rapid magma ascent recorded by water diffusion profiles in mantle olivine mechanisms and rates of magma ascent play a critical role in eruption dynamics but remain poorly constrained phenomena water dissolved in mantle minerals as hydrogen and partitioned into the magma during ascent may provide clues to quantifying magma ascent rates prior to eruption we determined the dehydration profiles in olivine crystals from peridotite mantle xenoliths within the pali aike alkali basalt from patagonia chile the results demonstrate that the amount of water stored in the uppermost mantle has likely been underestimated due to water loss during transport using experimental diffusion data for hydrogen we estimate that the xenoliths reached the surface from 60 70 km depth in several hours a surprisingly rapid rise comparable to ascent rates for kimberlite magmas

temperature and density of the tyrrhenian lithosphere and slab and new interpretation of gravity field in the tyrrhenian basin abstract the gravity anomaly field of the tyrrhenian basin and surrounding regions reflects the complex series of geodynamic events active in this area since the oligocene miocene they can resume in lithospheric thinning and asthenospheric rising beneath the tyrrhenian basin coexisting with the roll back subduction of the african plate margin westward sinking beneath the calabrian arc the geographic closeness between these processes implies an intense perturbation of the mantle thermal regime and an interference at regional scale between the related gravity effects a model of the litho asthenospheric structure of this region is suggested showing a reasonable agreement with both the evidences in terms of regional gravity anomaly pattern and the results concerning thermal state and petro physical features of the mantle the first phase of this study consisted of the computation of the isotherms in the crust mantle system beneath the tyrrhenian basin and afterwards of the density distribution within the partially melted upwelling asthenosphere the second phase consisted of a temperature density modelling of the slab subducting beneath the calabrian arc finally a 21 2 interpretation of gravity data was carried out by including as constraints the results previously obtained thus the final result depicts a model matching both gravity thermal and petrographic data they provide a a better definition of the thermal regime of the passive mantle rise beneath the tyrrhenian basin by means of the estimation of the moderate asthenospheric heating and b a model of lithospheric slab subducting with rates that could be smaller than generally suggested in previous works

reconstruction of the upper cretaceous chalks removed by dissolution during the cenozoic in the western paris basin during the cenozoic in the western paris basin atmospheric weathering of the chalks with flints of the upper cretaceous led to the creation of clay with flints a reconstitution of the chalks lost to dissolution is proposed and is based on the determination of the age of the parent chalks of the clay with flints and the quantification of the thickness of dissolved chalk the chalks affected by weathering range in age from turonian to maastrichtian thus confirming the deposition of calcareous sediments in the western paris basin up to the maastrichtian chalk weathering took place in situ as indicated by the preservation of the stratigraphic succession of the chalk in the clay with flints profiles weathering led to the dissolution of 20 200 m of chalk with regional variations the weathering rate varies between 2 1 and 14 5 m ma

viscosity of hydrous etna basalt implications for plinian style basaltic eruptions water dissolved in a silicate melt can strongly influence its physical properties and thus magma behavior during crystallization degassing foaming and fragmentation etna is a basaltic volcano whose activity is dominated by effusive eruptions which have long represented a threat to the densely populated surrounding area recently recognition of the products of a plinian eruption 122 b c has raised further issues for hazard assessment at etna and other basaltic volcanoes constraining the behavior of etna magma under conditions relevant to both effusive and explosive hazards requires viscosity data under conditions near the glass transition here we have investigated the viscosity of hydrous etna lava in order to better understand eruptive processes which characterize this volcano the experimental methods which have been used include piston cylinder synthesis of the hydrated melts micropenetration viscometry for low temperature viscosity measurements and near infrared spectroscopy for the evaluation of sample homogeneity and measurements of water content additionally scanning calorimetric determinations were performed to check whether incipient crystallization had occurred sample compositions were determined using electron microprobe analysis and 57fe mossbauer spectroscopy results from this study are compared with previous reports of trachytic phonolitic and model calc alkaline rhyolite hpg8 compositions the viscosity of the basaltic melt dry and wet has been parameterized in terms of temperature and water content via the non arrhenian equation log10 4 643 5 812 44 427 04 h2o t k 499 31 28 74 ln h2o where is the viscosity in pa s h2o is the water content in wt and t is the temperature in kelvin we observe that the viscosity of alkali basalt at more than 0 5 wt h2o is similar to that of an alkaline trachyte agnano monte spina eruption phlegrean fields and much higher than that of a peralkaline phonolite teide tenerife at similar silica contents and nbo t for water contents above 1 5 wt the viscosity of the basalt is similar to that of rhyolitic melts with similar water contents at temperatures ranging from 1 050 to 1 150 c and with water contents between 0 5 and 2 3 wt eruptive conditions the viscosities calculated using the equation defined in this study are 1 in reasonable agreement with those calculated using shaw s model and 2 much lower than those experimentally determined in a previous study however outside these temperature and water content ranges the agreement with shaw s model 1972 breaks down

