J. M. Salazar

Diffuse emission of carbon dioxide, methane, and helium‐3 from Teide Volcano, Tenerife, Canary Islands

Diffuse emission of CO2, CH4 and ³He was investigated in the summit crater of Teide volcano, Tenerife, Canary Islands. The results indicate that Teide volcano releases abundant CO2 not only from its active crater, but also from its flanks as diffuse soil emanations. The spatial distribution of these emanations correlates quite closely with that of geothermal anomalies and manifestations. Our flux measurements indicate a total output of 380 ton·day−1 of CO2 and 1.6 on·day−1 of CH4 by diffuse degassing. From the ³He/CO2 ratios and the CO2 flux we estimate a diffuse ³He flux of 0.052 mol·year−1. Isotopic analysis of carbon in soil gas samples from sites of high soil CO2 flux indicates a magmatic origin for the CO2, also reported by high ³He/ 4He ratios of 5.50 and 5.87 Ra. The origin of methane is more uncertain with low δ13C values suggesting a biogenic contribution.

Diffuse emission of CO2 from Miyakejima volcano, Japan

Abstract Two soil gas surveys were carried out in May and September 1998 at Miyakejima volcano, in the Izu Mariana arc, Japan. CO 2 flux values for May and September surveys ranged from 0.1 to 18,150 g m −2 day −1 and from 0.1 to 9685 g m −2 day −1 , respectively. Statistical graphical analysis showed three overlapping populations. The spatial distribution of these emissions correlated quite closely with the geothermal and geological characteristics of the studied area. The structure releasing higher CO 2 is the summit cone Oyama and surrounding areas, where the most obvious geothermal features occur. A total output about 100–150 t day −1 is estimated from this area. A good correlation was observed between soil CO 2 flux and soil temperature at the summit caldera indicating extensive condensation of fumarolic steam within the upper part of Miyakejima. Carbon isotopic analysis of selected samples inside the summit caldera ( δ 13 C–CO 2 =−0.90‰ to −5.70‰) suggests a mixing of carbon derived from marine limestone and magmatic CO 2 while a clear biogenic origin ( δ 13 C–CO 2 =−14.76‰ to −25.52‰) is observed for the diffuse degassing of CO 2 outside summit caldera.

Diffuse emission of carbon dioxide from Cerro Negro Volcano, Nicaragua, Central America

We report the first detailed soil CO2 efflux survey carried out at Cerro Negro volcano, Nicaragua, Central America. Soil CO2 efflux values ranged from 0.5 to 35,000 g m−2 d−1. Spatial distribution of CO2 efflux shows that volcanic gas is mainly released from the summit crater and the fissures opened during the August 1999 eruption. The total CO2 output of Cerro Negro is conservatively estimated to be 2,800 t d−1, based on a study area of 0.58 Km². Contour maps of soil CO2 efflux, ground temperature, and δ13C(CO2) distributions support an important magmatic contribution for the total output of CO2 at Cerro Negro volcano.

Precursory diffuse carbon dioxide degassing signature related to a 5.1 magnitude earthquake in El Salvador, Central America

Abstract Anomalous changes in the diffuse emission of carbon dioxide have been observed before some of the aftershocks of the 13 February 2001 El Salvador earthquake (magnitude 6.6). A significant increase in soil CO 2 efflux was detected 8 days before a 5.1 magnitude earthquake on 8 May 2001 25 km away from the observation site. In addition, pre- and co-seismic CO 2 efflux variations have also been observed related to the onset of a seismic swarm beneath San Vicente volcano on May 2001. Strain changes and/or fluid pressure fluctuations prior to earthquakes in the crust are hypothesized to be responsible for the observed variations in gas efflux at the surface environment of San Vicente volcano.

Helium-3 emission in and around Teide Volcano, Tenerife, Canary Islands, Spain

Air corrected ³He/4He ratios of fluid samples collected in and around Teide volcano, Tenerife, Canary Islands, ranged from 5.97 to 8.06 Ra. Helium-3 emission spatial distribution has a quite different pattern from other stratovolcanoes which show commonly a decreased trend for the ³He/4He ratio with respect to distance from the summit crater. This finding might be related to the volcano-tectonic setting differences between subduction-type and oceanic island-type statrovolcanoes. Radiogenic helium production is a significant process for the dilution of uprising derived-mantle fluids in and around subduction-type stratovolcanoes while it can be almost negligible for polygenetic volcanoes at oceanic islands.

Radon and helium in soil gases at Cañadas caldera, Tenerife, Canary Islands, Spain

Abstract The spatial distribution of soil radon was investigated at Canadas caldera, Tenerife, in two surveys carried out in the summers of 1992 and 1995 by using α-particle-sensitive cellulose nitrate films (Track-Etch) and emanometry, respectively. Soil helium was studied at several transects crossing different structural features of the area. Radon concentration measured by Track-Etch ranged from 1.0 to 1990 pCi/l while that measured by emanometry ranged from 0.1 to 618 pCi/l. Soil helium concentration varied from 5250 to 15 560 ppb with an average value of 6197 ppb. The spatial distribution of soil radon correlates quite closely with structural features (fractures, emission centers, etc.), where the main geothermal manifestations (fumaroles, steam ground and high subsurface temperature and gas contents) also occur. Areas showing high soil Rn concentrations occur at the summit of Teide, Roques de Garcia, caldera rim and south and east off side of the caldera. High soil helium anomalies correlate well spatially with those of radon, especially at the summit of Teide where relatively high 3 He/ 4 He isotopic ratios occur, suggesting a deep contribution for these emanations. Data indicate that radon and helium are supplied mostly from a deep source, with a minor contribution from U- and Th-rich shallow rocks and soils.

An in situ method for measuring CO2 flux from volcanic-hydrothermal fumaroles

Abstract The theory of an in situ method to measure CO 2 flux from hydrothermal fumarolic vents using a tracer gas technique is established and applied to vents and wells at three sites: Izu-Oshima (Japan), Kirishima (Japan) and Teide (Canary Islands, Spain) volcanoes. The method is done by releasing the tracer gas into the vent at known flux and by measuring CO 2 to the tracer gas ratio in situ. In Izu-Oshima, we carried out flux measurements at a low-temperature steam well (90°C) with three different tracer gas species (H 2 , CH 4 and SF 6 ) and three methods (GC, portable analyzers, FTIR) to evaluate this method. The calculated CO 2 flux showed an average of 2.3 t/day. In the case of Kirishima volcanic area, the flux measurement was tested at two low-temperature fumaroles on Iwoyama cone ( 2 fluxes were 9×10 −2 and 2×10 0 kg/day, respectively. The total CO 2 flux from the cone including both fumarolic and ground CO 2 discharges was estimated to be less than several tons per day. In Teide volcano, CO 2 flux measurements were performed in the fumarolic system of the summit crater (85°C). CO 2 flux from one fumarole reached levels up to 30.2 kg/day. We estimated that the CO 2 discharge from all fumaroles is roughly 30 t/day or less at maximum. These results imply that the total output of CO 2 flux from Teides fumarolic activity is much lower than the observed levels of CO 2 diffuse degassing from the summit crater.