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1.
In 1977 and 1981 a hydrothermal plume was detected at the East Pacific Rise (EPR) near 8°45′N and at MANOP Site M, 25 km east of the EPR, by anomalous222Rn and Mn concentrations. In 1981, samples were also taken for210Pb,210Po and226Ra analyses to determine if enhanced scavenging of these elements occurred in the plume. At both the ridge crest and at Site M, the 210Pb/226Ra ratios range from 0.09 to 0.35, which are among the lowest values ever measured. It appears that removal of the210Pb is occurring by processes operating at or near the seafloor. There is also significant 210Po/210Pb disequilibrium at both locations, which appears to increase away from the seafloor towards the plume (as indicated by elevated Mn concentrations). An in-situ water column scavenging process is suggested by correlation of [210Pb-210Po] and Mn concentrations in the plume. The residence time of the210Po is only about 1 year, which is close to its residence time in surface waters but quite short compared to typical deep sea values. Thus rapid scavenging of the polonium in the plume and relatively rapid settling of particles from the plume is suggested.  相似文献   

2.
Continuous monitoring of soil CO2 dynamic concentration (which is proportional to the CO2 flux through the soil) was carried out at a peripheral site of Mt. Etna during the period November 1997–September 2000 using an automated station. The acquired data were compared with SO2 flux from the summit craters measured two to three times a week during the same period. The high frequency of data acquisition with both methods allowed us to analyze in detail the time variations of both parameters. Anomalous high values of soil CO2 dynamic concentration always preceded periods of increased flux of plume SO2, and these in turn were followed by periods of summit eruptions. The variations were modeled in terms of gas efflux increase due to magma ascent to shallow depth and its consequent depressurization and degassing. This model is supported by data from other geophysical and volcanological parameters. The rates of increase both of soil CO2 dynamic concentration and of plume SO2 flux are interpreted to be positively correlated both to the velocity of magma ascent within the volcano and to lava effusion rate once magma is erupted at the surface. Low rates of the increase were recorded before the nine-month-long 1999 subterminal eruption. Higher rates of increase were observed before the violent summit eruption of September-November 1999, and the highest rates were observed during shorter and very frequent spike-like anomalies that preceded the sequence of short-lived but very violent summit eruptions that started in late January 2000 and continued until late June of the same year. Furthermore, the time interval between the peaks of CO2 and SO2 in a single sequence of gas anomalies is likely to be controlled by magma ascent velocity.Editorial responsibility: H. Shinohara  相似文献   

3.
A tripod-mounted correlation spectrometer was used to measure SO2 emissions from Pu`u `O`o vent, mid-ERZ, Kilauea, Hawaii between Episodes 33 and 34 (June 13 to July 6, 1985). In 24 repose days, 906 measurements were collected, averaging 38 determinations/day. Measurements reflect 13% of the total 576 hours of the repose and 42% of the bright daylight hours. The average SO2 emission for the 24-day repose interval is 167±83 t/d, a total of 4000 tonnes emitted for the entire repose. The large standard deviation reflects the puffing character of the plume. The overall rate of SO2 degassing gently decreased with a zero-intercept of 44–58 days and was interrupted by two positive peaks. The data are consistent with the gas emanating from a cylindrical conduit of 50 meter diameter and a length of 1700 meters which degasses about 50% of its SO2 during 24 days. This is in support of the Pu'u `O`o model of Greenland et al. (1987). 36 hours before the onset of Episode 34 (July 5–6, 1985), elevated SO2 emissions were detected while the magma column was extremely active ultimately spilling over during dome fountaining. A mid-repose anomaly of SO2 emission (June 21–22, 1985) occurs two days before a sudden increase in the rate of summit inflation (on June 24, 1985), suggesting magma was simultaneously being injected in both the ERZ and summit reservoir until July 24 when it was channelled only to the summit reservoir. This implies degassing magma is sensitive to perturbations within the rift zone conduit system and may at times reflect these disturbances. Periods of 7–45 min are detected in the daily SO2 emissions, which possibly reflect timing of convective overturn in the cylindrical magma body. If the 33–34 repose interval is considered representative of other repose periods, the ERZ reposes of Jan 1983–Jan 1986 ERZ activity, contributed 1.6 × 105 tonnes of SO2 to the atmosphere. Including summit fuming from non-eruptive fumaroles (2.7 × 105 tonnes SO2); 28% of the total SO2 budget from Kilauea between Jan 1983 to Jan 1986 was contributed by quiescent degassing, and the remainder was released during explosive fountaining episodes.  相似文献   