the effect of water on the viscosity of a haplogranitic melt under p t x conditions relevant to silicic volcanism the viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040 c have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques the micropenetration viscometry has been performed in the viscosity range of 109 pa s to 1012 pa s the samples ranged in water content from 0 4 to 3 5 wt for samples with up to 2 5 wt h2o the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements above this water content a measurable loss of water occurs during the viscometry the viscosity data illustrate an extremely nonlinear decrease in viscosity with added water the viscosity drops drastically with the addition of 0 5 wt of water and then shallows out at water contents of 2 wt an additional viscosity datum point obtained from the analysis of fluid inclusions in a water saturated hpg8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt these measurements may be compared directly with the data of hess et al 1995 in press for the effects of excess alkali and alkaline earth oxides on the viscosity of hpg8 also obtained at 1 bar the viscosity of the melts compared on an equivalent molar basis increases in the order h2o li2o na2o k2o rb2o cs2o bao sro cao mgo beo the extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of shaw 1972 for the range of water contents investigated here the speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger consequences for degassing calcalkaline rhyolite are discussed

short term biomarker variability in the monterey formation santa maria basin abstract forty eight samples of a 370 ft miocene monterey formation core from the union leroy 51 18 well santa maria basin california are examined for source rock potential and molecular distributions total organic carbon content rock eval pyrolytic yield vitrinite reflectance and distribution of regular steranes tricyclic and pentacyclic terpanes and aromatic steroids are measured the present day temperature range through the sampled interval is estimated to be less than 5 c maturity differences among samples are negligible source rock potential and molecular distribution differences are considered to arise entirely from the effects of source input diagenesis including lithologic effects and migration the sampled section is comprised of siliceous and phosphatic carbonate lithofacies and covariances between geochemical parameters source potential and biomarker distribution and lithofacies are recognized several molecular parameters change systematically with depth lithology including ethylcholestane ratios and c 20 and c 22 sterane and hopane epimer ratios the siliceous lithofacies possesses higher moretane hopane tricyclic pentacyclic terpane and sterane terpane ratios and lower mono mono tri aromatic steroid ratios bisnorhopane hopane ratios are up to 40 times higher in the phosphatic carbonate lithofacies than in the siliceous lithofacies increased relative concentrations of aromatic steroids are attributed to sulfur sterol reactions during very early diagenesis whereas increased relative bisnorhopane amounts are considered to result from massive input of specific anaerobic biota although kerogen elemental ratios change abruptly at the lithofacies boundary molecular distributions within the extractable organic matter change gradually beginning about 30 ft 9m above this interface this gradual change is tentatively attributed to upward migration of hydrocarbons originating in the phosphatic carbonate lithofacies probably through fractures within the lower part of the siliceous lithofacies interval

the influence of geometry on the ascent of magma in open fissures during steady eruption the flow conditions emitted mass flux exit velocity and exit pressure depend on the geometry of the conduit in which the eruption occurs this dependence is examined for the onedimensional isothermal ascent of a homogeneous basaltic magma with an aqueous volatile phase and newtonian rheology by fixing the geometry of the feeding fissure the mass flux flowing in steady conditions can be determined at any depth as well the magma pressure and vertical velocity flow behaviour is analysed for three fissure shapes constant width slowly upward narrowing and lenticular in all the cases examined the magma arrives at the earth s surface with a pressure greater than atmospheric the results are compared with those obtained when a lithostatic pressure gradient is assumed for the magma column some speculations are made moreover about the change in eruption style if conduit geometry varies during a non steady phase

mafic and ultramafic enclaves in ustica island lavas inferences on composition of lower crust and deep magmatic processes abstract ustica island southern tyrrhenian sea is constituted of quaternary alkaline volcanics a variety of enclaves representative of deep to supra crustal settings were recently found in a hawaiitic lava flow enclaves consist of i ultramafic meta cumulates i e clinopyroxenites and wherlites characterized by variably deformed porphyroclastic to granoblastic textures ii mafic cumulates i e gabbros amphibole and troctolites the first often characterized by frequent amphibole breakdown coronas olivine ti augite plagioclase magnetite ilmenite rhonite in response to an h 2 o decrease during the ascent while the troctolites interpreted as meta cumulates iii microsyenites consist of anorthoclase and fe clinopyroxene organized in a granular sub ipidiomorphic texture amphibole is absent in ustica lavas and is found only in some old now exposed sub intrusive volcanic bodies this evidence suggests a late appearance of amphibole on the liquidus at a high crystal content that inhibits further ascent of the magma the importance of the amphibole as a medium pressure liquidus phase in ustica mafic magmas is in the bearings on the geochemistry of lavas e g in buffering na and ti abundances in trace elements partitioning etc density measurements pointed out higher values for clinopyroxenites 3160 to 3300 kg m 3 than for gabbros ca 2900 kg m 3 given the density contrast between enclaves and host lavas 2790 kg m 3 and assuming appropriate rheological models we calculated a minimum ascent rate of 0 01 m s corresponding to an ascent time in the range of 5 29 days for a depth of entrapment of 25 km

