Magma Degassing Research Articles

Native Iron in Siberian Traps

The results of study of intrusive traps with a large-scale occurrence of native iron allowed us to identify general patterns of their composition and origin. Intrusive bodies are weakly differentiated; they feature a similar structure and mineralogical, petrochemical and geochemical composition. Two associations of rock-forming minerals were found in all the studied bodies, i.e. early deep (pre-chamber) and intra-chamber. Native iron forms nodular segregation, with a subordinate amount of cohenite, troilite and magnetite-wustite. Natural reduced iron can concentrate many elements, such as Ni, Co, Au and PGE. Their content in metal increases by hundreds or even thousands of times compared to the hosting silicate part. The formation of native iron is based on the fluid-magmatic interaction between magma substance and reducing components of the fluid, primarily of methane-hydrogen composition. As a result, dispersion of a primarily homogeneous basalt liquid into silicate and metallic components occurs. In the process of transfer, finely dispersed phases of iron form droplet-liquid segregations with a monomolecular layer of gas on their surface that prevents enlargement of metallic droplets. In the hypabyssal chamber, magma degassing occurs, including degassing from metallic spherules. The processes of droplet fusion and formation of native phase segregations begin.

The role of immiscible sulfides for sulfur isotope fractionation in arc magmas: Insights from the Talkeetna island arc crustal section, south-central Alaska

While sulfur isotopes have proven powerful tracers of sulfur (re)cycling within subduction zones, the origin of the 34S-enrichment seen in arc magmas remains a subject of debate, with competing hypotheses implicating both mantle and crustal processes. Herein, we investigate these competing models through the study of the Early–Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains, south-central Alaska, reporting the sulfur isotope systematics of rocks spanning a depth transect that extends from the upper mantle (∼0.9–1.2 GPa, ∼30–35 km) up into the mid-crust (∼0.2–0.5 GPa, ∼5–15 km). Marked by an increase in δ34S values from −1.28 to +5.61‰, these data reveal a significant isotope effect associated with melt differentiation, with 32S being preferentially sequestered from primitive basaltic melts into ultramafic and mafic cumulates, yielding progressively more evolved melts enriched in 34S. Leveraging a quantitative petrological model that constrains the liquid line of descent and sulfide saturation as a function of sulfur content, oxygen fugacity, pressure and temperature, we then demonstrate that, in the presence of dissolved oxidized sulfur species, the saturation of immiscible magmatic sulfides is capable of generating the observed sulfur isotope fractionation. While fluid saturation is unlikely to occur at lower crustal depths, ascending melts will almost certainly experience fluid saturation and degassing. As a result, sulfide immiscibility and magma degassing are not mutually exclusive and may, instead, represent complementary processes that combine to explain the observed range of positive δ34S compositions. In Talkeetna, crustal assimilation is unlikely to have played a role to increase δ34S values as there was little to no assimilation of pre-existing oceanic crust containing seawater sulfate during the formation of the Talkeetna Arc section. Furthermore, our model suggests that primitive melt in isotopic equilibrium with the most primitive ultramafic cumulates have mantle-like δ34S values between ∼0 and +1.1‰, which suggest limited input of 34S-enriched slab-derived sulfur into the sub-arc mantle. Collectively, these findings emphasize that mantle-like δ34S values for primitive arc melt may be common, while revealing the underappreciated importance of deep crustal crystallization of immiscible magmatic sulfides to generate positive δ34S compositions in erupted arc magmas.

Dynamics of Magma Chamber Replenishment Under Buoyancy and Pressure Forces

AbstractActive magma chambers are periodically replenished upon a combination of buoyancy and pressure forces driving upward motion of initially deep magma. Such periodic replenishments concur to determine the chemical evolution of shallow magmas, they are often associated to volcanic unrests, and they are nearly ubiquitously found to shortly precede a volcanic eruption. Here, we numerically simulate the dynamics of shallow magma chamber replenishment by systematically investigating the roles of buoyancy and pressure forces, from pure buoyancy to pure pressure conditions and across combinations of them. Our numerical results refer to volcanic systems that are not frequently erupting, for which magma at shallow level is isolated from the surface (“closed conduit” volcanoes). The results depict a variety of dynamic evolutions, with the pure buoyant end‐member associated with effective convection and mixing and generation of no or negative overpressure in the shallow chamber, and the pure pressure end‐member translating into effective shallow pressure increase without any dynamics of magma convection associated. Mixed conditions with variable extents of buoyancy and pressure forces illustrate dynamics initially dominated by overpressure, then, over the longer term, by buoyancy forces. Results globally suggest that many shallow magmatic systems may evolve during their lifetime under the control of buoyancy forces, likely triggered by shallow magma degassing. That naturally leads to long‐term stable dynamic conditions characterized by periodic replenishments of shallow partially degassed, heavier magma by volatile‐rich fresh deep magma, similar to those reconstructed from petrology of many shallow‐emplaced magmatic bodies.