4.
Excessive degassing of Izu-Oshima volcano: magma convection in a conduit   总被引：2，自引：0，他引：2  
Excess degassing of magmatic H2O and SO2 was observed at Izu-Oshima volcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1×104 kg/s using emission rates of magmatic H2O and SO2, and H2O and S contents of the magma. The minimum total volume of magma degassed during the 27-month period is estimated to be 2.6×108 m3. This volume is 20 times larger than that of the magma ejected during the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a magma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed magma spheres through a conduit filled with degassed magma. This process is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles.  相似文献   

5.
Activities of the naturally occurring radionuclides, 210Pb and 210Po, were measured in both dissolved (<0.45 μm) and particulate (>0.45 μm) phases from surface waters of the southern South China Sea. The average activity of particulate 210Pb, 0.23 Bq/m3 (n=23), accounted for about 12% of the total 210Pb, which corresponds with values of open oceans. Particulate 210Po, with an average activity of 0.43 Bq/m3, accounted for about 40% of the total 210Po, which was much higher than those of open and eutrophic oceans. The residence times of total 210Po and 210Pb in surface waters estimated from an irreversible steady-state model were 0.82 a and 1.16 a, respectively. The consistently high fractionation factor calculated either by scavenging rate constants (5.42) or Kd values (6.69) suggested that a significant fractionation occurred between 210Po and 210Pb during their removal from solution to particles and that the two radionuclides had different biogeochemical cycling pathways in the oligotrophic South China Sea. Furthermore, our results indicated that there exist different fractionation mechanisms between 210Po and 210Pb in different marine environments: in eutrophic ocean, plankton detritus and fecal pellets are the main carrier of 210Po and 210Pb, by which 210Po and 210Pb have been scavenged and removed; while in oligotrophic ocean, microbes could become the main carrier of 210Po and fractionate 210Po and 210Pb significantly as a result of scarce plankton detritus and fecal pellets. These results suggest the use of 210Po to trace marine biogeochemical processes relating to microbial activities and the cycling of sulfur group elements (S, Se, Te and Po).  相似文献   

6.
The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to 2000 ppm sulfur, but concentrations decrease rapidly with increasing SiO2 down to matrix glass values (50–100 ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (200 ppm S). Chlorine concentrations increase from 750 to 2200 ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2 Mt is not in accordance with a petrologic estimate of 0.09 Mt. Using a published value of 550 Mt of erupted material about 0.34 km3 fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26 km3), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3 Mt chlorine, which agrees well with an output of 0.47 Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.  相似文献   

