Search

Abstract Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (δ7Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and δ7Li are negatively correlated in both events, but mean δ7Li ratios and associated ranges of variation are distinct between them. We apply a previously developed reactive transport model (RTM) for the site to these event‐scale flow perturbations, but observed shifts in stream δ7Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream δ7Li signatures, we couple the RTM to a distribution of fluid transit times that evolve based on storm hydrographs. The approach guides appropriate flux‐weighting of fluid from the RTM over a range of flow path lengths, or equivalently fluid residence times. This flux‐weighted RTM approach accurately reproduces dynamic storm δ7Li‐discharge patterns distinguished by the antecedent conditions of the watershed.

Abstract We perform a set of reactive transport simulations in three‐dimensional fracture networks to characterize the impact of geochemical reactions on flow channelization. Flow channelization, a frequently observed phenomenon in porous and fractured subsurface rock formations, results from the spatially variable hydraulic resistance offered by a geological structure. In addition to geo‐structural features such as network connectivity, geometry, and hydraulic resistance, geochemical reactions, for example, dissolution and precipitation, can dynamically inhibit or enhance flow channelization. These geochemical processes can change the fracture permeability leading to increased flow channelization, which are localized connected regions of high volumetric flow rates that are seemingly ubiquitous in the subsurface. In our simulations, fractures partially filled with quartz are gradually dissolved until quasi‐steady state conditions are obtained. We compare the flow field's initial unreacted and final dissolved states in terms of flow and transport observations. We observe that the dissolved fracture networks provide less resistance to flow and exhibit increased flow channelization when compared to their unreacted counterparts. However, there is substantial variability in the magnitude of these changes which implies that the channelization strongly depends on the network structure. In turn, we identify the interplay between the particular network structure and the impact of geochemical dissolution on flow channelization. The presented results indicate that geological systems that have been weathering or reactive for longer times in older landscapes are likely to have increased flow channelization compared to their equivalent but younger counterparts, which implies a time dependence on flow channelization in fractured media.

Abstract The Pleiades Volcanic Field is made up of some 20 monogenetic, partly overlapping scoria and spatter cones, erupted in the last 900 ka, cropping out from the ice close to the head of the Mariner Glacier in northern Victoria Land, Antarctica. Erupted products vary from hawaiite to trachyte, defining a complete mild Na‐alkaline differentiation trend. Mafic samples are characterized by multi‐elemental patterns typical of OIB magmas, moderately low 87Sr/86Sr (0.7037) and high 143Nd/144Nd (0.51284), with a clear within‐plate affinity, indicating a subcontinental lithospheric source. With increasing SiO2, 87Sr/86Sr ratios increase up to 0.7052 and 143Nd/144Nd decrease to 0.51277, supporting the hypothesis of open‐system evolution, with significant crustal assimilation during fractional crystallization. The erupted volume of most evolved products (∼7 km3), according to fractionation models, suggests that primitive magmas should have been more than 10 times larger, indicating the occurrence of a large magma plumbing system, unexpected for a volcanic field of monogenetic scoria cones. The occurrence of a complete fractionation trend with large magma chambers and large assimilation rate is unusual, if not unique, among the alkali basaltic volcanic fields and it is matched by a climax of activity during the last glacial maximum (30 ka), as indicated by new 40Ar‐39Ar ages (30 ± 3 ka and 25 ± 2 ka) for samples from the two most prominent edifices. Therefore, we hypothesize a role of a thick ice cap in suppressing eruptions and ultimately leading to prolonged magma residence time in the subsurface, favoring significant fractionation coupled with unusual high rates of crustal assimilation.

Abstract Natural gas fields with economic helium (>0.3 He %) require the radioactive decay of crustal uranium (U) and thorium (Th) to generate He and tectonic/structural regimes favorable to releasing and concentrating He. An unknown is determining the role of faults and structural features in focusing deep‐seated He sources on shallow accumulations. We tested the correlation between high‐He wells (n = 94) and structural features using a new high‐resolution aeromagnetic survey in the Four Corners area, USA. A depth‐to‐basement map with basement lineaments/faults, an intrusion map, and a flattened basement structural high map were created using Werner deconvolution algorithms by combining magnetic, gravity, and topography data with magnetic and gravity depth profiles. We show quantitatively (via analysis of variance) that a non‐random process controls the relationship between He (>0.3%) and both basement faults and intrusions: 88% of high‐He wells occur

Abstract Understanding the age and dynamics of the overriding plates allows an assessment of competing subduction initiation hypotheses. The Izu‐Bonin‐Mariana margin in the Western Pacific is a key example of initiation and hence it is important to constrain the age and origin of the oldest igneous crust of the supra‐subduction Philippine Sea Plate. We present geochronological and geochemical data of igneous rocks from the oldest ocean basins of the Philippine Sea Plate: the West Philippine and Palau Basins. Basalts from these basins have enriched geochemical characteristics similar to the EM‐2‐like mantle component found in OIB‐like basalts associated with the Oki‐Daito mantle plume. Ages of basalts from the northernmost West Philippine Basin (WPB) and the Palau Basin range from 43.5 to 50.5 Ma, which is similar to the oldest samples associated with the Oki‐Daito mantle plume (48–50 Ma). This implies that the plume contributed to magmatism from the onset of basin formation. It also provides support for the proposition that rifting of the Mesozoic arc terrane and subsequent seafloor spreading of the WPB was triggered by the arrival of the Oki‐Daito mantle plume at the base of the lithosphere. The age of these Philippine Sea Basins implies that only the Mesozoic Daito Ridge Group and the Gagua Ridge existed as Philippine Sea Plate crust before subduction initiation. A major fault activity after 37 Ma in the northernmost WPB demonstrates that careful reconstruction of the Eocene Philippine Sea Plate is critical to understanding plate dynamics during subduction initiation in the Western Pacific.

Abstract This study provides a comprehensive characterization of various hydrothermal systems in Southern Peru ranging from the faulted Precordillera's steep topography up to the volcanic High Cordillera (>4,000 m asl). The objective is to investigate thermal anomalies that may potentially serve as new geothermal resources. Our integrated approach combines: (a) geochemistry from 14 hot springs sampled throughout the Tacna region, and (b) 3D numerical modeling of coupled groundwater and heat transfer considering topography and faults embedded in homogeneous permeability. Water and gas analysis indicates that the springs located near volcanoes discharge Na‐K‐Cl waters with high temperatures (>87°C), high Total Dissolved Solid concentrations (TDS >3,452 mg/L), and free gases dominated by CO2 (>90 vol%). Springs located along the regional faults in the Precordillera discharge Ca‐SO4 and Na‐K‐Cl waters with moderate temperatures (27–53°C), intermediate TDS concentrations (464–2,458 mg/L), radiocarbon ages between 1.4 and 7.9 kyr, and free gases dominated by N2 (>95 vol%). The Aruma springs, which are located at the transition between the High and the Precordillera, display intermediate characteristics. Numerical models accurately replicate the locations and temperatures of the fault‐related springs only for permeable faults (>10−14 m2), revealing the creation of 100‐km long thermal plumes along faults, locally rising up the 150°C‐isotherm to about ∼1,000 m below the surface. This approach clearly distinguishes the spring origins, which are volcanic in High Cordillera and tectonic in Precordillera. Moreover, we highlight that steep topographic gradient and permeable reverse faults in the Andean forearc may generate considerable thermal anomalies, opening perspectives for the geothermal exploration.