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

flow localization in fissure eruptions during a basaltic fissure eruption heat transfer from the hot magma to the surrounding rock causes a dramatic increase in the magmatic viscosity and solidification at the margins both viscosity contrast and solidification can amplify initial variations in the flow rate and lead to localization of the flow along the strike of the fissure however for typical parameters amplification driven by solidification is slower and significantly weaker than amplification driven by viscosity variations in fact for the parameters examined the amplification due to solidification is so weak that its effect is almost insignificant whereas viscosity variation provides a strong active mechanism for flow localization laboratory experiments illustrate viscous localization and suggest that this mechanism is robust the dependence of viscosity on temperature can cause a small change in the pressure of the magma chamber to lead to a large jump in the flow rate of magma through the fissure

haynesville and bossier mudrocks a facies and sequence stratigraphic investigation east texas and louisiana usa abstract upper jurassic haynesville and bossier shale gas plays of the northwestern gulf of mexico gom basin produce from organic rich marine transgressive to highstand mudrocks within mixed carbonate clastic depositional systems modern wireline log suites from 200 deep wells were used for detailed correlations and 10 cores throughout the upper kimmeridgian to lower tithonian basin were incorporated into detailed facies stratigraphic and lithologic analyses the haynesville shale lies within a 2nd order transgressive systems tract tst that encompasses back stepping ramp carbonates proximal and marine shales distal below a maximum flooding surface mfs this shale onlaps retrogradational carbonates and basement highs and is capped by the 2nd order mfs bossier shales and local sandstones prograde basinward above the haynesville shale and downlap the 2nd order mfs they grade upward and updip into the thick highstand systems tract hst fluvio deltaic sandstones of the cotton valley group distally organic rich facies developed in restricted bossier environments creating another productive gas shale opportunity several preexisting basement highs in the south and carbonate platforms in the northwest and west restrict and partition the basin thus influencing deposition of highly organic versus nonorganic and siliciclastic dominated versus carbonate dominated lithologies haynesville and bossier shales each compose three upward coarsening cycles that probably represent 3rd order sequences within the larger 2nd order tst and early hst respectively most of the three bossier 3rd order cycles are dominated by varying amounts of siliciclastics deposition of the haynesville mudrocks occurred below storm wave base under mostly dysoxic conditionsm allowing settlement of benthic bivalve communities and bioturbating organisms and oscillating periodically to more anoxic conditions on the seafloor however most of the sparse faunae found in these mudrocks are of planktonic origin bossier mudrocks exhibited more oxygenated and clastic rich lithologies related to progradation of the clastic shoreline and sea level highstand knowledge of lithologic stratigraphic geochemical and faunal variations across mudrock basins needs to be detailed so that shale gas basins might be successfully explored

dynamics of strombolian explosions inferences from field and laboratory studies of erupted bombs from stromboli volcano strombolian activity is characterized by repeated low energy explosions and is named after the volcano where such activity has persisted for around 2000 years i e stromboli aeolian islands italy stromboli represents an excellent laboratory where measurements of such explosions can be made from safe but close distances during a field campaign in 2008 two 15 cm diameter bombs were quenched and collected shortly after a normal explosion the bombs were characterized in terms of their textural chemical rheological and geophysical signatures the vesicle and crystal size distribution of the samples coupled with the glass chemistry enabled us to quantify variations in the degassing history and rheology of the magma resident in the shallow i e in last 250 m of conduit length the different textural facies observed in these bombs showed that fresh magma was mingled with batches of partially to completely degassed oxidized high crystallinity high viscosity evolved magma this magma sat at the top of the conduit and played only a passive role in the explosive process the fresh microlite poor vesiculated batch however experienced a response to the explosive event by undergoing rapid decompression integration of geophysical measurements with sample analyses indicates that popular bubble bursting models may not fit this case we suggest that the degassed magma forms a plug or rheological layer at the top of the conduit through which the fresh magma bursts in this model we need to consider the paradox of a slug ascending too fast through a magma of varying viscosity and yield strength

a case history of paroxysmal explosion at stromboli timing and dynamics of the april 5 2003 event abstract on april 5 2003 stromboli volcano italy produced the most violent explosion of the past 50 years the event was exceptionally well documented thanks to the presence on the island of several scientists and a large number of instruments deployed over the preceding months to monitor the effusive eruption that began in december 2002 integration of visual documentation deposit features and geophysical data allowed an accurate reconstruction of the explosive event and its dynamics the eruption consisted of a 8 min long explosive event which evolved through four phases whose timing was precisely recorded by an infrared thermometer located about 450 m from the summit crater phases 2 and 3 lasted 39 and 42 s respectively both had an impulsive character were responsible for ejecting almost the entire mass of the pyroclastic products phases 1 and 4 represented respectively a short lived precursory event and a waning tale during phase 2 meter sized ballistic blocks were launched with velocities of 170 m s to altitudes of up to 1400 m above the craters these fell on the volcano flanks and on the village of ginostra about 2 km distant from the vent a vertical jet rose above the craters which developed to feed a convective plume that reached a height of up to 4 km the calculated mass of the phase 2 fallout deposit and mass discharge rate were 1 1 1 4 108 kg and 2 8 3 6 106 kg s respectively during phase 3 a scoria flow deposit with an estimated volume of 0 9 1 1 104 m3 was erupted from the same vent that fed the ongoing sustained lava flow the average mass discharge rate for this phase was 2 5 3 1 105 kg s products emitted during phases 2 and 3 consisted of lithic and fresh magmatic material in similar proportions the juvenile fraction consisted of a deep originated almost aphyric highly vesicular pumice mingled with a shallow derived crystal rich moderately vesicular scoria similarities with the eruption dynamics of other historical paroxysms at stromboli makes the april 5 2003 explosion representative of these highly energetic events that constitute the most hazardous volcanic phenomena at stromboli volcano

