CO2-rich Vapor Research Articles

CO2 outgassing during collisional orogeny is facilitated by the generation of immiscible fluids

Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO2) from the continental crust. However, the processes of carbon mobilization are still poorly explored. Here, we use thermodynamic modeling to investigate the decarbonation of sediments metamorphosed under high geothermal gradients. Our modeling shows that immiscible CO2-rich vapors and hydrosaline brines are generated at these conditions, with different properties and mobility through the crust. The CO2-rich fluid fraction could rapidly rise toward the surface without interacting with the host rocks by carbo-fracturing the host rocks or through deep faults. The denser hydrosaline brines likely permeate the source rocks. When applied to the active Himalayan orogen, these observations reconcile measured CO2 fluxes at the surface and positive conductivity anomalies associated with micro-seismicity at depth. Our modeling shows that the continental crust represents a relevant reservoir of CO2 that can be efficiently degassed during hot collisions.

Pore scale modeling and evaluation of clogging behavior of salt crystal aggregates in CO2-rich phase during carbon storage

The optimal CO2 storage operation requires high permeability in the near-well region in order to keep it safe and cost-efficient. Nucleation and growth of salt crystals driven by the evaporation of formation water into under-saturated (dry) super-critical CO2 streams result in the changes in porosity and permeability of the near well-bore area. Permeability reduction is one of the main reasons for injectivity losses in the context of CO2 storage in saline aquifers. According to recent studies, during CO2 storage, salt crystals grow in two different forms: 1) single, large crystals in the aqueous phase, and 2) aggregates of micro-meter size salt crystals in the CO2-rich vapor phase. All previous numerical studies at pore-scale have addressed the formation of single, large crystals in the aqueous phase. In this work we have developed a 3D pore-scale reactive transport solver based on a D3Q19 advection-diffusion Lattice-Boltzmann model. The model takes for the first time salt nucleation into consideration via a new probabilistic approach to simulate the formation of micro-meter size salt crystal aggregates in the CO2-rich phase and their effect on changes in pore morphology and permeability. Comparing the results of porosity-permeability relations with some of the well-known clogging models, confirms the need for a new clogging model to capture the permeability reduction caused by salt aggregates.

Arsenian Pyrite and Cinnabar from Active Submarine Nearshore Vents, Paleochori Bay, Milos Island, Greece

Active, shallow-water (2–10 m below sea level) and low temperature (up to 115 °C) hydrothermal venting at Paleochori Bay, nearshore Milos Island, Greece, discharges CO2 and H2S rich vapors (e.g., low-Cl fluid) and high-salinity liquids, which leads to a diverse assemblage of sulfide and alteration phases in an area of approximately 1 km2. Volcaniclastic detritus recovered from the seafloor is cemented by hydrothermal pyrite and marcasite, while semi-massive to massive pyrite-marcasite constitute mounds and chimney-like edifices. Paragenetic relationships indicate deposition of two distinct mineralogical assemblages related to the venting of high-Cl and low-Cl fluids, respectively: (1) colloform As- and Hg-bearing pyrite (Py I), associated with marcasite, calcite, and apatite, as well as (2) porous and/or massive As-rich pyrite (Py II), associated with barite, alunite/jarosite, and late-stage hydrous ferric oxides. Mercury, in the form of cinnabar, occurs within the As-rich pyrite (Py I) layers, usually forming distinct cinnabar-enriched micro-layers. Arsenic in colloform pyrite I shows a negative correlation with S indicating that As1− dominates in the pyrite structure suggesting formation from a relatively reducing As-rich fluid at conditions similar to low-sulfidation epithermal systems. On the contrary, As3+ dominates in the structure of porous to massive pyrite II suggesting deposition from a sulfate-dominated fluid with lower pH and higher fO2. Bulk sulfide data of pyrite-bearing hydrothermal precipitates also show elevated As (up to 2587 ppm) together with various epithermal-type elements, such as Sb (up to 274 ppm), Tl (up to 513 ppm), and Hg (up to 34 ppm) suggesting an epithermal nature for the hydrothermal activity at Paleochori Bay. Textural relationships indicate a contemporaneous deposition of As and Hg, which is suggested to be the result of venting from both high-salinity, liquid-dominated, as well as CO2- and H2S-rich vapor-dominated fluids that formed during fluid boiling. The CO2- and H2S-rich vapor that physically separated during fluid boiling from the high-salinity liquid led to calcite formation upon condensation in seawater together with the precipitation of As- and Hg-bearing pyrite I. This also led to the formation of sulfuric acid, thereby causing leaching and dissolution of primary iron-rich minerals in the volcaniclastic sediments, finally resulting in pyrite II precipitation in association with alunite/jarosite. The Paleochori vents contain the first documented occurrence of cinnabar on the seafloor in the Mediterranean area and provide an important link between offshore hydrothermal activity and the onshore mercury and arsenic mineralizing system on Milos Island. The results of this study therefore demonstrate that metal and metalloid precipitation in shallow-water continental arc environments is controlled by epithermal processes known from their subaerial analogues.

