The ferric to total iron ratios (Fe3+/∑Fe) of garnets can be paired with thermodynamic mineral activity models to quantify the oxygen fugacity of garnet-bearing rocks. However, techniques with a high analytical and spatial resolution are necessary to distinguish differences in garnet Fe3+/∑Fe ratios at the percent level and to accurately measure garnets that are zoned or contain inclusions. We acquired conventional Fe Kα and high-resolution energy fluorescence detection (HERFD) Fe Kβ X-ray absorption near edge structure (XANES) spectra and electron microprobe flank method analyses on a suite of 27 peridotitic and eclogitic garnets with Fe3+/∑Fe ratios previously determined by Mössbauer spectroscopy to evaluate the precision of each technique. We examined variations in the energy and intensity of three XANES spectral features as a function of Fe3+/∑Fe ratios: 1) the intensity ratio of two-post edge features (I-ratio; Fe Kα only); 2) the energy of the Fe edge at 90% normalized intensity (E0.9; Fe Kα only) and 3) the pre-edge centroid energy (Fe Kα and HERFD Fe Kβ). In accordance with previous work, we find the energies of garnet pre-edge centroids are relatively insensitive to Fe3+/∑Fe ratios. The I-ratios of peridotitic and eclogitic garnets are offset from each other at low Fe3+/∑Fe ratios (≤0.13); I-ratio garnet XANES calibrations are composition-specific. The E0.9 feature is independent of garnet major element composition in spectra that have been corrected for the effects of self-absorption. We produce two Fe Kα garnet XANES calibrations based on variations in the E0.9 feature; one calibration with all garnet reference materials included (Fe3+/∑Fe up to 1.0; “all garnet calibration”) and another calibration specific to garnets with low Fe3+/∑Fe ratios (“low ferric calibration”). Fe3+/∑Fe ratios calculated from the mean of up to 25 flank method measurements on eight garnet reference materials fall within 4% absolute of a one-to-one correlation with Fe3+/∑Fe ratios measured by Mössbauer. The standard error of the mean Fe3+/∑Fe ratio calculated from flank method approaches the Mössbauer-determined Fe3+/∑Fe ratio within estimated error (3%) after three analyses. Flank method precision is enhanced at higher beam current; however, the precision of the flank method does not approach the precision of XANES under any microprobe analytical condition tested here. Garnet reference materials detailed here are available by request to the Smithsonian Institution.

Mineral chemistry records the pressure and temperature conditions of lithospheric processes. Active tectonic margins, however, are subjected to non-hydrostatic stresses wherein stress magnitudes vary directionally, and the impact of non-hydrostatic stress on mineral chemistry is uncertain. The work of materials scientists F. Larch & eacute; and J. Cahn provides a framework for quantifying how stress affects mineral chemistry. Crystallographically and mechanically anisotropic, multicomponent minerals will have different compositions as a function of their orientation under a fixed stress meaning that grain-to-grain compositional variation can be used to estimate stress. We develop two "orientation piezometry" methods that use the chemistry and orientations of multicomponent, anisotropic minerals to estimate stress. The first method uses chemistry and orientation ("coupled orientation piezometry") whereas the second method uses composition alone ("decoupled orientation piezometry"). We apply the methods to clinopyroxene and feldspar solid solutions using synthetic data sets. The first method determines the full stress tensor whereas the second method can only determine the differential stress magnitude unless additional a priori information is specified. Plausible scenarios for orientation piezometry include minerals undergoing diffusion creep, recrystallized grains formed during dislocation creep, and minerals grown statically under stress. Preliminary application of the decoupled piezometer to the famous eclogite facies shear zones on Holsn & oslash;y, Norway, suggests differential stresses in the range of 300-900 MPa, broadly consistent with previous estimates from the area. Thus, orientation piezometry techniques may provide valuable constraints on geodynamic processes and insights into long-standing geological problems such as the relationship between pressure and depth.

