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The term orogenic gold deposits has been widely accepted, but there has been continuing debate on their genesis. Early syn-sedimentary or syn-volcanic models and hydrothermal meteoric-fluid models are now invalid. Magmatic-hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships. The most plausible models involve metamorphic fluids, but the source of these fluids is equivocal. Intra-basin sources within deeper segments of the hosting supracrustal successions, the underlying continental crust, subducted oceanic lithosphere with its overlying sediment wedge, and metasomatized lithosphere are all potential sources. Several features of Precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic-fluid source. These include the presence of hypozonal deposits in amphibolite-facies domains, their anomalous multiple sulfur isotopic compositions, and problems of derivation of gold-related elements from devolatilization of dominant basalts in the sequences. The Phanerozoic deposits are largely described as hosted in greenschist-facies domains, consistent with supracrustal devolatilization models. A notable exception is the Jiaodong gold deposits of China, where ca. 120-Ma gold deposits are hosted in Precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization. Other deposits in China are comparable to those in the Massif Central and elsewhere in France, in that they are hosted in amphibolite-facies domains or clearly post-date regional metamorphic events imposed on hosting supracrustal sequences. If all orogenic gold deposits have a common genesis, the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package, or the associated metasomatized mantle wedge, with CO2 released during decarbonation and S- and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. Although this model satisfies all geological, geochronological, isotopic, and geochemical constraints, and is consistent with limited computer-based modeling of fluid release from subduction zones, the precise mechanisms of fluid flux are model-driven and remain uncertain. From an exploration viewpoint, the model re-emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction-related orogenic belts and importance of continental-scale lithosphere-tapping fault and shear zones to focus large volumes of auriferous fluid. It confirms the importance of the consistent spacing between world-class deposits, broadly equivalent to the depth of the Moho, as derived from empirical observations.

Oceanic lithosphere is generated at divergent plate boundaries and disappears at convergent plate boundaries. Seafloor spreading and plate subduction together constitute the physical coupling and mass conservation relationships to the movement of lithospheres on Earth. Subduction zones are a key site for the transfer of both matter and energy at converging plate boundaries, and their study has been the hot spot and frontier of Earth system science since the development of plate tectonics theory. As far as the dynamic regime and geothermal gradient of convergent plate margins are concerned, they have different properties in different stages of the subduction zone evolution. In general, the early low-angle subduction leads to compressional tectonism dominated by low geothermal gradients at the plate interface, and the late high-angle subduction results in extensional tectonism dominated by high geothermal gradients at the plate interface and its hanging wall. Active rifts are produced along suture zones through not only slab rollback or slab breakoff in the terminal stage of oceanic subduction but also foundering and thinning of the lithosphere in the post-subduction stage. Due to the differences and changes in the geometric and thermobaric structures of convergent plate margins, a series of changes in the type of metamorphism and magmatism can occur in active and fossil subduction zones. Dehydration and melting of the subducting oceanic crust are prominent at subarc depths, giving rise to fluids that dissolve different concentrations of fluid-mobile incompatible elements. The subduction zone fluids at subarc depths would chemically react with the overlying mantle wedge peridotite, generating metasomatites as the mantle sources of mafic magmas in oceanic and continental arcs. However, these metasomatites did not partially melt immediately upon the fluid metasomatism to trigger arc magmatism, and they did not melt until they were heated by asthenospheric convection due to rollback of the subducting slab. Therefore, recognition of the changes in the dynamic regime and geothermal gradient of subduction zones in different stages of plate convergence not only provides insights into geodynamic mechanisms of the tectonic evolution from subduction zones to orogenic belts, but also places constraints on the formation and evolution of different types of metamorphic and magmatic rocks within the advanced framework of plate tectonics.

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting.