7.
Between 1989 and 2001, five eruptions at Etna displayed a regular alternation between repose periods and episodes rich in gas, termed quasi-fire fountains and consisting of a series of Strombolian explosions sometimes leading to a fire fountain. This behaviour results from the coalescence of a foam layer trapped at the top of the reservoir which was periodically rebuilt prior to each episode (Vergniolle and Jaupart, J Geophys Res 95:2793–2809, 1990). Visual observations of fire fountains are combined with the foam dynamics to estimate the five degassing parameters characteristic of the degassing reservoir, i.e. the number of bubbles, gas volume fraction, bubble diameter, reservoir thickness and reservoir volume. The study of decadal cycles of eruptive patterns (Allard et al., Earth Sci Rev 78:85–114, 2006) suggests that the first eruption with fire fountains occurred in 1995 while the last one happened in 2001. The number of bubbles and the gas volume fraction increase smoothly from the beginning of the cycle (1995) to its end (2001). The increasing number of bubbles per cubic metre, from 0.61–20×105 to 0.1–3.4×109, results from cooling of the magma within the reservoir. The simultaneously decreasing bubble diameter, from 0.67–0.43 to 0.30–0.19 mm, is related to the decreasing amount of dissolved volatiles. Meanwhile, the thickness and the volume of the degassing reservoir diminish, from values typical of the magma reservoir to values characteristic of a very thin bubbly layer, marking the quasi-exhaustion of volatiles. The magma reservoir has a slender vertical shape, with a maximum thickness of 3,300–8,200 m and a radius of 240 m (Vergniolle 2008), making its detection from seismic studies difficult. Its volume, at most 0.58–1.4 km3, is in agreement with geochemical studies (0.5 km3) (Le Cloarec and Pennisi, J Volcanol Geotherm Res 108:141–155, 2001). The time evolution of both the total gas volume expelled per eruption, and the inter-eruptive gas flux results from the competition between the increasing number of bubbles and the decreasing bubble diameter. The smooth temporal evolution of the five degassing parameters also points towards bubbles being produced by a self-induced mechanism within the magma reservoir rather than by a magmatic reinjection prior to each eruption. The decadal cycles are therefore initiated by a magmatic reinjection, in agreement with a typical return time of 14–80 years (Albarède 1993). Hence, the 1995 eruption results from a fresh magma being newly emplaced while the magma from the following eruptions is progressively depleted in volatiles species until reaching a state of quasi-exhaustion in 2001. A magmatic reinjection of 0.13–0.6 km3 every few decades is sufficient to explain the expelled gas volume, including SO2. A scenario is also proposed for the alternation between gas-rich summit eruptions and gas-poor flank eruptions which are observed during decadal cycles. The scenario proposed for Etna could also be at work at Piton de la Fournaise and Erta ’Ale volcanoes.  相似文献   

8.
 An estimated average CO2 output from Etna's summit craters in the range of 13±3 Mt/a has recently been determined from the measured SO2 output and measured CO2/SO2 molar ratios. To this amount the CO2 output emitted diffusely from the soil (≈ 1 Mt/a) and the amount of CO2 dissolved in Etna's aquifers (≈ 0.25 Mt/a) must be added. Data on the solubility of CO2 in Etnean magmas at high temperature and pressure allow the volume of magma involved in the release of such an amount of this gas to be estimated. This volume of magma (≈ 0.7 km3/a) is approximately 20 times greater than the volume of magma erupted annually during the period 1971–1995. On the basis of C-isotopic data of CO2 collected in the Etna area and of new hypotheses on the source of Mediterranean magmas, significant contributions of CO2 from non-magmatic sources to the total output from Etna are unlikely. Such large outputs of CO2 and also of SO2 from Etna could be due to an anomalously shallow asthenosphere beneath the volcano that allows a continuous escape of gases toward the surface, even without migration of magma. Received: 7 August 1996 / Accepted: 9 November 1996  相似文献   

9.
The 1995–1996 eruption of Mt. Ruapehu has provided a number of insights into the geochemical processes operating within the magmatic-hydrothermal system of this volcano. Both pre-eruption degassing of the rising magma and its eventual intrusion into the convective zone of the hydrothermal system beneath the lake were clearly reflected in lake water compositions. The eruptions of September–October 1995 expelled the lake, and provided the first-ever opportunity to characterise gas discharges from this volcano. The fumarolic discharges revealed compositions typical of andesite volcanoes and strong interaction with the enclosing meteoric and hydrothermal system fluids. Some 1.1 MT of SO2 gas was released from the volcano between September 1995 and December 1996, whereas ca. twice this amount (2.2 MT equivalent SO2) was erupted as soluble (i.e. leachable) oxyanions of sulphur. Significantly more sulphur was released from the volcano over this period than can be accounted for from the magma volume actually erupted. The evidence suggests that a sizable component of the evolved sulphur was remobilised from the long-lived hydrothermal system within the volcano during the 1995–1996 activity.  相似文献   