The movement of carbon in subduction zones plays a crucial role in regulating the global carbon cycle, controlling Earth's climate, and maintaining its habitability. Recent work suggests that only a fraction of the carbon released from subducting slabs at sub-arc depths is ultimately released from volcanic arcs, necessitating the existence of hidden carbon reservoirs within the slab-to-arc pathways. However, the precise location of these reservoirs remains enigmatic. Slab fluid serves as the primary medium for carbon transport in subduction zones; thus, a comprehensive understanding of fluid-rock interaction during slab fluid migration is essential for reconciling the carbon flux imbalance between the slab and the arc. In this study, we explore rock carbonation along a fluid conduit in the Southwestern Tianshan HP metamorphic belt in northwest China. Field evidence and petrologic observation reveal significant carbonation of a siliciclastic metasediment at its contact with a high-pressure garnet-bearing calcite (formerly aragonite) vein. We find that rock carbonation (by progressive Febearing magnesite, dolomite, then aragonite precipitation) occurred when slab-derived carbonic fluids migrated through the metasedimentary sequence at approximately 80 km depth. Furthermore, modeling demonstrates that the metasedimentary layer atop the slab has the capacity to sequester 20%-50% of the fluid carbon from the ascending slab devolatilization flux. We propose that the metasedimentary veneer at the plate interface functions as a "carbon filter", hindering the transfer of carbon from the slab to the arc and helping to reconcile the carbon flux imbalance between the amount released by the slab and that emitted by the arc. This study also provides insights into decarbonation efficiency and mechanisms, carbon-transfer pathways, and temporal aspects of the subduction zone carbon cycle.

Some of the largest magmatic-hydrothermal copper ore deposits and deposit clusters are associated with arc-oblique fault systems. Whether this structural context impacts the geochemistry of hydrothermal fluids, including their copper contents, remains unknown. Here, we investigate the copper concentration and helium isotope signature of geothermal fluids as modern analogs of hydrothermal ore deposits in the Andes of central-southern Chile. We show that fault systems broadly parallel to the regional stress field facilitate the early release of fluids from deep primitive magmas. By contrast, fault systems oblique to the regional stress field prevent the early escape of fluids and promote magmatic enrichment in copper, volatiles, and ligands, enhancing the potential to form copper deposits. We conclude that the orientation of fault systems actively influences the copper budget of ascending hydrothermal fluids, explaining the contrasting distribution of metals along distinct structures often observed in porphyry-epithermal systems and other types of magmatic-hydrothermal deposits. The enrichment of copper in magmatic hydrothermal fluids is influenced by the orientation of fault systems in arcs, according to geochemical analyses of gases and hot spring waters from active fumarole fields in the Southern Volcanic Zone of Chile.

Intracrystalline chemical diffusion offers valuable insights into the durations of metamorphic and igneous processes. However, it can yield timescale estimates for orogenic and subduction zone events that are considerably shorter than those obtained via isotopic geochronology. One potential explanation that has been offered for the discrepancy is that the interdiffusion of species with different atomic or ionic radii may generate intracrystalline, compositional stresses that alter or limit diffusional relaxation. In this study we test this idea by developing and applying the compositional stress theory of materials scientists F. Larche and J. Cahn to garnet from the Barrovian sillimanite zone, Scotland. Relaxed contacts from the garnet, independent diffusion chronometers, and thermal modeling all indicate a >100 kyr duration for peak temperature metamorphism. Nonetheless, the garnet records sharp, mu m-scale variations in calcium and iron contents that standard diffusion treatments predict should relax in 1-10 kyr at peak temperature conditions. Our results show that the development of compositional stress during diffusional relaxation can explain the preservation of the observed short wavelength compositional oscillations at a >100 kyr timescale. Thus, it may be necessary to account for compositional stress when modeling diffusion in solid solutions with appreciable differences in their endmember molar volumes. This will be particularly relevant when considering sharp, mu m-scale chemical gradients involving grossular, the garnet endmember with the largest molar volume relative to pyrope, almandine, and spessartine. Neglecting compositional stress in such cases could result in the underestimation of the timescales of lithospheric processes by potentially orders of magnitude. The effects of compositional stress in garnet are predicted to be the most pronounced under amphibolite and blueschist-eclogite facies conditions. At lower temperatures diffusion is limited, and at higher temperatures both plastic deformation and more ideal solid solution behavior will act to diminish the impact of stress.

