Mapping of H2S fluxes from the ground using copper passive samplers: An application study at the Zolforata di Pomezia degassing area (Alban Hills,Central Italy)

A new method for measuring H₂S mass flux from the ground, based on the digital analysis of the interference colours produced by the sulphidation of copper passive samplers (CPS), is proposed and discussed in this article. CPS sulphidation has a wide range of linear responses to H₂S doses and can be used together the accumulation chamber method to estimate gas fluxes from natural degassing areas. These are often characterized by the presence of vent centred degassing areas (VCDAs), which are recognizable from the absence or rarefaction of vegetation due to high acid gas concentrations in the soil pores and in the air at ground level. A reference emission curve, accounting for the advective and diffusive components of the flux, can be modelled and used to estimate the total H₂S mass released from each VCDA. The application of this method can be supported by remote sensing analysis that helps identify VCDAs in the field in perivolcanic H₂S degassing areas.As an illustrative application, H₂S gas fluxes from the ground were measured in spring 2007 at the Zolforata di Pomezia degassing area (ZPDA, Alban Hills, Central Italy) using an accumulation chamber internally equipped with CPS. H₂S peak fluxes were measured over the vents after remote sensing assisted identification of the VCDAs. Further measurements were carried out in two ponds and one artificial channel bordering the study area. The total atmospheric flux released at the ZPDA, estimated to be about 1207.6 kg day^? 1, was calculated as the summation of the fluxes from all the H₂S sources, the background flux being negligible.     

Zones of activation and volcanism

The zones of tectono-magmatic activation (TMA) that bound and traverse the major volcanogenic belts in the Russian Northeast were previously thought to be satellites of these belts. We even suggested an appropriate term, “perivolcanic zones” [Sidorov et al., 1978]. However, these supra-subduction and their perivolcanic zones are generally radically different from the deep-seated TMA zones of the Chukchi or Omsukchan types in that they are less deep and are short-lived. In contrast to volcanogenic belts, which are characterized by numerous small volcanic features with numerous mineralization occurrences of base and noble metals among regional propilitization fields, it is exactly in the zones of activation of these types that long-lived, volcano-plutonic centers with major mineralization fields are formed.     

The Geochemistry of Volcanogenic Mineralization in the Northwestern Segment of the Pacific Ore Belt: Northeast Russia

This paper considers the geochemistry of volcanogenic mineralization in the northeastern segment of the Pacific Ore Belt (Northeast Russia). We give new evidence for the compositions and concentrations of trace and rare-earth elements (REE) in the ores of volcanogenic fields: Au-Ag epithermal (of various types and ages), Cu-Mo-Au porphyritic, Au-Bi related to granitoidal intrusions, Sn-Ag subvolcanic and kies polymetallic enriched in Au and Ag, as well as REEs in alkaline volcanic rocks. Geochemical signatures have been compiled for 17 formation types of volcanogenic fields. It was found that the ore-forming fluids in most fields belonged to an NaCl-H₂O hydrothermal system that was enriched in Cl relative to F; the values of Y/Ho in the ores of nearly all types correspond with the interval of ratios characteristic for present-day hydrothermal fluids in backarc basins; most of the ores that we studied had near-chondrite spectra with configurations similar to those of the REE spectra in volcanic rock sequences of the andesite-diorite series. Comparative analysis of REE spectra in the distribution of trace elements over classes of gold concentration shows synchronous enrichment of ores in similar sets of trace elements. The high Co/Ni ratio in volcanogenic ores probably reflects the superposition of a later magmatic fluid upon an earlier mineralization. Samples from ores of volcanogenic fields, except for Kuroko, show δCe and δEu varying from negative to mildly positive values, thus indicating low-oxidizing conditions during deposition. It was found for Au-Ag epithermal ores that they are enriched in a wide range of trace elements; they have low concentrations of REEs, the light REEs are more abundant than the heavy ones, and the Eu anomalies vary considerably from small negative to low and high positive values. The results provide evidence of an exhalation hydrothermal origin of the Khotoidokh field. It has been shown that the REEs in the ores of the Bol’shoe field are of the type that is most valuable to industry. The results can be used to deal with practical problems: determining the formation type and evaluating the industrial value of a field; detecting accessory components in ores; and discriminating between the types of geochemical anomalies (in rocks or in soil) and stray fluxes as to the potential of a field.     