10.
Measurements have been made of226Ra and both dissolved and particulate forms of210Pb and210Po in a vertical profile at 85°50′N, 108°50′W in the Arctic Ocean.In the upper water column226Ra shows a concentration maximum that is coincident with one in the nutrients, silicate, phosphate, and nitrate, while at the same depth, dissolved and particulate210Pb and210Po all show minimum concentrations. It is suggested that the concentration maxima are partly due to sources of the respective elements in the continental shelf sediments, the shelf waters being subsequently advected into the Arctic Ocean basins. The210Pb and210Po minima have similarly been explained by interaction between the shelf sediments and overlying waters. An estimate is made of the possible contributions of shelf sediments to the layer of silica-rich water which covers the Canada Basin at a depth of 100–150 m.Residence times have been calculated for dissolved210Pb and210Po at various depths in the water column. Surface water residence times of dissolved and particulate forms of these radionuclides are longer than in surface Atlantic waters, probably due to lower biological activity in the surface waters of the Canada Basin. An estimatee has been made of the average sinking velocity of particulate material.  相似文献   

11.
The distribution of210Po and210Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported210Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved210Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in210Po, with210Po/210Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for210Pb and a 0.6-year residence time for210Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,210Po maxima occur and unsupported210Po is frequently present. Calculations indicate that at least 50% of the210Po removed from the mixed layer is recycled within the thermocline. Similar calculations for210Pb suggest much lower recycling efficiencies.Comparison of the210Pb distribution with the reported distribution of226Ra at nearby GEOSECS stations has confirmed the widespread existence of a210Pb/226Ra disequilibrium in the deep sea. Vertical profiles of particulate210Pb were used to test the hypothesis that210Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate210Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred210Pb flux. It is proposed instead that an additional sink for210Pb in the deep sea must be sought. Estimates of the dissolved210Pb/226Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of210Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 107 cm2/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,210Po is deficient, on the average, by ca. 10% relative to210Pb; a corresponding enrichment is found in the particulate phase. Total inventories of210Pb and210Po over the entire water column, however, show no significant departure from secular equilibrium.  相似文献   

12.
Ambae (also known as Aoba), is a 38 × 16 km2 lozenge-shaped island volcano with a coastal population of around 10 000. At the summit of the volcano is lake Voui — one of the largest active crater lakes worldwide, with 40 × 106 m3 of acidic water perched 1400 m a.s.l. After more than 300 years of dormancy, Ambae volcano reawakened with phreatic eruptions through Voui in 1995, and culminating in a series of surtseyan eruptions in 2005, followed by a rapid and spectacular colour change of the lake from light blue to red in 2006. Integrating lake water chemistry with new measurements of SO2 emissions from the volcano during the 2005–2006 eruptive period helps to explain the unusual and spectacular volcanic activity of Ambae — initially, a degassed magma approached the lake bed and triggered the surtseyan eruption. Depressurization of the conduit facilitated ascent of volatile-rich magma from the deeper plumbing system. The construction of a cone during eruption and the high degassing destabilised the equilibrium of lake stratification leading to a limnic event and subsequently the spectacular colour change.  相似文献   

13.
Popocatépetl volcano in central Mexico has been erupting explosively and effusively for almost 4 years. SO2 emission rates from this volcano have been the largest ever measured using a COSPEC. Pre-eruptive average SO2 emission rates (2–3 kt/d) were similar to the emission rates measured during the first part of the eruption (up to August 1995) in contrast with the effusive–explosive periods (March 1996–January 1998) during which SO2 emission rates were higher by a factor of four (9–13 kt/d). Based on a chronology of the eruption and the average SO2 emission rates per period, the total SO2 emissions (up to 1 January 1998) are estimated to be about 9 Mt, roughly half as much as the SO2 emissions from Mount Pinatubo in a shorter period. Popocatépetl volcano is thus considered as a high-emission rate, passively degassing eruptive volcano. SO2 emission rates and SO2 emissions are used here to make a mass balance of the erupted magma and related gases. Identified excess SO2 is explained in terms of continuous degassing of unerupted magma and magma mixing. Fluctuations in SO2 emission rate may be a result of convection and crystallization in the chamber or the conduits, cleaning and sealing of the plumbing system, and/or SO2 scrubbing by the hydrothermal system.  相似文献   