The formation of copper-rich cumulates at the base of arc crusts has been proposed as a key process modulating the geochemical evolution of the continental crust and the genesis of giant ore deposits of copper. Despite the importance of these phenomena, the degree to which the lower crustal evolution of magmatic systems is influenced by open-system interaction and the assimilation of pre-existing crustal materials remains unclear. To tackle this issue, we provide direct isotopic constraints on the evolution of deep magmatic systems in arcs by measuring the osmium and oxygen isotope composition of hydrous copper-rich (similar to 730 mu g.g(-1) Cu) ultramafic cumulates formed at the base of the Acadian orogen (similar to 40 km deep) in the New England Appalachians (northeastern USA). The radiogenic Os-187/Os-188 initial ratios (ranging from 0.31 to 0.67) and the elevated delta O-18 values (8.97 +/- 0.42 parts per thousand for orthopyroxene; 9.25 +/- 0.26 for phlogopite) suggest a significant role of open-system magmatic differentiation, involving crustal assimilation, in the formation of these cumulates. Modeling of the Os-187/Os-188 and delta O-18 composition of the cumulates suggests that the observed isotopic compositions result from the initial evolution of parental magmas under sulfide-undersaturated conditions, followed by saturation after approximately 15% to 20% of assimilation and fractional crystallization progression. These results suggest that the assimilation of crustal material led to a drop in the magmatic system's f(O2) (similar to Delta FMQ < -1), triggering sulfide segregation and the formation of copper-rich cumulates. Our findings align with the hypothesis that magma-crust interactions can lead to the formation of lower crustal domains enriched with Cu, which may constitute a pre-stage in the formation of some porphyry copper deposits, particularly in collisional orogens.

Carbonate dissolution, the process by which carbonate minerals are stoichiometrically dissolved by an aqueous fluid, likely plays an important role in liberating carbon from subducting slabs and may thus make significant contributions to global carbon fluxes. It is therefore necessary to understand the fluid infiltration and flow geometries associated with this process. Despite its widespread use as a powerful tracer of fluid-rock interaction, strontium (Sr) has yet to be widely applied to carbonate dissolution scenarios. Previous work has identified a partially altered metacarbonate layer from Syros, Greece, that has undergone significant carbonate dissolution leading to ∼90% CO2 loss (Ague and Nicolescu, 2014). A bulk-rock compositional profile along this layer has a distinctive spike-shaped Sr profile. Sr concentrations are uniform in the altered region behind the reaction front, increase to form a spike just ahead of the front, and taper off to a baseline value moving farther into the unaltered metacarbonate rock. The origin of this type of spike shape and its potential utility for better understanding carbonate dissolution processes remain unaddressed. Here, we present a numerical diffusion-reaction model which demonstrates that both carbonate dissolution and multidimensional transport of Sr, in which the primary advective flow direction is perpendicular to the propagation of the diffusion front, are required to produce this shape. This model, as well as newly acquired Sr concentration data for calcite (former aragonite) and epidote, also indicate that the infiltration of a fluid with an elevated Sr concentration is necessary. We suggest that such spike-shaped features may aid in identifying carbonate dissolution in other settings, such as vein-selvage systems in metacarbonate rocks or reaction rinds on carbonate-bearing mélange blocks.

Large igneous province volcanism represents extensive mantle melting that has contributed to Earth’s chemical differentiation and lithospheric and climatic changes. Compositional heterogeneities in the mantle, such as accumulated recycled crust, may make key contributions to large igneous province activity. One class of rocks capable of producing distinctive mantle heterogeneities is the iron formations, uniquely dense Fe-rich sedimentary rocks formed in Earth’s early oceans. Although numerous iron formations were preserved on continents, with some becoming major Fe ore deposits, large amounts of iron formations may also have been recycled into the mantle, with uncertain consequences. Here we use statistical analysis of time series to show that from 3,200 to 1,000 Myr ago, most iron formation deposition ages are correlated with large igneous province activity 241 ± 15 Myr later, and that these events are coupled on long timescales. Linking observations from tectonics, geodynamics, mineral physics and seismology studies, we hypothesize that dense accumulations of subducted iron formations can form highly conductive Fe-rich zones in the lowermost mantle and facilitate the formation of thermal anomalies that produce mantle plume upwellings, and, ultimately, large igneous provinces. Although uncertainties remain regarding the precise nature of Archaean and Proterozoic convergent tectonics, facilitation of large igneous province activity by subducted iron formations would link Earth’s ocean chemistry to the pace of heat flow, crustal production and chemical differentiation.