Sulfide Emplacement and Migration in the Nova-Bollinger Ni-Cu-Co Deposit, Albany-Fraser Orogen, Western Australia

Abstract The Nova-Bollinger Ni-Cu sulfide deposit is associated with a small chonolith (tube-shaped) intrusion emplaced at lower crustal depths into granulite facies migmatite gneisses. The deposit comprises disseminated and net-textured ores within the intrusions and a high proportion of massive, semimassive, and breccia exocontact ores within the underlying country rocks. Internally disposed endocontact ores show typical magmatic textures including conventional net texture, leopard net texture characterized by the presence of centimeter-sized clots of olivine and intercumulus phases, and globular ores. Some of the globular ores show an association of sulfide blebs with clinopyroxene-carbonate intergrowths that may represent infilling of original CO2-rich vapor bubbles. The exocontact ores have an assemblage of textures indicative of emplacement into hot, soft country rocks at a large-scale melting-infiltration front. Characteristic features range from hard-walled extensional vein arrays to complex infiltrations of disseminated sulfide within chaotically folded paragneiss. Sulfide infiltration was accompanied by partial melting of the country rock, producing felsic leucosomes, some of them strongly enriched in garnet, mainly occupying vein walls and interpreted as the result of counterflow of displaced silicate partial melt. Coarse-grained pentlandite-chalcopyrite-pyrrhotite loop textures are characteristic of all ore types, down to the scale of the infiltrating sulfides within the gneisses, and are regarded as diagnostically magmatic textures generated by sulfide liquid fractionation and growth of high-temperature pentlandite by peritectic reaction between fractionated sulfide melt and early crystallized monosulfide solid solution. The highly distinctive features of the Nova-Bollinger ores are a consequence of their emplacement in the mid to lower crust under peak granulite facies conditions. Under these unusual conditions the timescales for cooling between the silicate solidus and sulfide solidus temperatures were of the order of millions of years, being controlled by the temperature-time path for the exhumation of the orogen as a whole. Sulfides solidified over a time period three orders of magnitude greater than the thousand-year timescale for the solidification of the host silicate magmas. Furthermore, timescales for deformation matched those for cooling and solidification, allowing the country rocks to undergo deformation during ore emplacement. Fluctuating strain rates during and after initial emplacement of the carrier magmas into the host intrusion caused episodes of brittle extension, allowing unusually efficient penetration of partially molten sulfide into heterogeneous, partially molten silicate country rock, resulting in an unusually extensive thermomechanical aureole compared with other mafic intrusion-hosted nickel systems globally.

Carbonatite and highly peralkaline nephelinite melts from Oldoinyo Lengai Volcano, Tanzania: The role of natrite-normative fluid degassing

Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO2 flux. The volcano is constructed of highly peralkaline [PI = molar (Na2O + K2O)/Al2O3 > 2–3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO2-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO2-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750–800 °C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natritess-normative, F-rich (7–14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2–7.9), iron-rich nephelinite melt coexisted following degassing of a CO2 + H2O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/ mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/ enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide.

Fluid inclusions and isotope (C, H, O, S, He, and Ar) study of the Xiaobaishitou skarn W–(Mo) deposit, East Tianshan, NW China

