gas evolution in eruptive conduits combining insights from high temperature and pressure decompression experiments with steady state flow modeling abstract in this paper we examine the consequences of bubble nucleation mechanism on eruptive degassing of rhyolite magma we use the results of published high temperature and pressure decompression experiments as input to a modified version of conflow the numerical model of mastin and ghiorso 2000 u s g s open file rep 00 209 53 pp and mastin 2002 geochem geophys geosyst 3 10 1029 2001gc000192 for steady two phase flow in vertical conduits synthesis of the available experimental data shows that heterogeneous nucleation is triggered at p 120 150 mpa and leads to disequilibrium degassing at extreme h2o supersaturation in this latter case nucleation is an ongoing process controlled by changing supersaturation conditions exponential bubble size distributions are often produced with number densities of 106 109 bubbles cm3 our numerical analysis adopts an end member approach that specifically compares equilibrium degassing with delayed disequilibrium degassing characteristic of homogeneously nucleating systems the disequilibrium simulations show that delaying nucleation until p 150 mpa restricts degassing to within 1500 m of the surface fragmentation occurs at similar porosity in both the disequilibrium and equilibrium modes 80 vol but at the distinct depths of 500 m and 2300 m respectively the vesiculation delay leads to higher pressures at equivalent depths in the conduit and the mass flux and exit pressure are each higher by a factor of 2 0 residual water contents in the melt reaching the vent are between 0 5 and 1 0 wt roughly twice that of the equilibrium model

magma chamber of the campi flegrei supervolcano at the time of eruption of the campanian ignimbrite a supereruption that occurred in the campi flegrei area italy ca 39 ka had regional and global scale environmental impacts and deposited the campanian ignimbrite ci we attempt to shed light on critical aspects of the eruption depth of magma chamber intensive pre eruptive magma conditions and the large volume magma plumbing system on the basis of information derived from analyzing melt inclusion mi data to achieve these aims we provide new measurements of homogenization temperatures and values of dissolved h2o within phenocryst hosted mis from pumices erupted during different phases of the ci eruption the mi data indicate that a relatively homogeneous overheated trachytic magma resided within a relatively deep magma chamber dissolved water contents in mis indicate that prior to the eruption the magma chamber underwent radical changes related to differential upward movement of magma decompression of the rising trachytic magma caused a decrease in water solubility and crystallization and trachytic bodies were emplaced at very shallow depths the proposed eruptive model links portions of the main magma chamber and apophyses with specific eruptive units

multiple levels of magma storage during the 1980 summer eruptions of mount st helens wa transitions in eruptive style explosive to effusive sustained to pulsatory are a common aspect of volcanic activity and present a major challenge to volcano monitoring efforts a classic example of such transitions is provided by the activity of mount st helens wa during 1980 where a climactic plinian event on may 18 was followed by subplinian and vulcanian eruptions that became increasing pulsatory with time throughout the summer finally progressing to episodic growth of a lava dome here we use variations in the textures glass compositions and volatile contents of melt inclusions preserved in pyroclasts produced by the summer 1980 eruptions to determine conditions of magma ascent and storage that may have led to observed changes in eruptive activity five different pyroclast types identified in pyroclastic flow and fall deposits produced by eruptions in june 12 july 22 and august 7 1980 provide evidence for multiple levels of magma storage prior to each event highly vesicular clasts have h2o rich 4 5 5 5 wt melt inclusions and lack groundmass microlites or hornblende reaction rims characteristics that require magma storage at p 160 mpa until shortly prior to eruption all other clast types have groundmass microlites ph20 estimated from both h2o bearing melt inclusions and textural constraints provided by decompression experiments suggest pre eruptive storage pressures of 75 40 and 10 mpa the distribution of pyroclast types within and between eruptive deposits can be used to place important constraints on eruption mechanisms fall and flow deposits from june 12 1980 lack highly vesicular microlite free pyroclasts this eruption was also preceded by a shallow intrusion on june 3 as evidenced by a seismic crisis and enhanced so2 emissions our constraints suggest that magma intruded to a depth of 4 km beneath the crater floor fed the june eruption in contrast eruptions of july and august although shorter in duration and smaller in volume erupted deep volatile rich magma if modeled as a simple cylinder these data require a step wise decrease in effective conduit diameter from 40 50 m in may and june to 8 12 m in july and august the abundance of vesicular intermediate to deep clast types in july and august further suggests that this change was effected by narrowing the shallower part of the conduit perhaps in response to solidification of intruded magma remaining in the shallow system after the june eruption eruptions from july to october were distinctly pulsatory transitioning between subplinian and vulcanian in character as originally suggested by scandone and malone 1985 a growing mismatch between the rate of magma ascent and magma disruption explains the increasingly pulsatory nature of the eruptions through time recent fragmentation experiments spieler et al 2004 suggest this mismatch may have been aided by the multiple levels at which magma was stored and degassed prior to these events

isotopic abundances inferences on solar system and planetary evolution members of the royal swedish academy ladies and gentlemen in thinking over why i am here today i recalled the story of tom sawyer a character of mark twain s on a beautiful bright saturday morning when all of his friends were playing he appeared by the wall in front of his aunt s house with a bucket of whitewash paint and a long handled brush before him stood about thirty meters of continuous wooden fence three meters tall he dipped his brush and passed it over a plank and then repeated it then he stopped and compared the insignificant whitewashed streak with the far reaching continent of unpainted fence and became melancholy then he took up the brush again and went to work a friend came by and teased him for working on saturday when he could be swimming or playing tom said it wasn t so bad because a boy didn t get the chance to whitewash a fence every day tom took a new brush stroke at the fence and stepped back to note the beauty of the effect his friend got interested and then asked permission to make a stroke or two he then offered tom the core of an apple he had just started to nibble on tom told him that it was doubtful that his friend could help because this was the front fence and that a very good and sophisticated job had to be done possibly only but one child in a thousand possibly two thousand could do it well after this denial his friend just had to try and offered tom the whole apple for a chance to paint then more young friends came around to attempt this challenging task by the end of the afternoon tom was the wealthiest boy in town he had twelve marbles a key that didn t unlock anything a piece of blue glass to look through a tin soldier a dog collar but no dog and lots of other valuable things the fence had been coated three times and if he had not run out of whitewash tom would have bankrupted every kid in the village my presence here is due to an ability to attract young talent to help whitewash the fence the only difference is that we have just started on the job

the campanian ignimbrite southern italy geochemical zoning insight on the generation of a super eruption from catastrophic differentiation and fast withdrawal more than ca 100 km3 of nearly homogeneous crystal poor phonolite and ca 100 km3 of slightly zoned trachyte were erupted 39 ka during the campanian ignimbrite super eruption the most powerful in the neapolitan area partition coefficient calculations equilibrium mineral assemblages glass compositions and texture were used to reconstruct compositional thermal and pressure gradients in the pre eruptive reservoir as well as timing and mechanisms of evolution towards magma chamber overpressure and eruption our petrologic data indicate that a wide sill like trachytic magma chamber was active under the campanian plain at 2 5 kbar before ci eruption thermal exchange between high liquidus 1199 c trachytic sill and cool country rocks caused intense undercooling driving a catastrophic and fast 102 years in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and an overpressure in the chamber that triggered the super eruption this process culminated in an abrupt reservoir opening and in a fast single step high decompression sanidine phenocrysts crystal size distributions reveal high differentiation rate thus suggesting that such a sill like magmatic system is capable of evolving in a very short time and erupting suddenly with only short term warning

the geological co2 degassing history of a long lived caldera the majority of the 100 holocene calderas on earth host vigorously active hydrothermal systems the heat and volatile budgets of which are sustained by degassing of deeply stored magma calderas may thus contribute a nontrivial although poorly quantified fraction of the global budget of magmatic volatiles such as co2 here we use original isotopic and petrological results from campi flegrei volcano italy to propose that hydrothermal calcites are natural mineral archives for the magmatic co2 that reacted with reservoir rocks during the geological history of a caldera we show that campi flegrei calcites identified in core samples extracted from 3 km deep geothermal wells formed at isotopic equilibrium with magmatic fluids having 18oh2o of 8 7 to 12 7 and 13cco2 of 1 5 this inferred fossil fluid composition is virtually identical to that of present day fumaroles demonstrating a stable carbon source during the caldera s 40 k y history we use the mass of calcites stored in the hydrothermal system to estimate that 12 gt of magmatic co2 reacted with the campi flegrei rocks during the caldera history this corresponds to a time averaged co2 flux of 800 t d 1 this long term co2 flux the first of its kind in the geological literature is similar to the present day soil co2 degassing flux 1100 200 t d 1 we conclude that the actual magmatic co2 degassing flux from calderas may be severely underestimated if subsurface calcite precipitation is not taken into account