Abstract The Sr‐Nd‐Hf‐Pb isotope geochemistry of the Late Miocene Tunceli Volcanics suggests that they are the products of mixed asthenospheric and lithospheric mantle melts. The combined elemental and mineral chemistry data additionally indicate that a pyroxenite component of lithospheric origin is involved in their genesis. Calculations favor melting depths of ∼2 GPa for the Tunceli lavas, that is, deeper than the current lithosphere‐asthenosphere boundary beneath Eastern Anatolia. Geochemical data suggest that during regional Neo‐Tethyan subduction, dense (i.e., pyroxenite‐bearing) domains formed by progressive melt intrusion into the lower lithosphere resulted in gravitational instabilities. This unstable density configuration eventually led to the foundering of the eastern Anatolian lithosphere in the Late Miocene, resulting in progressive melting of fusible pyroxenite‐bearing domains at asthenospheric depths. We demonstrate that these pyroxenitic melts mixed with ambient asthenospheric melts and generated the Tunceli lavas.

Abstract High‐resolution records from past interglacial climates help constrain future responses to global warming, yet are rare. Here, we produce seasonally resolved climate records from subarctic‐Canada using micron‐scale measurements of oxygen isotopes (δ18O) in speleothems with apparent annual growth bands from three interglacial periods—Marine Isotope Stages (MIS) 11, 9, and 5e. We find 3‰ lower δ18O values during MIS 11 than MIS 5e, despite MIS 11 likely being warmer. We explore controls on high‐latitude speleothem δ18O and suggest low MIS 11 δ18O values reflect greater contribution of cold‐season precipitation to dripwater from longer annual ground thaw durations. Other potential influences include changes in precipitation source and/or increased fraction of cold‐season precipitation from diminished sea ice in MIS 11. Our study highlights the potential for high‐latitude speleothems to yield detailed isotopic records of Northern Hemisphere interglacial climates beyond the reach of Greenland ice cores and offers a framework for interpreting them.

Abstract Provenance records from sediments deposited offshore of the West Antarctic Ice Sheet (WAIS) can help identify past major ice retreat, thus constraining ice‐sheet models projecting future sea‐level rise. Interpretations from such records are, however, hampered by the ice obscuring Antarctica's geology. Here, we explore central West Antarctica's subglacial geology using basal debris from within the Byrd ice core, drilled to the bed in 1968. Sand grain microtextures and a high kaolinite content (∼38–42%) reveal the debris consists predominantly of eroded sedimentary detritus, likely deposited initially in a warm, pre‐Oligocene, subaerial environment. Detrital hornblende 40Ar/39Ar ages suggest proximal late Cenozoic subglacial volcanism. The debris has a distinct provenance signature, with: common Permian‐Early Jurassic mineral grains; absent early Ross Orogeny grains; a high kaolinite content; and high 143Nd/144Nd and low 87Sr/86Sr ratios. Detecting this “fingerprint” in Antarctic sedimentary records could imply major WAIS retreat, revealing the WAIS's sensitivity to future warming.

Abstract Cave carbonates, seemingly growing in defiance of gravity, have attracted the community's interest for more than a century. This paper focusses on ‘helictites’, contorted vermiform speleothems with central capillaries. Petrographic, crystallographic and geochemical data of calcitic and aragonitic helictites (recent to 347 ka) from three caves in Western Germany are placed in context with previous work. Aragonitic helictites from one site, the Windloch Cave, form exceptionally large and complex structures that share similarities with the celebrated helictite arrays in the Asperge Cave in France. Aragonitic and calcitic helictites differ significantly in their crystal fabrics and internal geometry. Calcitic helictites are best described as a composite crystal fabric consisting of fibrous mesocrystals. Aragonite helictites display a complex fabric of acicular to platy crystals, some of which show evidence for growth‐twinning and perhaps crystallisation via a monoclinal precursor stage. The micro‐tomographic characterisation of several orders of channels and their complex architecture raises important questions regarding fluid migration and helictite architecture. In terms of their isotope geochemistry, helictites are depleted in 13C to various degrees, isotope values that are controlled by the mixing of fluids and mineralogy‐related fractionation. Regarding their δ18O values, most helictites overlap with other calcitic and aragonitic speleothems. Previous models explaining the twisted morphology of helictites are discussed from the viewpoint of fluid migration and CO2 degassing rates, mineralogy and helictite petrography. For the complex aragonitic helicities documented here, the most likely mechanisms to explain the contorted growth forms include the internal capillary network combined with localised (sector) growth at the helictite tip. The morphologically simpler calcitic helictites are best explained by capillary and surface flow. Future work should include geomicrobiology to assess the significance of induced mineralisation and transmission electron microscopy analysis to more quantitatively assign crystallographic properties.

Abstract The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada involved extrusion of continental flood basalts (CFBs) at 130–120 Ma and 100‐95 Ma and emplacement of an extensive sill and dike network that intersected the Carboniferous to Paleogene Sverdrup Basin. In this paper, we present new 40Ar/39Ar ages, major and trace elements, and Sr‐Nd‐Pb isotope ratios for HALIP lava, dikes, and sills from Bukken Fiord, NW Axel Heiberg Island, Canadian Arctic Islands. Our best constrained 40Ar/39Ar ages yield a weighted average of 124.1 ± 1 (2σ) Ma, coincident with the first pulse of tholeiitic CFB magmatism in the Arctic‐wide HALIP as exemplified by Isachsen Formation flood basalts on Axel Heiberg Island. The Bukken Fiord samples are plagioclase and clinopyroxene‐phyric tholeiitic basalts, are relatively evolved (3.2–6.5 wt% MgO), and share similar major and trace element compositions to typical HALIP tholeiites. Initial 143Nd/144Nd ranges from 0.51260 to 0.51291 and initial 87Sr/86Sr ranges from 0.70362 to 0.70776, while measured 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb range from 18.614 to 19.199, 15.534 to 15.630, and 38.404 to 39.054, respectively. The most primitive sample in this study has Sr‐Nd‐Pb isotope signatures that suggest an enriched plume‐derived mantle source for HALIP tholeiites. Most samples, however, possess relatively radiogenic isotope signatures that can be explained by moderate degrees of assimilation of Sverdrup Basin sedimentary rocks. Magma‐crust interaction in the HALIP plumbing system was likely widespread and may have increased the environmental impact of the HALIP, particularly if crustal carbon was volatilized.

Abstract Several compositional features of sedimentary deposits can be used to reconstruct environmental conditions of source areas. In this research, bulk X‐ray diffraction mineralogy, heavy and clay mineral assemblages and geochemistry obtained for modern deposits of the Cunene River and its tributaries Caculuvar–Mucope are integrated with geological/geomorphological characteristics of respective catchment areas to evaluate to what extent sediment production is spatially variable and source materials are differently affected by exogenous transformations. Detrital sources can be classified into four main types based on sediment composition: felsic, mafic, recycled and mixed. Source contributions obtained with unmixing models using distinct input data reveal some disagreements, with heavy mineral assemblages pointing to higher mafic contribution and bulk XRD‐mineralogy favouring the recycled sedimentary component. However, the three datasets coincide showing a lower supply from the Kalahari Basin than the large outcropping areas of its sedimentary units would suggest, which is attributed to the relatively low rainfall and relief in this region. Where multiple depositional cycles are involved in sediment production the composition of the river deposits, even their clay fractions, will not reflect coeval weathering transformations and an under‐estimation of the mafic component probably occurs. This research reveals how exogenous processes may deviate the composition of the produced sediment from a simple weighted by outcropping area average of the source units and compromise (palaeo)environmental interpretations based on sediment composition.