The Xiaobaishitou W–(Mo) deposit (36,264 tonnes; 0.78 wt% WO3) is located in the eastern segment of the Central Tianshan Terrane in the East Tianshan, Northwest China. The deposit comprises 96 economic orebodies that developed in skarns. The orebodies are distributed along the contacts between Triassic biotite granite and Mesoproterozoic limestone. Based on crosscutting relationships and mineral assemblages, mineralization processes at Xiaobaishitou can be divided into four stages: prograde skarn (stage I), retrograde skarn (stage II), quartz–sulfides (stage III), and calcite (stage IV). Stages II and III comprised the majority of W resources, with Mo mineralization occurring mainly in stage III. Stage II also produced minor Mo mineralization characterized by garnet and/or diopside skarns with a molybdenite–chalcopyrite–pyrite assemblages.Fluid evolution during the four stages of mineralization was determined by a detailed fluid inclusion study and six types of fluid inclusion assemblages (FIAs) were recognized: L0 (liquid-rich two-phase FIAs), V0 (vapor-rich two-phase FIAs), C1 (CO2-rich two-phase FIAs), C2 (CO2-rich vapor FIAs), rare L1 (liquid-rich three-phase FIAs with the transparent mineral) and M (melt inclusions). Fluid boiling in stage II, which is caused by fluid mixing (typically the mixing of magmatic water with meteoric water) and abrupt pressure release (possibly due to local fracturing), led to the formation of inhomogeneous fluids and the precipitation of scheelite. With pressure release and abundant meteoric water mixing, fluid boiling continued in stage III and led to precipitation of more scheelite and a large volume of molybdenite.Carbon, hydrogen, oxygen, sulfide, helium, and argon isotope data from stages I to IV indicate that the ore-forming fluids had a dominantly magmatic signature (crustal granite with minor mantle composition) and were diluted by meteoric waters, which started in stage II and developed in stage III.Base on mineral geothermometers, fluid inclusion microthermometry, and multiple isotopes (C, H, O, S, He, and Ar) studies, the fluid boiling in the stages II and III at Xiaobaishitou directly led to the formation of inhomogeneous fluids and scheelite/molybdenite precipitation. However, scheelite precipitation may have resulted from an increase in fluid pH, led mainly by fluid-rock interactions. The sulfide precipitation was likely controlled by the increases in pH and fO2, caused mainly by magmatic–meteoric fluid mixing. Meanwhile, the Mantle- and crust-derived magmatic–hydrothermal fluid and shallow emplacement with low-temperature and low-density ore-forming fluid primarily led to scheelite and molybdenum symbiosis at Xiaobaishitou deposit.

Hydrothermal fluid evolution of the Jintingling gold deposit in the Jiaodong peninsula, China: Constraints from U-Pb age, CL imaging, fluid inclusion and stable isotope

The Jintingling gold deposit, located in the Jiaodong peninsula, north China, contains vein-, dissemination- and stockwork-style ores hosted in the Jurassic Linglong granitic pluton. Hydrothermal activities in this deposit are divided into seven stages, including pre-ore massive K-feldspar alteration, specularite-quartz vein (V1), milky quartz ± sulfide vein (V2), quartz-pyrite vein (V3), massive quartz-sericite-pyrite alteration and gold mineralization, quartz-polymetallic sulfide vein (V4), and quartz-calcite vein (V5). Rhenium-Osmium dating was conducted on molybdenite from V2, yielding an isochron age of 123.3 ± 3.6 Ma. LA-ICP-MS zircon U-Pb dating yielded 121.5 ± 1.5 Ma (1σ, MSWD = 0.010) for a pre-ore diorite porphyrite dyke and 117.6 ± 1.2 Ma (1σ, MSWD = 0.018) for a post-ore diorite porphyry dyke, respectively. Thus, gold mineralization occurred at 122–118 Ma.Four types of fluid inclusions were identified in quartz veins, including CO2-H2O ± CH4 inclusions (type I), pure CO2 inclusions (type II), mineral-bearing inclusions (type III) and aqueous inclusions (type IV). Fluid inclusion studies demonstrate that the primary ore-forming fluids contain CO2-rich vapor (357–420 °C, 7.2–11.3 wt% NaCl equivalent) and critical fluids (368–400 °C). The two phases were likely formed by immiscibility of magmatic-hydrothermal fluids. Later on, the fluids evolved to a H2O-CO2-NaCl ± CH4 system which had temperatures of 300–364 °C and salinities of 2.0–9.7 wt% NaCl equivalent. The H2O-CO2-NaCl ± CH4 fluid system may experience phase separation, forming coexisted CO2-H2O ± CH4 inclusions (homogenized to vapor or liquid at temperature of 265–309 °C and 265–287 °C, salinities of 3.1–7.1 wt% and 4.3–7.1 wt% NaCl equivalent, respectively) and aqueous inclusions (homogenized to liquid at temperature of 255–294 °C and 3.7–7.6 wt% NaCl equivalent). Along with further evolution, the fluids shifted to a H2O-NaCl system, with temperatures of 163–258 °C and salinities of 0.5–9.0 wt% NaCl equivalent.Pyrite from the Jintingling deposit has δ34S values (5.5‰–6.1‰) similar to those of pyrite from other gold deposits in the Jiaodong district, indicating that they were related to the same hydrothermal ore-forming system. The δ13CPDB values of calcite (−4.5‰ to −5.4‰) in V5 are generally located in the range of magmatic-derived carbon. Fluids of V2, V3, V4 have δD from −64.4‰ to −87.1‰, and δ18O from 8.4‰ to 0.2‰, located in the compositional fields between magmatic water and meteoric water. Collectively, microthermonetric results and isotopic data indicate that the primary ore-forming fluids were most probably magmatic in origin, with incorporation of meteoric water in the later ore-forming stage. Precipitation of gold was triggered by cooling of fluids, fluid immiscibility and decrease of sulfur in the fluids.

