calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

development of a subvolcanic structure by the interaction of liquid filled cracks abstract the interaction of liquid filled cracks may be an important process for magma accumulation and the development of subvolcanic structures the horizontal interaction was studied through the stress analyses around pressured cracks in various regional stress fields the vertical interaction was studied using gelatin experiments buoyaney driven cracks coalesce more easily than pressure cracks the theory of the three dimensional crack interaction was discussed there are two types of crack interaction the interaction between liquid filled cracks may cause coalescence of cracks the interaction between a liquid filled crack and a solid filled crack never causes coalescence the stress relaxation by ductile or brittle behavior or by the driving out of magma decreases the local stress field generated by the accumulation of magma filled cracks as the regional differential stress increases and the minimum compressive principal stress becomes tensile magma filled cracks become parallel to each other as the supply rate of magma filled cracks becomes relatively large or as the regional differential stress decreases and the minimum compressive stress increases the arrangement of magma filled cracks varies from an offset pattern to a radial one because the local compressive stress field cannot be decreased this process leads to the development of subparallel or radial dikes in two dimensions or a dike sheet complex in three dimensions a large magma chamber such as a sphere or a cylinder is not always necessary for the development of a basaltic volcano in case of the interaction between liquid filled cracks magma filled cracks coalesce easily into one large crack and diminish the local compressive stress field last the theory of the crack interaction is applied to developments of various volcanoes the fissure pattern of hekla volcano iceland is explained in terms of the variation of the stress relaxation the difference in fissure pattern between izu oshima and miyakejima volcanoes japan is interpreted by the variation of the differential stress the local stress balance including the stress relaxation is kept in the developments of etna volcano italy and kilauea volcano hawaii

global positioning system measurements of ground deformation caused by magma intrusion and lava discharge the 1990 1995 eruption at unzendake volcano kyushu japan abstract global positioning system gps measurements made around unzendake volcano kyushu japan since january 1991 have detected ground deformation caused by magma intrusion and lava discharge in the intermittent phreatic and phreatomagmatic eruption stage the ground was inflating after growth of the lava dome and following frequent pyroclastic flows at unzendake volcano the ground began deflating these ground deformations are explained by the inflation and deflation of a mogi s source model a point source model located about 6 km west of the active crater at a depth of 11 km at an aseismic region the observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation the horizontal displacements at the reference gps station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other gps baseline systems these observations validate our estimates for the pressure source based on the relation between the deformation volume of the ground surface and the discharged volume of the lava it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater magma is estimated to be supplied to the reservoir at an average rate of 1 1 105 m3 day magma intrusion began in december 1989 at the latest and continued for 1 9 103 days

retrieving the stress field within the campi flegrei caldera southern italy through an integrated geodetical and seismological approach we investigated the campi flegrei caldera using a quantitative approach to retrieve the spatial and temporal variations of the stress field for this aim we applied a joint inversion of geodetic and seismological data to a dataset of 1 100 optical levelling measurements and 222 focal mechanisms recorded during the bradyseismic crisis of 1982 1984 the inversion of the geodetic dataset alone shows that the observed ground deformation is compatible with a source consisting of a planar crack located at the centre of the caldera at a depth of about 2 56 km and a size of about 4 4 km inversion of focal mechanisms using both analytical and graphical approaches has shown that the key features of the stress field in the area are a nearly subvertical 1 and a sub horizontal roughly nne ssw trending 3 unfortunately the modelling of the stress fields based only upon the retrieved ground deformation source is not able to fully account for the stress pattern delineated by focal mechanism inversion the introduction of an additional regional background field has been necessary this field has been determined by minimizing the difference between observed slip vectors for each focal mechanism and the theoretical maximum shear stress deriving from both the volcanic time varying and the regional constant field the latter is responsible for a weak nne ssw extension which is consistent with the field determined for the nearby mt vesuvius volcano the proposed approach accurately models observations and provides interesting hints to better understand the dynamics of the volcanic unrest and seismogenic processes at campi flegrei caldera this procedure could be applied to other volcanoes experiencing active ground deformation and seismicity

tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments remote prediction of gassy marine sediment properties is important for geohazard assessment gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity frequency and attenuation frequency relationships that depend on gas bubble size gas content and sediment elastic properties an acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at dibden bay southampton united kingdom a vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed a miniboomer in the seabed above the array was used to generate broadband 600 hz to 3000 hz acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m subaerial exposure at low tide and 2 35 m at high tide the calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum over 200 db m that shifts in frequency from 1050 hz at low tide to 1250 hz at high tide thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum supported by x ray computed tomography scan observations on a pressure core which also indicate that bubble shape depends on sediment type modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum implying that gas diffusion and nonspherical bubbles are significant

a reinterpretation of seismicity associated with the january 1983 dike intrusion at kilauea volcano hawaii in january 1983 a dike intrusion fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16 km segment of kilauea s middle east rift zone we searched the hawaiian volcano observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm consistent with generation near the propagating dike tip while others were active for several days the two multiplets nearest the origin of the seismic swarm include events from the preceding days and months most multiplets span only 50 to 100 m following relocation are located at about 3 to 4 km depth and appear to deepen downrift the catalog depths of those earthquakes in multiplets and those not in multiplets are similar suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface by analogy with a mechanical model used to explain a similar clustering of background seismicity in the upper east rift in 1991 we infer that the earthquakes are generated in regions of high stress concentration immediately above kilauea s deforming deep rift body this conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1

igneous or metamorphic hornblende phenocrysts as greenschist facies reaction cells in the half dome granodiorite california the half dome granodiorite yosemite national park california is recognized in the field by euhedral fresh looking black hornblende phenocrysts up to 2 cm in length this variety of granodiorite typifies intermediate age hornblende phyric units of cretaceous nested plutonic suites in the sierra nevada batholith although only inclusions of feldspar are evident in hand samples the phenocrysts are riddled with up to 50 inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt al 2 o 3 to magnesiohornblende with over 8 wt elemental zoning within the amphibole is highly irregular on the micrometer scale showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions the chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite suggesting that the non biotite inclusions are the result of metamorphism of the phenocrysts backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite pressure calculations using the al in hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization these hornblende phenocrysts would thus be unsuitable for geobarometry and caution must be used to avoid similarly zoned phenocrysts in the application of the al in hornblende geobarometer

bubble growth and rise in soft sediments the mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance we offer definitive evidence on the mode of bubble formation in the form of x ray computed tomographic images and comparison with theory natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids disks that grow either by fracturing the sediment or by reopening preexisting fractures in contrast bubbles in soft sandy sediment tend to be spherical suggesting that sand acts fluidly or plastically in response to growth stresses we also present bubble rise results from gelatin a mechanically similar but transparent medium that suggest that initial rise is also accomplished by fracture given that muddy sediments are elastic and yield by fracture it becomes much easier to explain physically related phenomena such as seafloor pockmark formation animal burrowing and gas buildup during methane hydrate melting

correlation of shape and size of methane bubbles in fine grained muddy aquatic sediments with sediment fracture toughness abstract gassy sediments contribute to destabilization of aquatic infrastructure air pollution and global warming in the current study a precise shape and size of the buoyant mature methane bubble in fine grained muddy aquatic sediment is defined by numerical and analytical modeling their results are in a good agreement a closed form analytical solution defining the bubble parameters is developed it is found that the buoyant mature bubble is elliptical in its front view and resembles an inverted tear drop in its cross section the size and shape of the mature bubble strongly correlate with sediment fracture toughness bubbles formed in the weaker sediments are smaller and characterized by a larger surface to volume ratio that induces their faster growth and may lead to their faster dissolution below the sediment water interface this may prevent their release to the water column and to the atmosphere shapes of the bubbles in the weaker sediments deviate further from the spherical configuration than those in the stronger sediments modeled bubble characteristics important for the acoustic applications are in a good agreement with field observations and lab experiments

trace element evidence for plagioclase recycling in calc alkaline magmas abstract trace element zoning in plutonic plagioclase feldspars from the val fredda complex southern adamello massif italy has been measured using secondary ion mass spectrometry sims the studied samples range from hornblende gabbro through diorite to granitoid with an overall range in plagioclase composition from an96 to an15 individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures there is equally extensive zoning in trace elements notably sr and ba broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition using a relationship betweenxan and the distribution coefficientsdsr anddba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone the calculated variations in ba and sr are greater than can be attributed to changes in pressure or temperature alone indicating that compositional variation in the coexisting melt was the principal cause of zoning the ba sr melt trend for each crystal describes a two stage crystallisation history the calcic cores of all crystals show melt trends of increasing sr and ba which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas in contrast the melt trends recorded by the rims vary from sample to sample but essentially show decreasing sr and ba consistent with fractionation of sodic plagioclase alkali feldspar and or biotite the plagioclase cores are thought to have crystallised in a common magma chamber at 6 10 kbar pressure while the rims crystallised at the emplacement level 1 5 kbar from interstitial melt whose composition is determined by that of the host magma the data suggest that early formed calcic plagioclase was recycled by the magma during fractionation vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re entrain cumulus crystals into more evolved magmas the occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale

buoyancy driven fracture ascent experiments in layered gelatine laboratory experiments on air filled fracture propagation in solidified homogeneous and layered gelatine have been carried out providing an analogue model for magma filled dikes ascending in the crust the effects of layering on fracture velocity and shape have been analyzed in detail the free surface is found to accelerate approaching fractures layering accelerates or decelerates fractures approaching discontinuities of the elastic parameters depending on the value of the rigidity contrast the shape of fractures are strongly influenced as they pass from one layer to another the observed cross sectional shape when crossing a layer interface and the acceleration with decreasing rigidity can be explained with theoretical models our experiments also reproduce the arrest of fractures in proximity of joints and the formation of sills in the layer below the interface these findings could help in the interpretation of accelerated seismicity and deformation rates observed in volcanic areas