14.
The emission rate of carbon dioxide escaping from the summit of Kīlauea Volcano, Hawai?i, proved highly variable, averaging 4900 ± 2000 metric tons per day (t/d) in June–July 2003 during a period of summit inflation. These results were obtained by combining over 90 measurements of COSPEC-derived SO2 emission rates with synchronous CO2/SO2 ratios of the volcanic gas plume along the summit COSPEC traverse. The results are lower than the CO2 emission rate of 8500 ± 300 t/d measured by the same method in 1995–1999 during a period of long-term summit deflation [Gerlach, T.M., McGee, K.A., Elias, T., Sutton, A.J. and Doukas, M.P., 2002. Carbon dioxide emission rate of Kīlauea Volcano: Implications for primary magma and the summit reservoir. Journal of Geophysical Research-Solid Earth, 107(B9): art. no.-2189.]. Analysis of the data indicates that the emission rates of the present study likely reflect changes in the magma supply rate and residence time in the summit reservoir. It is also likely that emission rates during the inflation period were heavily influenced by SO2 pulses emitted adjacent to the COSPEC traverse, which biased CO2/SO2 ratios towards low values that may be unrepresentative of the global summit gas plume. We conclude that the SO2 pulses are consequences of summit re-inflation under way since 2003 and that CO2 emission rates remain comparable to, but more variable than, those measured prior to re-inflation.  相似文献   

15.
Profiles of226Ra and dissolved210Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including226Ra and dissolved and particulate210Pb and210Po. These profiles show several distinct features: (1)226Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved210Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water210Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4)210Pb and210Po activities are essentially identical to each other in both particulate and dissolved phases although210Po activities appear somewhat lower; (5) about 20% of the210Pb and210Po in the water column residues on particulate matter.Assuming the atmospheric210Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm2 yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.  相似文献   

16.
The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl–SO4 and Cl–HCO3 hydrothermal waters, with peaks in SO4 and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO4 and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl–SO4 geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km2) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincón de la Vieja, Costa Rica).Editorial responsibility: H. Shinohara  相似文献   

17.
Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H2O, CO2, CO, SO2, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO2 and H2O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H2O/CO2 proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO2 flux determinations and measured gas ratios. In the case of CO2 and water, ∼ 1 and ∼ 0.4 m3 s− 1, respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.  相似文献   

18.
This paper presents the results of 7 years (Aug. 1999–Oct. 2006) of SO2 gas measurements during the ongoing eruption of Tungurahua volcano, Ecuador. From 2004 onwards, the operation of scanning spectrometers has furnished high temporal resolution measurements of SO2 flux, enabling this dataset to be correlated with other datasets, including seismicity. The emission rate of SO2 during this period ranges from less than 100 to 35,000 tonnes/day (t d− 1) with a mean daily emission rate of 1458 t d− 1 and a standard deviation of ± 2026 t d− 1. Average daily emissions during inferred explosive phases are about 1.75 times greater than during passive degassing intervals. The total amount of sulfur emitted since 1999 is estimated as at least 1.91 Mt, mostly injected into the troposphere and carried westwards from the volcano. Our observations suggest that the rate of passive degassing at Tungurahua requires SO2 exsolution of an andesitic magma volume that is two orders of magnitude larger than expected for the amount of erupted magma. Two possible, and not mutually exclusive, mechanisms are considered here to explain this excess degassing: gas flow through a permeable stagnant-magma-filled conduit and gas escape from convective magma overturning in the conduit. We have found that real-time gas monitoring contributes significantly to better eruption forecasting at Tungurahua, because it has provided improved understanding of underlying physical mechanisms of magma ascent and eruption.  相似文献   

