Article

Helium emission at Cumbre Vieja volcano, La Palma, Canary Islands

Authors:
  • Instituto Tenológico y de Energías Renovables
  • Instituto Volcanológico de Canarias (INVOLCAN)
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Abstract

We report herein the results of three soil helium surveys undertaken at Cumbre Vieja volcano, La Palma Island, Canary Islands. Helium content analyses at the surface environment of the volcano were carried out in the summers of 2002, 2003 and 2004. To estimate the diffusion contribution of helium emission, soil porosity was estimated indirectly from the granulometry data measured on a previous soil survey carried out in 1997. Helium enrichments in the soil layer with respect to the air concentration measured on Cumbre Vieja indicate a strong structural control in the degassing processes of the volcano and the excess helium seems to be emitted mainly along both N-S and N-W rifts of the volcano. He-4 flux values, calculated following a pure diffusive model, ranged between non-detectable values and 1.0 mg m(-2) d(-1) in 2002, 2003 and 2004 surveys. Total He-4 emission rate at Cumbre Vieja was estimated in the range of 18-38 kg d(-1). Isotopic composition of helium measured in soil gases along the N-S volcanic-rift of Cumbre Vieja showed small contributions of MORB-type helium increasing towards the southern part of the volcano. A He-3/He-4 ratio of 9.95 +/- 0.12 R-A was measured at a cold spring (geothermal) sample, showing a 7.2% contribution of plume-type helium. He-3 emission results in the range of 0.6-0.7 mol/yr support the hypothesis of contemporary degassing at Cumbre Vieja coming from magmas stored at different depths under La Palma island.

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... There is only one visible gas emanation point at La Palma. It is located inside Taburiente caldera (Dos Aguas, QP60 in Fig. 1b) and is a CO 2 -rich bubbling cold spring with the highest (R/R a ) c values reported for thermal fluids in the entire Canary Islands ranging between 9.42 ± 0.08 to 10.24 ± 0.07 (Pérez, 1994;Hilton et al., 2000;Padrón et al., 2012Padrón et al., , 2015. Since those values are clearly above the MORB range (Graham, 2002), some authors have suggested a small but persistent component of plume-type helium (Day and Hilton, 2011;Padrón et al., 2015) in accordance with the hot-spot model proposed for the origin of Canary Islands (Carracedo et al., 1998). ...
... Furthermore, the δ 13 C-CO 2 values (Fig. 4c) were similar to those reported for Teide fumaroles , pointing to an endogenous origin (Pineau and Javoy, 1983) (Fig. 5b). Under these conditions, the maximum (R/R a ) c ever measured at QP60, 10.24 ± 0.07 (Padrón et al., 2015), can be considered as an end-member (Padrón et al., 2012). Therefore, the decrease recorded in (R/R a ) c at QP60 could be related to an increase in crustal helium, 4 He crust , contribution (Tedesco, 1997;Capasso et al., 2005b;Padrón et al., 2013) as suggested by its estimation following the procedure described in Ballentine et al. (2002) (Fig. 4b). ...
... This deep helium contribution was accompanied by δ 13 C-CO 2 values measured at QP63, close to the MORB range (Pineau and Javoy, 1983) (Fig. 4c) and matching the highest values reported for this isotopic ratio in soil gases on La Palma (Padrón et al., 2015) with the exception of QP60. Together, these data suggest that this sampling point provides a preferential vent for deep magmatic gases (Capasso et al., 2005a;Padrón et al., 2012). As a result, QP63 showed an anomalous carbon dioxide concentration, visibly higher than the standard sea surface value (Sweeney et al., 2016). ...
Article
Monogenetic eruptions are the most common volcanic activity in the world. However, unrest monitoring data are scarce due to the long intervening quiescence periods. This study analyzes unrest signals recorded in one of the largest monogenetic fields in the Canary Islands, Cumbre Vieja (La Palma). Two seismic swarms were registered in October 2017 and February 2018 with b-values of 1.6 ± 0.1 and 2.3 ± 0.2 respectively suggesting an intense magmatic fluids contribution, gas and/or magma. Both swarms were linked to changes in gas emissions. Increases in hydrogen concentration, and (R/Ra)c up to 7.52±0.05, were recorded before the first swarm, at the sampling point closest to where seismicity was located, indicating a deep gas input. After the second swarm, increases in (R/Ra)c and thoron soil concentration were recorded at two locations. This dataset is compatible with a stalled magmatic intrusion at ca. 25 km depth, with an estimated volume between 5.5·10−4 km3 and 3·10−2 km3.
... In the Changbaishan volcanic field, hot springs are widely distributed and continually emitting gases, resulting in the emission of approximately 7.79 × 10 5 t of CO 2 into the atmosphere every year (Guo et al., 2014). Among the hot spring gases, helium can be used as a sensitive geochemical indicator for volatile provenance due to its inertness and low solubility (Padron et al., 2012). It is well documented that most 4 He is produced by radioactive decay within the crust and that most 3 He is still escaping from the Earth's interior (Morrison and Pine, 1955;Hilton, 1996;Padron et al., 2012;Zelenski et al., 2012). ...
... Among the hot spring gases, helium can be used as a sensitive geochemical indicator for volatile provenance due to its inertness and low solubility (Padron et al., 2012). It is well documented that most 4 He is produced by radioactive decay within the crust and that most 3 He is still escaping from the Earth's interior (Morrison and Pine, 1955;Hilton, 1996;Padron et al., 2012;Zelenski et al., 2012). Helium isotopes have been widely used to trace magma genesis and volcanic evolutions in different tectonic settings, as well as to monitor earthquakes and volcanoes (e.g., Van Soest, 2006, 2007;Ohno et al., 2011;Ohwada et al., 2012;Padron et al., 2012Padron et al., , 2013. ...
... It is well documented that most 4 He is produced by radioactive decay within the crust and that most 3 He is still escaping from the Earth's interior (Morrison and Pine, 1955;Hilton, 1996;Padron et al., 2012;Zelenski et al., 2012). Helium isotopes have been widely used to trace magma genesis and volcanic evolutions in different tectonic settings, as well as to monitor earthquakes and volcanoes (e.g., Van Soest, 2006, 2007;Ohno et al., 2011;Ohwada et al., 2012;Padron et al., 2012Padron et al., , 2013. When supplemented with CO 2 chemical and isotopic composition data, helium isotopic data can also be used to compute the percentages of mantle and slabderived fluids in magmatic mixtures (Sano and Marty, 1995;Sano et al., 2006). ...
Article
Changbaishan Volcano is located in northeastern China, approximately 1400 km west of the west Pacific subduction zone. Although the west Pacific plate and Changbaishan Volcano are spatially associated with each other, no previous evidence has demonstrated the existence of a direct material connection between the two. In this study, we utilize helium (3He/4He, CO2/3He) and carbon isotopes (δ13C) from the hot springs of Changbaishan Volcano to exclude the possibility of a direct material connection between the volcano and the west Pacific plate at source. A total of 22 gas samples were collected from three hot springs at Changbaishan Volcano in 2002, 2006, 2014 and 2015; isotopic and geochemical analyses were performed on these samples to trace the possible sources of these gases. Our analysis reveals that values for air-corrected 3He/4He ratios range from 3.98 RA to 6.03 RA (where RA represents the atmospheric 3He/4He ratio), CO2/3He ratios vary from 2.20 × 108 to 1.92 × 1011, and δ13C values vary from −7.9‰ to −1.6‰. By comparing these measured values to those of typical mantle and crustal sources, we can infer that hot spring gases from Changbaishan Volcano are mostly characterized by inputs from two isotopically distinct sources: deep mantle fluids and shallower, slab-derived fluids. Fluids liberated from the shallower magma chamber are likely to include ancient Izanagi subduction zone fluids, whereas fluids originating from deeper magma chamber likely consist of MORB-like asthenospheric mantle fluids. Based on these results, we suggest that helium and carbon isotopes in hot springs demonstrate the absence of a direct material connection between Changbaishan Volcano and the west Pacific plate.
... The only known visible gas emanation on La Palma Island is a CO 2 -rich bubbling cold spring in Taburiente caldera (Dos Aguas, DA in Fig. 1). Gas from this cold spring has the highest 3 He/ 4 He ratio measured in the Canary Islands, ranging from 9.5 to 9.95 R A (Pérez et al. 1994;Hilton et al. 2000;Padrón et al. 2012b), where R A is the atmospheric 3 He/ 4 He ratio (1.384×10 −6 , Clarke et al. 1976). This value is higher than those measured in any phenocrysts or geothermal gas samples in the archipelago (Pérez et al. 1994;Hilton et al. 2000;Day and Hilton 2011). ...
... The observed strong structural-spatial correlation in the emission of inert gases (helium) observed in Cumbre Vieja by Padrón et al. (2012b) and the absence of such correlation in the CO 2 emission, support the interpretation that any reactive gases (as CO 2 , SO 2 , H 2 S) released from depth by the volcanic system of Cumbre Vieja undergo chemical reactions or are almost completely dissolved in water during their rise. This observation is also supported by the chemical composition of groundwater extracted in the few existing vertical or horizontal wells at Cumbre Vieja (Veeger 1991;Pérez et al. 1996). ...
... Diffuse CO 2 increases observed in 2011 and 2013 seem to be preceded by an increase in the 3 He/ 4 He observed in Dos Aguas. The CO 2 / 3 He measured in the bubbling gases of this cold spring (0.5-1.7 10 10 ; Hilton et al. 2000;Padrón et al. 2012b) are within the range found in typical mantle environments (1-10 10 9 , Marty and Jambon 1987) and reasonably contain a significant CO 2 component from the present magmatic source of Cumbre Vieja. Assuming that the measured CO 2 / 3 He ratio in this cold spring is applicable for the whole island, as was suggested previously by Hilton et al. (2000), the rise in the 3 He emission would suggest corresponding increases in the deep-seated CO 2 release. ...
