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Along the Central Indian Ridge (CIR), the geochemical and isotopic signature of mid-ocean ridge basalts (MORB) from three segments between 8° and 12°S show a FOZO-like enrichment with elevated 3He/4He (R/R A) (> 10 R A), suggesting small-scale upwelling with a deep primordial mantle component. Based on complementary major and trace elements analysis as well as Sr, Nd and Pb radiogenic isotope compositions of MORB samples from the same location, we confirm the presence of a FOZO/C-like enriched signature characterized by high Pb isotope ratios (206Pb/204Pb = 18.1134–19.1481; 207Pb/204Pb = 15.4710–15.6146 and 208Pb/204Pb = 37.8625–39.0332), relatively low 87Sr/86Sr (0.702767–0.702974) and high 143Nd/144Nd (0.512989–0.513118). In agreement with recent seismological studies highlighting an asthenospheric anomaly centered under the Mascarene Basin and flowing beneath the CIR, we propose that this plume-like anomaly named Mascarene Basin Asthenosphere Reservoir (MBAR) is the source of the enriched FOZO/C-like signature observed along this portion of the CIR. Furthermore, our analyses reveal that the source of the MBAR anomaly may have a unique geochemical signature in the Indian Ocean, distinct from other Indo-African plumes such as the Réunion plume. Moreover, the primordial characteristics, and the location of the MBAR (next to the margin of the African LLSVP) suggest that this plume-like anomaly may be a disconnected blob/proto-plume of a broad mantle structure formed by different Indo-African plumes/upwelling (e.g., Afar and Bouvet plumes) and anchored in the lower mantle. • Sr-Nd-Pb isotope and trace element confirm a FOZO/C-like signature for CIR MORB. • Interaction between Mascarene Basin Asthenosphere Reservoir (MBAR) and CIR. • MBAR may be an aborted plume branch from the deep mantle, from the African LLSVP. • MBAR unlikely originated from the same plume branch conduit as the Réunion plume. • MBAR component may show a unique geochemical signature in the Indian Ocean. [ABSTRACT FROM AUTHOR]

This study investigated the hydrogeochemical variations in a scenario following a landslide event in the Soberbo pristine rainforest small catchment (13 km2). The selected catchment is underlain by granite gneiss and located at the Serra dos Órgãos a mountainous region in the Rio de Janeiro, Brazil, that faces frequent high-intensity rainfall events. Stream water sampling, water discharge, and pH and electrical conductivity measurements were conducted monthly from May 2018 to October 2019. In January 2019, a landslide occurred upstream. Concentrations of dissolved organic carbon (DOC), HCO 3 −, major elements (Ca, K, Mg, Na, and Si), and trace elements (Al, Ba, Ce, Fe, Mn, Nd, Sr, Rb, La, Pr and Sm) were determined. The Spearman correlation and the multivariate analysis (PCA) indicated that during baseflow, rock weathering hydrolysis reactions are the main source of Si, Na, Mg, Ba and Sr. On the other hand, the forest plays a significant role during the rainy months by remobilizing colloidal litter material (represented by the DOC concentration) closely associated with colloidal Al. The landslide promoted a change in local redox conditions, resulting in the early release of Mn-scavenged elements, and about a month later, Fe and LREE, following a deepening in reduced conditions. The Ce/La molar ratio appeared to be a reliable hydrogeochemical redox proxy. The landslide effects persisted for at least nine months. In this case, the traceability of natural geochemical impacts of landslides was achieved through the river hydrogeochemistry in a mountainous small catchment model area. [Display omitted] • Hydrogeochemistry of a small catchment was monitored before and after a landslide. • Ce/La molar ratio attested the anoxicity that mobilized Mn, Fe and LREE. • Forest role: DOC close association with colloidal Al and LREE in rainy periods. • Si, Na, Mg, Ba and Sr concentrations are related to baseflow. [ABSTRACT FROM AUTHOR]

The Ediacaran–Cambrian transition was one of the major transitions in Earth's history, with mass extinctions and radiation of animals. This event has been linked with perturbations in ocean geochemistry, including redox conditions and the oxygenation of Earth's surface environments. Here we present a highly resolved cadmium isotope record (δ114Cd) from shales deposited in a deep-water environment covering the Late Ediacaran to Cambrian Stage 4. Our δ114Cd data vary from −0.70 to +0.35 ‰ and demonstrate the importance of local redox-dependent burial phases on bulk sediment Cd isotope compositions, which are superimposed on global-scale changes in the Cd cycle. The heaviest Cd isotope composition recorded in the early Cambrian Liuchapo Formation is similar to the modern deep ocean, implying a broad similarity in global Cd cycling, perhaps involving similar organic-Cd burial rates. The provision of bio-limiting nutrients by continental weathering with rising oxygen concentrations in Earth's surface environments could have expanded the habitable area of the ocean and contributed to animal diversification. • Secular δ114/110Cd variation for the Ediacaran–Cambrian transition in South China. • Cd isotope composition is controlled by local redox condition and global Cd cycle. • Inferred Cambrian deep seawater δ114/110Cd values are comparable to modern ocean. • Near-modern primary productivity level in the early Cambrian ocean. [ABSTRACT FROM AUTHOR]

Igneous rocks in central Iran preserve proof of several episodes of Late Mesozoic–Cenozoic Neotethyan subduction, and record temporal and spatial variations from subduction to collisional geodynamic regimes that remain a subject of debate. We revisit magmatic rocks from the central Urumieh–Dokhtar magmatic arc (UDMA; 32° 30′ N to 36° 00′ N) and report new major and trace element analyses, whole-rock Sr–Nd–Pb-isotopic data and zircon U–Pb–Hf ages on plutonic rocks from the Kajan region. The data reveal magmatic pulses between 32 and 22 Ma that are represented by igneous rocks with geochemical and isotopic features resulting from melting of a metasomatized, enriched mantle. Modeling indicates the samples represent a mixture of 97% batch melt (from a source with 98.5% mantle melt +1.5% terrigenous sediment) with 3% upper continental crust. The oldest yielded age (i.e., 32 Ma) rules out a lull in magmatism in the central UDMA between 37 and 26 Ma and speaks against distinct flare-up magmatic episodes at ∼54–37 Ma and ∼ 20–5 Ma in the frontal arc. We hypothesize that magmatism records down-going slab tearing into two pieces beneath the central to SE UDMA, which then changes the geometry and buoyancy of subducting Neotethyan lithosphere. The down-going slab remained involved in central UDMA (i.e., rollback) during Early Miocene whilst arc retreat beneath the SE UDMA proceeded with detachment of subducted oceanic lithosphere (i.e., break-off). We hypothesize a diachronous collision in the UDMA, first in the southeastern segments, and subsequently in the central UDMA. These findings bear important implications for the geodynamic evolution of the Zagros orogen and run counter to a model suggesting that the collision initiated in the northwest to central and propagated progressively to the southeast along the Zagros suture zone. [ABSTRACT FROM AUTHOR]

Microbialites are microbial sedimentary structures that constitute some of the oldest traces of life on Earth. By their deposition in a wide range of sedimentary environments and their presence throughout most of geological time, the sedimentological and geochemical signatures they preserve represent important paleoenvironmental archives for understanding Earth's biological and geochemical co-evolution. Here we present a large microbialite collection containing >1370 curated specimens, covering all continents except Antarctica and spanning >3.5 Ga of Earth history, that is accessible to the international scientific community for examination and sampling at the Muséum National d'Histoire Naturelle (MNHN) in Paris, France. After cataloguing and evaluating the samples for their lithology, biogenicity, and inferred depositional environments, we characterized the collection for selected geochemical parameters, notably carbonate stable carbon and oxygen isotope ratios, as well as major, trace, and rare earth element compositions. Finally, we explore the different geochemical proxies analyzed with regards to their utility for reconstructing evolving Earth surface environments and/or microbial metabolisms via comparison of geochemical data from the MNHN Microbialite Collection to a compilation of similar proxy data for carbonates worldwide. We demonstrate that certain temporal trends previously recognized in carbonates worldwide (e.g., with respect to variations in C and O stable isotope compositions and redox sensitive trace element enrichments) are well reflected in this collection. Our findings highlight the utility of the MNHN Microbialite Collection and microbialites more generally for reconstructing the conditions associated with habitable environments in deep time and for tracing the response of microbial communities to the geochemical evolution of Earth's surface. • we present the MNHN Microbialite Collection. • a curated public sample archive with associated metadata. • carbonate C and O isotope data show trends comparable to carbonates in literature. • REE and redox-sensitive tracers also comparable to carbonates in literature. • carbonate microbialites may indeed record paleoenvironmental signatures. [ABSTRACT FROM AUTHOR]

The geochemistry of the Ría de Huelva estuary (Spain) has been widely studied owing to its unique conditions due to mixing of acid mine drainage (AMD), river water and seawater, which have been aggravated by the existence of an adjacent phosphate fertilizer plant and its associated phosphogypsum stack. To better understand the complex geochemistry of the overall estuarine system, we make use of (1) measured concentrations of relevant trace elements (rare earth elements (REEs) and metal(loid)s) in the sediments and waters of the estuary and (2) geochemical modeling which combines mixing and adsorption processes. Our study sheds light on the elemental distribution in the sediments and water in the water-mixing area. As seawater neutralizes the acidity of the river water (pH increases to 6.2) colloids of Fe- and Al-oxyhydroxysulphates (schwertmannite and basaluminite, respectively) precipitate as concentrations of aqueous Fe and Al increase. These phases have a high adsorption capacity and play a critical role in the geochemistry of the aquatic system by retaining Cu, Pb, Cr, As, P, REEs and smaller amounts of Zn, Co, Ni and Mn. The geochemical model reproduces the behaviour of the aqueous REEs, requiring high S/L ratios at pH ≥ 3.5 and participation of sediments and colloids in the reactions to match the field data. The evaluation of the effect of the adjacent phosphogypsum stack on the estuarine geochemistry shows that the stack has a notorious impact on the geochemistry of the surrounding estuarine environment owing to the release of high quantities of phosphate. [ABSTRACT FROM AUTHOR]

The geochemistry of arc rocks provides a window to probe compositional variability in mantle wedge and slab-derived inputs. However, quantitative estimation of their contribution is a great challenge. Here we report an integrated study for determination of variable contributions to early Paleozoic arc plutons (the Mafan diorites and gabbros), from the northern Hong'an orogen, central China. Zircon grains from the diorites yielded crystallization age of 445 ± 3 Ma, whereas the gabbros have zircon U Pb age of 432 ± 3 Ma. Although both the two mafic rocks suites exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li–Mo–O and radiogenic Sr–Nd–Hf isotope compositions, as well as major and trace element features. Geochemically, the diorites are medium-K calc-alkaline series with high and positive whole-rock ε Nd (t) (+3.8 to +4.5) and zircon ε Hf (t) values (+12.4 to +14.3), and normal mantle-like zircon δ18O values (5.1 ± 0.2 ‰). Contrarily, the gabbros are tholeiitic and have relatively low whole-rock ε Nd (t) (+ 2.3 to +3.1) and zircon ε Hf (t) (+5.0 to +8.5) values with normal mantle-like zircon δ18O values (5.4 ± 0.3 ‰). The diorites have δ98Mo values of −0.15 to −0.03 ‰, while the gabbros have δ98Mo values of −0.28 to −0.12 ‰. These values are slightly higher than those of the mid-ocean ridge basalts (MORB) (−0.20 ± 0.01 ‰). Whereas, δ7Li values of the studied diorites (2.15–4.49 ‰) and gabbros (2.89–5.33 ‰) are mostly consistent within the range of MORB. In combination with the above results, the primary magma of diorites was inferred to be derived from 1 to 5% partial melting of depleted lithospheric mantle metasomatized by slab-derived fluid with minor sediment-derived melts. While the gabbros were suggested to be generated by 5–10% partial melting of more depleted lithospheric mantle that had undergone perhaps extra 0.3% melt extraction, which had further been metasomatized by more subducted materials. In summary, both the slab-derived components and nature of the mantle wedge sources are quantitatively constrained for the mafic rocks in the Mafan plutons. Our new data suggest that the Mafan plutons exhibited the Mariana arc-affinity and represented products of an intra-oceanic subduction system in the northern Hong'an orogen. Combined with previously published data for the early Paleozoic mafic magmatic rocks along the Qinling-Tongbai-Hong'an orogenic belt, there exists an oceanic arc in the east (Hong'an orogen) and an Andean-type continental arc in the west (Qinling-Tongbai orogen). • The Mafan plutons are composed of diorites (ca. 445 Ma) and gabbros (ca. 432 Ma) with systematically geochemical variations. • Both more depleted mantle wedge and more slab-derived materials metasomatized are required for generating the Mafan gabbros. • The Mafan plutons represented products of an intra-oceanic subduction system. [ABSTRACT FROM AUTHOR]

