Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity

In order to investigate the variability of helium degassing in continental regions, its release from rocks and emission into the atmosphere, here we studied the degassing of volatiles in a seismically active region of northern Italy (MwMAX = 6) at the Nirano-Regnano mud volcanic system. The emitted gases in the study area are CH4–dominated and it is the carrier for helium (He) transfer through the crust. Carbon and He isotopes unequivocally indicate that crustal-derived fluids dominate these systems. An high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface permits to estimate the amount of He stored in the natural reservoirs. Our study demonstrated that the in-situ production of 4He in the crust and a long-lasting diffusion through the crust are not the main processes that rule the He degassing in the region. Furthermore, we demonstrated that micro-fracturation due to the field of stress that generates the local seismicity increases the release of He from the rocks and can sustain the excess of He in the natural reservoirs respect to the steady-state diffusive degassing. These results prove that (1) the transport of volatiles through the crust can be episodic as function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes

Long-term geochemical monitoring and extensive/compressive phenomena: case study of the Umbria Region (Central Apennines, Italy)

Long-term geochemical monitoring performed in the seismic area of the Umbria-Marche region of Italy (i.e. Central Apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical and isotopic variations of both the thermal springs as well as the gas vents. Coincident with the last seismic crisis, which struck the region in 1997-1998, an enhanced CO2 degassing on a regional scale caused a pH-drop in all the thermal waters as a consequence of CO2 dissolution. Furthermore, much higher 3He/4He isotope ratios pointed to a slight mantle-derived contribution. Radon activity increased to well above the ±2 sinterval of the earlier seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the single deep-located shock (March 26, 1998, 51 km deep). The anomalous CO2 discharge was closely related to the extensional movement of the normal faults responsible for the Mw 5.7, 6.0 and 5.6 main shocks that characterized the earlier seismic phase. In contrast, a clear compressive sign is recognizable in the transient disappearance of the deep-originating components related to the Mw 5.3, 51 km-deep event that occurred on March 26, 1998. Anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic crisis had terminated. We argue that the observed geochemical anomalies were driven by rock permeability changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the detection of anomalies reveals that a seismogenic process is developing. Indeed, comprehensive, long-term geochemical monitoring can provide new tools allowing us to better understand the development of seismogenesis

Mud volcanoes and methane seeps in Romania: main features and gas flux

Romania is one of the European countries with the most vigorous natural seepage of methane, uprising from pressurised natural gas and petroleum reservoirs through deep faults. The largest seepage zone is represented by large mud volcanoes, with CH4 >80% v/v, occurring on the Berca-Arbanasi hydrocarbon-bearing faulted anticline, in the Carpathian Foredeep. Smaller mud volcanoes have been identified in other areas of the Carpathian Foredeep, in the Transylvanian Depression and on the Moldavian Platform. New surveys carried out in Transylvania allowed us to discover the richest N2 mud volcano zone in the world (N2>90% v/v), with a remarkably high He content and a helium isotopic signature which highlights a contribution of mantle-derived source. The large mud volcanoes are generally quiescent, with rare explosive episodes and provide a methane flux in the order of 102-103 t km−2 y−1. Independently from mud volcanism, a remarkable dry macroseepage, however, has been found, with a degassing rate up to three orders of magnitude higher than that of mud volcanoes (i.e. 103-105 t km−2 y−1). The total gas flux from all investigated macroseepage zones in Romania is estimated in the range of 1500-2500 t y−1. The emission from microseepage, pervasively occurring throughout the hydrocarbon-prone basins, has yet to be assessed and added to the total gas output to the atmosphere

Long-term geochemical monitoring and extensive/compressive phenomena: case study of the Umbria Region (Central Apennines, Italy)

