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Spring waters from across the Costa Rica margin were analyzed for their Li and He isotope compositions to determine the utility of Li isotopes as a tracer of volatile sources in subduction zones. Li isotope ratios systematically decrease with increasing depth to the subducting slab: averaging +15.0‰ ± 9.2‰ in the outer forearc (<40 km to the slab), +9.3‰ ± 4.3‰ in the forearc (40–80 km to the slab), and +5.8‰ ± 2.8‰ in the arc (>80 km to the slab). In contrast, air-corrected ³He/4He values (reported relative to the ratio in air, RA) range from 0.4 to 7.5 RA and increase from predominantly crustal values near the trench to mantle values in the arc. Together, these data support progressive devolatilization of the subducting plate with slab-derived Li components sourced from shallowly expelled pore fluids in the outer forearc, sedimentary and/or altered oceanic crust contributing to the forearc, and limited slab input beneath the arc. [ABSTRACT FROM AUTHOR]

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ[sup.18]O value by ~1-3% from core to rim at a distance of ~120 ± 50 µm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in 8 O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ[sup.18]O in eclogite garnet cores range from 5.7 to 11.6%, preserving the composition of the eclogite protolith. The abrupt decrease in the δ[sup.18]O values of the garnet rims to values ranging from 3.2 to 11.2% suggests interaction with a lower δ[sup.18]O fluid during the final stages of growth during eclogite facies metamorphism (450-600°C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185-240°C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ[sup.18]O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ[sup.18]O garnet rims, and later as temperatures decrease, with rind formation. Keywords Franciscan Complex * Eclogite * Garnet * Oxygen isotopes * SIMS, Introduction Fluids are a primary means of mass transport in subduction zones with major consequences of arc magmatism, seismicity, and the chemical evolution of the crust and mantle. Tracing fluid [...]

Chlorine isotope ratios were determined for volcanic gas, geothermal well, ash, and lava samples along the Izu-Bonin-Mariana volcanic front, serpentinite clasts and muds from serpentine seamounts (Conical, South Chamorro, Torishima), basalts from the Guguan cross-chain, and sediments from Ocean Drilling Program (ODP) Sites 800, 801, 802, and 1149. There is no systematic variation in [[delta].sup.37]Cl values along the volcanic front in either gas or ash samples. In contrast, distinct variations occur across the arc, implying variations in the fluid source at different depths within the subduction zone. Serpentinite clasts and serpentine muds from the seamounts tap a source of ~30 km depth and have [[delta].sup.37]Cl values of structurally bound chloride of +0.4 [per thousand] [+ or -] 0.4 [per thousand] (n = 24), identical to most seafloor serpentinites, suggesting a serpentinite (chrysotile and/or lizardite to antigorite transition) fluid source. Tapping deeper levels of the subduction zone (~115-130 km depth), volcanic gases and ashes have [[delta].sup.37]Cl values averaging -1.1 [per thousand] [+ or -] 1.0 [per thousand] (n = 29), precisely overlapping the range measured in sediments from ODP cores (-1.1 [per thousand] [+ or -] +0.7 [per thousand], n = 11) and limited altered oceanic crust (AOC). Both sediments and AOC are possible Cl sources in the volcanic front. The Guguan cross-chain basalts come from the greatest depths and have an average [[delta].sup.37]Cl value of +0.2 [per thousand] [+ or -] 0.2 [per thousand] (n = 3), suggesting a second serpentine-derived source, in this case from antigorite breakdown at ~200 km depth.

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]