Article
Full-text available
We report herein the results of 13 soil CO2 efflux surveys at Cumbre Vieja volcano, La Palma Island, the most active basaltic volcano in the Canary Islands. The CO2 efflux measurements were undertaken using the accumulation chamber method between 2001 and 2013 to constrain the total CO2 output from the studied area and to evaluate occasional CO2 efflux surveys as a volcanic surveillance tool for Cumbre Vieja. Soil CO2 efflux values ranged from non-detectable up to 2,442 g m-2 d-1, with the highest values observed in the south, where the last volcanic eruption took place (Teneguía, 1971). Isotopic analyses of soil gas carbon dioxide suggest an organic origin as the main contribution to the CO2 efflux, with a very small magmatic gas component observed at the southern part of the volcano. Total biogenic and magmatic combined CO2 emission rates showed a high temporal variability, ranging between 320 and 1,544 t d-1 and averaging 1,147 t d-1 over the 220 km2 region. Two significant increases in the CO2 emission observed in 2011 and 2013 were likely caused by an enhanced magmatic endogenous contribution revealed by significant changes in the 3He/4He ratio in a CO2-rich cold spring. The relatively stable emission rate presented in this work defines the background CO2 emission range for Cumbre Vieja during a volcanic quiescence period.
... As a consequence of recent volcanic activity, the presence of an active geothermal system with associated passive degassing of volcanic gases along the N-S rift zone of Cumbre Vieja and its fault system is well documented [14][15][16] . High soil temperatures (90-130 °C) have recently been measured during a high CO 2 emission period 16 . ...
... Geological map of La Palma island (after Padrón et al.15 ).Content courtesy of Springer Nature, terms of use apply. Rights reserved ...
Article
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The study of geothermal systems is nowadays a topic of great importance because of the huge amount of energy that could be converted in electricity for human consumption from such sources. Among the various geophysical methods employed to study geothermal reservoirs, the magnetotelluric (MT) method is capable to reveal the internal structures of the subsurface and interpret the geological structures from the electrical resistivity. We present the first 3D resistivity model of La Palma (Canary archipelago, Spain) obtained from a dataset of 44 broadband magnetotelluric soundings distributed around the island. Our results highlight the presence of resistivity anomalies, spatially coinciding with density anomalies present in literature. In the north of the island, a high resistivity anomaly can be interpreted as the signature of an old intrusive body beneath the Taburiente caldera. In the south, a complex resistivity structure around the Cumbre Vieja volcanic ridge could be indicative of presence of an active geothermal system. In particular, low‐resistivity anomalies, located in a high‐fractured zone, have values compatible with clay alteration caps (illite and illite–smectite). Such a result suggests the presence of hot rocks, or a dike system, heating fluids in the interior of Cumbre Vieja volcanic system.
... This may be associated to some unrest on the island, as recently proposed by Torres-González et al. [20]; this same conclusion is suggested by considering our results together with the degassing results obtained in La Palma during a similar time period. Padrón et al. [72] reported the results of three soil helium surveys undertaken at the Cumbre Vieja volcano (2002, 2003 and 2004), which support the hypothesis of contemporary degassing at Cumbre Vieja from magmas stored at different depths under the island. ...
... An increase in the solid 3 He/ 4 He ratio was also observed toward the southern part of the Cumbre Vieja volcano [72]. It was interpreted as the result of residual degassing of volatiles from magma bodies stored at lithospheric levels beneath the southern part of the volcano, which were responsible for the last volcanic eruption of Cumbre Vieja in Teneguía in 1971. ...
Article
Full-text available
La Palma is one of the youngest of the Canary Islands, and historically the most active. The recent activity and unrest in the archipelago, the moderate seismicity observed in 2017 and 2018 and the possibility of catastrophic landslides related to the Cumbre Vieja volcano have made it strongly advisable to ensure a realistic knowledge of the background surface deformation on the island. This will then allow any anomalous deformation related to potential volcanic unrest on the island to be detected by monitoring the surface deformation. We describe here the observation results obtained during the 2006–2010 period using geodetic techniques such as Global Navigation Satellite System (GNSS), Advanced Differential Synthetic Aperture Radar Interferometry (A-DInSAR) and microgravimetry. These results show that, although there are no significant associated variations in gravity, there is a clear surface deformation that is spatially and temporally variable. Our results are discussed from the point of view of the unrest and its implications for the definition of an operational geodetic monitoring system for the island.
... Before the 2021 eruptive episode, Cumbre Vieja had produced 7 volcanic eruptions along the main north-south ridge in the historical period (post A.D. 1500) (Figure 1c). Due to the absence of visible gas emanations in the surface environment of Cumbre Vieja, geochemical monitoring of volcanic activity has been focused historically in diffuse degassing studies, paying attention mainly to CO 2 (Padrón et al., 2015) and He (Padrón et al., 2012;Torres-González et al., 2020). ...
... It is chemically inert, physically stable, non-affected by biogenic activity, barely soluble in water and almost non-adsorbable, which limits secondary influences on its ascent from source to surface and thereby represents a clear geochemical message from depth. Helium emission studies at Cumbre Vieja demonstrate a strong structural control upon the degassing processes of the system, with higher emission values focused along both N-S and N-W volcanic rifts (Padrón et al., 2012). The 3 He/ 4 He ratios of natural samples can vary by more than 3 orders of magnitude as the primordial 3 He/ 4 He ratio of (1.7-4.6) ...
Article
Full-text available
We report significant changes in the ³He/⁴He ratio measured at the Dos Aguas CO2‐rich cold mineral spring (La Palma, Canary Islands) that represent early precursory signals of the 2021 Tajogaite eruption at Cumbre Vieja volcano. Air‐corrected ³He/⁴He ratio has an average value of 9.80 RA in the last 30 years (1991–2021). Helium isotope ratios higher than the average value were observed during the period 2008–2013 suggesting an input of ³He‐rich less‐degassed magma beneath La Palma lithosphere. Magma ascended in 2017–2018 toward another reservoir located beneath Cumbre Vieja at 10–15 km depth, causing detectable seismic swarms located between 15 and 25 km, followed in 2020 by additional inputs of ³He‐rich less‐degassed magma that intruded at depth. Finally, in 2021 the buoyancy of magma and the pressure of the exsolved gases overcame the lithospheric resistance and a volcanic eruption took place between 19 September and 13 December 2021.
... Geochemical studies for geothermal exploration have been implemented while conducting extensive chemical characterization of distinct geothermal signatures existing in an investigation area (Liang et al. 2014). Radon is an excellent geochemical indicator (Ciotoli et al. 2004;Hanson et al. 2014;Rodriguez et al. 2015), which is a strong explorer for faults (Padrón et al. 2012). With cracks produced by the formation and movement of faults (Tao et al. 2019;Wang et al. 2020aWang et al. , 2020b, it not only provides a good channel for deep heat sources to reach the surface but also creates conditions for reservoir groundwater. ...
Article
Full-text available
Preliminary surveys found that Northern Jinan is rich in geothermal resources. To improve the prediction accuracy, we constructed an approach for predicting geothermal reservoir distribution using wavelet transform and self-organizing neural network (SOM). First, radon measurement and controlled-source audio-frequency magnetotellurics (CSAMT) field construction design was carried out, then the data collection, processing, and inversion were performed sequentially to obtain the radon and CSAMT data of each survey line. Next, wavelet transform was used to decompose the radon measurement data into different spatial and scale components and extract the low-frequency radon anomaly data of the target layer. Subsequently, the radon anomaly and resistivity data of the input sample dataset were input into the SOM for learning, which provided the classification results of the geothermal reservoirs. Finally, we assessed the SOM classification results and predicted favorable exploration areas depending on the drilling information and fault characteristics. Effective implementation contributes substantially towards understanding and developing geothermal resources.
... Eleazar Padrón 1,2 *, Nemesio M. Pérez 1,2 , Pedro A. Hernández 1,2 , Hirochika Sumino 3 , Gladys V. where D e = n 2 D m is the global diffusion coeffi cient (m 2 /s); dC is the variation of gas concentration (kg/m 3 ) along the depth dz (m); n is the effective porosity of the medium (%); and D m is the diffusion coeffi cient of helium in air (0.7 × 10 −4 m 2 /s at 25 ºC; Pandey et al., 1974). We used an average of 29% for the soil porosity, as measured on Cumbre Vieja volcano (La Palma Island, Canarian archipelago), a similar volcanic system with a comparable age and surface (Padrón et al., 2012). Soil helium effl ux data were used to construct spatial distribution maps by sequential Gaussian simulation, sGs (Cardellini et al., 2003), and then to estimate the total helium emission in the entire area of El Hierro Island. ...
Conference Paper
Full-text available
El Hierro Island is the southwesternmost and the youngest island of the Canary archipelago. Since 16 July, an anomalous seismicity at El Hierro island was recorded by IGN seismic network. After the occurrence of more than 10,000 seismic events, volcanic tremor was recorded since 05:15 of the October 10, by all of the seismic stations on the island, with highest amplitudes recorded in the southernmost station. During the afternoon of 12 October a large light-green coloured area was observed in the sea to the souht of La Restinga village (at the southernmost part of El Hierro island), suggesting the existence of a submarine eruption. Since October 12, frequent episodes of, turbulent gas emission and foaming, and the appearance of steamy lava fragments has been observed on the sea surface. As part of the volcanic surveillance of the island, the Instituto Volcanologico de Canarias (INVOLCAN) geochemical monitoring program is carrying out diffuse helium surveys on the surface environment of El Hierro (soil atmosphere). This nobel gas has been investigated because it has been considered an almost ideal geochemical indicator because it is chemically inert, physically stable, nonbiogenic, sparingly soluble in water under ambient conditions and almost non-adsorbable. At each survey, 600 sampling sites covering the whole island and following an homogeneous distribution are selected for helium measurements in the soil gases, The helium concentration gradients with respect to its value on air (5.24 ppm) allow us to estimate a pure diffusive emission rate of helium throughout the island. The first survey was carried out on the summer of 2003, when the island was on a quiescence period. At this survey, the amount of helium released by the volcanic system of El Hierro was estimated in 6 kg/d. Since the beginning of the seismic unrest, 13 helium emission surveys have been carried out. The helium emission rate has shown an excellent agreement with the evolution of the volcanic crisis of the island, reaching 30 kg/d on November 6, several days before the occurrence of the submarine eruption. A significant decrease to 13 kg/d was estimated almost 10 days after the beginning of the eruption, followed by a sudden increase to 38 kg/d several days before the largest seismic event of the volcanic crisis (M = 4.6) occurred on November 11. The results of the soil helium surveys performed at El Hierro Island prior and during a volcanic unrest period suggest that the emission of this noble gas is strongly controlled by the volcanic activity and its presence on the surface environment responds to the changes on the gas pressure at depth produced by the ascent of magma bodies.