The behaviour of metals in deep fluids of NE Iceland

In this contribution, we present some of the first data on the elemental signature of deep crustal fluids in a basalt-hosted, low-chloride magmatic-hydrothermal system. Down-hole fluid samples (850–1600 m) from wells in the Theistareykir and Krafla geothermal fields in the Northern Volcanic Zone of Iceland were combined with well-head samples of condensed vapor, cuttings of altered rock, and fresh basalt (being some of the first concentration data for volatile and semi-volatile elements (Sb, Tl, Bi, Cd and As) for this area of Iceland). Results show that the deep fluids are relatively enriched in base metals and (semi)-volatile metals (in particular Te, Hg, Re and Tl) compared to local basalt. We interpret this enrichment in volatile metals to reflect a significant element input from magma degassing. Boiling of this deep fluid results in a well-head fluid composition that is significantly depleted in most elements. This well-head fluid has a distinct elemental signature, including a depletion in Sb that is mirrored in the altered rocks, and a depletion in the base metals that shows their selective sequestration in scale minerals, likely sulphides. As expected, the element content and patterns in surface fluids can thus not be interpreted to directly reflect that of the deep reservoir fluid. The behaviour of elements in Theistareykir and Krafla fluids is consistent, and largely agrees with similar data obtained for the Reykjanes geothermal system in SW Iceland. We therefore posit that our results are representative for this geological setting and indicate a significant magmatic degassing cation input to deep fluids, variably modified by water–rock interaction.

Chlorine geochemistry of various geothermal waters in China: Implications for geothermal system geneses

Understanding the geneses of geothermal systems is critical for their efficient and sustainable exploitation. Chlorine is one of the most conservative elements in geothermal waters and capable of providing valuable information related to the formation and evolution of geothermal systems, thereby being a promising indicator for identifying geothermal system geneses. In this study, representative hydrothermal systems located in various tectonic regions in China, including Daggyai, Semi, and Gudui in plate margin as well as Xinzhou and Xiong’an in inland Basins, were selected for investigating the chloride sources of the geothermal waters and possible mechanisms of chlorine isotope fractionation occurring in the relevant geochemical processes. The chloride of geothermal waters in Tibet originated predominantly from magmatic fluids input, whereas water–rock interaction was the primary chloride source for the Xinzhou geothermal waters. As for the chloride in Xiong’an, it was primary derived from the dissolution of halite in carbonate rocks. In addition, the major processes resulting in the chlorine isotope fractionation were supposed to be magma degassing in the Tibetan geothermal systems and ion filtration in the Xiong’an geothermal system. Based on the discovered chloride sources and possible processes inducing non-negligible chlorine isotope fractionation, the geneses of various geothermal systems investigated in this study were proposed. This study suggests that stable chlorine isotopes, coupled with other geochemical parameters, have the potential for discriminating geothermal systems with different geneses.

Volatile trace metals deposited in ice as soluble volcanic aerosols during the 17.7.ka eruptions of Mt Takahe, West Antarctic Rift

Volatile metals are emitted at significant rates as gases and particulates from volcanoes, although their speciation, bioreactivity and longevity during atmospheric transport are essentially unknown. Ice cores provide detailed yet largely unexplored long-term records of volcanogenic volatile metals in air and precipitation. Here we evaluate the source and speciation of volatile metals (cadmium, lead, bismuth, and thallium) in Antarctic ice cores from the massive, halogen-rich and sulfur-poor ∼17.7 ka eruptions of Mt. Takahe, West Antarctic Rift. We show that these volatile, chalcophile metals were transported to the ice core as soluble aerosol, derived from magma degassing, in contrast to lithophile elements in the ice core that were transported as silicate ash. We use correlation analysis and chemical speciation modelling of the chlorine-rich volcanic plume to show that the volcanic metals cadmium, lead and bismuth were likely transported as water-soluble chloride aerosols in the atmosphere before they were scavenged from the plume by ice, water or ash and deposited onto the ice within 400 km of the vent. Our findings show that as well as recording trace metals sourced from much more distal regions, ice cores from Antarctica also record clear signatures of regional continental volcanism in the form of chloride aerosol.

