Holsn & oslash;y, Norway, offers a world-class natural laboratory for studying the impact of fluid on subducting lower crust. Holsn & oslash;y is composed of dry, metastable lower crustal granulite that was infiltrated by fluids along shear zones and seismic fractures during subduction. The infiltration facilitated the localized growth of eclogite facies mineral assemblages along the fluid flow pathways. The duration of the eclogite facies metamorphism, however, remains uncertain. Previous garnet diffusion chronometry studies have estimated timescales ranging from hundreds of years to millions of years based on diffusional relaxation between metastable granulite facies garnet cores and eclogite facies garnet rims and fractures. The shorter timescales are inferred from extremely sharp Ca gradients across chemical contacts present in some garnets whereas the longer timescales are from wider Mg and Fe profiles present in all garnets. The different timescale estimates have led to divergent models for the region's tectonometamorphic evolution. Here we show that the sharp Ca contacts can be explained by diffusion-induced compositional stress. As Ca is significantly larger than Mg and Fe, its movement strains the crystal lattice and generates stress that limits the relaxation of sharp chemical contacts. When compositional stress is accounted for, the sharp contacts yield timescales that are consistent with the wider Mg and Fe diffusion profiles. We determine that eclogite facies conditions (670-700 degrees C, 1.5-2.2 GPa) lasted a maximum of c. 300 kyr. The relatively short duration of eclogite facies conditions requires that multiple transient heating events were superimposed on a longer (>10(6) yr) overall timescale of metamorphism. Granulite facies garnet cores are surrounded by multiple generations of eclogite facies rims formed by interface-coupled dissolution-reprecipitation (ICDR) reactions. The garnet rims indicate two rapid, regional-scale fluid pulses and additional smaller, more localized pulses. The fluid pulses may be linked to episodes of seismic moment release as well as transient heating via exothermic hydration reactions and/or shear deformation. Our model results predict up to 400 MPa of differential stress at the garnet core-rim contacts, consistent with observed eclogite facies microfractures that extend into relic granulite facies garnet cores. The microfractures indicate that ICDR was aided by compositional stress: diffusion ahead of the reaction front generated stress and fracturing that created porosity for further ICDR. Thus, compositional stress can markedly impact both diffusion systematics and intracrystalline deformation. Together, these results show that despite their brevity, transient thermal, fluid flux, and/or baric episodes may exert the primary controls on the mineralogical and rheological development of subducted lithologies, in contrast to the long, slow burial and exhumation typically envisioned for regional metamorphism.

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.

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.

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.

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.

When hydrated, phosphides such as the mineral schreibersite, (Fe,Ni) 3 P, allow for the synthesis of important phosphorus-bearing organic compounds. Such phosphides are common accessory minerals in meteorites; consequently, meteorites are proposed to be a main source of prebiotic reactive phosphorus on early Earth. Here, we propose an alternative source for widespread phosphorus reduction, arguing that lightning strikes on early Earth potentially formed 10–1000 kg of phosphide and 100–10,000 kg of phosphite and hypophosphite annually. Therefore, lightning could have been a significant source of prebiotic, reactive phosphorus which would have been concentrated on landmasses in tropical regions. Lightning strikes could likewise provide a continual source of prebiotic reactive phosphorus independent of meteorite flux on other Earth-like planets, potentially facilitating the emergence of terrestrial life indefinitely.