... Eleazar Padrón 1,2 *, Nemesio M. Pérez 1,2 , Pedro A. Hernández 1,2 , Hirochika Sumino 3 , Gladys V. where D e = n 2 D m is the global diffusion coeffi cient (m 2 /s); dC is the variation of gas concentration (kg/m 3 ) along the depth dz (m); n is the effective porosity of the medium (%); and D m is the diffusion coeffi cient of helium in air (0.7 × 10 −4 m 2 /s at 25 ºC; Pandey et al., 1974). We used an average of 29% for the soil porosity, as measured on Cumbre Vieja volcano (La Palma Island, Canarian archipelago), a similar volcanic system with a comparable age and surface (Padrón et al., 2012). Soil helium effl ux data were used to construct spatial distribution maps by sequential Gaussian simulation, sGs (Cardellini et al., 2003), and then to estimate the total helium emission in the entire area of El Hierro Island. ...
Article
Full-text available
Significant increases in helium emissions from the soil and of 3He/4He ratios in groundwater on El Hierro Island (Canary Islands, Spain) were observed prior the 2011–2012 submarine eruption off the coast of the island. The changes of diffusive helium emissions rate were observed one month prior to the submarine eruption onset (October 12, 2011) and the major increase preceded increases in seismic energy release during the volcanic unrest. Measured 3He/4He ratios in groundwaters from a well in El Hierro Island increased from 2 to 3 RA to 7.2 RA one month prior the eruption onset to reach a peak of 8.2 RA (where RA is the 3He/4He ratio in air) indicating a dominant magmatic contribution to the dissolved gases in ground waters. 3He/4He values and diffusive helium emission studies have been extremely important for forecasting the onset of the volcanic unrest and subsequent volcanic eruption. An aseismic exsolution of magmatic gases from magma bodies beneath El Hierro Island through fractures and vertical permeability structures increased the diffusive helium emission rate prior to episodes of seismic-energy release associated to the volcanic unrest.
... Few comparable studies of soil He emission at volcanic areas have been reported to date. Padrón et al. (2012b) estimated the He emission from Cumbre Vieja volcano (La Palma, Canary Islands, Spain) considering only diffusion as main transport mechanism, obtaining a He emission for an area of 220 km 2 of between 15 and 38 kg day −1 . Using the same approach, Padrón et al. (2013) estimated the He emission across the entire island of El Hierro (Canary Islands, Spain) prior to and during to the 2011-2012 submarine eruptions. ...
Article
Full-text available
We report the first detailed study of diffuse emis-sion of carbon dioxide (CO 2), hydrogen sulfide (H 2 S), helium (He), and hydrogen (H 2) from the summit crater of Pico do Fogo volcano, Cape Verde. Diffuse CO 2 , H 2 S, He, and H 2 gas fluxes were measured at 57 sampling sites and ranged up to 12,800, 13, 1, and 6 g m −2 day −1 , respectively. Soil tempera-ture measurements at each sampling site were used to evaluate the heat flux. Most of the summit crater shows relatively high CO 2 efflux, with highest values close to the fumarolic area, suggesting a structural control of the degassing process. In contrast, H 2 S effluxes were negligible or very low at the sum-mit crater, except close to the fumarolic area where anoma-lously high CO 2 efflux and soil temperatures were also mea-sured. We estimate total CO 2 , H 2 S, He, and H 2 diffuse gas fluxes of 219 t day −1 , 25, 4, and 33 kg day −1 , respectively. Based on a H 2 O/CO 2 mass ratio of 1.52 measured at the fu-maroles, we estimate a diffuse steam flux from the summit crater of approximately 330 t day −1 . The enthalpy of this steam is equivalent to a heat flux of about 10.3 MW. The diffuse gas emission and thermal energy released from the summit crater of Pico do Fogo volcano are comparable to those observed at other volcanoes. Sustained surveillance of Pico do Fogo using these methods will be valuable for mon-itoring the activity of one of the most active volcanoes in the Atlantic Ocean.
... This is the case for helium (He). He has unique characteristics as a geochemical tracer: it is chemically inert and radioactively stable, non-biogenic, highly mobile and relatively insoluble in water (Reimer 1980;Ozima and Podosek 2002;Fu et al. 2005). There are two naturally occurring isotopes of helium: 4 He and 3 He, with an atmospheric 3 He/ 4 He ratio (R A ) of 1.384 9 10 -6 (Clarke et al. 1976). ...
Article
Full-text available
We report herein the results of soil gas geochemistry studies, focused mainly on nonreactive and/or highly mobile gases such as He and H2, in five mining licenses at Tenerife and Gran Canaria, Canary Islands, Spain, during 2011–2014. The primary objective was to sort the possible geothermal potential of these five mining licenses, thus reducing the uncertainty inherent to the selection of the areas with highest geothermal potential for future exploration works. By combining the overall information obtained by the statistical–graphical analysis of the soil He and H2 data, the spatial distribution of soil gas concentrations and the analysis of selected chemical ratios of the soil gas to evaluate the influence of deep-seating degassing, two of the five mining licenses (Garehagua and Abeque, both located in Tenerife Island) seemed to show the highest geothermal potential. These results will be useful for future implementation and development of geothermal energy in the Canaries, the only Spanish territory with potential high-enthalpy geothermal resources, thus the most promising area for high-enthalpy geothermal installations.
... fumaroles, hot springs) existing in an area under investigation. However, at those areas where there is a lack of visible geothermal manifestations in the surface environment, geochemical prospecting of soil gases and volatiles in the soil matrix itself can provide useful information on the location of areas where deep-seated fluids can reach the surface along active tectonic structures (Alparone et al., 2004;Giammanco et al., 2006;Hernández et al., 2000;Padrón et al., 2012;Voltattorni et al., 2010;Hanson et al., 2014). Gases released from active geothermal systems, might freely rise through the overlying cover to be detected in the soil surface. ...
... collected sequentially in copper tubes for later analysis of helium isotope ratios. Unlike the method of Padrón et al. (2012Padrón et al. ( , 2013 to evaluate diffuse helium emissions, in which samples are collected at 40 cm depth with a metallic probe, the accumulation chamber method does not require an evaluation of the soil porosity. ...
Article
The composition of atmospheric helium is generally considered to be constant (³He/⁴He = 1.39 × 10− 6) on a large spatial scale. However, local variations may arise in tectonically active areas due to focussed degassing of one of its two isotopes, for example degassing of mantle-derived ³He or crustal-derived ⁴He. If detected, such variations have the potential to trace open conduit degassing of magmatic bodies and/or diffusive emissions from volcanic and/or crustal sources. Here, we test the possibility of detecting such variations in the Afar region of north-eastern Ethiopia, which is located over a well-developed rift system. Special attention was paid to the Erta Ale volcanic edifice, where both magma lake activity and strong degassing occur. We conducted high-precision ³He/⁴He ratio measurements of air samples from this region using an analytical facility at the Centre de Recherches Pétrographiques et Géochimiques (CRPG), Nancy (France) that was specially designed for high-precision noble gas analyses.
... The primitive isotope of He, 3 He, gradually escapes from the Earth due to its lighter mass (Gilfillan et al., 2011). On the other hand, the heavier isotope 4 He is continuously generated via radioactive decay of 235 U, 238 U, and 232 Th, and it can be trapped in the gravity of the Earth ( Padrón et al., 2012). Consequently, the ratio of 3 He/ 4 He is higher in the Earth's mantle than in the crust (Mackintosh and Ballentine, 2012). ...
Article
Volcanic gases have been monitored to predict eruptions since their compositions are strongly affected by volcanic activities. Here, volcanic gas geochemistry is reviewed with the main focus placed on the potential use of volcanic gases as eruption precursors. First, volcanic activities are differentiated into four eruptive cycles: eruptive, post-eruptive, inter-eruptive, and pre-eruptive periods. Second, we discuss geochemical factors affecting volcanic gas compositions: magma origin and degassing, hydrothermal interactions, and meteorological variability. Third, volcanic gas data in previous studies are revisited to address the practical aspects in utilizing volcanic gas compositions to predict eruptions. As a magma is depressurized due to its uprising, volcanic gases become progressively exsolved. Generally, the fluxes of volcanic gases (e.g., CO2, SO2, H2S, etc.) increase considerably close to eruptions. Also, CO2/SO2, SO2/HCl, and SO2/HF ratios in volcanic gases tend to decrease at the imminence of eruptions. Nonetheless, such trends may not be encountered due to both kinetically limited degassing and secondary effects including hydrothermal interactions and meteorological variability. Due to the site specificity of the aforementioned effects, a comprehensive understanding of volcanic gas chemistry requires the knowledge of magma vents and reservoirs as well as hydrothermal systems below volcanoes. Accordingly, the integrated use of volcanic gas data with geophysical and geomorphological information can warrant a better prediction of eruptions.
... Other studies of soil 3 He emission in volcanic areas are rare. Padrón et al. [51] estimated a total output of 3 He in Cumbre Vieja volcano (La Palma, Canary Islands) of 0.6-0.7 mol y -1 considering a pure diffusive transport mechanism for an area of 220 km 2 . ...
Article
Full-text available
During July 2016, the first integrated heat flow, CO 2 , and ³ He emission survey was conducted across 0.5 km ² of the summit cone and crater of Teide volcano, Tenerife, Canary Islands, Spain. The thermal energy released from Teide summit cone by diffuse degassing was 2.2 MW, and the heat flux calculated through Dawson’s method was 8.1 MW, difference due to the comparison of purely convective areas as the crater with diffusive areas as the flanks of the volcano. Diffuse CO 2 output was 211±20 t d ⁻¹ , and ³ He emission was estimated to be within a range between 0.35 and 0.89 mol y ⁻¹ . The obtained values of diffuse degassing and heat fluxes are close to others obtained for similar volcanic areas. The calculation of ³ He/heat ratio for the first time in this volcanic system supports the presence of an important mantle source for the degassing of Teide volcano.
... The CO 2 content of air (ppm vol.) was measured continuously in the chamber over time and the accumulated gases were collected sequentially in copper tubes for later analysis of helium isotope ratios. Unlike the method of Padrón et al. (2012Padrón et al. ( , 2013 to evaluate diffuse helium emissions, in which samples are collected at 40 cm depth with a metallic probe, the accumulation chamber method does not require an evaluation of the soil porosity. ...