Water plays a critical role in the formation of granitic magmas and continental crust, but distinguishing water-present and water-absent anatectic scenarios using the geochemistry of granites is controversial. In this study we use an integrated approach that combines whole-rock major and trace element geochemistry, Ti-in-zircon thermometry, phase equilibrium modeling and trace element modeling to study the water regime that produced the Miocene granites from the Malashan-Gyirong area in central Himalaya to solve this controversy. The Gyirong granites have relatively low CaO, Sr and Ba and high Rb and (87Sr/86Sr) i and the Malashan granites have relatively high CaO, Sr and Ba and low Rb and (87Sr/86Sr) i , and were classified as group A and group B granites, respectively. Following previous interpretation, group A and group B granites are consistent with the products of water-absent and water-present melting of metasedimentary rocks, respectively. Ti-in-zircon thermometry yielded maximum values of 761–796 °C for the Gyirong granites and 730–764 °C for the Malashan granites. Distinct variation trends in zircon trace element compositions indicate that these two groups of granites were not linked by crystallization differentiation. Using average compositions of Proterozoic pelite as starting materials, phase equilibrium modeling was carried out at a variety of P–T–H 2 O conditions typical of the Himalayan orogen, P = 5, 10 and 15 kbar, T = 600–800 °C, H 2 O = 0–10 wt%. Pelite can produce melts with coupled CaO–Na 2 O contents for both groups at 10 kbar. Specifically, constraints from compositions and temperatures require that the bulk H 2 O content is ca. 1–2 wt% for group A granites and > ca. 4 wt% for group B granites. Compared with the maximum structural water content of the pelite at 10 kbar (1.77 wt%), this study testifies that group A granites formed under water-absent conditions and group B granites under water-present melting conditions. Modeling shows that water-present melting can produce melts with high Sr–Ba and low Rb contents resembling group B granites, while water-absent melting can produce melts with low Sr–Ba and high Rb contents resembling group A granites. This study highlights that water can indeed cause differences in granite geochemistry but a comprehensive investigation is required to better determine the role of water during crustal anatexis. [ABSTRACT FROM AUTHOR]

High-pressure/low-temperature rocks from two well-studied outcrops of the Cycladic Blueschist Unit on Syros are used to demonstrate the advantages of volume-coupled 87Rb/87Sr geochronology and geochemistry of in situ white mica. Single-spot Rb/Sr dates are interpreted based on the microstructural position of white mica combined with its chemical information, and the results are discussed within the tectonic framework of the island. The first outcrop on Katerghaki cape exposes rigid blocks of foliated eclogite within well-foliated blueschist, whereas the second outcrop on Delfini peninsula comprises massive eclogitic blocks, likely of metasomatic origin, hosted within blueschist that are interlayered with white mica schists. The rheological contrasts exhibited in both outcrops preserve (near-)peak HP/LT metamorphism/metasomatism (eclogites) followed by retrogression and deformation in lower P-T conditions (blueschist, micaschist). The white mica in eclogite from both outcrops are in equilibrium with the HP/LT mineral assemblages, do not show chemical zoning nor systematic chemical variations, and yield uniform single-spot Rb/Sr dates. Weighted averages of the Rb/Sr dates are 48.2 ± 3.7 Ma (MSWD: 0.9, n: 34; Katerghaki) and 44.5 ± 3.1 Ma (MSWD: 0.7, n: 28; Delfini), interpreted as (near-)peak HP/LT metamorphism and metasomatism, respectively, overlapping with published garnet Lu Hf and Sm Nd geochronology of the corresponding outcrops. White mica in the Katerghaki blueschist and the Delfini micaschist exhibit significant chemical zoning and correlations of Ti with major and trace elements (Al, Fe, Mg, Rb, Sr, Zn, Ba, Nb). Weighted age averages were calculated from Rb/Sr dates corresponding to high-Ti and low-Ti white mica compositions. The blueschist yields ages of 56.0 ± 10.6 Ma (MSWD: 2.2, n: 20/80) and 36.3 ± 2.1 Ma (MSWD: 0.9, n: 20/80), whereas the micaschist produced ages of 43.7 ± 3.5 Ma (MSWD: 0.5, n: 14/53) and 31.9 ± 2.7 Ma (MSWD: 1.0, n: 13/53), respectively. These results are interpreted to resolve the timing of fluid-mediated isotopic re-equilibration of white mica during deformation and retrogression (low-Ti), and reflect 87Sr/86Sr inheritance from the earlier HP/LT history (high-Ti). Overall, the results demonstrate that coupling 87Rb/87Sr geochronology with geochemistry of white mica in situ is advantageous for evaluating Rb/Sr datasets that may possess a single mica population (eclogites) or are affected by 87Sr/86Sr inheritance (blueschist, micaschist) in order to interpret geologically meaningful ages. [ABSTRACT FROM AUTHOR]

The compositional evolution of magmatic-hydrothermal fluids from the Sn W mineralized Cornubian batholith was investigated via in situ fluid inclusion LA-ICP-MS microanalysis and reveals a large degree of variation between intrusive stages and at the sample scale, with complexities due to superposition of several mechanisms affecting fluids chemistry during the protracted evolution of the batholith. Despite large ranges of salinities (from <1 wt% to 48 wt% NaCl eq.), the effects of individual processes such as magmatic differentiation, fluid saturation and boiling, fluid mixing and dilution can all be discerned on the basis of fluid inclusion trace element geochemistry combined with detailed petrography and microthermometry. In all samples, different fluid types (aqueous liquid-rich, vapor-rich and brine inclusions) were identified in variable proportions and microthermometry revealed complex temporal trends. Granite-related samples from the G5 intrusive stage are characterized by intermediate density fluids undergoing magmatic fractionation under pressure conditions above 1.5 kbar, indicated by successively decreasing fluid salinities with concomitant increases in Li, B, Rb and Cs concentrations. The latter two elements later strongly partitioned in the brine phase upon boiling, together with most of the other Cl-complexing elements (e.g., Fe, Mn, Pb, Zn). Those brines subsequently underwent progressive dilution with meteoric waters that caused a decrease in the concentration of all elements (besides B) and homogenization temperatures. In G3 granite from Dartmoor, on the other hand, brine inclusions represent the earliest fluid type while intermediate density fluids are absent, and a large salinity range in the fluid inclusion record associated with a decrease in homogenization temperatures represents their progressive dilution with meteoric fluid. The lack of intermediate density fluids at Dartmoor indicates fluid exsolution at relatively low pressures resulting in immediate phase separation into brine and vapor. All studied samples associated with ore mineralization display only the trend of dilution of high-salinity magmatic fluids. Their transition metal contents (e.g. Fe, Mn, Pb and Zn) show the expected positive correlation with salinity of the fluids. Conversely, the compositional trends observed for Sn and particularly W are more enigmatic, as they do not appear to significantly fractionate during most of the physicochemical processes outlined above and appear to be unrelated to fluid salinity, opposite of what would be expected for Sn as it is dominantly transported in Cl-complexes. This study highlights the key advantages of high-resolution geochemical fluid inclusion studies in discerning a variety of magmatic and post-magmatic processes in fluids in comparison to bulk fluid inclusion techniques or more simplistic fluid inclusions studies which might overlook important aspects of the typically complex history of fluids evolution in magmatic-hydrothermal systems. • High-pressure magmatic fractionation produces clear trends in fluids chemistry. • Meteoric waters ubiquitously diluted magmatic fluids in the Cornubian batholith. • In situ LA-ICP-MS provides high resolution for complex fluid inclusion record. [ABSTRACT FROM AUTHOR]

Belemnites were a type of cephalopod abundant in the Jurassic and Cretaceous, whose fossils have been extensively used for paleo-oceanographic studies. For this study, we analyzed 69 elements by Inductively Coupled Plasma-Sector Field Mass Spectrometry (ICP-SFMS) in 15 belemnite rostra from the West Rodiles section in the Asturian Basin, Northern Spain. We aim to determine if belemnite rostra carbonate chemistry reflects changes in seawater chemistry during the Late Pliensbachian–Early Toarcian time interval and examine how belemnites can be used to trace known drivers of the Early Toarcian Oceanic Anoxic Event (T-OAE) and associated oceanographic processes. Discarding drastic intra-species effects and assuming a similar influence of growth rates, diet, and other physiological processes for all analyzed belemnite specimens and taking into consideration the sizeable analytical uncertainty, we assess if determined belemnite rostra element chemistry reflects broad and relative changes in paleo-seawater chemistry. Many determined elements are present in only ppb amounts, and their interpretation is uncertain. We found that Mg, Mn, and P increase in the interval chronocorrelative to the T-OAE. This is interpreted to have resulted from an increase in these elements' inventory in seawater due to an increase in continental weathering and fluvial runoff associated with this global event. Iron, K, and Na contents decrease upwards in the section, potentially indicating that these elements became limited and likely hampered oceanic productivity. Our study also found a correspondence between a change in the behaviour of several elements, warming, the T-OAE negative CIE, and a reduction in the diversity and size of both calcareous nannofossils and belemnites in the study area. • We study Early Toarcian seawater chemistry and T-OAE drivers. • 69 elements in 15 Spain belemnites rostra were analyzed. • Mg, Mn, and P rose due to weathering. • Fe, K, and Na declined, impacting ocean productivity and reducing nannofossil size. [ABSTRACT FROM AUTHOR]

