Explore the vast range of titles available.

Continental arc magmas supply the ore-forming element budget of most globally important porphyry-type ore deposits. However, the processes enabling certain arc segments to preferentially generate giant porphyry deposits remain highly debated. Here we evaluate the large-scale covariation of key ore-forming constituents in this setting by studying silicate melt inclusions in volcanic rocks from a fertile-to-barren segment of the Andean Southern Volcanic Zone (33–40 °S). We show that the north-to-south, fertile-to-barren gradient is characterized by a northward increase in S and Cl concentrations and a simultaneous decrease in Cu. Consequently, we suggest that the concentration of S and Cl rather than the concentration of ore metals regulates magmatic-hydrothermal ore fertility, and that the loss of volatiles prior to arrival in the upper crust impacts ore-forming potential more than magmatic sulfide saturation-related ore metal scavenging. Earth’s largest copper deposits form in continental arcs, yet it is not well understood what determines whether a magmatic system generates economic mineralization or not. Here the authors show that the abundance of chlorine and sulfur, rather than the abundance of ore metals controls magmatic ore fertility.

The Arctic cryosphere is changing rapidly due to global warming. Northern Svalbard is a warming hotspot with a temperature rise of ~ 6 °C over the last three decades. Concurrently, modelled data suggest a marked increase in glacier runoff during recent decades in northern Svalbard, and runoff is projected to increase. However, observational data from before anthropogenic influence are sparse and the potential effects on the surface ocean are unclear. Here, we present a 200-year record of Ba/Ca ratios measured in annual increment-forming coralline algae from northern Spitsbergen as a proxy for past glacier-derived meltwater input. Our record shows a significant increasing trend in algal Ba/Ca ratios from the late-1980s onwards matching modelled regional runoff data, suggesting a drastic increase in land-based runoff. The rate of increase is unprecedented during the last two centuries and captures the impact of amplified warming on the coastal surface ocean in the high Arctic. The algal Ba/Ca runoff proxy offers an opportunity to reconstruct past land-based runoff variability in Arctic settings in high resolution, providing important data for validating and improving climate modelling studies.

We have reconstructed the compositional evolution of the silicate and carbonate melt, and various crystalline phases in the subvolcanic reservoir of Kerimasi Volcano in the East African Rift. Trace element concentrations of silicate and carbonate melt inclusions trapped in nepheline, apatite and magnetite from plutonic afrikandite (clinopyroxene–nepheline–perovskite–magnetite–melilite rock) and calciocarbonatite (calcite–apatite–magnetite–perovskite–monticellite–phlogopite rock) show that liquid immiscibility occurred during the generation of carbonatite magmas from a CO 2 -rich melilite–nephelinite magma formed at relatively high temperatures (1,100 °C). This carbonatite magma is notably more calcic and less alkaline than that occurring at Oldoinyo Lengai. The CaO-rich (32–41 wt%) nature and alkali-“poor” (at least 7–10 wt% Na 2 O + K 2 O) nature of these high-temperature (>1,000 °C) carbonate melts result from strong partitioning of Ca (relative to Mg, Fe and Mn) in the immiscible carbonate and the CaO-rich nature (12–17 wt%) of its silicate parent (e.g., melilite–nephelinite). Evolution of the Kerimasi carbonate magma can result in the formation of natrocarbonatite melts with similar composition to those of Oldoinyo Lengai, but with pronounced depletion in REE and HFSE elements. We suggest that this compositional difference results from the different initial parental magmas, e.g., melilite–nephelinite at Kerimasi and a nephelinite at Oldoinyo Lengai. The difference in parental magma composition led to a significant difference in the fractionating mineral phase assemblage and the element partitioning systematics upon silicate–carbonate melt immiscibility. LA-ICP-MS analysis of coeval silicate and carbonate melt inclusions provides an opportunity to infer carbonate melt/silicate melt partition coefficients for a wide range of elements. These data show that Li, Na, Pb, Ca, Sr, Ba, B, all REE (except Sc), U, V, Nb, Ta, P, Mo, W and S are partitioned into the carbonate melt, whereas Mg, Mn, Fe, Co, Cu, Zn, Al, Sc, Ti, Hf and Zr are partitioned into the silicate melt. Potassium and Rb show no preferential partitioning. Kerimasi melt inclusions show that the immiscible calcic carbonate melt is strongly enriched in Sr, Ba, Pb, LREE, P, W, Mo and S relative to other trace elements. Comparison of our data with experimental results indicates that preferential partitioning of oxidized sulfur (as SO 4 2− ), Ca and P (as PO 4 3− ) into the carbonate melt may promote the partitioning of Nb, Ta, Pb and all REE, excluding Sc, into this phase. Therefore, it is suggested that P and S enrichment in calcic carbonate magmas promotes the genesis of REE-rich carbonatites by liquid immiscibility. Our study shows that changes in the partition coefficients of elements between minerals and the coexisting melts along the liquid line of descent are rather significant at Kerimasi. This is why, in addition to the REE, Nb, Ta and Zr are also enriched in Kerimasi calciocarbonatites. We consider significant amounts of apatite and perovskite precipitated from melilite–nephelinite-derived carbonate melt as igneous minerals can have high LREE, Nb and Zr contents relative to other carbonatite minerals.