Thesis
Le rapport isotopique de l'hélium atmosphérique (RA = 3He/4He = 1.39 × 10-6) est lié aux flux naturels et anthropiques de 3He et de 4He en provenance de la Terre (et de l'espace pour 3He). Les activités industrielles ont émis des gaz riches en 4He radiogénique (exploitation des combustibles fossiles) et en 3He (activités nucléaires). Du fait de ces flux d'hélium, il a été proposé que le rapport 3He/4He dans l'air a varié dans le temps et dans l'espace. De telles variations pourraient permettre l'utilisation de ce rapport comme nouveau traceur des émissions atmosphériques de polluants anthropiques et/ou naturelles. Pour vérifier ces possibilités, nous avons entrepris la mesure haute-précision (2-6‰, 2σ) de la composition isotopique de l'hélium atmosphérique au Centre de Recherches Pétrographiques et Géochimiques (Université de Lorraine et Centre National de la Recherche Scientifique, Vandoeuvre-lès-Nancy, France). Nous avons effectué des analyses d'intercalation avec un spectromètre de masse double collecteur (Helix Split Flight Tube de la société Thermo Instruments). Au cours de ces analyses, plusieurs analyses individuelles d'un échantillon d'air sont effectuées en alternance avec un standard d'air. Nous n’avons détecté aucune évidence de variation temporelle du rapport 3He/4He dans l’air de France piégé dans des boules de pétanque (1965, 1990, 2010, 2013), dans des réservoirs métalliques (2010, 2016) et dans un carburateur de voiture (1910). Nous avons également re-analysé des échantillons d'air ancien prélevés dans des réservoirs métalliques depuis 1978 à Cape Grim (Tasmanie, Australie). En incluant les mesures de Mabry et al. (2015) pour certains de ces réservoirs, nous obtenons une tendance commune de -0.05 ± 2.46 ‰, montrant l'absence de variation statistiquement significative sur une période de 106 ans. Au niveau mondial, nous avons seulement détecté deux échantillons d’air sur seize (Dôme C, Antarctique; Tokyo, Japon) dont les compositions sont statistiquement distinctes, marquées par des excès en 3He de 2.0 ± 1.4 ‰, et 1.7 ± 1.5 ‰, respectivement (intervalle de confiance de 95%). Ces excès pourraient être liés aux flux d’hélium locaux par : (i) les précipitations aurorales en Antarctique ; (ii) les volcans et/ou les déchets nucléaires (Fukushima Daï-Ichi) au Japon. Dans la région de l’Afar (Éthiopie), nous avons détecté des excès en 3He de ~1 % au-dessus du lac de lave du volcan Erta Ale, pouvant être utilisés pour tracer des gaz réactifs (e.g. CO2) simultanément émis. Ces excès sont toutefois rapidement dilués dans les masses d’air régionales. L'isolation des gaz libérés par le sol du cratère de l'Erta Ale à l'aide d'un contenant inversé placé à sa surface a permis d'évaluer les flux diffusifs en 3He (via l'augmentation du rapport 3He/4He dans l'air isolé) et en CO2. Globalement, cette thèse conforte l’utilisation de RA comme standard inter-laboratoire. Dans certaines régions (Dôme C, Japon), une investigation plus poussée dans le temps devrait être effectuée pour contraindre les sources d’hélium atmosphérique pouvant engendrer de faibles variations locales
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We report the first detailed study on the types and distributions of active subaqueous fumaroles and surface diffuse CO2 degassing in the three main volcanic lakes of São Miguel Island (Sete Cidades, Fogo and Furnas), Azores archipelago, Portugal. The results of the surveys, carried out in May 2011 using a floating accumulation chamber and a dual beam 50 and 200 kHz echo sounder, revealed a very low surface CO2 degassing at the three lakes, in the range of 32-608 kg d⁻¹. However, dense subaqueous degassing plumes were found in the north of the Furnas crater lake (7.5-9 plumes per 100 m²), and moderate-density degassing in the Fogo (1.5-2 plumes per 100 m²) and Sete Cidades crater lakes (1-1.5 plumes per 100 m²). The echo sounder detected hydroacoustic signatures interpreted as acoustic flares, «puffing» bubble plumes or walls of bubbles associated with numerous subaqueous fumaroles. The recorded echograms show that the bubbles rise at average speeds of between 19 and 30 cm s⁻¹ at the bottom, with frequencies of release from 1-2 to 31 s. Most subaqueous fumaroles disappear due to the dissolution of CO2 before reaching the lake surface. These dissolution processes are enhanced by the pH range observed in the three volcanic lakes (c. 7-9). Observed dissolved CO2 values indicate that the pressure of this gas in the three lakes remained much lower than the hydrostatic pressure and the risk of a limnic eruption is therefore negligible. We suggest that the rising levels of CO2 from the subaqueous bubbles could constitute a critical fuel for subsurface phytoplankton layers, interpreted as horizontal acoustic layers with high backscattering values. The highest density of subaqueous bubbling correlates with recent submerged secondary craters formed around the caldera rims of the three Late Quaternary stratovolcano complexes of São Miguel Island. Our results emphasize the need to perform regular surface degassing studies as an important volcanic surveillance tool in the Azores archipelago.
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Diffuse degassing processes provide valuable information on geothermal reservoir characteristics not only in the context of monitoring, but also for exploration purposes. Areas with increased gas emissions can be indicative of major upflow zones from the reservoir through deep-reaching, permeable fault zones. These fault zones may act as preferential target areas for geothermal production drillings. In this study it is successfully demonstrated that diffuse degassing measurements can be used for the detection and characterization of permeable structural elements. The combination of following techniques has been applied at the Brady's geothermal system in the Basin-and-Range Province (Nevada, USA): accumulation chamber method for carbon dioxide and hydrogen sulfide measurements, alpha-spectroscopic measurements of radon and thoron activity concentrations, and gamma-spectroscopic measurements of selected nuclides.
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The construction of a small-size, magnetic sector, single focusing mass spectrometer (He-MS) for the continuous, on-site monitoring of He isotope ratios (³He/⁴He) is described. The instrument is capable of measuring ⁴He/²⁰Ne ratios dissolved in several different types of natural fluids of geochemical interest, such as groundwater and gas from hot springs, volcanoes and gas well fields. The ion optics of He-MS was designed using an ion trajectory simulation program “TRIO,” which permits the simultaneous measurement of ³He and ⁴He with a double collector system under a mass resolution power (M/ΔM) of >500. The presently attained specifications of the He-MS are; (1) a mass resolving power of ca. 490, sufficient to separate ³He⁺ from interfering ions, HD⁺ and H3⁺, (2) ultra-high vacuum conditions down to 3×10⁻⁸ Pa, and (3) a sufficiently high sensitivity to permit amounts of ³He to be detected at levels as small as 10⁻¹³ cm³STP (3×10⁶ atoms). Long term stability for ³He/⁴He analysis was examined by measuring the ³He/⁴He standard gas (HESJ) and atmospheric He, resulting in ∼3% reproducibility and ≤5% experimental error for various amounts of atmospheric He from 0.3 to 2.3×10⁻⁶ cm³STP introduced into the instrument. A dynamic range of measurable ³He/⁴He ratios with He-MS is greater than 10³ which was determined by measuring various types of natural fluid samples from continental gas (with a low ³He/⁴He ratio down to 2×10⁻⁸) to volcanic gas (with a high ³He/⁴He ratio up to 3×10⁻⁵). The accuracy and precision of ³He/⁴He and ⁴He/²⁰Ne ratios were evaluated by comparing the values with those measured using well established noble gas mass spectrometers (modified VG5400/MS-III and -IV) in our laboratory, and were found to be in good agreement within analytical errors. Usefulness of the selective extraction of He from water/gas using a high permeability of He through a silica glass wall at high temperature (700°C) is demonstrated.
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Consistent 3He/4He ratios have been measured for >25 years in geothermal fluids and gases from Cumbre Vieja, La Palma (9.4 ± 0.1RA, where RA is the 3He/4He of air), and Teide, Tenerife (6.8 ± 0.3RA), Canary Islands. Both locations are characterized by similar CO2/3He (~2 to 4 × 109), mantle-like δ13C (–3.3‰ to –4.4‰) and CO2 output (0.1–0.2 × 1010 mol yr–1). Helium isotopic differences between the islands cannot be explained by differential aging and 4He ingrowth in their mantle sources. Instead, distinct He reservoirs exist, with a high-μ (HIMU)–type mantle source for La Palma and a more enriched mantle, with possible lithospheric mantle influence, for Tenerife. Geothermal samples from the Canary Islands record a present-day He distribution distinct from higher 3He/4He in olivine from older eastern Canary Island lavas, indicating temporal variability in sources. Comparison of geothermal sample data versus olivine, pyroxene, and glass He isotope data for the Canary Islands, Azores, Cape Verde, Hawaiian islands, and Iceland reveals generally good correspondence, even across >1 m.y. of stratigraphy. However, in addition to the Canary Islands, there are examples of inter-island heterogeneity for He isotopes at Hawaii, the Azores, and within Iceland, preserved in hydrothermal samples, minerals, and glasses. In particular, in northwest Iceland, olivine separates from older lavas preserve higher 3He/4He than present-day geothermal samples from the same region. This difference likely reflects a reduced mantle-derived 3He input to Icelandic magmatism since the Miocene. Temporal variability in 3He/4He, assessed using geothermal and geological materials in conjunction, offers a powerful tool for examining heterogeneity and temporal evolution of mantle sources at intraplate volcanoes.
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Active/recent volcanism indicates the presence of high-enthalpy resources at depth, but sometimes there is not any evidence of endogenous fluids manifestations at surface, that confirms the presence of an active geothermal system. In this study we present an extensive study of published and new diffuse ⁴He and ³He emission and thermal energy released data from fifteen volcanic systems, confirming a direct relationship between diffuse helium emissions and thermal energy released associated with the rise of mantle fluids. The results are useful to infer the existence of deep and/or hidden geothermal resources and to estimate roughly its potential at those areas without obvious geothermal manifestations at the surface, but with measureable anomalous emissions of ⁴He and ³He. Tracing diffuse He emission anomalies at the surface environment of volcanic systems provides very important information for the exploration and discovery of geothermal resources at those areas where the resources are either hidden or lie at great depth.