Intracrystalline diffusion is an invaluable tool for estimating timescales of geological events. Diffusion is typically modeled using gradients in chemical potential. However, chemical potential is derived for uniform pressure and temperature conditions and therefore cannot be used to model diffusion when there are gradients in stress. Internal stress variations in minerals create gradients in strain energy which will drive diffusion. Consequently, it is necessary to have a method that incorporates stress variations into diffusion models. We derive a flux expression that allows diffusion to be modeled in ionic, crystalline solids under arbitrary stress states. Our derivation utilizes gradients in a thermodynamic potential called relative chemical potential which quantifies changes in free energy due to the exchanges of constituents on lattice sites under arbitrary stress conditions. We apply our derivation to the common quaternary garnet solid solution almandine-pyrope-grossular-spessartine. The rates and directions of divalent cation diffusion in response to stress are determined by endmember molar volume or lattice parameters, elastic moduli, and non-ideal activity interaction parameters. Our results predict that internal stress variations of one hundred MPa or more are required to shift garnet compositions by at least a few hundredths of a mole fraction. Mineral inclusions in garnet present a potential environment to test and apply our stress-driven diffusion approach, as stress variations ranging from hundreds of MPa to GPa-level are observed or predicted around such inclusions. The ability to model stress-induced diffusion may provide new information about the magnitudes of both intracrystalline stresses and the timescales during which they occurred, imparting a better understanding of large-scale tectono-metamorphic processes.

Data and computer code used to generate results for Keller et al. (2023) "Links between large igneous province volcanism and subducted iron formations." A readme file in the folder gives a description for each file. Data may also be accessed from the article link on the publisher's website.

Mineral compositions are used to infer pressures, temperatures, and timescales of geological processes. The thermodynamic techniques underlying these inferences assume a uniform, constant pressure. Nonetheless, convergent margins generate significant non‐hydrostatic (unequal) stresses, violating the uniform pressure assumption and creating uncertainty. Materials scientists F. Larché and J. Cahn derived an equation suitable for non‐hydrostatically stressed geologic environments that links stress and equilibrium composition in elastic, multi‐component crystals. However, previous works have shown that for binary solid solutions with ideal mixing behavior, hundreds of MPa to GPa‐level stresses are required to shift mineral compositions by a few hundredths of a mole fraction, limiting the equation's applicability. Here, we apply Larché and Cahn's equation to garnet, clinopyroxene, and plagioclase solid solutions, incorporating for the first time non‐ideal mixing behavior and more than two endmembers. We show that non‐ideal mixing increases predicted stress‐induced composition changes by up to an order of magnitude. Further, incorporating additional solid solution endmembers changes the predicted stress‐induced composition shifts of the other endmembers being considered. Finally, we demonstrate that Larché and Cahn's approach yields positive entropy production, a requirement for any real process to occur. Our findings reveal that stresses between tens and a few hundred MPa can shift mineral compositions by several hundredths of a mole fraction. Consequently, mineral compositions could plausibly be used to infer stress states. We suggest that stress‐composition effects could develop via intracrystalline diffusion in any high‐grade metamorphic setting, but are most likely in hot, dry, and strong rocks such as lower crustal granulites.