19.
226Ra,210Pb and210Po were measured in oceanic profiles at two stations near the Bonin and Kurile trenches.210Po is depleted by 50% on average relative to210Pb in the surface water. In the deep water,210Pb is about 25% deficient relative to226Ra. Based on the deficiency,210Pb residence time with respect to removal by particulate matter was estimated to be less than 96 years in the deep water.210Pb deficiency in the bottom water was significantly greater than that of the adjacent deep water, indicating more effective removal near or at the bottom interface.210Pb,210Po and Th appear to have similar overall rate constants of particulate removal throughout the water column.  相似文献   

20.
Comparison of the chemical characteristics of spring and river water draining the flanks of Poa´s Volcano, Costa Rica indicates that acid chloride sulfate springs of the northwestern flank of the volcano are derived by leakage and mixing of acid brines formed in the summit hydrothermal system with dilute flank groundwater. Acid chloride sulfate waters of the Rio Agrio drainage basin on the northwestern flank are the only waters on Poa´s that are affected by leakage of acid brines from the summit hydrothermal system. Acid sulfate waters found on the northwestern flank are produced by the interaction of surface and shallow groundwater with dry and wet acid deposition of SO2 and H2SO4 aerosols, respectively. The acid deposition is caused by a plume of acid gases that is released by a shallow magma body located beneath the active crater of Poa´s.No evidence for a deep reservoir of neutral pH sodium chloride brine is found at Poa´s. The lack of discharge of sodium chloride waters at Poa´s is attributed to two factors: (1) the presence of a relatively volatile-rich magma body degassing at shallow depths (< 1 km) into a high level summit groundwater system; and (2) the hydrologic structure of the volcano in which high rates of recharge combine with rapid lateral flow of shallow groundwater to prevent deep-seated sodium chloride fluids from ascending to the surface. The shallow depth of the volatile-rich magma results in the degassing of large quantities of SO2 and HCl. These gases are readily hydrolyzed and quickly mix with meteoric water to form a reservoir of acid chloride-sulfate brine in the summit hydrothermal system. High recharge rates and steep hydraulic gradients associated with elevated topographic features of the summit region promote lateral flow of acid brines generated in the summit hydrothermal system. However, the same high recharge rates and steep hydraulic gradients prevent lateral flow of deep-seated fluids, thereby masking the presence of any sodium chloride brines that may exist in deeper parts of the volcanic edifice.Structural, stratigraphic, and topographic features of Poa´s Volcano are critical in restricting flow of acid brines to the northwestern flank of the volcano. A permeable lava-lahar sequence that outcrops in the Rio Agrio drainage basin forms a hydraulic conduit between the crater lake and acid chloride sulfate springs. Spring water residence times are estimated from tritium data and indicate that flow of acid brines from the active crater to the Rio Agrio source springs is relatively rapid (3 to 17 years). Hydraulic conductivity values of the lava-lahar sequence calculated from residence time estimates range from 10−5 to 10−7 m/s. These values are consistent with hydraulic conductivity values determined by aquifer tests of fractured and porous lava/pyroclastic sequences at the base of the northwestern flank of the volcano.Fluxes of dissolved rock-forming elements in Rio Agrio indicate that approximately 4300 and 1650 m3 of rock are removed annually from the northwest flank aquifer and the active crater hydrothermal system, respectively. Over the lifetime of the hydrothermal system (100's to 1000's of years), significant increases in aquifer porosity and permeability should occur, in marked contrast to the reduction in permeability that often accompanies hydrothermal alteration in less acidic systems. Average fluxes of fluoride, chloride and sulfur calculated from discharge and compositional data collected in the Rio Agrio drainage basin over the period 1988–1990 are approximately 2, 38 and 30 metric tons/day. These fluxes should be representative of minimum volatile release rates at Poa´s in the last 10 to 20 years.  相似文献   

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