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We report herein the first results of an extensive soil gas survey performed on Timanfaya volcano on May 2011. Soil gas composition at Timanfaya volcano indicates a main atmospheric source, slightly enriched in CO2 and He. Soil CO2 concentration showed a very slight deep contribution of the Timanfaya volcanic system, with no clear relation to the main eruptive fissures of the studied area. The existence of soil helium enrichments in Timanfaya indicates a shallow degassing of crustal helium and other possible deeper sources probably form cooling magma bodies at depth. The main soil helium enrichments were observed in good agreement with the main eruptive fissures of the 1730-36 eruption, with the highest values located at those areas with a higher density of recent eruptive centers, indicating an important structural control for the leakage of helium at Timanfaya volcano. Atmospheric air slightly polluted by deep-seated helium emissions, CO2 degassed from a cooling magma body, and biogenic CO2, might be the most plausible explanation for the existence of soil gas. Helium is a deep-seated gas, exhibiting important emission rates along the main eruptive fissure of the 1730-36 eruption of Timanfaya volcano.
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The compositional features of fluids from both fumarolic discharges and productive geothermal wells of Ahuachapan-Chipilapa, Berlin-Chinameca, and San Vicente geothermal systems (El Salvador) are described and discussed in order to investigate the complex geochemical interactions involving geothermal fluids within the shallowest part of the hydrothermal circulation pathways. Our results highlight that secondary processes are able to strongly affect and modify the chemical characteristics of geothermal gases once they discharge to the surface as natural manifestations, mainly in relation to the chemical-physical properties of each gas species. The effects of both gas dissolution in shallow aquifers and gas-water-rock chemical interactions on gas discharge composition make it difficult to get a correct evaluation of the thermodynamic conditions that characterize the geothermal reservoirs by applying the common geoindicators based on the chemical equilibria of the H2O-CO2-H2-CH4-CO system. Differently, the composition of the C1-C2-C3 alkanes and the C3 and C4 alkane-alkene pair, established within the geothermal reservoirs under the control of chemical reactions, remains stable in samples collected from discharging gas vents. These results suggest that the relative abundances of hydrocarbons characterized by similar structure and molecular size seem to be mainly regulated by the diffusion velocity of gases through the liquid-dominated system. Therefore the chemical features of the light organic gas fraction of naturally discharging fluids can be successfully utilized for the evaluation of geothermal reservoir temperatures and redox conditions, providing useful indications in terms of geothermal exploration and exploitation. On this basis, the distribution, speciation, and relative abundances of light hydrocarbons can also be considered highly promising in geochemical monitoring of active volcanic systems.
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Measurements of 220Rn and 222Rn activity and of CO2 flux in soil and fumaroles were carried out on Mount Etna volcano in 2005–2006, both in its summit area and along active faults on its flanks. We observe an empirical relationship between (220Rn/222Rn) and CO2 efflux. The higher the flux of CO2, the lower the ratio between 220Rn and 222Rn. Deep sources of gas are characterized by high 222Rn activity and high CO2 efflux, whereas shallow sources are indicated by high 220Rn activity and relatively low CO2 efflux. Excess 220Rn highlights sites of ongoing shallow rock fracturing that could be affected by collapse, as in the case of the rim of an active vent. Depletion both in 220Rn and in CO2 seems to be representative of residual degassing along recently active eruptive vents.
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In March 1994, soil gases were sampled in the area of Vulcano Porto, on the island of Vulcano, using a grid of about 200 points/km2. Analysed gases were CO2, He and222Rn and, over a smaller area, H2S. Some of the samples were also analysed for the isotope composition of CO2C. Three anomalous CO2 degassing areas were identified: Grotta dei Palizzi, the area near the Telephone Exchange, and the area near the beach fumaroles. The behaviour of He and222Rn is different in these 3 areas. The concentration of He is much lower than that of atmospheric He (down to −3950 ppb) in the isthmus, and only in the area near Grotta dei Palizzi does it have values significantly higher than atmospheric ones (up to 1900 ppb). The activity of222Rn, always significantly positively related to CO2 concentrations, peaked in the 2 fumarole areas (isthmus, Telephone Exchange).
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Conditional sequential Gaussian simulations (sGs) have been applied for the first time to the study of soil diffuse degassing from different volcanic and nonvolcanic systems. The application regards five data sets of soil CO2 fluxes measured with the accumulation chamber methodology at the volcanic areas of Solfatara of Pozzuoli (Italy), Vesuvio cone (Italy), Nisyros (Greece), and Horseshoe Lake (California) and at the nonvolcanic degassing area of Poggio dell'Olivo (Italy). The sGs algorithm was used to generate 100 realizations of CO2 flux for each area. Probabilistic summaries of these simulations, together with the information given by probability plots, were used (1) to draw maps of the probability that CO2 fluxes exceed thresholds specific for a background flux, i.e., to define the probable extension of the degassing structures, (2) to calculate the total CO2 output, and (3) to quantify the uncertainty of the estimation. The results show that the sGs is a suitable tool to model soil diffuse degassing, producing realistic images of the distribution of the CO2 fluxes that honor the histogram and variogram of the original data. Moreover, the relation between the sample design and the uncertainty of estimation was investigated leading to an empirical relation between uncertainty and the sampling density that can be useful for the planning of future CO2 flux surveys.
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The emission rate of helium-3 by subaerial volcanism is re-evaluated by coupling He-3/CO2 ratios in volcanic gases with new estimate of the flux of carbon dioxide from both arc and nonarc volcanoes. Steady volcanic plume emissions are computed to supply 240-310 mole He-3/yr, most of which is due to nonarc volcanoes. Major eruptions represent minor additions on average, whereas diffuse He-3 emanations through volcanic piles may be important sources. Although not including this contribution, the above figure is equivalent to 20-40 percent of the He-3 flux from mid-ocean ridges and, thus, suggests that primordial He-3 degassing from subaerial volcanic regions may be larger than expected.
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The 3He/4He and 4He/20Ne ratios of 115 natural gas samples, including various types of gases from volcanoes, hot springs, mineral springs, water wells, petroleum fields, and natural gas fields, were measured using a mass spectrometer. The observed 3He/4He and 4He/20Ne ratios range from 7.47×10-8 to 9.65×10-6 and from 0.26 to 1100, respectively. The 3He/4He ratios reflect well the geotectonic structure of the Japanese Islands. In northeastern (NE) Japan there is a clear geographical difference in the 3He/4He ratios between the frontal arc (forearc) and volcanic arc (back arc) regions. Lower ratios were found in the trench side region and higher ratios in the back arc side. This result suggests that the mantle-derived helium in the volcanic arc region is associated with the diapiric uprise of a magma. Lower 3He/4He ratios in the frontal arc region may be due to radiogenic He produced by radioactive decay of U and Th in the crustal and sedimentary rocks. In southwestern (SW) Japan there is no clear geographical contrast in the 3He/4He ratios. Some samples in the frontal arc region show quite high 3He/4He ratios. The tendency of the 3He/4He ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from NE Japan. The high 3He/4He ratios observed in the frontal arc region in SW Japan may be indicative of renewed or incipient magmatism due to a descending yound and warm slab.
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U-Th-Ra disequilibria of basanites, tephrites, and phonolites from the A.D. 1585 eruption on La Palma, Canary Islands, constrain magma differentiation times in an ocean-island rift zone. The insignificant difference in (230Th)/(232Th) implies differentiation from basanite to phonolite in <15 k.y. 226Ra has a half-life of 1600 yr, however, and permits higher temporal resolution; (226Ra)/(230Th) disequilibria are highest in the phonolites (46%-54%) and basanites (44%-47%) and lowest in the tephrites (38%-41%). The higher 226Ra excesses in the end-member compositions model basanite-phonolite differentiation within 1550-1750 yr at a rate of 0.04% fractional crystallization per year. Such a short time interval is in sharp contrast to the ∼200 k.y. proposed for phonolite differentiation on the neighboring island of Tenerife and could reflect different volcanic systems, with a mantle-fed rift system on La Palma versus a crustal magma reservoir on Tenerife.
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Concentrations of soil gases (O 2, CO 2, N 2, H 2, He, Ar, Rn, and Hg vapor) and the efflux of carbon dioxide at the Ahuachapán Geothermal Field has been investigated. Soil pressure gradient and temperature were also measured. A grid of 196 points were sampled. Results show that higher concentrations of hydrothermal gases are found along fault zones. Soil gases seem to be good indicators of fault permeability. Two E-W trending zones with higher concentrations of different gases were identified suggesting buried E-W striking faults associated to the formation of the regional Central Graben or Medium Trough and related to the regional tectonics. This methodology looks promising to identify zones of vertical permeability and flow path that are not easily detected by other prospection methods.
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Kueishantao is a Holocene volcanic islet (<7,000 yrs) located at NE offshore Taiwan, and tectonically is part of western extension of the Okinawa Trough. Magmatic activities are considered very active around this area on the basis of the fact that on-land fumaroles and submarine hydrothermal systems are prevailing currently. Representative bubble gas samples from submarine hydrothermal vents were collected for gas composition and helium isotope analysis. The gases show similar compositions of low temperatre fumaroles in the world, i.e., with high CO2 and H2S but low SO2 and HCl contents. They exhibit consistent high 3He/4He ratios (7.3–8.4 RA, where RA is the 3He/4He ratio of air), probably the highest 3He/4He values of gases ever reported in active hydrothermal areas of the western Pacific region. Meanwhile, seawater samples around Kueishantao and other fluid samples from I-Lan Plain, the land area closest to the Kueishantao and also the southernmost part of the Okinawa Trough, show a significant excess of 3He compositions as well. This indicates that the mantle component plays an important role for their gas sources, and implies that the mantle fluids may have invaded into I-Lan Plain. The westward opening of the Okinawa Trough may have caused thinning of the continental crust and produced deep normal faults and hence, the primordial 3He is able to degas from mantle source region without significant crust contamination.
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Effective porosity in solute-transport analyses is usually estimated rather than calculated from tracer tests in the field or laboratory. Calculated values of effective porosity in the laboratory on three different textured samples were compared to estimates derived from particle-size distributions and soil-water characteristic curves. The agreement was poor and it seems that no clear relationships exist between effective porosity calculated from laboratory tracer tests and effective porosity estimated from particle-size distributions and soil-water characteristic curves. A field tracer test in a sand-and-gravel aquifer produced a calculated effective porosity of approximately 0.17. By comparison, estimates of effective porosity from textural data, moisture retention, and published values were approximately 50-90% greater than the field calibrated value. Thus, estimation of effective porosity for chemical transport is highly dependent on the chosen transport model and is best obtained by laboratory or field tracer tests.
