degassing processes at stromboli volcano inferred from short lived disequilibria 210 pb 210 bi 210 po in volcanic gases abstract volcanic aerosols and gases released by three active craters at stromboli volcano have been regularly collected since 1985 in this paper we present new evidence of the high volatility of some nuclides among radon daughters 210pb 210bi and 210po which are strongly fractionated leading to significant radioactive disequilibria in volcanic exhalations the very low volcanic activity in october 1996 allowed a separate sampling of each crater plume for the first time remote sampling of the bulk plume were also performed these data show that the chemical composition of volcanic aerosols remains constant within the first few hundred meters from their source vents ensuring the validity of remote sampling when the activity does not allow one to approach the active craters moreover it appears that there is no differentiation of gases from one crater to another suggesting that the geometry of the upper plumbing system of the volcano is rather simple gases being directly emitted from a shallow magma chamber without significant cooling inside the edifice on the basis of the assumption of a continuously replenished shallow magma reservoir in steady state we propose a dynamic model of degassing accounting for the variations of radionuclide contents and ratios observed in the gas phase since 1985 this model allows us to relate these variations to changes in the magma chamber dynamics namely the magma residence time inside the chamber and the escape time of gases from it both parameters being closely linked to the volcanic activity while gases are always emitted within a few hours after bubble nucleation suggesting that the chamber is no deeper than a few hundred meters magma residence time varies from less than 20 days during eruptive periods highly explosive or effusive periods to more than 200 days before the 1985 eruption the latter figure is explained by the storage at shallow depth of a poorly renewed magma batch that fed the eruption the variations of the residence time observed over the studied period suggest that the steady state dynamics of the stromboli magma chamber is sustained for rather short periods of a few months at most on basis of the knowledge of the deep undegassed magma supply and the magma residence time we estimate the volume of the shallow magma chamber at about 7 2 10 5 m 3 the good agreement of our results with previous estimates based on both geochemical and geophysical studies suggests that the measurement of radioactive disequilibria in the gas phase provides a robust tool for a deeper understanding of the volcanic activity and the magma dynamics beneath active volcanoes

magma at depth a retrospective analysis of the 1975 unrest at mount baker washington usa mount baker volcano displayed a short interval of seismically quiescent thermal unrest in 1975 with high emissions of magmatic gas that slowly waned during the following three decades the area of snow free ground in the active crater has not returned to pre unrest levels and fumarole gas geochemistry shows a decreasing magmatic signature over that same interval a relative microgravity survey revealed a substantial gravity increase in the 30 years since the unrest while deformation measurements suggest slight deflation of the edifice between 1981 83 and 2006 07 the volcano remains seismically quiet with regard to impulsive volcano tectonic events but experiences shallow 10 km long period earthquakes reviewing the observations from the 1975 unrest in combination with geophysical and geochemical data collected in the decades that followed we infer that elevated gas and thermal emissions at mount baker in 1975 resulted from magmatic activity beneath the volcano either the emplacement of magma at mid crustal levels or opening of a conduit to a deep existing source of magmatic volatiles decadal timescale multi parameter observations were essential to this assessment of magmatic activity

correlations between so2 flux seismicity and outgassing activity at the open vent of villarrica volcano chile 1 the characteristics of the open vent activity of villarrica volcano chile were studied in detail by integrating visual observations of the lava lake analysis of the seismic tremor and measurements of so2 flux the outgassing activity comprises a persistent gas plume emission from the bottom of the crater as well as frequent explosive events three main styles of bubble bursting were identified at the surface of the active lava lake seething magma small short lived lava fountains and strombolian explosions seething magma consists of continual burst of relatively small bubbles a few meters in diameter with varying strength over the entire surface of the lava lake small lava fountains seen as a vigorous extension of seething magma commonly have durations of 20 120 s and reach 10 40 m high above the lava lake correlations between seismicity and visual observations indicate that the seismic tremor is mostly caused by the explosive outgassing activity furthermore for different periods between 2000 and 2006 during which the activity remained comparable the real time seismic amplitude measurement system rsam and so2 emission rates show a very good correlation higher so2 emissions appeared to be related to higher levels of the lava lake stronger bubble bursting activity and changes in the morphology and texture of the crater floor background low levels of activity correspond to a lava lake located 80 m below the crater rim small and or blocky morphology of the roof seismic amplitude rsam lower than 25 units few volcano tectonic earthquakes and daily averages of so2 emissions lower than 600 mg d

stability and instability of quiescently active volcanoes the case of masaya nicaragua quiescently active volcanoes are enigmatic due to their restlessness but lack of eruptive activity i present a model of coupled conduit convection and foam accumulation to explain degassing behavior of masaya an active volcano in nicaragua that is currently emitting large amounts of gas but not erupting gas rich magma is transported through a conduit 2 6 m in radius and then released into a shallow reservoir the magma is degassed in the reservoir and forms a foam 1 3 m thick at the top with bubbles 20 60 m in diameter the foam layer is stable because the input of gas into the foam is balanced by gas release through a vent to the surface if the foam layer is destabilized the volcano can erupt explosively the most likely cause of foam destabilization is a large injection of volatile rich magma from deeper levels into the shallow conduit reservoir system thereby increasing magma fluxes and gas fluxes exsolving large gas bubbles and reducing surface tension of the magma

