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Exhumed high‐pressure/low‐temperature (HP/LT) metamorphic rocks provide insights into deep (∼20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist‐to‐eclogite facies oceanic‐ and continental‐affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P‐T‐D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn‐subduction return flow along similar P‐T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top‐to‐the‐S‐SW shear (prograde‐to‐peak subduction), top‐to‐the‐NE shear (blueschist facies exhumation), and then E‐W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi‐mineral Rb‐Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ∼50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ∼50–40 and ∼43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene‐Oligocene subduction channel proceeded at rates of ∼1.5–5 mm/yr and accommodated ∼80% of the total exhumation of this HP/LT complex. Key Points: Syros is a tectonic stack composed of three slices constructed by subduction and underplating; peak subduction ages young with structural depthThe subduction‐to‐exhumation transition is marked by kinematic rotation and cooling during decompressionMetamorphic geochronology indicates syn‐subduction exhumation occurred continuously in an Eocene‐Oligocene subduction channel [ABSTRACT FROM AUTHOR]

Silicate weathering is thought to increase and offset the rapid, massive input of CO2 into the atmosphere and ocean during the Paleocene‐Eocene Thermal Maximum (PETM), but few nonmarine records have been used to quantify this. We probe changes in silicate weathering intensity by measuring Li isotope ratios of bedrock and ancient floodplain deposits spanning the PETM in the Bighorn Basin, Wyoming (USA). Our results reveal a rapid increase in silicate weathering intensity during the PETM that remained high during at least the initial stage of climate recovery. Additionally, we determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils. Alongside increased temperatures and pCO2, the simplest explanation for this response relates to increased seasonal fluctuations in water table height in the floodplain that promote dissolution and precipitation reactions. These findings newly demonstrate that weathering on floodplains, like mountain hillslopes, responds to climate change. Plain Language Summary: The chemical breakdown of silicate minerals on continents promotes the withdrawal of CO2 from the ocean and atmosphere. This process is thought to be enhanced when CO2 rapidly enters the ocean and atmosphere, such as during past climate change events like the Paleocene‐Eocene Thermal Maximum (PETM). Although this enhancement of silicate weathering is found in global carbon cycle models and the chemistry of marine rocks, less terrestrial evidence exists for how, where, and to what extent silicate minerals weathered during the PETM. In this study, we measured the chemistry of bedrock and ancient floodplain sediment that span the PETM in the well‐studied Bighorn Basin, Wyoming (USA) to quantify changes in silicate weathering intensity. We find that silicate weathering intensity increases rapidly (within 7,000 years of the onset of the PETM) and remains elevated even as climate has begun to return to its pre‐perturbed state. We also determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils, suggesting active weathering responses on floodplains. Alongside high CO2 contents and warmer temperatures, the simplest explanation for this weathering response relates to soil hydrology, where increased water flow through soils caused by fluctuating water tables enhanced weathering. Key Points: A terrestrial silicate weathering response quantified during the Paleocene‐Eocene Thermal Maximum (PETM)Weathering intensity increases rapidly after PETM, with stronger responses found in floodplain soils farthest from ancient channelWeathering response in floodplain linked to hydroclimate and soil hydrology [ABSTRACT FROM AUTHOR]

Retrograde metamorphic rocks provide key insights into the pressure–temperature (P – T) evolution of exhumed material, and resultant P – T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P – T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P – T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4 –1.7 GPa between 500–550 ∘C , retrograde epidote growth at ∼1.3 –1.5 GPa between 400–500 ∘C , and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P – T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P – T -deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes. [ABSTRACT FROM AUTHOR]