the 2009 paroxysmal explosions at stromboli italy magma mixing and eruption dynamics three small scale paroxysmal explosions also called major explosions interrupted ordinary mild strombolian activity at stromboli on may 3 november 8 and 24 2009 products were largely confined to the summit area except in the november 24 event during which coarse pumiceous lapilli reached the coast emission of crystal poor pumice closely mingled with crystal rich products characterized the three events the textural and chemical study of minerals and glassy matrices revealed that the two end members are mingled together physically in the may 3 and november 24 pumice whereas november 8 products contain heterogeneous glass with intermediate compositions derived from chemical mixing between crystal rich and crystal poor magmas we here discuss the different degrees of interaction between the two magmas in the three explosions in terms of magma dynamics during small scale paroxysms

kerogen based characterization of major gas shales effects of kerogen fractionation abstract research into the origin and the mode of entrapment and expulsion of natural gas from unconventional plays requires the isolation and separation of kerogen in its purest and most intact form from the rock matrix this study expands on the comparative analysis of the effects that isolation methods conservative closed system versus conventional open system have on kerogen s elemental isotopic and physical properties four major gas shales including the barnett the marcellus the haynesville and a polish gas shale were chosen in addition the monterey shale though not strictly a gas shale was included to address the effects on sulfur rich type ii s kerogen results indicate that the kerogen residues from the conventional open system method showed lower recovery and higher mineral content than those from the conservative closed system method differences were manifested in the elemental analysis data where kerogens isolated using the open system method showed a significant deficit in the organic c h o s and n material balance furthermore the recovered residues show different sulfur content and 34s composition most likely attributable to differences in pyrite content nevertheless the relative abundances of the various macerals in the kerogen residues from the same parent shale are not very different neither was the bulk 13c composition of the recovered residues this is not particularly surprising considering that in all the five cases examined in this study the organic matter was fairly homogeneous

an image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the izu peninsula summary earthquake swarms in the area east of the izu peninsula central japan have been active and have been repeated intermittently since 1978 after 40 years of quiescence the activities were always accompanied by crustal deformations which were well modelled by dyke intrusions to study the process of a dyke intrusion precise hypocentres of the latest activity occurring in 1998 are obtained in this paper using waveform similarity by which new images of volcanic processes have been successfully acquired at several volcanoes the relocated hypocentres are mainly aligned on a thin vertical plane with a circular shape at a depth of 3 7 km and there is an aseismic area at the centre the normal direction to the plane coincides well with the direction of tectonic extensional stress around the hypocentral area and matches theoretical models well at the beginning of the activity a small fraction of the events occurred at greater depth where hypocentres align on a vertical line and migrate upward at a rate of 1 km h 1 the migration rate in this stage agrees well with a theoretical model of buoyancy driven dyke propagation after 1 day from the beginning earthquakes began to occur around the aseismic area and hypocentres spread on the thin circular plane described above during this activity the hypocentres seemed to migrate downwards and upwards from the centre the earthquakes are caused by shear fracture at the tip of the dyke and the migration of hypocentres reflects that the dyke expands downward and upward from the neutrally buoyant layer that is located at the centre of the swarm in this paper we also propose a process of magma intrusion based on the precise migration of hypocentres and show that the 1998 activity is composed of a few simple volcanic processes magma rising by buoyancy it staying at a neutral buoyancy point and spreading outward from the neutral point with elastic fracture by an inside excess magma pressure

the neogene and quaternary clay with flints north and south of the english channel comparisons of distribution age genetic processes and geodynamics weathered residual accumulations termed clay with flints argiles a silex overlying cretaceous deposits are widespread in southern england and northwestern france geological and pedological studies and some field surveys indicate their distribution nature age and origin in france the microfaunas preserved in the hollow flints of the clay with flints demonstrate the preservation of the parent chalk stratigraphy in the sections studied i e the clay with flints corresponds to in situ weathering profiles however in england and in france an important part of the clay component is derived from a thin veneer of basal tertiary sediment overlying sub tertiary marine erosion surfaces of various palaeogene transgressions in england as in france the youngest generation of clay with flints can be dated from late pliocene to quaternary it formed after removal of different cenozoic deposits on many plateaus and during the downcutting of pleistocene rivers the most visible effect of subsequent weathering profile development is irregular dissolution of the underlying chalk to produce deep karstic pipes into which the clay with flints has slumped other pedological processes include rubification clay illuviation and cryoturbation clay with flints also provides evidence of quaternary palaeoenvironments cenozoic continental palaeosurfaces and their vertical deformation copyright 2003 john wiley sons ltd