Long-term geochemical monitoring performed in the seismic area of the Umbria-Marche region of Italy (i.e. Central Apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical and isotopic variations of both the thermal springs as well as the gas vents. Coincident with the last seismic crisis, which struck the region in 1997-1998, an enhanced CO2 degassing on a regional scale caused a pH-drop in all the thermal waters as a consequence of CO2 dissolution. Furthermore, much higher 3He/4He isotope ratios pointed to a slight mantle-derived contribution. Radon activity increased to well above the ±2 sinterval of the earlier seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the single deep-located shock (March 26, 1998, 51 km deep). The anomalous CO2 discharge was closely related to the extensional movement of the normal faults responsible for the Mw 5.7, 6.0 and 5.6 main shocks that characterized the earlier seismic phase. In contrast, a clear compressive sign is recognizable in the transient disappearance of the deep-originating components related to the Mw 5.3, 51 km-deep event that occurred on March 26, 1998. Anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic crisis had terminated. We argue that the observed geochemical anomalies were driven by rock permeability changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the detection of anomalies reveals that a seismogenic process is developing. Indeed, comprehensive, long-term geochemical monitoring can provide new tools allowing us to better understand the development of seismogenesis

An overview of the geochemical characteristics of oceanic carbonatites: New insights from Fuerteventura carbonatites (Canary islands)

The occurrence of carbonatites in oceanic settings is very rare if compared with their continental counterpart, having been reported only in Cape Verde and Canary Islands. This paper provides an overview of the main geochemical characteristics of oceanic carbonatites, around which many debates still exist regarding their petrogenesis. We present new data on trace elements in minerals and whole-rock, together with the first noble gases isotopic study (He, Ne, Ar) in apatite, calcite, and clinopyroxene from Fuerteventura carbonatites (Canary Islands). Trace elements show a similar trend as Cape Verde carbonatites, almost tracing the same patterns on multi-element and REE abundance diagrams.3He/4He isotopic ratios of Fuerteventura carbonatites reflect a shallow (sub-continental lithospheric mantle, SCLM) He signature in their petrogenesis, and they clearly differ from Cape Verde carbonatites, i.e., fluids from a deep and low degassed mantle with a primitive plume-derived He signature are involved in their petrogenesis

First measurements of the Fe oxidation state of spinel inclusions inolivine single crystals from Vulture (Italy) with the in situ synchrotronmicro-Mossbauer technique

The redox state of the Earth's upper mantle (i.e., oxygen fugacity, f(O2)) is a key variable that influences numerous processes occurring at depth like the mobility of volatile species, partial melting, and metasomatism. It is linked to the oxidation state of peridotite rocks, which is normally determined through the available oxythermobarometers after measuring the chemical composition of equilibrated rock-forming minerals and the Fe3+ in redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe3+ / Sigma Fe in peridotites have been limited to those peridotites (e.g., harzburgites and lherzolites) for which an oxythermobarometer exists and where spinel (or garnet) crystals can be easily separated and measured by conventional Fe-57 Mossbauer spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a lherzolite with carbonatitic melts and, therefore, have recorded the passage of (metasomatic) fluids at mantle conditions. However, no oxythermobarometer exists to determine their equilibrium f(O2).The aim of this study was to retrieve the f(O2) of the mantle beneath Mt. Vulture volcano (Italy) through the study of a wehrlitic lapillus emitted during the last eruption (similar to 140 kyr ago) that contain olivines with multiple tiny spinel inclusions with sizes < 40 mu m. To our knowledge, the Fe oxidation state of these inclusions has been never determined with the Mossbauer technique due to their small sizes.Here, we present measurements of the Fe3+ / Sigma Fe using in situ synchrotron Mossbauer spectroscopy coupled with chemical and spectroscopic analysis of both host olivine and spinel inclusions.The results show Fe3+ / Sigma Fe ratios of 0.03-0.05 for olivine and 0.40-0.45 for the included spinels, the latter of which appear higher than those reported in literature for mantle spinel harzburgites and lherzolites. Given the evidence of the mantle origin of the trapped spinels, we propose that the high f(O2) (between 0.81 and 1.00 log above the fayalite-magnetite-quartz buffer; FMQ) likely results from the interaction between the pristine spinel lherzolite and a CO2-rich metasomatic agent prior to the spinel entrapment in olivines at mantle depths

Inferences on physico-chemical conditions and gas-water interaction by new quantitative approaches: The case of Panarea (Italy)

We have developed two new quantitative approaches to calculate temperatures in hydrothermal reservoirs by using the CO2-CH4-CO-H2 gaseous system and to model selective dissolution of CO2-H2S-N2-CH4-He-Ne mixtures in fresh and/or air saturated seawater. The anomalous outgassing starting November 2003 from the submarine exhalative system offshore Panarea island (Italy), was the occasion to apply such approaches to the extensive collection of volcanic gases. Gas geothermometry suggest the presence of a deep geothermal system at temperature up to 350°C and about 12 mol% CO2 in the vapor, which feeds the submarine emissions. Based on the fractional dissolution model, the rising geothermal vapor interacts with air-saturated seawater at low depths, dissolving 30-40% CO2 and even more H2S, modifying the pH of the aqueous solution and stripping the dissolved atmospheric volatiles (N2, Ne). Interaction of the liquid phase of the thermal fluids with country rocks, as well extensive mixing with seawater, have been also recognized and quantified. The measured output of hydrothermal fluids from Panarea exhalative field [1] accounts for the involvement of volatiles from an active degassing magma, nonetheless the climax of the investigated phenomenon is probably overcome and the system is new tending towards a steady-state. Our quantitative approaches allow us to monitor the geochemical indicators of the geothermal physico-chemical conditions and their potential evolution towards phreatic events or massive gas releases, which certainly are the main hazards to be expected in the area. The event at Panarea has in fact highlighted how hydrothermal systems can exhibit dramatic and sudden changes of their physico-chemical conditions and rate of fluid release, as a response to variable activity of feeding magmatic systems

First measurements of the Fe oxidation state of spinel inclusions in olivine single crystals from Vulture (Italy) with the in situ synchrotron micro-Mössbauer technique

The redox state of the Earth's upper mantle (i.e., oxygen fugacity, fO2) is a key variable that influences numerous processes occurring at depth like the mobility of volatile species, partial melting, and metasomatism. It is linked to the oxidation state of peridotite rocks, which is normally determined through the available oxythermobarometers after measuring the chemical composition of equilibrated rock-forming minerals and the Fe3+ in redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe3+ / ∑Fe in peridotites have been limited to those peridotites (e.g., harzburgites and lherzolites) for which an oxythermobarometer exists and where spinel (or garnet) crystals can be easily separated and measured by conventional 57Fe Mössbauer spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a lherzolite with carbonatitic melts and, therefore, have recorded the passage of (metasomatic) fluids at mantle conditions. However, no oxythermobarometer exists to determine their equilibrium fO2. The aim of this study was to retrieve the fO2 of the mantle beneath Mt. Vulture volcano (Italy) through the study of a wehrlitic lapillus emitted during the last eruption (∼ 140 kyr ago) that contain olivines with multiple tiny spinel inclusions with sizes &lt; 40 µm. To our knowledge, the Fe oxidation state of these inclusions has been never determined with the Mössbauer technique due to their small sizes. Here, we present measurements of the Fe3+ / ∑Fe using in situ synchrotron Mössbauer spectroscopy coupled with chemical and spectroscopic analysis of both host olivine and spinel inclusions. The results show Fe3+ / ∑Fe ratios of 0.03–0.05 for olivine and 0.40–0.45 for the included spinels, the latter of which appear higher than those reported in literature for mantle spinel harzburgites and lherzolites. Given the evidence of the mantle origin of the trapped spinels, we propose that the high fO2 (between 0.81 and 1.00 log above the fayalite–magnetite–quartz buffer; FMQ) likely results from the interaction between the pristine spinel lherzolite and a CO2-rich metasomatic agent prior to the spinel entrapment in olivines at mantle depths.</p

Active Degassing of Deeply Sourced Fluids in Central Europe: New Evidences From a Geochemical Study in Serbia

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 (&lt;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&nbsp;×&nbsp;109&nbsp;atoms&nbsp;m−2&nbsp;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