Chlorine, lithium, and boron are trace elements in rhyolite but are enriched in groundwater flowing through rhyolite because they tend to partition into the fluid phase during high‐temperature fluid‐rock reactions. We present a large data set of major element and δ37Cl, δ7Li, and δ11B compositions of thermal water and rhyolite from Yellowstone Plateau Volcanic Field (YPVF). The Cl/B, Cl/Li, δ37Cl (−0.2‰ to +0.7‰), and δ11B (−6.2‰ to −5.9‰) values of alkaline‐chloride thermal waters reflect high‐temperature leaching of chlorine, lithium, and boron from rhyolite that has δ37Cl and δ11B values of +0.1‰ to +0.9‰ and −6.3‰ to −6.2‰, respectively. Chlorine and boron are not reactive, but lithium incorporation into hydrothermal alteration minerals result​s in a large range of Cl/Li, B/Li, and δ7Li (−1.2‰ to +3.8‰) values in thermal waters. The relatively large range in δ7Li values of thermal waters reflects a large range of values in rhyolite. Large volumes of rhyolite must be leached to account for the chloride, lithium and boron fluxes, implying deep groundwater flow through rhyolite flows and tuffs representing Yellowstone's three eruptive cycles (∼2.1 Ma). Lower Cl/B values in acid‐sulfate waters result from preferential partitioning of boron into the vapor phase and enrichment in the near‐surface water condensate. The Cl/B, Cl/Li, δ7Li (−0.3‰ to +2.1‰), and δ11B (−8.0‰ to −8.1‰) values of travertine depositing calcium‐carbonate thermal waters which discharge in the northern and southern YPVF suggest that chlorine, lithium, and boron are derived from Mesozoic siliciclastic sediments which contain detrital material from the underlying metamorphic basement. Plain Language Summary: High concentrations of chlorine, lithium, and boron are typically measured in groundwater that chemically react at elevated temperatures with rocks such as rhyolite that have high silica (SiO2) concentrations. The elevated concentrations result from the partitioning of these elements into groundwater during chemical reactions. Understanding how the stable isotopes of chlorine, lithium, and boron partition between groundwater and rocks in continental hydrothermal systems can provide guidance for geothermal energy and mineral deposit exploration and for characterizing degassing in active magmatic systems. We present a large data set of major element and stable isotope compositions of thermal waters and rhyolite from the Yellowstone hydrothermal system. Results suggest that the ratios of chlorine to boron and chlorine to lithium and the stable isotope compositions of these three elements mostly reflect the compositions of the rocks with which the hot water reacts. Some lithium is subsequently incorporated into hydrothermal alteration minerals and boron is enriched in vapor when the groundwater boils. Key Points: Cl, Li, and B in alkaline‐chloride thermal water in the Yellowstone Caldera are derived from leaching large volumes of rhyoliteCl and B are not incorporated into hydrothermal alteration minerals, but small amounts of Li are transferred from thermal water to rockIn calcium‐carbonate thermal waters which deposit travertine, Cl, Li, and B are probably derived from Mesozoic siliciclastic sediments [ABSTRACT FROM AUTHOR]

Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca10(PO4)6(F,Cl,OH)2 solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for 37Cl/35Cl homogeneity using SIMS followed by δ37Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10‰ (1s) for the five samples with > 0.5 % m/m Cl and ± 0.19‰ (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38‰ between the mean 37Cl/35Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ37ClSMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 103δ37ClSMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25. [ABSTRACT FROM AUTHOR]

We combine elastic thermobarometry with oxygen isotope thermometry to quantify the pressure-temperature (P-T) evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ~1.4-1.7 GPa between 500-1550 °C, retrograde epidote growth at ~1.3-1.5 GPa between 400-500 °C, and a second stage of retrograde epidote growth at ~1.0 GPa and 400 °C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist-eclogite facies and subsequent retrogression under blueschist-greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure/low-temperature conditions. These P-T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P-T-deformation conditions of retrograde mineral growth in HP/LT metamorphic terranes. [ABSTRACT FROM AUTHOR]

We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47-3.25 wt%), to amphibole (563-4727 µg/g), phengite (610-1822 µg/g), omphacite (6.5-54.1 µg/g), and garnet (1.7-8.9 µg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192-515 µg/g), followed by amphibole (0.64-82.7 µg/g), phengite (1.2-2.1 µg/g), omphacite (<0.05-1.0 µg/g), and garnet (<0.05 µg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145-258 µg/g F and at least 7-11 µg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle. [ABSTRACT FROM AUTHOR]