Geochemistry of thermal waters in the Southern Volcanic Zone, Chile – Implications for structural controls on geothermal fluid composition

Thermal waters in the Southern Volcanic Zone (SVZ) of Chile between 36°–41°S were studied using major and selected trace element relationships to characterize their geochemistry, formation mechanisms, and to explore the influence of regional structural controls on fluid composition. Three sets of waters were identified based on physicochemical characteristics: (i) NaCl waters, (ii) acid-sulfate waters, and (iii) bicarbonate (HCO3) waters. NaCl waters are the most abundant type in the studied region and their chemistry is controlled by significant water-rock interaction. They are characterized by an alkaline pH (7.2–9.3), generally lower temperatures (avg: 55°C), and relatively high concentrations of Cl, Na, B, As, Li, and Cs. Acid-sulfate waters are typically associated with volcanoes and have a strong magmatic/volcanic component due to the absorption of magmatic vapors. They are acidic (pH<4), generally higher in temperature (avg: 85°C), and have elevated concentrations of SO4, Mg, and Ba. Bicarbonate waters are characterized by the highest concentrations of HCO3 (>892ppm) in the region and are similar in temperature (<47°C) and pH (>6.2) to NaCl waters. They have elevated concentrations of most cations (Ca, K, Na, Mg, Ba, Sr) as a result of intense shallow cation leaching due to the absorption of CO2-rich volcanic vapors on the peripheries of geothermal systems. The thermal waters were also characterized according to their spatial relation with the dominant fault systems of the region: the NNE-striking intra-arc Liquiñe-Ofqui Fault System (LOFS) and the WNW-striking Andean Transverse Faults (ATF). The inherent differences in fault nature between these fault systems constitutes the primary structural control influencing geothermal fluid development in the SVZ. The chemistry of waters spatially associated with the LOFS as a whole is defined by high vertical permeability networks and lack magmatic reservoir development. Therefore, these waters tend to have higher Cl/B ratios and strong correlations between trace alkali metals and Cl due to rapid, efficient upflow pathways. In contrast, waters spatially associated with the ATF have lower Cl/B ratios and show no correlation between trace alkali metals and Cl due to degassing magma chambers and decreased vertical permeability. The relationship between water type and structural domain in Cl/B ratios and trace metal behavior provides evidence that fault geometry and kinematics exert a fundamental control on geothermal fluid development in the SVZ of Chile.

Improved Understanding of CO2–Water Pretreatment of Guayule Biomass by High Solids Ratio Experiments, Rapid Physical Expansion, and Examination of Textural Properties

In this work, we provide a systematic study of CO2–water pretreatment of guayule biomass to optimize the residual ground bagasse from natural rubber extraction for hydrolysis and fermentation. Guayule biomass is mixed with water then loaded into a 250 mL reactor with exposure to a biphasic environment consisting of a CO2-rich vapor phase and water-rich liquid phase. The pressure is then rapidly released for a “physical expansion” effect. The pretreated biomass is enzymatically hydrolyzed, and the sugar concentration in hydrolysate is measured. Experimental runs are conducted in the temperature range of 145 to 210 °C and pressure range of 3.4 to 34 MPa. The solids ratio (dry solids mass/water mass) is between 0.17 and 1.7. The packing density is between 0.03 and 0.2 g of biomass per cm3 of the reactor, and the holding time ranges from 20 to 840 min. A 4-fold increase in the reactor volume is performed and optimized with a 1-L vessel. High-pressure carbon dioxide–water pretreatment increases surface area of...

Stabilization and Antifouling of Polymer Films on a Planar Surface by CO2 Pressurization.

In this article, we study the dewetting phenomenon of a polymer and carbon dioxide (CO2) mixture on a planar surface by combining density functional theory and the string method. It is found that dewetting is a first-order discontinuous phase transition. When the pressure is lower than the completely dewetting pressure (Pd), CO2 stabilizes the polymer films. The density fluctuation of the polymer decreases significantly with the inclusion of CO2. When the pressure is above Pd, the polymer film is depleted far away from the surface, leaving a thick layer of pure CO2 in the region near the surface. Pd is proportional to the surface energy strength. The CO2 molecules enhance the density fluctuation of the polymer during the dewetting process. The polymer-rich phase at the triple point dewets to a CO2-rich vapor film, as the CO2-rich liquid film near the surface is metastable. These results have promising application in the industry of fabricating polymer films and antifouling polymers on attractive surfaces.