Sulfur is an important volatile element involved in magmatic systems. Its quantification in silicate glasses relies on state-of-the-art techniques such as electronprobe microanalyses (EPMA) or X-ray absorption spectroscopy but is often complicated by the fact that S dissolved in silicate glasses can adopt several oxidation states (S6+ for sulfates or S2- for sulfides). In the present work, we use micro-Raman spectroscopy on a series of silicate glasses to quantify the S content. The database is constituted by 47 silicate glasses of various compositions (natural and synthetic) with S content ranging from 1179 to 13 180 ppm. Most of the investigated glasses have been synthesized at high pressure and high temperature and under fully oxidizing conditions. The obtained Raman spectra are consistent with these fO2 conditions and only S6+ is present and shows a characteristic peak located at ∼1000 cm-1 corresponding to the symmetric stretch of the sulfate molecular group (ν1 SO4 2-). The intensity of the ν1 SO42- peak is linearly correlated to the parts per million of S6+ determined by EPMA. Using subsequent deconvolution of the Raman spectra, we established an equation using the ratio between the areas of the ν1 SO42- peak and the silicate network species (Qn) in the high-frequency region: ppmS6+ = 34371 A SO4-2/AQn ± 609. We tested our calibration on several silicate glasses equilibrated under moderately reducing conditions (QFM+0.8 ≤ fO2 ≤ QFM+1.4) in which S is dissolved as both SO42- and S2- We also analyzed several olivine-hosted melt inclusions collected from Etna for which the fO2 and S speciation are unknown. For these samples, the S content estimated by the Raman calibration is systematically lower than the total S measured by EPMA. We combined both methods to estimate the S2- content not accounted for by Raman and derive the S speciation and fO2 conditions. The derived fO2 is consistent with the imposed fO2 for synthesized glasses and with current assumed fO2 conditions for basaltic melt inclusions from Etna.

Oxygen fugacity (fO2) is a fundamental variable affecting phase equilibrium in magmas, and in externally heated pressure vessel experiments it is typically controlled by using redox buffer assemblages. However, these do not allow fine enough resolution; for example, most arc magmas fall between the fO2 imposed by the neighboring Ni–NiO and Re–ReO2 buffers and so does the transition of S2− to S6+ in magmas. Here we propose a new method to quantitatively impose fO2 in hydrous high-P–T experiments in molybdenum hafnium carbide (MHC) pressure vessels by admixing small amounts of hydrogen into the Ar pressure medium. The thermodynamic calculation procedure used to determine the initial amount of hydrogen to be loaded to constrain desired fO2 values was verified by CoPd alloy redox sensor experiments to be accurate within ±0.3 log units for the pressure (P) – temperature (T) range of 940–2060 bar and 800–1100 ∘C. As hydrogen can be slowly lost from the pressure medium due to diffusion through the vessel walls at high T, we also determined the hydrogen permeability of the MHC alloy as a function of T. The such-obtained hydrogen permeability equation for the MHC alloy can be used to determine the rate of fO2 increase for any MHC pressure vessel configuration. As the rate of fO2 increase is slow (e.g., 0.36 log units per day in our setup at T= 1000 ∘C), we propose that H2 addition to the Ar pressure medium is an effective way to accurately impose fO2 in many types of experiments conducted in MHC vessels allowing experimentation up to T= 1200 ∘C and P= 300 MPa.