相山矿田铀矿地质研究进展与趋势

相山铀矿是华南最大的产于火山一侵入杂岩体中的热液脉型铀矿田。文章对相山铀矿地质研究的历史进行了简略回顾,重点阐述了铀矿地质研究的新进展,主要有：火山杂岩时代为早白垩世、火山岩浆具有反方向演化特点,成矿作用是一个相对连续的演化过程,铀矿类型归属斑岩型,属于与燕山晚期火山一斑岩作用有关的铀或铀一多金属矿床成矿系列。最后,文章展望了未来的研究方向。     

Isotopic geochemistry of acid thermal waters and volcanic gases from Zaō volcano in Japan

The chemical composition and D/H, and ratios have been determined for the acid hot waters and volcanic gases discharging from Zaō volcano in Japan. The thermal springs in Zaō volcano issue acid sulfate-chloride type waters (Zaō) and acid sulfate type waters (Kamoshika). Gases emitted at Kamoshika fumaroles are rich in CO₂, SO₂ and N₂, exclusive of H₂O. Chloride concentrations and oxygen isotope data indicate that the Zaō thermal waters issue a fluid mixture from an acid thermal reservoir and meteoric waters from shallow aquifers. The waters in the Zaō volcanic system have slight isotopic shifts from the respective local meteoric values. The isotopic evidence indicates that most of the water in the system is meteoric in origin. Sulfates in Zaō acid sulfate-chloride waters with δ³⁴S values of around +15‰, are enriched in ³⁴S compared to Zaō H₂S, while the acid sulfate waters at Kamoshika contain supergene light sulfate (δ³⁴S = + 4‰) derived from volcanic sulfur dioxide from the volcanic exhalations. The sulfur species in Zaō acid waters are lighter in δ³⁴S than those of other volcanic areas, reflecting the difference in total pressure.     

Gas hazard assessment in a densely inhabited area of Colli Albani Volcano (Cava dei Selci, Roma)

The northwestern flank of the Colli Albani, a Quaternary volcanic complex near Rome, is characterised by high pCO₂ values and Rn activities in the groundwater and by the presence of zones with strong emission of gas from the soil. The most significant of these zones is Cava dei Selci where many houses are located very near to the gas emission site. The emitted gas consists mainly of CO₂ (up to 98 vol%) with an appreciable content of H₂S (0.8–2%). The He and C isotopic composition indicates, as for all fluids associated with the Quaternary Roman and Tuscany volcanic provinces, the presence of an upper mantle component contaminated by crustal fluids associated with subducted sediments and carbonates. An advective CO₂ flux of 37 tons/day has been estimated from the gas bubbles rising to the surface in a small drainage ditch and through a stagnant water pool, present in the rainy season in a topographically low central part of the area. A CO₂ soil flux survey with an accumulation chamber, carried out in February–March 2000 over a 12 000 m² surface with 242 measurement points, gave a total (mostly conductive) flux of 61 tons/day. CO₂ soil flux values vary by four orders of magnitude over a 160-m distance and by one order of magnitude over several metres. A fixed network of 114 points over 6350 m² has been installed in order to investigate temporal flux variations. Six surveys carried out from May 2000 to June 2001 have shown large variations of the total CO₂ soil flux (8–25 tons/day). The strong emission of CO₂ and H₂S, which are gases denser than air, produces dangerous accumulations in low areas which have caused a series of lethal accidents to animals and one to a man. The gas hazard near the houses has been assessed by continuously monitoring the CO₂ and H₂S concentration in the air at 75 cm from the ground by means of two automatic stations. Certain environmental parameters (wind direction and speed; atm P, T, humidity and rainfall) were also continuously recorded. At both stations, H₂S and CO₂ exceeded by several times the recommended concentration thresholds. The highest CO₂ and H₂S values were recorded always with wind speeds less than 1.5 m/s, mostly in the night hours. Our results indicate that there is a severe gas hazard for people living near the gas emission site of Cava dei Selci, and appropriate precautionary and prevention measures have been recommended both to residents and local authorities.     