From a long-lived upper-crustal magma chamber to rapid porphyry copper emplacement: Reading the geochemistry of zircon crystals at Bajo de la Alumbrera (NW Argentina)

The formation of world class porphyry copper deposits reflect magmatic processes that take place in a deeper and much larger underlying magmatic system, which provides the source of porphyry magmas, as well as metal and sulphur-charged mineralising fluids. Reading the geochemical record of this large magmatic source region, as well as constraining the time-scales for creating a much smaller porphyry copper deposit, are critical in order to fully understand and quantify the processes that lead to metal concentration within these valuable mineral deposits. This study focuses on the Bajo de la Alumbrera porphyry copper deposit in Northwest Argentina. The deposit is centred on a dacitic porphyry intrusive stock that was mineralised by several pulses of porphyry magma emplacement and hydrothermal fluid injections. To constrain the duration of ore formation, we dated zircons from four porphyry intrusions, including pre-, syn- and post-mineralisation porphyries based on intersection relations between successive intrusion and vein generations, using high precision CA-ID-TIMS. Based on the youngest assemblages of zircon grains, which overlap within analytical error, all four intrusions were emplaced within 29 ka, which places an upper limit on the total duration of hydrothermal mineralisation. Re/Os dating of hydrothermal molybdenite fully overlaps with this high-precision age bracket. However, all four porphyries contain zircon antecrysts which record protracted zircon crystallisation during the ∼200 ka preceding the emplacement of the porphyries. Zircon trace element variations, Ti-in-zircon temperatures, and Hf isotopic compositions indicate that the four porphyry magmas record a common geochemical and thermal history, and that the four intrusions were derived from the same upper-crustal magma chamber. Trace element zoning within single zircon crystals confirms a fractional crystallisation trend dominated by titanite and apatite crystallisation. However, zircon cathodoluminescence imaging reveals the presence of intermediate low luminescent (dark) growth zones in many crystals from all intrusions, characterised by anomalously high Th, U and REE concentrations and transient excursions in trace element ratios. A return to the same fractionation trend after this excursion excludes external compositional forcing such as magma mixing. Instead we interpret the “dark-zones” to record zircon crystallisation during a transient event of rapid growth that resulted from mafic magma injection into the base of the magma chamber, releasing a CO2-rich vapour phase into the dacitic crystal mush. We propose that this vapour phase then migrated upwards to the apical part of the magma chamber from where it was expelled, together with successive batches of magma, to form the porphyry copper deposit within a short time-span of less than a few 10,000 years. The short duration of host rock emplacement, hydrothermal alteration and mineralisation presented in this study provides critical constraints on fluid storage in magma chambers and the genesis of large porphyry copper deposits.

Chemical and stable isotopic (B, H, and O) compositions of tourmaline in the Maocaoping vein-type Cu deposit, western Yunnan, China: Constraints on fluid source and evolution

This study determined the chemical and stable isotopic (B, H, and O) compositions of three stages of ore-related hydrothermal tourmaline in the newly discovered Maocaoping vein-type Cu deposit, western Yunnan, China, as well as the chemical and B isotopic compositions of magmatic tourmaline (tourmaline-G) from a neighboring peraluminous granitic intrusion that is contemporaneous with the Cu mineralization. The objective was to constrain the source and evolution of the CO2-rich ore-forming fluid.The Maocaoping cupriferous veins (types V1 and V2) are hosted in mylonitized and metamorphosed Jurassic sedimentary rocks. Early- to late-stage hydrothermal tourmaline occurs as vein-related alteration halos (tourmaline-A) in altered marble or schist, as tourmaline-V1 in type V1 veins, and as tourmaline-V2 in type V2 veins. Tourmaline-A, -V1, and -V2 belong to the alkalic group and consist mostly of dravite, with δ11B values of −3.9‰ to −1.1‰, −2.8‰ to +0.5‰, and −6.3‰ to −3.9‰, respectively. Tourmaline-G belongs to the alkalic group and consists of schorl, with δ11B values of −14.7‰ to −12.2‰. The similarities in chemical composition between marble- and schist-hosted tourmalines and between tourmalines in veins and alteration halos (indicating their formation at different water/rock ratios) suggest that the compositions of the hydrothermal tourmaline at Maocaoping were controlled mainly by the chemistry of ore fluid rather than that of the host rocks. The similarities together with the lack of systematic variations in δ11B values in individual tourmaline grains imply that mixing of multiple fluids cannot be important during the mineralization. The positive shift in δ11B values from tourmaline-A to tourmaline-V1 is interpreted to result from continuous precipitation of tourmaline in a closed fluid system. Relative to tourmaline-A and -V1, tourmaline-V2 is Fe-rich, Al-poor, and 11B-depleted. These differences may be explained by fluid phase separation and subsequent escape of the CO2-rich vapor phase during the formation of type V2 veins, consistent with the observations that these veins are extensional, contain large amounts of carbonate minerals, and include fluid inclusions trapped in the vapor–liquid two-phase field. The earliest tourmaline-A records the initial ore fluid chemistry. The calculated δ11B, δ18O, and δD values of the initial ore fluid in equilibrium with tourmaline-A at mineralization temperatures of 280 to 320°C are −0.3‰ to +3.0‰, +10.7‰ to +11.8‰ and −110‰ to −84‰, respectively. The δ11B values as well as the chemical compositions of tourmaline-A are much different from those of tourmaline-G, suggesting that the ore fluid at Maocaoping was not derived directly from the peraluminous granitic magma. The B–H–O isotopic features as well as enrichment of CO2 in fluid inclusions also suggest the ore fluid cannot solely be derived from basinal fluid or meteoric water. The simplest explanation is that the fluid was derived from a metamorphic fluid that had obtained isotopically lighter hydrogen via reaction with “organic” compounds (e.g., CH4, H2S). Alternatively, the ore fluid sourced from the mixed metamorphic fluid and basinal brine, meteoric water, or degassed magmatic fluid. It implies that the vein Cu ore formation in the Lanping Basin is unlikely as a result of a sole magmatic or basinal fluid activity and it needs the involvement of metamorphic fluid/process.