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and δ13C in soil gas were measured at three active subduction-related stratovolcanoes (Arenal and Poás, Costa Rica; Galeras, Colombia). In general, Rn, CO2 and δ13C values are higher on the lower flanks of the volcanoes, except near fumaroles in the active craters. The upper flanks of these volcanoes have low Rn concentrations and light δ13C values. These observations suggest that diffuse degassing of magmatic gas on the upper flanks of these volcanoes is negligible and that more magmatic degassing occurs on the lower flanks where major faults and greater fracturing in the older lavas can channel magmatic gases to the surface. These results are in contrast to findings for Mount Etna where a broad halo of magmatic CO2 has been postulated to exist over much of the edifice. Differences in radon levels among the three volcanoes studied here may result from differences in age, the degree of fracturing and faulting, regional structures or the level of hydrothermal activity. Volcanoes, such as those studied here, act as plugs in the continental crust, focusing magmatic degassing towards crater fumaroles, faults and the fractured lower flanks.
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Soil CO2 flux measurements were carried out along traverses across mapped faults and eruptive fissures on the summit and the lower East Rift Zone of Kilauea volcano. Anomalous levels of soil degassing were found for 44 of the tectonic structures and 47 of the eruptive fissures intercepted by the surveyed profiles. This result contrasts with what was recently observed on Mt. Etna, where most of the surveyed faults were associated with anomalous soil degassing. The difference is probably related to the differences in the state of activity at the time when soil gas measurements were made: Kilauea was erupting, whereas Mt. Etna was quiescent although in a pre-eruptive stage. Unlike Mt. Etna, flank degassing on Kilauea is restricted to the tectonic and volcanic structures directly connected to the magma reservoir feeding the ongoing East Rift eruption or in areas of the Lower East Rift where other shallow, likely independent reservoirs are postulated. Anomalous soil degassing was also found in areas without surface evidence of faults, thus suggesting the possibility of previously unknown structures.
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Diffusion measurements made for helium through porous cores including caprocks show that the effective diffusivity decreases with decreased permeability of the rock for gas-phase diffusion and for helium dissolved in water. A similar correlation is obtained of decreased diffusivity for increased threshold-displacement pressure for nitrogen displacing water from the rock. These correlations verify that parameters which indicate constrictions in the interstices of the rock also restrict diffusion.
Chapter
Magnetic data are presented from seven geophysical surveys which the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) has carried out off the Atlantic coast of Morocco since 1967. The observed elongated anomalies in the magnetic quiet zone parallel in part the Mesozoic M-anomalies which are observed further offshore, and in part the present shelf edge. The most westward anomaly is assumed to mark the initial rift zone, whereas the other anomalies lie on crust of continental origin. They may reveal a pattern of crustal weakness in this area.
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Noble gas isotopic ratios in mantle-derived samples require variability in the time-integrated ratio of volatile to lithophile elements in the Earth. Documentation of mantle 3He/4He variability is becoming increasingly complete, but for the heavier noble gases the picture is still partly clouded by the effects of atmospheric contamination of mantle samples. Nevertheless, clear variations in mantle Ne, Ar and Xe isotopic ratios exist, are apparently correlated with 3He/4He, and may be the product of varying degrees of mantle degassing. However, uncertainties in noble gas geochemical behavior and several conflicting observations leave open other possibilities. Recent Ne isotopic data are particularly important because they require that the atmosphere has not been closed to exchange with space. Derivation of much of the atmosphere from a source other than degassing of the mantle is a strong possibility that complicates efforts to model the geochemical evolution of the Earth.
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We have measured chemical compositions (CO2, CH4, N2, O2, H2S, H2, He, Ne and Ar), and isotopic compositions of light elements (δ13C, δ15N and δD) and of rare gases (He, Ne, Ar, Kr, and Xe) in seven gaseous and four fluid samples collected in Icelandic geothermal areas. Major chemical constituents of the gas samples are CO2, N2, H2 and H2S. Isotopic Compositions of Ne, Kr and Xe are indistinguishable from those of the atmosphere within the experimental accuracy of this study. There are slightly lower 40Ar/ 36Ar ratios than in the air for samples with extremely high 3He/4He ratios. No positive evidence for excess 129Xe is found. The measured 3He/4He ratios in the samples can be explained in terms of mixing among three end members: MORB-type He with a 3He/4He ratio of 1.1 × 10-5, plume-type He of 5.0 × 10-5, and atmospheric He of 1.4 × 10-6. Samples obtained from the northeastern part of the island contain typical MORB-type He, whereas, significant contributions (up to 33%) of plume-type He are apparent in samples from the southern region. Based on the 13C/12C ratios, CO2 in Icelandic gases is considered to be directly derived from magma, whereas CH4 may be formed by reaction of reduced carbon with water in the source magma. There is an inverse correlation between the 3He/4He ratios and δ15N values in Icelandic gases, suggesting co-genetic origin of He and N2.
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Diffuse degassing of CO2, CH4 and H2 was investigated at the surface environment of Cañadas caldera, Canary Islands, during the gas survey carried out in the summer of 1995. Soil CO2 concentration varied significantly from atmospheric levels to 30%, while soil CH4 and H2 contents ranged from 5 to 851 ppm and from 0.5 to 620 ppm, respectively. Soil CO2, CH4 and H2 distribution suggests that high diffuse degassing at Cañadas caldera is volcanic-structurally controlled. Anomalous soil H2 concentrations were identified at the summit of Teide and outside caldera boundaries, where the most recent eruption of Tenerife Island occurred. δ13C–CO2 data showed a magmatic, mixed magmatic–biogenic, and biogenic origin while a biogenic origin is suggested for soil CH4 at Cañadas caldera and its surroundings. By coupling the CO2/3He ratio with the 3He/4He ratio of fumarolic gas samples from the summit of Teide, we propose three possible sources for carbon: MORB-type, organic carbon and carbonate.
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Decreases in the helium concentration of soil-gas have been observed to precede six of eight recent central California earthquakes. Ten monitoring stations were established near Hollister, California and along the San Andreas Fault to permit gas collection. The data showed decreases occurring a few weeks before the earthquakes and concentrations returned to prequake levels either shortly before or after the earthquakes.
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Noble Gas Geochemistry gives a comprehensive description of the physical chemistry and cosmochemistry of noble gases, before leading on to applications for problem-solving in the earth and planetary sciences. There have been many developments in the use of the noble gases since publication of the first edition of this book in 1983. This second edition has been fully revised and updated. The book will be invaluable to graduate students and researchers in the earth and planetary sciences who use noble gas geochemistry techniques.
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Conditional sequential Gaussian simulations (sGs) have been applied to the study of soil diffuse degassing investigating the applicability of this technique to mapping diffuse CO2 degassing, quantifying the total CO2 output. The application regards five data set of soil CO2 fluxes measured with the accumulation chamber methodology at the volcanic areas of Solfatara of Pozzuoli (Italy), Vesuvio cone (Italy), Nisyros (Greece) and Horseshoe Lake (California) and at the non volcanic degassing areas of Poggio dell'Olivo (Italy). The sGs algorithm was used to generate 100 realizations of CO2 flux for each area. Probabilistic summaries of these simulations, together with the information given by probability plots, were used (i) to draw maps of the probability that CO2 fluxes exceed thresholds specific for a background flux, i.e., to define the probable extension of the degassing structures, (ii) to calculate the total CO2 output, and (iii) to quantify the uncertainty of the estimation. The results show that the sGs is a suitable tool to model soil diffuse degassing, producing realistic images of the distribution of the CO2 fluxes that honor the histogram and variogram of the original data. The maps of CO2 flux point out that in all the studied areas the gas is mainly released by delimited areas, i.e. diffuse degassing structures, generally associated to geological structures. The total CO2 output calculated for each study area and the uncertainty associated to these estimations were estimated in 1513 ± 136 t/d for Solfatara of Pozzuoli, 233 ± 28 t/d for Poggio dell'Olivo, 84 ± 3.82 t/d for Nisyros caldera, 104 ± 5 t/d for Horseshoe Lake and 0.95 ± 0.11 t/d for a small degassing area on the Vesuvio cone flank. The relation between the sample design and the uncertainty of estimation was also investigated leading to an empirical relation between uncertainty and the sampling density, that can be useful for the planning of future CO2 flux surveys.
Article
We used the closed chamber method to measure soil CO2 efflux over a three-year period at the Horseshoe Lake tree kill (HLTK)—the largest tree kill on Mammoth Mountain in central eastern California. Efflux contour maps show a significant decline in the areas and rates of CO2 emission from 1995 to 1997. The emission rate fell from 350 t d−1 (metric tons per day) in 1995 to 130 t d−1 in 1997. The trend suggests a return to background soil CO2 efflux levels by early to mid 1999 and may reflect exhaustion of CO2 in a deep reservoir of accumulated gas and/or mechanical closure or sealing of fault conduits transmitting gas to the surface. However, emissions rose to 220 t d−1 on 23 September 1997 at the onset of a degassing event that lasted until 5 December 1997. Recent reservoir recharge and/or extension-enhanced gas flow may have caused the degassing event.
Article
The Sangihe Arc is presently colliding with the Halmahera Arc in northeastern Indonesia, forming the world's only extant example of an arc-arc collision zone. We report the first helium and carbon isotopic and relative abundance data from the Sangihe Arc volcanoes as a means to trace magma origins in this complicated tectonic region. Results of this study define a north-south trend in 3He/4He, CO2/3He, and δ13C, suggesting that there are variations in primary magma source characteristics along the strike of the arc. The northernmost volcanoes (Awu and Karangetang) have higher CO2/3He and δ13C (up to 179 × 109 and −0.4‰, respectively) and lower 3He/4He (∼5.4 RA) than the southernmost volcanoes (Ruang, Lokon, and Mahawu). Resolving the arc CO2 into component structures (mantle-derived, plus slab-derived organic and carbonate CO2), the northern volcanoes contain an unusually high (>90%) contribution of CO2 derived from isotopically heavy carbonate associated with the subducting slab (sediment and altered oceanic basement). Furthermore, the overall slab contribution (CO2 of carbonate and organic origin) relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc. These observations may be caused by greater volumes of sediment subduction in the northern arc, along-strike variability in subducted sediment composition, or enhanced slab-derived fluid/melt production resulting from the superheating of the slab as collision progresses southward.