Abstract Breakup of the proposed greater Ontong Java Nui large igneous province during the Cretaceous Normal Superchron can be constrained by the opening of the Ellice Basin (EB) separating the Ontong Java and Manihiki Plateaus and the Osbourn Basin separating the Manihiki and Hikurangi Plateaus. Dating of recovered dredged samples using plagioclase 40Ar/39Ar and zircon U/Pb geochronology methods indicates that spreading was well underway in the EB by 118 Ma with full spreading rates up to 3X faster than any observed today of 30–45 cm/yr and spreading likely continued until 112‐108 Ma. Ellice Basin samples show diverse geochemical affinities ranging from mid‐ocean ridge basalt (MORB) or Ontong Java‐like to more enriched OIB‐like. Pb and Nd isotopes from six samples contain varying influences from Pacific MORB and possibly Ontong Java. The geochemistry shows a lack of a clear mantle plume influence despite EB's close temporal and spatial relationship to Ontong Java, while some data resemble the Louisville Seamounts. This compositional diversity complements morphological differences among dredge sites and shows that both in situ MORB and younger overprinted features related to the nearby Tuvalu Seamounts were sampled. 40Ar/39Ar geochronology confirms the age of International Ocean Discovery Program Site U1365 near the Osbourn Trough (OT) to be 102.60 ± 0.26 Ma (2σ, n = 18). This age constrains the timing of a spreading reorientation event observed in the OB to coincide with a global plate reorganization event around 105 Ma and estimates the cessation of spreading at the OT to 96 Ma.

Abstract At the Trans‐Atlantic Geotraverse hydrothermal field, metalliferous sediments cover extinct hydrothermal mounds and the surrounding seafloor. Here, we report the morphological, mineralogical and geochemical processes that deposit these sediments, remobilize their metals, and affect their preservation. We found that the initial sediment metal tenor is controlled by physical transport of hydrothermal material from its source, followed by diagenetic redistribution and potentially diffuse fluid flow after high‐temperature hydrothermal activity has ceased. We distinguished three different environments: (a) proximal metalliferous sediments on top of extinct mounds are mainly derived from oxidative weathering of primary sulfide structures and are predominantly composed of Fe oxyhydroxides with low contents of Cu, Co, and Zn; metal enrichments in specific layers are likely related to upward flow of low‐temperature hydrothermal fluids; (b) medial distant metalliferous sediments found at the base of the mounds, deposited by mass transport, contain cm‐thick layers of unsorted sulfide sands with high base metal contents (e.g., up to 28% Cu); these buried sulfides continue to undergo dissolution, resulting in metal release into porewaters; (c) distal metalliferous sediments, found in depositional basins a few hundreds of meters from the extinct mounds, include fining‐upwards sequences of thin sulfide sand layers with Fe oxyhydroxides and were deposited by recurrent turbiditic flows. Dissolved metals (e.g., Cu2+ and Mn2+) diffuse upwards under reducing conditions and precipitate within the sediment. Hence, when using hydrothermal sediments to construct reliable geochronological records of hydrothermal activity, distance from source, local seafloor morphology, mass‐transport and depositional, and diagenetic modification should all be considered.

Abstract The Craters of the Moon (COM) Volcanic Field in Idaho is the largest mostly Holocene lava field in the conterminous U.S. and the site of the most recent volcanism in the Yellowstone‐Snake River Plain province. This prominent example of a polygenetic fissure field has produced over 60 eruptions over the past 15,000 years from the Great Rift fissure system. The eight most recent lava flows, known as Period A, were erupted between about 2,500 and 2,000 years ago. In this study, major and trace element and Pb‐Sr‐Nd isotope data were obtained, with several samples collected from each flow from widely spaced locations to evaluate heterogeneity in their source magmas. The results reveal three previously unrecognized compositional groups that may represent separate magma batches, and that assimilation and fractional crystallization (AFC) controlled the evolution of the entire Period A suite. The three oldest flows were periodic withdrawals from a body of highly evolved and poorly homogenized magma, and four later flows represent successive samplings from a stratified body of more primitive magma. These two flow groups are related via variable AFC modeled with Magma Chamber Simulator software, with Neogene rhyolite as a plausible crustal assimilant. A third compositional type that requires a different parent magma and AFC history is represented by a single flow erupted from vents a few km away from the others. The compositional evolution and variability observed in this sequence of flows provide new insights into how magmas are generated, modified, and delivered in such polygenetic fissure systems.

Abstract The end‐Permian extinction and its aftermath altered carbonate factories globally for millions of years, but its impact on platform geometries remains poorly understood. Here, the evolution in architecture and composition of two exceptionally exposed platforms in the Nanpanjiang Basin are constrained and compared with geochemical proxies to evaluate controls on platform geometries. Geochemical proxies indicate elevated siliciclastic and nutrient fluxes in the basal Triassic, at the Induan—Olenekian boundary and in the uppermost Olenekian. Cerium/Ce* shifts from high Ce/Ce* values and a lack of Ce anomaly indicating anoxia during the Lower Triassic to a negative Ce anomaly indicating oxygenation in the latest Olenekian and Anisian. Uranium and Mo depletion represents widespread anoxia in the world's oceans in the Early Triassic with progressive oxygenation in the Anisian. Carbonate factories shifted from skeletal in the Late Permian to abiotic and microbial in the Early Triassic before returning to skeletal systems in the Middle Triassic, Anisian coincident with declining anoxia. Margin facies shifted to oolitic grainstone in the Early Triassic with development of giant ooids and extensive marine cements. Anisian margins, in contrast, are boundstone with a diverse skeletal component. The shift in platform architecture from ramp to steep, high‐relief, flat‐topped profiles is decoupled from carbonate compositions having occurred in the Olenekian prior to the onset of basin oxygenation and biotic stabilisation of the margins. A basin‐wide synchronous shift from ramp to high‐relief platforms points to a combination of high subsidence rate and basin starvation coupled with high rates of abiotic and microbial carbonate accumulation and marine cement stabilisation of oolitic margins as the primary causes for margin up‐building. High sea water carbonate saturation resulting from a lack of skeletal sinks for precipitation, and basin anoxia promoting an expanded depth of carbonate supersaturation, probably contributed to marine cement stabilisation of margins that stimulated the shift from ramp to high‐relief platform architecture.

Abstract Over the past decades, the burial realm, the most prolonged and arguably the least well‐understood diagenetic environment, has received significant research attention. Despite remarkable progress driven by exploratory drilling, outcrop analogue studies and experimental work, the scientific theories defining the burial sub‐domains are inconsistently presented in the literature. This paper reviews the concepts, processes and products that characterise the burial realm from the viewpoint of the carbonate geoscientist. Typical features of carbonate burial (fluid types, porosity evolution, diagenetic fabrics, patterns in isotope geochemistry) in epicontinental and marine basins are presented and discussed. A step towards an improved conceptual delimitation and a subdivision of the carbonate burial realm is taken, and an intuitive terminology is proposed. The very shallow limit of the burial realm is placed within the upper sediment column (redox boundary; centimetres to tens of metres). In the shallow (marine) burial domain (down to depths of many hundreds of metres), carbonate dissolution and reprecipitation, sediment dewatering and grain reorganisation take place. Interstitial waters are mainly marine (and subordinate meteoric) in origin and the system is fluid‐dominated. Under ongoing burial, physical and chemical compaction reduces pore space. At burial depths of ca 750 m, initial sediment porosities (40–80%) are reduced to about 30%. The intermediate burial domain (hundreds of metres to about 2 km; T 100°C), marine formation fluids are increasingly modified by rock–fluid interaction and replaced by saline brines. The transition from the deep burial to the very low‐grade metamorphic domain is placed at depths of 12–15 km (T > 250°C). Here, carbonates undergo recrystallisation into meta‐carbonate and equigranular marble fabrics.