CO2-fluxing collapses metal mobility in magmatic vapour

Magmatic systems host many types of ore deposits, including world-class deposits of copper and gold. Magmas are commonly an important source of metals and ore-forming fluids in these systems. In many magmatic-hydrothermal systems, low-density aqueous fluids, or vapours, are significant metal carriers. Such vapours are water-dominated shallowly, but fluxing of CO2-rich vapour exsolved from deeper magma is now recognised as ubiquitous during open-system magma degassing. Furthermore, we show that such CO2-fluxing leads to a sharp drop in element solubility, up to a factor of 10,000 for Cu, and thereby provides a highly efficient, but as yet unrecognised mechanism for metal deposition.

Copper partitioning between felsic melt and H2O–CO2 bearing saline fluids

Analysis of fluid inclusions from porphyry copper deposits reveals that magmatic vapor and brine are vital for the removal of copper from arc magmas and its transport to the site of ore deposition. Experiments in melt–vapor–brine systems allow for investigation of the partitioning of copper between silicate melts and volatile phases at magmatic conditions. The presence of CO2 affects both the pressure at which a melt saturates with respect to volatile phases. Therefore, the partitioning of copper among felsic (rhyolitic) melt, vapor and brine in CO2-bearing experiments was examined to provide insights into copper partitioning and the generation of porphyry copper and related deposits.We present results from experiments performed at 800°C and 100MPa in CO2-bearing melt–vapor–brine systems with XCO2v+b=0.10 and 0.38. The compositions of vapor and brine inclusions, and run-product glasses, were determined during the course of this investigation. Microthermometric measurements of fluid inclusions show an increase in the salinity of the magmatic brine (∼65 to ∼70wt% NaCleq) and decrease in the salinity of the vapor (∼3.5 to ∼1wt% NaCleq) as XCO2 is increased from 0.10 to 0.38. The partitioning of copper between brine and vapor (DCub/v±2σ) increases from 25 (±6) at XCO2=0.10, to 100 (±30) at XCO2=0.38. The partitioning of copper between vapor and melt (DCuv/m±2σ) decreases from 9.6 (±3.3) at XCO2=0.10, to 2 (±0.8) at XCO2=0.38. These data demonstrate that copper partitioning in sulfur-free, CO2-bearing systems is controlled by the changes in the salinity of the vapor and brine that, in turn, are functions of XCO2. No change in the apparent equilibrium constants for Cu–Na exchange was observed in Fe-bearing experiments which supports a salinity-dependent model for copper partitioning.An existing model (MVPart) for ore metal partitioning between melt and volatile phases was modified to incorporate partitioning data from CO2-bearing experiments. The model can be used to predict trends in efficiency of removal of copper from melts into exsolving CO2-bearing magmatic volatile phases at a fixed pressure and temperature. The CO2-MVPart model predicts that CO2-rich (XCO2=0.38) magmatic vapor will remove half of the available copper from the melt, without a contribution from the brine, compared to low CO2 (XCO2≤0.10) systems. Thus, periods of CO2-rich vapor exsolution are not expected to be associated with efficient removal of copper from shallow felsic melts.