calibration shifts in a lacoste and romberg gravimeter comparison with a scintrex cg 3m the reliability and ruggedness of lacoste and romberg gravity meters has meant that their use has extended over recent years from gravity mapping to the investigation of small gravity changes 10 gal with time when using the instrument at the limit of its capability limitations become apparent some instruments drift more than others and some are prone to excessive tares however with regular servicing these problems are minimized here we demonstrate clear evidence for calibration changes of the order of 1000 ppm for lacoste and romberg meter g513 which for gravity differences of over 50 mgal represents a highly significant induced error of over 50 gal data collected over 15 years using nearly 4000 mgal of the 7000 mgal range of the instrument illustrates that long periods of stability may be interspersed with periods lasting several months while the calibration changes by up to 0 1 this is comparable with the calibration drift of a new scintrex cg 3m instrument although the process causing the change may not be the same

gravity changes as a precursor to volcanic eruption at poas volcano costa rica monitoring of gravity changes at active volcanoes has considerable potential for understanding magma chamber dynamics and for detecting eruptive precursors the technique essentially aims to identify changes in sub surface mass or density corresponding to departures from the free air gradient fag or bouguer corrected terrain corrected fag bcfag respectively1 2 such gravity changes of the order of 10 100 gal have been reported previously through eruptive periods3 6 here we present a ten year data set measured at poas volcano costa rica in which gravity increases in 1987 1989 relative to both the fag and the bcfag preceded the ash cloud eruptions of april may 1989 these results constitute the first unambiguous detection of gravity changes as a precursor to eruption and we interpret them in terms of the intrusion of an ascending relatively dense magma body

constraints on density changes in the funnel shaped caldera inferred from gravity monitoring of the lusi mud eruption abstract from its inception may 29 2006 the lusi mud eruption has continuously erupted fluids boiling mud and clasts through large active vents approximately 100 m in diameter in 2016 we conducted a dynamic gravity survey dg using a network built over four locations and two continuous gravity monitoring cg experiments to monitor the eruption activity the cg was done for 8 days from june 2nd to june 10th and for 9 days from august 20th to august 29th 2016 atmospheric pressure and atmospheric temperature variations were recorded during each experiment to constrain potential environmental effects and earth and oceanic tide effects were removed from gravity signals cg and dg atmospheric pressure effects were removed from cg gravity signals at the station closest to the hydrothermal pond the dg survey results show a gravity increase of 0 009 mgal month 1 which we interpret as the growth of the mud edifice in the central area cg monitoring shows that gravity variations occur at a period of 12 13 h with amplitudes reaching up to 0 020 mgal we interpret this as relating to density variation of the rising mud mixture fluids coarse mud clasts the observed 12 13 h period variations appear to indicate that tides may have some control on the density change of rising mud mixture by triggering the release of gas trapped at depth our 3d gravity results around the lusi vents show that density variations range from 100 kg m 3 to 775 kg m 3 similarly vent diameters better constrain density contrasts occurring within the caldera zone which is more likely to range between 400 and 450 kg m 3 and is equivalent respectively to 27 and 31 of gas ratio change over time

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

characteristics and origin of the gas and condensate in the yinggehai basin offshore south china sea evidence for effects of overpressure on petroleum generation and migration three superimposed pressure systems have developed in the yinggehai basin the strongly overpressured rocks have abnormally low vitrinite reflectance ro and rock eval tmax values and display abnormal c15 hydrocarbon distributions such organic maturity anomalies were confirmed to be the result of overpressure retardation commercial gas fields with carbon dioxide content up to 93 and gas shows with nitrogen content more than 50 have been found in the basin methane and carbon dioxide dominated gases have low c2 hydrocarbon contents less than 4 and heavy methane 13c and d values and the associated condensates have high diamondoid indices and very low c25 saturated hydrocarbon concentration at the same time the methane and carbon dioxide dominated gases have high branched and cyclic light hydrocarbon contents and associated condensates have abnormally low heptane and isoheptane values low sterane isomerization values and abnormally high concentrations of isoprenoid hydrocarbons relative to adjacent n paraffins these distinct characteristics seem to result from the effects of overpressure on both hydrocarbon generation and migration

coupled use of cospec and satellite measurements to define the volumetric balance during effusive eruptions at mt etna italy using etna as a case study location we examine the balance between the volume of magma supplied to the shallow volcanic system using ground based so2 data and the volume erupted using satellite thermal data we do this for three eruptions of mt etna italy during 2002 to 2006 we find that during the three eruptions 2 3 107 m3 or 24 of the degassed volume remained unerupted however variations in the degree of partitioning between supplied vsupply and erupted verupt magma occur within individual eruptions over the time scales of days consequently we define and quantify three types of partitioning in the first case vsupply verupt i e less lava is erupted than is supplied in such a case magma can erupt in an explosive manner as occurred during phase ii of the 2002 2003 eruption or remain within or below the edifice in the third case vsupply verupt i e all supplied magma is erupted during 2002 2006 over a total of 280 days of eruptive activity this balancing case applied to 50 of the time