In order to trace the cycling of fluid-mobile elements (FMEs) through subduction zone forearcs, we collected water samples from two warm and 16 cold springs along the subaerially exposed forearc of the Hikurangi subduction zone in New Zealand. Water samples were analyzed for their cation and anion concentrations, as well as their B, Li, Cl, and O stable isotope compositions. Fluids discharging through the prism have high concentrations of Cl (2400-16,000 mg/L), Br (6-70 mg/L), I (0.4-72 mg/L), Sr (0.1-200 mg/L), B (3-130 mg/L), Li (0.1-13 mg/L), and Na (33-6600 mg/L), consistent with data from previous studies. Most of these elements decrease overall in concentration from north to south, have a concentration peak in the central part of the margin, and have had limited concentration variability during the last three decades. Because Li, Cl, and B are all fluid-mobile elements, their incompatibility potentially limits modification by fluid-rock interaction, making them reliable tracers of fluid source. δ37Cl, δ11B, and δ7Li values range from -1.3‰ to +0.4‰ (n = 36), +11.8‰ to +41.9‰ (n = 25), and -3.1‰ to +29.0‰ (n = 29), respectively. Despite the change in concentrations along the margin, there is no corresponding trend in isotopic composition. Chlorine and boron isotope compositions are consistent with fluids dominated by seawater (δ37Cl = 0‰; δ11B = 40‰) and sedimentary pore fluids (δ37Cl ≈ -8‰ to 0‰; δ11B > ~17‰). Br/Cl (0.0025-0.005) and I/Cl (0.00005-0.007) weight ratios also support a dominant seawater and pore-fluid source. Lithium isotope data also suggest fluids sourced from seawater (δ7Li = +31‰) as well as dehydrating sediments and/or modified by interaction with local sediment. The fluid geochemical data cannot be explained by a change in the fluid source along the margin, but rather by a change in the upper-plate structural permeability. In the north, extension likely results in a highly permeable forearc, whereas transpression in the south traps fluids within the upper plate and dilutes saline fluids with groundwater. Such changes in upper-plate structural permeability may influence fluid pressure conditions within the forearc, which in turn may influence the observed change in slip behavior on the interface from north Hikurangi (aseismic creep) to south Hikurangi (deep locking). This work highlights the role the upper plate may play in the geochemical modification and transport of slab-derived fluids, and supports the long-standing but poorly documented assumption that seawater and pore fluids are expelled at shallow levels in the subduction zone (<15 km) and therefore play a limited role in the transport of FMEs to great depths in subduction zones. [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]

Subducting serpentinized lithosphere has distinct δD and δ18O values compared to normal mantle. Slab-derived fluids that infiltrate the mantle wedge can alter its oxygen and hydrogen isotope composition, raising or lowering the δ18O and δD values depending on the nature of the subducted components. Hydrous minerals in peridotite xenoliths from the Colorado Plateau (southwestern USA) have δD values (up to -33‰) much higher than average mantle (-80%c), but similar to δD values of olivine-hosted melt inclusions within arc basalts, suggesting a slab-derived fluid source. Oxygen isotope ratios of olivine from these xenoliths are similar to average mantle, yet display a strong negative correlation with clinopyroxene Ce/Sm, a proxy of metasomatism. This correlation is most simply explained by metasomatism from fluids derived from the serpentinized portion of the Farallon slab. Although δ18O values of mantle minerals span a narrow range, integration of stable isotope data with other geochemical tracers can provide new constraints on modern and ancient subduction-related processes, potentially providing a method for probing Archean lithospheric mantle for evidence of early subduction. [ABSTRACT FROM AUTHOR]

The chlorine isotope composition of thermal fluids from Iceland were measured in order to evaluate the source of chlorine and possible chlorine isotope fractionation in geothermal systems at divergent plate boundaries. The geothermal systems studied have a wide range of reservoir temperatures from 40 to 437 °C and in-situ pH of 6.15 to 7.15. Chlorine concentrations range from 5.2 to 171 ppm and δ 37 Cl values are −0.3 to + 2.1 ‰ ( n = 38 ). The δ 37 Cl values of the thermal fluids are interpreted to reflect the source of the chlorine in the fluids. Geothermal processes such as secondary mineral formation, aqueous and vapor speciation and boiling were found to have minimal effects on the δ 37 Cl values. However, further work is needed on incorporation of Cl into secondary minerals and its effect on Cl isotope fractionation. Results of isotope geochemical modeling demonstrate that the range of δ 37 Cl values documented in the natural thermal fluids can be explained by leaching of the basaltic rocks by meteoric source water under geothermal conditions. Magmatic gas partitioning may also contribute to the source of Cl in some cases. The range of δ 37 Cl values of the fluids result mainly from the large range of δ 37 Cl values observed for Icelandic basalts, which range from −0.6 to + 1.2 ‰ . [ABSTRACT FROM AUTHOR]