Thermodynamic modeling in active tectonic settings typically makes the assumption that stress is equal in all directions. This allows for the application of classical equilibrium thermodynamics. In contrast, geodynamic modeling indicates that differential, or non-hydrostatic, stresses are widespread. Non-hydrostatic equilibrium thermodynamics have been developed by past workers [1], but their application to geological systems has generated controversy in recent years [2-5]. Therefore, we seek to clarify how stress influences the chemical potential of non-hydrostatically stressed elastic solids. To quantify this, we consider the effects of stress variation on the equilibrium between the single-component polymorph pairs of kyanite/sillimanite, quartz/coesite, calcite/aragonite, and diamond/graphite. The stress on each interface of a solid can be decomposed into components normal to the interface and parallel to the interface. In our work, we determine the shift in the temperature of equilibrium on fixed interfaces between polymorph pairs as a function of both interface-normal and interface-parallel stress variation. We find that the influence of normal stress variation on the equilibrium temperature of polymorphs is approximately two orders of magnitude greater than interface-parallel stress variation. Thus, at a fixed temperature, normal stress determines the chemical potential of a given interface to first order. Consequently, high-pressure polymorphs will preferentially form normal to the maximum stress, while low-pressure polymorphs, normal to the minimum stress. Nonetheless, interface-parallel stress variations can meaningfully affect the stability of phases that are at or near equilibrium. We demonstrate the surprising result that for a given polymorph pair, a decrease in interface-parallel stresses can make a high-pressure polymorph more stable relative to a low-pressure polymorph on the given interface. The effects of non-hydrostatic stress on mineral assemblages are most likely to be seen in dry systems. Many reactions in metamorphic systems are fluid-mediated, and fluids cannot sustain non-hydrostatic stress. Consequently, in systems with interconnected, fluid-filled porosity, mineral assemblages will tend to form at a pressure approximately equal to the fluid pressure. In contrast, in dry systems all reactions occur directly between solids which can sustain non-hydrostatic stress. This facilitates the application of non-hydrostatic thermodynamics. Consequently, dry rocks containing polymorphs such as such as quartzites, marbles, and peridotites represent ideal lithologies for the testing and application of these concepts. By influencing the chemical potential of solid interfaces, non-hydrostatic stress alters the thermodynamic driving force and subsequent kinetics of polymorphic reactions. This likely results in preferential orientations of polymorphs which could influence seismic anisotropy and potentially generate seismicity. [1] Larché, F., & Cahn, J. W. (1985). Acta Metallurgica, 33(3), 331-357. https://doi.org/10.1016/0001-6160(85)90077-X [2] Hobbs, B. E., & Ord, A. (2016). Earth-Science Reviews, 163, 190-233. https://doi.org/10.1016/j.earscirev.2016.08.013 [3] Powell, R., Evans, K. A., Green, E. C. R., & White, R. W. (2018). Journal of Metamorphic Petrology, 36(4), 419-438. https://doi.org/10.1111/jmg.12298 [4] Tajčmanová, L., Podladchikov, Y., Powell, R., Moulas, E., Vrijmoed, J. C., & Connolly, J. A. D. (2014). Journal of Metamorphic Petrology, 32(2), 195-207. https://doi.org/10.1111/jmg.12066 [5] Wheeler, J. (2018). Journal of Metamorphic Petrology, 36(4), 439-461. https://doi.org/10.1111/jmg.12299

Gibbs free energy, the fundamental thermodynamic potential used to calculate equilibrium mineral assemblages in geological systems, does not apply to non‐hydrostatically stressed solids. Consequently, there is debate over the significance of non‐hydrostatic stress in petrological and geophysical processes. To help resolve this debate, we consider the effects of non‐hydrostatic stress on the polymorph pairs kyanite/sillimanite, graphite/diamond, calcite/aragonite, and quartz/coesite. While these polymorphs are most relevant to metamorphic processes, the concepts developed are applicable to any single‐component solid reaction. We quantitatively show how stress variations normal to an interface alter equilibrium temperatures of polymorph pairs by approximately two orders of magnitude more than stress variations parallel to an interface. Thus, normal stress controls polymorph stability to first order. High‐pressure polymorphs are expected to preferentially nucleate normal to and grow parallel to the maximum stress and low‐pressure polymorphs, the minimum stress. Nonetheless, stress variations parallel to an interface allow for the surprising possibility that a high‐pressure polymorph can become more stable relative to a low‐pressure polymorph as stress decreases. The effects of non‐hydrostatic stress on mineral equilibrium are unlikely to be observed in systems with interconnected, fluid‐filled porosity, as fluid‐mediated reactions yield mineral assemblages at approximately constant pressures. In dry systems, however, reactions can occur directly between elastic solids, facilitating the direct application of non‐hydrostatic thermodynamics. Non‐hydrostatic stress is likely to be important to the evolution of metamorphic systems, as preferential orientations of polymorphic reactions can generate seismicity and may influence fundamental rock properties such as porosity and seismic anisotropy.