Mineralogy, geochemistry and sulfur (S) isotope composition of native sulfur (S0) precipitated at intraoceanic and intracontinental back-arc rift, submarine and subaerial volcanic arc, sediment and sediment-free mid-ocean ridge, hot spot, accretionary wedge, and submarine and continental cave settings were investigated with a main goal to understand the mode of formation of all the types of native S at the modern seafloor. Native S occurs as various depositional forms: chimneys, colloform flows, liquid ponds, slabs; fills in cavities and pore space of the sediment, vesicles and cracks in volcanic rocks; cements and coats the sediment; stains the conduits or fills in pores of the sulfide chimneys; forms fine-grained layers within the sediment; coats the walls, stalactites and stalagmites in the caves. Mineralogically, the native S from the seafloor is pure rhombic S0 with negligible impurities of metal sulfides, aluminosilicates, and carbonates inferred from the chemistry data. Mineral interrelations and S isotope data suggest that native S from different geologic settings has different origin. In the sulfide chimneys and mounds at the mid-ocean ridges, native S appears to be a result of oxidative alteration of primary pyrrhotite. The native S from back-arc rifts, volcanic arcs and hot spots can be a result of either disproportionation of magmatic SO 2 (δ34S < 0 ‰), or synproportionation of magmatic SO 2 and H 2 S (δ34S > 0 ‰). The native S from the sediments in anoxic brine-filled deeps (accretionary wedge setting) is a result of bacterial sulfate reduction and consequent sulfide (δ34S < 0 ‰) oxidation. The native S coating the cave walls and forms also has a bacterial origin (δ34S < 0 ‰). • Native S precipitates in a variety of seafloor settings. • Oxidative alteration of pyrrhotite results in S0 formation at the mid-ocean ridges. • S0 forms via disproportionation of SO 2 at volcanic arcs, back-arcs and hot spots. • SO 2 - H 2 S synproportionation results in S0 formation at volcanic arcs and back-arcs. • Bacterial sulfate reduction and sulfide oxidation play a role in S0 precipitation. [ABSTRACT FROM AUTHOR]

Halite deposits have long been utilized for interrogating past climate conditions. Microthermometry on halite fluid inclusions has been used to determine ancient water temperatures. One notable obstacle in performing microthermometric measurements, however, is the lack of a vapor bubble in the single-phase liquid inclusions at room temperature. (Pseudo-) isochoric cooling of the inclusions to high negative pressures, far below the homogenization temperature, has commonly been needed to provoke spontaneous vapor bubble nucleation in the liquid. High internal tensile stress in soft host minerals like halite, however, may induce plastic deformation of the inclusion walls, resulting in a wide scatter of measured homogenization temperatures. Nucleation-assisted (NA) microthermometry, in contrast, employs single ultra-short laser pulses provided by a femtosecond laser to stimulate vapor bubble nucleation in metastable liquid inclusions slightly below the expected homogenization temperature. This technique allows for repeated vapor bubble nucleation in selected fluid inclusions without affecting the volumetric properties of the inclusions, and yields highly precise and accurate homogenization temperatures. In this study, we apply, for the first time, NA microthermometry to fluid inclusions in halite and we evaluate the precision and accuracy of this thermometer utilizing (i) synthetic halite crystals precipitated under controlled laboratory conditions, (ii) modern natural halite that precipitated in the 1980s in the Dead Sea, and (iii) Late Pleistocene halite samples from a sediment core from Death Valley, CA. Our results demonstrate an unprecedented accuracy and precision of the method that provides a new opportunity to reconstruct reliable quantitative temperature records from evaporite archives. • Fluid inclusions trapped in halite host crystals are valuable paleoclimate archives. • Primary halite inclusions act as physical thermometers at time of entrapment. • Nucleation-assisted microthermometry is precise and accurate in halite inclusions. • Formation temperatures are measured for Lab-grown, Dead Sea, & Death Valley halites. [ABSTRACT FROM AUTHOR]

Understanding the chemical composition of materials within the subducting slab is crucial for studying slab dehydration and melting mechanisms. Previous research has primarily focused on examining sediments and altered basalts collected near the seafloor, which has overlooked the chemical changes occurring within the forearc subduction channel. The Mariana forearc serpentinite mud volcanoes offer a unique opportunity to investigate metabasite xenoliths, providing valuable insights into the compositional changes of subducting slab materials at forearc depths. In this study, we present Pb-Nd-Hf isotopic data for blueschist-facies mafic clasts obtained from the Asùt Tesoru serpentinite seamount in the Mariana forearc. These data are combined with existing Sr isotope and trace element data to address the aforementioned issues. The mafic clasts display Nd Hf isotopic signatures reminiscent of the HIMU- and EM1-type basalts discovered in the Magellan seamounts. However, their Sr Pb isotopes display mixed signatures, indicating the influence of sediments and altered mid-ocean ridge basalts. These findings underscore the importance of serpentinizing fluids resulting from the interaction between sediment pore water and forearc mantle peridotite in altering the Sr Pb elemental and isotopic compositions of subducted slab materials. This process is achieved through interactions between slab materials and serpentinizing fluids within the subduction channel. It is important to emphasize that the blueschist-facies mafic clasts themselves do not directly serve as source materials for arc volcanism. However, their Sr Pb isotopes can undergo further modifications through exchanges with unaltered seamount basalts facilitated by serpentinizing fluids within the deep subduction channel, which extends from depths of 30 to 100 km. In this scenario, the mafic clasts with modified isotopic signatures could potentially contribute as source materials for arc volcanism. • The protolith of the metabasite is HIMU- and EM1-type seamount basalts. • The seamount basalts interacted with serpentinizing fluid in the subduction channel. • Serpentinizing fluid modified the Sr Pb isotopes of the metabasite. [ABSTRACT FROM AUTHOR]

The impact of warming, acidification, and deoxygenation on deep-sea environments is a growing concern. Historical records are sparse, particularly at high latitudes, making climate change projections challenging. Indirect proxies, such as trace element composition of marine carbonates, such as coral skeletons, can offer an alternative method to fill data gaps but have not been realised. Here, using Laser Ablation Triple-Quadrupole Inductively Coupled Plasma Mass Spectrometry (LA-QQQ-ICP-MS), we examined micrometre-scale element variation within and between individual colonies of the bamboo coral Keratoisis sp. obtained from the Eastern Canadian Arctic. These data are used to assess the influence of biological variability on geochemical tracers for reconstructing past environmental conditions (temperature: Mg/Ca, Li/Mg, Sr/Ca, Ba/Ca, U/Ca; [Ba] SW : Ba/Ca). We place these data into context, based on a survey of literature data, using refined calibrations for high-Mg calcitic Octocorals. We find reproducible (2σ relative coefficient of variation) values of Mg/Ca (3%) and Ba/Ca (6%) along the radial growth axis of all colonies and internodes of Keratoisis sp., indicating that these signals are likely suitable for environmental reconstructions. After revising the available multi-taxa calibrations for Mg/Ca (0.316 ± 0.026 °C/mmol/mol, R2 = 0.87, p < 0.001) and Ba/Ca ([Ba/Ca μmol/mol] = 0.148 ± 0.005 [Ba SW nmol/kg], R2 = 0.97, p < 0.001), we show that vital effects within and among Keratoisis sp. colonies strongly influence reconstructed temperature and [Ba] SW , but this can be somewhat mitigated by combining multiple internode transects from one colony into a single composite series. Despite the ontogenetic variability, all colonies reveal a gradual deep-water cooling trend since the early 21st century and synchronised, multi-year spikes in Ba/Ca (and hence [Ba] SW) that suggest substantial and coherent barium inputs to the seafloor. Our study confirms the reliability of Mg/Ca and Ba/Ca proxies in high-Mg bamboo corals for detecting multi-annual temperature and seawater barium variations in cold-water environments, but further investigation into micro-scale element behaviour influenced by biotic processes in these corals is needed to enhance confidence in reconstructions at finer spatial and temporal resolutions. We conclude that employing empirical calibrations based on multi-taxa approaches can increase the certainty of capturing regional changes in the environment more accurately than a single species calibration, while leveraging multiple element series to account for biological-induced variability improves single colony reconstructions. • Trace element patterns in Arctic Keratoisis vary between and within colonies. • Leveraging element patterns across internodes improves reproducibility of signals. • Integrating intra-specificity in living proxies enhances deep-water reconstructions. [ABSTRACT FROM AUTHOR]

Tonalitic–trondhjemitic–granodiorite (TTG)-like leucosomes, which are formed from anatectic melts, record crustal growth and evolution and thus provide constraints on the process of anatexis. However, potential Mg isotope disequilibrium during crustal anatexis makes interpreting the genesis of anatectic melts challenging, thereby potentially hindering the application of the Mg isotope system in investigations of crustal evolution and crustal anatexis. A key factor in Mg isotope fractionation during anatexis is the behaviour of garnet, which becomes preferentially enriched in light Mg isotopes compared with other Mg-hosting minerals during partial melting processes. In this study, a combination of petrography, whole-rock geochemistry, and mineral microanalysis reveals that garnets in leucosomes from the Dulan area of the North Qaidam orogen, were formed by dehydration melting of hornblende. Garnet-bearing leucosomes yield low δ26Mg values (−0.70‰ to −0.33‰) compared with garnet-free leucosomes (−0.19‰ to −0.16‰) and global arc rocks (−0.35‰ to 0.06‰). The different Mg isotopic compositions are attributed to the effect of peritectic garnets on the Mg isotope system in anatectic melts. Quantitative modelling further indicates that the lower Mg isotopic values of garnet-bearing leucosomes require the incorporation of ∼5% peritectic garnets into anatectic melts during magma ascent. Our results demonstrate the influence of peritectic garnets on the Mg stable isotope system and highlight that this influence on the Mg isotopic compositions of peritectic minerals during crustal anatexis must be considered in interpretations of crustal evolution using the Mg isotope system. • The Grt-bearing TTG-like leucosomes show low δ26Mg values than Grt-free ones. • The Grt-bearing leucosomes could be explained by incongruent melting and garnet entrainment. • Garnet entrainment is first-order factor of δ26Mg values of TTG-like melts. [ABSTRACT FROM AUTHOR]

This study seeks to refamiliarize the geochemical community with Niggli Numbers and explore how Paul Niggli's largely forgotten scheme for representing major elements as equivalent molecular numbers that are combined as groups, sums and ratios. This scheme provides simple and effective tools for tackling many modern geochemical problems – particularly to reduce complexity and degrees of freedom where there are requirements to handle large volumes of data. Niggli Numbers provide an alternative to ratio-based methods for the long-recognised problem of the constant sum and data closure in geochemistry and offer a means to represent minerals and their relationships in a straightforward and consistent chemical framework. The Niggli calculation scheme follows simple rules that are easily automated, is flexible enough to accommodate missing elements or partial analyses, and is appropriate for all rock types. Following a short review of the most useful Niggli binary diagrams and where common minerals plot in Niggli space, a series of case studies are presented to illustrate potential applications of the scheme in modern petrology and resource exploration. These cover the following: magmatic evolution of a layered intrusion such as the Bushveld Complex; contamination of magmas by country rocks; tracing hydrothermal alteration in volcanogenic massive sulfide and porphyry Cu Mo deposits; and chemostratigraphy of sedimentary sequences. [Display omitted] • Niggli Numbers are simple and efficient tools for geochemical interpretation. • Niggli Number plots can effectively relate bulk chemistry to mineralogy. • Niggli Numbers help to illustrate mineralogical controls that vary with stratigraphy. • Niggli Numbers are an alternative to ratio-based methods for the closed sum problem. • Applications: evolution/contamination of magmas, alteration and chemostratigraphy. [ABSTRACT FROM AUTHOR]

