petroleum systems

This review examines new techniques and applications of organic petrography in source-rock reservoir petroleum systems that have occurred along with development of the global ‘shale revolution’ in energy resources. The review is limited to techniques and instrumentation that provide spatially resolved information, typically at or below microscales, for dispersed organic matter occurring in situ in samples of shale and mudrock. A brief summary of ion beam sample preparation is followed by discussion of the most common analytical techniques and applications. Advantages and limitations of each technique, including requisite sample preparation, types of information generated [e.g., molecular or elemental (isotopic) abundance], sensitivity, and resolution are discussed. In a few cases, techniques not yet applied to organic petrology of shale or mudrock are described (e.g., X-ray photoelectron spectroscopy imaging), in anticipation of near-term future application. The most common in situ techniques applied for characterization of organic matter in shale and mudrock include optical (including fluorescence) and electron microscopies, Raman, fluorescence, and infrared spectroscopies, and surficial measurements via force microscopy. Techniques that show growing application to organic petrography of shale include tip-enhanced photothermal infrared spectroscopy, mass spectrometry imaging, and synchrotron-based spectroscopies, among others. It is anticipated that the future of dispersed organic matter petrography will hold continued development of integrated instrument techniques (e.g., simultaneous or sequential correlative microscopies and/or spectroscopies of the same location), increased instrumental resolution, increased use of multiscale and multimodal organic petrography investigations, and three-dimensional imaging and chemical speciation mapping applications via multiple analytical approaches.

International Journal of Coal Geology↗

USGS2023

Chromium in minerals and selected aquifer materials

Between 1952 and 1964, hexavalent chromium, Cr(VI), was released into groundwater from a Pacific Gas and Electric Company (PG&E) compressor station in Hinkley, California, in the western Mojave Desert 80 miles northeast of Los Angeles, California. In 2015, the extent of anthropogenic Cr(VI) in groundwater in Hinkley and Water Valleys was uncertain, but some Cr(VI) in groundwater may be naturally occurring from rock and aquifer material. To evaluate potential sources of natural Cr(VI), chromium and other selected trace-element concentrations were measured by inductively coupled plasma-mass spectrometry (ICP-MS), with multi-acid digestion, on 34 samples of surficial alluvium and core material from Hinkley and Water Valleys, California, and on 2 samples of alluvium from the mafic Sheep Creek fan to the southwest. Chromium concentrations in Hinkley and Water Valleys ranged from 2 to 110 milligrams per kilogram (mg/kg), with a median concentration of 14 mg/kg; concentrations were highest in weathered mafic hornblende diorite associated with Iron Mountain. High chromium concentrations also were present within fine-textured materials and visually abundant iron- and manganese-oxide coatings on the surfaces of mineral grains. For comparison, chromium concentrations as high as 170 mg/kg were measured in mafic alluvium from the Sheep Creek fan. In contrast, chromium concentrations were lowest in Mojave-type deposits (Mojave River stream and lake margin deposits), with a median of 6 mg/kg. Chromium concentrations measured by ICP-MS compared favorably with concentrations measured by portable (handheld) X-ray fluorescence (pXRF; chapter B), on the basis of least-squares regression results and a coefficient of determination (R 2 ) of 0.97. Minerals in bulk samples and the heavy (dense) mineral fractions isolated from those samples were identified using optical techniques, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Quartz and feldspar were the most abundant minerals, especially within recent and older Mojave River deposits. Chromium concentrations were as high as 1,250 mg/kg in the heavy-mineral fraction, with specific gravity greater than 3.32. Chromium was not commonly detected in the light-mineral fraction, with specific gravity less than 2.85. Most chromium within the heavy-mineral fraction was substituted within magnetite mineral grains less than 100 micrometers (μm) in diameter, and almost no chromite was present within the heavy-mineral fraction. Although magnetite is resistive to weathering, weathering of magnetite to hematite was identified (1) in Miocene materials underlying unconsolidated deposits in the western subarea of Hinkley Valley and (2) in alluvium within Water Valley that contains weathered minerals eroded from Miocene rock. Less-dense, more easily weathered chromium-containing amphiboles, such as actinolite in older Mojave River alluvium and hornblende in locally derived alluvium from Iron Mountain, were identified optically. Magnetite was not identified in weathered hornblende diorite and was less abundant in locally derived materials and in Miocene materials than in Mojave-type deposits. A comparison of ICP-MS data and sequential extraction data shows that approximately 90 percent of chromium in aquifer material within Hinkley and Water Valleys was not extractable and was interpreted to reside within unweathered mineral grains. Most extractable chromium was within the strong acid extractable fraction. Chromium within the weakly sorbed, and specifically sorbed extractable fractions in oxide accumulations within the regulatory Cr(VI) plume is potentially mobile into groundwater with changes in ionic strength or pH. Although Hinkley and Water Valleys are regionally low in chromium, natural geologic sources of chromium may be present in aquifer materials penetrated by wells completed in (1) weathered hornblende diorite bedrock underlying the western subarea; (2) Miocene deposits underlying the western subarea and unconsolidated material in the northern subarea and Water Valley containing basalt or weathered minerals eroded from Miocene deposits; (3) unconsolidated material containing visually abundant iron- and manganese-oxide coatings on the surfaces of mineral grains that are present near the water table and near lithologic or geologic contacts; and (4) brown clay and mudflat/playa deposits in the northern subarea. Brown clay and mudflat/playa deposits in the eastern subarea near Mount General have a low-chromium, felsic mineralogy similar to Mojave River deposits and do not contain high concentrations of chromium; however, manganese(IV) oxides within these materials may facilitate oxidation of trivalent chromium, Cr(III), to Cr(VI).

California↗