the clay with flints of the western paris basin a potential aggregate resource with future increasing restrictions on extraction of alluvial materials for aggregate production the paris basin in particular is faced with the problem of substituting up to 62 more than 50 mt year of its requirements from other sources the clay with flints formation to the north west west and south east of paris has been the subject of intense studies aimed at determining its suitability as a potential source of substitute aggregate material the formation is located much closer to paris than alternative resources and because the cost of transport constitutes a major factor in the price of aggregates proximity to consumption centres is of prime importance two potential resources targets have been identified within the clay with flints the slope located clay with flints named bief a silex in france despite being rich in flint form deposits that are too small for industrial scale exploitation in addition the biefs a silex tend to be located in forested areas and subjected to environmental constraints where the plateau located clay with flints attains a thickness in excess of 30 m notably in northern eure et loir and southern eure departments an increase in the proportion of flint from 10 m downwards opens up the possibility of dry processing separation of the fine grained clayey matrix either direct from the quarry or after open air drying wet processing has been shown to be unrealistic because of the vast amounts of water that would be required the plateau deposits show far fewer environmental constraints and the resource potential for substituting a significant part of alluvial material consumption in the paris basin is excellent

magmatic evolution and ascent history of the aries micaceous kimberlite central kimberley basin western australia evidence from zoned phlogopite phenocrysts and uv laser 40ar 39ar analysis of phlogopite biotite the neoproterozoic aries kimberlite was emplaced in the central kimberley basin western australia as a n nne trending series of three diatremes infilled by lithic rich kimberlite breccias the breccias are intruded by hypabyssal macrocrystic phlogopite kimberlite dykes that exhibit differentiation to a minor high na si olivine phlogopite richterite kimberlite and late stage macrocrystic serpentine diopside ultramafic dykes mineralogical and geochemical evidence suggests that the high na si olivine phlogopite richterite kimberlite was derived from the macrocrystic phlogopite kimberlite as a residual liquid following extended phlogopite crystallization and the assimilation of country rock sandstone and that the macrocrystic serpentine diopside ultramafic dykes formed as mafic cumulates from a macrocrystic phlogopite kimberlite chemical zonation of phlogopite biotite phenocrysts indicates a complex magmatic history for the aries kimberlite with the early inheritance of a range of high ti phlogopite biotite xenocrysts from metasomatized mantle lithologies followed by the crystallization of a population of high cr phlogopite phenocrysts within the spinel facies lithospheric mantle a further one to two phlogopite biotite overgrowth rims of distinct composition formed on the phlogopite phenocrysts at higher levels during ascent to the surface ultra violet laser 40ar 39ar dating of mica grain rims yielded a kimberlite eruption age of 815 4 4 3 ma 95 confidence 40ar 39ar laser profiling of one high ti phlogopite biotite macrocryst revealed a radiogenic 40ar diffusive loss profile from which a kimberlite magma ascent duration from the spinel facies lithospheric mantle was estimated assuming an average kimberlite magma temperature of 1000 c yielding a value of 0 23 2 32 days for the north extension lobe of the aries kimberlite

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

the viscosity of planetary tholeiitic melts a configurational entropy model abstract the viscosity of silicate melts is a fundamental physical property controlling mass transfer in magmatic systems viscosity can span many orders of magnitude strongly depending on temperature and composition several models are available that describe this dependency for terrestrial melts quite well planetary basaltic lavas however are distinctly different in composition being dominantly alkali poor iron rich and or highly magnesian we measured the viscosity of 20 anhydrous tholeiitic melts of which 15 represent known or estimated surface compositions of mars mercury the moon io and vesta by concentric cylinder and parallel plate viscometry the planetary basalts span a viscosity range of 2 orders of magnitude at liquidus temperatures and 4 orders of magnitude near the glass transition and can be more or less viscous than terrestrial lavas we find that current models under and overestimate superliquidus viscosities by up to 2 orders of magnitude for these compositions and deviate even more strongly from measured viscosities toward the glass transition we used the adam gibbs theory a g to relate viscosity to absolute temperature t and the configurational entropy of the system at that temperature sconf which is in the form of log a e b e ts conf heat capacities cp for glasses and liquids of our investigated compositions were calculated via available literature models we show that the a g theory is applicable to model the viscosity of individual complex tholeiitic melts containing 10 or more major oxides as well or better than the commonly used empirical equations we successfully modeled the global viscosity data set using a constant ae of 3 34 0 22 log units and 12 adjustable sub parameters which capture the compositional and temperature dependence on melt viscosity seven sub parameters account for the compositional dependence of be and 5 for sconf our model reproduces the 496 measured viscosity data points with a 1 root mean square deviation rmsd of 0 12 log units across 13 orders of measured melt viscosity the model performed well in predicting the viscosity of lunar and martian melts not used in calibration and should be used to calculate lava flow velocities and fluxes for anhydrous basaltic volcanism on other moons and planets