The Cariboo Plateau in central British Columbia, Canada, hosts the largest aerial extent of low temperature lacustrine magnesite in the world. Magnesite was the most abundant carbonate mineral in 21 of 33 sampled alkaline lakes and 20 of 22 sampled playas. Calcite, hydromagnesite and very high magnesium calcite (VHMC, also called disordered dolomite) were also found in closed basin alkaline lakes on the plateau. Carbonate mineralogy was strongly associated with lake water geochemistry, particularly total alkalinity. VHMC was the dominant carbonate mineral in lakes with the highest alkalinities (>1000 mEq/kg), magnesite was the dominant carbonate mineral in lakes in the middle range of alkalinities (generally 70 to 800 mEq/kg) and hydromagnesite with minor amounts of magnesite was the phase assemblage in lakes with lower relative alkalinities (<110 mEq/kg). The geochemical trends for the activity of CO 3 2−, Ca2+ and Mg2+ suggest that these constituents in all Cariboo water bodies are controlled by the precipitation of Ca- and/or Mg‑carbonate minerals. The waters are near equilibrium with metastable crystalline Ca‑carbonate and Mg‑carbonate minerals. We hypothesize that carbonates present in the lakes likely formed via non-classical crystallization pathways based on the relationship between aqueous geochemistry and mineralogy and the chemical composition of the Ca-bearing magnesites. Non-classical crystallization offers new models for thinking about magnesite formation in lacustrine environments. • Cariboo Plateau has largest aerial extent of low temperature lacustrine magnesite • Alkalinity likely controls carbonate mineralogy (VHMC, magnesite, hydromagnesite) • Lake water geochemistry is controlled by Ca- and/or Mg-carbonate precipitation • Non-classical crystallization offers new models for natural carbonate formation [ABSTRACT FROM AUTHOR]

Geochemistry of iron and trace metals in seep carbonates and its dependence on anaerobic oxidation of methane (AOM)-driven diagenesis in cold seeps are not well documented. Here we characterize geochemistry of Fe and trace metals in carbonate nodules collected at an active cold seep in the middle Okinawa Trough (OT) impacted by hydrothermal plumes. The carbonate nodules in the studied core are dominated by aragonite even at sulfate-low core bottom. Most aragonite at the core bottom may have formed much earlier close to the seafloor under high sulfate concentration. Additionally, porewater at the depth may not be developed to a condition for the formation of calcite dominating over aragonite due to constant downward replenishment of sulfate across through shallow SMTZ (40 cm below sediment surface). As indicated by δ13C and δ18O of the carbonates, anaerobic oxidation of thermogenic methane from gas hydrate dissociation is the dominant fluid source for carbonate precipitation. It can be inferred from the existence of vivianite that Fe reduction coupled to AOM (Fe-AOM) may have ever been prevalent at the hydrothermal Fe-impacted seep site. Enhanced inputs of Mo, Co, Cu, Ni, and Zn associated with hydrothermal Fe-oxyhydroxides-organic colloids is the main source for their high enrichment in the carbonate nodules, and clay minerals and AOM-related microbial organics may have played an important role in their partitioning in the nodules. Uranium enrichment in the carbonate nodules is mainly through enhanced U delivery associated with Fe-oxyhydroxides-scavenged organic colloids and additional U enrichment induced by U(VI) reduction. U(IV) incorporation into carbonate lattices dominates U partitioning in the carbonate nodules over the upper 60 cm of the seep sediments, but the incorporation is substantially dampened at depth due to an increase in concentration of dissolved inorganic carbon, which consequently may have rendered U(IV) adsorption on clay minerals as the important U sink in the nodules. Our results suggest that the exceptional enrichments of Co, Cu, Ni, Zn, Mo, and U in seep carbonates may be used as a paleo-proxy to differentiate cold-seep carbonates whose formation has been co-impacted by hydrothermal plumes. • Aragonite is the dominant carbonate mineral even at sulfate-poor core bottom. • Carbonate formation is related to AOM and hydrate dissociation. • Hydrothermal Fe input enhances trace metal enrichment in carbonate nodules. • Clays and AOM-microbial organics play an important role in trace metal partitioning. [ABSTRACT FROM AUTHOR]

The Alborz Mountains in northern Iran form part of the Tethyan orogenic belt and surround the South Caspian Basin. The geology of the western Alborz Mountains is dominated by Eocene mafic to intermediate high-K calc-alkaline-alkaline shoshonitic and minor Oligo-Miocene magmatic rocks, displaying arc geochemical characteristics (e.g., negative Nb, Ta, Ti anomalies). Cenozoic magmatism across this region in western Asia has been explained by a diversity of contrasting geodynamic models involving (multiple slab) subduction and slab-breakoff. The aim of this study is to better constrain the geodynamic setting of magmatism during regional convergence through the investigation of the relatively unstudied Alborz-Azerbaijan magmatic belt. Incompatible trace element geochemistry of Eocene lavas from this belt is distinctive and indicates that they were generated by relatively low-degrees of partial melting of the subcontinental lithospheric mantle with a contribution of asthenosphere melts. Miocene lavas from the Alborz and northern Urmia–Dokhtar magmatic arc (UDMA) share a common arc geochemical signature. Zircon εHf (t) values of the Miocene magmatic rocks from the Alborz and northern UDMA range from −0.4 to 11.7, suggesting incorporation of older continental crust mixed with a more juvenile component. New thermochronological data (fission track and (U-Th)/He on apatite) from the late Eocene plutonic bodies in the Tarom area track exhumational cooling at moderate rates following rapid post-emplacement magmatic cooling at ca. 40 Ma. The geochemical data in conjunction with geological and published geophysical results imply a bending or disruption in the subducting slab under the Tarom area, associated with slab roll-back during the Eocene. This process led to the arc-front displacement and a greater contribution of deep enriched mantle in the Alborz magmas compared to those from the high-flux magmatic event along the Alborz and Urmia–Dokhtar magmatic arc (UDMA), triggered by asthenospheric upwelling and mixing with melts derived from earlier metasomatized subcontinental lithospheric mantle. • Track the tectono-magmatic evolution of Alborz mountains via geochemical data. • Investigation of geochemical differences and similarities between Alborz and UDMA magmas. • Neotethyan slab tear influenced the magmatism type and location of arc front. [ABSTRACT FROM AUTHOR]

Ophiolitic mantle rocks play a crucial role in understanding deep geochemical cycling and the transfer of components associated with slab-mantle interactions. Geochemical and geochronological and geochemical analysis of zircon and TiO 2 polymorphs (e.g., rutile) present within mantle rocks of the Eocene New Caledonian ophiolite reveal discrete magmatic and xenocrystic populations amongst harzburgite from Me Maoya and chromitite from Tiébaghi massifs. Most mineral grains examined have a xenocrystic origin and are concentrated particularly in the harzburgite. Source tracking involved the use of U Pb dating and trace element analysis. They are inferred to have been recycled from the northern end of the Norfolk Ridge that formed the leading edge of a thin continental slab of Gondwana affinity which had previously rifted from eastern Australia together with other elements of Zealandia. The slab was drawn into an intra-oceanic subduction zone beneath the proto-Loyalty arc immediately before the emplacement of the forearc ophiolite today represented by the New Caledonian Peridotite Nappe. Geochemical and geochronological data of these xenocrystic grains support a scenario in which these minerals were recycled into the mantle wedge at moderate mantle depths (above 60–80 km depth) within the subduction channel. In contrast, Eocene U Pb ages of zircon and rutile grains, mostly found within the Tiébaghi chromitite, imply their formation through magmatic processes. These minerals' distinctive geochemical characteristics (e.g., HFSE enrichment in rutile) likely indicate an association with late-stage percolation of relatively enriched fluids and melt. Overlapping ages of the Eocene zircon and rutile grains, Ar Ar pargasite ages, and Zircon U Pb ages of cross-cutting dikes in the Tiébaghi chromitite provide evidence that it cooled at ca. 47–46 Ma. This study demonstrates that mineral geochemistry of mantle rocks, when combined with data from geochronology, mineralogy, and petrology, is critical to enhance our comprehension of forearc mantle wedge evolution. It also highlights the potential for studying recycled solid mineral phases, tracing their thermal evolution, and characterizing potential subducted source terranes. [ABSTRACT FROM AUTHOR]

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C1-C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (δ13C = - 53.5‰ V-PDB; D/H around - 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C1-C4, CO2]) relative to the other gas types and the virtual lack of 14C-CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1-C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C-CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area. © 2009 Elsevier B.V. All rights reserved.

High-resolution stable-isotope ratio data (δ18O, δ13C) were used to study growth strategies of two bivalve species, Pecten jacobaeus (calcitic shell) and Glycymeris pilosa (aragonitic shell) from the North Adriatic Sea. The principal objectives of this study were to identify the period of the year when the growth line is formed in the shell of two target species, to identify the main growing season of these two species, to identify the environmental drivers of shell growth, and to evaluate the potential applicability of δ18O and δ13C values for the reconstruction of environmental variability. Samples were collected from the North Adriatic Sea by commercial bean trawl (P. jacobaeus , December 2013 and January 2014, N = 4) and SCUBA diver (Glycymeris pilosa , March 2016, N = 3). Samples for the oxygen (δ18O) and carbon (δ13C) isotope composition of the calcium carbonate were collected by drilling the outer shell layer across several annual cycles. Temporal and spatial temperature and salinity values inside the investigated area were simulated using the 3D numerical ocean model - ROMS. The δ18O cycles corresponded to the number of seasonal growth marks observed on the external shell surface of both target species, thereby confirming the annual periodicity of these growth patterns. In February 2012, extreme cooling of the water column accompanied by dense water formation occurred in the Adriatic Sea - an event recorded by P. jacobaeus shells. This study indicates that P. jacobaeus and G. pilosa have contrasting shell growth strategies. Pecten jacobaeus grows during winter and slows shell growth during the warmest part of the year, and thereby may be an interesting archive for winter conditions. Due to its longevity and continuous growth during the warmest part of the year, G. pilosa is a promising archive for the reconstruction of summer seawater temperatures. • Pecten jacobaeus and Glycymeris pilosa from the North Adriatic have contrasting growth strategies • Growth slowdown of P. jacobaeus occurred during summer while G. pilosa grew at slowest rates during winter • Both species formed growth lines in fall, indicating that sea temperature is not the only factor regulating shell growth • Studied species can be used as archives of environmental variability in the Adriatic and Mediterranean Seas [ABSTRACT FROM AUTHOR]

Trace metals are essential micronutrients for phytoplankton, as well as useful tracers of biogeochemical processes in aquatic systems. The largest Arctic rivers are known to play an important role in the cycling of trace metals in the Arctic Ocean; however, these systems account for little more than half of Arctic freshwater discharge, leaving a major gap in our understanding of the trace metal geochemistry from smaller river systems. This is especially problematic for rivers draining the Arctic coastal margins, as these continuous permafrost systems are anticipated to be particularly sensitive to climate change. This study presents the first observations of dissolved and particulate trace metal concentrations and lead isotopes in 14 rivers in the Canadian Arctic Archipelago (CAA). Dissolved concentrations of aluminum, iron, manganese, nickel, copper, zinc, cadmium, and lead display clear spatial variability in their distributions. Small rivers located in the central islands of the CAA have remarkably low concentrations of these trace metals, while the rivers draining the southern continental landmass have higher values. This geographic variability in dissolved metal concentrations reflects both differences in bedrock geology and the South-North environmental gradients in the Archipelago. We extrapolate these findings to estimate the dissolved trace metal concentrations for other rivers draining the CAA based on their geographic location. Elemental ratios measured in the suspended particulate matter in several rivers indicates a significant enrichment of nickel, copper, zinc, and lead over the crustal composition. In particular, lead isotope composition shows an extremely large range, varying from the highly radiogenic Tree River (206Pb/207Pb: 1.5121) in the south, to the relatively unradiogenic Glacier River (206Pb/207Pb: 1.0153) in the central CAA. Lack of correlation between the Pb isotopic composition and Pb enrichment, suggests Pb is derived from within the watershed. This work presents the first observations of dissolved and particulate trace metal concentrations in small coastal draining rivers from the CAA, an important first step in developing a baseline against which to evaluate future change in the high Arctic. • Dissolved trace metals concentrations display clear spatial variability in their distributions • The variability in dissolved concentrations is influenced by the bedrock geology and the South-North gradients of environmental conditions • The elemental ratios measured in the suspended particulate matter indicates a significant enrichment of Ni, Cu, Zn and Pb • Pb isotopic composition shows an extremely large range, from highly radiogenic to relatively unradiogenic values [ABSTRACT FROM AUTHOR]