petrology of the 1991 1995 eruption at unzen effusion pulsation and groundmass crystallization abstract effusive eruption of dacite magma 2 1 108 m3 during 1991 1995 formed a lava dome at the summit of unzen volcano japan the effusion rate was highest at the beginning 4 0 105 m3 day 4 6 m3 s and decreased roughly with time to almost zero before this pattern was repeated with a second pulse of magma supply the whole rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance the more mafic the more abundant the phenocrysts the pattern of chemical variation with time shows some difference from that of the effusion rate all phenocrysts in dacite plagioclase hornblende biotite quartz and magnetite show evidence of disequilibrium with melt although a glomerophyric aggregation of phenocrysts suggests coexistence with each other phenocrysts are isotopically heterogeneous from species to species the calculated initial melt composition was rhyodacite and was nearly constant throughout the activity in contrast the bulk phenocryst population is andesite a model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments or xenocrysts by parental rhyodacite magma it is suggested that when effusion rate was high less viscous crystal poor magma exited from the reservoir groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate the higher the effusion rate the more evolved the compositions groundmass crystallinity increased with decreasing effusion rate from 33 to 50 textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top the fe ti oxide temperature 880 780 c was low when the crystallinity was high micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit porosity of dome lavas was lower at lower effusion rates collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low frequency earthquakes phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction when effusion waned degassing and the resultant crystallization proceeded more completely so that the magma became too viscous to flow in the conduit top and behaved as a plug resulting in a temporary halt of effusion in turn groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling the plug was scavenged when rising excess pressure overcame its effective strength then the second pulse of magma supply began strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason

recent seismicity and tomographic modeling of the mount etna plumbing system the monitoring of seismic activity in eastern sicily southern italy has been recently improved in the framework of the poseidon project to investigate both tectonics and volcanic processes of mount etna this effort has produced a homogeneous and complete data set which we use to image the deep structure of the volcano and to define the space and time distribution of the recent seismicity encompassing the 1995 eruption diffuse eruptions between 1995 and 1997 a further magma intrusion started in 1997 and an increase of volcanic activity in july 1998 we inverted p wave arrival times from 307 selected local earthquakes to obtain a three dimensional velocity model of the volcano with a simultaneous inversion for hypocenters and velocity parameters the new tomographic images permit us to define the structure beneath the volcano from the surface down to 18 km depth the main structural feature revealed by our inversion is a high velocity body located beneath the central craters whose lateral extent increases from 6 km between 18 and 9 km depth to 12 km between 9 and 3 km depth near the surface the fast anomaly branches in two separate high vp regions which are located below the summit craters and the eastern flank valle del bove respectively the high velocity features are interpreted as high density cumulates of solidified magma that intruded the shallow crust we hypothesize that magma ascends the crust within the relatively small high velocity conduit below 9 km depth and is stored at depth shallower than 9 km within the broad high velocity region as also suggested by petrological data a sharp increase of seismicity in 1997 with earthquakes occurring at the border of the high velocity body suggests that mount etna sustained recent episodes of intrusions which possibly herald future eruptions no large low velocity anomaly interpretable as a melted magma chamber is imaged in the upper 18 km of the crust while a broad low velocity anomaly in the uppermost mantle revealed by regional seismic data possibly indicates the magma source region at depth earthquake occurrence from the deep source to the shallow intrusive area helps to trace the magma migration and the feeding of the volcanic system

growth of disk shaped bubbles in sediments disc shaped methane bubbles often observed in marine sediments result from growth in a medium that elastically resists expansion of the bubbles and yields by fracture we have modeled this process to obtain estimates of growth times by using a reaction diffusion model coupled to a linear elastic fracture mechanics lefm for comparison we also modeled the growth of a constant eccentricity bubble in a nonresistant medium discoidal bubbles that grow in sediments that obey lefm grow much faster than spherical bubbles two to fourfold faster for the times and conditions tested here and become more eccentric with time aspect ratios falling from 0 3 to 0 03 ove r8do fgrowth in addition their growth is not continuous but punctuated by fracture events furthermore under some conditions lefm predicts that bubble growth can become arrested which is not possible for a bubble in a nonresistant medium even for nonspherical bubbles cessation of growth occurs when the dissolved gas concentration gradient near the bubble surface disappears as a result of the increase in bubble gas pressure needed to overcome sediment elasticity copyright 2003 elsevier science ltd 1 introduction the production of volatile substances usually methane dur ing diagenesis can lead to saturation of the pore waters and the subsequent formation of gas bubbles the presence of such bubbles can engender significant geochemical and geotechnical consequences as previously discussed in martens and klump 1980 and hovland and judd 1992 including greatly en hanced methane fluxes to the atmosphere through ebullition given the important effects of bubbles the dearth in our knowl edge of their rates of formation and of the processes that control those rates is surprising in particular little solid or systematic estimates of these rates exist and what we do have is largely in the form of crude deductions of seasonal growth that are based on limited observations in martens and klump 1980 recently we undertook a research program to redress this paucity of information by a combined program of bubble growth experiments and theoretical modelling our first effort in this field produced a theoretical model of the growth for a spherically shaped bubble in a sediment that does not offer resistance to the expansion of the bubble boudreau et al 2001a b this model balances the methanogenic production of the volatile solute with its diffusion to the growing bubble to obtain an equation for the radius of the bubble r t with time

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

methane bubble growth in fine grained muddy aquatic sediment insight from modeling abstract methane ch 4 is the most abundant hydrocarbon and one of the most important greenhouse gases in the atmosphere ch 4 bubble growth and migration within muddy aquatic sediments are closely associated with sediment fracturing in this paper we present the modeling of buoyancy driven ch 4 bubble growth in fine grained muddy aquatic sediment prior to the beginning of its rise we designed a coupled mechanical reaction transport numerical model that enables a differential fracturing over the bubble front as it occurs in nature when the fracturing increment stays constant at the bubble head and subsides towards bubble tail during bubble growth we show that this differential fracturing over the bubble front controls the bubble shape and size temporal evolution and is significantly affected by the critical stress intensity factor of the muddy sediment the intercalated stages of elastic expansion and fracturing during the bubble growth shorten with time as the bubble approaches its terminal size prior to its ascent our simulations reveal a high asymmetry in the bubble shape growing with time with respect to its initial symmetric penny shaped configuration it was found that the bubble grows allometrically while the importance of the bubble surface area growth with time we also confirmed the earlier predictions about the inverted tear drop bubble cross section just prior to the beginning of its rise modeling of the terminal bubble characteristics will permit prediction of the delivery of gaseous methane from the sediment to the atmosphere via the water column

continuous mixing of crystal mush and replenished magma in the ongoing unzen eruption zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing the dacite was formed by mixing of relatively high and low temperature t end member magmas the magnetite phenocrysts derived from the low t magma are reversely zoned by the mixing with high t magma a diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching for the mixed dacite erupted from may 1991 to may 1993 the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr this indicates that the mixing was continuous during the effusion the invariability of the other mixing signatures such as the thickness of reaction rims around biotite phenocrysts also supports the continuous mixing model low t end member magma is estimated by mass balance calculation to be a crystal rich mush of dacitic composition these observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion the proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes

degassing of stratified magma by compositional convection abstract a new model is proposed for passive degassing from sub volcanic magma chambers the water content in stably stratified shallow magma chamber will be equated to its solubility at the upper boundary by convection water from a lower layer high in water content can enrich the contact zone of the upper layer and lead to further convective overturn of this boundary layer a complete set of equations describing convection with bubble formation and dissolution is reduced to a simplified form by assuming a small bubble content the development and pattern of flow driven by vesiculation is modeled numerically in a 2d magma chamber for relatively low raleigh numbers 5 105 bubbles rising from the magma will collect near the roof in a layer of 8 10 vol and then escape upward to fumaroles the stokes flux of bubbles escaping from an andesitic magma with viscosity 104 p and a top surface of about 500 500 m corresponds with observed total magmatic water fluxes of 35 kg s pressure within the chamber is buffered by elastic and local visco elastic deformations in the solid rocks bounding the chamber to the range between ambient and close to lithostatic values in a chamber closed to fresh magma inputs the decrease in volume due to such gentle volatile escape lowers the reference pressure bubbles flux from the lower layer induced by variation of the saturation level around stratification boundary may be efficient mechanism for the water transport between layers

imaging the multi level magma reservoir at mt etna volcano italy 1 the continuous gps network operating on mt etna with its 36 stations is currently one of the largest worldwide the aim of this network is the evaluation of volcanic hazard and the modelling of the active sources in this paper we propose an in depth analysis and modelling of continuous gps data collected at mt etna from may 2008 to december 2010 the analyzed period has been divided into four different coherent phases 1 14 may 2008 02 august 2008 deflation of the entire gps network 2 02 august 2008 14 june 2009 deflation of the summit area and inflation at lower heights 3 14 june 2009 21 may 2010 inflation of the entire gps network 4 21 may 2010 31 december 2010 inflation at medium and low heights and end of the inflation in the summit area analytical models indicate a non uniform deformation style revealing spaced sources acting at different time on different segments of a multi level magma reservoir the etnean plumbing system imaged here is depicted as an elongated magma reservoir that extends from the volcano body downwards to about 6 5 km below sea level b s l sloping slightly towards the north west with storage volumes located at about 6 5 2 0 and 0 0 km b s l the changes in position of the modelled pressure sources during the analyzed time intervals indicate that throughout the 2008 eruptive period the deformation field was mostly driven by the upward migration of magma on the other hand the pattern of deformation recorded after the end of the eruption strongly suggests a significant contribution of the magma overpressure generated by the gas boiling thus outlining the importance of volatiles content in magma

the acoustic turbid layer in muddy sediments of eckernfoerde bay western baltic methane concentration saturation and bubble characteristics in eckernfoerde bay in the western baltic an enclosed basin with organic rich mud acoustic turbidity has been observed since 1952 to investigate the relationships between acoustic properties and free gas we developed a method for rapid sub sampling of gravity cores to determine the total methane content from temperature measurements determinations of the salinity of the pore water and data from the literature saturation limits were calculated for each core it is demonstrated that there is a certain amount of free gas in the mud of eckernfoerde bay but subbottom depths of the oversaturation and of the acoustic turbidity do not correspond the existence of free gas implies the existence of bubbles in the sediment which are regarded to be responsible for the acoustic turbidity bubble sizes observed by x ray computed tomography while still under in situ pressure and temperature vary between 1 and 10 mm equivalent diameter the distribution of bubbles is discontinuous occurring in depth zones from 2 to 20 cm thickness