slope clay with flints biefs a silex as indicators of quaternary periglacial dynamics in the western part of the paris basin france the lithology and distribution of the clay with flints occurring on slopes called biefs a silex in french suggests a twofold classification of these deposits according to slope angle biefs a silex formed as a solifluction deposit under quaternary periglacial conditions during valley development two biefs a silex facies suggest two phases of development between 800 000 and 20 000 yr bp copyright 2003 john wiley sons ltd

methodological issues raised by laser particle size analysis of deposits mapped as clay with flints from the palaeolithic site of dickett s field yarnhams farm hampshire uk abstract lower and middle palaeolithic stone tools evidence of the earliest occupation of britain are frequently to be found in association with deposits mapped as clay with flints which cap the highest chalk downland hilltops and plateaux of southern england these superficial deposits exhibit great lithological variability i e clay with flints sensu lato including plateau drift clay with flints sensu stricto loess and waterlain brickearths on both a local and regional geographical scale field observations suggest that particular facies of the clay with flints deposits in addition to the presence of solution features may be implicated in the retention of palaeolithic artefacts on these high levels detailed interpretation of sedimentological analyses is therefore required to identify a preservation potential correlation for palaeolithic sites on deposits mapped as clay with flints much of what is generally known about the sedimentology of these deposits is found in literature where the classic sieve and sedimentation methods pipette or hydrometer have been adopted to determine quantitatively particle size distribution of soils refinement in particle size methodologies of the fine fraction through the introduction of laser diffraction granulometry makes it difficult to compare laser research results with published data based on the use of the sieve sedimentation methods this presents problems when attempting to use the available data for both intra and inter palaeolithic site analysis our objective was to find suitable limits for the clay silt and sand fractions to enable calibration between the sieve sedimentation methods and laser diffraction granulometry which underestimates the clay fraction the resultant methodologies described here allow such calibrations to be made with confidence by applying these methodologies geologists soil scientists geomorphologists archaeologists and other workers can access important comparative sedimentological data that would otherwise not be available for interpretation of the depositional context of the sediments soils and any artefacts they may contain samples from the palaeolithic site of dickett s field yarnhams farm holybourne hampshire provided the focus of this work

comparative analysis and geological significance of kerogen isolated using open system palynological versus chemically and volumetrically conservative closed system methods abstract characterization of kerogen the insoluble organic fraction of sedimentary rocks is of key importance in investigations of the geochemistry of ancient depositional systems paleo climates basin thermal history and petroleum generation systematics furthermore exploitation of organic rich oil shales as unconventional petroleum resources requires understanding of the physical chemistry of the kerogen conversion reactions and the coking behavior these issues can be effectively addressed only if pure representative kerogen is obtained from the rock matrix using chemically and volumetrically conservative methods that preclude losses alteration or fractionation of the organic matter isolation of representative and pure kerogen concentrates from the rock matrix is difficult many methods have been used over the years with various degrees of success most are based on traditional palynological processing approaches in open systems that commonly yield fractionated products generally contaminated with pyrite and insoluble neo fluorides a few are based on complex sealed system procedures that purport to conserve the entire kerogen with no fractionation no alteration and no contamination this study focuses on a comparative analysis of kerogens isolated with the conventional open system approach hcl hf heavy liquid separation and kerogens concentrated with a fully automated conservative closed system method hcl hf crcl 2 under an inert n 2 atmosphere source rock samples of various richness kerogen types ages depositional environments lithologies and thermal histories were selected for kerogen isolation using the two approaches kerogen recovery efficiency progress of the processes and quality of the isolated kerogens were monitored via mass balance calculations organic elemental analysis and x ray diffraction analysis the inherent inadequacy in the open system approach for geochemical research and the improved features of the chemically and volumetrically conservative method are documented

application of organic petrography in north american shale petroleum systems a review abstract organic petrography via incident light microscopy has broad application to shale petroleum systems including delineation of thermal maturity windows and determination of organo facies incident light microscopy allows practitioners the ability to identify various types of organic components and demonstrates that solid bitumen is the dominant organic matter occurring in shale plays of peak oil and gas window thermal maturity whereas oil prone type i ii kerogens have converted to hydrocarbons and are not present high magnification sem observation of an interconnected organic porosity occurring in the solid bitumen of thermally mature shale reservoirs has enabled major advances in our understanding of hydrocarbon migration and storage in shale but suffers from inability to confirm the type of organic matter present herein we review organic petrography applications in the north american shale plays through discussion of incident light photographic examples in the first part of the manuscript we provide basic practical information on the measurement of organic reflectance and outline fluorescence microscopy and other petrographic approaches to the determination of thermal maturity in the second half of the paper we discuss applications of organic petrography and sem in all of the major shale petroleum systems in north america including tight oil plays such as the bakken eagle ford and niobrara and shale gas and condensate plays including the barnett duvernay haynesville bossier marcellus utica and woodford among others our review suggests systematic research employing correlative high resolution imaging techniques and in situ geochemical probing is needed to better document hydrocarbon storage migration and wettability properties of solid bitumen at the pressure and temperature conditions of shale reservoirs