The average contents F = 788 ppm and Cl < 45 ppm of the clay fraction from Amazon River mouth sediments represent the composition of a large portion of weathered South American continental crust prior to any contact with high salinity formation waters. The data show a large fractionation between F and Cl with the latter being strongly leached during weathering along with Na and Ca. The one-order-of-magnitude fractionation is comparable to Cl fractionation from degassing during early Earth accretion. In contrast, F is strongly retained in the hydroxyl sites of neoformed clay minerals, mainly illite, as well as in detrital apatite and muscovite. The high average ratio F/Cl >10 of the clay fraction from Amazon River sediments are comparable only to Cl-poor MORB and evolved granite and rhyolite. The high content Cl = 180 ppm and low F/Cl = 3 of average shale is due to interaction of the sediments with high salinity diagenetic fluids. The Seridó schist belt is used to assess the behavior of F and Cl during shale metamorphism and anatexis. Its ratio F/Cl >10 ppm is high compared to average shale and is related to the strong partitioning of Cl into metamorphic fluids. The obtained data suggest that the Cl partitioned into shales during diageneses is released back into fluids produced during metamorphic conditions. The experimental data indicate that anatexis of mica schist significantly fractionates F/Cl ratios producing schist residue with F/Cl > 38 and peraluminous melts with F/Cl = 6. • F and Cl geochemistry • Amazon River mouth clay fraction • Seridó Belt amphibolite grade metapelititic schist • Experimental schist anatexis • Weathering–diagenesis–metamorphism–anatexis cycle [ABSTRACT FROM AUTHOR]

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a 40Ar/39Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 ± 0.0031 Ma for BHs relative to an age of 98.79 ± 0.54 Ma for GA1550 and a calibration factor (R BHs/GA1550) of 0.096719 ± 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; R TCs/BHs of 2.90874 ± 0.00067), Fish Canyon Tuff (FCs; R FCs/BHs of 2.88339 ± 0.00088), and rhyolite of Alder Creek (ACs; R ACs/BHs of 0.12028 ± 0.00024). These calibration factors yield ages of 28.344 ± 0.011 Ma, 28.099 ± 0.013 Ma, and 1.1809 ± 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to ±0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 ± 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes. Zircons from the rhyolite of Bodie Hills are strongly zoned in U and Th, and U/Pb geochronologic analyses suggest multiple generations of zircon growth. The youngest ages indicate a crystallization age of 9.97 ± 0.08 Ma (2σ); arguably similar to the 40Ar/39Ar results when residence time is considered, but previous episodes of zircon growth began at least 400 ka prior to eruption. Precise (U - Th)/He thermochronology of these zircons is difficult because crystal-to-crystal variations in the magnitude and complexity of their U + Th zoning complicates accurate alpha ejection calculations. Replicate conventional (single-crystal) (U - Th)/He ages are more widely dispersed than predicted by analytical uncertainties, but laser ablation (U - Th)/He ages are more reproducible, with an inverse variance-weighted mean of 9.71 ± 0.58 Ma. • Characterization of the Bodie Hills sanidine standard with 40Ar/39Ar, U/Pb, and (UTh)/He geochronology and geochemistry. • Intercalibration of a high K/Ca, highly radiogenic 9.8-Ma sanidine standard with widely used 40Ar/39Ar mineral standards. • Further documentation of mass fractionation of Ar isotopes during stepwise heating argues against its use for standards. • SHRIMP U - Pb analyses of Bodie Hills zircons show at least 400 ka of pre-emplacement history similar to Fish Canyon Tuff. [ABSTRACT FROM AUTHOR]

A suite of crustal xenoliths from the Bearpaw Mountains, Montana, USA, was examined using zircon geochronology, whole-rock geochemistry and petrology in order to investigate mid- to lower crustal geological processes along the northern margin of the Wyoming Craton. The xenoliths are hosted within Eocene alkaline intrusive and volcanic rocks ("minettes") and include mafic to intermediate granulites and amphibolites. Whole-rock geochemistry and geothermobarometry indicate igneous protoliths of alkaline to sub-alkaline composition, which recrystallized at lower to middle crustal conditions (642–817 °C and 3.5–9.4 kbar). Mantle-normalized trace element patterns for all xenoliths show a subduction signature, with extreme enrichments in large-ion lithophile (LILE) and fluid mobile elements (Cs, Rb, Ba, Pb, Sr) relative to high-field strength elements, together with relative Zr-Hf and Nb-Ta-Ti depletions. While late-stage modification of the xenolith compositions by the magma host cannot be excluded, simple binary mixing models between the host lava and xenoliths indicate that such a process was insignificant, and implies that the xenoliths preserved their original geochemical signatures. SEM-CL imaging of zircon reveal complex internal structures and indicates the presence of both magmatic and metamorphic zircon. In the simplest cases, magmatic grains are recognized by weakly preserved, altered oscillatory zoning. In more complex magmatic grains, high Th/U (x ¯ = 1.2) zircon appear recrystallized in the presence of a melt phase, resulting in resorbed internal domains and exterior morphology. Metamorphic zircon with low Th/U (0.03–0.8, x ¯ = 0.2) is present as whole grains or rims with uniform domains devoid of recognizable internal microstructures. Zircon U-Pb dating produces a spectrum of dispersed Late Archean to Early Paleoproterozoic ages (2000–2534 Ma), including a dominant mid-Proterozoic age population that records both magmatic (1834 ± 19 and 1874 ± 10 Ma) and metamorphic (1772 ± 5–1788 ± 4 Ma) events. Proterozoic ages are correlated with lithospheric processes along the cratonic north-western margin (Great Falls Tectonic Zone and Montana Metasedimentary Terrane), involving continental arc magmatism and collision-related metamorphism. The protolith to xenoliths containing mid-Proterozoic (ca. 1834–1874 Ma) magmatic zircon ages were formed by subduction-induced melting of the overlying mantle wedge. Xenoliths containing older, dispersed magmatic zircon ages (ca. 2000–2534 Ma) do not produce a significant age population that can be well correlated with events within the Wyoming Craton/GFTZ, and their protolith was likely produced by melting of a subduction-related metasomatized mantle in the Late Archean or earlier. Both Proterozoic and Late Archean protolith magmas are envisaged as new additions to the Wyoming Craton that resided within middle to lower crustal levels of the continental lithosphere before being subjected to collision-related metamorphism and recrystallization at ca. 1770 Ma. • Zircons from crustal xenoliths of the Wyoming Craton yield ages between ca. 1800 and 2500 Ma. • Proterozoic (ca. 1800 Ma) ages can be correlated with subduction-related processes along the north-western cratonic margin • Whole-rock and trace-element signatures are suggestive of xenolith formation in a convergent margin setting • Xenoliths are envisioned as new melts added to the continental crustal lithosphere that were subsequently metamorphosed. [ABSTRACT FROM AUTHOR]

This study addresses a specific component associated with mass transport complexes in marine systems: the role of hyperpycnal flows, dense shelf water cascading, submarine canyons, distributary channels, and other transport mechanisms in transferring organic matter from continental and shallow marine settings into deep-marine environments. We speculate that during the Eocene, allowing for only 0.1‰ of shelf carbon to be preserved through transport mechanisms would account for up to 13.7% of all organic carbon burial. As such, the potential to mobilize through this mechanism large quantities of organic carbon is significant. Our case study focuses on a 150 m Eocene sequence composed of organic-rich chalks interleaved with displaced neritic limestones. TOC values range between 1.5 and 14%, averaging 4.5%. Displaced limestones are composed of a variety of poorly cemented mud- and wackestones, with low-diversity assemblages of large benthic foraminifera associated with planktonic foraminifera, suggesting deposition under low-energy conditions within the oligophotic zone on the outer ramp. Transport overprints include soft-sediment deformation, partially lithified rip-ups, folds, small diapirs, bed-scale imbrication, brecciation and syn -sedimentary shear. These features indicate detachment, movement and emplacement following initial sedimentation, in some cases more than once. Emplacement occurs into a chalk facies that can vary in appearance from darker (higher TOC) and lighter (lower TOC) lithofacies. Combination between the sedimentological, petrophysical, and elemental analyses indicates shifts between autochthonous and allochthonous sedimentation, whereas the organic geochemical analysis reveals a correlation between modes of sedimentation and preservation/composition of organic matter. Organic richness seems to increase within intervals of allochthonous sedimentation, with lower TOC values within intervals of autochthonous sedimentation. Organic matter preservation is enhanced due to poor oxygenation of the sea floor, further depleted by rapid burial beneath mass-transport deposits, increasing sedimentation rates and thus organic matter preservation. Horizons rich in organic matter may be derived from three different sources: organic matter with a fingerprint of terrestrial sources (e.g., enhanced contribution of plant leaf waxes) transported from nearshore environments; an allochthonous marine fingerprint with sulfurized hopanoids, which seem to be reworked from pre-existing Cretaceous organic-rich carbonates entrained within fined-grained micro-turbidites in the para-autochthonous facies; and productivity-derived organic matter deposited on the seafloor of the deep marine environment. This study demonstrates how transport mechanisms allow for the long-term burial of organic carbon in marine systems. When taking into consideration similar processes reported to occur in the world oceans today, it is clear that sediment transport and long-term burial of organic carbon is a fundamental part of the global carbon cycle. [ABSTRACT FROM AUTHOR]

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an "Archean seawater signature" with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification. • REE + Y analyses of 3.45–2.45 Ga greenalite-bearing cherts and BIFs. • Textures and chemistry preserved by early silicification. • Dales Gorge BIFs preserve seawater signature. • Joffre and Marra Mamba BIFs have hydrothermal fluid influence. • Greenstone-belt cherts and BIFs have dominant hydrothermal contribution. [ABSTRACT FROM AUTHOR]

Rare earth elements (REE including Y—REY) in phosphorites have been used widely as a proxy for seawater chemistry at the time of their formation. However, it is unclear whether metamorphism can lead to REE remobilization and alteration of primary seawater signatures recorded in phosphorites. In this study, we examine similarly aged metamorphosed phosphorite ore deposits from the northeastern margin of the Yangtze Block (Jinping and Xinpu ore sections), with contemporaneously deposited unmetamorphosed equivalents in the southwestern margin of the Yangtze Block (Weng'an ore section). New insights have been provided into the migration of rare earth elements and the redistribution of Zn and O isotopes in metamorphosed phosphorite, which suggests that the varying degrees of metamorphic fluid–rock reaction during the metamorphism process significantly reduced the δ18O value of phosphate (13.7‰ ± 2.2‰ of Jinping phosphate and 7.3‰ ± 1.8‰ of Xinpu phosphate) and added light Zn isotope. These metamorphic fluids were likely generated by the metamorphic dehydration of upper slab uplift during the plate subduction in the Triassic. The negative correlation between the O (and Zn) isotope compositions and REY contents in P-rich rocks suggests that REY were likely re-enriched during the recrystallization process of apatite. Preferential migration of LREE led to significant LREE and MREE enrichment in Haizhou metamorphosed P-rich rocks compared to Weng'an unmetamorphosed phosphorites. [Display omitted] [ABSTRACT FROM AUTHOR]

