We report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central‐western Serbia), a mega‐suture zone at the boundary between Eurasia and Africa plates. ...Thirty‐one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO2‐dominated, N2‐dominated, and CH4‐dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle‐derived component in Serbia. δ13C values range from −20.2‰ to −0.1‰ (versus PDB), with the more negative compositions observed in N2‐dominated samples. The carbon‐helium relationship indicates that these negative δ13C compositions could be due to isotopic fractionation processes during CO2 dissolution into groundwater. In contrast, CO2‐rich samples reflect mixing between crustal and mantle‐derived CO2. Our estimated mantle‐derived He flux (9.0 × 109 atoms m−2 s−1) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep‐mantle fluids through the crust.
Key Points
Chemical and isotopic composition of natural gas manifestations along the Serbian Vardar zone are controlled by mixing processes and fraction during water‐gas‐rock interactions in shallow crustal layers
Mantle‐derived He flux of 2 orders of magnitude higher than normally found in stable continental areas are estimated
Mantle volatiles and heat are sourced directly from the mantle supporting the asthenosphere up‐rise and delamination processes at the mantle‐crust boundary recognized in the studied area
This study provides the first micro‐thermometric data of fluid inclusions (FIs) in mafic loose (disaggregated) xenocrysts and ultramafic xenoliths in explosive products of the melilitite‐carbonatite ...Mt. Vulture volcano (southern Italy). Pure CO2 late stage FIs hosted in rock‐forming minerals of wehrlite xenoliths and clinopyroxene xenocrysts were trapped at the local crust‐mantle boundary (32 km). In contrast, trapping pressures within the loose olivine xenocrysts are from 3.2 to 4.5 kbar (8–13 km). Considering the ongoing degassing of mantle‐derived CO2 rich gases, together with seismic evidences of the presence of low amount of melts at depth, and the tectonic control of the past volcanic activity, our study opens new perspective about the hazardous nature of the “quiescent” melilitite‐carbonatite volcanoes.
Plain Language Summary
The study of fluid inclusions (FIs) (small amount of fluid trapped within minerals) provides important information on variable environments and magmatological processes in which the host minerals were formed. Investigation of the FIs with respect to their composition, trapping pressure and temperature, allow us to constrain magma ascent history. To understand the last explosive volcanic activity of Mt. Vulture volcano (southern Italy), we investigated FIs in mafic minerals and mantle fragments brought to the surface by a melilitite‐carbonatite magma. Our results show the presence of CO2‐rich FIs with trapping pressure corresponding to a depth of 32 km in mantle fragments, and a shallower depth (8–13 km) in mafic mineral. Estimates on magma ascent rate show rapid ascent dynamics to the surface. Our study emphasizes the importance of a multidisciplinary approach that combine geochemistry and petrology to investigate a volcanic system even if the volcano is considered “quiescent,” as is the case of Mt. Vulture volcano, where mantle degassing is still ongoing.
Key Points
Micro‐thermometric analyses show the occurrence of high‐density CO2‐rich fluid inclusions hosted by minerals within wehrlite xenoliths
Ascent rate between melilitite‐carbonatite (≈20 m/s) and kimberlite (≈45 m/s) magma is comparable
Melilitite‐carbonatite volcanoes can be hazardous even after long time of quiescence (>105 years)
Ciomadul is the youngest volcano in the Carpathian‐Pannonian Region, Eastern‐Central Europe, which last erupted 30 ka. This volcano is considered to be inactive, however, combined evidence from ...petrologic and magnetotelluric data, as well as seismic tomography studies, suggests the existence of a subvolcanic crystal mush with variable melt content. The volcanic area is characterized by high CO2 gas output rate, with a minimum of 8.7 × 103 t/year. We investigated 31 gas emissions at Ciomadul to constrain the origin of the volatiles. The δ13C–CO2 and 3He/4He compositions suggest the outgassing of a significant component of mantle‐derived fluids. The He isotope signature in the outgassing fluids (up to 3.10 Ra) is lower than the values in the peridotite xenoliths of the nearby alkaline basalt volcanic field (R/Ra 5.95 Ra ± 0.01), which are representative of a continental lithospheric mantle and significantly lower than MORB values. Considering the chemical characteristics of the Ciomadul dacite, including trace element and Sr–Nd and O isotope compositions, an upper crustal contamination is less probable, whereas the primary magmas could have been derived from an enriched mantle source. The low He isotopic ratios could indicate a strongly metasomatized mantle lithosphere. This could be due to infiltration of subduction‐related fluids and postmetasomatic ingrowth of radiogenic He. The metasomatic fluids are inferred to have contained subducted carbonate material resulting in a heavier carbon isotope composition (δ13C is in the range of −1.4‰ to −4.6‰) and an increase of CO2/3He ratio. Our study shows the magmatic contribution to the emitted gases.
