Volume 109, Issue 12

Small amounts of water (tens to hundreds of parts per million) can have a profound effect on the properties of mantle peridotites, including viscosities, conductivities, and melting temperatures. Measuring the water content of nominally anhydrous minerals (NAMs) has provided insight into the amounts of water contained within mantle rocks. However, converting from NAM water contents to the activity of H 2 O is non-trivial. Equilibria involving amphibole can be used to determine values of the activity of H 2 O ( a H 2 O ) at the time of mineral equilibration. This approach yields low values of the activity of H 2 O (<0.3) for four peridotite xenoliths from Southeastern Australia. These four xenoliths also record values of oxygen fugacity ( f O 2 ) that range from −0.2 to −1.2 log units below the fayalitemagnetite-quartz buffer. All these values of f O 2 are inconsistent with the presence of a CH 4 -rich fluid (too oxidizing), and the lowest value of oxygen fugacity, as recorded by one sample, is inconsistent with the presence of a CO 2 -rich fluid.

Fission track analysis is a thermochronologic method for dating rocks and reconstructing their low-temperature thermal histories. We investigate the influence of the apatite composition on the etching of fossil confined fission tracks and its consequences for the fission track method. We conducted step-etch experiments with 5.5 M HNO 3 at 21 °C on samples with etch pit diameters ( Dpar ) spanning most of the range for natural apatites (Panasqueira: 1.60 μm; Slyudyanka: 2.44 μm; Brazil: 3.92 μm; and Bamble: 4.60 μm) to determine their apatite etch rates v R (the rate at which each lattice plane is displaced parallel to itself) as a function of crystallographic orientation (ϕ′). Our measurements revealed significant differences between the four samples. We fitted three-parameter functions, v R = a ( Dpar )ϕ′ e b ( Dpar )ϕ′ + c , describing v R as a function of the angle to the apatite c -axis for our hexagonal samples (excluding Bamble) and Durango apatite. Both parameters a and b exhibit a linear correlation with Dpar , whereas the constant c is small (~0.1 μm/min) and its between-sample variation is negligible at the resolution of our measurements. Bamble exhibits a different, bimodal relationship between v R and ϕ′, which we fitted with a sum of two sine functions. In all cases, including Bamble, there is a striking correlation between the angular frequencies of horizontal confined tracks and the magnitude of the apatite etch rate v R perpendicular to the track axes. This result shows that the sample of confined tracks selected for measurement and modeling is to a much greater degree determined by the etching properties of the apatite sample than by geometric or subjective biases. The track etch rate v T is constant along most of the track length but varies from track to track. The mean v T correlates with Dpar , so that tracks etch to their full lengths in a shorter time in faster etching apatites. The mean rate of length increase between etch steps, v L , also correlates with Dpar . The length increments of individual tracks are however irregular. This points to an intermittent structure at the ends of the tracks.

Inorganic X-ray amorphous materials (iXAMs) such as vitreous phases, minerals having an insufficient number of repeating structural units to diffract X-rays, and inorganic solids with exclusively structural short-range order are ubiquitous in soils and are relevant for numerous environmental processes but are notoriously difficult to identify and quantify. To test for the quantification and chemical composition of iXAMs in soil, we prepared four mineral mixtures containing quartz, calcite, feldspars, and clay minerals in different proportions typical of soils and amended them with 10–70 wt% iXAMs in the form of a 1:1 weight mixture of ferrihydrite and opal-A. We quantified these iXAMs in mineral mixtures by analyzing powder X-ray diffraction (PXRD) data using the Rietveld method and compared the results for different sample preparation techniques (conventional and spray drying) based on the internal standard method in Rietveld analysis. The mineral mixtures were also analyzed for their chemical composition by X-ray fluorescence (XRF) spectrometry, and mass-balance calculations combining Rietveld and XRF data were carried out to estimate the chemical composition of iXAMs in mineral mixtures. Both sample preparation methods showed no significant difference in determined iXAM contents and yielded accurate results for iXAM contents within ±3 wt% at the 95% confidence level (2σ). The relative accuracy deteriorated with decreasing iXAM content but remained below 10% for iXAM contents >10 wt% (mean = 3%). The precision of iXAM content quantification in mineral mixtures prepared by spray drying was slightly better, though statistically equivalent to the conventionally prepared mixtures (2σ = 1.49 and 1.61 wt%). The average precision of both sample preparation methods was ±2 wt% at the 95% confidence level. Levels of detection and quantification of iXAMs in spray-dried mineral mixtures containing 1–10 wt% iXAMs were estimated at 0.8 and 4.0 wt%, respectively. The chemical composition of iXAMs in terms of major oxides was accurately assessed by mass-balance calculations with average relative errors for nominal SiO 2 and Fe 2 O 3 contents of 9.4 and 4.3%, respectively (range = 0.02–54.7%). Even though adsorbed H 2 O and structural H 2 O/OH – as quantified by the loss on ignition comprised an important portion of the iXAMs (15.3 wt%), their LOI in mineral mixtures as derived from mass-balance calculations could only be quantified with an average relative error of 67.2% (range = 1.30–371%). We conclude that iXAMs in soil and related geomaterials present at levels >4 wt% can be quantified by Rietveld analysis of PXRD data with an accuracy of ±3 wt% at best. Combined results of Rietveld and XRF analyses can yield accurate results for the chemical composition of iXAMs within a relative error of 10% for major oxides, provided iXAM contents exceed 10 wt%, and the content and chemical composition of all crystalline mineral phases are accurately assessed. The results presented in this study lay the foundation to explore iXAM contents and chemical compositions in soils and to examine their impact on soil physicochemical properties and biogeochemical element cycles.

Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg 2 GeO 4 using diamond-anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg 2 GeO 4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type ( Cmc 2 1 ) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a third-order Birch Murnaghan fit to the experimental data, we obtained V 0 = 305.1(3) Å 3 , K 0 = 124.6(14) GPa, and K0′ $\begin{array}{} \displaystyle K_{0}^{\prime} \end{array}$ = 3.86 (fixed) for forsterite-type Mg 2 GeO 4 and V 0 = 263.5(15) Å 3 , K 0 = 175(7) GPa, and K0′ $\begin{array}{} \displaystyle K_{0}^{\prime} \end{array}$ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs, and interpreting the results of shock compression experiments.

The analysis of optical microscopic image data is crucial for the identification and characterization of mineral phases and, thus, directly relevant to the subsequent methodology selections of further detailed petrological exploration. Here, we present a novel application of Swin Transformer, a deep learning algorithm to classify mineral phases such as arsenopyrite, chalcopyrite, gold, pyrite, and stibnite in images captured by optical microscopy. To speed up the training process and improve the generalization capabilities of the investigated model, we adopt the “transfer learning” paradigm by pre-training the algorithm using a large, general-purpose image data set named ImageNet-1k. Furthermore, we compare the performances of the Swin Transformer with those of two well-established Convolutional Neural Networks (CNNs) named MobileNetv2 and ResNet50, respectively. Our results highlight a maximum accuracy of 0.92 for the Swin Transformer, outperforming the CNNs. To provide an interpretation of the trained models, we apply the so-called Class Activation Map (CAM), which indicates a strong global feature extraction ability of the Swin Transformer metal mineral classifier that focuses on distinctive (e.g., colors) and microstructural (e.g., edge shapes) features. The results demonstrate that the deep learning approach can accurately extract all available attributes, which reveals the potential to assist in data exploration and provides an opportunity to carry out spatial quantization at a large scale (centimeters-millimeters). Simultaneously, boosting the learning processes with pre-trained weights can accurately capture relevant attributes in mineral classification, revealing the potential for application in mineralogy and petrology, as well as enabling its use in resource explorations.

Magmatic fluid degassing within shallow magma chambers underneath the ore bodies is critical to the formation of porphyry Cu-Au deposits (PCDs). Yet, it remains unclear how the fluid degassing influences the development of PCDs. Here, geochemical data of apatite, amphibole, and plagioclase from ore-bearing and coeval barren porphyries have been analyzed in Sanjiang metallogenic belt, China. The ore-bearing porphyries normally exhibit high and wide X F /X Cl (31.76–548.12) and X F /X OH (0.779–7.370) ratios of apatites, which are evidently higher than those of the barren porphyries (X F /X Cl of 1.03–26.58; X F /X OH of 0.686–3.602). Combined with the continuous variation features of Cl/OH ratios and H 2 O contents of melts calculated by amphiboles, as well as fluid migration models, we constrained the mechanisms of fluid degassing within shallow magma chambers underneath PCDs. There are three different ways of fluid degassing, while only fluid degassing via fluid channel stage can migrate and focus the metal-rich fluids effectively, conducive to the development of PCDs. The mechanisms of magmatic fluid degassing processes are further controlled by the storage depths of magma chambers and initial H 2 O contents of the magmas revealed by the compositions of amphibole, plagioclase, and thermodynamic modeling. Magmas with shallower storage depths and higher initial H 2 O contents are more likely to experience extensive and focused fluid degassing, leading to the generation of PCDs. This study demonstrates the potential utility of integrated mineral analyses and thermodynamic modeling for investigating the mechanisms of magmatic fluid degassing in porphyry systems, as well as for identifying prospective buried PCDs.

Polarized Fourier transform infrared (FTIR) microspectroscopy of the OH-stretching region of hydroxylapatite-chlorapatite solid solutions presents novel problems for the assignment of peaks to specific OH-Cl pairs. Crystal structure refinements of Hughes et al. (2016) identified new positions for column anions in synthetic mixed Cl-OH apatites, with three different column anion arrangements depending on composition. These structural refinements, combined with bond-valence calculations, allow for interpretation of the OH-stretching region. A peak at 3574 cm –1 is identified as that from end-member hydroxylapatite. A second major peak at 3548 cm –1 is only found in mixed chlorapatite-hydroxylapatite solid solutions, as is a third peak at 3592 cm –1 . Both represent perturbations of the OH-stretching vibration as compared to hydroxylapatite, to lower and higher frequency, respectively. Both of the new peaks are the result of a Cl b -OH sequence, with adjacent anions in crystallographically similar positions, both above or both below adjacent mirror planes. One configuration has the hydrogen atom pointed toward the chlorine atom. The second has the hydrogen of the OH group pointed away from the chlorine atom. Both configurations present novel problems. The shift to lower wavenumber at 3548 cm –1 is characteristic of hydrogen bonding in fluorapatite-hydroxylapatite mixtures, yet the distance between O(H) and Cl b is too great to allow it. The shift of OH-stretching vibrations to lower wavenumber is produced through changes in polarization of intervening Cl-Ca2′ (or Ca2) and Ca2(′)-O3 bonds, which are affected by the presence of the large chlorine atom. Lowering the OH-stretching vibration mimics the expected effect of chlorine on a neighboring OH group in the apatite c -axis column, though without hydrogen bonding. The shift to higher wavenumbers, i.e., higher frequency at 3592 cm –1 , is the opposite of that expected for hydrogen bonding between column anions in the apatite mineral group. It is ascribed to the interaction between an adjacent Cl b and the oxygen end of an adjacent OH dipole. This pairing places an oxygen and a chlorine atom in close proximity. Possible means of accommodation are discussed. A ubiquitous peak at 3498 cm –1 represents hydrogen bonding between an OH and the OH a site, with an interoxygen distance of about 2.9 Å. Published modeling supports the hypothesis that the OH a site is occupied by an O rather than an OH. However, no clear counterpart to this pairing is observed in crystal structure refinements for specimens lacking OH a , although the infrared absorbance is present. The existence of oxyapatite is inferred from studies of plasma-sprayed biomaterials, but the crystallographic details of the substitution have remained elusive. A minor shoulder at 3517 cm –1 does not have a clear counterpart in the structural refinements. Sequences of three columnar anions (e.g., OH-Cl-OH or Cl-OH-OH) can be ruled out, but an unequivocal assignment awaits further research.

The thermal equation of state (EoS) of a natural schorl has been determined at high temperatures up to 673 K and high pressures up to 15.5 GPa using in situ synchrotron X-ray diffraction combined with a diamond-anvil cell. The pressure-volume ( P - V ) data were fitted to a third-order Birch-Murnaghan EoS with V 0 = 1581.45 ± 0.25 Å 3 , K 0 = 111.6 ± 0.9 GPa, and K0′ $\begin{array}{} \displaystyle K_0' \end{array}$ = 4.4 ± 0.2; additionally, when K0′ $\begin{array}{} \displaystyle K_0' \end{array}$ was fixed at a value of 4, V 0 = 1581.04 ± 0.20 Å 3 , and K 0 = 113.6 ± 0.3 GPa. The V 0 (1581.45 ± 0.25 Å 3 ) obtained by the third-order Birch-Murnaghan EoS agrees well with the V 0 (1581.45 ± 0.05 Å 3 ) measured at ambient conditions. Furthermore, the axial compression data of schorl at room temperature were fitted to a “linearized” third-order Birch-Murnaghan EoS, and the obtained axial moduli for the a - and c -axes are K a = 621 ± 9 GPa and K c = 174 ± 2 GPa, respectively. Consequently, the axial compressibilities are β a = 1.61 × 10 –3 GPa –1 and β c = 5.75 × 10 –3 GPa –1 with an anisotropic ratio of β a :β c = 0.28:1.00, indicating axial compression anisotropy. In addition, the compositional effect on the axial compressibilities of tourmalines was discussed. Fitting our pressure-volume-temperature ( P - V - T ) data to a high-temperature third-order Birch-Murnaghan EoS yielded the following thermal EoS parameters: V 0 = 1581.2 ± 0.2 Å 3 , K 0 = 110.5 ± 0.6 GPa, K0′ $\begin{array}{} \displaystyle K_0' \end{array}$ = 4.6 ± 0.2, ( ∂K T /∂T ) P = –0.012 ± 0.003 GPa K –1 and α V0 = (2.4 ± 0.2) × 10 –5 K –1 . These parameters were compared with those of previous studies on other tourmalines, and the potential factors influencing the thermal EoS parameters of tourmalines were further discussed.

Due to their widespread occurrence in several geological settings, omphacite inclusions could be used for elastic Raman geothermobarometry. However, the Raman scattering of complex silicate minerals entrapped in a host depends on both the chemical composition and elastic strain developed during the metamorphic pathway, which makes the task very challenging. Here, as a very first step to probe the potential of omphacite to be used as a mineral inclusion in elastic geothermobarometry, we report the pressure dependence of the Raman spectra of omphacite crystals with the same composition, approximately Jd 43 Di 57 , but having different symmetry because of the existence ( P 2/ n ) or absence ( C 2/ c ) of chemical order at the six- and eight-coordinated cation sites. The experimental results are complemented by ab initio quantum mechanical simulations on fully ordered omphacite (Jd 50 Di 50 ). We demonstrate that the position of the well-resolved Raman peak near 688 cm –1 , arising from Si-O-Si bond bending, is very sensitive to pressure but independent of the state of chemical order, which makes it promising to be utilized in Raman geobarometry. The width of this peak varies with chemical order but not with pressure and therefore can be used to constrain the temperature of inclusion entrapment, because the chemical order is indicative of the closure temperature of the cation-exchange reaction. However, further detailed analyses on the compositional variation of the Raman spectra of omphacite is required before considering omphacite-in-garnet systems to be suitable for Raman elastic geothermobarometry.

Machine learning improves geochemistry discriminant diagrams in classifying mineral deposit genetic types. However, the increasingly recognized “black box” property of machine learning has been hampering the transparency of complex data analysis, leading to challenges in deep geochemical interpretation. To address the issue, we revisited pyrite trace elements and proposed the use of the “Decision Map,” a cutting-edge visualization technique for machine learning. This technique reveals mineral deposit classifications by visualizing the “decision boundaries” of high-dimensional data, a concept crucial for model interpretation, active learning, and domain adaptation. In the context of geochemical data classification, it enables geologists to understand the relationship between geo-data and decision boundaries, assess prediction certainty, and observe data distribution trends. This bridges the gap between the insightful properties of traditional discriminant diagrams and the high-dimensional efficiency of modern machine learning. Using pyrite trace element data, we construct a decision map for mineral deposit type classification, which maintains the accuracy of machine learning while adding valuable visualization insight. Additionally, we demonstrate two applications of decision maps. First, we show how decision maps can help resolve a dispute concerning the genetic type of a deposit whose data were not used in training the models. Second, we demonstrate how the decision maps can help understand the model, which further helps find indicator elements of pyrite. The recommended indicator elements by decision maps are consistent with geologists’ knowledge. This study confirms the decision map’s effectiveness in interpreting mineral genetic type classification problems. In geochemical classification, decision maps mark a shift from conventional machine learning to a visually insightful approach, thereby enhancing the geological understanding derived from the model. Furthermore, our work implies that decision maps could be applicable to diverse classification challenges in geosciences.

The Luobusa chromitite and ophiolite present a captivating geological feature marked by peculiar mineralogical and geochemical characteristics. Abundant platinum-group minerals (PGM), base-metal sulfides (BMS), and PGE-sulfides and alloys in the chromitite reveal a multistage genesis, encompassing partial mantle melting, melt-rock interactions, and dynamic shifts in oxygen and sulfur fugacities ( f O 2 , f S 2 ). The geochemical signatures and PGE patterns of these mineral inclusions elucidate the evolutionary process of the Luobusa ophiolite, tracing its transition from a sub-ridge environment to a sub-arc setting. The variable ΣPGE values (40–334 ppb) in chromitite, coupled with notably lower ΣPGE values (10–63 ppb) in dunite imply extensive melt fractionation and melt-rock interactions. Coexisting well-crystallized Os-Ir alloys alongside interstitial BMS likely reflect low f S 2 and high temperatures during the early formational stages, whereas abundant anhedral or irregular sulfarsenide and pyrite inclusions in chromite point to lower temperatures and higher f S 2 during the late stages. The trace element composition of pyrite inclusions displays some of the characteristics of mid-ocean ridge (MOR) and oceanic island rocks, manifesting the interplay of diverse magmatic sources during the evolution of the Luobusa ophiolite.

Petrologic investigation of the El Ali IAB iron meteorite (Somalia) revealed three new minerals: elaliite [ Fe82+ $\begin{array}{} \displaystyle {\rm Fe_{8}^{2+}} \end{array}$Fe 3+ (PO 4 )O 8 , IMA 2022-087], elkinstantonite [Fe 4 (PO 4 ) 2 O, IMA 2022-088], and olsenite [KFe 4 (PO 4 ) 3 , IMA 2022-100]. The name elaliite recognizes the occurrence of this mineral within the El Ali meteorite, originally located at 4° 17′ 17″N, 44° 53′ 54″E. Elkinstantonite is named after Linda (Lindy) Elkins-Tanton (b. 1965), a planetary scientist and professor in the School of Earth and Space Exploration at Arizona State University. The name olsenite is in honor of Edward J. Olsen (1927–2020), the former Curator of Mineralogy and Meteorites at the Field Museum of Natural History in Chicago (1960–1991). The new minerals and their names have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association. The holotype specimens of elaliite, elkinstantonite, and olsenite consist of the polished block mount with accession number MET11814/2-1/EP1 deposited in the University of Alberta Meteorite Collection. Elaliite, elkinstantonite, and olsenite occur along with wüstite, troilite, sarcopside, and Ca-bearing graftonite within inclusions in the iron-nickel metal (kamacite, 9.4 wt% Ni) that makes up the bulk of the El Ali sample. The empirical formulas determined by electron probe microanalysis for elaliite, elkinstantonite, and olsenite are: ( Fe7.9432+Fe1.0203+ $\begin{array}{} \displaystyle {\rm Fe_{7.943}^{2+}Fe_{1.020}^{3+}} \end{array}$Cr 0.010 Ni 0.006 Ca 0.004 Mn 0.004 ) Σ8.987 (P 0.932 Si 0.077 S 0.005 ) Σ1.014 O 12 ,( Fe3.9472+ $\begin{array}{} \displaystyle {\rm Fe_{3.947}^{2+}} \end{array}$Mn 0.016 Ni 0.003 Ca 0.001 Cr 0.001 ) Σ3.968 (P 1.986 Si 0.014 S 0.013 ) Σ2.013 O 9 , and (K 0.820 Na 0.135 Ca 0.004 ) Σ0.959 (Fe 3.829 Mn 0.050 ) Σ3.879 (P 2.972 S 0.058 Si 0.017 ) Σ3.047 O 12 , respectively. Electron backscatter diffraction was used to confirm the crystal structures of the three new minerals. Raman spectra for all three minerals are also presented.