Diffuse volcanic degassing and thermal energy release from Hengill volcanic system, Iceland

We report the first detailed study of spatial variations on the diffuse emission of carbon dioxide (CO₂) and hydrogen sulfide (H₂S) from Hengill volcanic system, Iceland. Soil CO₂ and H₂S efflux measurements were performed at 752 sampling sites and ranged from nondetectable to 17,666 and 722?g?m^?2?day^?1, respectively. The soil temperature was measured at each sampling site and used to evaluate the heat flow. The chemical composition of soil gases sampled at selected sampling sites during this study shows they result from a mixing process between deep volcanic/hydrothermal component and air. Most of the diffuse CO₂ degassing is observed close to areas where active thermal manifestations occur, northeast flank of the Hengill central volcano close to the Nesjavellir power plant, suggesting a diffuse degassing structure with a SSW?CNNE trend, overlapping main fissure zone and indicating a structural control of the degassing process. On the other hand, H₂S efflux values are in general very low or negligible along the study area, except those observed at the northeast flank of the Hengill central volcano, where anomalously high CO₂ efflux and soil temperatures were also measured. The total diffuse CO₂ emission estimated for this volcanic system was about 1,526?±?160?t?day^?1 of which 453?t?day^?1 (29.7?%) are of volcanic/hydrothermal origin. To calculate the steam discharge associated with the volcanic/hydrothermal CO₂ output, we used the average H₂O/CO₂ mass ratio from 12 fumarole samples equal to 88.6 (range, 9.4?C240.2) as a representative value of the H₂O/CO₂ mass ratios for Hengill fumarole steam. The resulting estimate of the steam flow associated with the gas flux is equal to 40,154?t?day^?1. The condensation of this steam results in thermal energy release for Helgill volcanic system of 1.07?×?10¹⁴?J?day^?1 or to a total heat flow of 1,237?MW_t.     

Relationships between mineralization and silicic volcanism in the Central Andes

Studies of late Tertiary silicic volcanic centres in the Western and Eastern Cordilleras of the Central Andes show that three volcanic environments are appropriate sites for mineralization: (1) ring-fracture extrusions post-dating large calderas; (2) similar extrusions within ignimbrite shields; and (3) isolated, small silicic volcanoes. Subvolcanic tin mineralization in the Eastern Cordillera is located in silicic stocks and associated breccias of Miocene age. The Cerro Rico stock, Potosi, Bolivia, contains tin and silver mineralization and has an intrusion age apparently millions of years younger than that of the associated Kari Kari caldera. Similar age relationships between mineralization and caldera formation have been described from the San Juan province, Colorado. The vein deposits of Chocaya, southern Bolivia, were emplaced in the lower part of an ignimbrite shield, a type of volcanic edifice as yet unrecognized in comparable areas of silicic volcanism. The El Salvador porphyry copper deposit, Chile, is related to silicic stocks which may have been intruded along a caldera ring fracture. Cerro Bonete, Chile, provides a modern example of the volcanic superstructure which may have overlain isolated mineralized stocks and breccia pipes such as that of Salvadora at Llallagua, Bolivia.Existing models for the genesis of porphyry copper deposits suggest that they formed in granodioritic stocks located in the infrastructure of andesitic stratovolcanoes. Sites of porphyry-type subvolcanic tin mineralization in the Eastern Cordillera of Bolivia are distinguished by the absence of such andesitic structures. The surface expression of a typical subvolcanic porphyry tin deposit was probably an extrusive dome of quartz latite porphyry, sometimes related to a larger caldera structure. Evidence from the El Salvador porphyry copper deposit in the Eocene magmatic belt in Chile suggests that it too may be more closely related to a silicic volcanic structure than to an andesitic stratovolcano.The dome of La Soufriere, Guadeloupe is proposed as a modern analog for the surface expression of subvolcanic mineralization processes, the phreatic eruptions there suggesting the formation of hydrothermal breccia bodies in depth. Occurrence of mineralized porphyries, millions of years after caldera formation, does not necessarily indicate that intrusions and mineralization are not genetically related to the sub-caldera pluton, but may be a consequence of the long thermal histories (1–10 million years) of the lowermost parts of large plutons. Caldera formation can only inhibit mineralization by dispersal of ore metals when these are of magmatic origin, and ignimbrites should not be taken as being unlikely to be associated with porphyry mineralization. Whether ore metals are of wall rock or magmatic origin, the key to understanding the relationships between silicic volcanism and mineralization lies in the fractionation of trace elements within large zoned magma chambers during their igneous history, and their subsequent hydrothermal migration. Small, highly mineralized intrusions formed late in a caldera cycle (such as the Cerro Rico) may be due to the introduction of fresh supplies of mafic magma into the lower parts of the main pluton.     