Plain Language Summary
Determining the fluxes and composition of gases in active and dormant volcanoes could help to constrain their origin. Ciomadul is the youngest volcano of the Carpathian‐Pannonian Region, Eastern‐Central Europe, where the last eruption occurred 30 ka. Its eruption chronology is punctuated by long quiescence periods (even >100 kyr) separating the active phases; therefore, the long dormancy since the last eruption (30 ka) does not unambiguously indicate inactivity. Knowing if melt‐bearing magma resides in the crust is fundamental to evaluate the nature of the volcano. Isotopic compositions of helium (3He/4He) and carbon (δ13CCO2) are important tools for the study of the origin of the gases. We show that the isotope variation of the emitted gases suggests a metasomatized lithospheric mantle origin for the primary magmas. This is consistent with a degassing deep magma body existing beneath Ciomadul, and this long‐dormant volcano cannot be considered as extinct.
Key Points
CO2 emissions at Ciomadul, Eastern‐Central Europe, suggest a still‐active plumbing system beneath the volcano in spite of long dormancy
The CO2 and He isotope compositions provide evidence for significant contribution of magma‐derived volatiles, up to 80%
Isotopic signatures of gases indicate that primary magmas could have derived from a mantle source modified by subduction‐related fluids
Post‐orogenic back‐arc magmatism is accompanied by hydrothermal ore deposits and mineralizations derived from mantle and crustal sources. We investigate Zannone Island (ZI), back‐arc Tyrrhenian ...basin, Italy, to define the source(s) of mineralizing hydrothermal fluids and their relationships with the regional petrological‐tectonic setting. On ZI, early Miocene thrusting was overprinted by late Miocene post‐orogenic extension and related hydrothermal alteration. Since active submarine hydrothermal outflow is reported close to the island, Zannone provides an ideal site to determine the P‐T‐X evolution of the long‐lived hydrothermal system. We combined field work with microstructural analyses on syn‐tectonic quartz veins and carbonate mineralizations, X‐ray diffraction analysis, microthermometry and element mapping of fluid inclusions (FIs), C, O, and clumped isotopes, and analyses of noble gases (He‐Ne‐Ar) and CO2 content in FIs. Our results document the evolution of a fluid system of magmatic origin with increasing mixing of meteoric fluids. Magmatic fluids were responsible for quartz veins precipitation at ∼125 to 150 MPa and ∼300°C–350°C. With the onset of extensional faulting, magmatic fluids progressively interacted with carbonate rocks and mixed with meteoric fluids, leading to (a) host rock alteration with associated carbonate and minor ore mineral precipitation, (b) progressive fluid neutralization, (c) cooling of the hydrothermal system (from ∼320°C to ∼86°C), and (d) embrittlement and fracturing of the host rocks. Both quartz and carbonate mineralizations show noble gases values lower than those from the adjacent active volcanic areas and submarine hydrothermal systems, indicating that the fossil‐to‐active hydrothermal history is associated with the emplacement of multiple magmatic intrusions.
Key Points
Deciphering the fossil‐to‐active hydrothermal system on Zannone Island in which magmatic and meteoric fluids mixed
Polyphase and long‐lived hydrothermal activity associated with mantle‐ and crustal‐derived magmas
Fluid mixing and fluid‐rock interaction led to fluid neutralization, cooling, embrittlements, alteration, and minor ore minerals
The Zirconium and Hafnium concentrations in worldwide fumaroles fed by magmatic fluids reveal that the Zr/Hf ratio is inversely related to the temperature of emission. Lower Zr/Hf ratio values below ...the chondritic signature are found in fluids having the highest temperature while super‐chondritic Zr/Hf ratio values are found in lower temperatures. Sub‐chondritic values of the Zr/Hf ratio may be related to larger volatility of Hf‐chloride gas species with respect to Zr‐Cl gas species, while super‐chondritic ratios may correspond to fluid‐rock processes resulting from cooling of uprising magmatic fluids. We propose that sub‐chondritic Zr/Hf ratio values in fumaroles associated with high temperature may be an appropriate marker of fast magmatic rising representing a new sensitive tool for volcanic risks strategies.