Article
The terrestrial degassing fluxes of He from continental crust, oceanic crust, mid-ocean ridges, subduction volcanism and hot-spot volcanism are estimated and/or reviewed. The only significant terrestrial sources of 3He are mid-ocean ridge (MOR) and subduction volcanism. The degassing of the continental crust (approximately equivalent to whole crustal production) represents ∼60% of the total terrestrial 4He degassing flux with MOR volcanism supplying an additional ∼22%. These terrestrial flux estimates are evaluated with regard to an atmospheric He budget, and the results strongly suggest a steady-state He atmosphere.To achieve a continental crustal degassing flux approximately equivalent to the whole crustal production, 4He must be released from the solid phase and transported from ∼40 km of crust to the atmosphere. A discussion of the mechanisms involved in this degassing process suggests the release of 4He by both small-scale and tectonic forcing functions. A much simplified model of pore-fluid transport in the crust suggests that this degassing flux can only be achieved by enhanced fluid/gas transport (advection/convection ). Fluid transport in the upper brittle crust is likely to be discontinuous, while in the lower ductile crust rapid transport by grain boundary diffusion is possible and consistent with theory. These results are in agreement with recent observations and theories on the magnitude of crustal fluid transport and strongly suggest that crustal fluid transport is ubiquitous, rapid and large-scale.
Article
Noble gases within the crust originate from three main sources: the atmosphere, introduced into the crust dissolved in groundwater; the mantle, in regions of magmatic activity; and those produced in the crust by the result of radioactive decay processes. The continental crust contains approximately 40% of the terrestrial radioelements (Rudnick and Fountain 1995) that produce noble gases and, after the mantle and the atmosphere, forms the third major terrestrial noble gas reservoir (neglecting the core). In addition to these sources, contributions from interplanetary dust particles (IDP), cosmic ray interaction with the crustal surface and anthropogenic noble gases can in some cases be a significant source of noble gases in crustal materials. The use of noble gases to understand the role of fluids in different geological settings relies on their low natural abundance and chemical inertness. The low abundance of noble gases in crustal systems and their distinct isotopic character means that contributions from these different sources can often be resolved and quantified. With this, information is gained about the source of associated fluids, the environment from which they originated the physical manner in which they have been transported to the sampling site and the different phases that may have interacted within the crustal fluid system. This is only possible, however, with a detailed understanding of the processes that control the concentration and isotopic composition of the noble gases in different crustal environments. The first part of this chapter deals with the three different mechanisms of noble gas production within the crust—radiogenic, nucleogenic, fissiogenic (Fig. 1⇓). We show how production ratios are affected not only by the source region radioelement concentration, but in the case of nucleogenic reactions, also by the spatial distribution and concentration of the target elements. For completeness we consider cosmogenic noble gas production rates and Interplanetary …
Article
We detail here the general concepts behind using noble gases as a tracer of crustal fluid processes and illustrate these concepts with examples applied to oil-gas-ground-water systems, mineralizing fluids, hydrothermal systems and ancient ground-waters. Many of the concepts and processes discussed here are also directly applicable to the study of young ground and surface-water systems (Kipfer et al. 2002, this volume). Noble gases in the Earth are broadly derived from two sources; noble gases trapped during the accretionary process (often called ‘primitive’, ‘juvenile’ or ‘primordial’ noble gases), and those generated by radioactive processes (e.g., Ballentine and Burnard 2002, this volume). Differentiation of the Earth into mantle and continental crust, degassing and early processes of atmosphere loss has resulted in the formation of reservoirs in which the abundance pattern and isotopic compositions of primitive noble gases have been variably altered. Combined with their different radioelement concentrations (U, Th, K) producing radiogenic noble gases, the mantle, crust and atmosphere are now distinct in both their noble gas isotopic composition and relative elemental abundance pattern. Fluids that originate from these different sources will contain noble gases that are therefore isotopically distinct and resolvable (Fig. 1⇓). Because the noble gases are chemically inert even if these fluids are lost through reaction or masked by addition of similar species from different sources, a conservative record of their presence and origin is preserved by the noble gases. Once resolved, the noble gas abundance patterns from the respective sources are particularly important, as these are sensitive to physical processes of fractionation. For example, from the distinct fractionation patterns it is possible to distinguish between for example, diffusive or advective transport processes. Similarly the abundance patterns enable the interaction of different phases to be identified and quantified. In a system that has fluids sourced from …
Article
In order to better predict soil sinks of methane, we need to examine soil methane ¯ux patterns and responses to altered soil moisture regimes. Estimates of the global atmospheric CH 4 budget must also account for ¯uxes in the vast boreal region. We measured methane ¯uxes into the soil surface, methane concentrations, water content, and temperature in the soil pro®le in two interior Alaskan forests, over two growing seasons. At each site, a 0.10 ha rain-shelter limited summer precipitation from entering the soil. Limiting summer precipitation at the upland site generally increased that site's soil uptake of methane. Average rates of soil methane uptake among upland plots ranged from 0.10 to 0.95 mg m À2 day À1 . At the ¯oodplain site, limiting precipitation decreased the soil methane uptake of that site, and the rates here ranged from À0.02 to 0.57 mg m À2 day À1 . Using soil pro®le methane concentrations, we calculated CH 4 ¯uxes using Fick's Law. Our inability to precisely measure the concentration gradient across the soil surface resulted in calculated ¯ux estimates that more likely represent ¯uxes within the soil pro®le. Methane sources and sinks in the soil pro®le also confounded the comparison of measured and calculated ¯uxes.
Article
The abundances and isotopic compositions of noble gases in two samples from ultramafic xenoliths in alkali basalt, a young kaersutitic amphibole separated from a peridotite xenolith from Dish Hill, California and an ancient whole-rock lherzolite xenolith from Baja California, are reported and compared with the results of analyses on other mantle samples. In addition to previously recognized excesses of 3He and 129Xe, our results indicate that ambient gases in the mantle show a general enrichment of the lighter-mass nonradiogenic isotopes of Ar, Kr and Xe, and Ar with 40Ar/36Ar = 3 · 102.
Article
Two Italian areas, characterized by different seismological histories, were investigated to enhance the basic knowledge of gas migration mechanisms during earthquakes. Sharp variations occur in the movement and concentration of some gaseous species due to the evolution of the local stress regime. The first area (Colpasquale) is located in the central Italian region of Marche and provided a good location to study gas migration in a seismically active region. The area was devastated by a sequence of shallow earthquakes over a 3 month-long period (September–December, 1997). The occurrence of this catastrophic event, as well as the long duration of the “seismic sequence”, presented a unique opportunity to study gas migration in a zone undergoing active displacement. Soil gas surveys were performed 1 day, 1 week, 1 year and 2 years after the main shock (Ms 5.6) in the Colpasquale area. In particular, results highlight a change in the Rn distribution during the three monitoring years indicating a variation of gas migration that may be linked to the evolution of the stress regime.The second study area is located in the Campidano Graben (southern part of Sardinia Island). This area is characterized by seismic quiescence, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. The consistently low values observed for all analyzed gases suggest that the studied area is likely characterized by sealed, non-active faults that prevent significant gas migration. The comparison of data from both studied areas indicate that soil gas geochemistry is useful to locate tectonic discontinuities even when they intersect non-cohesive clastic rocks near the surface and thus are not visible (i.e., “blind faults”).
Article
The magma activity of the North Luzon Arc is considered to have ceased, due to collision with the Asian continental margin, since late Miocene. New ÿssion track dates of zircons taken from dike swarms in the central Coastal Range of East Taiwan show very young ages of ∼0:5 Ma. Furthermore, the high 3 He= 4 He ratios (∼2:5 RA; RA is the air ratio) of hot spring gases near the same area indicate that more than 30% of a mantle-derived source component is necessary to account for the helium composition. These results suggest the existence of later remnant magmatic activity (more than 4 Ma) in the central Coastal Range. Therefore, arc magmatism may survive longer than the previously expected after collision.
Article
The observations of short-term decreases in helium soil-gas concentrations along the San Andreas Fault in central California have been correlated with subsequent earthquake activity. The area of study is elliptical in shape with radii approximately 16080 km, centered near San Benito, and with the major axis parallel to the Fault. For 83 percent of theM>4 earthquakes in this area a helium decrease preceded seismic activity by 1.5 to 6.5 weeks. There were several earthquakes without a decrease and several decreases without a corresponding earthquake. Owing to complex and unresolved interaction of many geophysical and geochemical parameters, no suitable model is yet developed to explain the observations.
Article
Wide variations were measured in the diffuse CO2 flux through the soils in three selected areas of Mt Etna between August 1989 and March 1993. Degassing of CO2 from the area of Zafferana Etnea-S. Venerina, on the eastern slope of the volcano, has been determined to be more strongly influenced by meteorological parameters than the other areas. The seasonal component found in the data from this area has been excluded using a filtering algorithm based on the best fitting equation calculated from the correlation between CO2 flux values and those of air temperature. The filtered data appear to have variations temporally coincident with those from the other areas, thus suggesting a common and probably deep source of gas. The highest fluxes measured in the two most peripheral areas may correlate well with other geophysical and volcanological anomalous signals that preceded the strong eruption of 1991–1993 and that were interpreted as deep pressure increases. Anomalous decreases in CO2 fluxes accompanied the onset and the evolution of that eruption and have been interpreted as a sign of upward migration of the gas source. The variations of CO2 flux at the 1989 SE fracture have also given interesting information on the timing of the magmatic intrusion that has then fed the 1991–1993 eruption.
Article
Two helium surveys were carried out at Phlegraean Fields in July and September 1983 aiming at locating loci of discontinuities (fractures) in the area on the basis of variations in helium concentrations following the bradyseismic crisis then in progress.
Article
Soil gas investigation is a useful tool to detect active faults. The sudden appearance of soil gas anomalies in zones of deep-reaching faults represents a promising potential precursor of earthquakes and volcanic eruptions. In volcanic areas the development of soil gas monitoring techniques is particularly important, as they can represent, together with remote sensing techniques, the only geochemical methods that can be safely applied during volcanic unrest, when it becomes impossible or too dangerous to sample crater fumaroles. A soil gas survey was carried out in June 1993 at the main island of Thera, in the Santorini volcanic complex. CO2 flux and CO2 and helium concentrations were measured at 50 cm depth for 76 points covering the entire island, with a spacing of 500 m or less. Several anomalous soil degassing sites have been detected. The main anomalies correspond to the Kolumbos line and to the Kameni line, two volcano-tectonic fault systems that controlled all the historic volcanic activity of Santorini. A third anomaly is related to a gas-leaking fault cutting the geothermal field of southern Thera. Soil gas data, together with geovolcanological and seismological evidence, indicate that the Kolumbos and Kameni lines are the most probable sites for future volcanic or seismic reactivation, and provide the basis for the establishment of a new geochemical monitoring technique at Thera.