Understanding the long-term evolution of Earth's plate–mantle system is reliant on absolute plate motion models in a mantle reference frame, but such models are both difficult to construct and controversial. We present a tectonic-rules-based optimization approach to construct a plate motion model in a mantle reference frame covering the last billion years and use it as a constraint for mantle flow models. Our plate motion model results in net lithospheric rotation consistently below 0.25∘ Myr−1, in agreement with mantle flow models, while trench motions are confined to a relatively narrow range of −2 to +2 cm yr−1 since 320 Ma, during Pangea stability and dispersal. In contrast, the period from 600 to 320 Ma, nicknamed the “zippy tricentenary” here, displays twice the trench motion scatter compared to more recent times, reflecting a predominance of short and highly mobile subduction zones. Our model supports an orthoversion evolution from Rodinia to Pangea with Pangea offset approximately 90∘ eastwards relative to Rodinia – this is the opposite sense of motion compared to a previous orthoversion hypothesis based on paleomagnetic data. In our coupled plate–mantle model a broad network of basal mantle ridges forms between 1000 and 600 Ma, reflecting widely distributed subduction zones. Between 600 and 500 Ma a short-lived degree-2 basal mantle structure forms in response to a band of subduction zones confined to low latitudes, generating extensive antipodal lower mantle upwellings centred at the poles. Subsequently, the northern basal structure migrates southward and evolves into a Pacific-centred upwelling, while the southern structure is dissected by subducting slabs, disintegrating into a network of ridges between 500 and 400 Ma. From 400 to 200 Ma, a stable Pacific-centred degree-1 convective planform emerges. It lacks an antipodal counterpart due to the closure of the Iapetus and Rheic oceans between Laurussia and Gondwana as well as due to coeval subduction between Baltica and Laurentia and around Siberia, populating the mantle with slabs until 320 Ma when Pangea is assembled. A basal degree-2 structure forms subsequent to Pangea breakup, after the influence of previously subducted slabs in the African hemisphere on the lowermost mantle structure has faded away. This succession of mantle states is distinct from previously proposed mantle convection models. We show that the history of plume-related volcanism is consistent with deep plumes associated with evolving basal mantle structures. This Solid Earth Evolution Model for the last 1000 million years (SEEM1000) forms the foundation for a multitude of spatio-temporal data analysis approaches.

Aqueous fluids produced by dehydration of the downgoing slab facilitate chemical exchange in subduction zones, but the efficiency of fluid-mediated redox transfer as a mechanism to deliver oxidized material from the slab to the sub-arc mantle remains hotly debated. Here we report the first direct measurements of the oxidation state of experimentally produced slab fluids using in situ redox sensors. Our experiments show that the dehydration of natural antigorite serpentinite at shallow subduction zone conditions (1 GPa, 800 °C) produces moderately oxidizing fluids (QFM + 2) with elevated concentrations of Na, K, Ca, and Mg. The composition and redox of the experimental fluids are then used to parameterize a thermodynamic reactive transport model to investigate the interaction of slab fluid with the sub-arc mantle from 1–4 GPa and 700–900 °C. Recently determined equation of state parameters for aqueous fluids at high pressures now enables thermodynamic modeling of aqueous fluid–rock interactions at conditions relevant to deep subduction zones for the first time. Our thermodynamic modeling demonstrates that aqueous fluid can efficiently oxidize Fe in mantle minerals via the reduction of H+ to H2 in the fluid. We estimate that < 1–3 kg of serpentinite-derived fluid at 850–900 °C is required to increase the Fe3+/ΣFe in 1 kg of sub-arc mantle from MORB-like values (0.15) to those of primitive arc basalts (0.2–0.3). We calculate that a slab fluid flux of 1.4 × 109–1.4 × 1014 kg year−1 is required to oxidize sufficient sub-arc mantle to produce the average annual flux of magmas at arcs, which overlaps with the estimated range of H2O flux in subduction zones.

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid–rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet–phengite quartzite. The B isotopic composition is variable (δ11B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid–rock interaction during peak metamorphic conditions. This interpretation is supported by fluid–rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ11B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ11B values (δ11B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ11B values that cluster around 0% (δ11B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid–rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.

We estimate depth‐dependent azimuthal anisotropy and shear wave velocity structure beneath the Alaska subduction zone by the inversion of a new Rayleigh wave dispersion dataset from 8 to 85 s period. We present a layered azimuthal anisotropy model from the forearc region offshore to the subduction zone onshore. In the forearc crust, we find a trench‐parallel pattern in the Semidi and Kodiak segments, while a trench‐oblique pattern is observed in the Shumagins segment. These fast directions agree well with the orientations of local faults. Within the subducted slab, a dichotomous pattern of anisotropy fast axes is observed along the trench, which is consistent with the orientation of fossil anisotropy generated at the mid‐ocean ridges of the Pacific‐Vancouver and Kula‐Pacific plates that is preserved during subduction. Beneath the subducted slab, a trench‐parallel pattern is observed near the trench, which may indicate the direction of mantle flow. Plain Language Summary The azimuthal anisotropy of seismic waves refers to the directional dependence of the seismic wave propagation speed. We present a comprehensive azimuthal anisotropy model of the Alaska subduction zone to a depth of 200 km, revealing anisotropy caused by local faults and fractures, fossil anisotropy inherited from the oceanic plate within the subducted slab, and sub‐slab mantle flow. The along‐strike variation of crustal anisotropy indicates variations in the stress regime in the forearc region. The along‐strike variation of anisotropy within the subducted slab identifies different origins of the subducted slab. Our model contributes to the understanding of the anisotropic structure and the sources of anisotropy in subduction zones. Key Points A new model of depth‐dependent azimuthal anisotropy of the Alaska subduction zone is built based on a new surface wave dataset The along‐strike variation in the azimuthal anisotropy of the forearc crust is caused by faults and fractures Azimuthal anisotropy within the subducted slab is controlled by fossil anisotropy produced at different mid‐ocean ridges