Porphyry copper and tin-bearing porphyries a discussion of genetic models

The paper briefly describes the characteristics of the Chilean porphyry copper deposits, emphasizing the volcanic characteristics observed in the youngest ore bodies. Mention is made of the polymetallic ore bodies that are associated with rhyolitic porphyries intruding Jurassic sedimentary formations in the north of Chile, and the similarities and differences between the Chilean porphyry coppers and the Bolivian tin-bearing porphyries are pointed out.The models proposed by Sillitoe (1972), Brousse and Oyarzún (1971), and Mitchell and Garson (1972), are discussed and a new model is proposed agreeing with that of Mitchell and Garson for the genesis of the tin-bearing porphyries (the transportation of Sn as volatile halogenide by F liberated in the Benioff zone), but suggesting different generating mechanism for the porphyry coppers and polymetallic deposits related with porphyries. This mechanism, as a part of the global tectonic model, is based on the separation at depth of hydrogen sulphide from pyrite and water introduced with the upper layers of the lithospheric plate. The ascending migration of H₂S to high levels in the crust would be responsible for the porphyry copper mineralization by segregating sulphides of Fe, Cu, Mo and other sulphophile metals contained in the calc-alkaline magmas, brines, or sedimentary-volcanic formations intruded by the porphyries. The excess of H₂S would give rise to the formation of sulphur deposits normally present in the volcanic belt situated along the same line but at a higher level than those of the emplacement of porphyry coppers.The generation of copper and tin-bearing porphyries would be a consequence of the upward migration of volatile substances from different depths in the subduction zone, which explain its different emplacement with regards to the continental margins.     

Lateral variations of H2O contents in quaternary magmas of Northeastern Japan

Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan.Recent experimental studies on calc-alkaline andesite-dacite under H₂O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H₂O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H₂O content and bulk chemical composition of the magma.It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H₂O contents decrease, towards the volcanic front in the East Japan volcanic arc.Such lateral variations in the H₂O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H₂O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H₂OMg₂SiO₄z.sbnd;CaMgSi₂O₆z.sbnd;SiO₂, the decrease of H₂O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model.     

The relationship of uranium isotopes to oxidation/reduction in the Edwards carbonate aquifer of Texas

The concentration of dissolved uranium and²³⁴U/²³⁸U alpha activity ratio (“A.R.”) were determined in water samples from 23 locations in the Edwards carbonate aquifer of south central Texas by isotope dilution methods and alpha spectrometry. This aquifer consists of two parts, an updip oxidized portion and a downdip reduced portion. At some places the boundary is associated with faulting and at others it is not. The boundary between the two portions of the aquifer can be located by an abrupt change in chemical properties of the water such as a large increase in concentration of Cl^?, SO₄^2? and total dissolved solids, the presence of H₂S and a decrease in Eh in moving from updip to downdip. Compared with the oxidized samples, the uranium concentration is much lower and the A.R. higher in the reduced samples so that the uranium from each portion falls in a distinct field. The oxidized aquifer samples show very little variation in the measured uranium parameters even though, in some cases, there is evidence that the water has flowed through some tens of kilometers of aquifer. Samples collected near the boundary at those places not associated with faulting yield dissolved uranium values which fall in neither field and which, for the most part, cannot result from mixing of the oxidized and reduced waters. These samples probably result from changes in location of the oxidation-reduction boundary.     