volcano tectonic seismicity at unzen volcano japan 1985 1999 abstract the eruption of unzen volcano in southwestern japan began on november 17 1990 and an extrusion of a dacite lava dome lasted from may 20 1991 until early february 1995 the seismic network operated by the shimabara earthquake and volcano observatory of kyushu university sevo located about 4000 volcano tectonic earthquakes during the period from 1985 to 1999 the seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion the hypocenters were mostly distributed within a 12 20 km area on the western side of the volcano an inclined seismic aseismic boundary shallows eastward to the summit probably reflecting the thermal structure beneath the volcano within 4 km west from the summit however the seismicity is very low it was suggested that the high temperature ductile and low q bodies exist there we obtained focal mechanism solutions for about 90 earthquakes among them orientations of p axes for the earthquakes occurring on the upper side of the inclined seismic aseismic boundary suggest a pressure source located below this boundary although most of the focal mechanisms have t axes nearly horizontal in the n s direction consistent with the n s extensional tectonic stress several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit this is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings

gas domes in soft cohesive sediments accumulation of gaseous methane can generate seabed domes in soft cohesive sediments such structures can in turn lead to seafloor instability act as precursors to pockmark formation and arguably pose a threat to seafloor drilling if they contain significantly overpressured gas future melting of gas hydrates within the seabed due to global warming will likely lead to a significant long term release of methane which could potentially produce a new and abundant generation of gas domes and associated pockmarks despite their geological and practical significance our understanding of gas dome formation in marine sediments has been limited to observations and qualitative analyses to provide a quantitative understanding we conducted small scale laboratory doming experiments we found that thin layers of clayey sediment behave elastically over the range of deformations needed to create seabed domes the observed behavior is well described by elastic thin plate mechanics from which it is possible to predict the gas pressure required to create natural domes our results suggest that observed shallow dome geometries require surprisingly small overpressures to form however large overpressures can build under increasingly thicker and stiffer layers of sediment

analytical model of surface uplift above axisymmetric flat lying magma intrusions implications for sill emplacement and geodesy abstract in this paper we develop a new axisymmetric analytic model of surface uplift upon sills and laccoliths based on the formulation of a thin bending plate lying on an elastic foundation in contrast to most former models also based on thin bending plate formulation our model accounts for i axi symmetrical uplift ii both upon and outside the intrusion the model accounts for shallow intrusions i e the ratio a h 5 where a and h are the radius and depth of the intrusion respectively the main parameter of the model is the elastic length l which is a function of the elastic properties of the bending plate and of the elastic foundation the model exhibits two regimes depending on the ratio a l when a l a l 5 the uplift is mostly restricted upon the intrusion when the elastic foundation is very stiff our model converges towards that of a clamped plate we provide as supplementary material a matlab function that calculates the surface uplift from the set of system and control parameters we discuss three possible applications of our model i the model can be used to describe sill propagation by introducing a propagation criterion for realistic values our model reproduces well the behavior of horizontal intrusions simulated in experiments ii the model can also be used to compute the critical size of saucer shaped sills it shows for instance that a soft elastic foundation favors the horizontal spreading of sills before they form inclined sheets iii we show that the classical mogi point source model cannot be used to constrain sill properties from the surface uplift we thus propose that our model can be used as a valuable alternative to both simple analytical models like mogi s and more complex numerical models used to analyze ground deformation resulting from sill intrusions in active volcanoes

cl incorporation into successively zoned amphiboles from the ramnes cauldron norway amphibole from alkali granites in the ramnes cauldron in the oslo rift was altered hydrothermally by corrosion and growth through multiple events of fluid circulation the alteration developed successive zones of amphibole at the crystal margins resulting in a 1 brownish ferro edenitic hornblende core fe 2 deep bluish green hastingsitic hornblende zone hh and 3 light greenish fe rich actinolite rim fa the edenitic core preserves the original igneous amphibole composition the cl content of amphibole strongly increases from the fe 0 78 0 82 wt to the hh zone 2 07 2 96 wt and abruptly decreases in the fa rim 0 01 0 36 wt in the cl rich hh amphibole zone amphibole has characteristically high cl content 2 96 wt 0 82 atoms per formula unit apfu and high concentrations in both 4 al 1 73 apfu and a site occupancy 0 86 apfu with a large k na 1 k value of 0 47 both 4 al and a site occupancy increase systematically with positive correlation with cl content throughout the three amphibole zones on the other hand fe 21 content is not so simply correlated to the cl content based on crystal structure considerations on cl rich amphiboles the cation substitutions are illustrated by structural geometrical constraints for 4 al and by a chemical constraint for fe 21 these contributions for cl incorporation

gas bubble cavities in deltaic muds lake powell delta glen canyon national recreation area hite utah abstract cavities produced by migrating gas mainly methane emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments the delta of the colorado river in lake powell glen canyon national recreation area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids sub meter scale to tens of meter scale structures including craters salses and sand and mud volcanoes are present on the modern lake powell delta abundant sub centimeter scale cavities are found in excavations through crater sediments and exposed along cut bank exposures of the colorado river in cross section the cavities are subvertical irregular to sigmoidal to lenticular shaped 1 3 mm short axes up to 3 0 cm long axes and 1 cm intermediate axes cavities are developed in cm scale laminated silty clay and clay graded beds the tops of the cavities are wider than their bases the cavities observed in the lake powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to molar tooth carbonate structures the characteristics of the cavities and the geometries and deformation of the surrounding delta mud host are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments fractures developed as gas migrated into pre existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation recent sudden decompression related to rapid lake level drops of lake powell may have enhanced the release of gas primarily bacterially produced leading to the development of these soft sediment deformation structures

magma reservoir conditions beneath tsurumi volcano sw japan evidence from amphibole thermobarometry and seismicity abstract calcic amphibole phenocrysts in the tsurumidake summit ts lava which was produced during the most recent eruption at tsurumi volcano sw japan at around 7 5 10 5 ka have been analyzed to determine the pre eruptive conditions such as temperature t pressure p oxygen fugacity fo2 sio2 content sio2melt and feo mgo ratio feo mgomelt of coexisting silicate melts in the magma reservoir beneath the volcano although most of the amphibole phenocrysts have been completely decomposed to a fine grained opaque symplectite 6 of the grains remain intact the degree of amphibole breakdown dab defined as the ratio of the area of symplectite to the area of symplectite plus relict amphibole in each phenocryst varies from 20 to 100 compositional zoning was not observed in the amphibole grains however two distinct groups of amphibole phenocrysts have been identified based on their chemical compositions group i amphiboles which are relatively si poor and 6 al rich and have a relatively high mg 100mg mg fe2 and group ii amphiboles which are si rich and 6 al poor and have a relatively low mg empirical equations for geothermobarometry oxygen barometry and chemometry that exclusively rely on the amphibole composition were applied to estimate the t p fo2 sio2melt feo mgomelt conditions of the silicate melts with which the amphibole crystals equilibrated the results show that group i and group ii amphiboles equilibrated with andesitic melts group i melts and dacitic rhyolitic melts group ii melts respectively the t p fo2 conditions of group i melts were estimated as 374 483 mpa 13 9 17 9 km depth 950 c and nno 1 3 respectively and those of group ii melts were 93 242 mpa 3 4 9 0 km depth 824 913 c and nno 0 6 1 7 respectively the estimated t p fo2 sio2melt feo mgomelt conditions were almost constant for group i melts whereas the t nno and feo mgomelt values of group ii melts decreased with increasing sio2melt the dab decreased as the temperature increased low dab grains are absent in group ii amphiboles our results suggest that 1 a deeper reservoir at a depth of 16 km stored an andesitic melt coexisting with group i amphiboles at 950 c 2 a chemically zoned reservoir at shallower depth 6 4 km was filled with dacitic rhyolitic melts that coexisted with group ii amphiboles at 824 913 c 3 the ts lava was the product of magma mixing of at least three components including the andesitic melt derived from the deeper reservoir the dacitic and rhyolitic melts from the shallower reservoir and a mafic magma derived from the lower crust and or upper mantle and 4 the ts lava eruption occurred within a month or less of the magma mixing

a phenomenological model for precursor volcanic eruptions intense explosions of relatively short duration frequently precede large explosive and effusive volcanic eruptions by as much as weeks to months in the case of very viscous magmas1 2 3 4 5 6 in some cases such pre eruption activity has served as a sufficient warning to those living in the vicinity to evacuate and avoid calamity1 precursor events seem to be related to the formation of a magma pathway to the surface but their precise interpretation is a long standing puzzle it has been inferred from theoretical studies that exsolution of volatiles might create an almost separate gas pocket at the tip of a propagating dyke7 8 9 here we explain the role that such a process may have using a laboratory study of the transient propagation of a liquid filled crack with a gas pocket at its tip that grows with time we show that once the gas pocket acquires sufficient buoyancy to overcome the fracture resistance of the host solid the dynamics of the gas pocket rather than those of the liquid determine the velocity of the crack tip furthermore we find that the gas can ultimately separate from the liquid we propose that fast moving gas rich pockets reaching the surface ahead of the main liquid filled fissure could be the origin of many precursor eruptions

apatite in the dike gabbro transition zone of mid ocean ridge evidence for brine assimilation by axial melt lens we present textures and halogen and trace element compositions of apatites in intrusive rocks from the dike gabbro transition zone of a fast spreading mid ocean ridge which was formed at the east pacific rise and recently drilled by iodp hole 1256d these data are used to discuss the properties of parental magmas and seawater derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust in general zoning of apatites from three different lithologies tonalites diorites and gabbros is common and shows a consistent evolution trend with depletion in cl and rees from core to rim the cores are usually homogenous in composition and interpreted as magmatic origin whereas zones with lower cl and rees are disseminated with heterogeneous concentrations indicating exchanges with hydrothermal fluids the apatite cores in tonalites are rich in both f and cl with x ap f proportion of fluorapatite end member up to 0 5 and x ap cl proportion of chlorapatite end member up to 0 4 in contrast the apatite cores in gabbros have high x ap cl up to 0 85 and very low x ap f s in some parts of the apatite crystals mainly rim can be explained by removal of these components via hydrothermal fluids based on available f cl oh exchange coefficients for apatite melt the very high cl oh and cl f ratios and high cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole bearing metabasalts but indicate that an assimilation of high cl brines must have occurred similarly the low f chlorapatites in gabbros also imply an assimilation of high cl brines the source of high cl fluids in the axial magmatic system may result from seawater derived fluids which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling

an accurate and efficient method for including the effects of topography in three dimensional elastic models of ground deformation with applications to radar interferometry topography has a large effect on the results predicted by elastic surface deformation models in regions of significant relief in some cases topography may have more influence on the predicted deformation field than do model source parameters we have developed an approximate analytical technique for including topographic effects that retains most of the computational simplicity of elastic half space models while providing an accurate representation of topographic effects we use a series expansion of the elastic half space solution with a small slope approximation yielding a set of higher order corrections the integrated effect of these corrections is evaluated using fourier methods we investigate the effectiveness of our method by comparing predicted results for a tilted triaxial ellipsoid with those predicted by finite element models the resulting displacements and displacement gradients are in good agreement with finite element results both for relatively smooth topography synthetically generated and for the greater relief in the vicinity of mount etna volcano we then compare the results of our method with those predicted by traditional elastic half space models using different reference elevations and with a previously proposed method of estimating topographic effects our new method provides a significantly better fit to the finite element results than do the other methods our method is able to accurately portray both the number and the horizontal pattern of fringes in a synthetic interferogram when compared with finite element results in particular the method accurately represents the broadening of fringes that is observed in regions of high relief as well as reproducing the location of the fringe center and the topographically induced deviation of the fringes from a regular pattern elastic half space models typically show a fringe center that is displaced with respect to the finite element results indicating that parameter inversions based on such a model would provide incorrect estimates of the horizontal source position the lack of topographically generated fringe distortions in elastic half space results would also likely lead to inaccuracies in the predicted magnitude and orientation of proposed deformation sources our new technique over comes these limitations and should be useful when evaluating either traditional geodetic results or the greater areal coverage provided by interferometric observations

hydraulic fracturing of soil during laboratory experiments part 1 methods and observations hydraulic fracturing appears to have useful environmental and geotechnical applications but the details of fracture morphology and propagation in soil are poorly known to identify those details

intrusive mechanism of the 2008 2009 mt etna eruption constraints by tomographic images and stress tensor analysis abstract since january 2008 several geophysical parameters have evidenced a recharging phase at mt etna volcano culminating with an effusive eruption that began on may 13 2008 seismic activity recorded at mt etna from january 2007 to may 2008 was analyzed in order to provide seismological constraints to the volcano dynamics leading to the eruption a total of 336 selected earthquakes with m l 1 5 were used as data source for this study specifically we calculated 3d velocity and attenuation tomography including a 3d relocation of the events and we computed 53 selected fault plane solutions fpss that were used for stress tensor inversion the most important result obtained from the joint analysis of v p v p v s and p wave attenuation is an anomalous zone with normal to high v p values between 3 5 and 4 5 km s and low v p v s values 1 64 which partially overlaps with a low q p values 50 volume located along a ns trending channel beneath the central crater this can be interpreted as a shallow volume characterized by high temperature where the magma is located with the presence of supercritical fluids the analysis of seismic stress tensor evidenced an extensional regime in the depth range 3 13 km with a vertically oriented 1 this finding may suggest an extensional stress regime probably related to the kinematic response of the volcanic edifice to both a deep magmatic intrusion and a condition of decreased regional compressive stress facilitated by sliding processes of the eastern flank of the volcano

temporal and spatial variation of local stress fields before and after the 1992 eruptions of crater peak vent mount spurr volcano alaska we searched for changes in local stress field orientation at mount spurr volcano alaska between august 1991 and december 2001 this study focuses on the stress field orientation beneath crater peak vent the site of three eruptions in 1992 and beneath the summit of mount spurr local stress tensors were calculated by inverting subsets of 140 fault plane solutions for earthquakes beneath crater peak and 96 fault plane solutions for earthquakes beneath mount spurr we also calculated an upper crustal regional stress tensor by inverting fault plane solutions for 66 intraplate earthquakes located near mount spurr during 1991 2001 prior to the 1992 eruptions and for 11 months beginning with a posteruption seismic swarm the axis of maximum compressive stress beneath crater peak was subhorizontal and oriented n67 76 e approximately perpendicular to the regional axis of maximum compressive stress n43 w the strong temporal correlation between this horizontal stress field rotation change in position of the 1 3 axes relative to regional stress and magmatic activity indicates that the rotation was related to magmatic activity and we suggest that the crater peak stress field rotation resulted from pressurization of a network of dikes during the entire study period the stress field beneath the summit of mount spurr also differed from the regional stress tensor and was characterized by a vertical axis of maximum compressive stress we suggest that slip beneath mount spurr s summit occurs primarily on a major normal fault in response to a combination of gravitational loading hydrothermal circulation and magmatic processes beneath crater peak online material regional and local fault plane solutions

methane emission through ebullition from an estuarine mudflat 1 a conceptual model to explain tidal forcing based on effective stress changes ebullition is an important pathway for transport of methane ch4 to the atmosphere in wetlands water level changes have been suggested to trigger ebullition especially in tidally flooded areas although the controlling mechanisms remain uncertain bubble transport in submerged sediment represents a multiphase dynamic interaction between gaseous and solid phases under the modulation of a liquid phase an unvegetated sediment monolith was retrieved from an estuarine mudflat area at a tidal marsh site and maintained in a saturated state laboratory measurements on the mud monolith confirmed that not only ebbing tides but also flooding tides could trigger ebullition releases of gas bubbles we develop a changing stress for simulating ebullition csse model to describe mechanisms controlling bubble expansion in response to water level changes to unify these observations decreases in water level are assumed to lower the effective stress surrounding isolated trapped gas bubbles driving upward transport via bubble expansion and deformation with associated fracturing of overlying sediments increases in relative permittivity suggest that additional water invades macropores with associated pore expansion during the initial stage of increases in water level we propose that subsequent matrix expansion under lowered effective stress on rising tides also leads to fracture propagation and bubble release our findings demonstrate the importance of effective stress changes in triggering ebullition from mudflat areas in tidal wetlands modulated by the mechanical properties of shallow soft sediments

dusty and honeycomb plagioclase indicators of processes in the uchino stratified magma chamber izu peninsula japan abstract two types of melt inclusion bearing plagioclase coexist in the uchino basalt izu peninsula japan 1 dusty plagioclase with an ab rich core mantled by a dusty zone consisting of more an rich plagioclase and fine melt inclusions and 2 honeycomb plagioclase with a central core containing larger melt inclusions the dusty plagioclase probably forms through partial dissolution of ab rich plagioclase as dusty zones truncate the concentric zonal structure of the ab rich inner core the honeycomb plagioclase is formed by skeletal growth under supercooling conditions this is indicated by the euhedral geometry of the boundary between the inner inclusion free and the outer inclusion bearing zones of the honeycomb plagioclase which has a concentric inner core bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims the core rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry respectively this suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization these features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber

interaction of highly saline fluid and olivine gabbro experimental simulation of deep hydrothermal processes involving amphibole at the base of the oceanic crust abstract natural occurrences of cl bearing amphibole indicate crystallization in the presence of a highly saline fluid amphibole with varying cl contents 0 1 to 5 wt cl found in lower oceanic crustal rocks demonstrates the activity of saline hydrothermal fluids at depth here we present an experimental study done in cold seal pressure vessels cspv and internally heated pressure vessels ihpv to illustrate the process by which gabbro hosted amphibole rich parageneses evolve in the presence of a hydrothermal saline fluid the starting material used was a natural gabbroic rock containing high ti magnesio hastingsite with the addition of a moderately to highly saline fluid nacl h2o with xnacl 0 02 0 07 and 0 24 6 20 and 50 wt nacl we evaluated a range of conditions from hydrothermal 500 750 c to magmatic 900 c at pressures of 200 mpa and fo2 close to nno fluid rock mass ratios used were 0 2 and 1 in subsolidus hydrothermal experiments and 0 07 in partial melting magmatic experiments new amphibole was formed on rims of the starting high ti magnesio hastingsite with product compositions corresponding to magnesio hastingsite high si magnesio hornblende tschermakite edenite hastingsite and ferro pargasite with varying cl contents up to 0 47 wt cl our results from subsolidus experiments demonstrate a decrease in olivine plagioclase and clinopyroxene in the starting rock and formation of new amphibole with decreased ival and ti with respect to starting amphibole product amphibole does not display any correlation between cl and fe2 ival and k in contrast to natural highly cl rich amphibole suggesting that these correlations hold true only at cl contents higher than those attained in our experiments in addition we found that increasing nacl in the fluid correlates with increased maximum cl contents in amphibole compositional variations found in product amphibole highlight the heterogeneities in fluid infiltration and cl activity that account for the complexity of hydrothermal fluid rock interactions in deep oceanic geological systems

the sr nd and o isotopic studies of the 1991 1995 eruption at unzen japan abstract the magma generation at unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization afc the magma in the unzen volcano is estimated to consist of about 50 80 of residual magma f and about 30 70 assimilated crustal material a relative to the original magma concerning the 1991 1995 eruption it is estimated that the magma formed as the result of mixing of about 50 60 crustal material and about 55 65 of residual magma an alternative magma eruption model for the 1991 1995 eruption is proposed here in the early stage the isotopic characteristics of 1991 eruption are defined by afc process in the deeper magma chamber later the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption the minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber the crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent therefore the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber in contrast the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history the 1991 1995 eruptive rocks of the unzen volcano show their characteristics in sr and nd isotopic values independent of their two previous eruptions however the isotopic values of early eruptive product could represent the original magma value this result also supports the previous work of chen et al 1993 chen c h depaolo d j nakada s shieh y n 1993 relationship between eruption volume and neodymium isotopic composition at unzen volcano nature 362 831 834 that suggested the end of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption

experimental petrology of the 1991 1995 unzen dacite japan part ii cl oh partitioning between hornblende and melt and its implications for the origin of oscillatory zoning of hornblende phenocrysts high temperature pressure experiments were carried out to determine the chlorine hydroxyl exchange partition coefficient between hornblende and melt in the 1992 unzen dacite cl in hornblende and melt was analyzed by electron microprobe whereas oh in hornblende and melt was calculated assuming anion stoichiometry of hornblende and utilizing the dissociation reaction constant for h2o o 1 4 2 oh in water saturated melt respectively the partition coefficient strongly depends on the mg mg fe ratio of hornblende and is expressed as ln k1 1 4 cl oh hb cl oh melt 1 4 2 37 4 6 mg mg fe hb at 2 3 kbar and 800 850 c the twofold variation in cl content in the oscillatory zoned cores of hornblende phenocrysts in the 1991 1995 dacite cannot be explained by the dependence of the cl oh partition coefficient on the mg mg fe hb ratio and requires c 80 variation of the cl oh ratio of the coexisting melt available experimental data at 200 mpa on cl oh fractionation between fluid and melt suggest that c 1 2 1 8 wt degassing of water from the magma can explain the required 80 variation in the cl oh ratio of the melt the negative correlation between al content and mg mg fe ratio in the oscillatory zoned cores of the hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber

modeling fracture propagation and seafloor gas release during seafloor warming induced hydrate dissociation the stability of marine methane hydrates and the potential release of methane gas to the ocean and atmosphere have received considerable attention in the past decade sophisticated hydraulic thermodynamic models are increasingly being applied to investigate the dynamics of bottom water warming hydrate dissociation and gas escape from the seafloor however these models often lack geomechanical coupling and neglect how overpressure development and fracture propagation affect the timing rate and magnitude of methane escape in this study we integrate a geomechanical coupling into the widely used tough hydrate model it is shown that such coupling is crucial in sediments with permeability 10 16 m2 as fracture formation dramatically affects rates of dissociation and seafloor gas release the geomechanical coupling also results in highly nonlinear seafloor gas release which presents an additional mechanism for explaining the widely observed episodic nature of gas flares from seafloor sediments in a variety of tectonic and oceanographic settings

ground deformation patterns at mt etna from 1993 to 2000 from joint use of insar and gps techniques abstract combined gps measurements and radar interferometry insar have been applied at mt etna to study the ground deformation affecting the volcano both over the long 1993 2000 and short term 1997 1998 and 1998 2000 the aim was to better understand the dynamics of the volcano during the magma recharging phase following the 1991 93 eruption since 1993 insar and gps data indicate that mt etna has undergone an inflation a deep intrusion was detected by insar on the western flank of the volcano between march and may 1997 in the following months this intrusion rose up leading to a seismic swarm occurring in january 1998 in the western sector this now shallow intrusion is confirmed by gps data from 1998 to 2000 a general deflation affecting the upper part of the volcano was detected over the whole study period a continuous eastward to south eastward motion of the eastern sector of the volcano was also evidenced the analytical inversions of gps data inferred a plane dipping about 12 ese located beneath the eastern flank of the volcano at a depth of 1 4 km b s l the movement along this plane is able to reproduce the observed south eastward motion of a sector bounded northward by the pernicana fault westward by the north east rift and the south rift and southward by the mascalucia tremestieri trecastagni fault system insar data have validated this model

amphibole perspective to unravel pre eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes a case study from ciomadul volcano se carpathians ciomadul is the youngest volcano in the carpathian pannonian region produced crystal rich high k dacites that contain abundant amphibole phenocrysts the amphiboles in the studied dacites are characterized by large variety of zoning patterns textures and a wide range of compositions e g 6 4 15 wt al2o3 79 821 ppm sr often in thin section scale and even in single crystals two amphibole populations were observed in the dacite low al hornblendes represent a cold 900 c mafic magma amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage 8 12 km this was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple zoned and composite amphiboles this includes reheating by 200 c and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population including pargasites breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes suggesting that pre eruptive heating and mixing could take place within days or weeks before the eruption the decompression rim of pargasites suggests around 12 days of magma ascent in the conduit several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the ciomadul but in our dacite the two amphibole population can be found even in a single crystal composite amphiboles our study indicates that high al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone thus they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes the simple zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the al tschermak substitution can be recognized suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation

grain displacive gas migration in fine grained sediments support for this research was provided by the kaust endowment g e abelskamp edited the manuscript data sets presented as part of this study are available from the kaust repository http hdl handle net 10754 630965

amphibole record of the 1964 plinian and following dome forming eruptions of shiveluch volcano kamchatka abstract shiveluch is one of the most active explosive volcanoes worldwide during the last linian eruption in 1964 and the following 1980 current time dome forming eruptions shiveluch has produced andesites and dacites sio2 60 64 wt containing variably zoned compositionally and texturally diverse amphibole phenocrysts in this work we attempt to decode the complex zoning of the amphibole crystals in the 55 year series of pumice dome rocks and mafic enclaves in order to reconstruct the most recent evolution of the volcano plumbing system the amphibole zoning in shiveluch andesites reveals correlation with the style and date of eruption high al cores mantled by low al rims in amphiboles from the 1964 plinian eruption record a drastic decrease of pressure and rapid magma ascent from the lower crust to the shallow magma chamber typically unzoned and often opacitized low al crystals from the early dome building episodes in 1980 1981 and 1993 1995 reflect magma crystallization in the shallow magma chamber complexly zoned amphiboles from andesites erupted in 2000s indicate replenishment of the shallow magma chamber with mafic magma and syn eruptive mixing processes amphibole based barometric calculations obtained by different approaches indicate that the shiveluch plumbing system is complex and comprises two mafic and silicic magma storage zones at 15 20 km and 5 6 km depths we suggest that both episodes of the plinian eruption in 1964 and the extensive dome growth in 2001 2016 were driven by influx of mafic magma in the shallow storage zone beneath shiveluch the mafic replenishment likely preceded the 1964 plinian eruption and repeatedly occurred during the period of extensive dome growth in 2001 2016 the variable styles of the recent shiveluch eruptions may be controlled by the relative volume of the mafic recharges and their thermal and viscosity effects on the efficiency of magma mixing

sediment damage caused by gas exsolution a key mechanism for mud volcano formation abstract gassy sediments are common in marine environments and are characterized by a specific mechanical behavior significantly different from that of water saturated sediments it is shown that gas causes damage and initiates fractures in sediments to define the controlling parameters dominating the damage process during gas exsolution and its consequences in terms of hydro mechanical behavior we developed a specific consolidation apparatus to test sediments collected from the active absheron mud volcano indeed mud volcano formation is initiated by gas exsolution and subsequent mud generation at depth from stratified sediments gas was generated in the samples by circulating carbonated water through the fine grained sediments then decreasing the total pressure particular attention was given to the impact of gas saturation and associated damage and fractures on p wave velocity sediment compressibility permeability and preconsolidation pressure results show that fracture geometry is mainly controlled by the degree of gas saturation and preconsolidation pressure the damage level increases with the degree of gas saturation while the elastic modulus of sediments is degraded experimental data show that sediments do not entirely recover their original mechanical behavior when gas dissipates finally the experimental data confirm that gas exsolution and expansion is a key mechanism for mud generation

volcano deformation and eruption forecasting recent advances in global positioning system gps tilt and interferometric synthetic aperture radar insar have greatly increased the availability of volcano deformation data these measurements combined with appropriate source models can be used to estimate magma chamber depth and to provide information on chamber shape and volume change however kinematic models cannot constrain magma chamber volume and provide no predictive

viscosity of andesite melts and its implication for magma mixing prior to unzen 1991 1995 eruption abstract the viscosity of an iron bearing melt with composition similar to unzen andesite was determined experimentally in the high 10 9 10 10 5 pa s and low 5 1000 pa s viscosity range using a parallel plate viscometer and the falling sphere method respectively falling sphere experiments were carried out in an internally heated argon pressure vessel and in a piston cylinder apparatus at 1323 to 1573 k and 200 to 2000 mpa creep experiments were performed in the temperature range of 747 845 k at 300 mpa the water content of the melt varies from nominally dry to 6 2 wt h 2 o the fe 2 fe tot ratio was determined for each sample in the quenched glass using a colorimetric method pressure has minor influence on the viscosity compared with the effect of temperature water content main compositional parameter controlling the viscosity or with the fe 2 fe tot ratio especially important at low water content of the melt based on our new viscosity data and literature data with measured fe 2 fe tot ratio we propose a new empirical equation to estimate the viscosity in pa s of andesitic melts as a function of temperature t in k water content w in wt and fe 2 fe tot ratio the derived relationship reproduces the experimental data 87 in total in the viscosity range from 10 0 5 to 10 13 pa s with a 1 standard deviation of 0 17 log units however application of this calculation model is limited to fe 2 fe tot fe tot 0 3 and to temperatures above t g moreover in the high viscosity range the variation of viscosity with water content is constrained only by few experimental data and needs verification by additional measurements the viscosity data are used to interpret mixing processes in the unzen magma chamber prior to 1991 1995 eruption we demonstrate that the viscosities of the rhyolite and andesite melts from the two end member magmas are nearly identical prior and during mixing enabling efficient magma mixing

dissolution origin and syn entrapment compositional change of melt inclusion in plagioclase abstract in the system di ab an h 2 o melt inclusions in plagioclase were reproducibly formed in a series of partial dissolution experiments and the compositional differences from the surrounding matrix melt were evaluated at 200 mpa and 1050 1150 c a seed crystal of an 59 was partially dissolved into the matrix melt which was in equilibrium with more calcic plagioclase than the seed the reaction proceeded inward from the crystal surface by a dissolution and recrystallization process to form a reaction zone composed of melt channels and recrystallized calcic plagioclase the melt channels were choked necked off by the recrystallized plagioclase at the crystal surface to create melt inclusions the experimentally formed inclusions having square and zigzag shapes up to a few tens of micrometers in size are similar in texture to some common types of natural melt inclusion since the partial dissolution of phenocrysts can occur during various magma mixing processes it should be taken into consideration as one of the inclusion formation mechanisms when trying to interpret information from natural melt inclusions especially for those in hydrous arc magmas the chemical composition of the reproduced melt inclusions deviates from the matrix melt basically along the plagioclase liquidus isotherm the molar ratio of ca ca na and the mgo concentration in the synthesized melt inclusion are up to 17 and 61 relative lower than those in the matrix melt respectively this compositional difference is caused by a combination of factors 1 supersaturation with dissolved plagioclase component 2 insufficient diffusive homogenization in the melt channels due to rapid dissolution of plagioclase and 3 the choking off effect on the basis of the experimental results a model for diffusion in a growing melt channel in dissolving plagioclase was constructed which revealed that the concentration of incompatible elements with low diffusivities can be significantly lower in the melt inclusions than that in the matrix melt these results demonstrate the importance of petrography for interpretation of inclusion data and of more experimental studies on melt inclusion origin