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

40ar 39ar dating of latest pleistocene 41 ka marine tephra in the mediterranean sea implications for global climate records abstract fifty one 40 ar 39 ar laser heating analyses of sanidine crystals in the c 13 tephra layer recovered out of a piston core raised from 3000 m depth in the tyrrhenian sea yielded apparent ages between 25 and 112 ka twenty four of these measurements define an age probability maximum and generate an isochron of 41 1 2 1 ka 2 with a 40 ar 36 ar intercept of 288 21 that is indistinguishable from the modern atmosphere however the entire population is skewed toward older apparent ages with smaller peaks at 48 ka 55 ka 67 ka and 110 ka the composition of glass shards in the c 13 tephra overlaps that of the sio 2 rich phase of the 150 km 3 campanian ignimbrite which erupted from the campi flegrei caldera 130 km northwest of the core site although the 41 1 2 1 ka isochron is 15 older than k ar and 14 c ages for the campanian ignimbrite it is indistinguishable from recent 40 ar 39 ar age determinations the campanian ignimbrite eruptions were also chemically similar to and correlated with the y 5 ash layer the most voluminous and widely dispersed late pleistocene tephra in the mediterranean region the age of the y 5 layer was constrained originally at 38 0 2 5 ka on the basis of astronomical dating of o isotope stages the 40 ar 39 ar ages astronomical dating plus chemical similarities permit correlation of the c 13 tephra layer with both the campanian ignimbrite and the y 5 tephra layer the older ages preserved in the c 13 tephra layer may reflect xenocrysts incorporated during the explosive eruption accompanying collapse of the campi flegrei caldera 41 ka the c 13 tephra layer is correlated with a 10 be peak in an adjacent marine core o isotope stage 3 12 and immediately preceded a cooling episode corresponding to heinrich event 4 thus the age of these features is constrained by radioisotope dating the 41 ka peak in 10 be deposition is a global phenomenon whose age matches that of the laschamp geomagnetic excursion thus our results provide a new tie point among several polar ice core and marine proxy records of paleoclimate during the past 50 kyr precise age information independent of radiocarbon dating can be recovered from latest pleistocene marine tephra using the 40 ar 39 ar laser heating method despite the presence of multiple components that may have been incorporated during or after its deposition

al correlated26mg excess in a large ca al rich inclusion of the leoville meteorite abstract minerals from a large 1 5 cm diameter type b inclusion of the c3v carbonaceous chondrite leoville have been analyzed mass spectrometrically for their mg isotopic composition 25 mg 24 mg ratios were found to be indistinguishable in all cases from those of terrestrial mg while in anorthites 26 mg is overabundant by up to 15 the excess 26 mg 26 mg is linearly correlated with the 27 al 24 mg ratio with the constant of proportionality 26 mg 27 al 5 9 0 3 10 5 this is in agreement with values reported for allende and another type b inclusion from leoville it is at variance with the twenty times higher ratio found in leoville hibonites

inferring volcanic degassing processes from vesicle size distributions both power law and exponential vesicle size distributions vsds have been observed in many different types of volcanic rocks we present results of computer simulations and laboratory analogue experiments which reproduce these findings and show that the distributions can be interpreted as the product of continuous bubble nucleation resulting from non equilibrium degassing this ongoing nucleation causes the bubbles to evolve through an exponential size distribution into a power law size distribution as nucleation and growth progress these findings may explain the apparent contradiction between present models of bubble growth in magmas which predict that degassing in explosive eruptions is a non equilibrium process and models of conduit flow which assume perfect equilibrium degassing the process of continuous nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurization and slow volatile diffusion

the age of the neapolitan yellow tuff caldera forming eruption campi flegrei caldera italy assessed by 40ar 39ar dating method abstract the neapolitan yellow tuff nyt is the product of the largest known trachytic phreatoplinian eruption it covered an area larger than 1000 km2 with an estimated volume of about 40 km3 of erupted magma during the course of the eruption a caldera collapsed within the previously formed campanian ignimbrite caldera the resulting nested structure strongly influenced the following volcanic activity in the campi flegrei caldera as previous dating of the nyt does not converge toward a unique result a new set of 40ar 39ar age determinations has been carried out to better constrain the age of the eruption two variants of the 40ar 39ar dating method were applied to determine the age of the nyt eruption 1 single crystal total fusion sctf on an individual phenocryst of feldspar and 2 laser incremental heating lih on bulk aliquots of feldspar phenocrysts the results of the sctf analyses show that the overall sample weighted mean age derived from the conventional age calculation is 15 6 0 8 ka a weighted mean of the isochron age is 15 3 1 2 ka 2 and has been assumed as the best indicator of age to be derived from the sctf analyses the lih analyses results show that plateau ages vary from 15 4 0 5 to 14 5 0 5 ka the overall weighted mean age of the isochron results is 14 9 0 4 ka 2 this result has been assumed as the reference age for the nyt eruption and agrees with the sctf age the new age obtained for the nyt deposits is of great relevance for the understanding of the evolution and the present state of the campi flegrei caldera and collocates the nyt in a crucial stratigraphical position to date the climatic oscillations that occurred between the late glacial and the holocene

geochemical study of misoa formation crude oils centro lago field lake maracaibo western venezuelanbasin abstract the aim of this study is to evaluate hydrocarbon distributions c15 and c15 in themisoa formation oils from the centro lago field lake maracaibo western venezuela detailed geochemical analyses of 38 crude oils from various producing sands b x c2 c3 c4 c5 and c6 indicate that the oils are derived from the same source rock la luna formation andthat they were formed during the main phase of hydrocarbon generation and expulsion migration fractionation is considered as a possible contributor to changes in crude oilcomposition however little variations in the organic facies of la luna formation or multiplecharge process reservoir filling could explain the compositional differences on the basis of theanalysis c15 organic compounds we differentiated various crude oil families in the centrolago field the reservoir is composed of four principal oil families b x crude oils in cla 237and cla 346 c2 crude oils c3 and c4 oils of the south dome except cla 233 for c3 andcl 24 and cla 238 for c4 and c4 north dome and c5 oils form an oil family with similarchemical composition

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

a 3d imaging textural characterization of pyroclastic products from the 1538 ad monte nuovo eruption campi flegrei italy abstract the explosive volcanic event of the 1538 ad monte nuovo eruption volcanic explosivity index vei 2 in the campi flegrei high risk caldera italy has a strategic significance in the framework of volcanology and volcanic hazard of caldera forming magmatic systems in fact it represents the last and unique historical eruption of the highly populated phlegraean restless caldera and its precursory and eruptive phenomena are well known because they were described in detail by contemporaneous eyewitnesses in this study a set of samples representative of the complete stratigraphic sequence of the monte nuovo eruption was characterized using phase contrast synchrotron radiation computed microtomography and quantitatively investigated through the development of a new protocol for 3d textural analysis of highly vesiculated volcanic rocks previous studies of products from this eruption are available in the literature mostly based on 2d imaging techniques and thus provide a useful data set for comparison the 3d textural measurements allow us to investigate the subvolcanic processes mechanisms and timing of magma degassing that occurred during magma ascent in the conduit for each stage of the eruption and their relationship with the variations in the eruptive style described in the contemporaneous accounts of the eruption this information is fundamental for the definition of a volcanic eruption scenario for such low vei events often recurrent in the history of the caldera and is useful both for hazard assessment and emergency planning

computer simulation of transport and deposition of the campanian y 5 ash analyses of grain size and modal composition of the campanian tuff ash layer y 5 from 11 deep sea cores have been carried out this layer represents ash fall that has been correlated with the 38 000 y b p campanian ignimbrite thunell et al 1979 a deposit formed by the largest eruption documented in the mediterranean region during the late pleistocene barberi et al 1978 the bulk deposit is bimodal in grain size and dominated by glass shards the calculated mean grain size of the coarse mode of the individual size distributions decreases with distance from the source and progressively approaches a near constant fine mode of approximately 13 microns distal samples are unimodal in grain size these data combined with a set of vertical profiles of wind 10 year average have been used as input to a computer model that simulates fallout of tephra modelling indicates that the downwind variation of grain size of the coarse mode can be accurately reproduced with transport of ash between 5 and 35 km the observed fine mode of the deposit cannot however be generated by transport of ash as individual particles at these elevations such transport would result in deposition of virtually all of the fine ash beyond the studied area deposition of fine ash within the studied distance of 1600 km from source can only occur by fallout as particle aggregates from a high eruption plume or as individual particles from co ignimbrite ash clouds with a maximum elevation of 3 km the large volume of ash in the fine mode 70 wt and the irregularity in azimuth of low level winds argue against major low level transport of co ignimbrite ash rather the ash may have been derived from both a plinian eruption column and high altitude clouds of co ignimbrite ash with settling of fine ash as particle aggregates