Pre-accretionary mineralization of Japan

Abstract The metallogeny of Japan can be grouped into four environments: (1) Paleozoic-Mesozoic stratiform Cu and Mn deposits within accretionary complexes, (2) Cretaceous-Paleogene post-accretionary deposits related to felsic magmatism in a continental-margin are environment, (3) Miocene epigenetic and syngenetic deposits related to felsic magmatism during back-arc opening, and (4) late Miocene-Quaternary volcanogenic deposits in an island-are environment. Group (1) deposits were a major source of Cu and Mn for the Japanese mining industry, and this style of mineralization is reviewed here. The stratiform Cu and Mn deposits were formed on the sea floor during the late Paleozoic to Mesozoic, and were subsequently accreted to active continental margins mainly in Jurassic to Cretaceous age. The Cu sulfide deposits, termed Besshi type, are classified into two subtypes: the Besshi-subtype deposit is related to basaltic volcanism, probably at a mid-oceanic ridge or rise; the Hitachi subtype is related to bimodal volcanism, probably in a back-arc or continental rift. Most of the Besshisubtype deposits occur in the Sanbagawa metamorphic belt, with some occurrences in weakly metamorphosed Jurassic and Cretaceous accretionary terrains. This subtype is divided into two groups: the sediment-barren group is hosted by basalt-chert sequences; whereas the sedimentcovered group is hosted by basalt-shale sequences. Both subtypes are characterized by S isotope trends similar to those of sea-floor sulfide deposits now forming at mid-oceanic ridges. The Hitachi-subtype deposits occur in late Paleozoic volcanic-sedimentary sequences and lack pelagic sediments. These deposits are characterized by association of sphalerite- and barite-rich ores. The Mn deposits occur mainly in Middle Jurassic to Early Cretaceous accretionary complexes containing abundant chert beds of Triassic to Jurassic age. Their locations are well separated from those of the Cu sulfide deposits. The Mn deposits are divided into two types: the Mn type, hosted by chert, and the Fe-Mn type, sandwiched between chert and basaltic volcanic rocks. The Mn-type ores appear to have deposited on the deep-sea floor further from the site of hydrothermal activity than the Fe-Mn type. Primary Mn precipitates may have been transformed to rhodochrosite and other Mn-minerals during diagenesis. Many of the Mn deposits were significantly metamorphosed during intrusion of Cretaceous granitoids, resulting in a very complex mineralogy.     

Evolution of fluid geochemistry at the Turrialba volcano (Costa Rica) from 1998 to 2008

Turrialba (10°02′N, 83°45′W) is a 3,349-m high stratovolcano belonging to the Holocene “Cordillera Central” volcanic belt of Costa Rica. The summit consists of three EW-oriented craters (East, Central, and West). Since its last eruptive phase (1864–1866), the Central and West craters have displayed modest fumarolic activity, with outlet temperatures clustering around 90°C. In 2001, seismic swarms, ground deformation, and increasing fumarolic activity occurred. From 2005 to 2008, new fumarolic vents opened between and within the Central and West craters, and along the western and southwestern outer flanks of the volcanic edifice. These physical changes were accompanied by a drastic modification in the gas chemistry that can be divided in three stages: (1) hydrothermal (from 1998 to autumn 2001), characterized by the presence of H₂O, CO₂, H₂S, and, to a very minor extent, HCl and HF; (2) hydrothermal/magmatic (autumn 2001–2007), with the appearance of SO₂ and a significant increase of HCl and HF; and (3) magmatic-dominated (2007–2008), characterized by increased SO₂ content, SO₂/H₂S > 100, and temperatures up to 282°C. Accordingly, gas equilibrium in the CO₂-CH₄-H₂ system suggests a progressive evolution of the deep fluid reservoir toward higher temperatures and more oxidizing conditions. The chemical–physical modifications of Turrialba in the last decade can be interpreted as part of a cyclic mechanism controlling the balance between the hydrothermal and the magmatic systems. Nevertheless, the risk of rejuvenation of the volcanic activity cannot be excluded, and an appropriate seismic, ground deformation, and geochemical monitoring program is highly recommended. Turrialba lies at a distance of 35 and 15 km from San José and Cartago, respectively, the two largest cities in Costa Rica.     