Relative alignments of mineral exsolutions and their host crystals can be described by crystallographic orientation relationships (COR). Exsolved phases in garnet from high‐grade metamorphic rocks and igneous rocks may have COR, but the complexity of COR distributions has thus far restricted their use to identifying exsolved phases. Classification of COR also remains mineral‐specific, leaving doubt as to what information COR preserve. To test how COR may be standardized, we calculated mismatch of low‐index crystallographic planes (d‐value ratios) and crystallographic directions (rows of atoms) between precipitates and garnet and defined search criteria for structural alignments likely to be energetically favourable. We analysed published electron backscatter diffraction (EBSD) data for apatite, rutile, ilmenite, corundum, and quartz precipitates in garnet (ntot = 1,296) for the presence of these alignments. Our method predicts between 88% and 98% of observed alignments across the studied minerals and requires only calculations using unit cell parameters. We further show that each exsolved mineral forms COR predicted by the edge‐to‐edge matching model which was developed to describe unambiguous exsolution textures in alloys. Edge‐to‐edge matching aligns atoms at the host–precipitate interface by parallelism or near‐parallelism of crystallographic planes of similar spacing and parallelism of crystallographic directions (rows of atoms) of similar length on the edges of those planes. Edge‐to‐edge matches likely facilitate coherent to semi‐coherent interfaces by lowering surface free energy and strain energy, stabilizing precipitates. These matches are defined by criteria applicable to all minerals, making them an ideal tool for classifying, discovering and interpreting COR of diverse precipitates in garnet. This approach may also predict COR in other geological mineral pairs (e.g., exsolved feldspars). We find that edge‐to‐edge matching may explain the stability of the needle‐shaped morphologies commonly observed for exsolution textures in garnet. Edge‐to‐edge matching COR distributions can be tested as proxies of the state of the host rock at the time of exsolution to evaluate factors such as temperature, cooling rate, degree of undercooling, and/or strain. Patterns of edge‐to‐edge matching COR may be paired with geothermobarometry and/or petrochronology to provide a powerful new tool for studying the histories of granulite and eclogite facies metamorphic and igneous rocks.

Understanding the behavior of chalcophile elements during the evolution of arc magmas is critical to refining models for the formation and distribution of porphyry copper deposits used in mineral exploration. Because magmas in continental arcs undergo copper depletion during their early differentiation, a widely held hypothesis posits that the removed copper is locked at the base of the crust in copper-rich cumulates that form due to early sulfide saturation. Testing this hypothesis requires direct evidence for such copper-rich reservoirs and a comprehensive understanding of the mechanisms driving sulfide saturation. Interaction between oxidized magmas and reducing crustal material in island arcs has been shown to be an efficient process causing sulfide saturation. However, the extent to which crustal assimilation impacts the flux of chalcophile elements during magmatism in thick continental arcs remains to be established. Here, we provide a deep perspective into these problems by studying a suite of subarc cumulate rocks from the Acadian orogen, New England (USA). These cumulates record the imprint of subduction zone magmatism and represent the residues left behind during the genesis of intermediate to evolved Acadian magmas (ca. 410 Ma). We find that the most primitive Acadian cumulates are enriched in copper (up to ~730 µg g–1) hosted by sulfide phases, providing direct evidence for the formation of lower crustal copper-rich reservoirs. The Acadian cumulates reveal a wide range of δ34S values, from –4.9‰ in the ultramafic rocks to 8‰ in the most evolved mafic rocks. The negative δ34S values observed in the most primitive and copper-rich cumulates (avg –3‰) reflect the assimilation of isotopically light sulfur from surrounding sulfidic and graphite-bearing metasedimentary rocks (δ34S of –19 to –12‰), whereas the more evolved cumulates with positive δ34S signatures may have formed from different magma batches that experienced less sediment assimilation. The assimilation of these reducing metasedimentary rocks caused a critical drop in oxygen fugacity (~DFMQ –2.5 to –1.9; FMQ = fayalite-quartz-magnetite buffer) in the evolving magmas, ultimately leading to extensive sulfide saturation and the consequent formation of copper-rich subarc cumulates. Assimilation-driven sulfide saturation may be a common process at the root of thickened arc crusts that triggers the formation of lower crustal copper-rich reservoirs, which play a pivotal role in the fate of copper during arc magmatism. Thus, deeply buried reducing metasedimentary crustal material at the base of continental arcs can act as a barrier to the magmatic flux of chalcophile elements and may play a crucial role in the genesis and distribution of porphyry copper deposits.