The chlorine isotope composition of Earth’s interior can place strong constraints on deep-Earth cycling of halogens and the origin of mantle chemical heterogeneity. However, all mantle-derived volcanic samples studied for Cl isotopes thus far originate from submarine volcanic systems, where the influence of seawater-derived Cl is pervasive. Here, we present Cl isotope data from subglacial volcanic glasses from Iceland, where the mid-ocean ridge system emerges above sea level and is free of seawater influence. The Iceland data display significant variability in d37Cl values, from -1.8‰ to +1.4‰, and are devoid of regional controls. The absence of correlations between Cl and O isotope ratios and the lack of evidence for seawater-derived enrichments in Cl indicate that the variation in d37Cl values in Icelandic basalts can be solely attributed to mantle heterogeneity. Indeed, positive correlations are evident between d37Cl values and incompatible trace element ratios (e.g., La/Y), and long-lived radiogenic Pb isotope ratios. The data are consistent with the incorporation of altered lithosphere, including the uppermost sedimentary package, subducted into the Iceland mantle plume source, resulting in notable halogen enrichments in Icelandic basalts relative to lavas from adjacent mid-ocean ridges. [ABSTRACT FROM AUTHOR]

In order to provide constraints on the sources of chlorine in spring waters associated with arc volcanism, the major/minor element concentrations and stable isotope compositions of chlorine, oxygen, and hydrogen were measured in 28 thermal and mineral springs along the Cascade Range in northwestern USA. Chloride concentrations in the springs range from 64 to 19,000 mg/L and δ Cl 37 values range from + 0.2 ‰ to + 1.9 ‰ ( average = + 1.0 ± 0.4 ‰ ), with no systematic variation along or across the arc, nor correlations with their presumed underlying basement lithologies. Additionally, nine geochemically well-characterized lavas from across the Mt. St. Helens/Mt. Adams region of the Cascade Range ( Leeman et al., 2004, 2005 ) were analyzed for their halogen concentrations and Cl isotope compositions. In the arc lavas, Cl and Br concentrations from the volcanic front are higher than in lavas from the forearc and backarc. F and I concentrations progressively decrease from forearc to backarc, similar to the trend documented for B in most arcs. δ Cl 37 values of the lavas range from −0.1 to + 0.8 ‰ (average = + 0.4 ± 0.3 ‰ ). Our results suggest that the predominantly positive δ Cl 37 values observed in the springs are consistent with water interaction with underlying 37 Cl-enriched basalt and/or altered oceanic crust, thereby making thermal spring waters a reasonable proxy for the Cl isotope compositions of associated volcanic rocks in the Cascades. However, waters with δ Cl 37 values > + 1.0 ‰ also suggest additional contributions of chlorine degassed from cooling magmas due to subsurface vapor–liquid HCl fractionation in which Cl is lost to the aqueous fluid phase and 37 Cl is concentrated in the ascending magmatic HCl vapor. Future work is necessary to better constrain Cl isotope behavior during volcanic degassing and fluid–rock interaction in order to improve volatile flux estimates through subduction zones. [ABSTRACT FROM AUTHOR]

• Altered seafloor basalts preferentially incorporate F relative to altered gabbros, which contain more Cl and Br. • The efficiency of halogen devolatilization from subducting ocean crust is Cl ≈ Br > I ≫ F. • At high P-T, lower crustal gabbros can be converted to halogen-enriched eclogites. • Subducted ocean crust represents the largest flux of F, Cl, Br, and I to the mantle. • Approximately 50% of the initial Cl and 95% of initial Br and I is lost before the arc. In order to constrain the geochemical cycling of halogens (F, Cl, Br, I) during subduction of altered ocean crust (AOC), this study compares bulk halogen concentrations from seven seafloor AOC drill cores (n = 21) and exhumed eclogites (n = 22) from three paleo-subduction settings (Raspas Complex, Ecuador; Zambezi Belt, Zambia; Cabo Ortegal, Spain). As ocean crust undergoes hydrothermal alteration, basalts and gabbros preferentially incorporate different halogens. Fluorine is predominantly hosted in basalts (averaging 155 μg/g); Cl and Br are enriched in gabbros (averaging 241 μg/g and 225 ng/g, respectively). During prograde metamorphism of AOC, F behaves compatibly and is decoupled from the heavy halogens (Cl, Br, I). Eclogite samples retain F in concentrations (30–160 μg/g) similar to seafloor values (20–190 μg/g). Chlorine and Br are strongly coupled and effectively mobilized during prograde metamorphism, with high-pressure (HP) samples containing between 5–15 μg/g Cl and 17–69 ng/g Br. Elevated F/Cl, Br/Cl, and I/Cl in eclogites relative to MORB and AOC ratios support the decoupling of halogens during metamorphism as Cl is most efficiently expelled from the slab; the mobility of halogens in subducting AOC is as follows: Cl ≈ Br > I ≫ F. In conjunction with published values of halogen abundances in seafloor serpentinites, marine sediments, and HP metaserpentinites and metasediments, the results of this study are used to estimate global halogen fluxes. Mass balance estimates indicate that up to 2% of the initial F, 50% of the initial Cl, 93% of the initial Br, and 97% of initial I entering subduction zones is lost between the trench and eclogite facies. Subducted AOC represents a substantial halogen reservoir to arc magma depths and may represent the most significant carrier of halogens to the deep mantle. [ABSTRACT FROM AUTHOR]

Abstract: Despite the potential of chlorine isotopes to trace inputs of fluids from the subducting slab, few studies so far have used this tool to investigate the petrogenesis of arc magmas. Here we report stable chlorine isotope data (δ 37Cl values) and Cl concentrations of volcanic rocks from five Quaternary volcanoes of Ecuador situated on an across-arc transect encompassing the frontal arc (Pilavo, Pululahua, Pichincha volcanoes), main arc (Chacana caldera complex) and back-arc (Sumaco volcano). We find that changes in δ 37Cl values across the arc correlate with slab fluid indices (Ba/La, Pb/Ce). The overall decrease in δ 37Cl values away from the trench can be interpreted in the frame of previous petrogenetic models of Ecuadorian volcanoes, according to which magmas are formed by a steadily decreasing melt fraction of the mantle induced by a steadily decreasing amount of fluids released by the subducted slab away from the trench. The high δ 37Cl values of the frontal arc volcanoes (up to +3‰) imply that the Cl carried by slab fluids derives, at least partly, from 37Cl-rich subducted terrigenous sediments plus subordinate amounts of altered oceanic crust and serpentinite. The anomalously high Cl contents (up to 0.2 wt.% Cl) and high δ 37Cl values (up to +1.5‰) of the back-arc Sumaco volcano can be explained by preferential partial melting of mantle portions metasomatized by slab fluids during a Jurassic subduction event. Superimposed on the first order changes of δ 37Cl values across the arc, we observe that δ 37Cl values at each volcanic center are systematically lowered due to intracrustal evolution processes occurring in magmatic reservoirs at mid-crustal levels. [Copyright &y& Elsevier]

Abstract: Serpentinized abyssal peridotites sampled by the Ocean Drilling Program Leg 209 along the mid-Atlantic Ridge near the 15°20′N Fracture Zone have been analyzed for oxygen, hydrogen, and chlorine isotope compositions in order to determine isotopic behavior under a wide range of serpentinization conditions and place constraints on fluid history. Oxygen and hydrogen thermometry suggests peak serpentinization temperatures of 300–500 °C. Serpentine separates have low δD values possibly due to a magmatic fluid component or low-temperature exchange during seafloor weathering. Chlorine geochemistry focused on three holes: 1274A and 1272A (serpentinized peridotites) and 1268A (serpentinite locally altered to talc). Concentrations of both, water-soluble chloride (WSC) and structurally bound chloride (SBC) are significantly lower at Hole 1268A compared to Holes 1274A and 1272A. The δ 37Cl values for WSC and SBC of serpentinites in Holes 1274A and 1272A are slightly positive (avg. WSC=0.20‰, n =22 and avg. SBC=0.35‰, n =22), representing typical seawater-hydration conditions commonly determined for abyssal peridotite. The SBC of serpentinites from Hole 1268A are also positive (avg.=0.63‰); whereas, the SBC in talc-dominated samples is negative (avg.=−1.22‰). The WSC of both talc- and serpentine-dominated samples are also negative (avg.=−0.15‰). We interpret the chlorine isotope data to preserve a record of multiple fluid events. As seawater hydrated the peridotite, 37Cl was preferentially incorporated into the forming serpentine and water-soluble salts, yielding similar δ 37Cl values on a regional scale as sampled by Holes 1268A, 1274A and 1272A. The resultant pore fluid was left depleted in 37Cl. Locally (Hole 1268A), this evolved fluid was remobilized possibly due to the initiation of hydrothermal circulation in response to emplacement of a mafic magma body. The low δ 37Cl pore fluids attained elevated SiO2 and sulfur concentrations due to interaction with the gabbroic intrusion and, when ascending through the surrounding serpentinite, caused formation of isotopically negative talc. This secondary fluid also flushed the preserved serpentinite of its previously formed salts, resulting in negative δ 37ClWSC values. The δ 37ClSBC values of the serpentinite samples remained unmodified by reaction with the secondary fluid. [Copyright &y& Elsevier]

In magmatic settings, water behaves as an incompatible species and should be depleted during melting and enriched during metasomatism. Previous studies have identified correlations between nominally anhydrous mineral (NAM) water content ([H 2 O]) and indices of metasomatism or melt extraction, seemingly confirming this behavior in the mantle. However in detail, these correlations are ambiguous and do not reflect robust controls on NAM [H 2 O]. We measured orthopyroxene (opx) and clinopyroxene (cpx) [H 2 O] in variably hydrated and metasomatized peridotite xenoliths from the Navajo volcanic field (NVF) that sample the Colorado Plateau subcontinental lithospheric mantle (SCLM), an endmember of SCLM hydration and metasomatism. These xenoliths span a wide range of pyroxene [H 2 O] (opx from 50 to 588 ppm wt. H 2 O; cpx from 38 to 581 ppm wt. H 2 O), but NAM [H 2 O] does not correlate with either indices of melt depletion or metasomatism. Growth of hydrous minerals suggests higher water activity than in anhydrous peridotites, and therefore hydrous-mineral-bearing xenoliths and anhydrous xenoliths should have different NAM [H 2 O] and water activities. However, when the two groups are compared no significant differences can be found in either NAM [H 2 O] or water activity. We propose that the high diffusivity of hydrogen in the mantle allows for equilibration of water activity in the mantle over sub-kilometer length scales over geologic time. Such diffusive equilibration reduces water activity variability and results in the blurring and destruction of correlations between NAM [H 2 O] and indices of metasomatism or melt extraction. As a result of diffusive equilibration of water, there is a large difference in the variability of concentration between NAM [H 2 O] (spanning ∼2 orders of magnitude) and similarly incompatible elements such as Ce in the same peridotites (spanning ∼4 orders of magnitude). This difference in behavior explains why H 2 O/Ce ratios in mantle peridotites are highly variable relative to those of basalts. [ABSTRACT FROM AUTHOR]

Skarns are one of the few direct genetic links between magma and base metals in ore deposits, yet temporal relationships between skarns and their assumed causative plutons are rarely confirmed with radiometric formation ages. Skarns have been notoriously difficult to date directly due to a limited primary mineral assemblage, generally garnet and pyroxene. Discrepancies in timescales of skarn formation versus pluton emplacement emphasize our tenuous understanding of temporal relationships between plutons and hydrothermal systems including the duration of hydrothermal activity and skarn mineral growth rates. Here we apply a new method for U–Pb dating grossular-andradite (grandite) garnet via LA-ICP-MS and present the first comprehensive study to focus on U–Pb dates of primary skarn garnet. We present U–Pb age constraints on garnets from multiple skarns within the Jurassic and Cretaceous arcs of the North American Cordillera. Two case studies (Darwin and Empire Mountain skarns) illustrate the effectiveness of the U–Pb grandite method by demonstrating that garnet U–Pb ages agree within error of the pluton- and skarn-hosted U–Pb zircon ages. Two additional study localities (Black Rock and Mojave skarns) demonstrate the ability of grandite U–Pb ages to: 1) directly identify a causative igneous event, and 2) record timing of major tectonic events, specifically extensional to trans-tensional shifts in the Jurassic Mojave of California. [ABSTRACT FROM AUTHOR]