mechanical response of sediments to bubble growth modeling the process of bubble growth in sediments requires an understanding of the physics that controls bubble shape and the interaction of the growing bubble with the sediment to acquire this understanding we have conducted experiments in which we have injected gas through a fine capillary into natural and surrogate sediment samples and have monitored pressure during bubble growth to provide information about stress and strain in gas injection studies with natural sediment samples we have observed two modes of bubble growth behavior one of these modes characterized by a saw tooth record of pressure as the bubble grows is consistent with fracture of the medium observations indicate that bubble growth by fracture should correspond to bubbles that are coin or disk shaped this shape is confirmed in observations of bubbles in natural sediments and in our studies of bubble injection into gelatin a surrogate sediment material interpretation of the stress strain results for bubble growth also required that we measure young s modulus e the measurements show e to be near 0 14 mn m2 which differs by more than 4 orders of magnitude from values that have been reported in the literature our measurements of e give substantially better estimates of bubble shape than are predicted using the literature values our data are interpreted with linear elastic fracture mechanics lefm which predicts that the critical pressure for bubble growth will depend on the bubble volume v raised to the 1 5 power while evidence of substantial heterogeneity in sediment properties is apparent in our results this v 1 5 dependence is confirmed through application of lefm theory we have determined the critical stress intensity factor k1c a material property and the principal determinant of bubble shape and growth by fracture our values of k1c range from 2 8 10 4 mn m 3 2 to 4 9 10 4 mn m 3 2 for our natural sediment samples from cole harbor nova scotia we have also estimated the critical stress intensity factor for eckernforde bay samples by analyzing published images of natural bubbles the k1c obtained in this way is similar to our cole harbor results and is 5 5 10 4 mn m 3 2

first order description of the mechanical fracture behavior of fine grained surficial marine sediments during gas bubble growth 1 bubbles in sediments imaged via computed tomography ct scanning and in surrogate transparent material gelatin are well described geometrically as eccentric oblate spheroids while sediments are undoubtedly visco elasto plastic solids only part of that complex behavior appears to influence significantly the formation and shape of gas bubbles specifically the shape of these bubbles can be explained if the mechanical response of fine grained sediment is approximated by linear elastic fracture mechanics lefm to determine the adequacy of the lefm approximation for gas bubble growth in fine grained sediments a number of gas bubbles were injected and grown in natural sediments while monitoring the size and shape using an industrial ct scanner a comparison of measured inverse aspect ratios iars of the injected bubbles with calculated iars from pressure records provides support for the lefm theory deviations from lefm are observable in the data but as bubbles grow larger they trend more closely toward the theory the use of lefm has been shown to describe gas bubble growth in shallow coastal sediments to first order

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

insight into mt etna italy kinematics during the 2002 2003 eruption as inferred from seismic stress and strain tensors 1 seismic activity linked to the 2002 03 mt etna eruption was investigated by analyzing the md 2 3 earthquakes the results of 3d relocation were used to compute fault plane solutions and a selected dataset was inverted to determine stress and strain tensors the analysis revealed a complex kinematic response of the eastern flank dominated by fast stress propagation and reorientation we hypothesize that a vertical dike intruded the southern flank generating an extensional regime that triggered a radial intrusion in the northeast sector of the volcano the combined effects gave rise to a rotation of the stress tensor that controlled the activation of the pernicana fault system the volcanic and tectonic interactions produced a second reorientation of the stress tensor causing a structural response in the southeast lower flank the overall result of the deformation processes observed during the eruption was an e w extension on the eastern flank of the volcano

exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt implications for chlorine and metal transport abstract the compositional variation of a cl bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt na k c1 h 2 o with attention to the effect of vaporliquid immiscibility on the aqueous fluid above critical pressures for the aqueous fluid p 1 5 kb at 800 c where the fluid is single phase the cl concentration in the aqueous fluid c cl aq gradually decreases with crystallization of a silicate melt under rayleigh fractionation as demonstrated by previous studies however the variation of ca is quite different below critical pressures where vapor liquid immiscibility occurs in particular at low pressures p cl h 2 o ratio in a melt is small and ca will increase with crystallization a high cl h 2 o ratio at low pressure results in liquid exsolution and decreasing ca with crystallization eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase during this simultaneous exsolution the composition of the exsolving vapor and liquid phases the mass ratio of these phases and the cl concentration of the silicate melt will all remain constant comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid therefore simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor liquid immiscibility model calculations at 0 6 and 1 2 kb indicate that major amounts of cl in the system will be distributed to the liquid phase particularly at the lower pressure this suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions

h2o in rhyolitic glasses and melts measurement speciation solubility and diffusion dissolved h2o in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution in this paper major progress on several aspects of the h2o melt or glass system is reviewed consistency among a variety of data is investigated discrepancies are evaluated and confusion is clarified on the infrared measurement of total h2o and species concentrations calibration for a variety of glasses has been carried out at room temperature the measurements for h2o in rhyolitic glasses have undergone the most scrutiny resulting in the realization that absorptivities for the near infrared bands depend on total h2o content although the variation of the absorptivities does not seem to significantly affect the determination of total h2o it does affect the determination of molecular h2o and oh species concentrations calibration of the infrared technique for h2o in rhyolitic glasses still needs much improvement especially at high total h2o furthermore it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies hence it will be necessary to carry out calibrations in situ at high temperatures on h2o speciation results from two experimental approaches the quench technique and the in situ technique are very different leading to controversy in our understanding of true speciation a solution is presented to reconcile this controversy it is almost certain that the quench technique does not suffer from a quench problem but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature accurate calibration at high temperatures is necessary for the quantitative application of the in situ technique to h2o speciation in silicate melts and glasses on h2o solubility in silicate melts recent experimental work has significantly expanded the t p range of solubility measurements and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions i present a solubility model for rhyolitic and quasi rhyolitic melts over a wide range of t and p 500 1350 c 0 8 kbar by incorporating the role of speciation the solubility model is able to recover the experimental solubility data and has extrapolative value although the partial molar volume of h2o derived from the solubility model differs from that derived from density measurements on h2o diffusion recent studies on h2o diffusion in a quasi rhyolitic melt at 800 1200 c 0 5 5 kbar and up to 7 total h2o not only provide important new diffusion data but are also challenging earlier understanding of h2o diffusion based on data in rhyolitic glasses at 400 550 c 1 bar and 0 2 1 8 total h2o a comparison between the earlier model and recent data is made the recent high temperature diffusivities at total h2o 2 can be predicted by the earlier model however at higher total h2o the earlier model fails new work is under way to understand the diffusion mechanisms at high h2o contents

gas invasion into water saturated unconsolidated porous media implications for gas hydrate reservoirs abstract gas transport through unconsolidated porous media has been a recent focus in studies of hydrate reservoir dynamics and is important in numerous other geophysical contexts we conduct experiments to examine the controls on gas invasion through sieved sands that are suspended within the water column as air is injected slowly from below we monitor the thickness of the free gas column that accumulates beneath the sand to gauge the difference between the gas and liquid pressure or overpressure just prior to gas invading and passing through the sand layer comparing the threshold overpressures for initial gas invasion using different particle sizes our experiments demonstrate the control of capillary forces when the effective stress supported by particle contacts is sufficiently large at lower effective stresses smaller sediment overburdens we observe gas invasion before the capillary controlled threshold overpressure is reached these results demonstrate a transition from capillary invasion at high effective stresses to fracture dominated invasion at low effective stresses moreover video documentation reveals evidence for transitions from capillary to fracture dominated behavior within single invasion episodes as the rising gas nears the sediment surface after the initial invasion episode in each experiment we infer that observed reductions in the overpressure needed for subsequent invasions are caused by bubble break up and retention within the sediment column these experiments highlight the importance of effective stress and pore size in determining the mode of gas passage through unconsolidated sediments we discuss the implications of our findings for the dynamics of gas hydrate reservoirs and other geophysical systems

generation of porphyritic and equigranular mafic enclaves during magma recharge events at unzen volcano japan mafic to intermediate enclaves are evenly distributed throughout the dacitic 1991 1995 lava sequence of unzen volcano japan representing hundreds of mafic recharge events over the life of the volcano this study documents the morphological textural chemical and petrological characteristics of the enclaves and coexisting silicic host lavas the eruptive products described in this study appear to be general products of magma mingling as the same textural types are seen at many other volcanoes two types of magmatic enclaves referred to as porphyritic and equigranular are easily distinguished texturally porphyritic enclaves display a wide range in composition from basalt to andesite are glass rich spherical and porphyritic and contain large resorbed plagioclase phenocrysts in a matrix of acicular crystals and glass equigranular enclaves are andesitic non porphyritic and consist of tabular medium grained microphenocrysts in a matrix glass that is in equilibrium with the host dacite magma porphyritic enclaves are produced when intruding basaltic magma engulfs melt and phenocrysts of resident silicic magma at their mutual interface equigranular enclaves are a product of a more prolonged mixing and gradual crystallization at a slower cooling rate within the interior of the mafic intrusion

partitioning of fluorine and chlorine between biotite and granitic melt experimental calibration at 200 mpa h 2 o experiments from 640 to 680 c 200 mpa h2o at fo2 nno employing a natural f rich vitrophyric rhyolite from spor mountain utah assessed the effect of variable mg 100mg mg mn fe on the partitioning of fluorine and chlorine between biotite bt and melt over this temperature interval bt fluorite quartz is the sole liquidus phase partition coefficients for fluorine between biotite and glass dfbt melt show a strong dependence on the mg of bt dfbt melt varies from 1 5 to 7 2 over the range of mg from 21 to 76 a strong linear correlation between dfbt melt and mg has a slope of 9 4 and extrapolates through the origin i e dfbt melt 0 at mg 0 an annite siderophyllite solid solution in these experiments dclbt melt values 1 to 6 in the same experiments vary inversely with mg the al content of biotite does not vary with the aluminum saturation index asi molar al2o3 alkali and alkaline earth oxides of melt but two exchange mechanisms involving al appear to operate in these micas 1 alvi aliv siiv mgiv and mgiv 2aliv 2siiv iv the effects of other components such as li or other intensive parameters including fo2 have yet to be evaluated systematically at comparable mg of bt however the spor mountain rhyolite yields higher dfbt melt values than an li rich strongly peraluminous melt previously investigated the results indicate that the mg of bt exerts the principal control on halogen partitioning with asi and t as second order variables the experimental partition coefficients compare well with other experimental results but not with most volcanic rocks magmatic bt from most rhyolites records higher dfbt melt due to reequilibration with degassed h2o depleted magma and perhaps with f2o 1 exchange that may accompany oxidation fe3 o fe2 oh 1 this behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near morococala bolivia bt is sharply zoned with f rich rims but bt core melt inclusion pairs fall on our experimental curve for dfbt melt these experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks for plutonic rocks the actual f content of melt not a relative activity ratio involving hf species can be reasonably estimated if the mica has not undergone subsolidus reequilibration this information is potentially useful for some shallow level ca poor magmas that are thought to be rich in f e g a and s type granites but do not conserve f well as rocks