numerical evidence enabling reconciliation gravity and height changes in volcanic areas s u m m a r y gravity and height changes reflecting magma accumulation in subsurface chambers are evaluated using finite element models in order to resolve controversial relationships observed in some volcanic areas when significant gravity changes occur without any significant deformation or vice versa it is often difficult if not impossible to jointly explain the observations using the popular mogi model here we explore whether these discrepancies can be explained by magma compressibility and source geometry effects compression of resident magma and expansion of the chamber wall act concurrently to accommodate newly added magma gravityheight ratios are found to mainly depend on i geometry of the sources which control the volume expansion of the chamber ii magma compressibility which affects the contraction of the magma resident in the chamber and iii depth of the sources our numerical results show that when magma compressibility and non spherical sources are taken into account significant gravity variations can indeed be successfully reconciled with negligible height changes this may be the case at etna volcano where gravity changes about 40 gal without any significant deformation below 5 cm were observed during the 1994 1995 inflation period the numerical results point to the accumulation of a 1 4 1010 kg mass into an elongated source simulating a shallow storage region supplying the summit craters

investigation of at vent dynamics and dilution using thermal infrared radiometers at masaya volcano nicaragua abstract in order to develop a detailed understanding of the dynamics of gas puffing gas release as a series of distinct pulses and more sustained degassing steady plumes of gas during persistent volcanic degassing measurements of gas mass flux are required in the vicinity of the volcanic vent masaya volcano nicaragua a persistently degassing system is an ideal location for measuring the dynamics of releases of volcanic gas in the first few seconds of their propagation we carried out two field experiments during february 2002 and march 2003 during which thermal infrared thermometers were targeted into the degassing vent at masaya to record thermal variations related to variations in the at vent gas emission over short on the order of seconds time scales the thermometers recorded an oscillating signal as gas puffs passed through the field of view detailing variations in the degassing process developing over time scales of seconds these data were processed to extract puff frequencies amplitudes durations emission velocities and volumes these data showed that over time periods of hours the total gas flux was stable with little variation in the puffing frequency however between the 2002 and 2003 data set we noted an increase in mean plume temperature puffing frequency puff emission velocity and puff volume as well as a decrease in mean puff duration these changes were consistent with a thermal data derived increase in emitted gas flux from 4 2 10 7 m 3 d 1 to 6 4 10 7 m 3 d 1 between the two campaigns turbulent gas puffs entrain surrounding air and quantifying the magnitude of air entrainment or dilution represents a major challenge for the measurement of total volcanic gas emissions our observations of small gas puffs suggest that they behave as turbulent buoyant thermals and we use equations for mass momentum and buoyancy coupled with the standard entrainment assumption for turbulent buoyant flows to estimate the gas puff dilution the theoretically calculated dilution of 0 09 and 0 24 between emission and detection yields total so 2 mass fluxes of 209 t d 1 and 864 t d 1 for 2002 and 2003 respectively this compares well with uv spectrometer so 2 fluxes of 470 and 680 t d 1 for february 2002 and march 2003 respectively

the generation and expulsion of gases in ya131 gas field south china sea implication of laboratory pyrolysis results abstract laboratory pyrolysis methods were used to determine the source and thermal maturity of the gas accumulation in the ya131 gas field south china sea the mudstone source rock samples used in the pyrolysis experiments were selected from the yacheng and meishan formations in the ying qiong basin the total amount of gases generated and expelled from the yacheng formation mudstone is larger than that from the meishan formation mudstone suggesting that the gas generation potential of the yacheng formation is higher than that of the meishan formation the similarity between the gases from the ya131 field and the gases produced at the higher pyrolysis temperatures used in the maturation study in terms of wetness the amounts of c 1 c 2 c 3 and the percentages of c 1 c 2 c 4 and c 5 c 7 implies that the gases of the ya131 field are high maturity gases however there are distinct differences between the normalised molecular compositions of the c 6 and c 7 species of the gases from the ya131 field and those of the pyrolysis gases produced in the laboratory comparison of the stable carbon isotope composition of the n alkanes pristane pr and phytane ph in the saturated hydrocarbon fraction of the condensate from the ya131 field with that of rock extracts from the yacheng meishan ying huang formations indicates no definite correlation between the condensate and the extracts these observations suggest that there may be additional source rocks for the ya131 field existing within the deeper part of the basin

rapid response of a hydrologic system to volcanic activity masaya volcano nicaragua hydrologic systems change in response to volcanic activity and in turn may be sensitive indicators of volcanic activity here we investigate the coupled nature of magmatic and hydrologic systems using continuous multichannel time series of soil temperature collected on the flanks of masaya volcano nicaragua one of the most active volcanoes in central america the soil temperatures were measured in a low temperature fumarole field located 3 5 km down the flanks of the volcano analysis of these time series reveals that they respond extremely rapidly on a time scale of minutes to changes in volcanic activity also manifested at the summit vent these rapid temperature changes are caused by increased flow of water vapor through flank fumaroles during volcanism the soil temperature response 5 c is repetitive and complex with as many as 13 pulses during a single volcanic episode analysis of the frequency spectrum of these temperature time series shows that these anomalies are characterized by broad frequency content during volcanic activity they are thus easily distinguished from seasonal trends diurnal variations or individual rainfall events which triggered rapid transient increases in temperature during 5 of events we suggest that the mechanism responsible for the distinctive temperature signals is rapid change in pore pressure in response to magmatism a response that can be enhanced by meteoric water infiltration monitoring of distal fumaroles can therefore provide insight into coupled volcanic hydrologic meteorologic systems and has potential as an inexpensive monitoring tool