Abstract The Permian–Triassic boundary sediments at Ursula Creek accumulated in a continental shelf basin, or on the continental slope of the western Canadian passive margin, at 30°N palaeolatitude along the eastern Panthalassic Ocean margin. The area lay within the cold northerly ocean currents at the junction of westerly and north‐easterly trade wind belts, the latter causing summer coastal upwelling. The shift from uppermost Permian grey radiolarian cherts and grey shales to lowermost Triassic grey and black shales and fine‐grained dolomites is typical of deep‐water Panthalassic sediments. The palaeogeographical situation and palaeoenvironments are comparable to those of the present Canadian north‐western Pacific margin. The Ursula Creek section reveals the progressive decline of seafloor oxygen values in the Changhsingian Stage), followed by the persistent development of euxinic conditions in the latest Changhsingian and throughout the Early Triassic; a transition that coincides with the disappearance of a siliceous sponge fauna and the loss of diverse radiolarian populations. Much of the detrital sediment was supplied by summer north‐east Trade winds from the deserts of western North America, although variable amounts may have come across the Panthalassic Ocean as dust from contemporary volcanic eruptions. Relative palaeoproductivity changes show no consistent change in productivity across the Permian–Triassic boundary producing results that are comparable with those from the similar Opal Creek section to the south‐east. The Ni/Co, Cu/Zn, U/Al and Th/U ratios indicate variable redox conditions in all sections, but with a tendency for oxic conditions to change to dysoxic across the Permian–Triassic boundary. The lack of consistent element geochemical changes across the boundary accompanied by significant isotopic changes, here and elsewhere, suggests that atmospheric and oceanic chemistry rather than physical changes, like provenance and sea‐level changes, drove Permian–Triassic environmental changes and extinctions.

Abstract Although many studies have attempted to reconstruct millennial‐scale hurricane patterns using various proxy‐based methods, it is still unclear what the most effective proxies are to identify storm surge deposits in different environmental settings. This study quantitatively compares the application of grain‐size, loss‐on‐ignition, stable isotopes, X‐ray fluorescence, and palynological proxies in paleotempestology from an organic‐rich environment in the Florida Everglades. The nonparametric tests indicate that only 9 among the 27 parameters (mean diameter, %water, %organic, %carbonate, Ca, Sr, Ca/Ti, Cl/Br, and marine microfossils) exhibited significant differences between storm‐surge and in situ deposits. The principal component analysis shows that five marine indicators (Sr, Ca, Ca/Ti, %Carbonate, and Marine microfossils) have the closest association with the allochthonous samples, while Cl/Br and Mz are the most sensitive proxies in low‐ and high‐energy environments, respectively. Moreover, organic geochemical proxies (e.g., δ13C and δ15N of bulk sedimentary organic matter) are ineffective for identifying storm‐surge deposits in organic‐rich mangrove environments.

Abstract Carbonate clumped isotope geochemistry has primarily focused on mass spectrometric determination of m/z 47 CO2 for geothermometry, but theoretical calculations and recent experiments indicate paired analysis of the m/z 47 (13C18O16O) and m/z 48 (12C18O18O) isotopologues (referred to as Δ47 and Δ48) can be used to study non‐equilibrium isotope fractionations and refine temperature estimates. We utilize 5,448 Δ47 and 3,400 Δ48 replicate measurements of carbonate samples and standards, and 183 Δ47 and 195 Δ48 replicate measurements of gas standards from 2015 to 2021 from a multi‐year and multi‐instrument data set to constrain Δ47 and Δ48 values for 27 samples and standards, including Devils Hole cave calcite, and study equilibrium Δ47‐Δ48, Δ47‐temperature, and Δ48‐temperature relationships. We compare results to previously published findings and calculate equilibrium regressions based on data from multiple laboratories. We report acid digestion fractionation factors, Δ*63‐47 and Δ*64‐48, and account for their dependence on the initial clumped isotope values of the mineral.

Abstract This paper reports new software, PRIMELT3‐P, for computing the pressure and temperature range over which a primary basaltic magma was formed by adiabatic decompression and fractional melting of fertile mantle peridotite. The underlying model was developed to explore the meaning of magmatic pressure and its connection to the physics of mantle melting, melt extraction, and melt migration. We present a comparison of the results of the batch melting model FractionatePT3 (Lee et al., 2009, https://doi.org/10.1016/j.epsl.2008.12.020) and the new fractional melting model PRIMELT3‐P, each applied to basalts from various localities, with a particular focus on Hawai'i and Iceland. The unexpected result is that pressures for batch melting and fractional melting are quite similar across a broad range of potential temperatures, agreeing to within ±0.29 GPa (1σ) when applied to magmas that pass tests for derivation from peridotite sources. Although samples from both Hawai'i and Iceland yield a range of final pressures of melting, the minimum final pressure in each location coincides with independent geophysical constraints—the depth to the lithosphere‐asthenosphere boundary beneath Hawai'i and to the Moho below Iceland. These results clarify the meaning of primary magma pressure, but the concept remains complex. We suggest that computed temperature and pressure arrays arising from heterogeneous peridotite‐source primary magmas may reflect melt transport and mixing in a thermally zoned mantle plume with a complex network of branching melt channels that nucleate over a range of depths.

Abstract When plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant‐lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction‐related, tectonically‐exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non‐plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt‐enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite‐spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole‐rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low‐Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically‐exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co‐genetic melts. Because such rocks could have formed in either plate or non‐plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings.

Abstract This study presents an extensive geochemical data set of 23 samples from St. Barthélemy Island, which belongs to the extinct branch of the Lesser Antilles arc and is currently exposed in the northern part of the subduction forearc. Samples were selected to represent all lithologies and main periods of magmatism, that is, Middle‐Late Eocene, Early Oligocene and Late Oligocene. They show enrichment in light rare earth element/medium rare earth element, large ion lithophile elements (Rb and Ba) and isotopic characteristics, suggesting mixing between the mantle and a subduction component (oceanic crust + sediments). Trace element ratios suggest that primary magmas were generated in a normal mid‐oceanic ridge basalt‐type mantle‐wedge that underwent 8%–18% partial melting in the spinel‐stability field. The sediment contribution was low (0.1%–1%) irrespective of the age of the samples. This is similar to what is observed for the northern Lesser Antilles active branch. St. Barthélemy Island shares strong similarities with St. Martin Island, located on the same extinct arc branch, which suggests a similar geodynamic evolution. Oligocene samples displayed an increase in incompatible elements in the magma source, suggesting an increase in sediment melts, which could be correlated with a drastic change in the tectonic regime at that time, characterized by stretching perpendicular to the trench and subsequent basin opening. On Δ7/4Pb versus 206Pb/204Pb, the samples showed a similar trend for both active and extinct islands of the northern Lesser Antilles, suggesting negligible changes in the nature of the magma sources.