volatile variations in magmas related to porphyry cu au deposits insights from amphibole geochemistry duolong district central tibet ore forming fluid exsolution in a shallow magma chamber is a critical step in the formation of porphyry cu mo au deposits but one for which direct evidence is rarely found here ci abundance major trace element content and h isotope analysis of amphibole in diorite and barren granodiorite porphyry samples from the duolong porphyry epithermal cu au district are presented in order to reveal processes associated with fluid exsolution and magma evolution low al mg hornblende formed in ore bearing diorite at 750 860 degrees c and similar to 80 200 mpa on primitive mantle normalized diagrams these low al amphibole grains have slightly convex upward ree patterns with distinctly negative eu anomalies and negative anomalies in pb sr eu zr hf and ti anomalies suggesting crystallization from the same arc magma after plagioclase and magnetite crystallization a large variation in estimated melt h2o similar to 6 to 3 wt and ci content 0 09 0 38 wt as well as low 8d values 103 to 113 o indicate that the magma underwent large scale fluid exsolution contributing to the formation of duolong cu au mineralization additionally amphibole from a barren granodiorite porphyry shows two distinct populations distinguished by their a1 content this intrusion has low al amphibole mghornblende which formed at similar conditions to low al amphiboles in the diorite 790 870 degrees c and similar to 100 230 mpa whereas high al amphibole tschermakite formed at similar to 880 970 degrees c and degrees 210 400 mpa some high al amphibole phenocrysts have slightly convex ree patterns with no negative eu anomalies a depletion in nb zr and hf and positive sr ba and pb anomalies likely consistent with amphibole crystallization from more mafic basaltic andesitic melts low al amphibole in the granodiorite porphyry shows different compositional trends compared to those in the diorite suggesting that they crystallized from different magmas considering the evidence for mafic magma replenishment suggested by the positive correlation between al iv and mg values in high al amphibole the low al amphibole in the barren granodiorite porphyry likely crystallized from a new hybrid magma with mafic to intermediate felsic magma compositions meanwhile smaller variations in ci content 0 08 0 24 wt in low al amphibole was dominantly controlled by magma evolution shown by variable mg rather than fluid exsolution thus the low 8d values 102 to 122 o were likely inherited from evolved dioritic magmas that underwent fluid exsolution importantly the ci compositional variation of amphibole has a potential application as a powerful tool to identify ore bearing and barren intrusions within porphyry cu mo au deposits

stepwise accumulation and ascent of magmas one of the currently popular theories on magma ascent is that it mainly occurs by propagating hydrofractures dykes and that magma viscosity is the primary rate controlling factor this theory is based on mathematical models for single hydrofractures under idealised conditions we simulated magma ascent with air ascending through gelatine and observed that the air ascended in batches following paths made by their predecessors multiple batches accumulate at obstacles along the path although magma viscosity may control ascent rate during movement obstacles ultimately control the size and average ascent velocity of ascending batches we propose that step wise movement of magma batches is the mechanism of primary accumulation and ascent from the partially molten source rock of a magma to its first emplacement site and therefore the main ascent mechanism for granitic magmas classical dyking is the mechanism for secondary ascent from a magma chamber

strain and stress fields in the southern apennines italy constrained by geodetic seismological and borehole data summary we present an improved evaluation of the current strain and stress fields in the southern apennines italy obtained through a careful analysis of geodetic seismological and borehole data in particular our analysis provides an updated comparison between the accrued strain recorded by geodetic data and the strain released by seismic activity in a region hit by destructive historical earthquakes to this end we have used nine years of gps observations 2001 2010 from a dense network of permanent stations a data set of 73 well constrained stress indicators borehole breakouts and focal mechanisms of moderate to large earthquakes and published estimations of the geological strain accommodated by active faults in the region although geodetic data are generally consistent with seismic and geological information previously unknown features of the current deformation in southern italy emerge from this analysis the newly obtained gps velocity field supports the well established notion of a dominant ne sw oriented extension concentrated in a 50 km wide belt along the topographic relief of the apennines as outlined by the distribution of seismogenic normal faults geodetic deformation is however non uniform along the belt with two patches of higher strain rate and shear stress accumulation in the north matese mountains and in the south irpinia area low geodetic strain rates are found in the bradano basin and apulia plateau to the east along the ionian sea margin of southern italy in southern apulia and eastern basilicata and calabria geodetic velocities indicate nw se extension that is consistent with active shallow crustal gravitational motion documented by geological studies in the west along the tyrrhenian margin of the campania region the tectonic geodetic field is disturbed by volcanic processes comparison between the magnitude of the geodetic and the seismic strain rates computed using a long historical seismicity catalogue allow detecting areas of high correlation particularly along the axis of the mountain chain indicating that most of the geodetic strain is released by earthquakes this relation does not hold for the instrumental seismic catalogue as a consequence of the limited time span covered by instrumental data in other areas e g murge plateau in central apulia where seismicity is very low or absent the yet appreciable geodetic deformation might be accommodated in aseismic mode overall the excellent match between the stress and the strain rate directions in much of the apennines indicates that both earthquakes and ground deformation patterns are driven by the same crustal forces

ground deformations and volcanic processes as imaged by cgps data at mt etna italy between 2003 and 2008 1 we analyze the kinematic and crustal deformations of mt etna from 2003 to 2008 as imaged by the mt etna continuous gps cgps network etn net through a careful analysis of gps time series six coherent phases of crustal deformations have been identified three inflation phases and three deflation phases superimposed on a major inflation of the volcanic edifice since 2001 the inversions of gps velocities have enabled 1 a better understanding of the evolution of the volcanic sources acting beneath the volcano 2 analysis of the strain rate patterns and 3 a delineation of potential coupling between volcanic sources and the observed ground deformations the modeling of the pressure sources has shown a separation between inflation and deflation sources the deflation sources show an upward migration from 5 5 toward 2 0 km b s l while the inflation sources are located within 5 5 and 4 0 km b s l our results indicate that the kinematic and ground deformations of the mid upper eastern flank are driven by the interplay between the effect of the magmatic sources and a southeastward motion furthermore clockwise rotations have been detected that prevailed over the eastern motion of the flank during the inflation phases preceding the 2004 2005 and 2006 eruptions finally the accordance between the higher geodetic shear strain rates and the area with the highest seismic energy release shows that measured geodetic shear strain rates can provide useful information on the potential occurrence of seismic activity

magma intrusion mechanisms and redistribution of seismogenic stress at mt etna volcano 1997 1998 terra nova 23 339 348 2011 abstract several volcanoes worldwide have shown changes in their stress state as a consequence of the deformation produced by the pressurization of a magmatic body this study investigated seismic swarms occurring on the western flank of mt etna in january 1997 january 1998 integrating seismic observations and geodetic data we constrained the seismogenic fault system and on the basis of stress tensor inversion and shmax analyses we inferred an inflating pressure source located at 5 5 km b s l beneath the west portion of summit area evaluation of coulomb failure stress related to the proposed model showed how a large part of the seismogenic fault underwent a significant coulomb failure stress increase 500 kpa we inferred the presence of a sub vertical faulted region potentially weak n50 e oriented beneath the western sector of mt etna this structure could be brought closer to failure thereby generating seismic swarms as the effect of elastic stress transfer induced by movement and or overpressure of magmatic masses within the upper crust under the volcano

petrology and fe ti oxide reequilibration of the 1991 mount unzen mixed magma a dacitic magma 64 5 wt sio2 a mixture of phenocryst rich rhyodacite and an aphyric mafic magma was erupted during the recent 1991 1995 mount unzen eruptive cycle the experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas and the nature of the mixing process the preeruption rhyodacitic magma was at a temperature of 790 20 c according to fe ti oxide phenocryst cores and at a depth of 6 to 7 km 160 mpa according to al in hornblende geobarometry the mafic magma that mixed with the rhyodacite is found as andesitic 54 to 62 wt sio2 enclaves in the erupted magma and was essentially aphyric when intruded phase equilibria indicate that an aphyric andesite at 160 mpa is 1030 c h2o saturated and possibly as high as 1130 c 2 wt h2o the composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt sio2 using these compositions and temperatures the temperature of the unzen magma after mixing is estimated to be at least 850 to 870 c the groundmass fe ti oxide microphenocrysts and those in pargasite bearing reaction zones around biotite phenocrysts both give 890 20 c temperatures the oxide oxide contacts give temperatures of 910 20 c the 900 30 c postmixing temperatures are consistent with phase equilibria experiments which show that the magma was not above 930 c at 160 mpa our fe ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption which is a shorter time than is calculated using available diffusion data there is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma

the physics of bubbles in surficial soft cohesive sediments abstract this paper reviews current understanding of the physics of bubbles in soft unlithified cohesive sediments certainly in the top 100 m from the sediment water interface but likely much deeper experimental evidence primarily ct scans and internal pressure records indicate that near surface bubbles grow by elastically compressing and fracturing these sediments which results in thin irregular disks of gas these data do not suggest either plastic or fluid behavior on the part of the bulk sediment in addition there is no hint of capillary invasion of gas into the pores of fine grained sediments the growth rate of an elastic fracturing disk shaped bubble can vastly exceed that of a spherical bubble depending on the eccentricity of the bubble this effect results from the far more favorable surface to volume ratio of flattened bubbles the initial rate of rise of bubbles in cohesive sediments also appears to be governed by a fracture process which is driven by pseudo buoyancy the rate of bubble rise could be controlled by various possible mechanisms but only the predictions from a visco elastic fracture propagation model seem to produce rise velocities that are not ballistic experimental evidence demonstrates that a rising bubble can deviate from its path to the surface if it encounters a zone of sediment with lower lateral fracture toughness such a bubble will begin to move in a lateral direction but stall because pseudo buoyancy is no longer operative subsequent bubbles will do the same and accumulate in the low fracture toughness zone this explains the occurrence of shallow sub seafloor domes of gas and modeling of this process predicts only modest over pressures in such structures