Stratigraphic and Spatial Extent of HALIP Magmatism in Central Spitsbergen

AbstractRapid extensive magmatism may have a profound effect on global climate by liberating and releasing greenhouse gases to the atmosphere through contact metamorphism of lithologically heterogeneous host rocks and degassing of magma and associated lava flows. The high Arctic Archipelago of Svalbard offers accessible, superbly exposed outcrops revealing Early Cretaceous magmatism associated with the High Arctic Large Igneous Province (HALIP). In this contribution, we investigate the onshore‐offshore intrusive complex of central Spitsbergen formed due to HALIP activity, that is, the Diabasodden Suite. This is the most “data‐rich” part of Svalbard due to past petroleum exploration and research drilling. In this area, the predominantly dolerite intrusions are emplaced in a range of host rocks ranging from Permian carbonate‐dominated successions to organic‐rich shale‐dominated successions of Middle Triassic and Late Jurassic‐Early Cretaceous age. Two hundred sixty five individual igneous intrusions, covering 72 km2, are exposed onshore in the study area. This equates to approximately 0.14–2.5 km3 of emplaced magma. In addition, subsurface characterization using borehole, seismic and magnetic data indicates that an area of additional ca. 3,000 km2 is affected by magmatism (magma volume 3.2–195.2 km3). Wireline logs in boreholes characterize both intrusions and associated aureoles. Aureoles with very low resistivity indicate occurrence of organic‐rich shales suggesting past fluid circulation and de‐gassing. This study forms the foundation for quantifying HALIP‐related magmatism in the data‐poorer parts of Svalbard, and other circum‐Arctic basins.

The heterogeneity of the Mexican lithospheric mantle: Clues from noble gas and CO2 isotopes in fluid inclusions

The abundance of mantle-derived rocks and lavas, in combination with its tectonic evolution, render Mexico a perfect laboratory to investigate the chemical and the isotopic heterogeneity of the lithospheric mantle. New data on the composition of noble gases and CO2in Mexican mantle xenoliths and lavas is reported. Our samples consist of six ultramafic nodules from the Durango Volcanic Field (DVF) and the San Quintin Volcanic Field (SQVF), monogenetic complexes belonging to the Mexican Basin and Range province; and four lavas from the Sierra Chichinautzin (SCN), a Quaternary monogenetic volcanic field located in the Mexican volcanic arc. Ne and Ar isotopes in fluid inclusions reveal mixing between atmospheric and MORB-like fluids (e.g.,40Ar/36Ar < 1,200). DVF and SQVF nodules record low40Ar/36Ar and4He/20Ne that confirm the existence of recycled atmospheric-derived noble gases in the local mantle. The averages of the Rc/Ra ratios (3He/4He corrected for atmospheric contamination) measured in Mexican localities are within the MORB-like range: DVF= 8.39 ± 0.24 Ra, SQVF = 7.43 ± 0.19 Ra and SCN lavas = 7.15 ± 0.33 Ra (1σ). With the aim of assessing the isotopic variability of the Mexican lithospheric mantle, the above results were compared with similar data previously obtained from ultramafic nodules found in the Ventura Espiritu Santo Volcanic Field (VESVF), another Quaternary monogenetic volcanic complex belonging the Basin and Range. The higher3He/4He ratios in DVF relative to those reported for the VESVF and the SQVF are explained as reflecting different ages of mantle refertilization, triggered by the retreating of the Farallon slab (∼40 Ma ago) and associated delamination slab processes. We propose that the DVF mantle was refertilized more recently (<10 Ma ago) than the mantle beneath the SQVF and VESVF (∼40–20 Ma ago). On the other hand, He-Ne-Ar compositions of SCN olivines share similarities with VESVF xenoliths, suggesting a relatively homogeneous lithospheric mantle in central Mexico. Finally, DVF and the SCN samples exhibit δ13C values within the MORB range (comparable to other values previously reported in fluid inclusions and fumaroles from Popocatépetl, Colima—Ceboruco volcanoes). While we explain the MORB-like carbon signatures of the DVF samples as the result of the above-mentioned refertilization process, the SCN signatures likely reflect either (i) trapping of isotopically fractionated CO2derived from magmatic degassing or (ii) a mantle source unaffected by subduction-related crustal carbon recycling.

Nitrogen, helium, and argon reveal the magmatic signature of fumarole gases and episodes of outgassing from upper-crustal magma reservoirs: The case of the Nisyros caldera (Aegean Arc, Greece)

The chemical composition of gases emitted by active volcanoes reflects both magma degassing and shallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derived fluids. Untangling the magmatic fluid endmember within surface gas emission is therefore challenging, even with the use of well-known magma degassing tracers such as noble gases. Here, we investigate the deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogen and noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collected from 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles are admixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The N2, He, and Ar composition of the magmatic endmember is calculated by reverse mixing modeling and shows N2/He = 31.8 ± 4.5, N2/Ar = 281.6, δ15N = +7 ± 3 ‰, 3He/4He = 6.2 Ra (where Ra is air 3He/4He), and 40Ar/36Ar = 551.6 ± 19.8. Although N2/He is significantly low with respect to typical values for arc volcanoes (1,000–10,000), the contribution of subducted sediments to the Aegean Arc magma generation is reflected by the positive δ15N values of Nisyros fumaroles. The low N2/He ratio indicates N2-depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/rhyodacitic) melt in a high-crystallinity reservoir. We compare our 2018–2021 data with N2, He, and Ar values collected from the same fumaroles during a hydrothermal unrest following the seismic crisis in 1996–1997. Results show additions of both magmatic fluid and ASW during this unrest. In the same period, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such as CO2/CH4 and He/CH4 ratios, an increase of ∼50 °C in the equilibrium temperature of the hydrothermal system (up to 325 °C), and greater amounts of vapor separation. These variations reflect an episode of magmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by the magmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of the hydrothermal system. Reverse mixing modeling of fumarolic N2-He-Ar has therefore important ramifications not only to disentangle the magmatic signature from gases emitted during periods of dormancy, but also to trace episodes of magmatic outgassing and better understand the state of the upper crustal reservoir.

Sulfur Isotopic Fractionation of the Youngest Chang'e‐5 Basalts: Constraints on the Magma Degassing and Geochemical Features of the Mantle Source

AbstractThe unusually prolonged volcanism at the Chang'e‐5 (CE‐5) landing site remains a mystery. To constrain the geochemical features of the CE‐5 mantle source, we performed in situ sulfur isotope analysis on sulfides of the CE‐5 basalts. The modal abundance of sulfides is ∼0.1 wt%, dominated by troilite with trace cubanite, chalcopyrite and pentlandite, yielding a S‐abundance of ∼360 ± 180 ppm for the bulk CE‐5 basalts. Our analysis shows a decreasing trend of δ34S in the magma with crystallization and degassing, suggesting ∼40% degassing loss of S, and accordingly the maximum S‐abundance in CE‐5 primitive magma was calibrated to approximately 600 ± 300 ppm. Considering an extensive fractional crystallization following a low‐degree partial melting of the CE‐5 basalts, the estimated sulfur abundance in the mantle source is ∼1–10 ppm. This result suggests a strong depletion of volatiles in the CE‐5 mantle source, which may be caused by prolonged magmatic activity.

Contrasting Volcanic Deformation in Arc and Ocean Island Settings Due To Exsolution of Magmatic Water

AbstractTwo of the most widely observed co‐eruptive volcanic phenomena—Ground deformation and volcanic outgassing—Are fundamentally linked via the mechanism of magma degassing and the development of compressibility, which controls how the volume of magma changes in response to a change in pressure. Here we use thermodynamic models—Constrained by petrological data—To reconstruct volatile exsolution and the consequent changes in magma properties. We use the fraction of SO2 exsolved during decompression to predict co‐eruptive SO2 flux and magma compressibility to predict co‐eruptive surface deformation (both normalized by erupted volume). We conduct sensitivity tests using properties of typical basalts to assess how varying magma volatile content, crustal properties, and chamber geometry affect co‐eruptive deformation and degassing. We find that magmatic H2O content has the most impact on both SO2 flux and volume change. Our findings have general implications for typical basaltic systems in arc and ocean island settings. The higher water content of arc magmas makes them more compressible than ocean island magmas and leads to muted or non‐existent deformation being observed during arc eruptions. Our models are consistent with observation: Deformation has been detected during 48% of basaltic eruptions in ocean island settings (16/33) during the satellite era (2005–2020), but only 11% of basaltic eruptions in arc settings (7/61).

Geochemistry of Hydrothermal Fluids From the E2-Segment of the East Scotia Ridge: Magmatic Input, Reaction Zone Processes, Fluid Mixing Regimes and Bioenergetic Landscapes

The compositions of hydrothermal fluids in back-arc basins (BABs) can be affected by the influx of magmatic fluids into systems that are dominated by reactions between basement rocks and seawater-derived fluids. The East Scotia Ridge (ESR) in the Scotia Sea hosts such hydrothermal systems where the role of magmatic fluid influx has not yet been addressed. During expedition PS119 in 2019, three chimneys were sampled from the E2 segment. These samples were analysed for their chemical and isotopic composition along with fluid inclusions in corresponding precipitates. Our data provide evidence for the temporal evolution of hydrothermal fluids in this remote back-arc system. Salinity variations in anhydrite-hosted fluid inclusions indicate that phase separation takes place in the subseafloor. Moderate-temperature (<53°C) fluids from the newly discovered E2-West hydrothermal vent field and high-temperature (>320°C) fluids from the E2-South area were sampled. Depletions in fluid-mobile elements, ΣREE and low δ18OH2O show that the basement in this root zone has been leached since the previous sampling in 2010. The results indicate that high-temperature fluid-rock interactions are key in setting the composition of the fluids with cation-to-chloride ratios suggesting a common root zone for both vent sites. The concentrations of dissolved gases provide new insights in the connection between magmatic degassing and its influence on endmember vent fluid composition. Specifically, stable isotope (O, H) data and elevated CO2 concentrations point to a minor influx of magmatic vapour. Stable sulphur isotopes provide no evidence for SO2 disproportionation suggesting a H2O-CO2 dominated nature of these vapours. The concentrations of conservative elements in the E2-W fluid reflects subseafloor mixing between E2-S endmember fluid and seawater. In contrast, non-conservative behaviour, and depletion of Fe, H2, and H2S point to a combination of sub-surface abiotic and biotic reactions affecting these fluids. Similarly, E2-W fluids show evidence for H2S and CH4 being metabolized in the subseafloor. Thermodynamic computations confirm that the E2 system is dominated by sulphide oxidation as a major catabolic pathway. Our results indicate that the conditions at E2 are favourable to hosting a robust subseafloor biosphere.

Carbon stable isotope constraints on CO2 degassing models of ridge, hotspot and arc magmas

Carbon dioxide emissions from volcanoes are important parameters to constrain in order to fully understand the Earth system, especially the effect of volcanic forcing on climate. The characterization of carbon concentration in magmas has been used to constrain volcanic fluxes. Because of low CO2 solubility in silicate melts, however, CO2 is significantly degassed from magmas due to decompression during transfer to the surface. The measurement of the carbon stable isotope ratio (13C/12C expressed as δ13C-values) in natural submarine glasses has been a helpful geochemical tool to study magma degassing. Carbon stable isotope fractionation at magmatic temperature between CO2 in vesicles and carbonate ions dissolved in the melt is still large enough to cause variations in δ13C-values. This variability can be used to deduce the mode of degassing (open vs. closed system, equilibrium vs. kinetic) operating in a given magmatic system.In this study, I present a review of the existing carbon isotope data for magmas of three different settings (ridge, hotspot and arc). This review allows to (1) investigate the diversity of degassing modes operating within a given setting and (2) compare the prevailing degassing mode between these three settings.Except for rare undersaturated samples and for volatile-rich, vesicular popping rocks, Mid-Ocean Ridge Basalts (MORBs) are predominantly extensively degassed and supersaturated in CO2 reflecting incomplete degassing during their last degassing step. Such a behavior is also reflected in their vesicle-dissolved carbon isotopic fractionations that are generally smaller than equilibrium values stemming from kinetic/diffusive effects. By contrast, the CO2 + H2O gas phase in hotspot and arc magmas is predominantly in chemical equilibrium with the melt because of volatile-rich initial conditions (and thus larger vesicularity) enhancing vapor-melt chemical and isotopic equilibrium. This larger initial volatile content is responsible for the extensive open-system (Rayleigh distillation) degassing generally observed in hotspot and arc magmas.This review highlights the fact that one single degassing model cannot explain the geochemical evolution of all magmas. Also, no single model can be assigned to one specific type of magma (i.e., a significant diversity of degassing mode exists within a given type of magma, notably in MORBs). It is important to take into account these observations when degassing corrections are applied on the basis of noble gas (He/Ne or He/Ar) ratios. It appears helpful to characterize the carbon concentration and stable isotope ratios in vesicles and dissolved in the glass in order to (1) identify the mode of degassing operating in a given magmatic system and (2) apply the most appropriate degassing correction to these magmas. It is only after this critical correcting step has been achieved that an assessment of initial magmatic CO2 contents can be reasonably undertaken.

The critical role of magma degassing in sulphide melt mobility and metal enrichment

Much of the world’s supply of battery metals and platinum group elements (PGE) comes from sulphide ore bodies formed in ancient sub-volcanic magma plumbing systems. Research on magmatic sulphide ore genesis mainly focuses on sulphide melt-silicate melt equilibria. However, over the past few years, increasing evidence of the role of volatiles in magmatic sulphide ore systems has come to light. High temperature-high pressure experiments presented here reveal how the association between sulphide melt and a fluid phase may facilitate the coalescence of sulphide droplets and upgrade the metal content of the sulphide melt. We propose that the occurrence of a fluid phase in the magma can favour both accumulation and metal enrichment of a sulphide melt segregated from this magma, independent of the process producing the fluid phase. Here we show how sulphide-fluid associations preserved in the world-class Noril’sk-Talnakh ore deposits, in Polar Siberia, record the processes demonstrated experimentally.

Sampling Volcanic Plume Using a Drone-Borne SelPS for Remotely Determined Stable Isotopic Compositions of Fumarolic Carbon Dioxide

Both chemical and isotopic compositions of concentrated volcanic plumes are highly useful in evaluating the present status of active volcanoes. Monitoring their temporal changes is useful for forecasting volcanic eruptions as well. Recently, we developed a drone-borne automatic volcanic plume sampler, called SelPS, wherein an output signal from a sulfur dioxide (SO2) sensor triggered a pump to collect plume samples when the SO2 concentration exceeded a predefined threshold. In this study, we added a radio transmission function to the sampler, which enabled our operator to monitor real-time SO2 concentration during flights and thus obtain more concentrated volcanic plume samples through precise adjustment of the hovering position. We attached the improved SelPS to a drone at Nakadake crater, Aso volcano (Japan), and successfully obtained volcanic plume samples ejected from the crater more concentrated than those obtained by using previous version of SelPS in 2019. Additionally, we found a significant linear correlation between the reciprocal of the concentration and isotopic ratios for the 2H/1H ratios of H2, 18O/16O ratios of CO2, and 13C/12C ratios of CO2 within the plume samples. Based on the isotopic ratios of fumarolic H2 (δ2H = −239 ± 6‰) and fumarolic CO2 (δ13C = −3.58 ± 0.85‰ and δ18O = +22.01 ± 0.68‰) determined from the linear correlations, we estimated the apparent equilibrium temperatures (AETs) with magmatic H2O simultaneously and precisely for the first time in erupting volcanoes, assuming hydrogen isotope exchange equilibrium between H2 and H2O (AETD = 629 ± 32°C) and oxygen isotope exchange equilibrium between CO2 and H2O (AET18O = 266 ± 65°C). We found that the AET18O was significantly lower than the AETD in the crater. While the temperature of the magmatic gases was originally 600°C or more, most of the gases cooled just beneath the crater to temperatures around the boiling point of water. The improved SelPS enable us to determine both AETD and AET18O in eruptive volcanoes, wherein fumaroles are inaccessible. Simultaneous and precise determination of both the AET18O and AETD can provide novel information on each volcano, such as the physicochemical conditions of magma degassing and the development of fluid circulation systems beneath each volcano.

Isotopic signatures of magmatic fluids and seawater within silicic submarine volcanic deposits

Addressing questions of magma ascent, volcanic eruption dynamics, and volatile fluxes requires accurate measurement of magmatic water concentrations in volcanic glass. Glass in volcanic rocks in the deep-sea environment can experience rehydration (addition of external water) across a range of temperatures, and from different sources (e.g., seawater and hydrothermal fluids), which can lead to overestimation of magmatic-H2O. This study used H and O isotopes (δD and δ18O) to identify sources of rehydration in pumice and lava from the deep-sea 2012 volcanic eruption of Havre volcano, Kermadec Arc, and from a variety of older (tens to thousands of years) silicic submarine deposits across the Izu-Bonin Arc and Lau Basin. We find that old seafloor pumices were rehydrated up to 6 wt.% H2O by the diffusion of cold seawater over 100s to 1000s of years, and thus, enriched in δD, bulk-δ18O, and water-in-glass (wig) δ18O up to −30‰, +9‰ and −5‰ respectively. By contrast, the young Havre deposits exhibit a much wider range of both isotopic enrichment and depletion with δD = −50 to −120‰, δ18Obulk = +5.7 to +6.2‰, and δ18Owig = −10 to +4‰, depending on the eruptive units. Using magmatic degassing and vapor δD-H2O modeling, a volatile-melt δ18O-geothermometer, and previous textural studies of Havre 2012 deposits, we identify multiple high-temperature rehydration sources, timescales, and mechanisms. δD-depleted pumices were likely rehydrated by vapor co-existing in bubbles and vesicles at temperatures around and below the glass transition (320–670 °C) over timescales of a few minutes during clast cooling above the eruptive vent. Conversely, δD-enriched Havre pumice and lava from different deposits were most likely rehydrated by heated, δD-enriched seawater at glass temperatures > 100 °C. These results provide a natural confirmation of recent experimental findings, which tackle the fundamentals of H and O diffusion and isotope exchange in silicate materials at temperatures of 100–400 °C. Addressing the effects of rehydration, and thus accuracy of H2O measurements, in volcanic glasses and crystals is important for improving our understanding of conduit processes during volcanic eruptions, and the kinetics of glass hydration and alteration.

Complex magmatic processes recorded by clinopyroxene phenocrysts in a magmatic plumbing system: A case study of mafic volcanic rocks from the Laiyang Basin, southeastern North China Craton

Understanding the evolution of magmatic plumbing systems requires an understanding of the mechanisms of magma ascent from the magma source to the surface. We undertook textural and in situ geochemical studies of clinopyroxene phenocrysts in Early Cretaceous mafic volcanic rocks from the Laiyang Basin, China. Four distinct clinopyroxene populations were recognized based on their textures and zoning patterns in the mafic volcanic rocks (types 1, 2, 3, and 4). Type 1 is reversely zoned and contains rounded, irregularly resorbed, low-Mg cores (Mg# = 60–75) surrounded by clear, high-Mg mantles (Mg# = 75–90). Type 2 phenocrysts are typically normally zoned and characterized by relatively high-Mg cores (Mg# > 85) and thin lower Mg# rims (Mg# = 70–85). Type 3 comprises strong oscillatory zoned crystals (Mg# = 72–85) that are generally normally zoned and, in some cases, exhibit substantial sector zoning. Type 4 crystals are euhedral and unzoned phenocrysts. The textural and compositional features of the clinopyroxenes record complex magmatic processes including fractional crystallization, recycling of early formed crystals, magma convection, and magma recharge and subsequent mixing. Clinopyroxene–melt equilibrium pairs in the volcanic rocks yielded a relatively wide range of temperatures (984–1264°C) and pressures (0.1–10.1 kbar). The P–T array defines two main magma storage zones: (1) a deep, high-T, magma reservoir (~22–28 km), and (2) a shallow, low-T, magma reservoir (~8–19 km). At pressures of >3 kbar, the amount of water in melts in equilibrium with the clinopyroxene is in the range 1.5–3.0 wt.%, but at pressures of <3 kbar, the melt water contents decrease to 0.1 wt.%, indicative of extensive magma degassing. The clinopyroxene phenocrysts have a wide range of 87Sr/86Sr ratios from 0.70683 to 0.71366. The broad range of 87Sr/86Sr ratios (0.70796–0.71171) for the most primitive clinopyroxene (Mg# > 89) might have been inherited from a heterogeneous mantle source. However, some type 1 crystals with more radiogenic Sr isotopic compositions may record assimilation of crustal material during evolution of the mantle-derived magma. The textural and compositional features of the studied clinopyroxene phenocrysts suggest that Laiyang volcano is fed by a vertically extensive magmatic plumbing system that extends from shallow to deep levels, in which geochemically distinct magmas crystallized and mixed under variable physicochemical conditions.

Thermal springs and active fault network of the central Colca River basin, Western Cordillera, Peru

Thermal waters and vapor discharges (hot springs, geysers, solfataras, and fumaroles) are common phenomena in volcanic regions at active plate boundaries, and the Central Andes are no exception. The Colca River basin in S Peru is a highly diversified and complex thermal region with unresolved questions on the origin of thermal fluids, reservoir temperature, and connections with tectonic and/or volcanic activity. To answer these, we used hydrogeochemical analysis of 35 water samples from springs and geysers, together with isotopic (δ18O and δD) analysis, chemical and mineral studies of precipitates collected in the field around these outflows, and field observations. We aimed (1) to recognize the geochemistry of thermal waters and precipitates in the central part of the Colca River basin, (2) to identify fluid sources and their origin, (3) to estimate the temperature of a potential geothermal reservoir, and (4) to discuss the regional active tectonic and volcanic framework of this geothermal region and mutual relationships.Our results corroborate a heterogeneous and complex geothermal system in the central part of the Colca River basin, with contrasting hydrogeochemical and physical properties, variable isotope composition, different reservoir temperatures, and associated precipitates around thermal springs. Processes controlling water chemistry are closely related to the Ampato-Sabancaya magmatic chamber's activity and tectonic structures that allow complex interactions of meteoric waters with magmatic fluids and gases. With a considerable gradient of pressure owing to local relief and deep incision in the Colca Canyon, these processes led to the differentiation of the thermal waters into three main groups. (1) Chloride-rich, mainly sodium chloride, thermal waters are of meteoric origin but mature within the geothermal reservoir possibly fed by magma degassing. These waters' chemical and isotopic composition results from water-rock interaction and mixing with magmatic waters within the reservoir. These waters discharge at the bottom of the Colca Canyon and Valley, presenting a broad hydrogeochemical spectrum and highly variable mineral phases precipitating at the outflows. The reservoir temperature estimated for these waters ranges from 180 to 200 °C. The group of hottest springs and geysers at the bottom of the Colca Canyon waters are fully equilibrated, with the reservoir temperature ~ 240 °C. (2) Sulfate-rich waters are shallow meteoric waters heated by ascending gases that form an independent group referring to the local water circulation, often controlled by tectonic barriers. (3) Bicarbonate-rich waters are the intermediate meteoric waters, divided into two hydrochemical groups: waters partially equilibrated with reservoir rocks and more similar to chloride-rich waters or additionally enriched with SO4 and more similar to sulfate-rich waters.Studied thermal springs show a clear spatial correlation with active and seismogenic crustal W- to NW-tracing normal and strike-slip faults. These act as barriers to infiltrating meteoric waters, provide pathways to hydrothermal solutions and gases assisting in meteoric water heating, and yield passages for pressured by lithostatic load and heated waters to ascend to the surface.

Magmatic and phreatomagmatic contributions on the ash-dominated basaltic eruptions: Insights from the April and November–December 2005 paroxysmal events at Karthala volcano, Comoros

Basaltic eruptions are commonly associated with lava emissions and relatively weak explosive activities, but they can sometimes produce strong explosive eruptive phases. In April and November 2005, two paroxysmal eruptive events occurred within the summit crater of Karthala basaltic shield volcano (Grande Comore Island, Comoros), which hosted a water lake before each of these events. Both 2005 ash plumes spread across the Comoros Archipelago and heavily impacted the whole Grande Comore Island. Associated deposits on the volcano summit are extremely fine-grained (up to 50 wt% of fine ash <63 μm for some analyzed layers) and rich in millimeter-sized rounded accretionary lapilli aggregates. Field observations, as well as textural and chemical analyses performed on both coarse- and fine-grained pyroclasts permit to identify juvenile and non-juvenile components and quantify their peculiar characteristics. Coarse ash (710–1000 μm) mainly consists of juvenile pumice particles (vesicle number density NV = 4.5 104 mm−3 and gas to melt ratio VG/VL = 1.5, on average), characterized by glassy groundmasses and representative of magma portions ascending quickly within the eruptive conduits (up to 10 m s−1). A relatively low amount of juvenile scoria particles are also observed in the coarse ash fractions, which are characterized by magma degassing (NV = 4.7 104 mm−3 and VG/VL = 0.5 on average) and associated crystallization (occurrence of dendritic microlites). Non-juvenile fragments (from blocks to coarse ash) are dense lava or intrusive fragments. Their amount decreases exponentially towards the fine ash fractions, which are mainly composed of juvenile, blocky, dense and glassy particles that are characterized by unambiguous textural signs of brittle fragmentation (hackle lines, stepped features and cracks). We support that Molten Fuel-Coolant Interactions between highly porous fast ascending basaltic magmas and external waters occurred during the paroxysmal phases of the studied eruptions, leading to a brittle-dominant and efficient regime of magma fragmentation. Variable but large amount of fine ash grains through the stratigraphic depth of the deposits can be ascribed to the brittle failure of the vesicle walls of the initial porous magma. Concurrently, thermohydraulic explosions caused the host rock fragmentation at shallow level, generating the relatively coarse non-juvenile particles. A short-lived episode of intense lava fountaining associated with steam explosions eventually occurred at the end of the November 2005 paroxysm, forming the last and relatively coarse tephra layer at the top of the studied eruptive sequence. Each paroxysmal phase lasted about a day as each associated water lake and shallow water table progressively vaporized and dried away. Both eruptions ended with lava pond and weak lava fountaining activities confined within the summit crater. We conclude that the contributions of both magmatic processes and phreatomagmatic interaction mechanisms ultimately generated the grain size, grain component and grain texture variabilities observed within the paroxysmal deposits. This work contributes to a better understanding of the generation of unusual fine ash from basaltic explosions as well as their eruptive dynamics and associated mechanisms, from magma ascent in the conduit to the fragmentation level and the interaction with intra-crateric lake waters.

Experimental and Observational Constraints on Halogen Behavior at Depth

Halogens are volatile elements present in trace amounts in the Earth’s crust, mantle, and core. They show volatile behavior and tend to be incompatible except for fluorine, which makes them key tracers of fluid-mediated and/or melt-mediated chemical transport processes. Even small quantities of halogens can profoundly affect many physicochemical processes such as melt viscosity, the temperature stability of mineral phases, the behavior of trace elements in aqueous fluids, or the composition of the atmosphere through magma degassing. Experiments allow us to simulate deep-Earth conditions. A comparison of experimental results with natural rocks helps us to unravel the role and behavior of halogens in the Earth’s interior.