Abstract Fluid pressure is a key parameter in earthquake mechanics, controlling seismic failure and plate coupling in convergent zones. Yet fluid pressure is also extremely difficult to quantify at seismogenic depth, which limits our knowledge of the stress state in accretionary prisms. Here, we show that the geochemical record of exhumed hydrothermal quartz veins may be used to place quantitative bounds on fluid pressure variations in subduction zones. The studied veins come from sediments accreted and exhumed by plate convergence in southwestern Japan. Quartz in veins displays growth rims of contrasted bright blue/dark brown cathodoluminescence (CL) colors, high/low Al concentrations, and low/high fluid inclusion abundance. Because Si‐Al substitution (and charge compensation by Li) strongly depends on the rate of quartz precipitation and Si solubility, Al‐Li concentrations must be sensitive to fluid pressure. This is confirmed by fluid inclusions, the density of which, converted into trapping pressures, record fluid pressure drops by up to ∼70 MPa from CL‐brown, Al‐Li‐poor rims to CL‐blue, Al‐Li‐rich quartz rims. CL‐blue rims grow at a fast rate, high Si supersaturation and low fluid pressure whereas CL‐brown rims grow at a slower pace, lower Si supersaturation, and higher fluid pressure. Quartz trace element chemistry thus offers a promising tool to quantify deep fluid pressure variations and their relationships to earthquakes.

Abstract Tephra layers preserved in marine sediments are strong tools to study the frequency, magnitude and source of past major explosive eruptions. Thirty‐seven volcanoes from the Ecuadorian and Colombian arc, in the northern Andes, experienced at least one eruption during the Holocene. The volcanic hazard is therefore particularly high for the populated areas of the Andes and in particular cases for the coastal region, and it is crucial to document such events to improve hazard assessment. The age and distribution of deposits from major Holocene eruptions have been studied in the Cordillera, but no descriptions of distal fallouts have been published. In this study, we focused on 28 Holocene tephra layers recorded in marine sediment cores collected along the northern Ecuador—Southern Colombia margin. New lithological, geochemical and isotope data together with 14C datings on foraminifers allow us to determine the age and volcanic source of marine tephra, and to propose a first land‐sea correlation of distal tephra fallouts. We show that at least seven explosive eruptions from Guagua Pichincha, Atacazo‐Ninahuilca, Cotopaxi, and Cerro Machín volcanoes left tephra deposits recorded in marine cores over 250 km away from their source. Volume estimates of emitted tephra range between 1.3 and 6.0 km3 for the tenth century Guagua Pichincha, ∼5 ka Atacazo‐Ninahuilca, ∼6.7 and ∼7.9 ka Cotopaxi events, suggesting that they were eruptions of Volcanic Explosivity Index of 5. The distribution of these deposits also brings new constraints for a better evaluation of the volcanic hazard in Ecuador.

Abstract Despite the scientific and economic significance of kimberlites and related magmas, their origin is unclear. Here, we address this issue using whole‐rock and perovskite‐derived Sr‐Nd‐Hf isotopes for the three occurrences of kimberlite, lamproites and ultramafic lamprophyres (UMLs) in Finland. Mesoproterozoic olivine lamproites at Lentiira‐Kuhmo and UMLs at Kuusamo have different isotopic signatures yet were emplaced contemporaneously at 1,200 Ma in response to an extensional regime linked to the Baltica‐Laurentia breakup. The low εNd(i) and εHf(i) values of the olivine lamproites are consistent with an enriched subcontinental lithospheric mantle (SCLM) source while UML compositions are intermediate between those of typical kimberlites and lamproites and are interpreted to reflect mixing of asthenospheric melts and ∼10%–15% of melts sourced from metasomatized SCLM. The ∼750 Ma Kuusamo kimberlites, are probably linked to the mantle plume activity that initiated Rodinia's break‐up, exhibiting homogenous isotopic compositions which mirror the prominent Geodynamic and Source Constraints for ∼1,200 Ma Olivine Lamproite and Ultramafic Lamprophyre Magmatism in Eastern Finland (PREMA)‐like signature of kimberlites globally. By contrast, ∼620–585 Ma Kaavi‐Kuopio kimberlites show limited range in εNd(i) and 87Sr/86Sr(i) but have remarkably heterogeneous εHf(i) (+6.5 to −6.3) with a temporal trend toward lower εHf(i) values. While the Kuusamo kimberlites erupted during the early stage of continental break‐up, the Kaavi‐Kuopio kimberlites were emplaced when Rodinia break‐up was completed, coeval with formation of the Central Iapetus large igneous province. Magmas forming the Kaavi‐Kupio kimberlites may have formed from an upwelling mantle source similar to PREMA modified by increasing incorporation (up to 10%) of recycled crustal material through time accounting for the trend toward lower εHf(i) in these kimberlites.

Abstract The Lucky Strike vent field, located on the Mid‐Atlantic Ridge (MAR), is hosted on enriched mid‐ocean ridge basalt associated with the nearby Azores hotspot. In this study, we present bulk rock geochemistry coupled with in situ sulfur isotope analysis of hydrothermal samples from Lucky Strike. We assess the geological controls on the differences in the major and trace element content and sulfur isotopic composition of the hydrothermal deposits within the vent field. The hydrothermal deposits contain elevated concentrations of elements typically enriched in mid‐ocean basalt (E‐MORB), such as Mo, Ba, and Sr, compared to typical values for other hydrothermal deposits hosted on the MAR. The range in sulfur isotope compositions of hydrothermal marcasite and chalcopyrite (−2.5 to 8.7‰) is similar to the range recorded at other sediment‐free basalt‐hosted seafloor hydrothermal sites. However, at Lucky Strike, the Capelinhos vent, situated 1.4 km east of the main field, is enriched in 34S (by ∼3.5‰ for both marcasite and chalcopyrite), relative to the main field. This difference reflects contrasting subseafloor fluid/rock interactions at these two sites, including subseafloor sulfide precipitation at the main field that results in

Abstract We document the geochemistry of calcite veins in the Late Cretaceous Tikihore Formation (Raukumara Peninsula, New Zealand) to characterize their fluid composition and source and to help establish the age of subduction initiation at the Hikurangi margin of the Australia‐Pacific plate boundary. The calcite veins occur within normal faults offsetting turbidites that accumulated in a lower slope basin. Vein calcite trace metal content and rare earth element patterns are consistent with a seawater‐derived brine composition. Oxygen isotope (δ18O) values range from −6.1 to +8.4‰ and are −0.2‰ VPDB on average; positive δ13C values of up to +28‰ VDPB reflect methanogenesis. Oxygen isotope temperature data indicate that calcite vein mineralization occurred at temperatures in the range of 29°C–48°C. This is markedly less than the maximum burial temperature experienced by the host rocks, which we estimate to be 104 ± 10°C at 30–27 Ma from the inverse modeling of apatite fission track data. The vein calcite has a 28.5 ± 4.9 Ma U‐Pb age. From these data, we infer that the succession above Tikihore Formation was removed by slumping, thereby resulting in fluid overpressure in the reservoir, followed by hydraulic fracturing and the precipitation of the vein calcite. Ultimately, the data presented here from the Tikihore veins are consistent with subduction initiation at 30–27 Ma, based on the U‐Pb age of the vein calcite and modeling of apatite fission track data for the host sandstone, corroborated by the 30–27 Ma timing of back thrusting on the Taranaki Fault and related foredeep development in eastern Taranaki Basin.