The Kletno deposit in the Śnieżnik Massif (Central Sudetes, SW Poland), mined for Fe, U, Ag, Cu, fluorite, and marble through the ages, developed at the contact of marbles and orthogneiss. Here, we present a new Fe-Ti-V-ore (containing up to 14.07 wt. % Fe, 2.05 wt. % Ti, and 2055 ppm V in bulk rock) and ornamental- to gem-quality talc prospect at the southwest margin of this deposit. This newly documented Fe-Ti-V mineralization is hosted in hornblendites, dolomite veins, and chlorite schists, which, along with talc, envelopes a tectonic slice of serpentinite. Hornblendites are interpreted as metamorphosed ferrogabbros, derived from the same mafic melts as adjacent barren metagabbros. The oxygen and carbon isotope compositions of metabasites and dolomite veins (amphibole δ18O values = 8.8-9.3‰; carbonate δ18O values = 12.8-16.0‰, and δ13C values = -8.3‰ to -7.2‰), in combination with those of the country marbles (carbonate δ18O and δ13C values = 23.2‰ and +0.1‰, respectively), suggest that mineralization-bearing hornblendites formed due to interaction of the mafic magma with CO2 released during the decarbonation of the sediments. The CO2-bearing fluid interaction with gabbros likely caused carbonation of the gabbros and formation of the dolomite veins, whereas talc formed due to Si-rich fluids, possibly derived from a mafic intrusion, interaction with serpentinite, or due to the metasomatism of the serpentinite-gabbro assemblage. Moreover, fluids leaching Fe and Ti from the adjacent sediments can mix with the mafic magma causing enrichment of the magma in Fe and Ti. Consequently, the mineralization-bearing ferrogabbros became even more enriched in Fe and Ti, which can be linked with the formation of Fe-Ti-V ore bodies. [ABSTRACT FROM AUTHOR]

Oxygen isotope analyses of skarn minerals have long been used to fingerprint the variable fluid sources involved in skarn formation. The Empire Mountain skarn of the Sierra Nevada (California, USA) batholith is identified as a low‐δ18O skarn and is thought to form due to surface fluid involvement that was enhanced by fractures that formed during host rock brecciation. Although geochemically well characterized, the Empire Mountain skarn is less understood in terms of its hydrodynamic history. In this study, we develop a two‐dimensional model of oxygen isotope transport during high‐temperature fluid‐rock interactions to assess the mechanisms by which low‐δ18O garnets could form exclusive of brecciation. Highlighting regions nearest to the intrusion that could form garnet, we make three primary observations: (1) the oxygen isotope composition of the fluids, and not temperature, dominantly controls δ18Ogarnet values; (2) >6‰ increases in δ18Ogarnet, from negative to positive values, are observed over the maximum time frame of garnet thermodynamic stability; and (3) incremental emplacement of the intrusion can produce oscillations in δ18Ogarnet values. Without invoking brecciation, we find that low‐δ18O garnets can form without the presence of surface fluids; they instead source 18O‐depleted pore fluids from adjacent units. Further, surface fluids that do not equilibrate with the surrounding rock at depth become low‐δ18O fluid sources during later stages of pluton emplacement. This study underscores that pore fluids at depth, regardless of their equilibrium state, can act as dormant low‐δ18O fluid sources and may be responsible for low‐δ18O‐valued garnets. Plain Language Summary: An ancient hydrothermal system in the Sierra Nevada of California—Empire Mountain—is composed of garnets whose chemistry suggests that water from the surface of the Earth flowed several kilometers into the crust during the beginning stages of hydrothermal activity. The chemistry of the garnets also show that sources of fluid abruptly changed as the hydrothermal activity proceeded. Previous studies have attributed the presence of surface water and the abrupt changes in fluid source to the formation of fracture networks in the rocks that surrounded the causative magma body. However, we seek alternative explanations for these two observations using a numerical model. Our model simulates hydrothermal fluid flow and heat transport at Empire Mountain to assess changes in garnet chemistry during hydrothermal activity. Our results show that large fluctuations in garnet chemistry can occur without the formation of fractures. Additionally, we find that the chemistry of pore waters at depth and surface waters can be similar; instead of forming in the presence of surface water, garnets may form in the presence of chemically similar pore waters. Our findings highlight that the chemistry of garnets that form during hydrothermal activity may not be faithful recorders of distinct fluid sources during their growth. Key Points: Pore fluids at depth can have low oxygen isotope ratios, regardless of equilibrium stateSkarn garnets with low oxygen isotope ratios may form in the presence of these pore fluidsLarge variation in garnet oxygen isotope ratios may be due to influx of spatially heterogeneous pore fluids at depth [ABSTRACT FROM AUTHOR]

The Picacho Mountains (SE Arizona, USA) are composed of a variety of Paleogene, Late Cretaceous, and Proterozoic granite and gneisses that were deformed and exhumed along the gently south to southwest dipping detachment shear zone associated with the Picacho metamorphic core complex. The detachment shear zone is divided into three sections that record a progressive deformation gradient, from protomylonites to ultramylonites, and breccia. New thermochronological data from mylonite across the footwall of the detachment shear zone associated with the Picacho metamorphic core complex suggest that the footwall was exhumed through about ~300°C between 22 and 18 Ma by progressive incisement of the footwall of the detachment shear zone. Combined geochronological and oxygen and hydrogen stable isotope data of metamorphic silicate minerals reveal that mylonite recrystallization occurred in the presence of a deep‐seated metamorphic/magmatic fluid, and experienced a late stage meteoric overprint during the development and exhumation of the detachment shear zone. Quartz‐biotite and quartz‐hornblende geothermometry from the base to the top of the detachment shear zone yield equilibrium temperatures ranging from 630 to 415°C, respectively. This temperature trend is attributed to an insulating effect caused by rapid slip and juxtaposition of cool hanging wall on top of a hot footwall. It is suggested that rapid cooling of the top of the detachment shear zone caused strain to migrate toward lower structural level by incisement of the footwall of the shear zone. Progressive strain front migration into the ductile footwall produced hydromechanical anisotropies parallel to the detachment shear zone, effectively saturating the footwall with magmatic/metamorphic fluids and preventing downward flow of meteoric fluids. The combined microstructural, geochronological, and stable isotope results presented in this study provide insight on the dynamic feedback between deformation and fluid flow during the evolution of a detachment shear zone. Key Points: Thermochronological data suggest that the PMDSZ was exhumed through about 300°C between 22 and 18 Ma by progressive incisementO&H stable isotope data of metamorphic minerals reveal that the mylonite recrystallized in the presence of a metamorphic/magmatic fluidProgressive incisement caused by deformation front migration, leading to the development of hydromechanical anisotropies [ABSTRACT FROM AUTHOR]

Pervasive interaction between rock and fluid produces regional-scale alteration and/or metasomatism of rock units. The middle and upper crustal rocks from the Bamble sector (SE Norway) show a complex history of metamorphic and metasomatic events, accompanied by ore deposit formation, which provides an excellent context for investigation of regional-scale fluid-rock interaction. Halogens are important agents of rock alteration and ore metal transportation in such hydrothermal regimes. Here we use F, Cl, Br and I concentrations and stable chlorine isotope ratios (δ37Cl) to trace the metasomatic evolution of two gabbroic bodies and understand the interplay between localized and pervasive fluid flow in the upper crust. An alteration sequence was sampled from pristine gabbro to an amphibolitic shear zone, showing progressive amphibolitization and scapolitization caused by pervasive fluid ingress. Halogen concentrations and ratios suggest the evolution of a single fluid, causing enrichment of Cl, Br and I in samples nearest the shear zone. Owing to the differences in fluid-mineral distribution coefficients, fluid-mineral interaction resulted in either enrichment or depletion of halogens in the fluid as it reacted pervasively with the gabbro. Although scapolite formation leads to progressive desalination of the fluid near the shear zone, increasing Cl concentrations in amphibole near the gabbro reflect an increase in salinity towards the unaltered gabbro owing to desiccation of the fluid. The formation of alteration minerals mainly governs changes in the halogen chemistry of the fluid, showing that halogens do not behave fully conservatively during fluid-rock interaction. Nevertheless, the evolving fluid has a feedback on the stability of alteration minerals such as scapolite and biotite, which form only close to the shear zone. The Br/Cl and I/Cl ratios of the most highly altered samples are 3 × 10-3 and 25 × 10-6, respectively, and overlap with the range of ratios measured for marine pore fluids. Remobilization of evaporites would have produced distinctly lower halogen ratios and can be ruled out as a fluid source. The unaltered gabbro has δ37Cl values near 0% and a similar value is inferred for the infiltrating fluid. Minimally altered samples, noted by the presence of <20 vol. % amphibole, have negative δ37Cl values (average = -0·6 ± 0·1%). The δ37Cl values increase (up to + 1% nearest the shear zone) with increasing evidence of fluid-rock interaction towards the shear zone. Mixing of different Cl reservoirs (i.e. gabbro and fluid) cannot explain the observed changes in the δ37Cl values. Instead, isotopic fractionation owing to Rayleigh distillation with a fractionation factor of 1·0010 can explain the observed negative δ37Cl values of the alteration sequence by the evolution of a single fluid during fluid-rock interaction. An additional kinetic component during fractionation may have enhanced the observed changes in chlorine isotope values. We speculate that these processes are the reason for the large spread of δ37Cl values found in many stable Cl studies for single reservoirs. [ABSTRACT FROM AUTHOR]