stratigraphy and physical parameters of the plinian phase of the campanian ignimbrite eruption the campanian ignimbrite eruption 39 ka was the most powerful eruptive event of the campi flegrei caldera southern italy this event coincided with the onset of a cold climatic phase and the paleolithic transition from neanderthals to modern humans the eruption started with a sustained column that emplaced a widespread pyroclastic fall deposit covering an area 4000 km2 within the 15 cm isopach for the first time we present the complete longitudinal variation from the coarse and 10 m thick proximal down to 15 km sequence through the well stratified pumice lapilli deposit in medial areas 30 80 km to the distal tephra hundreds to thousands of kilometers distant the plinian pumice fall deposit shows a strong lateral heterogeneity due to variations in stratification grading and abundance of components all campanian ignimbrite fall layers display plinian dispersal variation of grain size with stratigraphic height suggests that the convective plume was far from steady state initially the plume dispersed 1 3 km3 of tephra toward the ene n75 e from a height of 29 km layer a a gradual increase in intensity resulted in inverse grading of layer b column height increased from 26 to 37 km at a vertical velocity of 3 6 m s it had a main dispersal axis to the east n97 e and a secondary lobe to the southeast n137 e during this phase a maximum volume of 1 73 km3 of tephra was emplaced accessory lithics concentrated in layer c are possibly due to vent clearing after partial blockage of the vent the 33 km high column dispersed ejecta to the east n95 e with only 0 2 km3 of tephra erupted during this phase during the successive gradual decline in eruption intensity column height decreased from 38 to 32 km at a velocity of 4 2 m s a normally graded layer d with a volume of 1 km3 accumulated to the east n95 e the sustained column phase ended with a pulsating and partially collapsing column that reached 23 km in height and dispersed 1 1 km3 of a stratified and lithic rich succession layer e to the southeast n112 e if we include the distal co plinian deposit a total volume of 7 8 km3 dense rock equivalent dre of magma was released containing 0 09 km3 of accessory lithics tephra mass for the single layers is on the order of 1011 kg and the total mass is 2 1012 kg 2 1013 kg including the co plinian ash mass discharge rates ranged from 0 9 to 6 7 108 kg s the calculated magnitude of the sustained column phase is 6 3 the duration of the plinian phase of this eruption based on the ratio of two parameters erupted mass divided by discharge rate is estimated to have been 20 h including co plinian ash this study shows that plinian deposits are not always homogeneous and as for pyroclastic current deposits can show an articulate architecture only the complete reconstruction of vertical and lateral variations in components stratification and grading might provide insights into the temporal and spatial evolution of the sustained plume

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

oil composition variation and reservoir continuity unity field sudan abstract a suite of oils from stacked reservoirs in the unity field in sudan has been analyzed by various geochemical techniques for molecular information to elucidate the geological processes which cause variations in oil composition and their resulting oil fingerprints in different reservoir units analyses of these highly paraffinic oils indicate that the chromatographic fingerprint variations are due to differences in the abundances of saturated compounds including branched and cyclic alkanes neither aromatics nor nso compounds have any significant effect on the observed fingerprint variations this association of saturates instead of aromatics and nso compounds with the fingerprint variations precludes rock fluid interactions as a cause of the variations biomarker analyses show that variations in thermal maturity and organic facies of the source rock are responsible for the fingerprint variations thermal maturity increases with the depth of the reservoir suggesting a multiple charge process for the oils to fill these reservoirs over an extended period of time apparently the source rock generated and expelled progressively more mature oils and little mixing occurred during migration thus knowledge of oil compositional variations from one reservoir to another organic facies variation and source rock maturity combined with tectonic history may help explain charging and timing of oil emplacement

the possible role of extensional faults in localizing magmatic activity a crustal model for the campanian volcanic zone eastern tyrrhenian sea italy abstract three crustal geological sections of the campanian volcanic zone italy were reconstructed by integrating basin architecture and deep well stratigraphies in addition mapping of the fault system and of large volume ignimbrites was carried out a linked fault system has been identified which was responsible for asymmetric subsidence contemporaneous with the eruption of ignimbrites late quaternary extension is characterized by a wnw ese stretching axis nne ssw normal faults and reactivation of inherited structures structural stratigraphic and palaeogeographical analyses reveal evidence for up to 750 m of subsidence which has occurred at a mean rate of up to 4 9 mm a 1 over the past 154 ka these rates suggest that extensional tectonics was responsible for the regional subsidence a 6 km deep seismic reflector was associated with a magmatic reservoir underlying campi flegrei by matching the structural and stratigraphic architecture with published geophysical and geochemical data a crustal tectonomagmatic model was constructed that displays high angle faults that root into a low angle detachment which in turn roots into a deep sill like magma reservoir this model suggests the possible role of extensional faults in localizing magmatic activity as faults controlled magma rise from deep to shallow reservoirs and to the surface during ignimbrite eruptions supplementary material seismic reflection profiles and interpretations are available at http www geolsoc org uk sup18451

anisotropy of magnetic susceptibility studies of depositional processes in the campanian ignimbrite italy the late pleistocene trachytic campanian ignimbrite underlies much of the campanian plain near naples italy and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source the campi flegrei caldera at sites within 15 km of the campi flegrei anisotropy of magnetic susceptibility ams principal directions indicate that in the absence of significant topography deposition came from a flow moving in a roughly radial direction ams studies of the more distal ignimbrite reveal downhill and or downvalley flow directions prior to deposition even where these directions are at high angles to a generally radial transport direction from the vent on the flanks of roccamonfina volcano flow was directly downhill as if the source of the ignimbrite was the summit of the volcano in most localities the ignimbrite consists of a single massive deposit in a few localities in the apennine mountains however the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys a detailed study of the ignimbrite in the flat titerno river valley near massa shows that the ams fabrics are not due to late stage creeping during deposition or compaction well defined but non parallel ams fabrics from vertical and lateral sections in the massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit clast compositions and ams axes at mondragone indicate that the pyroclastic flow encountered the monte massico massif and was partially blocked so that flow during deposition was toward the campi flegrei similar ams data from sites along the edge of the campanian plain indicate back flow off the first ridge of the apennine mountains reached at least 5 km from their base the campanian ignimbrite was deposited from a density stratified pyroclastic flow the depositional system consisted of the lower denser portion of the current and was controlled by topography the grouping of the ams axes is interpreted to indicate that deposition occurred under laminar flow conditions

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

ecosystem impact of the campanian ignimbrite eruption in late pleistocene europe the dating of the campanian ignimbrite ci eruption to 37 000 cal yr b p draws attention to the coincidence of this volcanic catastrophe and the suite of coeval late pleistocene biocultural changes that occurred within and outside the mediterranean region these included the middle to upper paleolithic cultural transition and the supposed change from neanderthal to modern homo sapiens anatomy a subject of sustained debate no less than 150 km3 of magma were extruded in the ci eruption the signal of which can be detected in greenland ice cores as widespread discontinuities in archaeological sequences are observed at or following the ci event a significant interference with ongoing human processes in mediterranean europe is hypothesized

magma degassing and crystallization processes during eruptions of high risk neapolitan volcanoes evidence of common equilibrium rising processes in alkaline magmas abstract compositional textural and experimental data on products from explosive and effusive eruptions of neapolitan volcanoes campi flegrei and somma vesuvio allow us to constrain degassing and fragmentation conditions during eruptions of alkaline magmas significant differences in compositional and textural features have been recognized between lavas scoria and pumice resulting respectively from effusive moderately and extremely explosive eruptions pumice samples have highly vesicular glassy matrix low microlite number density and moderate to high water content crystal size distributions csd are steep with high intercept values the narrow microlite size range indicates single nucleation event scoria products are characterized by moderate vesicularity and water content they have high number density of microlites which are bimodal in size csd show distinct inflections that are explained as two crystal populations growing in distinct time lava samples generally have low vesicularities moderate to high microcrystalline groundmass and low glass water content the comparison between textural and compositional features of natural rocks with samples obtained by decompression experiments allows us to conclude that degassing processes during magma ascent occurs in near equilibrium conditions even at high decompression rate moderate to long magma rise times calculated in the order of a few days produce open degassing responsible for moderately explosive to effusive activity short magma rise times calculated in the order of a few hours result in closed system degassing that allow explosive fragmentation when the volume of growing bubble reaches a fixed threshold vesicularity and water content measured on matrix glass of pumice indicate that this process occurs at pressure of 10 30 mpa in these conditions degassing fragmentation and in turn the eruptive style is strongly influenced by initial conditions in the magma chamber volatile content temperature pressure instead of decompression rate in contrast with that observed for rhyolitic melts these differences have important consequences in terms of volcanic hazards and risk the low viscosity alkaline magma is able to maintain efficient degassing even during the final stage of magma ascent favoring in the case of closed system fragmentation and explosive activity