The metaturbidite‐hosted, ∼380 Ma Dufferin gold deposit, Meguma terrane, northeastern Appalachian Orogen (Nova Scotia, Canada) is an orogenic gold deposit with mineralized saddle reef‐type quartz veins hosted by metasandstones and black slates in a tightly folded anticline. Together with native gold inclusions, genetically related hydrothermal carbonaceous material (CM) in veins occurs as pyrobitumen in cavities and along fractures/grain boundaries proximal to vein contacts and wallrock fragments. Integrating several microanalytical methods we document the precipitation of gold via coupled fluid‐fO2 reduction (via interaction with CM) and pH increase. These changes in fluid chemistry destabilized gold bisulfide complexes, leading to efficient Au precipitation from a gold‐undersaturated (0.045 ± 0.024 ppm Au; 1σ; n = 58 fluid inclusions) aqueous‐carbonic fluid (H2O‐NaCl‐CO2 ± N2 ± CH4). The proposed mineralization mechanism is supported by: (a) a complementary decrease in Au and redox‐sensitive semimetals (As, Sb), and increase in wall rock‐derived elements (i.e., Mg, K, Ca, Sr, Fe) concentrations in fluid inclusions with time; (b) a corresponding decrease in the XCO2, consistent with CO2 removal via reduction/respeciation and late carbonate precipitation; and (c) gold embedding in, or on, the surface of CM inside mineralized cavities and fractures. Despite mineralizing fluids transporting low concentrations of Au far from saturation, precipitation of gold was locally evidently high where such fluids interacted with CM, contributing to the overall gold endowment of Meguma deposits. This work re‐emphasizes CM as a potential prerequisite for efficient gold precipitation within the overall genetic model for similar orogenic metasedimentary settings globally where the presence and/or role of CM has been documented. Plain Language Summary The Dufferin gold deposit in Nova Scotia, Canada, formed ∼380 million years ago within metamorphosed sedimentary rocks called the Meguma Group. The deposit contains gold‐bearing quartz veins sandwiched between layers of tightly folded rocks. This study focused on unraveling the mechanisms behind some of the gold deposition within this deposit, specifically where associated with carbonaceous matter (CM). We found a close association between gold and CM, which represents organic matter preserved in the rocks. CM is abundant within small cavities throughout the quartz veins that also contain appreciable gold occurring as microscopic particles in the CM. By using a variety of analytical techniques, we determined that efficient gold mineralization occurred in response to specific chemical changes to the gold‐carrying fluid, including a decrease in the oxidation potential and a decrease in the acidity of the fluid through interaction with the CM‐rich rocks. Such changes to the fluid caused gold to become insoluble and form particles that were deposited in the rocks and vein material. Importantly, despite the fluid having a low concentration of dissolved gold, it exhibited a remarkable ability to deposit significant quantities of the precious metal, underscoring the important role of CM in facilitating efficient gold precipitation from fluids. Key Points First fluid gold concentrations (0.045 ± 0.024 μg/g) measured from an economic, Meguma‐type metasediment‐hosted gold deposit Fluid Au, S, As, W, and B concentrations comparable to Alpine and Variscan metamorphic fluids hosted in uneconomic, Au‐poor vein systems Gold and carbonaceous matter are coeval and co‐distributed in flysch wallrocks and vein laminae