Porphyry copper±molybdenum±gold deposits (PCDs) are the most representative magmatic-hydrothermal metallogenic system above subduction zones with important economic value. Previous studies revealed that large PCDs are generally formed from initial arc magmas (from subduction-induced partial melting of the mantle wedge), which eventually ascend to the shallow crust (3–5 km) for mineralization after a series of complex evolution processes. These processes include (1) the dehydration or partial melting of subducting slab, which induces partial melting of the metasomatized mantle wedge; (2) the ascent of mantle-derived magma to the bottom of the lower crust, which subsequently undergoes crustal processes such as assimilation plus fractional crystallization (AFC) or melting, assimilation, storage and homogenization (MASH); (3) the magma chamber formation at the bottom of the lower, middle and upper crust; (4) the final emplacement and volatilization of porphyry stocks; and (5) the accumulation of ore-forming fluids and metal precipitation. Despite the many decades of research, many issues involving the PCD metallogenic mechanism still remain to resolve, such as (1) the tectonic control on the geochemical characteristics of ore-forming magma; (2) the reason for the different lifespans of the long-term magmatic arc evolution and geologically “instantaneous” mineralization processes; (3) the source of ore-forming materials; (4) the relative contributions of metal pre-enrichment to mineralization by the magma source and by magmatic evolution; and (5) the decoupling behaviors of Cu and Au during the pre-enrichment. These issues point out the direction for future PCD metallogenic research, and the resolution to them will deepen our understanding of the metallogenesis at convergent plate boundaries.

Vanadium is a multivalent element that may speciate as V2+; V3+; V4+ and V5+ in silicate and oxide phases. The relative abundance of V in planetary materials can be used as a proxy for oxygen fugacity (fO2) when its partitioning behavior has been calibrated with controlled laboratory experiments. Here we present the results of 20 piston-cylinder experiments executed over a 10-log unit range of fO2 at temperatures from 800 to 1230 °C, at 1.8–2 GPa, to quantify the partitioning of V between garnet, clinopyroxene, rutile and hydrous silicate melt under conditions relevant to eclogite melting in subduction zones. In all experiments, the partitioning of V between phases is controlled nearly equally by fO2 and by temperature (and/or compositional effects that are directly related to temperature). Vanadium is most compatible in experimental rutile, followed by clinopyroxene, then garnet. Calculated mineral/melt partition coefficients are ≥ 1 for all three phases in our experimental series. The high compatibility of V in eclogitic minerals results in negligible mass transfer of V during eclogite melting under all fO2 conditions investigated. Oxidized species of V are more soluble in rutile compared to garnet and clinopyroxene, leading to a linear increase in rutile/cpx and rutile/garnet inter-mineral partition coefficients as fO2 increases. We calibrate the partitioning of V among rutile-cpx and rutile-garnet pairs as an fO2 proxy for natural rocks and test its application to eclogitic xenoliths from the Koidu kimberlite suite (Sierra Leone). Our application yields spurious fO2 values for Koidu, indicating the V systematics of natural systems are likely much more complex than predicted by our experiments. Further work is needed to characterize the partitioning of V between eclogitic minerals over an extended range of mineral solid solutions, pressures, and temperatures before a V-oxybarometer may be applied to natural metamorphic systems with confidence.

Alaska's tectonic complexity makes it ideal for probing upper mantle deformation. We present a 3D shear‐wave anisotropy model obtained by inverting 7,985 SKS splitting intensity measurements from 261 broadband stations using 941 events from 2000 to 2023. Our multi‐scale model resolves lateral and vertical variations of shear‐wave azimuthal anisotropy. In the subduction zone, the fast axis in the sub‐slab mantle below the forearc and offshore regions changes with depth from trench‐normal to trench‐parallel, in agreement with predictions of geodynamic model for retreating slabs. In the asthenosphere below the backarc region, fast axes are mainly oriented northeast–southwest, which is consistent with southwestward mantle flow and extrusion tectonics, and suggests strong lithosphere–asthenosphere coupling. Anisotropy weakens below ∼200 km, indicating an increased vertical olivine alignment due to slab steepening. A two‐layer anisotropic structure in the backarc region suggests the presence of lithosphere fabrics unrelated to recent dynamics.