Abstract We report results from a hydrothermal plume survey along the southernmost Chile Rise from the Guamblin Fracture Zone to the Chile Triple Junction (CTJ) encompassing two segments (93 km cumulative length) of intermediate spreading‐rate mid‐ocean ridge axis. Our approach used in situ water column sensing (CTD, optical clarity, redox disequilibrium) coupled with sampling for shipboard and shore based geochemical analyses (δ3He, CH4, total dissolvable iron (TDFe) and manganese, (TDMn)) to explore for evidence of seafloor hydrothermal venting. Across the entire survey, the only location at which evidence for submarine venting was detected was at the southernmost limit to the survey. There, the source of a dispersing hydrothermal plume was located at 46°16.5’S, 75°47.9’W, coincident with the CTJ itself. The plume exhibits anomalies in both δ3He and dissolved CH4 but no enrichments in TDFe or TDMn beyond what can be attributed to resuspension of sediments covering the seafloor where the ridge intersects the Chile margin. These results are indicative of sediment‐hosted venting at the CTJ.

Abstract Located in northern Dominican Republic, the Early Cretaceous Rio Boba mafic‐ultramafic plutonic sequence constitutes a lower crust section of the Caribbean island arc, made up by gabbroic rocks and subordinate pyroxenite. Modal compositions, mineral chemistry, whole‐rock compositions and thermobarometric calculations indicate that pyroxenites and gabbronorites represent a cumulate sequence formed by fractionation of tholeiitic magmas with initially very low H2O content in the lower crust of the arc (0.6–0.8 GPa). Melts evolved along a simplified crystallization sequence of olivine → pyroxenes → plagioclase → Fe‐Ti oxides. The magmatic evolution of the Rio Boba sequence and associated supra‐crustal Puerca Gorda metavolcanic rocks is multi‐stage and involves the generation of magmas from melting of different sources in a supra‐subduction zone setting. The first stage included the formation of a highly depleted substrate as result of decompressional melting of a refractory mantle source, represented by a cumulate sequence of LREE‐depleted island arc tholeiitic (IAT) and boninitic gabbronorites and pyroxenites. The second stage involved volumetrically subordinate cumulate troctolites and gabbros, which are not penetratively deformed. The mantle source was refractory and enriched by a LILE‐rich hydrous fluid derived from a subducting slab and/or overlying sediments, and possibly by a LREE‐rich melt. The third stage is recorded in the upper crust of the arc by the Puerca Gorda “normal” IAT protoliths, which are derived from an N‐MORB mantle source enriched with a strong subduction component. This magmatic evolution has implications for unraveling the processes responsible for subduction initiation and subsequent building of the Caribbean island arc.

Abstract The formation of mafic magmatic provinces are significant geological events that can drive mass extinctions and continental rifting and can influence basin evolution, petroleum prospectivity and mineralization. Buried magmatic provinces, however, are rarely identified and difficult to define. The Northwest Shelf of Australia contains large volumes of potentially interconnected mafic igneous material across several sedimentary basins. However, limited study and a lack of surface exposure have prevented detailed description and classification of these rocks. In this study, the distribution and composition of these mafic igneous rocks are described using an integrated geophysical and geochemical approach, which included over 10,000 km line length of 2D seismic data, well log data and chemical analysis of samples from 14 wells across the Browse, Roebuck, Canning and North Carnarvon basins. Using this combined data set, we demonstrate interconnectivity of buried mafic igneous rocks across the Northwest Shelf and calculate a total surface area exceeding 280,000 km2 and a cumulative minimum volume of ∼140,000 km3. Petrology and geochemistry of samples indicate they are basaltic and doleritic with alkaline and sub‐alkaline compositions and formed in a continental rift setting. Collectively, the igneous rocks meet the criteria for classification as a mafic magmatic province (MMP) and closely match the criteria required for classification as a large igneous province (LIP). Emplacement of the newly defined Northwest Shelf MMP may represent hotspot magmatism that could have initiated rifting of the Cimmerian Block from NW Australia during the Permian and could have potential for future, large scale CO2 sequestration and storage.

Abstract The Ordovician (Hirnantian; 445 Ma) hosts the second most severe mass extinction in Earth history, coinciding with Gondwanan glaciation and increased geochemical evidence for marine anoxia. It remains unclear whether cooling, expanded oxygen deficiency, or a combination drove the Late Ordovician Mass Extinction (LOME). Here, we present combined iodine and sulfur isotope geochemical data from three globally distributed carbonate successions to constrain changes in local and global marine redox conditions. Iodine records suggest locally anoxic conditions were potentially pervasive on shallow carbonate shelves, while sulfur isotopes suggest a reduction in global euxinic (anoxic and sulfidic) conditions. Late Katian sulfate‐sulfur isotope data show a large negative excursion that initiated during elevated sea level and continued through peak Hirnantian glaciation. Geochemical box modeling suggests a combination of decreasing pyrite burial and increasing weathering are required to drive the observed negative excursion suggesting a ∼3% decrease of global seafloor euxinia during the Late Ordovician. The sulfur datasets provide further evidence that this trend was followed by increases in euxinia which coincided with eustatic sea‐level rise during subsequent deglaciation in the late Hirnantian. A persistence of shelf anoxia against a backdrop of waning then waxing global euxinia was linked to the two LOME pulses. These results place important constraints on local and global marine redox conditions throughout the Late Ordovician and suggest that non‐sulfidic shelfal anoxia—along with glacioeustatic sea level and climatic cooling—were important environmental stressors that worsened conditions for marine fauna, resulting in the second‐largest mass extinction in Earth history and the only example during an icehouse climate.

Abstract Widespread anoxic events affected the Tethys Ocean during the Mesozoic. The Ocean Anoxic Event 1a (Early Aptian), expressed as the Selli Level or Goguel Level (GL) in European basins. The GL was deposited in the French Vocontian Basin, a semi‐enclosed basin connected to the Tethys Ocean. This study presents an integrated approach (Rock Eval, clay minerals, grain‐size, inorganic geochemistry and molecular biomarkers), applied to four sections in the basin distributed along a proximal‐distal transect. This study shows that the GL was perturbed by turbidites. In particular, the results demonstrate: (a) a homogeneous redox status of the basin that experienced oxic to suboxic conditions, according to trace element distributions and (b) low organic matter contents (total organic carbon ca 1 wt%) in the three sections where turbiditic deposits are observed. The distal, condensed section exhibits the highest organic matter contents (total organic carbon >3 wt%). In addition, the presence of gammacerane and isorenieratene derivatives in the distal sections suggests that the water‐column was intermittently stratified, with hypoxia developing in the photic zone. This stratification did not result from strong surface productivity but more likely from: (a) limited renewal of deep water in the basin; (b) reputedly high surface‐water palaeotemperatures during the Early Aptian; and (c) the influx of waters, possibly depleted in free oxygen and in some dissolved trace elements, into the basin. The turbiditic inputs, in addition to organic matter dilution in the sediments and a brief rupture of the water‐column stratification in the proximal areas of the basin, ventilated the sea floor and more specifically re‐oxidized the sediment‐water interface as well as underlying sediments. Such episodes of benthic re‐oxygenation could have altered the long‐term palaeoredox record, even in the distal sections where reducing conditions prevailed during deposition. In the area deprived of turbiditic input, sedimentary condensation, coupled with low oxygen conditions, furthered organic matter preservation and concentration.