The extreme geochemical enrichment of post-collisional potassium-rich lava in the Alpine-Himalayan orogenic belt has led researchers to hypothesize that enrichment is inherited from a metasomatized mantle source potentially incorporating crustal components. However, direct verification of metasomatic processes remains challenging due to the scarcity of mantle rocks preserving metasomatism records. Here, we report two groups of mantle xenolith entrained in Tibetan ultrapotassic lavas. Integrated petrographic observations, whole-rock geochemistry, and in-situ microanalysis reveal that subcontinental lithospheric mantle exhibits extreme enrichment in both isotopes and incompatible elements. Textural evidence of vein networks and melt pockets in xenoliths indicate the coexistence of carbonate and silicate metasomatic regimes. Considering subduction-collision background, we propose that the recycling of Indian continental materials during collision substantially contributes to the metasomatic enrichment of the Tibetan subcontinental lithospheric mantle. The Himalayan-Tibetan orogenic belt’s subcontinental lithospheric mantle contains both carbonate and silicate metasomatic regimes, with Tibetan mantle metasomatic enrichment likely due to Indian continental material recycling, according to analysis of mantle xenolith from Tibetan ultrapotassic lavas.

A new approach was developed to measure the water content of silicate glasses using Raman spectroscopy, which is independent of the glass matrix composition and structure. Contrary to previous studies, the compositional range of our studied silicate glasses was not restricted to rhyolites, but included andesitic, basaltic and phonolitic glasses. We used 21 glasses with known water contents for calibration. To reduce the uncertainties caused by the baseline removal and correct for the influence of the glass composition on the spectra, we developed the following strategy: (1) application of a frequency-dependent intensity correction of the Raman spectra; (2) normalization of the water peak using the broad T-O and T-O-T vibration band at 850-1250 cm-?1 wavenumbers (instead of the low wavenumber T-O-T broad band, which appeared to be highly sensitive to the FeO content and the degree of polymerization of the melt); (3) normalization of the integrated Si-O band area by the total number of tetrahedral cations and the position of the band maximum. The calibration line shows a -0.4 wt% uncertainty at one relative standard deviation in the range of 0.8-9.5 wt% water and a wide range of natural melt compositions. This method provides a simple, quick, broadly available and cost-effective way for a quantitative determination of the water content of silicate glasses. Application to silicate melt inclusions yielded data in good agreement with SIMS data. [PUBLICATION ABSTRACT]

The D. João de Castro is a submarine volcano with known hydrothermal activity located in the Terceira ultra-slow spreading rift within the Azores triple junction (ATJ). Several well-known mafic and ultramafic rock-hosted seafloor hydrothermal systems lay along the Mid-Atlantic Ridge, to the north and to the south of the Azores platform, yet little is known about seafloor hydrothermal activity, ore-metal availability, and magmatic–hydrothermal interactions within the ATJ. Here, we investigate multi-phase melt inclusions hosted in early formed phenocrysts (olivine, clinopyroxene and plagioclase), and metallic precipitates found in groundmass vesicles. Combining detailed petrographic observations with geochemical data and thermobarometry calculations, we assess P–T conditions of early formed phenocrysts, melt pathways towards surface, timing of sulfide saturation and composition of immiscible sulfide melts. Results show that D. João de Castro is characterized by a multi-level magmatic system where primary melt segregated from the upper mantle and moved up through the oceanic crust with little residence time. Sulfide saturation with the formation of immiscible magmatic sulfide liquid (Fe–Ni–Cu) occurred early in primitive magmas with clinopyroxene and olivine crystallization and continued during plagioclase crystallization. At shallower levels, the magmatic degassing of volatiles carrying base metals (Cu–Zn–Pb–Co) and Ba have contributed to the element budget of the D. João de Castro hydrothermal system. The study of multi-phase melt inclusions and vesicles at D. João de Castro submarine volcano contributes to the understanding of source to surface magmatic processes at the Terceira Rift and underline the importance of magmatic degassing into seafloor hydrothermal systems.