origin source mixing and thermal maturity of natural gases in the panyu lower uplift and the baiyun depression pearl river mouth basin northern south china sea multiple natural gas fields have been discovered in the baiyun depression and the adjoining panyu lower uplift in the pearl river mouth basin northern south china sea the natural gases are associated with condensate and are characterized by relatively heavy carbon isotopes with methane and ethane 13c values ranging from 44 2 to 33 6 and 30 0 to 25 4 respectively nearly all methane and ethane are derived from oil prone type ii kerogen in the wenchang formation source rock whereas the heavy hydrocarbon gases propane butanes and pentanes are derived from both the wenchang and enping type iii kerogen formations based on an integrated comparison of carbon isotopic compositions of the natural gases typical type i ii and type iii kerogen derived gases and the enping and wenchang kerogens the gases from the eastern parts of the baiyun depression and the panyu lower uplift mainly originate from secondary oil cracking and primary kerogen cracking respectively the gases from the northern slope of the baiyun depression are a mixture of oil cracking and kerogen cracking gases both oil cracking and kerogen cracking gases were mainly generated from the wenchang formation source rock in the maturity range of 1 5 2 5 vitrinite reflectance with a corresponding present day depth range of 5400 6500 m 17 700 21 300 ft the apparent contribution of the wenchang formation to the discovered gas accumulations demonstrates that it is the most important source rock in the area instead of the enping formation the search for more gas derived from oil cracking will be the next natural gas exploration direction in the baiyun depression

carbon isotopic composition of individual light hydrocarbons evolved from pyrolysis of source rocks from ying qiong basins china abstract light hydrocarbons are an important part of natural gas and are commonly used in gas source rock correlation this study determines carbon isotopic compositions of individual light hydrocarbons generated from three potential source rocks pyrolyzed at different temperatures by using gc irms technique with combined glass tube pyrolysis the results indicate that for the natural gases from the ying qiong basins it is difficult to further identify the origin of gas in terms of carbon isotope data alone light hydrocarbons from pyrolysates can be divided into two main generation stages at the lower temperature stage light hydrocarbons are formed directly from primary cracking of kerogen and their carbon isotopic distributions accord with isotope kinetic fractionation at the higher temperature stage isotopic distributions of light hydrocarbons become more complex due to the mixing of light hydrocarbons with different generation mechanisms e g secondary cracking of heavy hydrocarbons in addition experimental results also prove that an accumulative effect may be one of major factors that cause the part reversal in carbon isotope ratios among light hydrocarbon components from the ying qiong basins

magma movements in etna volcano associated with the major 1991 1993 lava eruption evidence from gravity and deformation the 1991 1993 eruption was probably the largest on mt etna for 300 years since then the volcano has entered an unusually quiescent period a comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before during and after the eruption the gravity and deformation changes observed before the eruption 1990 1991 record the intrusion of magma into the summit feeder and the sse trending fracture system which had recently been active in 1978 1979 1983 and 1989 creating the feeder dyke for the 1991 1993 eruption in the summit region gravity changes between 1992 and 1993 spanning the end of the eruption reflect the withdrawal of magma from the conduit followed more recently 1993 1994 by the re filling of magma in the conduit up to pre eruption levels in contrast in the vicinity of the fracture zone gravity has remained at the 1991 1992 level indicating that no withdrawal has occurred here rather magma has solidified in the fracture system and sealed it such that the 1993 1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano

a dynamic balance between magma supply and eruption rate at kilauea volcano hawaii the dynamic balance between magma supply and vent output at kilauea volcano is used to estimate both the volume of magma stored within kilauea volcano and its magma supply rate throughout most of 1991 a linear decline in volume flux from the kupaianaha vent on kilauea s east rift zone was associated with a parabolic variation in the elevation of kilauea s summit as vent output initially exceeded then lagged behind the magma supply to the volcano the correspondence between summit elevation and tilt established with over 30 years of data provided daily estimates of summit elevation in terms of summit tilt the minimum in the parabolic variation in summit tilt and elevation or zero elevation change occurs when the magma supply to the reservoir from below the volcano equals the magma output from the reservoir to the surface so that the magma supply rate is given by vent flux on that day the measurements of vent flux and tilt establish that the magma supply rate to kilauea volcano on june 19 1991 was 217 000 10 000 m3 d or 0 079 0 004 km3 yr this is close to the average eruptive rate of 0 08 km3 yr between 1958 and 1984 in addition the predictable response of summit elevation and tilt to each east rift zone eruption near puu oo since 1983 shows that summit deformation is also a measure of magma reservoir pressure given this the correlation between the elevation of the puu oo lava lake 4 km uprift of kupaianaha and 18 km from the summit and summit tilt provides an estimate for magma pressure changes corresponding to summit tilt changes the ratio of the change in volume to the change in reservoir pressure dv dp during vent activity may be determined by dividing the ratio of volume erupted to change in summit tilt dv dtilt by the ratio of pressure change to change in summit tilt dp dtilt this measure of dv dp when combined with laboratory measurements of the bulk modulus of tholeitic melt provides an estimate of 240 50 km3 for the volume of kilauea s magma reservoir this estimate is much larger than traditional estimates but consistent with seismic tomographic imaging and geophysical modeling of kilauea s magma system