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

kerogen asphaltene sulfur relationships in sulfur rich monterey oils abstract monterey oils in the santa maria basin onshore and offshore are generally low maturity oils because of low thermal exposure relatively shallow burial for short times they are generated as heavy high sulfur oils because of the composition of the unusual type ii source kerogens which are exceptionally rich in organic sulfur 8 14 these high sulfur kerogens which we designate type ii s appear to generate oil at significantly lower thermal exposures than ordinary type ii kerogens with less than 6 s in contrast to common type ii kerogens which generate oil largely by cleavage at oxygen bridge groups and carbon carbon bonds eliminating co2 h2o and hydrocarbon fragments type ii s kerogens have an additional complement of weak bonds associated with bound sulfur the preferential cleavage at weak sulfur linkages tends to produce larger fragments leading to high initial amounts of asphaltenes resins and sulfur rich aromatics together with smaller amounts of saturated hydrocarbons elemental analyses of monterey kerogens and asphaltenes clearly support the concept that the asphaltenes are essentially fragments of kerogen cleaved sufficiently to be soluble or peptized in this heavy oil system solubilization of asphaltenes is attributed to their interactions with the abundant resins and aromatics this sysstem is sufficiently stable under source and reservoir conditions to allow expulsion and migration for reasonable distance without significant fractionation or changes in gross properties initial asphaltenes have essentially the same atomic h c ratios as the source kerogen but have significantlyy lower o c ratios and slightly lower s c ratios the n c ratios are nearly the same for kerogens and their derived asphaltenes with increasing maturity the h c ratios in asphaltenes decrease much like maturation changes in kerogen the s c ratio also decreases significantly however the o c ratios initially low in asphaltenes and n c ratios increase slightly due to material balance caused by other losses these element composition and atomic ratio relationships allow some important inferences concerning source kerogen composition from analysis of asphaltenes alone this can be useful when potential source rock samples are not available for analysis and or when a choice between alternate source rock possibilities must be made

intra grain sr isotope evidence for crystal recycling and multiple magma reservoirs in the recent activity of stromboli volcano southern italy over the last several hundred years stromboli has been characterized by steady state strombolian activity the volcanic products are dominated by degassed and highly porphyritic hp magma black scoria bombs lapilli and lava flows of basaltic shoshonitic composition periodically about one to three events per year more energetic explosive eruptions also eject light coloured volatile rich pumices with low phenocryst content lp magma that have more mafic compositions than the hp magma an in situ major and trace element and sr isotope microanalysis study is presented on four samples chosen to characterize the different modes of activity at stromboli a lava flow 1985 1986 effusive event a scoria bomb from the normal present day activity of stromboli april 1984 and a scoria and coeval pumice sample from a recent more explosive eruption september 1996 plagioclase an

it takes three to tango 2 bubble dynamics in basaltic volcanoes and ramifications for modeling normal strombolian activity this is the second paper in a two part series examining numerical simula tions of buoyancy driven ow in the presence of large viscosity contrasts in the rst pa per we demonstrated that a combination of three numerical tools an extended ghost uid type method the level set approach and the extension velocity technique accurately simulate complex interface dynamics in the presence of large viscosity contrasts in this paper we use this three fold numerical method to investigate bubble dynamics in the conduits of basaltic volcanos with a focus on normal strombolian eruptions strombolian type activity named after the famously episodic eruptions at stromboli volcano is char acterized by temporally discrete fountains of incandescent clasts the mildly explosive nature of normal strombolian activity as compared to more eusive variants of basaltic volcanism is related to the presence of dissolved gas in the magma yielding a complex two phase ow problem we present a detailed scaling analysis allowing identication of the pertinent regime for a given ow problem the dynamic interactions between gas and magma can be clas sied into three non dimensional regimes based on bubble sizes and magma viscosity re solving the uid dynamics at the scale of individual bubbles is not equally important in all three regimes as long as bubbles remain small enough to be spherical their dy namic interactions are limited compared to the rich spectrum of coalescence and breakup processes observed for deformable bubbles in particular once inertia ceases to be neg ligible one key nding in our simulations is that both large gas bubbles and large conduit lling gas pockets slugs are prone to dynamic instabilities that lead to their rapid breakup during buoyancy driven ascent we provide upper bound estimates for the maximum sta ble bubble size in a given magmatic system and discuss the ramications of our results for two commonly used models of normal strombolian type activity the rise speed dependent rsd model and the collapsing foam cf model