stability of bubbles in a linear elastic medium implications for bubble growth in marine sediments 1 methane bubbles in muddy fine grained sediments grow initially through a process of elastic expansion punctuated by discrete fracture events lefm growth the ability of the surrounding sediments to support a stress and actively resist expansion can under conditions of low gas production or high sediment toughness result in the cessation of growth and the presence of stable bubbles thus it is possible for a bubble to stop growing despite the presence of a source in the sediments that continues to produce gas this contrasts with growth of bubbles in a fluid medium which cannot support a stress and so will continue to grow as long as a surrounding source provides gas this no growth condition is the result of the coupling between gas supply methane production or supersaturation and the sediment mechanics here we quantify this condition and present a criterion for the switch between no growth and the lefm growth regimes we apply this theory to the sediments of eckernforde bay in the kiel bight germany and despite the absence of measurements for the key sediment mechanical properties we can provide a qualitative explanation for the sizes and shapes of the observed bubble population with depth in the sediment we also show how the release of hydrostatic pressure can stimulate growth by pushing otherwise stable bubbles into the lefm growth regime this could provide a mechanism for the release of bubbles during periods of low water such as during low tide or wave events

a joint inversion of ground deformation and focal mechanisms data for magmatic source modelling the paucity of geodetic data acquired on active volcanoes can make the understanding of modelling magmatic systems quite difficult in this study we propose a novel approach which allows improving the parameter estimation of analytical models of magmatic sources e g shape depth dimensions volume change etc by means of a joint inversion of surface ground deformation data and p axes of focal plane solutions the methodology is first verified against a synthetic dataset of surface deformation and strain within the medium and then applied to real data from an unrest episode occurred before the may 13 2008 eruption at mt etna italy the main results clearly indicate the joint inversion improves the accuracy of the estimated source parameters by about 70 the statistical tests indicate that the source depth is the parameter with the highest increment of accuracy in addition a sensitivity analysis confirms that displacements data are more useful to constrain the pressure and the horizontal location of the source than its depth while the p axes better constrain the depth estimation

microlite transfer by disaggregation of mafic inclusions following magma mixing at soufriere hills volcano montserrat the soufriere hills volcano on montserrat has for the past 12 years been erupting andesite with basaltic to basaltic andesite inclusions the andesite contains a wide variety of phenocryst textures and strongly zoned microlites analysis of minor elements in both phenocrysts and microlites allows us to put detailed constraints on their origins compositions of clinopyroxene from overgrowth rims on quartz and orthopyroxene and coarse grained breakdown rims on hornblende are identical to those from the mafic inclusions indicating that these rims form during interaction with mafic magma in contrast resorbed quartz and reversely zoned orthopyroxenes form during heating microlites of plagioclase and orthopyroxene are chemically distinct from the phenocrysts being enriched in fe and mg and al and ca respectively however microlites of plagioclase orthopyroxene and clinopyroxene are indistinguishable from the compositions of these phases in the mafic inclusions we infer that the inclusions disaggregated under conditions of high shear stress during ascent in the conduit transferring mafic material into the andesite groundmass the mafic component of the system is therefore greater than previously thought the presence of mafic derived microlites in the andesite groundmass also means that care must be taken when using this as a starting material for phase equilibrium experiments

magmatic controls on eruption dynamics of the 1950yrb p eruption of san antonio volcano tacana volcanic complex mexico guatemala abstract san antonio volcano in the tacana volcanic complex erupted 1950 yr b p with a pelean type eruption that produced andesitic pyroclastic surges and block and ash flows destroying part of the volcano summit and producing a horse shoe shaped crater open to the sw between 1950 and 800 yr b p the eruption continued with effusive andesites followed by a dacite lava flow and a summit dome all from a single magma batch all products consist of phenocrysts and microphenocrysts of zoned plagioclase amphibole pyroxene magnetite ilmenite set in partially crystallized groundmass of glass and microlites of the same mineral phases except for the lack of amphibole included in the andesitic blocks of the block and ash flow deposit are basaltic andesite enclaves with elongated and ellipsoidal forms and chilled margins the enclaves have intersertal textures with brown glass between microphenocrysts of plagioclase hornblende pyroxene and olivine and minor proportions of phenocrysts of plagioclase hornblende and pyroxene a compositional range obtained of blocks and enclaves resulted from mixing between andesite 866 c 22 and basaltic andesite enclaves 932 c 22 which may have triggered the explosive pelean eruption vestiges of that mixing are preserved as complex compositional zones in plagioclase and clinopyroxene rich reaction rims in amphibole in the andesite whole rock chemistry geothermometry experimental petrology and modeling results suggest that after the mixing event the eruption tapped hybrid andesitic magma 900 c and ended with effusive dacitic magma 825 c all of which were stored at 200 mpa water pressure a complex open system evolution that involved crustal end members best explains the generation of effusive dacite from the hybrid andesite amphibole in the dacite is rimmed by reaction products of plagioclase orthopyroxene and fe ti oxides produced by decompression during ascent amphibole in the andesite however lacks such rims because the andesite was at 866 22 c and the dacite was at 825 c the reaction rims indicate that the andesitic magma ascended at 0 023 m s 1 during the explosive phase of the eruption whereas the dacitic magma rose more slowly at 0 002 0 004 m s 1

numerical models of volcanotectonic earthquake triggering on non ideally oriented faults 1 calculation of inflation induced coulomb stress changes on faults of varying orientation indicates that local faults of almost any orientation are susceptible to slip in response to dike inflation additionally as dikes typically inflate in the direction of regional minimum compressive stress the sense of inflation induced strike slip faulting predicted by coulomb stress change models is opposite to that expected from regional tectonic forces observations of volcanotectonic vt earthquake fault plane solutions with pressure p axes oriented 90 to regional maximum compression confirm this prediction in detail imperfect agreement between coulomb stress change models and the ranges of fault plane orientations observed in vt fault plane solution studies and observed locations of vt seismicity indicates that the orientation and location of pre existing faults around the conduit ultimately determine the seismic response of host rock to an inflating dike

transient growth of an isolated bubble in muddy fine grained sediments abstract methane bubbles in fine grained sediments have been shown to grow initially by elastic expansion and fracture a previous growth model assumed quasi steady state diffusion in which the methane porewater concentration quickly adjusted to changes in bubble geometry gardiner b s boudreau b p and johnson b d 2003a growth of disk shaped bubbles in sediments geochim cosmochim acta 67 8 1485 1494 here we present a finite element model that solves the transient form of the reaction diffusion equation and the coupled linear elastic fracture mechanics lefm in so doing we also employ a new theory for the post fracture bubble sizes based upon the full principles of lefm our findings indicate that the quasi steady state assumption is flawed due to violation of conservation of mass during fracture events when the new model is applied to sediment conditions found at cape lookout bight nc usa it is found that bubbles grow somewhat faster than previously thought a reference bubble of 0 5 cm 3 will form in about 6 days 2 5 days quicker than the old model predicted moreover typical bubbles of 0 04 cm 3 for this site can grow in as little as a day and a half we examined the sensitively of the finite element model to the various parameters in order to gain an understanding of how bubbles may behave under different sediment conditions the influence of tides on bubble growth through the process of rectified diffusion was also examined and it was found that this had little influence upon growth

release of multiple bubbles from cohesive sediments 1 methane is a strong greenhouse gas and marine and wetland sediments constitute significant sources to the atmosphere this flux is dominated by the release of bubbles and quantitative prediction of this bubble flux has been elusive because of the lack of a mechanistic model our previous work has shown that sediments behave as elastic fracturing solids during bubble growth and rise we now further argue that bubbles can open previously formed partially annealed rise tracts fractures and that this mechanism can account for the observed preferential release at low tides in marine settings when this mechanical model is applied to data from cape lookout bight nc usa the results indicate that methanogenic bubbles released at this site do indeed follow previously formed rise tracts and that the calculated release rates are entirely consistent with the rise of multiple bubbles on tidal time scales our model forms a basis for making predictions of future bubble fluxes from warming sediments under the influence of climate change

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

initial rise of bubbles in cohesive sediments by a process of viscoelastic fracture 1 an understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid e g magmas and hydrofractures appear not to be applicable to bubbles in soft sediments and this paper presents a new model for gas bubble rise in soft fine grained cohesive sediments bubbles in such sediments are essentially dry little if any free water and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics which assume the existence of a spectrum of flaws within the sediment fabric by extending this theory we predict that bubbles initially rise by preferential propagation of a fracture in a sub vertical direction we present a criterion for initial bubble rise once rise is initiated the speed of rise is controlled by the viscoelastic response of the sediments to stress using this new bubble rise model we estimate rise velocities to be of the order of centimeters per second we again show that capillary pressure plays no substantive role in controlling bubble growth or rise

predictions of the acoustic scattering response of free methane bubbles in muddy sediments the response of the sediments of eckernfoerde bay germany to acoustic remote sensing previously has been attributed qualitatively to gas features which were postulated to exist in situ within the sediment the existence of features as small as 0 5 mm equivalent spherical radius has now been confirmed by x ray computed tomography of cores taken and scanned at in situ pressures the interaction of an acoustic pulse from the acoustic sediment classification system ascs with this type of gassy sediment was modeled and the results are presented here the bubble scattering response model includes both the effects of nonspherical bubbles and of sediment resistance to shear deformation as expressed by the dynamic shear modulus model predictions made using the observed gas bubble distribution agree with normal incidence ascs data exhibiting extended returns i e greater than the source pulse length from the seafloor bubble layers as well as high attenuation within the bubble layers spectrograms of ascs and modeled return pulses were also similar these results suggest that the acoustic return characteristics of the near surface sediments of eckernfoerde bay are dominated by scattering from a distribution of gas bubbles with depth

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods