Volume 110, Issue 1

The distribution of water reservoirs in the deep Earth is critical to understanding geochemical evolution and mantle dynamics. Phase D is a potential water carrier in the slab subducted to the uppermost lower mantle (ULM), and its seismic velocity and density characteristics are important for seismological detection in water reservoirs, but these properties remain poorly constrained. Here, we calculate the seismic velocities and density of Mg-end-member phase D (MgSi 2 H 2 O 6 ) under the ULM conditions using first-principles calculations based on the density functional theory. The velocities of phase D are higher than those of periclase and slightly lower than those of bridgmanite by 0.5–3.4% for V P and by 0–1.9% for V S between 660 and 1000 km depths. Considering its relatively low content, phase D can hardly produce a low-velocity anomaly in the ULM observed by seismological studies. However, its strong elastic anisotropy may contribute significantly to the observed seismic anisotropy at a similar depth. Additionally, phase D dehydrates into bridgmanite and stishovite at the ULM, producing insignificant velocity changes but a substantial density increase of ~14%. Therefore, the dehydration is probably too weak to generate discontinuities associated with velocity jumps. In contrast, it may account for seismic discontinuities sensitive to impedance changes, particularly density jumps, near the dehydration depth observed in some subduction zones.

This study investigates the effect of pressure on diffusion creep of dry San Carlos and synthetic (prepared by sol-gel method) olivine. We prepared dry (water content <9 ppm wt) fine-grained (<1 μm grain size) olivine and deformed the samples (both San Carlos and sol-gel olivine) in the same sample assembly under high pressure ( P = 2.9–8.8 GPa) and moderate temperatures ( T = 980–1250 K) at a fixed strain rate. The evolution of the sample’s strength was studied using radial X-ray diffraction from various diffraction planes. We found that San Carlos and sol-gel olivine show similar rheological behavior (when normalized to the same grain size). Stress estimated by the radial X-ray diffraction increases with time and initially shows similar values for all diffraction planes. In many cases, stress values start to depend on the diffraction planes in the later stage, and time dependence becomes minor. The microstructural observations show that grain size increases during an experiment. The results are interpreted using a theory of radial X-ray diffraction and the theoretical models of diffusion and dislocation creep. We conclude that the initial stage of deformation is by diffusion creep, but deformation in the later stage is by dislocation creep. For dislocation creep, our results are in reasonable agreement with previous low-temperature dislocation creep results after correcting the temperature effect. For diffusion creep, we obtain an activation volume of 7.0 ± 2.4 cm 3 /mol that is substantially smaller than the values reported on dislocation creep but agrees well with the results on grain growth. By comparing the present results on dry olivine with the previous results on wet (water-saturated) olivine, we found that water enhances diffusion creep but only modestly compared to dislocation creep. The difference in the pressure and water content dependence between diffusion and dislocation creep has an important influence on the dominant deformation mechanisms of olivine in the upper mantle.

Raman spectroscopy is a rapid, nondestructive analysis technique used in various scientific disciplines, including mineralogy, chemistry, materials science, and biology. The analysis of Raman spectra and the identification of specific substances in unknown samples can be complex and time-consuming due to the large database of Raman spectra. The Raman Match application was developed to simplify and automate the sample identification process through a search and match method. The application integrates the well-established RRUFF Raman database with the Python programming language. It provides a user-friendly graphical interface to load Raman spectra, identify and fit peaks, match peaks to the reference libraries, visualize the results, and generate publication-ready figures. The application offers a swift and automated method for mineral identification using Raman spectroscopy in laboratory and field settings and during planetary exploration missions to extraterrestrial environments with constraints on time and resources.

Raman spectroscopy is a powerful technique in the context of planetary exploration because it provides information on mineral identification, chemistry, and abundance. For Raman spectrometers with large spot sizes, multiple mineral phases can be investigated by collecting a single Raman spectrum. There is a lack of methodology for quantifying mineral species in mixtures due to the independent signal strengths of different materials in Raman spectra. Two techniques are presented in this work for quantifying common rock-forming minerals: partial least-squares multivariate analysis and a novel approach called Raman cross-section proxies (numerical metrics associated with specific Raman features). This paper targets 20 mineral species relevant to the mineralogy of the planet Mars. Mineral end-member samples and 187 binary mineral-mineral mixtures (mixture of two distinct minerals) are used for multivariate modeling. Eighteen diamond-mineral mixtures are used to derive Raman cross-section proxies. Mineral abundance proportions are predicted for the binary mineral-mineral mixtures with known mineralogical content to evaluate the efficacy of the two quantitative methods. Technique performance is mineral dependent. The root mean square error for unseen predictions (RMSE-P) using Raman cross-section proxies ranges from ±3.2–17.0 vol%. For the multivariate models, the cross-validated root mean square error (RMSE-CV) ranges from ±8.8 to 26.2 vol%. Although these error estimates are not directly comparable, they provide the most accurate error estimate currently available. Different scenarios may favor the use of one or the other of the two quantitative methods. This work provides fundamental groundwork that can be applied to common rock-forming minerals on terrestrial planets, including Mars. Quantification of mineral abundances aids in answering critical geologic questions related to ancient primary and altered rocks as well as planetary processes and conditions.

The compositions of cotectic olivine-spinel pairs in mafic magmas provide information on the oxygen fugacity of their host liquid, which can be accessed with a thermodynamic analysis of the olivinespinel-liquid peritectic reaction: 3Fe2SiO4oliv +O2sys =2Fe3O4sp +3SiO2melt  $$\begin{array}{} \displaystyle 3 \mathrm{Fe}_2 \mathrm{SiO}_4^{\text {oliv }}+\mathrm{O}_2^{\text {sys }}=2 \mathrm{Fe}_3 \mathrm{O}_4^{\text {sp }}+3 \mathrm{SiO}_2^{\text {melt }} \end{array}$$ The extraction of redox information from cotectic olivine-spinel pairs requires a well-defined silica activity value ( aSiO2melt  $\begin{array}{} \displaystyle a_{\mathrm{SiO}_2}^{\text {melt }} \end{array}$) for the melt of interest, as well as a method to calculate aFe2SiO4oliv  $\begin{array}{} \displaystyle a_{\mathrm{Fe}_2 \mathrm{SiO}_4}^{\text {oliv }} \end{array}$ and aFe3O4sp $\begin{array}{} \displaystyle a_{\mathrm{Fe}_3 \mathrm{O}_4}^{\mathrm{sp}} \end{array}$ from chemical analyses of olivine and spinel, respectively. In this work, we develop a new olivine-spinel- aSiO2melt  $\begin{array}{} \displaystyle a_{\mathrm{SiO}_2}^{\text {melt }} \end{array}$(OSaS) oxygen barometer that utilizes MELTS to obtain values of aSiO2melt  $\begin{array}{} \displaystyle a_{\mathrm{SiO}_2}^{\text {melt }} \end{array}$, which are used with values aFe2SiO4oliv  $\begin{array}{} \displaystyle a_{\mathrm{Fe}_2 \mathrm{SiO}_4}^{\text {oliv }} \end{array}$ and aFe3O4sp $\begin{array}{} \displaystyle a_{\mathrm{Fe}_3 \mathrm{O}_4}^{\mathrm{sp}} \end{array}$ determined from established solution models for olivine and spinel. We find that two implementations of the spinel-liquid peritectic equilibria can successfully generate magmatic oxygen fugacity values: (1) using a combination of mineral activity models from the literature (classical-OSaS) and aSiO2melt  $\begin{array}{} \displaystyle a_{\mathrm{SiO}_2}^{\text {melt }} \end{array}$ determined from MELTS, and (2) directly from chemical potentials obtained from MELTS, where a correction is added to the MELTS-derived chemical potentials for the magnetite component of the spinel (MELTS-OSaS). The two implementations of the OSaS were tested by using each model to recover the experimentally reported f O 2 values for a data set consisting of 50 olivine-spinel-glass assemblages derived from 14 published experimental studies. This data set was filtered to remove potential disequilibrium phase assemblages, experiments with failed redox buffers, and poor-quality EMP analyses. Data quality metrics for the data set filtration included Fe-Mg partitioning between olivine and melt, Fe-Mg partitioning between olivine-spinel, and an examination of whether aSiO2melt  $\begin{array}{} \displaystyle a_{\mathrm{SiO}_2}^{\text {melt }} \end{array}$ values were consistent with the reported phase assemblages. The classical-OSaS implementations reproduced the log f O 2 values reported from the experimental data set with a standard error estimate (SEE) of ±0.39, root mean standard error (RMSE) of ±0.40 and average deviation of ±0.31. In testing the MELTS-OSaS model, we identified that the solution model for magnetite underpredicted the values of aFe3O4sp $\begin{array}{} \displaystyle a_{\mathrm{Fe}_3 \mathrm{O}_4}^{\mathrm{sp}} \end{array}$; therefore, we used the 50 experiments to assign a correction to the MELTS-predicted chemical potentials of magnetite. We tested the MELTS-OSaS model with the magnetite correction on a data set of 18 additional buffered experiments, filtered for redox equilibrium and not included in the original experimental data set. We find that the MELTs-OSaS model, which includes the correction for the magnetite chemical potential, reproduces log f O 2 values for the 18 experiments with a SEE of ±0.20, RMSE of ±0.23, and average deviation of ±0.18. The OSaS oxybarometer can return magmatic log f O 2 values with a standard error of ±0.20 to 0.39 log units, depending on the model selected, provided that the olivine-spinel cotectic temperature is known to an accuracy of ±25 °C, the H 2 O content of the melt can be estimated within ±1.5 wt%, and that the crystallization pressure of the olivine-spinel pair is <500 MPa. We also propose that the OSaS models can be applied to experimental run products to determine or confirm oxygen fugacity values. We additionally suggest that the careful application of the OSaS oxybarometer can provide a reliable and robust alternative for performing redox studies on samples that do not contain sufficient glassy material to support the application of spectroscopic techniques (i.e., XANES and Mössbauer).

The depths of crustal reservoirs within volcanic systems may experience transitions over time. Here, we report the crystal and bulk rock compositions of the shield-forming basaltic lavas of the Tianchi composite volcano in the intraplate Changbaishan Volcanic Field, NE China, to constrain crustal magmatic evolution with time. We investigated samples covering the entire basaltic stratigraphic sequence, consisting of the Toudao (TD), Baishan (BS), and Laofangzixiaoshan (LFZ) units from bottom to top, respectively. The core compositions of olivine macrocrysts vary among the three units, i.e., the TD and BS olivine phenocrysts can be divided into two populations: a high-Fo population (~Fo 76–80 ) and a low-Fo population (~Fo 72–74 ). The LFZ unit only exhibits a high-Fo population (~Fo 77–80 ). Phase equilibria modeling using rhyolite-MELTS suggests that the high-Fo populations were stored at depths of ~20 km for the TD and BS units and ~15 km for the LFZ unit. The low-Fo populations crystallized at shallow depths, i.e., 15 km for the TD unit and 13 km for the BS unit. We employ dynamic Fe-Mg interdiffusion modeling with constantly adapting boundary conditions in zoned olivine macrocrysts to constrain the magmatic environments and timescales during the pre- and posteruption phases, enabling us to clarify the magmatic histories recorded by the two olivine populations. The dynamic Fe-Mg interdiffusion modeling considers the variable boundary conditions caused by crystal growth and composition variations of melts during magma cooling. Calculated results suggest that the high-Fo populations from the TD and BS units recorded prolonged timescales ranging from six months to more than two years with lower cooling rates and slower crystal growth rates. These characteristics reflect a relatively hot and slow-cooling magmatic environment, and the modeled timescales correspond to the sum of the time, including shallow storage, magma ascent, and further cooling within the lava flows. Conversely, the high-Fo population from the LFZ unit and the low-Fo populations from the TD and BS units record shorter timescales (<140 days) with higher cooling rates and faster crystal growth rates. These results indicate relatively cold and highly undercooled magmatic environments; hence, the timescales record magma ascent in the conduits and further cooling during lava emplacement. Our study demonstrates that the Tianchi basaltic plumbing system experienced a structural transition over time. In detail, the TD and BS magmas experienced multi-stage stalling and ascent, first accumulating in deep reservoirs and then ascending to shallow reservoirs for storage before the eruption. The LFZ magmas accumulated in a mid-crustal reservoir, followed by a direct ascent to the surface without additional residence.

In orogenic systems, pyrite hosts both free-native and lattice-bound gold. The processes governing gold habit, however, remain uncertain and subject to widespread debate. In this study, we employ micro-to-nanoscale trace element mapping alongside crystallographic characterization of gold-rich pyrite from the Kanowna Belle deposit (Western Australia) to probe the mechanisms influencing gold precipitation. Our examination reveals a complex chemical zoning in mineralized pyrite samples, characterized by an As-depleted core (As ≤2.5 wt%; Au ≤149 ppm) and rim (As ≤2.5 wt%; Au ≤264 ppm), separated by an As-Au-rich banded mantle (As ≤4.5 wt%; Au ≤2251 ppm). Pyrite structure in the mantle domain shows that low-angle boundaries mostly follow the shape of As-rich oscillatory zones. Gold in the pyrite mantle domain occurs as: (1) lattice-bound Au associated with As-rich oscillatory zoning; (2) gold micro-inclusions formed in equilibrium with arsenopyrite inclusions hosted within convoluted As-depleted pyrite domain; and (3) Au-filled late-stage fractures that crosscut the pyrite mantle. The shift in pyrite chemical composition from Au- and As-poor pyrite cores to Au- and As-rich mantles suggests that lattice-bound Au may have been integrated into As-rich zones via chemisorption during rapidly changing fluid conditions. Conversely, we propose that the gold inclusions hosted within the As-depleted convoluted domain form through a coupled dissolution re-precipitation process driven by fluid infiltration along low-angle boundaries. This study underscores the significance of linking arsenic distribution and crystallographic characteristics to comprehensively understand the controls on both gold form and distribution in gold deposits.

The replacement of chalcopyrite in NaCl solutions was investigated from 310 to 365 °C and 15 to 25 MPa with ex-situ surface characterization techniques and in-situ electrochemical methods. The replacement products and reactions were analyzed ex situ using Raman spectroscopy and a scanning electron microscope (SEM) with an energy-dispersive X-ray spectrometer (EDS). Chalcopyrite dissolution was monitored by open circuit potential (OCP), polarization curves, and electrochemical impedance spectroscopy (EIS). The ex-situ and in-situ measurements show that: (1) elevated temperature and pressure promote the oxidative dissolution of chalcopyrite by different pathways; (2) the altered chalcopyrite surface consists of an outer layer of hematite and an inner layer of covellite; (3) at elevated temperature, removal of more copper from chalcopyrite surface enhances chalcopyrite dissolution, and favors hematite formation over covellite in the alteration rind; and (4) at elevated pressure, more iron ions diffuse through the passive covellite layer by point defects, promoting chalcopyrite replacement by covellite over hematite. This study provides experimental evidence on the replacement of chalcopyrite by covellite, the precipitation of hematite, and the exchange of Fe/Cu ions between chalcopyrite and hydrothermal brine.

Critical rare metal deposits are strategic resources as these metals have numerous applications in hightech industries. Among the critical rare metals, natural stannum (Sn) is mainly found in the Sn-oxide mineral cassiterite (SnO 2 ) and is closely associated with granite or pegmatite. Carbonatite and alkaline rocks are more likely to contain vast amounts of critical rare metals, especially REEs and niobium (Nb). Here we report the presence of abundant cassiterite (SnO 2 ) and evaluate potential Sn mineralization in the Bayan Obo Fe-Nb-REE deposit in northern China, the largest REE deposit worldwide. This represents the first reported evidence of Sn enrichment in a carbonatite-hosted REE deposit. REE-Fe ores are dominantly mined in the Bayan Obo deposit. Disseminated, banded, and massive ores contain tens to hundreds of parts per million Sn, and vein-type ores are notably rich in Sn (up to 1500 ppm). In-situ micro-zonation mineralogical analyses identified two occurrences of cassiterite and several Sn-rich minerals in these REE-Fe ores. Abundant early-stage nanoscale cassiterite inclusions are present within magnetite grains in banded and massive REE-Fe ores, and ubiquitous late-stage granular cassiterite, Sn-rich rutile, titanite, and bafertisite are present in vein-type REE-Fe ores. Multiple U-Th-Pb dating of monazite and columbite-Mn in association with cassiterite yields peak ages of 425 Ma and 419 ± 18 Ma, respectively, revealing coeval Sn and Nb mineralization. We conclude that Sn was derived from carbonatitic magmas, and the dense distribution of cassiterite inclusions in magnetite marked the pre-enrichment of Sn in the Bayan Obo deposit. Subsequent Early Paleozoic hydrothermal events led to reactivation and further Sn mineralization. Similar to Nb, Sn was mineralized in the Bayan Obo deposit, possibly forming economically important resources. Our study highlights the potential of Sn mineralization associated with carbonatite-hosted REE deposits.

Ion-adsorption-type rare earth element (REE) deposits are the source of more than 90% of global heavy REEs (HREEs). Thus, understanding the ore genesis of REEs, particularly the distribution characteristics and enrichment mechanisms of HREEs, is vital for efficient exploration and mining of ion-adsorption-type REE deposits worldwide. The characteristics and petrogenesis of bedrock and the aqueous mobility of REEs are known to be important factors controlling REE accumulation and fractionation in the weathering crust of REE deposits. However, the effect of REE adsorption on secondary minerals, a crucial step in deposit formation, remains poorly understood. This problem was addressed by the study described herein, which involved a systematic analysis of the complete weathering profile (78 m) of the Renju ion-adsorption-type REE deposit in South China and a simulated adsorption experiment. Clay minerals and iron (Fe) (hydr)oxides are the dominant REE adsorbents in the weathering crust and most are micro-to-nanosized particles. Thus, the fine-particle fraction (<2 μm) was separated from field samples to disclose better their effects on the concentration and redistribution of REEs. Phase compositions and morphologies were characterized by X-ray diffraction, Mössbauer spectrometry, and scanning/transmission electron microscopy (SEM/TEM), which revealed that various clay minerals and Fe (hydr)oxides form composites along the profile of the deposit. Composites of ferrihydrite-kaolinite, goethite-kaolinite/halloysite, and hematite-kaolinite/halloysite were found to be distributed in the semi-weathered, completely weathered, and topsoil layers, respectively, with different sizes and shapes. The concentrations and partition patterns of REEs in different occurrence states were distinguished after sequential extraction. Ion-exchangeable-REEs were the major state and enriched in the upper completely weathered layer. These species were found to be adsorbed onto kaolinite and halloysite via electrostatic attraction without obvious fractionation. Fe (hydr) oxides were determined to comprise ca. 20% of REEs at most depths and over 50% of REEs in the topsoil and semi-weathered layer. It was found that Fe (hydr)oxides scavenge REEs through complexation and oxidation, resulting in HREE enrichment and a positive cerium (Ce) anomaly, respectively. In addition, compared with crystalline Fe (hydr)oxides, amorphous Fe (hydr)oxides immobilize more REEs but exhibit weaker preferential adsorption of HREEs. The above-described findings are consistent with the results of simulated experiments for REE adsorption onto a clay mineral-Fe (hydr)oxide composite (Bt-60d), which was obtained from hydrothermal processing of biotite. Furthermore, the distributions and stabilities of LREEs and HREEs were distinguished by TEM-energy-dispersive spectroscopy (EDS) of Bt-60d before and after REE extraction by ammonium sulfate. The phase transformation pathways of clay minerals and Fe (hydr)oxides and their different enrichment and fractionation characteristics in REEs were also discussed in terms of the structure and surface properties of minerals, adsorption mechanisms, and variations in chemical properties across the REE group. The results shed new light on how clay minerals and Fe (hydr)oxides affect the enrichment and fractionation of REEs in ion-adsorption-type deposits.

Arc magmas above subduction zones hold abundant fluid-mobile elements attributed to fluids released from the dehydrating subducted oceanic crust. However, the quantity of trace elements in the fluids and their evolution with the metamorphic processes during subduction and exhumation are still unclear. The precipitation sequence of vein minerals preserves the nature of multi-stage high-pressure (HP) metamorphic fluids and the fingerprint of mass exchange in deep subduction zones. In this contribution, we conducted detailed petrological studies and phase equilibria modeling on a unique HP omphacite-rich vein and its host eclogite from the Chinese southwestern Tianshan. The host eclogite consists mainly of garnet, omphacite, epidote, glaucophane, phengite, quartz, and rutile. Garnet in the eclogite records prograde subduction and early exhumation characterized by decompression heating at P-T conditions of ~2.4–2.6 GPa and 460–540 °C. The embedded omphacite-rich vein has similar mineral assemblage to the host eclogite. Garnet grains in this vein are predominantly distributed along or intersect the vein wall, which records similar eclogite-facies metamorphic conditions to the host eclogite. Omphacite is dominant in the vein, while epidote and glaucophane occur interstitially. Phase equilibria modeling reveals sequential growth of garnet-dominated, omphacite-dominated, and epidote-dominated assemblages from fluids originating from the breakdown of different hydrous minerals. These lines of evidence suggest that the formation of multi-stage HP fluids are a continuous long-term process with spontaneous short-distance transport and sequential mineral precipitation. Calculated fluid compositions demonstrate that the fluids released by lawsonite breakdown during exhumation have great potential to modify the trace element systematics of arc magmas. Our findings reveal the nature and evolution of multi-stage HP metamorphic fluids from internal sources during subduction and exhumation of oceanic crust, providing valuable insights into the chemical compositions of arc magmas.

High-calcic (~95% anorthite) plagioclase is the key mineral comprising the primary lunar crustal suites that cover over 60% of the Moon’s surface. Pristine crystals of similar high-calcic plagioclase are rare occurrences on Earth, which creates a roadblock to using terrestrial material as lunar crustal analogs. We discuss the potential of a particular megacrystic anorthite (An 95.51±0.31 ) occurring in the basaltic lava flows of the island arc volcano in Miyake-jima, Japan, as a material analog. A comprehensive analytical routine for the Miyake-jima anorthites has been performed to explore intra- and inter-crystalline heterogeneities in major, minor, and trace elements. These anorthites show flat concentration gradients across core profiles for all major elements (Si, Al, Ca, Na), minor elements (Mg, Fe), and most trace elements (La, Ce, Pm, Nd, Eu). Comparing the chemical composition of the samples with that of different lunar crustal suites like ferroan anorthosites, high-magnesium suites, and high-alkali suites shows that the Miyake-jima anorthites are overlapping or depleted in most minor and trace elements except for a slight enrichment in Li, Ti, Fe, Sr, Eu, Ba, and Pb. Given the low abundance of most trace elements in the Miyake-jima anorthites, we can treat this sample suite as a “blank slate,” which provides the opportunity to dope the crystalline matrix with the elements of interest at different levels and use them for geochemical, petrologic, and spectroscopic studies. The lack of typical magmatic zoning and overlapping elemental compositions across the different megacrysts make the Miyake-jima anorthites very well suited as a lunar crustal material analog. Highly calcic, crystalline anorthite is shown to have unique spectral signatures from less calcic anorthite, and intermediate and sodic compositions of plagioclase feldspar as calcium and iron contents control the wavelength position and shape of the diagnostic spectral features in the thermal infrared region of the electromagnetic spectrum. Thus, near- and thermal infrared spectral measurements of the Miyake-jima anorthites highlight the importance of developing chemically and mineralogically consistent terrestrial material analogs for remote sensing studies.

Over the past three decades, advances in crystal nucleation and growth have led to the understanding that crystallization proceeds through various pathways, ranging from the conventional atom-by-atom model to the particle aggregation- or amorphous transformation-based non-classical modes. Here, we present a novel mineralization mechanism exemplified by a lunar chromite formed via solid-liquid interface reactions through investigations of a lunar breccia returned by the Chang’e 5 mission. The chromite occurs in the middle of a whisker-shaped intergrowth structure made by olivine at the bottom and nanospheres of troilite and metallic iron at the top. Morphological observation and size statistics of the nanospheres, including those within the whisker structure and the others dispersed in glass, suggest the nanophases attached to olivine with coherent crystallographic orientations, possibly through an oriented aggregation process. The chromium deficiency in the olivine near the interface between olivine and chromite suggests that Cr in chromite originated from olivine, but the significantly reduced ferrous concentration in the glass surrounding chromite indicates the iron was derived from surrounding impact-induced glass. Based on laboratory observations and simulated calculations of energy and lattice mismatch, we propose that chromite crystallized at the interface between troilite and olivine in the impact melts, during which the nanospheres were lifted and transported away from olivine surface and formed a mushroom-shaped structure. This finding suggests that oriented attachment growth, chiefly confined to homogeneous systems thus far, can also occur in heterogeneous systems far from equilibrium, such as that produced by the impacts. It is conceivable that the studied crystallization pathway occurring on the heterogeneous interfaces may have been a common mineralization mode at highly nonequilibrium conditions.