clinopyroxene liquid trace element partitioning in natural trachyte trachyphonolite systems insights from campi flegrei southern italy trace element partition coefficients between clinopyroxenes and associated glassy matrix cpx l d have been determined for 13 ree hfse4 5 u th sr pb sc and v from combined la icp ms emp analyses in selected trachytes and trachyphonolites from campi flegrei composition of clinopyroxene and glass is pretty homogeneous in the trachyphonolites pointing to an overall attainment of the equilibrium conditions in trachytes conversely phases show some compositional heterogeneity due to the presence of clinopyroxene xenocrysts that requested a more careful petrographic and geochemical inspection of the samples to assess the equilibrium clinopyroxene composition in the trachyte clinopyroxenes ree are compatible from nd to lu cpx l d up to 2 9 like y ti sc and v the cpx l d for eu is lower than those of the adjacent ree highlighting eu2 contribution high d values are also shown by u th pb zr hf nb and ta relatively to basaltic and andesitic systems whereas the d sr is roughly similar to that found for less evolved magmas trachyphonolites are characterized by an overall decrease of the cpx l d for highly charged cations with the exception of v and by a slight increase of d sr ree are still compatible from nd to lu cpx l d up to 2 1 like ti y sc and v this variation is also predicted for ree and y by models based on the elastic strain theory being consistent with the slightly lower polymerization degree estimated for the trachyphonolites however the observed cpx l d ree y are matched by the modelled ones only considering very low t 825 c which are believed unlikely this mismatch cannot be attributed to effects induced by the water rich composition of the trachyte trachyphonolite suite since they would lower the observed cpx l d ree y moreover the anomalous inflections of measured cpx l d for hree suggests some crystal chemical control such as the entrance of these elements in a site distinct from m2 it is concluded that the large cpx l d determined for trachytes and trachyphonolites are likely induced by hitherto unconstrained changes of the z3 activities related to the composition of melt and or solid all these considerations strongly highlight the importance of a direct characterization of trace element partitioning in natural samples from magmatic systems poorly characterized by experimental studies

petrologic and almg isotopic clues to the accretion of two refractory inclusions onto the leoville parent body one was hot the other wasn t abstract two type b refractory inclusions found within several centimeters of each other in the foliated leoville meteorite differ greatly in their styles of deformation textures and mg al isotopic characteristics one is a subspheroidal type b1 with textures similar to those observed previously in many type b1s from other cv3 meteorites and whose deformation fracturing some granulation has been almost entirely brittle in nature its isotopic composition shows 26 mg 24 mg correlated with 27 al 24 mg which although disturbed indicates a lower limit for initial 26 al 27 al of 4 9 10 5 the other inclusion is a highly elongate l w 6 object whose long axis lies in the plane of foliation of the leoville meteorite it has a predominantly metamorphic texture but relict islands of an earlier and more normal type b precursor are preserved in its core the isotopic compositions of the two textural variants in this inclusion are different anorthite in the metamorphic region mostly showing little or no excess 26mg 26 al 27 al 0 6 that would be attributable to in situ decay of 26al whereas anorthite in the relict islands shows well resolved excess 26mg that gives a lower limit for initial 26 al 27 al of 4 6 10 5 the effects of shock metamorphism on the leoville meteorite are clearly seen in both inclusions including brittle deformation and local shock melting but are not responsible for the major textural differences between the two inclusions the most likely origin for the deformed and recrystallized features of the elongate inclusion is that it accreted onto the leoville parent body while still very hot and deformed into its present shape at the instant it impacted if the resetting of the al mg system occurred during this event it must have taken place at least 3 3 million years after formation of the precursor inclusion the possibility that other deformed leoville components accreted while hot should not be discounted

case studies of plagioclase growth and residence times in island arc lavas from tonga and the lesser antilles and a model to reconcile discordant age information abstract the time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved however crystals and liquids can have different ages leading to apparently contradictory information from different dating methods two case studies are presented from island arc lavas that highlight such dichotomies plagioclase crystal size distributions and u th ra isotope data are presented for lavas from late and metis shoal in tonga and from soufriere st vincent in the lesser antilles in all three lavas mineral data yield apparent u th ages of 30 50 kyr yet 226 ra excesses are preserved in both the minerals and the whole rocks the crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations furthermore a cumulate from soufriere yields the same u th age as its host lava on 226 ra ba versus time evolution diagrams curves for melts in equilibrium with plagioclase provide apparent ages of 1700 3300 yr for tonga but for soufriere they are displaced below that of the whole rock our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age the apparent u th ages reflect mixtures of old cores and young rims and indicate a volume integrated age of 10s of kyr for the time for initial crystal growth crystal liquid segregation the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption the 226 ra excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma because crystal growth segregation and entrainment can all appear to occur in 100s to a few thousand years at most the bulk of the u th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches plagioclase growth rates of 3 9 10 13 cm s 1 would reconcile the combined ra th and crystal size distribution data magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times indicating complex phenocryst liquid behaviour beneath island arc volcanoes

hydrothermal fluid venting in the offshore sector of campi flegrei caldera a geochemical geophysical and volcanological study the ongoing unrest at the campi flegrei caldera cfc in southern italy is prompting exploration of its poorly studied offshore sector we report on a multidisciplinary investigation of the secca delle fumose sdf a submarine relief known since antiquity as the largest degassing structure of the offshore sector of cfc we combined high resolution morpho bathymetric and seismo stratigraphic data with onshore geological information to propose that the present day sdf morphology and structure developed during the initial stages of the last cfc eruption at monte nuovo in ad 1538 we suggest that the sdf relief stands on the eastern uplifted border of a n s trending graben like structure formed during the shallow emplacement of the monte nuovo feeding dike we also infer that the high angle bordering faults that generated the sdf relief now preferentially allow the ascent of hot brines with an equilibrium temperature of 179 c thereby sustaining hydrothermal degassing on the seafloor systematic vertical seawater profiling shows that hydrothermal seafloor venting generates a sizeable co2 ph and temperature anomaly in the overlying seawater column data for the seawater vertical profile can be used to estimate the co2 and energy heat outputs from the sdf area at 50 tons day 0 53 kg s and 80mw respectively in view of the cause effect relationship with the monte nuovo eruption and the substantial gas and energy outputs we consider that the sdf hydrothermal system needs to be included in monitoring programs of the ongoing cfc unrest this article is protected by copyright all rights reserved

tephrostratigraphy and tephrochronology in the philippi peat basin macedonia northern hellas greece abstract three tephra layers have been identified in the upper 15 m of a 190 m section of peat beneath the philippi fen they provide significant lithological and chronological markers throughout the fen and the aegean region the upper tephra pht1 consists of shards of a transparent calcalkaline felsic glass and fragments of plagioclase augite hypersthene and rare hornblende magnetite apatite and quartz peat directly beneath pht1 gave a radiocarbon approximate age of 13 000 yr the middle tephra layer pht2 which resembles pht1 in chemistry and petrography but contains more crystals and lithic fragments rests on peat dated ca 18 000 14 c yr bp the lower tephra pht3 has colorless to brown glass shards with a trachytic chemistry and a mineral assemblage of sanidine sodic plagioclase biotite aegirine augite hornblende titanite and apatite bracketing radiocarbon ages imply that pht3 accumulated about 30 000 14 c yr bp the likely ages of pht1 and pht2 together with their mineralogical and chemical characteristics suggest that these tephras came from the volcanic field of thera in the hellenic arc pht2 particularly was derived from a major known explosive eruption ca 18 000 yr bp the cape riva eruption correlative to the y 2 tephra layer evidence for pht3 suggests derivation from the campanian province of italy and correlation with the campanian ignimbrite and the y 5 ash beneath the mediterranean sea

kinetics of bubble nucleation in a rhyolitic melt an experimental study of the effect of ascent rate in order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma we performed three series of experiments decompressed at rates of either 1000 167 or 27 8 kpa s the experiments were carried out in an externally heated pressure vessel at 800 c and in the pressure range 260 59 mpa the starting material was a crystal free and bubble free rhyolitic glass containing 7 0 wt dissolved h2o in all the decompression experiments homogeneous bubble nucleation began at 90 2 mpa that is 150 mpa below the water saturation pressure of the silicate liquid 240 mpa the degree of supersaturation phon required to trigger homogeneous bubble nucleation was almost independent of decompression rate phon is the difference between the saturation pressure and the nucleation pressure nucleation pressure decreased by 3 mpa for a 36 fold increase in decompression rate these results are in good agreement with the classical theory of nucleation assuming a rhyolite h2o surface tension of 0 106 n m 1 our major experimental finding is that after a short nucleation event the nucleation rate dropped and the bubble number density n reached a stationary value that was strongly sensitive to decompression rate 6 8 mm 3 at 27 8 kpa s 470 mm 3 at 167 kpa s and 5800 mm 3 at 1000 kpa s the smaller value of n at low decompression rate was compensated by a larger mean bubble size so that at a given pressure vesicularity was almost independent of decompression rate the experimental values of n can be reproduced within a factor 0 3 1 4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas the nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation once a critical value of n is attained diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure the strong correlation between bubble number density and decompression rate has fundamental volcanological implications if we extrapolate the experimental data to the typical ascent rates of silicic magmas we obtain bubble number densities 10 3 101 mm 3 for homogeneous nucleation 10 1 103 mm 3 for heterogeneous nucleation that are orders of magnitude smaller than those measured in most natural pumices we therefore propose that the large values of n in silicic pumices may be due to two successive nucleation events 1 a first event which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles and 2 a second nucleation event yielding a very large number of small bubbles and presumably related to the dramatic increase of decompression rate that precedes fragmentation the small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

composition and correlation of volcanic ash beds of silurian age from the eastern baltic sanidine composition and bulk geochemistry of volcanic ash beds from the east baltic indicate the subalkaline nature of the volcanism near the margins of the baltica plate during the silurian several bentonites in the wenlock include a previously unknown sanidine with 48 to 58 mol of the na ca component in contrast to the earlier telychian volcanism sodium rich sanidine occurs in ash beds which originate from relatively moderately evolved dacitic magma the studied material from two drill cores integrated with previous research enables production of a more complete list of 49 volcanic eruption layers for the lower to middle wenlock in the east baltic this updated list of bentonites characterized by their sanidine compositions forms a good basis for future integrated bio and chemostratigraphic correlations in northern europe

controlled source nonlinear tomography a powerful tool to constrain tectonic models of the southern apennines orogenic wedge italy the anatomy of the southern apennines is a matter of compelling debate both thick and thin skinned tectonic models are proposed due to paucity of deep seismic constraints we present new and well defined information on the velocity structure of the mountain range by using an innovative nonlinear tomographic technique applied to refraction wide angle reflection data the velocity interface model shows a high velocity region with values exceeding 7 km s at 8 9 km depth such large velocities cannot be related to sedimentary rocks and imply the presence of shallow crystalline rocks i e the basement involvement in the thrust belt the integration of our results with other geophysical data local earthquake tomography bouguer and magnetic anomalies confirms this interpretation which strongly supports thick skinned tectonics and has significant implications for the seismotectonics and hydrocarbon potential of the southern apennines our inversion approach can be effective in other orogenic wedges when seismic reflection profiling fails to uniquely resolve deep complex structures

evolution of caal rich bodies in the earliest solar system growth by incorporation abstract many type b ca al rich inclusions cai s in the allende carbonaceous chondrite contain two types of spinel structures framboids and palisades framboids are clumps of spinel grains generally in situ by solid state growth processes palisades are texturally different and consist of ovoid shells of spinel grains that appear in thin sections as long arcs or rings with diameters ranging from 50 m up to 2 cm no in situ formation process seems able to explain the variety of sizes and morphologies of palisades nor the different compositions and textures of the enclosed and enclosing materials we therefore suggest that palisades are the spinel rims of smaller earlier formed type b cai s that were incorporated into other cai material in various ways by capture into liquid drops by solid condensate overgrowths and by the partial melting or welding of agglomerates containing the bodies as some type b bodies have been found inside type a host material it appears that in at least some regions and probably generally type a cai s formed after type b we propose that type b cai s are residues from the heating and incomplete evaporation of interstellar dust during the accretion of the protosolar nebula and that type a cai s are later condensates from completely evaporated dust whose exotic 16 o rich component became more diluted in the gas phase

bubbledrive 1 a numerical model of volcanic eruption mechanisms driven by disequilibrium magma degassing abstract we have developed a numerical model for magma dynamics in volcanic conduits that involves conduit flow driven by a detailed treatment of bubble growth the bubbledrive 1 model can be characterized by several unique aspects including treatment of the transient problem specific eruption triggers disequilibrium degassing a full scale bubble growth module and steady state eruption in the case of constant recharge from below the general conditions for the model are homogeneous two phase flow a simple fragmentation threshold pre existing bubbles and parameters corresponding to magma compositions of dacite and rhyolite the advantage of using a full scale bubble growth model in our treatment of conduit flow makes it possible to track the level of oversaturation throughout the conduit in space and time the model accounts for dynamic decompression in which the evolving interaction between decompression rate due to magma ascent and diffusive addition of gas into bubbles due to decreased solubility with decreasing pressure serves as the driving force of magma flow in the conduit the ability of the model to run the transient case enables us to explore specific triggers to initiate an eruption which proceeds with duration and eruption energy depending on conduit geometry and several other factors our numerical study revealed several notable features of volcanic systems that may shed additional light on natural systems each of the model runs was directed at elucidating specific aspects of eruptions and the suite of runs indicated some common features that may play a key role in all silicic eruptions we find that conduit geometry is critical in determining the nature of eruption conduits lacking a magma chamber have smooth and relatively quiet eruption styles conduits that include a magma chamber involve eruptions that start out like those without chambers but then evolve into a long and uniform eruption that ends with a violent explosion as the chamber is emptied the uniform phase is maintained by vesiculation within the chamber and the blast occurs when the fragmentation level descends down into the magma chamber resistance to flow imposed by the narrow conduit above the chamber acts as a buffer stabilizing the eruption during the steady state phase until the chamber is depleted in material discharge and vent velocity are proportional to the conduit chamber depth and are controlled by the initial concentration of dissolved volatiles maximum oversaturation is observed immediately below the fragmentation level of the magma column oversaturation at the vent remains remarkably constant at 1 wt of h2o for all cylindrical conduits with no recharge from below oversaturation at the vent is much more complex for conduits that contain magma chamber at 2 km depth our numerical study indicates that the primary factors that control eruption behavior are magma composition and conduit shape this suggests that future observation programs could provide valuable information by determining sub edifice structure so that better constraints on geometry can be derived

a petrographic chemical and isotopic study of calcium aluminum rich inclusions and aluminum rich chondrules from the axtell cv3 chondrite petrographic compositional and isotopic characteristics were studied for three calcium aluminum rich inclusions cais and four plagioclase bearing chondrules three of them al rich from the axtell cv3 chondrite all seven objects have analogues in allende cv3 and other primitive chondrites yet axtell like most other chondrites contains a distinctive suite of cais and chondrules in common with allende cais cais in axtell exhibit initial 26 al 27 al ratios 26 al 27 al 0 ranging from 5 10 5 to 2 ma after the first cais as in other cv3 chondrites some objects in axtell show evidence of isotopic disturbance axtell has experienced only mild thermal metamorphism 600 c probably not enough to disturb the al mg systematics its cais and chondrules have suffered extensive metasomatism probably prior to final accretion these data indicate that cais and chondrules in axtell and other meteorites had an extended history of several million years before their incorporation into the axtell parent body these long time periods appear to require a mechanism in the early solar system to prevent cais and chondrules from falling into the sun via gas drag for several million years before final accretion we also examined the compositional relationships among the four plagioclase bearing chondrules two with large anorthite laths and two barred olivine chondrules and between the chondrules and cais three processes were examined 1 igneous differentiation 2 assimilation of a cai by average nebular material and 3 evaporation of volatile elements from average nebular material we find no evidence that igneous differentiation played a role in producing the chondrule compositions although the barred olivine compositions can be related by addition or subtraction of olivine methods 2 and 3 could have produced the composition of one chondrule axch 1471 but neither process explains the other compositions our study indicates that plagioclase bearing objects originated through a variety of processes

time scales of magma formation ascent and storage beneath subduction zone volcanoes there is now sucient information to attempt an integrated model for melt gener ation transfer and storage beneath subduction zone volcanoes fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr to as little as less than 1 kyr before eruption this supports models in which uid addition is closely linked to partial melting though there may also be evidence for a component of decompression melting the timing of the onset of uid addition may be linked to the rate of subduction i e water supply rate and the angle of subduction and consequently the thermal structure of the mantle wedge in contrast contributions from subducted sediments to subduction zone lava sources appear to occur some 350 kyr 4 myr before eruption evidence for partial melting of the sediment component combined with the short uid transfer times phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate new 226 ra data permit only a short period of time between uid addition and eruption this requires rapid melt segregation magma ascent by channelled ow and minimal residence time within the lithosphere typically the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs inferred from some geophysical data to lie within the lithospheric mantle mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers where they can evolve to dacitic compositions via fractionation typically combined with assimilation on time scales of a few thousand years or less

simulations of convection with crystallization in the system kalsi2o6 camgsi2o6 implications for compositionally zoned magma bodies

experimental study of water degassing from phonolite melts implications for volatile oversaturation during magmatic ascent abstract we have experimentally studied degassing of phonolite melts between 50 and 200 mpa and 825 and 850 c our results indicate that decompressed phonolite melts contain equilibrium water contents regardless of decompression rate or final pressure in addition the total porosities of our experiments agree with predicted equilibrium values regardless of final pressure or decompression rate combined observations of mean bubble diameters porosities and arrangement of bubbles in our experiments indicate that degassing occurs preferentially in the large bubble fraction of the population because of this retardation of growth in the small bubble population occurs this may lead to bimodal bubble size distributions in completely degassed phonolites our results indicate that water saturated phonolites at temperatures 800 c can degas in equilibrium while rhyolites follow a disequilibrium trend at decompression rates of 0 5 1 0 mpa s thus hotter water saturated phonolitic magmas are likely to maintain equilibrium magma water contents during ascent at rates up to 20 m s which are typical of highly explosive eruptions these results may apply to eruptions such as the 79 ad eruption of vesuvius which is the type location for plinian style explosive eruptions however caution must be used when applying these results to lower temperature more differentiated and water undersaturated phonolite magmas if fragmentation can occur at depths of 2 km or greater in the conduit our results indicate that highly explosive eruptions of phonolitic magma may occur in the absence of water oversaturation in the magma during ascent fast ascent rates and high shear stresses in the conduit may provide a better explanation for fragmentation in this case

the effect of diffusive re equilibration time on trace element partitioning between alkali feldspar and trachytic melts abstract we present new experimental data on major and trace element partition coefficients between alkali feldspar and trachytic melt experiments were conducted at 500 mpa 870 890 c to investigate through short disequilibrium and long near equilibrium experiments the influence of diffusive re equilibration on trace element partitioning during crystallization our data show that ba and sr behave compatibly and their partition coefficients are influenced by re equilibration time orthoclase or content growth rate and cation order disorder high field strength elements hfse and rare earth elements except eu are strongly incompatible but alkali feldspar efficiently fractionates light lree from heavy rare earth elements hree our crystallization experiments reveal a strong influence of disequilibrium crystal growth on the partitioning of ba and sr in particular short duration experiments show that rapid alkali feldspar crystal growth after nucleation promotes disordered growth and less selectivity in the partitioning of compatible trace elements that easily enter the crystal lattice e g ba and sr this produces partition coefficients of compatible elements higher than those obtained through long duration experiments in which growth is slower and more selective on approach to equilibrium decreasing growth rate and timescales of diffusive re equilibration facilitates the incorporation of compatible elements within the crystal lattice through ordered growth on the basis of the charge and size of the octahedral site resulting in lower partition coefficients of ba and sr therefore our results indicate that besides crystal chemical effects i e lattice strain the substitution mechanisms during growth with different degrees of ordering of ba and sr in alkali feldspar are strongly influenced by diffusive re equilibration and crystal growth kinetics this implies that partition coefficients of ba and sr can be used to establish timescales of crystallization processes under pre and syn eruptive conditions the application of our results to alkali feldspar in rocks from campi flegrei constrain the magma residence time at subliquidus conditions in a reservoir to a maximum of 6 days under disequilibrium conditions and to a minimum of 9 days upon approaching near equilibrium conditions

vesiculation of may 18 1980 mount st helens magma the may 18 1980 eruption of mount st helens washington produced both white and gray pumice similar in composition but varying in phenocryst microlite and vesicle content the white pumice has fewer phenocrysts no microlites and higher vesicularity and is thus less dense than the gray in addition vesicles in the white pumice are larger and more interconnected than those in the gray both white and gray have effective crystal free vesicularities 85 7 and 72 2 respectively close to the traditionally accepted fragmentation vesicularity of 75 77 given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites the presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity but appears to have aided bubble nucleation and hindered bubble expansion and coalescence thus magmas with microlites may fragment at a lower bulk vesicularity than those without microlites fragmented microlite bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation

geochemical study on a well in the western canada basin relation of the aromatic distribution pattern to maturity of organic matter abstract thirty one rock samples from a western canada basin well have been analysed for aromatics using glass capillary gas chromatography with simultaneous flame ionization and sulfur selective flame photometric detection a uniform aromatic distribution pattern with a marked predominance of di and tricyclic aromatic hydrocarbons was observed throughout a depth interval of 3000 meters comprising cretaceous and jurassic sediments the very high relative abundance of sulfur aromatics at two narrow intervals in the triassic and mississippian is attributed to a major change in facies gradual changes with increasing depth have been observed for a series of compound ratios which had been calculated from the isomers of methyl and dimethylphenanthrene and their parent compounds these changes reflect the increasing thermal maturation of the sedimentary organic matter a methylphenanthrene index mpi has been introduced which exhibited a very good correlation with vitrinite reflectance data the mpi is the first maturity parameter which is based on a series of aromatic hydrocarbons this hydrocarbon internal maturity parameter permits the recognition and definition of maturity levels in extracts which can be compared to the maturity e g vitrinite reflectance of source rocks

quantifying volcanic ash dispersal and impact of the campanian ignimbrite super eruption 1 we apply a novel computational approach to assess for the first time volcanic ash dispersal during the campanian ignimbrite italy super eruption providing insights into eruption dynamics and the impact of this gigantic event the method uses a 3d time dependent computational ash dispersion model a set of wind fields and more than 100 thickness measurements of the ci tephra deposit results reveal that the ci eruption dispersed 250 300 km 3 of ash over 3 7 million km 2 the injection of such a large quantity of ash and volatiles into the atmosphere would have caused a volcanic winter during the heinrich event 4 the coldest and driest climatic episode of the last glacial period fluorine bearing leachate from the volcanic ash and acid rain would have further affected food sources and severely impacted late middle early upper paleolithic groups in southern and eastern europe citation costa a a folch g macedonio b giaccio r isaia and v c smith 2012 quantifying volcanic ash dispersal and impact of the campanian ignimbrite super eruption geophys res lett 39 l10310 doi 10 1029 2012gl051605

timing of magma extraction during the campanian ignimbrite eruption campi flegrei caldera a core drilled within the northern part of the city of napoli has offered the unique opportunity to observe in one single sequence the superposition of the four pyroclastic flow units emplaced during the campanian ignimbrite ci eruption such a stratigraphic succession has never been encountered before in natural or in man made exposures therefore the ci sequence was reconstructed only on the basis of stratigraphic correlations and compositional data in literature the occurrence of four superposed ci flows together with all the data available in literature allowed us to better constrain the chemical stratigraphy of the deposit and the compositional structure of the ci magma chamber the ci magma chamber includes two cogenetic magma layers separated by a compositional gap the upper magma layer was contaminated by interaction with radiogenic fluids the two magma layers were extruded either individually or simultaneously during the course of the eruption in the latter case they produced a hybrid magma but no evidence of input of new geochemically and isotopically distinct magma batches just prior or during the eruption has been found comparison with the exposed ci deposits has permitted reconstruction of variable eruption phases and related magma withdrawal and caldera collapse episodes the eruption was likely to have began with phreatomagmatic explosions followed by the formation of a sustained plinian eruption column fed by the simultaneous extraction from both magma layers towards the end of this phase the upward migration of the fragmentation surface and the decrease in magma eruption rate and or activation of fractures formed an unstable pulsating column that was fed only by the most evolved magma layer this plinian phase was followed by the collapse of the eruption column and the beginning of caldera formation at this stage expanded pyroclastic flows fed by the upper magma layer in the chamber generated during the following major caldera collapse episode the maximum mass discharge rate was reached and both magma layers were tapped generating expanded pyroclastic flows towards the end of the eruption only the deeper and less differentiated magma layer was tapped producing more concentrated pyroclastic flows that traveled short distances

carboxylic acids as indicators of oil migration ii case of the mahakam delta indonesia the acid distributions of oils and coals from a wide depth range in the mahakam delta indonesia were studied in order to determine the effects of oil migration the crude oils of the mahakam delta indonesia are particularly well suited for such a study since the depth of the reservoir can be directly related to the migration distance of the oil two processes seem to control the changes in the acidic biomarker distributions during oil migration solubilization of immature organic matter during migration and to a much lesser extent geochromatography both processes are reflected differently in the acid distributions of the oils the solubilization process is most clearly reflected in variations taking place with increasing degree of migration increasing shift towards long chain acids with a higher even predominance and increase in cyclic acids typical of immature zones a geochromatographic effect may be seen in the depletion of short vs long chain acids with increasing vertical migration distance

phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite the campanian ignimbrite is a 4200 km trachyte phonolite pyroclastic deposit that erupted at 39 3 0 1ka within the campi flegrei west of naples italy here we test the hypothesis that campanian ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts 5 10 wt of crustal rock skarns and foid syenites during crystallization comparison of observed crystal and magma compositions with results of phase equilibria assimilation fractionation simulations melts is generally very good oxygen fugacity was approximately buffered along qfm 1 where qfm is the quartz fayalite magnetite buffer during isobaric fractionation at 0 15 gpa 6 km depth the parental melt reconstructed from melt inclusion and host clinopyroxene compositions is found to be basaltic trachyandesite liquid 51 1 wt sio2 9 3 wt mgo 3 wt h2o a significant feature of phase equilibria simulations is the existence of a pseudo invariant temperature 883 8c at which the fraction of melt remaining in the system decreases abruptly from 0 5 to50 1 crystallization at the pseudo invariant point leads to abrupt changes in the composition properties density dissolved water content and physical state viscosity volume fraction fluid of melt and magma a dramatic decrease in melt viscosity from 1700 pa s to 200 pa s coupled with a change in the volume fraction of water in magma from 0 1 to 0 8 and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger thermal models suggest a timescale of 200 kyr from the beginning of fractionation until eruption leading to an apparent rate of evolved magma generation of about 10 3 km year in situ crystallization and crystal settling in density stratified regions as well as in convectively mixed less evolved subjacent magma operate rapidly enough to match this apparent volumetric rate of evolved magma production

magnesium isotopic compositions of olivine spinel and hibonite from the murchison carbonaceous chondrite the mg isotopic compositions of olivine spinel and hibonite from the murchison carbonaceous chondrite have been determined by secondary ion mass spectrometry the olivines exhibit only marginal effects they have no detectable intrinsic mass fractionation however after correcting for mass fractionation the mean residual 26mg24mg for the five grains is slightly in excess of the terrestrial value blue spinel which often contains inclusions of hibonite and perovskite is characterised by positive fractionation enriched in heavier isotopes ranging from 1 to 46 permil amu and with a roughly constant residual of 1 04 0 25 2 m permil these characteristics can be derived from an initial terrestrial mg isotopic composition through varying degrees of mass fractionation and the addition of 26mg from 26al decay the hibonites show both positive and negative fractionation and residuals three deep blue granular hibonites show large fractionation from 10 to 17 permil amu with residuals ranging from 1 8 to 9 7 permil the remaining five colourless hibonites are closer to the terrestrial composition the fractionation for these grains varies from 5 to 3 permil amu and the residual from 7 0 to 2 5 permil there is a hint of a correlation between fractionation and residual effects

magma dynamics beneath kameni volcano thera greece as revealed by crystal size and shape measurements size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from kameni volcano greece can be modelled by mixing two populations of crystals each with overlapping linear crystal size distributions csd termed microlites and megacrysts the magmas bearing the microlites and megacrysts started to crystallise 6 13 and 24 96 years respectively before each eruption the dates of initiation of crystallisation of the megacrysts indicate that they are left overs of earlier injections of new magma into a shallow chamber some magma remains after each eruption and continues to crystallise new magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma eruption followed 6 13 years after mixing such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied just topped up from time to time

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

a chemostratigraphic study of the campanian ignimbrite eruption campi flegrei italy insights on magma chamber withdrawal and deposit accumulation as revealed by compositionally zoned stratigraphic and facies framework abstract petrochemical analyses of juvenile samples from twenty stratigraphic sections of the campanian ignimbrite medial deposits located from 30 to 79 km from the vent are presented here sampling has accurately followed a well defined stratigraphic framework and the new component facies scheme the campanian ignimbrite succession is formed by a basal plinian pumice fall deposit overlain by a complex architecture of pyroclastic density current deposits emplaced from a single sustained pyroclastic density current through a mechanism of vertical and lateral accretion the deposit is broadly zoned from more evolved trachyte at its base to less evolved trachyte at its top and is similarly less evolved with increasing distance from the area of emission irregular chemical trends are locally observed and interpreted to represent only a limited patchy record of the entire vertical geochemical trend the petrochemical variation observed horizontally was ascribed to changes in the flow dynamics and interaction between the advancing flow and the underlying topography the results of this study were used to propose a unified volcanological petrological model for the campanian ignimbrite eruption taking into account the emplacement of both the proximal i e the breccia museo formation and medial deposits

crystal mush reactivation by magma recharge evidence from the campanian ignimbrite activity campi flegrei volcanic field italy abstract processes of crystal mush remobilization by mafic magma recharges are often related to the outpouring of large volumes of silicic melt during caldera forming eruptions this occurred for the campanian ignimbrite ci eruption campi flegrei italy which produced a voluminous trachy phonolitic ignimbrite in southern central italy about 40 ka ago we focussed on the proximal ci deposits at san martino that are composed of a main sequence of early erupted crystal poor units and a late erupted post caldera collapse crystal rich upper pumice flow unit upfu detailed micro analytical geochemical data were performed on glasses and crystals of pyroclasts from these deposits and coupled with sr nd isotopic measurements on glasses results show that the ci eruption was fed by two distinct melts for the early erupted units and the late upfu respectively the glasses of the early erupted units have negative eu anomalies and show more evolved compositions and higher nd isotope ratios than those of the upfu which have positive eu eu the magmas of the early units formed the main volume of eruptible melt of the ci reservoir and are interpreted as having been extracted from cumulate crystal mush without a vertical geochemical gradient within the magma reservoir the data indicate that the generation of the distinctive upfu melts involved the injection of a new batch of mafic magma into the base of the ci reservoir the mafic magma allowed heating and reactivation of the ci crystal mush by melting of low or sanidines low an plagioclases leaving high an plagioclases and high mg clinopyroxenes as residual phases and a crystal mush melt made of 20 of the initial mush interstitial melt with a composition similar to the early erupted units and 80 of sanidine melt when the mush crystallinity was sufficiently reduced the mafic magma was able to penetrate into the reactivated crystal mush mixing with variable proportions of crystal mush melt and generating cooler hybrid melts which underwent further crystallization of high or sanidine at variable degrees 10 25 finally possibly a short time before the eruption the upfu magmas were able to mix and mingle with the crystal poor eruptible melts still persisting in the ci reservoir at the time of upfu emission we suggest that the complex mechanisms described for the magma evolution feeding the ci eruption may occur whenever a crystal mush is reactivated by new mafic magma inputs

rejuvenation of the fish canyon magma body a window into the evolution of large volume silicic magma systems voluminous unzoned phenocryst rich pyroclastic deposits considered as erupted batholiths provide a unique opportunity to investigate magmatic processes in silicic magmas the fish canyon tuff a well documented example of these monotonous ignimbrites displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near isobaric reheating from 720 to 760 8c these observations along with a high crystallinity 45 and near solidus mineral assemblage suggest that the fish canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption rejuvenation was triggered by intrusion of water rich mafic magmas at the base of the fish canyon mush but the mechanisms of heat transfer remain poorly understood the growth of amphibole during reheating requires addition of mafic components but the absence of any measurable gradients and the paucity of mafic enclaves in the fish canyon magma rule out a reheating event dominated by convective mixing with a mafic magma closed system processes such as heat conduction and convective self mixing could not account for the transport of externally derived mafic components we performed numerical simulations of upward percolation of a hot low density h2o co2 fluid phase gas sparging through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems sparging by 20 40 km 3 of gas extracted from 3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by 40 8c in 150 200 k y moreover the vertical thermal gradient after 150 k y in most of the mush is small 25 8c in the upper 65 gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal poor rhyolites by expelling interstitial melt however our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites

heterogeneous bubble nucleation on fe ti oxide crystals in high silica rhyolitic melts the nucleation of h2o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 740 800 c and 50 175 mpa decompressed by 20 70 mpa and held at the lower pressures for 10 s before being quenched the hydration conditions were subliquidus and all samples contain blocky magnetite needle shaped hematite plagioclase magnetite is abundant at 800 c and high pressures whereas hematite becomes more abundant at lower temperatures and pressures bubbles nucleated in a single event in all samples with the number density nt of bubbles varying between 2 107 and 1 109 cm 3 at low degrees of supersaturation one to a few bubbles nucleate on faces of magnetite but at medium to high degrees of supersaturation multiple bubbles nucleate on single magnetite grains on hematite one to a few bubbles nucleated at the ends of the needle shaped crystals at medium supersaturations but formed along their entire lengths at high supersaturations nt increases as water diffusivity decreases indicating that the number of bubbles nucleated is influenced by their growth which depletes the melt with respect to h2o and lowers supersaturation if volcanic eruptions are triggered by bubble formation in magmas stored in shallow level magma chambers then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed whereas homogeneous nucleation is unlikely to trigger eruptions

a significant middle pleistocene tephra deposit preserved in the caves of mulu borneo article i nfo article history received 23 may 2011 available online xxxx a distinctive white sediment in the caves of mulu sarawak borneo is a well preserved tephra representing a fluvially transported surface air fall deposit re deposited inside the caves we show that the tephra is not the younger toba tephra formerly considered as most likely the shards are rod shaped with elongate tubular vesicles the largest grains 170 m in length of rhyolitic composition and 87 sr 86 sr ratio of 0 70426 0 00001 u th dating of associated calcites suggest that the tephra was deposited before 125 4 ka and probably before 156 2 ka grain size and distance from closest potential source suggests an eruption of vei 7 prevailing winds grain size thickness of deposit location of potential sources and sr isotopic ratio limit the source to the philippines comparisons with the literature give the best match geochemically with layer 1822 from ku et al 2009a dated by ocean core stratigraphy to 189 ka this tephra represents a rare terrestrial repository indicating a very substantial plinian ultra plinian eruption that covered the mulu region of borneo with ash a region that rarely receives tephra from even the largest known eruptions in the vicinity it likely will be a valuable chronostratigraphic marker for sedimentary palaeontological and archaeological studies

tephra dispersal during the campanian ignimbrite italy eruption implications for ultra distal ash transport during the large caldera forming eruption the campanian ignimbrite eruption dispersed ash over much of the central eastern mediterranean sea and eastern europe the eruption started with a plinian phase that was followed by a series of pyroclastic density currents pdcs associated with the collapse of the plinian column and the caldera the glass compositions of the deposits span a wide geochemical range but the plinian fallout and pdcs associated with column collapse the lower pumice flow only erupted the most evolved compositions the later pdcs the breccia museo and upper pumice flow erupted during and after caldera collapse tap a less evolved component and intermediate compositions that represent mixing between the end members the range of glass compositions in the campanian ignimbrite deposits from sites across the central and eastern mediterranean sea allow us to trace the dispersal of the different phases of this caldera forming eruption we map the fallout from the plinian column and the plumes of fine material associated with the pdcs co pdcs across the entire dispersal area this cannot be done using the usual grain size methods as deposits in these distal regions do not retain characteristics that allow attribution to either the plinian or co pdc phases the glass compositions of the tephra at ultra distal sites 1500 km from the vent match those of the uppermost pdc units suggesting that most of the ultra distal dispersal was associated with the late co pdc plume that was generated during caldera collapse

crystal size distribution analysis of plagioclase in experimentally decompressed hydrous rhyodacite magma article i nfo this study presents crystal size distributions csd of plagioclase forming during decompression experiments of hydrous rhyodacite magma samples were annealed at 130 mpa subjected to continuous decompression at either 2 mpa h 1 or 0 5 mpa h 1 and then quenched along their respective decompression paths at 20 mpa intervals samples display concave up curved csd plots which result from a combination of two crystal populations those which formed during the anneal period and those which formed during decompression the csds also display a decrease in crystal number density at 3 7 m a size range that is easily resolvable with the imaging strategy employed the downturn at small sizes is presumably due to insufficient compensation for the intersection probability effect in converting 2d measurements to 3d size distributions crystal nucleation and growth rates derived from csds using standard assumptions are compared with values obtained using 2d measurements of bulk crystal populations batch methods csd calculated nucleation rates are substantially low relative to the batch values in fact csd derived volumetric nucleation rates may underestimate actual nucleation rates by up to two orders of magnitude in contrast growth rates from csds are consistently higher than batch rates although plagioclase growth rates are relatively constant during decompression at a given rate the average growth rate in the rapidly decompressed series is approximately five times faster than the crystal growth rate in the slowly decompressed series because crystal growth rate depends on decompression rate csds are incapable of revealing decompression timescales or magma ascent rates without independent knowledge of crystal growth rate published by elsevier b v

two models for the formation of magma reservoirs by small increments abstract it is now clear that magma reservoirs develop over long time intervals out of a large number of individual intrusions evidence from some large volcanic deposits including those of the bishop tuff california and fish canyon tuff colorado indicates that such reservoirs may experience pervasive and rapid heating before catastrophic eruptions injection of hot primitive melt in the reservoir is often invoked to explain these observations but this requires the rapid emplacement of a large magma volume at least 25 km 3 in less than 100 years for the bishop tuff and efficient heat transfer through a thick cumulate pile within a magma body that grows incrementally temperatures rise progressively and may eventually lead to a permanent volume of melt temperature and crystallization follow two completely different paths depending on the thickness of individual intrusive sheets in one limit corresponding to thick sheets crystallization proceeds at equilibrium initial magma batches crystallize completely and a permanent body of melt develops progressively once ambient temperatures in the complex rise above the solidus in the other limit corresponding to thin sheets crystallization is kinetically controlled such that initial intrusions do not crystallize completely and preserve a glassy residue once temperatures within the growing magma pile reach a certain threshold value nucleation and growth of crystals get reactivated in the residual glass crystallization then proceeds catastrophically in a positive feedback loop involving latent heat release and temperature rise thus an initial phase with no evolved melt present ends with the sudden formation of a large volume of evolved melt in a crystal mush after this initial phase new intrusions get emplaced in heated surroundings and follow a thermal path similar to that of the equilibrium case with melts that become increasingly more primitive with time evidence for kinetic controls on crystallization in the natural environment is provided by the significant amounts of glass up to 40 that are found in thin isolated basaltic sills and dykes owing to sluggish crystallization kinetics more glass should be generated in andesitic and dacitic magmas emplaced in similar conditions devitrification of such quantities of glass accounts for the rapid heating of a thick sill complex the two models of magma reservoir formation involve different requirements the intrusion thickness must be smaller than a critical value in the kinetic model and the average magma input rate must exceed a threshold value in the equilibrium model for a given injection rate a permanent magma reservoir forms in a shorter time interval and over a smaller cumulative magma thickness in the kinetic model than in the equilibrium one the kinetic model is such that the amount of evolved melt generated increases with decreasing injection rate in contrast to the equilibrium model for which the opposite relationship holds

the origin of composition gaps at south sister volcano central oregon implications for fractional crystallization processes beneath active calc alkaline volcanoes abstract south sister is an active calc alkaline strato volcano located in west central oregon in the high cascades south sister contains lavas that range from basalt 50 sio 2 to rhyolite 75 sio 2 within this compositional spectrum the lavas are restricted to three distinct populations basalt basaltic andesite andesite and rhyolite compositional textural and mineralogic criteria indicate that the basaltic andesites are hybrid mixtures of basalt and andesite magma this implies that the system is characterized by basalt andesite and rhyolite magma populations that are separated from one another by composition gaps major minor and trace element modeling suggests that the three magma populations are related to one another primarily through fractional crystallization we propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ fractional crystallization occurs within a downward descending solidification front extraction and eruption of the interstitial fractionated liquid now separated from the parental liquid by a composition gap occurs only after the percentage of crystallization within the solidification front has exceeded the appropriate critical crystallinity value for the given parental magma we envision a magmatic plumbing system characterized by multiple independently solidifying bodies of magma thus at any given time discrete bodies of basalt andesite and rhyolite magma may exist interaction and mixing among these bodies could generate the continuous range of lava compositions that characterize most calc alkaline volcanic complexes