Abstract The Guryul Permian–Triassic sediments were deposited on a passive margin in a partially enclosed basin, at a latitude of about 45°S along the southern margin of the Neotethys Ocean. The closest modern analogy is the Japan Sea rotated into a southern hemisphere orientation. Guryul element variations are subdued, mostly lithological, and take place across the change from the dominantly sandy shallow‐water Permian Zewan Formation into the dominantly shaly deeper‐water latest Permian–Triassic Khunamuh Formation, and not at the palaeontological Permian–Triassic boundary. The overall geochemistry of the Guryul sediments is very similar to that of the Ulleung Basin in the Japan Sea, where the sediments are 40%–60% eolian from loess and redeposited loess in the North China Basin. The overall Guryul geochemistry indicates that the sediments were derived from dominantly silica‐rich continental rather than silica‐poor sources although with some more silica‐poor inputs at times. Element ratios suggest increasing aridity and wind abrasion across the Permian–Triassic boundary. Various geochemical redox proxies suggest mainly oxic depositional conditions, with episodes of anoxia, but with little systematic variation across the extinction boundary. Productivity proxies suggest a slight decrease across the Permian–Triassic boundary, and at least one more decreased interval in the Early Triassic. The similar geochemistry of other localities indicate that the Guryul geochemical changes are regional in extent and representative of the North Indian Permian–Triassic passive margin. The lack of consistent element geochemical changes across the boundary accompanied by significant C, S, and other isotopic changes suggests that atmospheric and oceanic chemistry rather than physical changes, such as provenance, climate and sea‐level changes, drove the Permian–Triassic environmental changes and extinctions at least on the mid‐latitude Tethyan shelf of northern India.

Abstract We present PetroChron Antarctica, a new relational database including petrological, geochemical and geochronological data sets along with computed rock properties from geological samples across Antarctica. The database contains whole‐rock geochemistry with major/trace element and isotope analyses, geochronology from multiple isotopic systems and minerals for given samples, as well as an internally consistent rock classification based on chemical analysis and derived rock properties (i.e., chemical indices, density, p‐velocity, and heat production). A broad range of meta‐information such as geographic location, petrology, mineralogy, age statistics and significance are also included and can be used to filter and assess the quality of the data. Currently, the database contains 11,559 entries representing 10,056 unique samples with varying amounts of geochemical and geochronological data. The distribution of rock types is dominated by mafic (36%) and felsic (33%) compositions, followed by intermediate (22%) and ultramafic (9%) compositions. Maps of age distribution and isotopic composition highlight major episodes of tectonic and thermal activity that define well known crustal heterogeneities across the continent, with the oldest rocks preserved in East Antarctica and more juvenile lithosphere characterizing West Antarctica. PetroChron Antarctica allows spatial and temporal variations in geology to be explored at the continental scale and integrated with other Earth‐cryosphere‐biosphere‐ocean data sets. As such, it provides a powerful resource ready for diverse applications including plate tectonic reconstructions, geological/geophysical maps, geothermal heat flow models, lithospheric and glacial isostasy, geomorphology, ice sheet reconstructions, biodiversity evolution, and oceanography.

Abstract Magmatic activity that accompanied the collision between Arabia and Eurasia at ∼27 Ma, provides unique opportunities for understanding the triggers and magma reservoirs for collisional magmatism and its different styles in magmatic fronts and back‐arcs. We present new ages and geochemical‐isotopic results for magmatic rocks that formed during the collision between Arabia and Eurasia in NE Iran, which was a back‐arc region to the main magmatic arcs of Iran. Our new zircon U‐Pb ages indicate that collisional magmatism began at ∼24 Ma in the NE Iran back‐arc, although magmatism in this area started in the Late Cretaceous time and continued until the Pleistocene. The collisional igneous rocks are characteristically bimodal, and basaltic‐andesitic and dacitic‐rhyolitic components show significant isotopic differences; εNd(t) = +4.4 to +7.4 and εHf(t) = +5.4 to +9.5 for mafic rocks and εNd(t) = +0.2 to +8.4 and εHf(t) = +3.4 to +12.3 for silicic rocks. The isotopic values and modeling suggest that fractional crystallization and assimilation‐fractional crystallization played important roles in the genesis of felsic rocks in the NE Iran collisional zone. Trace element and isotopic modeling further emphasize that the main triggers of the magmatism in NE Iran comprise a depleted to the enriched mantle and the Cadomian continental crust of Iran. Our results also emphasize the temporal magmatic variations in the NE Iran back‐arc from Late Cretaceous to Pleistocene.

Abstract The 4.5 km deep IDDP‐2 drill hole was drilled at Reykjanes, Iceland, in an active seawater‐recharged hydrothermal system on the landward extension the Mid‐Atlantic Ridge. Drilling targeted a well‐defined hydrothermal up‐flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt‐hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659 m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659 m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30 to An99 and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase‐separated brine, indicated by co‐existing vapor‐rich and salt + vapor‐rich fluid inclusions. Seawater‐derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

Abstract Spatio‐temporal changes of upper mantle structure play a significant role in generating and maintaining surface topography. Although geophysical models of upper mantle structure have become increasingly refined, there is a paucity of geologic constraints with respect to its present‐day state and temporal evolution. Cenozoic intraplate volcanic rocks that crop out across eastern Australia provide a significant opportunity to quantify mantle conditions at the time of emplacement and to independently validate geophysical estimates. This volcanic activity is divided into two categories: age‐progressive provinces that are generated by the sub‐plate passage of mantle plumes and age‐independent provinces that could be generated by convective upwelling at lithospheric steps. In this study, we acquired and analyzed 78 samples from both types of provinces across Queensland. These samples were incorporated into a comprehensive database of Australian Cenozoic volcanism assembled from legacy analyses. We use geochemical modeling techniques to estimate mantle temperature and lithospheric thickness beneath each province. Our results suggest that melting occurred at depths ≤80 km across eastern Australia. Prior to, or coincident with, onset of volcanism, lithospheric thinning as well as dynamic support from shallow convective processes could have triggered uplift of the Eastern Highlands. Mantle temperatures are inferred to be ∼50–100°C hotter beneath age‐progressive provinces that demarcate passage of the Cosgrove mantle plume than beneath age‐independent provinces. Even though this plume initiated as one of the hottest recorded during Cenozoic times, it appears to have thermally waned with time. These results are consistent with xenolith thermobarometric and geophysical studies.

Abstract The sedimentological, geochemical, physical and magnetic properties of 40 surface and basal sediment samples of box cores collected throughout the Canadian Arctic Archipelago (CAA) from the Canadian Beaufort Shelf to Lancaster Sound were analyzed to determine the sedimentary processes that operate within the CAA during the pre‐ and post‐industrial periods (i.e., before and after 1900 Common Era or CE). In addition, the chronology of seven selected regional cores was established using 210Pb measurements, where the base is dated between 1550 and 1820 CE. These cores provide an opportunity to robustly compare post‐1900 sedimentary conditions with those of the colder Little Ice Age period (LIA; ∼1500–1900 CE). The different properties combined with multivariate statistical analyses result in the identification of three regional provinces with distinct sedimentary characteristics: (a) the West province (Mackenzie Shelf/Slope, west Banks Island and M'Clure Strait) typified by detrital associations (Fe‐Rb‐Ti‐Zn), high organic matter inputs, dominance of magnetite and low‐coercivity minerals and high aluminosilicate contents; (b) the Intermediate Zone (Amundsen and Coronation gulfs) distinguished by Si‐Al‐Zr‐Sr‐K associations, Mn oxyhydroxides precipitation, constant high magnetic grain concentrations and a mixture between marine and terrigenous organic matter; and (c) the East Province (Queen Maud Gulf, Victoria and Barrow straits, and Lancaster and Eclipse sounds) described by high detrital carbonate inputs, marine organic matter, a dominance of high‐coercivity minerals and high magnetic concentrations in pre‐industrial samples. Our results confirm that the pre‐ and post‐industrial sedimentary dynamics are controlled by sediment supplies from the river discharges in the West and Intermediate provinces, whereas the East province is more influenced by sea ice and coastal erosion. Basal sediment samples from the seven 210Pb‐dated cores suggest an intensification of the Mackenzie, Coppermine, and Ellice rivers runoff, extensive sea‐ice cover and consequently sediment transport by the latter during the LIA period.