a global synthesis of lava lake dynamics abstract active lava lakes represent a variety of open vent volcanism in which a sizable body of lava accumulates at the top of the magma column constrained by the vent and or crater geometry the longevity of lava lakes reflects a balancing of cooling and outgassing occurring at the surface and input of hot and gas rich magma from below due to their longevity and relative accessibility lava lakes provide a natural laboratory for studying fundamental volcanic processes such as degassing convection and cooling this article examines all seven lakes that existed at the time of writing in 2018 located in the pacific antarctica africa and south and central america these lakes span all tectonic environments and a range of magma compositions we focus on analysis of the lake surface motion using image velocimetry which reveals both similarities and contrasts in outgassing and lake dynamics when comparing the different lakes we identify two categories of lake behavior organized erta ale nyiragongo k lauea after 2011 and erebus and chaotic villarrica masaya marum this division does not map directly to lake size viscosity gas emission rate or temperature instead when examined together we find that the lakes follow a linear relationship between average surface speed and the ratio of total gas flux to lake surface area this relationship points to the combined importance of both flux and lake size in addition to the total volume of gas emission and suggests that a shared deep mechanism controls the supply of heat and gas to all lakes on the other hand the differences between chaotic and organized lakes highlight the important role of the geometry of the conduit lake transition which superimposes a shallow signal on that of the deep circulation the spatial patterns of surface motion we document suggest that the release of gas bubbles at chaotic lakes is more efficient i e bubbles are less likely to be retained and recycled compared with organized lakes in addition the data presented here indicate that the solidified crust of organized lakes plays a role in regulating convection and outgassing in lava lakes

transfer of sulfur in subduction settings an example from batan island luzon volcanic arc philippines sulfur abundances have been determined in silicic and basaltic melt inclusions in olivines from harzburgitic xenoliths and a basaltic lava sample all from batan island in mantle xenoliths olivines fo80 91 are present as neoblasts or in finely recrystallized patches the most magnesian olivines fo89 7 91 cao 85 cao 0 25 wt and their melt inclusions cao al2o3 from 0 8 to 1 15 have recorded early stages of crystallization the sulfur concentrations for these calc alkaline basaltic melts are estimated between 1720 and 3200 ppm with a mean value at 2550 ppm 1 390 and s cl ratio at nearly 1 this is in agreement with the idea that arc basaltic melts may contain high concentrations of sulfur s 2000 ppm at 1200 c however the heterogeneous distribution of s and its partitioning between silicate melts h2o rich vapor and s bearing solid phases as illustrated by the batan mantle xenoliths would result in highly variable sulfur concentrations in island arc basaltic magmas mostly controlled by fo2 and fs2

geology compositional heterogeneities and geochemical origin of the yacheng gas field qiongdongnan basin south china sea the yacheng gas field is located in the footwall of the no 1 fault the boundary fault between the yinggehai and qiongdongnan basins the main reservoir is the fan delta sandstones in the lingshui formation the seals are meishan for m ation shales near the no 1 fault and lingshui formation shales away from the no 1 fault all strata are normally pressured in the gas field except for the meishan formation the meishan formation is overpressured near the no 1 fault in the gas field and in the adjacent yinggehai basin away from this fault into the qiongdongnan basin the overpressure diminishes an obvious thermal anomaly occurs below 3600 m in the gas field this anomaly characterized by an abrupt increase in drill stem test and fluid inclusion homogenization temperatures vitrinite reflectance ro and rock eval tmax and by an abnormally low temperature ro tmax gradient diminishes away from the yinggehai basin the gases and condensates have abnormally high aromatic hydrocarbon contents and show obvious heterogeneities away from the no 1 fault the c2 hydrocarbon content and c2 scn increase carbon dioxide content decreases d13c values for methane ethane and carbon dioxide become lighter the heptane and isoheptane values decrease and the relative contents of aromatic hydrocarbons both in c6 c7 light hydrocarbons and in the condensates decrease such heterogeneities reflect the reservoir filling process and origin of the gas field the gas field was charged from both the qiongdongnan and the yinggehai basins hydrocarbons sourced from the qiongdongnan basin have relatively low maturities whereas hydrocarbons from the yinggehai basin have relatively higher maturities and seem to have been in association with hydrothermal fluids the hydro thermal fluids from the yinggehai basin in which methane ethane carbon dioxide and especially aromatic hydrocarbons dissolved under the high temperature and high pressure subsurface conditions migrated along the no 1 fault and caused the abnormally high concentration of aromatic hydrocarbons as well as the thermal anomalies in the gas field especially near the no 1 fault