Plutons associated with a 1.4 Ga magmatic event intrude across southwestern Laurentia. The tectonic setting of this major magmatic province is poorly understood. Proposed melting models include anorogenic heating from the mantle, continental arc or transpressive orogeny, and anatexis from radiogenic heat buildup in thickened crust. Re-Os analyses of refractory mantle xenoliths from the Navajo volcanic field (NVF; central Colorado Plateau) yield Re depletion ages of 2.1-1.7 Ga, consistent with the age of the overlying Yavapai and Mazatzal crust. However, new Sm-Nd isotope data from clinopyroxene in peridotite xenoliths from NVF diatremes show a subset of xenoliths that plot on a ca. 1.4 Ga isochron, which likely reflects mantle melt production and isotopic resetting at 1.4 Ga. This suggests that Paleoproterozoic subcontinental lithospheric mantle was involved in the 1.4 Ga magmatic event. Our constraints support a subduction model for the generation of the 1.4 Ga granites but are inconsistent with rifting and anorogenic anatexis models, both of which would require removal of ancient lithosphere. [ABSTRACT FROM AUTHOR]

Among the magmatic volatiles, chlorine (Cl) is degassed at shallow depths offering the opportunity to investigate the behavior of magmatic degassing close to the surface, and the possible occurrence of chemical and isotopic fractionation related to gas/melt partitioning. However, it is still unclear if the isotopic composition of Cl (δ 37 Cl) can be used as a proxy of magmatic degassing. In this work, we investigate the concentrations of chlorine and sulfur, and the Cl isotope composition of rocks and plume gases collected at Stromboli volcano, Aeolian Islands, Italy. This volcano was chosen because it is characterized by persistent eruptive activity (i.e., Strombolian explosions) and by the presence of magma at very shallow levels in the conduits. Rocks belonging to the different magmatic series erupted throughout the formation of the volcano have δ 37 Cl values ranging between − 1.0 and + 0.7‰. The isotopic composition seems independent of the Cl concentration of the rocks, but shows a negative correlation with SiO 2 content. Plume gases have a greater isotopic compositional variability than the rocks (− 2.2‰ ≤ δ 37 Cl ≤ + 1.5‰) and the composition seems related to the level of volcanic activity at Stromboli. Gases collected in 2011–2013 during days of ordinary eruptive activity are characterized by δ 37 Cl values ranging from + 0.3 to + 1.5‰ and S/Cl molar ratios between 1.4 and 2.2, similar to previous S/Cl measurements performed at Stromboli with other techniques. Plume gases collected in July 2014, in days of high-level eruptive activity preceding the onset of the 2014 effusive eruption, have negative δ 37 Cl values (− 2.2‰ ≤ δ 37 Cl ≤ − 0.1‰) and S/Cl between 0.9 and 1.2, which are among the lowest S/Cl values measured at this volcano. The amplitude of the volcanic tremor and the variation in the inclination of very long period (VLP) seismic signal polarization clearly indicate that in July 2014 the intensity and frequency of Strombolian explosions, as well as the level of magma in the conduits, were higher than normal. This suggests that when magmatic degassing occurs at very shallow depths (magma residing at few tens of meters below the craters), isotopic fractionation between gaseous and dissolved chlorine (lnα gas-melt > 0) can occur, resulting in a depletion of 37 Cl in the melt. Finally, we argue that the range of δ 37 Cl values measured in rocks and gases at Stromboli overlaps most of the known mantle reservoirs (i.e., DMM, HIMU and EM-EM1), indicating that the transfer of chlorine from the mantle to the surface occurs without significant isotopic fractionation. This work opens new perspectives for better constraining the isotope signature of chlorine of different magmatic systems and mantle reservoirs on the Earth. [ABSTRACT FROM AUTHOR]

The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., “gabbro glacier” vs. “multiple sills”). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the “multiple sills” model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC. Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5–5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt–crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially overprinted clinopyroxene Sr and Nd isotope compositions of LOC-derived gabbros. Although minor assimilation of Pacific crust by Hawaiian melts cannot be excluded, the range of oxygen isotope compositions recorded in Hawaiian lavas and cumulates cannot be generated by assimilation of the in situ LOC gabbros, which have relatively uniform and MORB-like δ 18 O values. To first order, the isotopic heterogeneity observed in Hawaiian melts appears to derive from the heterogeneous plume source(s), rather than assimilation of local oceanic crust. [ABSTRACT FROM AUTHOR]