Submarine volcanic activity releases large amounts of gases and metals in the water column, affecting biogeochemical cycles and ecosystems at a regional and local scale. In 2018, Fani Maoré submarine volcano erupted 50 km offshore Mayotte Island (Comoros Archipelago, Indian Ocean). Active eruptive plumes were observed in May 2019 at and around the summit with acoustic plumes rising 2 km into the water column coupled to strong geochemical anomalies. Between May 2019 and October 2020, three research cruises monitored the eruptive activity. Here, we report spatial and temporal variability of water column chemistry above the volcano, focusing on dissolved gases, trace metal concentrations, and physico-chemical parameters. In May 2019, concentrations above 800 nM in CH 4 and H 2 were measured throughout the water column, with Total Dissolvable Mn and Total Dissolvable Fe concentrations above 500 nM, and CO 2 values of 265 μM. Strong water column acidification was measured (0.6 pH unit) compared to the regional background. From May 2019 to October 2020, we observed a general decrease in gas concentrations, and an evolution of the TDMn/TDFe ratios similar to previously reported values in other submarine volcanic contexts, and consistent with a decrease of the eruptive activity at the volcano. In October 2020, a rebound of high H 2 concentrations resulted from new lava flows, which were identified by seafloor observation using deep-towed camera, 5 km further than the volcano summit. During 2 years timespan of our observations (2019–2020), He, CO 2 and CH 4 concentrations correlate highlighting a magmatic origin of dissolved gases. δ13C-CH 4 values of −34‰ vs. vPDB might suggest magma/sediments interaction during the magma ascent, and potential thermal cracking of organic matter, although abiotic methane generation cannot be ruled out given the volcanic context. Weak correlations between H 2 and excess of 3He suggest complex processes of H 2 from magmatic degassing, lava/seawater interaction, and oxidation processes in the water column. Strong and correlated Fe, Mn and Si water column anomalies are also consistent with fluid-rock reactions induced by acidic fluids rich in magmatic volatiles. Water column acidification appears to be associated with the release of CO 2 -rich fluids. A year after the main eruptive event, the system seems to be back to steady-state highlighting the buffer capacity and resilience of the seawater column environment. • Massive gases released in the water column during the eruption. • Emitted gases respond in an uncoupled way due to original settings of the volcano. • Strong water column acidification due to the release of CO 2 -rich fluids. • Water column enrichments in iron and manganese by fluid-rock interactions. • Helium isotope signatures show evidence of a change in the magma path. [ABSTRACT FROM AUTHOR]

Sulfur isotopic values (δ34S) in sedimentary organic matter and pyrite provide essential information about the global S cycle. The measurement of δ34S values is commonly used to reconstruct paleo-environmental conditions of sediment deposition. In petroleum geochemistry, δ34S values are used to trace relationships between disparate elements of petroleum systems. However, the determination of organic and pyrite δ34S values requires a tedious and time-consuming (weeks) stepwise procedure. Here we used the latest version of the Rock-Eval ® 7S analyzer, which can quantify different S-fractions in sedimentary rocks. We have coupled the Rock-Eval® pyrolysis cycle with a multi-collector inductively coupled plasma mass spectrometer (RE/MC-ICPMS). The analysis requires a small sample size (5 to 90 mg), and minimum sample preparation of acid treatment to avoid the carbonate effect on the pyrolyzed pyrite-S. The RE/MC-ICPMS system allows for measuring the pyrolysable organic-S and pyrite-S and directly determining δ34S values from a decarbonated rock. The new method was applied to a set of immature sedimentary rocks of various paleoenvironmental origins and geographic locations. The rapid analysis by the RE/MC-ICPMS, which takes only 20 min per sample, achieved an average precision of 0.5‰, for peaks above 5 nanomole of S. Accuracy was determined relative to a common wet chemistry method followed by an offline EA-IRMS analysis and reached an agreement of <1‰. • Coupling a Rock eval 7S analyzer and MC-ICPMS enables rapid and reliable results. • Simultaneous δ34S measurement of pyrite and organic S from decarbonated rocks. • The δ34S values are affected by the presence of a carbonate matrix and low pyrite content. • The average precision and accuracy of the method are 0.5 (σ) and up to ±1 ‰, respectively. [ABSTRACT FROM AUTHOR]

The short-lived radiogenic 182Hf/182W (t½ = 9 Ma) and 146Sm/142Nd (t½ = 103 Ma) systems are useful tools to investigate differentiation processes in Earth's early history. Mantle reservoirs that have been preserved over long geological timescales could carry signatures formed early on in Earth's history and later could act as sources to modern mantle plumes. Negative μ182W anomalies in combination with high 3He/4He ratios have been detected globally as differently sloping negative trends in plume-derived ocean island basalts (OIB) from individual hotspots. Large low shear velocity provinces (LLSVP) and ultra-low velocity zones (ULVZ) are seismically anomalous regions in the lower mantle that have been suggested as sources of primordial mantle components near the core-mantle boundary (CMB) and accessed by deep-rooted mantle plumes. In this work, seven samples from the islands Hiva Oa and Fatu Hiva of the Marquesas Archipelago, showing a range in 3He/4He ratios, have been investigated motivated by its location above the Pacific LLSVP and a recently discovered "mega-ULVZ". Despite elevated 3He/4He ratios of up to 14.4 R/R A , new high precision measurements using thermal ionization mass spectrometry revealed non-anomalous μ182W (−0.9 ± 4.9 ppm) and μ142Nd (1.3 ± 2.4 ppm, 95% confidence interval (c.i.)). These mark the Marquesas Archipelago as the first hotspot to show a decoupling of 3He/4He ratios and μ182W in OIB. A mixing model between an elevated 3He/4He-negative-μ182W and an EM2 or a HIMU endmember suggest that the addition of recycled material is not responsible for this decoupling. Instead, like many previously studied OIB, the Marquesas plume is most likely dominated by an LLSVP component proposed to contain high 3He/4He ratios and μ182W of 0. The unique observed absence of negative μ182W anomalies, on the other hand, implies a lack of involvement of the ULVZ source in the plume's melting region, despite the hotspot being located above a recently discovered mega-ULVZ. This may imply that the expression of ancient mantle domains in hotspot geochemistry is temporally variable, thus, delaying the arrival or involvement of ULVZ material in the origin of the Marquesas OIB. • Marquesas OIB exhibit statistically significant variability in μ142Nd but isotope compositions are unresolved from modern BSE values. • Unlike all other OIB globally, the Marquesas samples show no resolvable negative μ182W anomalies despite elevated 3He/4He ratios. • The global association of negative μ182W with seismically anomalous mantle domains (ULVZ) cannot be observed in the Marquesas hotspot. • The lack of μ182W anomalies in elevated 3He/4He Marquesan OIB suggests temporal variability of ULVZ source contribution in mantle plumes. [ABSTRACT FROM AUTHOR]

The chemical composition of soil represents to a large extent the primary mineralogy and geochemistry of the source bedrock, the effects of pre- and post-depositional weathering and element mobility, either by leaching or mineral sorting with the addition of secondary products such as clays. Agricultural soil in Europe (0–20 cm, 33 countries, 5.6 million km2) was sampled during the continental-scale soil mapping project (GEochemical Mapping of Agricultural and grazing land Soil - GEMAS). Total element concentrations were determined by wavelength dispersive X-ray fluorescence spectrometry. Bulk content of major elements (SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, CaO, Na 2 O, K 2 O, P 2 O 5) in soil has been used to calculate weathering indices such as chemical index of alteration (CIA), reductive and oxidative mafic indices of alteration (MIA), the change in mass balance (τ calculation relative to immobile Nb) for soil derived from silicate bedrocks (granite, gneiss and schist) at the European continental-scale, which are useful tools for evaluating chemical weathering trends. The greatest variation of CIA values is exhibited by soil derived from granite, followed by soil derived from schist. The CIA results show clearly that Ca and Na are removed from agricultural soil (0–20 cm) during soil development. Geographical distribution of weathering indices shows that weak chemical weathering of agricultural soil samples, sourced from gneissic and granitic bedrock, occurs in the Fennoscandian Shield; intermediate chemical weathering in granite, schist and gneiss occurring in the Iberian Peninsula and France, and intense chemical weathering in schist sourced agricultural soil occurring in areas with intense rainfall and moderate temperature (Wales, England, Ireland and western Iberia). In addition, values of CIA and MIA, higher than those for standard rock compositions, suggest more active enrichment/depletion processes during soil formation and subsequent soil weathering/maturation. The range of the elemental mass transfer coefficient τ is wide for agricultural soil derived from gneiss and granite, and narrow for agricultural soil sourced from schist parent rocks. The weathering behaviour of chemical elements, assessed by using the τ mass transport model, suggests an order of susceptibility of K ≈ Na > Al > Fe > Ca > Mg > Ti. Weathering indices and gain-loss mass transfer coefficients, tested on agricultural soil samples, provide an insight to the weathering processes affecting the silicate parent rocks and their impact on soil development at the European scale. This can have further implications for evaluation of soil nutritional conditions and mitigating of soil erosion and deprivation. • Spatial variation in weathering grade studied by geochemistry of agricultural soil. • Chemical indices of alteration identify weathering patterns in soil parent materials. • Ternary diagrams are ideal tools for studying weathering trends in soil data sets. • Distribution of major elements in soil controlled by lithology and weathering. [ABSTRACT FROM AUTHOR]

Phosphate rock bears both geologically and environmentally significant information. Rare earth elements and yttrium (i.e., REY) characteristics have been commonly used for reconstructing the redox conditions of depositional environments and the effects of post-depositional diagenetic alteration on phosphate rock. In addition, phosphate rock is typically enriched in a range of trace elements such as uranium (U) and cadium (Cd) that can be dispersed as contaminants into the environment with phosphate mining and phosphate fertilizer application. Here we report the lead (Pb) isotope compositions combined with Pb and REY concentrations of both global sedimentary and igneous phosphate rocks, aiming to evalute the geological origin of phosphate rocks over time and the potential of using them for environmental tracing. Phopshate rock samples analyzed in this study were sourced from major economic phosphate deposits in the world, including China, Southern Tethys (e.g., Morocco, Tunisia, Israel), the U.S., India, South Africa and Russia. Our results show a wide range of 208Pb/204Pb (35.70 to 60.58), 207Pb/204Pb (15.20 to 18.25), and 206Pb/204Pb (16.369 to 71.806) ratios in the phosphate rocks, with sedimentary phosphate rocks being significantly more radiogenic than igneous rocks. The majority of sedimentray phosphate rocks show a notable isotopic overprinting by non-radiogenic terrestrial Pb, except for those from Israel and Morocco that have the most radiogenic Pb isotope compositions. Correspondingly, phosphate rocks with more radiogenic Pb isotope ratios show relatively pristine seawater REY features, likely suggesting their preservation of the original oxic seawater conditions and/or minimal diagenetic alteration. In contrast, phosphate rocks with less radiogenic Pb isotope compositions show REY indications for more anoxic seawater redox conditions and/or greater diagenetic alteration. We further evaluate the potential utility of Pb isotopes for tracing the associated contamination with phosphate rock mining and fertilizer application in the environment. In most cases, the radiogenic Pb isotope composition of phosphate rocks and corresponding P-fertilizers is distinctive from both natural crustal Pb and major anthropogenic Pb sources (e.g., Pb ore deposits and pesticides), which provides a great advantage for applying Pb isotopes as an environmental tracer for metal(loid) contamination from phosphate sources. The combination of Pb isotope ratios and REY proxies could further constrain the Pb source discrimination. Overall, this study provides new Pb isotopic and REY geochemical data on global phosphate rocks and fertilizers, which lays the groundwork for future regional and local studies on both their geological and environmental implications. • First set of Pb content and isotope data on global phosphate rocks are reported. • Israeli and Moroccan samples have the most radiogenic Pb isotope composition. • Pb and REY variations indicate depositional conditions and diagenetic influences. • Distinct Pb isotopic and REY variations can help trace P-fertilizer contamination. [ABSTRACT FROM AUTHOR]