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

plinian airfall deposits of basaltic composition two widespread scoria fall deposits locally known as the fontana lapilli and san judas formation are demonstrated to have issued from masaya caldera complex nicaragua and to be the first documented plinian airfall deposit of basaltic composition data from maps of median grain size average maximum pyroclast diameter and average maximum diameter of lithic fragments provide conclusive evidence of the plinian nature of both deposits the volume within the 1 mm isopach of the fontana lapilli is 12 km3 3 4 km3 dense rock equivalent and within the san judas formation it is 1 2 km3 0 35 km3 dre theoretical eruption parameters calculated for the fontana lapilli and san judas formation respectively include muzzle velocity 170 m s and 150 m s column height 50 km and 18 km volume eruption rate 2 105 m3 s and 1 104 m3 s and duration 2 h and 10 h these are all similar to the calculated parameters for silicic plinian eruptions basaltic magmas are generally considered incapable of erupting in the manner characteristic of plinian eruptions recognition of them argues that factors other than high viscosity may be of fundamental importance

sulfur and halogen gases at masaya caldera complex nicaragua total flux and variations with time santiago crater of masaya caldera complex near managua nicaragua entered a phase of intense magmatic degassing in late 1979 the flux of so2 increased rapidly from a previous average of 380 t d 1 to approximately 1200 t d 1 in february 1980 we report on results of a study of the gas flux and geochemistry conducted as part of a broader interdisciplinary study of the volcano and its impact on the adjacent environment masaya is an unusual volcano because of its low shield like form consistent tholeiitic basaltic composition frequent activity and especially because of the approximately 25 year cycle of major non eruptive degassing crises we have used a combination of techniques involving remote sensing using a correlation spectrometer cospec of the flux of so2 and direct filter sampling of the ratios of the sulfur and halogen gases and aerosols to quantify the flux of all of these species the three stage filters successively trap aerosols halogen acid gases and sulfur dioxide and are analyzed by ion chromatography we have used the ratios of various species with the cospec measurements of the flux of so2 to calculate that the average flux of so2 is 1275 t d 1 hcl is 830 t d 1 hf is about 16 t d 1 and hbr is at most 1 t d 1 h2s and so4 are only minor sulfur species the flux of so2 and hcl represents the largest reported noneruptive sustained volcanic release of these species in the world masaya contributed 7 of the total so2 estimated to have been released by volcanoes worldwide in 1981 the flux of so2 has declined by about 9 each year since 1979 and can be modeled as a first order kinetic rate law with a decay constant of approximately 0 04 yr 1 approximately 2 2 106 tons of so2 has been released between september 1979 and september 1985 the ratio s cl has declined more rapidly than the flux of so2 reflecting the greater solubility of hcl than so2 in the magma the so2 flux at santiago varies more widely within a single day than observed at other degassing volcanoes and may be controlled at least in part by effects of the solid earth tide an area larger than 500 km2 has been severely impacted downwind of the volcano downwind ground level concentrations of so2 of 0 25 ppm are frequently encountered acid rain unusually chloride rich has a ph as low as 2 63 s cl ratios in the gas plume increase downwind probably as a result of the high solubility and dry deposition velocity of hcl preliminary analyses of primary concentrations of sulfur and chlorine in the magma of the present lava lake are used to calculate that at least 1 2 km3 has been completely degassed with respect to sulfur to provide the observed flux this implies that 10 km3 have been degassed by previous crises in the past century and further suggests a magma supply rate of approximately 0 1 km3 yr comparable with kilauea or etna volcanoes during historic time only two lava flows have been extruded implying an unusually low ratio of extrusion intrusion of only 0 0007

a geochemical and geophysical investigation of the hydrothermal complex of masaya volcano nicaragua abstract masaya volcano nicaragua is a persistently active volcano characterized by continuous passive degassing for more than 150 years through the open vent of santiago crater this study applies self potential soil co 2 and ground temperature measurements to highlight the existence of uprising fluids associated to diffuse degassing structures throughout the volcano the diffuse degassing areas are organized in a semi circular pattern and coincide with several visible and inferred surface volcanic structures cones fissure vents and likely consist of a network of buried faults and dykes that respectively channel uprising flow and act as barrier to gravitational groundwater flow water depths have been estimated by multi scale wavelet tomography of the self potential data using wavelets from the poisson kernel family compared to previous water flow models our water depth estimates are shallower and mimic the topography typically less than 150 m below the surface between 2006 and 2010 the depths of rising fluids along the survey profiles remained stable suggesting that hydrothermal activity is in a steady state this stable activity correlates well with the consistency of the volcanic activity expressed at the surface by the continuously passive degassing when compared to previous structural models of the caldera floor it appears that the diffuse degassing structures have an important effect on the path that shallow groundwater follows to reach the laguna de masaya in the eastern part of the caldera the hydrogeological system is therefore more complex than previously published models and our new structural model implies that the flow of shallow groundwater must bypass the intrusions to reach the laguna de masaya furthermore these diffuse degassing structures show clear evidence of activity and must be connected to a shallow magmatic or hydrothermal reservoir beneath the caldera as such the heat budget for masaya must be significantly larger than previously estimated