Abstract While most volcanic arcs show a distinctive spatial relationship to subducting plates, there are many examples where volcanoes occur in anomalous locations. These are commonly also geochemically anomalous relative to the composition of more typical subduction‐related rocks. Using Holocene volcanoes in South America as a case study, we document the spatial and geochemical patterns along the Andean volcanic belt. To determine whether spatial variations are also geochemically anomalous, we assess a series of geochemical indices that provide information on the depth and degree of melting, and the role of metasomatic subduction inputs in melt generation. We use these parameters to develop a scoring system, with the lowest and highest scores indicating “typical” and “anomalous” arc melting processes, respectively. Typical arc magmatism is defined as melts generated in the sub‐arc mantle wedge through slab‐derived fluid metasomatism, with or without contributions from subducted sediments. In contrast, we show that anomalous volcanism in South America appears to relate to geometric anomalies in the subducting Nazca plate (e.g., beneath Sumaco, Laguna Blanca and Payun Matru), or to areas affected by variations in mantle flow due to the proximity to the slab edge (Crater Basalt Volcanic Field). By establishing relationships between anomalous magmatism and slab structure, we propose that similar geochemical fingerprints could be used to explore the magmatic response to slab deformation and/or tearing in older arc systems, particularly in cases where the three‐dimensional slab structure is no longer detectable.

Abstract Subduction‐related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Abstract The Tierra Blanca (TB) eruptive suite comprises the last four major eruptions of Ilopango caldera in El Salvador (≤45 ka), including the youngest Tierra Blanca Joven eruption (TBJ; ∼106 km3): the most voluminous event during the Holocene in Central America. Despite the protracted and productive history of explosive silicic eruptions at Ilopango caldera, many aspects regarding the longevity and the prevailing physicochemical conditions of the underlying magmatic system remain unknown. Zircon 238U‐230Th geochronology of the TB suite (TBJ, TB2, TB3, and TB4) reveals a continuous and overlapping crystallization history among individual eruptions, suggesting persistent melt presence in thermally and compositionally distinct magma reservoirs over the last ca. 80 kyr. The longevity of zircon is in contrast to previously determined crystallization timescales of

Abstract The Okavango Delta in northern Botswana is an endorheic ecosystem formed by 150,000 islands surrounded by a flood plain. The vegetation of these islands displays an external tree ring enclosing a barren interior domain with evaporitic deposits. We sampled the groundwater into piezometers across an island at the southwest of the Delta, and drilled sediments. The geochemical analysis of the water exhibits two different compositions: a fresh water in the floodplain and a saline water in the interior part. These inner samples display enrichments in heavy metals, metalloids and critical metals such as, Rare Earth Elements (REE). The latest show a continuous enrichment from LREE to HREE and positive Ce and Eu anomalies, associated to alkaline pH. Overall, the two types of displayed REE patterns fingerprint two different water sources. The geochemistry of sediments indicates the same composition whatever the sample location and no traces of evaporitic deposits was found. The variations in the water table depth in the piezometers shows different hydrological behaviors in between the floodplain and the barren zone which does not follow the flood cycle. Thus, we document two different aquifers with independent hydrological behaviors: a surface fresh‐water aquifer in the floodplain connected to the channels, and a saline confined aquifer below the islands. The contrasted geochemical composition of the waters explains the distribution of the vegetation which survives through access to fresh water. The hydrological architecture of this island in the southwestern Delta differs from other islands, highlighting the hydrological complexity of the Okavango Delta.

Abstract Chemical weathering in the Himalayan river basins plays a key role in the global carbon cycle that controls the climate. In this contribution, the spatial distribution of dissolved major ions and (inorganic) carbon isotopic compositions of the Teesta River, a major tributary of the Brahmaputra River, has been investigated to constrain solute sources, weathering patterns, and acids involved in these processes. Mixing diagrams and piper plots of major ions indicate a dominant solutes supply by weathering of silicates and carbonates. The average sulfate concentration of the Teesta samples (92 ± 47 µM) is significantly higher than that reported earlier for atmospheric deposition (∼5 µM) in this basin. The carbon isotopic compositions of the dissolved inorganic fractions (δ13CDIC) of the samples vary between −11.87‰ and −3.82‰ (mean: −7‰ ± 2‰). Comparison of δ13CDIC – SO4/(SO4 + HCO3) – HCO3/(Ca + Mg) trends with their corresponding values expected for H2SO4 and H2CO3 acid‐mediated weathering indicates that both these acids contribute to chemical weathering in the Teesta river catchment. Inversion modeling of major ions and δ13CDIC data have been used to quantify the impact of carbonic (H2CO3) and sulfuric acids (H2SO4) on the Teesta water chemistry. On average, the fractions of cations derived from H2CO3‐mediated weathering of carbonates (0.27 ± 0.14) and silicates (0.20 ± 0.09) are comparable with H2SO4‐mediated carbonate (0.38 ± 0.18) and silicate (0.15 ± 0.11) weathering in this basin. These results show that the amount of CO2 uptake (via H2CO3‐mediated silicate weathering) and release (via H2SO4‐mediated carbonate weathering) are in balance for the Teesta River.

Abstract The spatial distribution of the geochemical domains hosting recycled crust and primordial (high‐3He/4He) reservoirs, and how they are linked to mantle convection, are poorly understood. Two continent‐sized seismic anomalies located near the core‐mantle boundary—called the Large Low Shear Wave Velocity Provinces (LLSVPs)—are potential geochemical reservoir hosts. It has been suggested that high‐3He/4He hotspots are spatially confined to the LLSVPs, hotspots sampling recycled continental crust are associated with only one of the LLSVPs, and recycled continental crust shows no relationship with latitude. We reevaluate the links between LLSVPs and isotopic signatures of hotspot lavas using improved mantle flow models including plume conduit advection. While most hotspots with the highest‐3He/4He can indeed be traced to the LLSVP interiors, at least one high‐3He/4He hotspot, Yellowstone, is located outside of the LLSVPs. This suggests high‐3He/4He is not geographically confined to the LLSVPs. Instead, a positive correlation between hotspot buoyancy flux and maximum hotspot 3He/4He suggests that it is plume dynamics (i.e., buoyancy), not geography, which determines whether a dense, deep, and possibly widespread high‐3He/4He reservoir is entrained. We also show that plume‐fed EM hotspots (enriched mantle, with low‐143Nd/144Nd), signaling recycled continental crust, are spatially linked to both LLSVPs, and located primarily in the southern hemisphere. Lastly, we confirm that hotspots sampling HIMU (“high‐μ,” or high 238U/204Pb) domains are not spatially limited to the LLSVPs. These findings clarify and advance our understanding of deep mantle reservoir distributions, and we discuss how continental and oceanic crust subduction is consistent with the spatial decoupling of EM and HIMU.