Evaluation of the Physical and Chemical Properties of Eyjafjallajökull Volcanic Plume Using a Cloud-Resolving Model

The Eyjafjallajökull volcanic eruption, which occurred on April 14, 2010, caused many environmental, air traffic and health problems. An attempt has been made to demonstrate for the first time that certain improvements could be made in the quantitative prediction of the volcanic ash parameters, and in the accounting of the processes in the immediate vicinity of the volcano, using a cloud-resolving model. This type of explicit modeling by treatment of volcanic ash and sulfate chemistry parameterization, with input of a number parameters describing the volcanic source, is the way forward for understanding the complex processes in plumes and in the future plume dispersion modeling. Results imply that the most significant microphysical processes are those related to accretion of cloud water, cloud ice and rainwater by snow, and accretion of rain and snow by hail. The dominant chemical conversion rates that give a great contribution to the sulfate budget are nucleation and dynamic scavenging and oxidation processes. A three-dimensional numerical experiment has shown a very realistic simulation of volcanic ash and other chemical compounds evolution, with a sloping structure strongly influenced by the meteorological conditions. In-cloud oxidation by H₂O₂ is the dominant pathway for SO₂ oxidation and allows sulfate to be produced within the SO₂ source region. The averaged cloud water pH of about 5.8 and rainwater pH of 4.5 over simulation time show quantitatively how the oxidation may strongly influence the sulfate budget and acidity of volcanic cloud. Compared to observations, model results are close in many aspects. Information on the near field volcanic plume behavior is essential for early preparedness and evacuation. This approach demonstrates a potential improvement in quantitative predictions regarding the volcanic plume distribution at different altitudes. It could be a useful tool for modeling volcanic plumes for better emergency measures planning.     

Basical characteristics of fluid geologic process of interlayer oxidation zone sandstone-type uranium deposit

This paper reveals the physicochemical properties such as component, formulation, genesis, tem-perature, pH, Eh, salinity and pressure of all main alteration fluid of interlayer oxidation zone sand-stone-type uranium deposits after studying the geologic process and geochemistry of internal typical sandstone-type uranium deposits such as Shihongtan deposit in the Turpan-Hami basin, 512 deposit in the Yili basin, Dongsheng deposit in the Ordos basin. The composition of fluid can be divided into two parts based on the analysis of inclusion: one can be affirmed as atmospheric water with ordinary temperature epigenesist according to the character of hydrogen and oxygen isotope of inclusion, the other is natural gas containing gaseous hydrocarbon like CH₄, and CO₂ as well as a little H₂S, CO, H₂, N₂ and so on, it always contains a small quantity of hydrocarbon liquid in petroliferous basins. The fluid property of oxidation alteration zone is always oxidation alkaline, and neutrality or weak acid-weak alkaline and reducibility during the metallizing process, but at secondary reduction or deoxidization zone it becomes strong reduction alkaline. Oxygenic groundwater in the fluid is the activate and mig-ratory medium of uranium element, but the gaseous hydrocarbon like CH₄ as well as H₂, H₂S, CO from natural gas is the important sedimentary reducer of uranium mineral; the transformation of pH,Eh in fluid environment is the main reason for the formation of uranium metallization.     

CO2 and H2S concentrations in the atmosphere at the Solfatara of Pozzuoli

The CO₂ and H₂S concentration in the Solfatara atmosphere has been measured. The concentrations of both gases are higher neraby the more active areas and decrease away from them. A sharp horizontal and vertical gradient of the CO₂ content has been recognized.Such gradient is assumed to result from a diffusion of gas from the ground to the atmosphere.The total output of CO₂ has been computed based on a turbulent diffusion model. The obtained value is in good agreement with previously abserved values (Italiano et al., 1984).The feasibility of monitoring the atmosphere of Solfatara for either gas hazard and surveillance of volcanic activity has also been evaluated.     