The Himalayan leucogranites have received particular attention as they record the continental crust-building processes of a collisional orogenic system. Deciphering the magmatic evolution processes and potential sources is essential for understanding the origin of the Himalayan leucogranites. In-situ monazite Th-Pb ages, trace element geochemistry, and Nd isotope ratios have been determined with whole-rock geochemical data for the leucogranites and their country rocks in the Yadong area, southern Tibet. Monazites in most of the studied leucogranites are marked by multiple age populations with variable REE ratios and wide ranges of Nd isotope ratios, reflecting a complex history of magmatic processes. Abundant inherited monazites are identified in the Yadong leucogranites, and most of them are excluded from the source regions due to their distinguished Nd isotope compositions from the magmatic monazites. The inherited monazites share close affinities in geochronological dates and geochemical compositions with those in the country rocks, strongly suggesting a major origin of the country-rock contamination. This highlights the contamination-assimilation processes in the genesis of the Yadong leucogranites. It is worth noting that xenocrystal monazites from the metasedimentary country rock are less preserved in the leucogranites than those from the granitoid country rock, which probably reflects the different assimilation reactions for different types of country rock. Assimilation of the metapelitic rocks released low content of xenocrystic monazite and high volume of partial melt into the leucogranites, whereas assimilation of the granitoid rocks released high content of xenocrystic monazite. In addition to the contamination-assimilation processes, the uniform Nd isotope data with large variations in REE ratios for magmatic monazites in some leucogranite samples suggest a subsequent closed-system fractional crystallization process during the magma evolution. Besides, some magmatic monazites in the muscovite granites show a significant tetrad effect in the Chondrite-normalized REE patterns, indicating that the magma system has evolved into an aqueous fluid-available environment. This supports an important role of the magmatic differentiation process in the formation of the Yadong leucogranites. Taken together, a petrogenic model involving AFC (assimilation and fractional crystallization) processes is proposed, and the geochemical variations of the Yadong leucogranites are mainly attributed to the post-source processes. It is suggested that the rock geochemistry of the Himalayan leucogranites often dose not directly reflect the nature and the petrogenetic processes in the source regions. [ABSTRACT FROM AUTHOR]

The youngest explosive eruptions of the Sete Cidades volcano, São Miguel, Azores, are recorded by a series of relatively small-volume (<0.03 to 0.13 km3 DRE) trachytic pyroclastic deposits termed the Pepom tephra deposits. While dominated by crystal-poor to crystal-moderate (<10%) pumice clasts, these deposits also contain a suite of dense glassy clasts of broadly similar crystallinity. The obsidian clasts from a single deposit vary in texture from entirely dense to those that are moderately vesicular and typically single clasts will contain multiple textural domains. The majority (∼71%) of these dense clasts have compositions both from bulk rock and in-situ glass measurements that are identical to those of the pumice clasts within the same deposit. We interpret these dense clasts to reflect sintering of previously fragmented magma at shallow levels in the conduit prior to being re-entrained and erupted with the vesicular magma, in agreement with recent studies focussing on textural observations. Notably, across the exposed volcanic stratigraphy of São Miguel obsidian domes, flows/coulees are not preserved, arguing against the idea that the dense glass clasts within the Pepom tephras are sourced from existing surficial rocks. In contrast, the neighbouring island of Terceira exhibits domes and coulees with large obsidian bands that cut through the crystalline groundmass. Most silicic rocks of Santa Bárbara and Pico Alto volcanoes on Terceira are peralkaline, comenditic to pantelleritic in composition, and at similar conditions (e.g., temperature and water content) have lower viscosities than the trachytic Pepom obsidian clasts. However, the Santa Bárbara silicic lavas on Terceira (the less peralkaline suite) have more obsidian than the more peralkaline Pico Alto domes and coulees indicating that while peralkalinity, developed during magmatic evolution in the crust, may play a role, sintering occurring at shallow levels within the conduit likely is more important in producing obsidian. • Obsidian pyroclasts are common in the youngest trachytic tephras erupted from Sete Cidades volcano, São Miguel, Azores. • Within a single clast vesicularity and groundmass crystallisation may vary significantly. • Obsidian glass compositions are mostly the same as co-erupted pumices suggesting that they are predominantly juvenile. • Sintering within the vent can produce such obsidians over timescales of seconds to hours. [ABSTRACT FROM AUTHOR]

Rare earth elements plus yttrium (REY), well-known critical metals for many new and emerging technologies and as proxies in geological events, are one type of the most important high-tech metals in ferromanganese (Fe-Mn) crusts. The REY(III) are accumulated in Fe-Mn crusts through adsorption by Fe-Mn (oxyhydr)oxides. However, negative correlations between Mn and REY in some Fe-Mn crusts reveal a controversial role of Mn oxides in REY enrichment, which remains equivocal. In this study, texture, structure, mineralogy, and microstratigraphic geochemistry with the growth of a Fe-Mn crust (MP2D32A) from the Line Islands archipelago were analyzed using multiple microanalysis methods. The Fe-Mn crust is characterized by laminar structures, stromatolite-like structures, and micronodules that are primarily composed of low-crystalline Fe-vernadite and ferrihydrite. Throughout the profiles of MP2D32A, the contents of elements are more or less interdependent. Analysis of elemental correlations from the MP2D32A samples and 847 collected data sets from non-phosphatized hydrogenetic Fe-Mn crusts shows Fe plays a positive role and Mn plays multiple roles in REY accumulation during Fe-Mn crust formation. The REY content increases initially and then decreases with increasing Mn content and Mn/Fe ratio and the turning points (i.e., highest REY contents) mainly occur at an Mn content of ∼18–26% and an Mn/Fe content ratio of ∼0.8. These fundamental results provide novel insights into understanding the influence of Fe and Mn in REY accumulation in the Fe-Mn crusts and are very significant in enhancing our understanding of the REY geochemical processes in the ocean. • REY displays a positive correlation with Fe in hydrogenetic Fe-Mn crusts. • REY contents increase initially and decrease with Mn contents and Mn/Fe ratios. • Multiple roles of Mn in REY enrichment are due to changes in oceanic environments. [ABSTRACT FROM AUTHOR]

Results of International Ocean Discovery Program (IODP) Expeditions on the northern continental margin of the South China Sea (SCS) have nullified the early notion that the SCS is a magma-poor margin. However, there are continuing debates on how the rapid transition from continent to ocean is supported by geochemical and petrologic data, and whether such a process was related to plate subduction or mantle plume activities. Here we present the results of bulk-rock and plagioclase phenocryst geochemistry of the basalts generated during the early spreading stage (initial and steady ocean stages) of SCS basin extension, which were drilled at IODP Sites U1500 and U1503, respectively. We combined these findings with published data from the late spreading stage of the SCS basin, which provides an excellent opportunity to examine the magmatic processes associated with the evolution of the SCS basin. Our results revealed that the SCS basalts generally exhibit higher Al contents than global Mid-Ocean Ridge Basalts, particularly at lower MgO contents, which is similar to the characteristics of modern arc basalts. Nevertheless, their origins are site-specific and complex. For instance, some of the Site U1503 basalts originated through fractional crystallization of water-rich parental magmas with the lowest degree of melting, while the basalts from the other sites, including Site U1500, were primarily generated by the accumulation of plagioclase. More specifically, the water-rich magmas from Site U1503 displayed strong subduction signals in terms of trace elements, and their correlations with Al 2 O 3 content suggest that they are the product of partial melting of the mantle wedge, closely related to the northward subduction of proto-SCS. This subduction had a significant effect on triggering the opening of the SCS, rather than a mantle plume. Moreover, the Site U1500 basalts exhibit a significant increase in plagioclase An values with the increase of host magma Al contents, indicating that the plagioclase floatation mechanism cannot account for the origin of their plagioclase accumulation. Therefore, we propose that the high abundance of plagioclase in Site U1500 basalts requires rapid ascent of magma, supporting a rapid rifting and strong magmatism during the initial opening of the SCS. [ABSTRACT FROM AUTHOR]

Springs in Sagehen basin, California, were used to document the effect of chemical weathering on the chemical evolution and composition of groundwater in a high elevation catchment. Geochemical tracer ages were determined with chlorofluorocarbons (CFCs) and tritium/3 He dating techniques. The spring water ages range from less than 5 years to almost 40 years. Mass balance calculations performed by NETPATH were combined with spring water ages to calculate chemical weathering rates observed throughout the basin, which range from 0.0116 to 0.0018 and from 0.0036 to 0.0006 mmol 1-1 year-1, for plagioclase and hornblende, respectively. Major cation concentrations, pH, and spring water conductivity were found to correlate positively (R2 = 0.7) with spring water age. This suggests that shallow groundwater, as represented by the springs, is a chemically evolving system. © 2001 Elsevier Science B.V. All rights reserved.

We report mineralogical, elemental (major and trace elements) and Sr Nd isotopic data for suspended particulate matter (SPM) samples from the Amazon, Orinoco and Maroni Rivers collected on a monthly basis over a one-year long hydrological cycle. The aim of this study was i) to characterize the mineralogical and geochemical composition of major South American tropical rivers and ii) to evaluate the effect of seasonal hydroclimate variations and changes in sediment discharge on the composition of these SPM. In addition to displaying particular mineralogical and geochemical signatures (e.g. Al/Si ratios, weathering indices), the Amazon, Orinoco and Maroni SPM are characterized by marked differences in 87Sr/86Sr (0.7213 ± 0.0030, 0.7288 ± 0.0018 and 0.7335 ± 0.0019, respectively), and εNd values (−10.6 ± 0.6, −14.1 ± 0.3 and −23.7 ± 1.2), which reflect differences in source rock lithology. While we find no effect of the hydrological cycle on the geochemistry of Orinoco SPM, particulate εNd and Cr/Th signatures fluctuate with the hydrological cycle in the Maroni basin, indicating that they are controlled by variation in rainfall distribution linked to the latitudinal migration of the Intertropical Convergence Zone (ITCZ). In contrast to Maroni and Orinoco SPM, the Amazon SPM are characterized by significant Sr isotope annual variability correlated with suspended sediments discharge and a small but significant Nd isotopic variability over the year. This latter variation is related to seasonal changes in the rainfall distribution patterns across the Amazon basin, associated with latitudinal migrations of the ITCZ. This suggests that the geochemical composition of the SPM exported over the year from the Amazon Basin faithfully responds to hydroclimate changes related associated with the migration of the rain belt over regions of contrasted geochemical signatures. These findings have implications for the application of Sr and Nd isotopes as provenance proxies in sedimentary archives and paleoclimatic studies. The Sr isotopic composition of exported SPM appears to be mostly controlled by weathering processes and/or mineralogical sorting, rather than being indicative of sediment provenance. In contrast, the relationship documented between Nd isotopes and the hydrological variability indicates that their application to archives of past river sediment discharges can provide unique insights on paleo-hydroclimate changes over tropical South America. • Distinct SPM geochemical signatures exported by tropical South American rivers. • Control of source rock lithology and hydrological cycles on these SPM signatures. • Potential applications for provenance study and past hydroclimate changes. [ABSTRACT FROM AUTHOR]