Petrological cannibalism: the chemical and textural consequences of incremental magma body growth

The textures of minerals in volcanic and plutonic rocks testify to a complexity of processes in their formation that is at odds with simple geochemical models of igneous differentiation. Zoning in plagioclase feldspar is a case in point. Very slow diffusion of the major components in plagioclase means that textural evidence for complex magmatic evolution is preserved, almost without modification. Consequently, plagioclase affords considerable insight into the processes by which magmas accumulate in the crust prior to their eventual eruption or solidification. Here, we use the example of the 1980–1986 eruptions of Mount St. Helens to explore the causes of textural complexity in plagioclase and associated trapped melt inclusions. Textures of individual crystals are consistent with multiple heating and cooling events; changes in total pressure (P) or volatile pressure ( $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ ) are less easy to assess from textures alone. We show that by allying textural and chemical analyses of plagioclase and melt inclusions, including volatiles (H2O, CO2) and slow-diffusing trace elements (Sr, Ba), to published experimental studies of Mount St. Helens magmas, it is possible to disambiguate the roles of pressure and temperature to reconstruct magmatic evolutionary pathways through temperature–pressure–melt fraction (T– $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ –F) space. Our modeled crystals indicate that (1) crystallization starts at $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ > 300 MPa, consistent with prior estimates from melt inclusion volatile contents, (2) crystal cores grow at $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ = 200–280 MPa at F = 0.65–0.7, (3) crystals are transferred to $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ = 100–130 MPa (often accompanied by 10–20 °C of heating), where they grow albitic rims of varying thicknesses, and (4) the last stage of crystallization occurs after minor heating at $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ ~ 100 MPa to produce characteristic rim compositions of An50. We hypothesize that modeled $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ decreases in excess of ~50 MPa most likely represent upward transport through the magmatic system. Small variations in modeled $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ , in contrast, can be effected by fluxing the reservoir with CO2-rich vapors that are either released from deeper in the system or transported with the recharge magma. Temperature fluctuations of 20–40 °C, on the other hand, are an inevitable consequence of incremental, or pulsed, assembly of crustal magma bodies wherein each pulse interacts with ancestral, stored magmas. We venture that this “petrological cannibalism” accounts for much of the plagioclase zoning and textural complexity seen not only at Mount St. Helens but also at arc magmas generally. More broadly we suggest that the magma reservoir below Mount St. Helens is dominated by crystal mush and fed by frequent inputs of hotter, but compositionally similar, magma, coupled with episodes of magma ascent from one storage region to another. This view both accords with other independent constraints on the subvolcanic system at Mount St. Helens and supports an emerging view of many active magmatic systems as dominantly super-solidus, rather than subliquidus, bodies.

In-situ geochemistry of sulfides in highly metasomatized mantle xenoliths from Kerguelen, southern Indian Ocean

Mantle xenoliths from the Kerguelen Archipelago record a complex multistage history involving a high degree (15 to 25%) partial melting that created a harzburgitic mantle completely stripped of Base Metal Sulfides (BMS), followed by pervasive melt-rock reaction with alkaline melts above the Kerguelen mantle plume. Subsequent reaction of the highly refractory protolith with small volumes of carbonate-rich silicate melts led to a re-enrichment in BMS (up to 0.05wt.%). Two BMS precipitation mechanisms are suggested: immiscibility from the silicate-carbonate melt and sulfidation reactions from a CO2-rich supercritical fluid. In-situ analyses of chalcophile and siderophile elements (major and trace levels) in the BMS shed new light on their origin. The BMS phases that precipitated via immiscibility are metal-rich sulfide melts which progressively evolved toward Ni and Cu-rich end-members by cumulate fractionation of monosulfide solid solution (mss) during percolation inside the peridotites. Some cumulate mss have elevated and fractionated IPGE contents (200–900× C1-Chondrite abundances), indicating random digestion of preexisting Os, Ir, Ru-rich PGM by the percolating sulfide melt. The BMS that precipitated by sulfidation reactions from a CO2-rich vapour phase are subsolidus exsolution products from Cu-bearing but Ni-poorer mss. They have the highest concentrations of PGEs and show selective enrichment in S, Pd, Pt and Os over Cu, S, Ir, Ru and Rh. Their PGE compositions confirm experimental data, which demonstrate that S, Pd, Pt and Os can be efficiently transported in a CO2-rich supercritical fluid. Superchondritic (S/Se), (Os/Ir) and (Pd/Pt) in both bulk-rocks and individual sulfides are inferred to be the geochemical fingerprints of sulfide crystallisation from a CO2-rich vapour exsolved from a highly evolved carbonate-rich metasomatic melt.

XAS evidence for the stability of polytellurides in hydrothermal fluids up to 599 C, 800 bar

Although the crustal abundance of tellurium (Te) is about half of that of gold (Au), several classes of Au deposits are highly enriched in Te. Our understanding of the nature of this Au-Te association is hampered by the lack of experimental studies of Te geochemistry at elevated temperature. We characterized the structure of polytelluride solutions from room temperature to 599 °C at 800 bar using in situ X-ray absorption spectroscopy. Both ab-initio XANES and EXAFS fits show that polytellurides are stable up to the highest temperature, with planar structures (four- or threefold coordination of Te) giving way to linear chains (e.g., Te2-2ion) at temperatures above ~200 °C. This is the first experimental confirmation of the thermal stability of polytelluride species. The data show that polytellurides play an important role in Te transport in reduced S-rich or CO2-rich solutions and vapors.

マグマ供給系における深部二酸化炭素の輸送現象

Carbon dioxide fluxing is a recently proposed process in which a large amount of CO2-rich vapour migrates in crustal magmatic systems. Such a proposal was derived from analytical studies on glass inclusions that have higher CO2/H2O ratios than those usually expected from a simple degassing process. The fluxing may be a universal phenomenon that occurs in various geological settings. However, its mechanisms, including the transport mode, duration and source, are still unclear. In this paper I review recent case studies on vapour transport in active volcanoes and discuss unsettled points and future studies.

Fluid evolution in zoned Cordilleran polymetallic veins — Insights from microthermometry and LA-ICP-MS of fluid inclusions

Fluid inclusion analysis through the paragenetic sequence of one symmetrically zoned vein sample is used to reconstruct the P–T–X fluid evolution of a porphyry intrusion-related Cordilleran polymetallic vein from Morococha, central Peru. Results record an evolution from initial deep-seated precipitation of quartz–pyrite and base metal sulphides to final near-surface deposition of carbonates, demonstrating progressive mineralisation during uplift and erosion. This is the first detailed study addressing meso- to epithermal Zn–Pb–Ag–Cu-rich ore in a magmatic–hydrothermal system by combination of fluid inclusion microthermometry with laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) quantifying metal, as well as sulphur concentrations in the evolving hydrothermal fluid. Scanning electron microscopy cathodoluminescence (SEM-CL) imaging of quartz and detailed transmitted- and reflected-light petrography provide textural evidence that early, moderately saline (4–5 wt.% NaCl eq.) and CO 2-bearing fluids with homogenisation temperatures of 340°–380 °C precipitate Cu-bearing minerals. In this open hydrothermal system the fluids record decreasing salinities, CO 2-contents and temperatures, while Zn-, Pb-, and Ag-sulphides precipitate. Fluids related to early precipitation in the vein have metal contents of several 1000 μg/g S and Fe, over 1000 μg/g Cu, 100 μg/g Pb, 10 μg/g Ag, and several 100 μg/g Zn. Sulphur concentrations in the fluid are sufficiently high to precipitate all metals in solution as sulphides. The latest generation of fluid inclusions associated with abundant carbonate precipitation in the centre of the vein has homogenisation temperatures ranging from 260° to 220 °C, low metal concentrations, and no measurable CO 2. During vein formation, cooling and several kilometres of erosion resulted in “telescoping” of consecutively precipitated mineral assemblages. The deep input fluid dominating in the early vein stage is interpreted to be of magmatic origin, most likely a single phase magmatic fluid of intermediate salinity and density. It cooled to an aqueous liquid, separated minor CO 2-rich vapour, and was eventually diluted by meteoric water in the late stages of vein formation when the progressively eroded land surface was only several hundred meters above the vein location.

Segregating gas from melt: an experimental study of the Ostwald ripening of vapor bubbles in magmas

Diffusive coarsening (Ostwald ripening) of H2O and H2O-CO2 bubbles in rhyolite and basaltic andesite melts was studied with elevated temperature–pressure experiments to investigate the rates and time spans over which vapor bubbles may enlarge and attain sufficient buoyancy to segregate in magmatic systems. Bubble growth and segregation are also considered in terms of classical steady-state and transient (non-steady-state) ripening theory. Experimental results are consistent with diffusive coarsening as the dominant mechanism of bubble growth. Ripening is faster in experiments saturated with pure H2O than in those with a CO2-rich mixed vapor probably due to faster diffusion of H2O than CO2 through the melt. None of the experimental series followed the time1/3 increase in mean bubble radius and time−1 decrease in bubble number density predicted by classical steady-state ripening theory. Instead, products are interpreted as resulting from transient regime ripening. Application of transient regime theory suggests that bubbly magmas may require from days to 100 years to reach steady-state ripening conditions. Experimental results, as well as theory for steady-state ripening of bubbles that are immobile or undergoing buoyant ascent, indicate that diffusive coarsening efficiently eliminates micron-sized bubbles and would produce mm-sized bubbles in 102–104 years in crustal magma bodies. Once bubbles attain mm-sizes, their calculated ascent rates are sufficient that they could transit multiple kilometers over hundreds to thousands of years through mafic and silicic melt, respectively. These results show that diffusive coarsening can facilitate transfer of volatiles through, and from, magmatic systems by creating bubbles sufficiently large for rapid ascent.