Distinguishing Iron-Reducing from Sulfate-Reducing Conditions

Ground water systems dominated by iron‐ or sulfate‐reducing conditions may be distinguished by observing concentrations of dissolved iron (Fe²⁺) and sulfide (sum of H₂S, HS^?, and S⁼ species and denoted here as “H₂S”). This approach is based on the observation that concentrations of Fe²⁺ and H₂S in ground water systems tend to be inversely related according to a hyperbolic function. That is, when Fe²⁺ concentrations are high, H₂S concentrations tend to be low and vice versa. This relation partly reflects the rapid reaction kinetics of Fe²⁺ with H₂S to produce relatively insoluble ferrous sulfides (FeS). This relation also reflects competition for organic substrates between the iron‐ and the sulfate‐reducing microorganisms that catalyze the production of Fe²⁺ and H₂S. These solubility and microbial constraints operate in tandem, resulting in the observed hyperbolic relation between Fe²⁺ and H₂S concentrations. Concentrations of redox indicators, including dissolved hydrogen (H₂) measured in a shallow aquifer in Hanahan, South Carolina, suggest that if the Fe²⁺/H₂S mass ratio (units of mg/L) exceeded 10, the screened interval being tapped was consistently iron reducing (H₂～0.2 to 0.8 nM). Conversely, if the Fe²⁺/H₂S ratio was less than 0.30, consistent sulfate‐reducing (H₂～1 to 5 nM) conditions were observed over time. Concomitantly high Fe²⁺ and H₂S concentrations were associated with H₂ concentrations that varied between 0.2 and 5.0 nM over time, suggesting mixing of water from adjacent iron‐ and sulfate‐reducing zones or concomitant iron and sulfate reduction under nonelectron donor–limited conditions. These observations suggest that Fe²⁺/H₂S mass ratios may provide useful information concerning the occurrence and distribution of iron and sulfate reduction in ground water systems.     

Gas and hydrogen isotopic analyses of volcanic eruption clouds in Guatemala sampled by aircraft

Gas samples were collected by aircraft entering volcanic eruption clouds of three Guatemalan volcanoes. Gas chromatographic analyses show higher H₂ and S gas contents in ash eruption clouds and lower H₂ and S gases in vaporous gas plumes. H isotopic data demonstrate lighter isotopic distribution of water vapor in ash eruption clouds than in vaporous gas plumes. Most of the H₂O in the vaporous plumes is probably meteoric. The data are the first direct gas analyses of explosive eruptive clouds, and demonstrate that, in spite of atmospheric admixture, useful compositional information on eruptive gases can be obtained using aircraft.     

Monitoring and mapping of hydrogen sulphide emissions across an active geothermal field: Rotorua,New Zealand

Hydrogen sulphide (H₂S) is one of a number of gaseous species associated with geothermal activity in the Taupo Volcanic Zone (TVZ), New Zealand. The city of Rotorua is located within Rotorua Caldera in the TVZ and is one of the few urban areas in the world where a large population (>60,000 people) is frequently exposed to geothermal emissions. In order to evaluate the health hazard from long-term exposure to H₂S being emitted from the Rotorua geothermal field, a passive sampler has been developed to measure concentrations of H₂S at many locations across the city simultaneously. In contrast to other passive or pump-based samplers, the sampler is inexpensive, easily mass-manufactured, and involves the reaction of H₂S with silver halide contained in treated photographic paper. H₂S-exposed paper shows a distinct colour change from white to dark brown as H₂S concentrations increase and is sensitive to concentrations between ≪30 and around 1000 ppb. Rotorua city can be divided into three regions—an area of low H₂S concentration in the west, a ‘corridor’ of high concentrations running north–south through the city centre where H₂S is being emitted, and an area of medium concentration to the east which is influenced by the prevailing wind direction, creating a plume from the central corridor. The data give new insight into the subsurface routes of degassing in the Rotorua geothermal field, by showing the surface expression of the main upflow zone and the direction of the conjectured faulting below.     

Forecast of volcanic eruptions by chemical methods

From the magmatic emanations differentiation point of view it is possible to calculate some ratios such as F/CO₂, Cl/CO₂, SO₂/CO₂, SO₂/H₂S, H₂S/CO₂ and CO₂/N₂ in the tumarolic gases for the forecasting of volcanic activity. In order to predict the cruptions of a volcano it is needed to select several fumaroles or hot springs having different regimes of variation of the above ratios. The study of some fumaroles composition at the Asama. Mihara, Kirishima and other volcanoes in Japan showed a close connection between volcanic gas compositions and state of the volcanoes.