trace elements in the particulate phase of the plume of masaya volcano nicaragua masaya volcano nicaragua has been passively degassing since 1993 this gas crisis provides a unique opportunity for investigating the degassing of magmatic volatiles here we studied the chemical composition of the volcanic particulate phase collected directly from the plume the objective was to gain new insights into the volatilities and emission rates of trace elements the results indicate enrichments in the aerosol phase of all analyzed trace elements including not only those known to be volatile such as cu and te but also elements considered as refractory such as rare earth elements rees and th rees were fractionated with a significant enrichment of light rare earth elements lrees over heavy rare earth elements hrees this pattern suggests degassing of lrees as chlorides in general the emission of most trace elements in volcanic plumes is governed by the primary degassing of chloride compounds however sem eds data and chemical analysis of aerosol filters allow us to state that trace elements can also be released as sulphate and chloro sulphate compounds the trace element emission rates at masaya are significant and place this volcano as a major gas emitter however for some elements like cu masaya s contribution to the global volcanic flux seems disproportionate probably pointing to the poor dataset available for other degassing volcanoes

correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components the natural distribution of carbon isotopes between hydrocarbon gas components is used for 1 determining a gas s maturity 2 correlating a reservoired gas to its source 3 correlating one reservoired gas with another and 4 recognizing gas mixtures calculated separations of carbon isotopes between the normal alkane components of a natural gas have been related to source rock maturity by use of a single continuous diagram independent of source type actual data from a wide variety of geologic settings and geologic ages confirm this relationship and demonstrate its applicability to the source rock levels of organic metamorphism ranging from 8 to 13 covering the entire range of oil and wet gas generation at greater maturities the wet gas components are found to undergo thermal degradation losing their usefulness for correlation three examples showing indigenous gas west texas non indigenous gas gippsland basin australia and gas mixtures from multiple sources southeastern alberta illustrate exploration applications

shallow open system evolution of basaltic magma beneath a subduction zone volcano the masaya caldera complex nicaragua abstract lavas and pyroclastic material from the masaya caldera complex have a number of distinctive geochemical features relative compositional homogeneity low al2o3 and high feo contents a tholeitic differentiation trend and elevated large ion lithophile lil element concentrations e g ba 800 ppm on cmas projections their compositions always fall on or near low pressure cotectics in addition the basalts of masaya have unusually high 87sr 86sr and 10be masaya has exhibited medium term compositional cycles best exhibited by the sawtoothed changes in tio2 and feo mgo there are also a number of longer term compositional changes which are abrupt and generally coincide with caldera formation many of the geochemical characteristics of masaya coupled with a number of volcanological observations indicate masaya is underlain by a large shallow open system magma chamber perhaps on the order of 10 km3 in size although fractional crystallization is a significant magmatic process in masaya s open system chamber magma mixing contamination is equally important magma mixing is necessary to explain the discontinuous stratigraphic changes in magma composition observed at masaya and crustal contamination is necessary to explain their generally elevated 87sr 86sr and lil element concentrations two components therefore have been admixed into the magma chamber of masaya a lil poor basaltic component such has been erupted from the nearby nejapa and granada cinder cones and a lil rich acidic component such has been erupted from the nearby calderas of apoyo and apoyeque admixtures of the former have dominated admixtures of the latter ended with caldera formation ironically open system behavior has exerted fundamental control on the maintenance of relative compositional homogeneity

gravity changes and passive so2 degassing at the masaya caldera complex nicaragua an understanding of the mechanisms responsible for persistent volcanism can be acquired through the integration of geophysical and geochemical data sets by interpreting data on micro gravity ground deformation and so2 flux collected at masaya volcano since 1993 it is now clear that the characteristically cyclical nature of the activity is not driven by intrusion of additional magma into the system rather it may be due in large part to the blocking and accumulation of gas by restrictions in the volcano substructure the history of crater collapse and formation of caverns beneath the crater floor would greatly facilitate the trapping and storage of gas in a zone immediately beneath san pedro and the other craters another mechanism that may explain the observed gravity and gas flux variations is the convective overturn of shallow pre existing degassed cooled dense magma that is replaced periodically by lower density hot gas rich magma from depth buoyant gas rich magma rises from depth and is emplaced near the surface resulting in the formation and fluctuation of a low density gas rich layer centred beneath nindiri and santiago craters as this magma vigorously degasses it must cool increase in density and eventually sink five stages of activity have been identified at masaya since 1853 and the most recent data suggest that the system may have been entering another period of reduced degassing in 2000 this type of analysis has important implications for hazard mitigation because periods of intense degassing are associated with poor agricultural yields and reduced quality of life a better understanding of persistent cyclically active volcanoes will allow for more effective planning of urban development and agricultural land use