The Ambohimirahavavy igneous complex (Ampasibitika intrusion) in Madagascar hosts a world-class rare-earth element (REE) bearing ion-adsorption clay (IAC) resource. In this study, we present an in-depth chemical and mineralogical characterisation of selected samples from two regolith profiles at Ambohimirahavavy with a focus on REE geochemistry, mineralogy and associations with clay mineralogy. μ-Synchrotron X-ray fluorescence mapping was combined with traditional characterisation techniques in order to ascertain REE distribution within varying clay mineralogy. The ion-adsorption clays consist of aluminosilicate clay minerals (e.g. kaolinite, illite, smectite) formed through in-situ lateritic weathering of REE-rich host rocks. The new data reveal a high level of inhomogeneity in the samples at micro- to macroscopic levels. The ores are characterised by negative cerium anomalies, which are similar to those reported from the South China IAC ores, but contrast with positive anomalies reported in many regolith profiles developing on acid rocks, including in Madagascar. Ce-L III edge XANES spectroscopy shows that despite the negative Ce anomaly in the pedolith, Ce exists in trivalent form (mostly associated with clay minerals) and as Ce(IV); the latter is at least partially associated with textures reminiscent of the rhizosphere and with Mn- (and Fe-) oxides, highlighting the importance of micro-environments for Ce oxidation. The μSXRF images also reveal that a significant portion of REEs is associated with Zr; SEM imaging does not show the presence of Zr or REE (nano)-minerals, and the association is likely to reflect the decomposition of REE-Zr-rich host minerals and differential sorption of Zr and REEs on clay minerals. The Ambohimirahavavy ores are LREE-enriched, but contain HREE concentrations that are similar to those of the South China ores, making them an economically viable REE source. REE concentrations in the two studied profiles were particularly high in the saprolith relative to the pedolith, further contrasting with the South China ores, and indicating different mechanisms of ore formation. • Characterisation of a REE-rich ion-adsorption clay deposit in Madagascar • Significant differences in REE behaviour compared to South China ores • Samples with deep negative Ce anomalies still contain a significant amount of Ce(IV). • Zirconium-REE association at micron-level in ion-adsorption clay deposits [ABSTRACT FROM AUTHOR]

Sedimentary redox proxies are usually employed to reconstruct the paleo-redox conditions of bottom water environments, assuming that porewater and bottom water dissolved oxygen concentrations are similar. Using a combination of geochemical and magnetic techniques, we investigate the relationship between iron speciation and mineralogy in recent (~1760–2009 CE) sediments retrieved from the Santa Barbara Basin (SBB) – a modern silled basin with low-oxygen (dissolved O 2 < 10 μmol/kg) and sporadically anoxic (no O 2 detected) bottom waters. Magnetic analyses reveal that biogenic magnetite is preserved in SBB sulfidic porewaters on at least decadal to centennial time scales. Highly-reactive Fe (oxyhydr)oxides remain preserved despite observed sulfidic porewaters, indicating incomplete pyrite conversion, and producing low Fe py [pyrite Fe]/Fe HR ratios. We attribute this observation to restricted porewater reaction kinetics under high sedimentation rates. Our results also reveal non-steady state diagenesis caused by instantaneous depositional events (e.g., turbidites and flood layers). The most reducing water column suggested by Fe speciation coincided with the Macoma layer, where in situ colonization of hypoxia-intolerant bivalve shells argues for the most oxygenated bottom water in the 250 year record. A turbidite potentially introduced fresh unsulfidized Fe HR that buffered upward-diffusing sulfide from underlying sediments. Subsequent pyrite precipitation following re-establishment of sulfidic porewaters could have facilitated a "false positive" interpretation. In comparison, redox-sensitive metal enrichments (Mo EF , U EF , and Re EF) were not obscured by post-depositional diagenesis and appear to accurately record redox geochemistry at the sediment-water interface. • Fe (oxyhydr)oxides preserved in SBB sulfidic porewaters on centennial timescales. • Incomplete pyrite conversion in sulfidic porewaters due to rapid burial leads to low Fe py /Fe HR. • Non-steady state diagenesis can overprint redox signals in Fe speciation. • Trace metal enrichments controlled by redox conditions at sediment water interface were not overprinted in SBB. [ABSTRACT FROM AUTHOR]

Dolomite rarely occurs in modern marine environments, while it is ubiquitous in the geological record. This apparent contradiction made the formation mechanism of dolomite a long-standing problem haunting sedimentologists. However, dolomite is a constituent of many modern seep carbonates, providing an opportunity to investigate the conditions favoring dolomite formation at low temperatures in the shallow subseafloor. This study reports the petrology, mineralogy, carbon and oxygen stable isotopic compositions, as well as sulfur isotopic compositions of both carbonate-associated sulfate (CAS) and chromium reducible sulfur (CRS) of a set of dolomite-bearing carbonates from two seep sites (GC140 and GB382) on the northern slope of the Gulf of Mexico. The δ13C values of carbonates vary from −50.0‰ to 3.2‰ (V-PDB), confirming that carbonate minerals mostly resulted from methane oxidation. The corresponding δ18O values range from 2.6‰ to 5.9‰ (V-PDB), reflecting formation close to equilibrium with seawater composition. All carbonate samples with dolomite show similar petrologies and lack skeletal remains of seep-dwelling metazoans. The homogenous texture of microcrystalline dolomite agrees with its formation in the shallow subseafloor. Dolomite-bearing samples reveal δ18O CAS /δ34S CAS slopes of 0.425 and 0.435 for the two study sites, respectively. Such high slopes are significantly different from the smaller slopes of seep carbonates consisting of calcite from Gulf of Mexico seeps, suggesting low overall sulfate reduction rates during dolomite formation. This explanation is also supported by relatively high δ34S CRS values ranging from −29.4‰ to 19.0‰ (V-CDT) and the positive correlation of these values with dolomite contents. Interestingly, an approximately stoichiometric dolomite sample yielded the most positive δ34S CRS value, indicating a formation environment typified by low replenishment of seawater sulfate at a supposedly relatively deep sulfate methane transition zone (SMTZ). Overall, these results confirm that dolomite formation is facilitated where the SMTZ is situated at relatively great depth, where sulfate reduction rates are slowed down due to the buildup of high contents of dissolved sulfide and low seawater sulfate replenishment. • CAS and CRS geochemistry were applied to dolomite-bearing seep carbonates. • Low CAS contents confirm dolomite formed at greater depth during sulfate reduction. • High δ18O CAS /δ34S CAS slope suggests low sulfate reduction rate during dolomite formation. • Elevated δ34S CRS values indicate low replenishment of seawater sulfate. • Understanding dolomite formation at seeps helps in to disentangle the "dolomite problem". [ABSTRACT FROM AUTHOR]

Abstract The Singhbhum Craton in eastern India, one of the four major Archean cratons of the Indian Shield, is underlain by Paleoarchean granitoids (trondhjemite-tonalite-granodiorite i.e. TTGs, and potassic granites) that are encircled by greenstone belts rich in banded iron formation known as Iron Ore Group. This study presents whole rock major-trace element geochemistry, Sr and Nd isotopes, K-feldspar common Pb isotopes, along with in-situ zircon U–Pb ages and Hf isotope data from the granitoids and the greenstone volcanic rocks to characterize their source(s) and to constrain the evolution of the Paleoarchean continental crust. Zircon U–Pb ages show that these granitoids were emplaced during two magmatic episodes at ca. 3.47–3.44 Ga and 3.36–3.28 Ga. The greenstone volcanic rocks in the eastern part of the craton are basalts and basaltic andesites with rare komatiites, whereas those in the western part are basaltic-andesites and andesites. Whole rock Sm–Nd isotope regression lines for volcanic rocks from the eastern (including komatiites) and western regions yield ages of 3746 ± 340 Ma (MSWD = 169) and 2961 ± 420 Ma (MSWD = 3.3), respectively. The major and trace element data show that the volcanic rocks of the eastern and western greenstone belts had a different petrogenesis. The granitoids are highly evolved with SiO 2 contents up to 76 wt%. The major and trace element data indicate that the granitoids of the Singhbhum Craton were derived by partial melting of a mafic crust at variable depths (ca. 10–15 kbar). The isotopic compositions of Pb measured on leached K-feldspars from the granitoids show undepleted mantle-like characteristics and negate the possible involvement of much older evolved crust in the formation of these rocks. The whole rock initial Nd isotopic compositions (εNd i = −0.3 to +2.2) and zircon in-situ initial Hf isotopic compositions (εHf i = −0.3 to +2.0) of the granitoids calculated using zircon crystallization ages indicate that they were derived from a near-chondritic reservoir. These data suggest that only a limited volume of continental crust formed prior to ca. 3.5 Ga, and major crust extraction from the mantle started at ca. 3.5 Ga. Using this time as the start of major crustal growth and concomitant mantle depletion, Nd and Hf mantle depletion histories are proposed for the Singhbhum Craton that can be described by the equations εNd t = −2.85 × (t Ga) + 10, and εHf t = −4.68 × (t Ga) + 16.4. These curves do not change the model ages for young rocks, but they make a significant difference for the Archean rocks. Highlights • Granitoids of the Singhbhum Craton were emplaced in two magmatic episodes at ca. 3.47–3.44 Ga and ca. 3.36–3.28 Ga. • Hafnium and Nd isotope data of most of these granitoids indicate their derivation from a near-chondritic source. • Lead isotopes in K-feldspars show no evidence for much older evolved crustal material in petrogenesis of these granitoids. • Major continental crust formation started at ca. 3.5 Ga. [ABSTRACT FROM AUTHOR]

Abstract Rare earth elements (REE) and yttrium (Y), termed as REY, are recently found to be extremely enriched in some pelagic sediments, which could be an important resource for REY in the future. However, their enrichment mechanism remains controversial. Because biogenetic apatite is one of the most important REY carriers in REY-rich deep-sea sediment, it is critical to identify its mineralogical and geochemical characteristics by using combined methods of in situ chemical analysis and atomic resolution chemical imaging. Scanning electron microscope and transmission electron microscopy studies reveal that bioapatite fossils mainly consist of aligned hydroxyapatite nano-plates. Laser ablation-inductively coupled plasma-mass spectrometry analyses show that the REY are not uniformly distributed within the studied bioapatite fossils. Concentration profiles from the measurements and modeling analyses indicate that the major REY should be incorporated into the bioapatite fossil from seawater as well as the pore water during the early stage of diagenesis. Z-contrast imaging results reveal the following two possible coupled substitutions in the apatite: REE3+ + Na+ ↔ 2Ca2+ and/or REE3+ + Si4+ ↔ Ca2+ + P5+, which is distinguished from those ion-absorption-type REE deposits on land. Our study indicates that bioapatite fossils are prone to undergo alteration during diagenesis, and thus cautious examination should be taken when applying REE signal of bioapatite as a proxy for palaeo-ocean environmental reconstruction. Graphical abstract Unlabelled Image Highlights • Z-contrast was used to image substituted REY of bioapatite in atomic resolution. • The extraction of REY in the REY-rich deep-sea sediment needs strong acid. • Bioapatite obtains REE from seawater and/or pore water during early diagenesis. • Histology controls the REE in-diffusion pathway within bioapatite fossils. • Cautious examination is required for using REE signals in bioapatite for palaeoceanographic study. [ABSTRACT FROM AUTHOR]