Plain Language Summary
Understanding volcanoes dynamics is critical in evaluating volcanic risks and fundamental for the Earth system comprehension. The behavior of trace elements and their isotopes in fumaroles can be explored for evaluating imminent volcanic actions. Because of large crystal‐chemical similarities of Zr and Hf in rocks and minerals, the Zr‐Hf ratio is nearly constant to 36.6 ± 2.9 in meteorites and primitive rocks (“chondritic” reference.) We found that fumarole fluids at 1060°C–1084°C have sub‐chondritic Zr/Hf signature (Zr/Hf between 24 and 29) when fast degassing of magma occurs while, fumarole fluids of lower temperature have super‐chondritic or chondritic Zr/Hf values according to the intensity of rock‐fluid interaction processes. The Zr/Hf ratio of fumarole fluids can be used as a new tool for volcanic risk evaluation as possible tracer of fast magmatic arriving.
Key Points
Subchondritic to chondritic Zr/Hf values inversely related to temperature occur in fumaroles fed by pristine magmatic fluids
Superchondritic Zr/Hf values occur in fumaroles fed by hydrothermally contaminated volcanic fluids
These pieces of evidence agree with the different volatility of Zr and Hf halides in magmatic gas and Zr and Hf speciation in hydrothermal fluids
Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's ...accretion versus that subducted from the Earth's surface is unclear1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ15N (the fractional difference in15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7-10, in fact represent dominantly air-derived N2 that experienced15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12-15, its N2/36Ar and N2/3He ratios are indistinguishable from those ofthe convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution ofmantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ15N values may both be dominantly primordial features.
We investigated the geochemistry of the fumaroles at the summit area of Mt. Etna, including sulfur speciation and the content of acidic gases. The carbon-isotope composition of the Etnean plume was ...also measured in order to compare it to that of fumaroles. Two types of fumaroles were identified: (i) low-temperature fumaroles, which are dominated by CO
2 with minor amounts of SO
2 and H
2S, and negligible chlorine contents, and (ii) high-temperature fumaroles, which are strongly air-contaminated and characterized by appreciable amounts of volcanogenic carbon, sulfur, and chlorine. As recognized by
Martelli et al. (2008), both groups of fumaroles are fed by the degassing of an underlying magma; nevertheless, compositional data clearly show that secondary processes affect the composition of the fluids once they leave the magma body. Here a model of cooling and condensation of fluids is proposed to explore such postmagmatic processes. The model, which uses Etnean plume geochemistry as starting composition of fluids exsolved from magma, shows that SO
2 and H
2S control the redox conditions of the gas mixture during the cooling, until the reactions involving CO/CO
2 and H
2/H
2O ratios are fully quenched at temperatures around 350–450
°C. The dissolution of gases in water, subsequent to condensation, must occur at thermobaric conditions over 50
bar and 260
°C, which allows (a) total removal of HCl, (b) partial removal of sulfur species while preserving the SO
2/H
2S ratio, and (c) the C/S ratio to increase by almost 10-fold relative to that in the plume. The observed CH
4/CO
2 ratios are higher than those calculated for the Etnean magmatic gas, and hence they provide evidence of modest contributions from peripheral hydrothermal fluids during the migration of magmatic gases toward the surface in both low- and high-temperature fumaroles. Due to the peculiar thermodynamic conditions, the model predicts that carbon isotopes do not experience any postmagmatic fractionation, and hence the isotopic composition of the fumaroles is representative of magmatic carbon. Measurements of the carbon-isotope composition of the plume corroborate these findings.
►After gases exsolve from the magma they migrate and cool along the rock. ► Scrubbing processes lead to total removal of HCl. ► CH
4 increases due to a modest contribution from shallow hydrothermal fluids.
We report the results of a geochemical study of gas emissions along a NE–SW transect in southern Italy in order to test the hypothesis that the region around Monte Vulture is affected by degassing of ...mantle-derived fluids through a lithospheric discontinuity. We also investigated lavas from the Monte Vulture volcano displaying 3He/4He (up to ~6.0 Ra) and Sr isotopes that are consistent with an origin in mantle that has had minimal pollution from subducted Adriatic slab. Similar 3He/4He in fluids from around Mt. Vulture indicate that the deep volcanic system is still degassing. Mantle-derived He occurs in fluids along the length of the Vulture line, reinforcing the hypothesis that it is a deep tectonic discontinuity along which mantle fluids and/or melts advect to the surface. The CO2/3He ratios are highly variable (2.7×108–2.15×1011) in response to processes such as gas dissolution into aquifers, addition of crustal gases and degassing fractionation.
► Lavas display 3He/4He (up to ~6.0 Ra) suggesting an origin in asthenospheric mantle. ► The measured mantle value is the highest of the Italian peninsular magmatism. ► Similar 3He/4He in fluids indicate that degassing of mantle-derived fluids is present. ► The degassing of mantle fluids along Vulture line is generated by a vertical slab. ► We underline the geodynamic analogies linking Mt. Vulture and Mt. Etna volcanoes.
The mephitis of Maschito, known since historical times as Lago Fetente (Smelly Lake) -although the lake is now dry-, is located 20 km from the Mt. Vulture volcanic edifice (Southern Italy). It is ...placed along the same regional tectonic discontinuity where some maars are located, close to the boundary between the foredeep and the Apulian foreland. About 300 m2 of surface is lacking in flora, while dead animals are frequently found all around it. The smelly exhalations are mainly composed of CO2 (∼98%), and, in lesser amounts, of H2S, N2, CH4 and other hydrocarbons. He, Ne and Ar occur in trace amounts. The CO2 isotopic composition is in the range of that of the main active Italian volcanic gases. The helium isotopic ratio (4.7 Ra) fits with the values measured in Mt. Vulture volcano and particularly with the olivine and pyroxene fluid inclusions of mantle xenoliths ejected during its last volcanic activity (140,000 years). The 40Ar/36Ar isotopic ratio of ∼320 supports some minor non-atmospheric contributions. The C/3He ratio (2.9 × 109) is in the typical range of magma released fluids, while δC(CH4)13 and δD(CH4) values fall in the field of thermogenic methane.
The amount of CO2 released is about 3200 tons/year. The flux of mantle-derived helium (>7 × 1010 atoms m−2 s−1) is at least three orders of magnitude higher than that of a stable continental crust. This study strongly supports the possibility that Maschito manifestations are fed by a geothermal system, which is powered by a degassing melt, bearing in mind that the Maschito gas emissions fall along the same fault system of the Monticchio maars, which formed during Mt. Vulture volcano's last activity.
•The mephitis of Maschito is a an area of 300 m2 lacking in flora, where dead animals are frequently.•The C/3He ratio (2.9 × 109) is in the typical range of magma released fluids.•The amount of CO2 released is about 3200 tons/year.•The flux of mantle-derived helium is orders of magnitude higher than that of a stable continental crust.•Maschito manifestations are fed by a geothermal system, which is powered by a degassing melt.
Here we report new data on the sulfur isotopic compositions (δ34S) of fumarolic and plume gases collected at Mount Etna volcano during 2008–2009. While low‐temperature fumaroles are affected by ...postmagmatic processes that modify the pristine isotopic signature, high‐temperature and plume gases allow establishment of aδ34S range of ∼0 ± 1‰ for magmatic SO2. We compared our data with those from S dissolved in primitive melt inclusions from 2002 lava and in whole rocks that erupted during the past two thousand years. Such a comparison revealed that δ34S is systematically lower for magmatic gases than for sulfur dissolved in the melt. We modeled how isotopic fractionation due to magma degassing process may vary δ34S value in both the melt and gaseous phases. This modeling required assessment of the fractionation factor (αgas‐melt). The most recent measurements on the oxidation state of sulfur in basaltic melt inclusions indicate that nearly all S is dissolved as sulfate (S6+), which would be possible in oxidized magmatic systems (ΔNNO ≥ 1). Under these conditions the exsolved gaseous phase is depleted with respect to the melt and the proposed model fits both gas and melt data, and constrains the Etnean magmatic δ34S to 1.0 ± 1.5‰. It is remarkable that the assessed redox conditions—which are significantly more oxidizing than previously thought—are able to explain why the dominant sulfur species measured in the Etnean plume is SO2.
Key Points
Sulfur magmatic signature at Etna is estimated as d34S=1(+/‐)1.5 per mil
Magma degassing is able to explain a lower d34S in gases with respect to melt
Redox conditions of NNO >/= 1 must exist in the shallow plumbing system
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