Article
 The relationships between soil gas emissions and both tectonic and volcano-tectonic structures on Mt. Etna have been studied. The investigation consisted of soil CO2 flux measurements along traverses orthogonal to the main faults and eruptive fissures of the volcano. Anomalous levels of soil degassing were found mainly in coincidence with faults, whereas only 49% of the eruptive fissures were found to produce elevated CO2 soil fluxes. This result suggests that only zones of strain are able to channel deep gases to the surface. According to this hypothesis, several previously unknown structures are suggested. Based on our geochemical data, new structural maps of different areas of Etna are proposed. The soil CO2 fluxes observed in this study are higher than those measured in a 1987 study, and they are consistent with the higher level of volcanic unrest during the current study.
Chapter
The primary goal of this work is to present a geostatistical software library known as GSLIB. An important prerequisite to geostatical case studies and research is the availability of flexible and understandable software. Flexibility is achieved by providing the original FORTRAN source code. A detailed description of the theoretical background along with specific application notes allows the algorithms to be understood and used as the basis for more advanced customized programs. The three main chapters of this guidebook are based on the three major problem areas of geostatistics: quantifying spatial variability (variograms), generalized linear regression techniques (kriging), and stochastic simulation. Additional utility programs and problem sets with partial solutions are given to allow a full exploration of GSLIB and to check new software installations.
Article
An extensive compilation of recently acquired geophysical reconnaissance data has allowed the Mesozoic magnetic lineations (The Eastern Keathley sequence) to be identified and mapped in detail for the area off northwest Africa lying between Madeira and the Cape Verde Islands. These anomalies were generated as one limb of a symmetric spreading center (Paleo Mid-Atlantic Ridge) from about 107 to 153 m.y.B.P. Offsets in the lineation pattern serve to identify fracture zone traces whose trends are approximately east-west. The seaward boundary of the marginal quiet zone does not precisely define an isochron due to the presence of a variable width transition zone of intermediate amplitude magnetic anomalies. Crust underlying the marginal quiet zone was generated, at least in part, during the Jurassic, Graham normal polarity epoch. The quiet zone boundary is not offset significantly on opposite sides of the Canaries lineament as previously suggested. A possible counterpart of the U.S. east coast magnetic anomaly is observed in some areas near the shelf/slope break of Spanish Sahara and Mauritania. The presence of relatively high-amplitude (but not-correlatable) magnetic anomalies seaward of the Mesozoic sequence and presumably generated during the Cretaceous, Mercanton normal polarity epoch remains a paradox.
Article
La Palma (Canary Islands) represents an oceanic island volcano with an active rift zone, inferred to have formed during the last 800 ka following southward growth of the former radial–symmetrical stratovolcano Taburiente. We carried out clinopyroxene–melt thermobarometry and microthermometry of fluid inclusions to reconstruct the evolution of the magma plumbing systems over time and to understand the genetic relationship between Taburiente and the presently active Cumbre Vieja rift zone. Clinopyroxene–melt equilibria of phenocryst rims and glassy groundmass indicate pressures of 0.60–1.04 GPa (∼19–34 km depth) for Taburiente, 0.47–1.17 GPa (16–40 km) for the former Cumbre Nueva rift arm of Taburiente, and 0.50–0.78 GPa (16–26 km) for Bejenado volcano that formed after collapse of the Cumbre Nueva rift. These pressures are interpreted to reflect depths of magma storage and major crystal fractionation. CO2-dominated fluid inclusions hosted by clinopyroxenes and olivines indicate pressures of formation or re-equilibration within an overall range of 0.25–0.61 GPa (∼8–19 km depth). Respective frequency maxima are at 0.41–0.50 GPa for Taburiente dunite xenoliths, 0.26–0.43 GPa for Cumbre Nueva ankaramites, and 0.26–0.32 GPa for Bejenado cumulate xenoliths. These pressures are interpreted to reflect levels of temporary magma stagnation during ascent. Our data show that the magma pathways during all volcanic phases including the presently active Cumbre Vieja rift [Klügel, A., Hansteen, T.H., Galipp, K., 2005. Magma storage and underplating beneath Cumbre Vieja volcano, La Palma (Canary Islands). Earth and Planetary Science Letters 236, 211–226] are characterized by two distinct storage levels: a system of prolonged storage within the upper mantle, and a system of short-term stagnation within the lower crust or near to the Moho. Both the mantle and crustal storage systems show a migration to shallower levels from 1.0 Ma to present, probably as a result of changing thermomechanical properties of the mantle and crust and possibly stoping. Our combined barometric data and field observations suggest that the extinct Taburiente/Cumbre Nueva and the active Cumbre Vieja represent two distinct volcanoes with separate magma plumbing systems. In this case, the present rift configuration does not reflect continuous growth of the Taburiente shield volcano during the last 800 ka. None of the La Palma volcanoes shows any indicators of a long-lived shallow magma reservoir where magmas fractionate and from which rift zones emanate, which is an important difference to Hawaiian shield volcanoes characterized by shallow subcaldera magma chambers.
Article
Two strong flank eruptions occurred in July–August 2001 and from late October 2002 to late January 2003 at Mt. Etna volcano. The two eruptions mainly involved the upper southern flank of the volcano, a particularly active area during the last 30 years, damaging several tourist facilities and threatening some villages. The composite eruptive activity on the upper southern flank of Mt. Etna during 2001–2003 has confirmed “a posteriori” the results of a multidisciplinary study, started well before its occurrence by combining geological, seismic and geochemical data gathered in this part of the volcano. We were able, in fact, to highlight fractured zones likely to be re-activated in the near future in this area, where the largest majority of eruptive fissures in the recent past opened along N120° to N180° ranging directions.
Article
We report the first measurements of CO_2 solubility in molten basalt at pressures comparable to those at which submarine basalts erupt. A basalt from the Juan de Fuca ridge was equilibrated with CO_2-rich vapor at 1200°C, 100–1500 bar for up to four hours. After quenching, the glass was analyzed for dissolved carbonate ions by infrared spectroscopy. No forms of dissolved CO_2 other than carbonate were detected. CO_2 solubility is roughly a linear function of pressure at these low pressures. The experimentally determined solubility differs from previous estimates based on CO_2 concentrations of submarine glasses, on CO_2 solubilities in basaltic liquids at significantly higher pressures, and on CO_2 concentrations of glasses equilibrated with H_2O-CO_2 vapor. Our results are compatible with those obtained previously at higher pressures on a molten Kilauea tholeiite only if there is a significant positive dependence of carbonate solubility on temperature. CO_2 contents of mid-ocean ridge glasses measured by infrared spectroscopy are generally higher than would be expected based on solubilities at the hydrostatic pressures for the water depths from which the glasses were recovered, but the lowest dissolved CO_2 contents agree with the experimentally determined solubilities. We propose that submarine glasses with low CO_2 contents were quenched from magmas that were able to degas because they rose slowly from depth. The common occurrence of glasses with dissolved CO_2 contents in excess of the experimentally determined solubility suggests they were quenched from magmas that ascended too rapidly to degas fully. In conjunction with our solubility data, the highest CO_2 contents allow minimum estimates of depths to magma chambers. Depths of ⩾ 2.3 km beneath the ridge are indicated for the East Pacific Rise at 21°N, in agreement with geophysical constraints.
Article
Despite the abundance of SO2(g) in magmatic gases, precursory increases in magmatic SO2(g) are not always observed prior to volcanic eruption, probably because many terrestrial volcanoes contain abundant groundwater or surface water that scrubs magmatic gases until a dry pathway to the atmosphere is established. To better understand scrubbing and its implications for volcano monitoring, we model thermochemically the reaction of magmatic gases with water. First, we inject a 915°C magmatic gas from Merapi volcano into 25°C air-saturated water (ASW) over a wide range of gas/water mass ratios from 0.0002 to 100 and at a total pressure of 0.1 MPa. Then we model closed-system cooling of the magmatic gas, magmatic gas-ASW mixing at 5.0 MPa, runs with varied temperature and composition of the ASW, a case with a wide range of magmatic–gas compositions, and a reaction of a magmatic gas–ASW mixture with rock. The modeling predicts gas and water compositions, and, in one case, alteration assemblages for a wide range of scrubbing conditions; these results can be compared directly with samples from degassing volcanoes. The modeling suggests that CO2(g) is the main species to monitor when scrubbing exists; another candidate is H2S(g), but it can be affected by reactions with aqueous ferrous iron. In contrast, scrubbing by water will prevent significant SO2(g) and most HCl(g) emissions until dry pathways are established, except for moderate HCl(g) degassing from pH<0.5 hydrothermal waters. Furthermore, it appears that scrubbing will prevent much, if any, SO2(g) degassing from long-resident boiling hydrothermal systems. Several processes can also decrease or increase H2(g) emissions during scrubbing making H2(g) a poor choice to detect changes in magma degassing.
Article
Variations of He gas concentration are widely applied in studies devoted to the location of faults and to monitor seismic activities. Up to now, its migration mechanism in soil is not fully understood. A systematic soil gas survey across an active fault in NW Taiwan provides the opportunity to closely examine the mechanism of He migration in the fault zone. Significant spatial and temporal correlations observed between soil N2 and He gas support the hypothesis that N2 is the probable carrier gas for He emission in the studied area. Based on N2/Ar ratios and N2 isotopic results, the excess soil N2 in this study is considered to be largely derived from ancient atmospheric air which was dissolved in groundwater. Furthermore, observations rule out the possibility of CO2 being the dominant carrier gas for He in the studied area based on the C and He isotopic compositions and the relationship between concentrations of these gases. At least two soil gas sources, A and B, can be identified in the studied area. Source A is an abiogenic gas source characterized by excess N2 and He, and very low O2 and CO2 content. Source B, on the other hand, is a mixture of biogenic gas and atmospheric air. The development of the fault system is an important factor affecting the degree of mixture between sources A and B. Therefore, variations of soil gas composition, in particular those derived from source A, could be a useful proxy for tracing faults in the area.