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

permanent tremor of masaya volcano nicaragua wave field analysis and source location the masaya volcano nicaragua is a basaltic caldera in a subduction zone the permanent source of the volcanic tremor was located inside santiago crater at the lava lake s position and 400 m below the ne rim and therefore corresponds to superficial magma activity we used two tripartite arrays 90 m side one semicircular array r 120 m in 1992 and two semicircular arrays r 60 m and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993 we used both a cross spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field using the delays therefore by intersecting the back azimuth wave directions from the arrays we could pinpoint the source additionally the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor the tremor s wave field is composed of comparable quantities of dispersed rayleigh and love surface waves whose phase velocities lie in the ranges 730 1240 m s at 2 hz and 330 550 m s at 6 hz the dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers the resulting velocity models are similar for the northern and southern parts of the volcano after geometrical spreading corrections q2hz 14 and q3hz 31 were determined along the northern linear array the typical low velocities and low q corresponding to the cone structure and are similar to those of other basaltic volcanoes like puu oo hawaii and klyuchevskoy kamchatka

a total volatile inventory for masaya volcano nicaragua nerc project magma dynamics at persistently degassing basaltic volcanoes a novel approach to linking volcanic gases and magmatic volatiles within a physical model ne f004222 1 and ne f005342 1

integration of micro gravity and geodetic data to constrain shallow system mass changes at krafla volcano n iceland new and previously published micro gravity data are combined with insar data precise levelling and gps measurements to produce a model for the processes operating at krafla volcano 20 years after its most recent eruption the data have been divided into two periods from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at krafla is decaying exponentially the net micro gravity change at the centre of the caldera is shown using the measured free air gradient to be 85 gal for the first and 100 gal for the second period after consideration of the effects of water extraction by the geothermal power station within the caldera the net gravity decreases are 73 17 gal for the first and 65 17 gal for the second period these decreases are interpreted in terms of magma drainage following a mogi point source model we calculate the mass decrease to be 2 1010 kg year reflecting a drainage rate of 0 23 m3 s similar to the 0 13 m3 s drainage rate previously found at askja volcano n iceland based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems we suggest a pressure link between askja and krafla at deeper levels at the lower crust or the crust mantle boundary after the krafla fires co rifting pressure decrease of a deep source at krafla stimulated the subsequent inflow of magma eventually affecting conditions along the plate boundary in n iceland as far away as askja we anticipate that the pressure of the deeper reservoir at krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber possibly initiating a new rifting episode we have demonstrated that by examining micro gravity and geodetic data our knowledge of active volcanic systems can be significantly improved

volcanic structure of the southern sector of mt etna after the 2001 and 2002 eruptions defined by magnetotelluric measurements two magnetotelluric mt surveys were carried out on the mt etna volcano after two of the most intense eruptions of the last 30 years which took place in summer 2001 and winter 2002 2003 surveying was pursued for two main reasons first we sought to contribute to the definition of the first order structure and physico chemical state temperature fluids melts of a volcano that has been extensively explored and monitored by means of various geophysical methods but where only few electrical and electromagnetic surveys have been performed secondly we acquired mt data in the same sites in the two different surveys with the aim of monitoring the possible changes of the first order structure since conditions are expected to vary on an active volcano such as etna and are supposed to be linked to the eruptive events soundings have been acquired in an e w 10 km long profile across the southern flank of mt etna at a distance of almost 6 km south from the central crater the first survey was carried out three months after the 2001 eruption inverse models define a pronounced 4 km thickness low resistivity section at a depth of about 1 km b s l to the west to the east a low resistivity section is still present but appears deeper thinner and more resistive and a shallow low resistivity anomaly also exists the shallow anomaly to the east is tentatively correlated with altered and clayey volcanic units and or temporary groundwater storage the deep anomalies are interpreted as being due to melt storage at shallow depths which was not exhausted during the eruption this would be confirmed by the abundance of lava erupted within one year from the end of the survey the few good sites retrieved in the second survey carried out a few weeks after the eruption of 2002 2003 confirm the picture defined in the first survey and provide a better definition of the bottom of the deep anomaly located in the sedimentary basement

4d gravity changes associated with the 2005 eruption of sierra negra volcano galapagos sierra negra volcano the most voluminous shield volcano in the galapagos archipelago and one of the largest basaltic calderas in the world erupted on october 22 2005 after more than 25 years of quiescence gps and satellite radar interferometryinsar monitoring of the deformation of the caldera floor in the months prior to the eruption documented extraordinary inflation rates 1c m day the total amount of uplift recorded since monitoring began in 1992 approached 5 m at the center of the caldera over the eight days of the eruption the caldera floor deflated a maximum of 5 m and subsquently renewed its inflation but at a decelerating rate to gain insight into the nature of the subsurface mass density changes associated with the deformation gravity measurements performed in 2005 2006 and 2007 are compared to previous measurements from 2001 2002 when the volcano underwent a period of minor deflation and magma withdrawal the residual gravity decrease between 2001 2002 and 2005 is among the largest ever recorded atan active volcano950 gal and suggests that inflation was accompanied by a relative density decrease in the magmatic system forward modeling of the residual gravity data in 4d from 2002 to 2005 gives an estimate of the amount of vesiculation in the shallow sill required to explain the observed gravity variations geochemical constraints from melt inclusion and satellite remote sensing data allow us to estimate the pre eruptive gas content of the magma and place constraints on the thickness of the gas rich sill necessary to produce the gravity anomalies observed results suggest that reasonable sill thicknesses 700 800 m and bubble contents 10 50 volume can explain the large decrease in residual gravity prior to eruption following the eruption 2006 and 2007 the deformation and gravity patterns suggest re equilibration of the pressure regime in the shallow magma system via a renewed influx of relatively gas poor magma into the shallow parts of the system