Single-crystal optical spectra of corundum (Al 2 O 3 ) and the Al 2 SiO 5 polymorphs andalusite, kyanite, and sillimanite, containing both Fe 2+ -Fe 3+ and Fe 2+ -Ti 4+ intervalence charge transfer (IVCT) absorption bands were measured at temperatures up to 1000 °C. Upon heating, thermally equilibrated IVCT bands significantly decreased in intensity and recovered fully on cooling. These trends contrast with the behavior of crystal field bands at temperature for Fe, Cr, and V in corundum, kyanite, and spinel. The effects of cation diffusion and aggregation, as well as the redistribution of band intensity at temperature, are also discussed. The loss of absorption intensity in the visible and near-infrared regions of the spectrum of these phases may point to a more general behavior of IVCT in minerals at temperatures within the Earth with implications for radiative conductivity within the Earth.

Sulfur (S) in the mantle is conventionally assumed to be exclusively stored in accessory sulfide phases, but recent work shows that the major silicate minerals that comprise >99% of the mantle could be capable of hosting trace amounts of S. Assessing the incorporation of trace S in nominally S-free mantle minerals and determining equilibrium S partitioning between these minerals and basaltic melt requires analyzing small experimental phases with low S contents. Here, we develop a protocol for EPMA analysis of the trace levels of S in silicate phases. We use a suite of natural and experimental basaltic glass primary and secondary standards with S contents ranging from 44 ppm to 1.5 wt%. The effects of beam current and counting time are assessed by applying currents ranging from 50 to 200 nA and total counting times between 200 and 300 s at 15 kV accelerating voltage. We find that the combination of 200 nA beam current with a 200 s counting time (80 s peak, 60 s each for upper and lower background, respectively) achieves precise yet cost-effective measurements of S down to a calculated detection limit of ~5 ppm and a blank-derived, effective detection limit of ~17 ppm. Close monitoring of the S peak intensity and position throughout the duration of each spot also shows that high currents and extended dwell times do not compromise the accuracy of measurements, and even low S contents of 44 ppm can be reproduced to within one standard deviation. Using our developed recipe, we analyzed a small suite of experimental clinopyroxenes (Cpx) and garnets (Gt) from assemblages of silicate partial melt + Cpx ± Gt ± sulfide, generated at 1.5 to 3.0 GPa and 1200 to 1300 °C. We find S contents of up to 71 ± 35 ppm in Cpx and 63 ± 28 ppm in Gt and calculate mineral-melt partition coefficients ( (Dsmin/melt) $\begin{array}{} \displaystyle (D_{\text{s}}^{\text{min/melt}}) \end{array}$) of up to 0.095 ± 0.064 and 0.110 ± 0.064 for DsCpx/melt and DsGt/melt, $\begin{array}{} \displaystyle D_{\text{s}}^{\text{Cpx/melt}} ~\text{and}~D_{\text{s}}^{\text{Gt/melt}}, \end{array}$ respectively. The sulfur capacity and mineral-partitioning for Cpx are in good agreement with SXRF measurements in a prior study by Callegaro et al. (2020), serving as an independent validation of our EPMA analytical protocol.