diffusion and redox reactions of sulfur in silicate melts although large scale transport in magmas is usually controlled by convection diffusion of sulfur in silicate melts nevertheless plays a crucial role in the kinetics of various magmatic processes the most important process is probably the degassing of magmas and melts initiated by an oversaturation of the melt with respect to dissolved volatiles such oversaturation can be caused in nature i e by decompression when a magma ascends to the earth surface but the degassing of melts is also of major interest for technical applications e g during fining of melts in industrial glass production the kinetics of volcanic eruptions and the fining of glass melts are basically controlled by the dynamics of bubble nucleation and growth and these properties are strongly affected by the diffusion of volatiles from the melt into bubbles nemec 1980a b sparks et al 1994 muller simon 2011 this volume hence understanding the diffusion of volatiles in melts provides a necessary tool for modeling bubble nucleation and growth however the diffusivities of volatiles such as h2o co2 so2 and h2s in the melt do not only affect the degassing rate they may also cause a fractionation of volatiles between gas phase and melt as well due to different diffusivities for instance in hydrous magmas the diffusion of h2o is usually much faster than co2 and sulfur diffusion baker et al 2005 zhang and ni 2010 so rapid disequilibrium removal of bubbles from the melt may cause an artificial enrichment of some components in the bubbles other processes which may be governed by sulfur diffusion in the melts are the dissolution and precipitation of minerals in glass manufacturing crucial problems are the kinetics of dissolution of the raw materials in the melt batch and of

an analytical model for gas overpressure in slug driven explosions insights into strombolian volcanic eruptions strombolian eruptions common at basaltic volcanoes are mildly explosive events that are driven by a large bubble of magmatic gas a slug rising up the conduit and bursting at the surface gas overpressure within the bursting slug governs explosion dynamics and vigor and is the main factor controlling associated acoustic and seismic signals we present a theoretical investigation of slug overpressure based on magma static and geometric considerations and develop a set of equations that can be used to calculate the overpressure in a slug when it bursts slug length at burst and the depth at which the burst process begins we find that burst overpressure is controlled by two dimensionless parameters v which represents the amount of gas in the slug and a which represents the thickness of the film of magma that falls around the rising slug burst overpressure increases nonlinearly as v and a increase we consider two eruptive scenarios 1 the standard model in which magma remains confined to the vent during slug expansion and 2 the overflow model in which slug expansion is associated with lava effusion as occasionally observed in the field we find that slug overpressure is higher for the overflow model by a factor of 1 2 2 4 applying our model to typical strombolian eruptions at stromboli we find that the transition from passive degassing to explosive bursting occurs for slugs with volume 24 230 m 3 depending on magma viscosity and conduit diameter and that at burst a typical strombolian slug with a volume of 100 1000 m 3 has an internal gas pressure of 1 5 bars and a length of 13 120 m we compare model predictions with field data from stromboli for low energy puffers mildly explosive strombolian eruptions and the violently explosive 5 april 2003 paroxysm we find that model predictions are consistent with field observations across this broad spectrum of eruptive styles suggesting a common slug driven mechanism we propose that paroxysms are driven by unusually large slugs large v

the viscosity of shoshonitic melts vulcanello peninsula aeolian islands italy insight on the magma ascent in dikes abstract the viscosity of shoshonitic melts and glasses from vulcanello peninsula vulcano island italy was experimentally determined at temperatures between 733 k and 1673 k the water content of the melts varies from 0 03 to 4 75 wt h 2 o the micropenetration technique was employed at ambient pressure in the high viscosity range 10 9 10 12 pa s falling sphere s experiments were performed at 500 and 2000 mpa in the low viscosity range 10 0 5 10 3 pa s results show a decrease of about 2 orders of magnitude in viscosity if 3 wt of water is added to the dry melt at 1300 k at high temperature the viscosity of vulcanello melts is intermediate between that of andesitic and basaltic melts in contrast at low temperatures 1050 k the shoshonitic melt is characterized by a lower viscosity with respect to the two previous melts the general viscosity models of giordano et al giordano d mangiacapra a potuzak m russell j k romano c dingwell d b di muro a 2006 an expanded non arrhenian model for silicate melt viscosity a treatment for metaluminous peraluminous and peralkaline liquids chemical geology 229 42 56 for dry silicate melts is in good agreement with the experimental data in the high temperature range less than 0 3 log units but shows significant differences in the low temperature range up to 1 1 log units the calculation model for hydrous silicate melts from huy and zhang huy h zhang y 2007 toward a general viscosity equation for natural anhydrous and hydrous silicate melts geochimica et cosmochimica acta 71 403 416 reproduces the new viscosity data in the high temperature range within 0 6 log units but shows as well large discrepancies in the low t range for hydrous melts deviations up to 2 log units thus we propose a specific calculation model to predict the viscosity of the shoshonitic melts as a function of temperature and water content this model reproduces the experimental data 24 measurements with a 1 standard deviation of 0 17 log units the viscosity data are used to constrain the ascent velocity of shoshonitic magmas from vulcanello within dikes using petrological data temperature and crystal content of the magma and volcanological information geometrical parameters of the eruptive fissure and depth of magma storage we estimate the time scale for the ascent of magma from the main reservoir to the surface results show time scales in the order of hours to few days we conclude that the rapid ascent of poorly evolved melts from moho depths should be taken into account for the hazard assessment of vulcano island

viscosity of high k basalt from the 5th april 2003 stromboli paroxysmal explosion abstract the 5th april 2003 paroxysmal event was the strongest explosion that has occurred at stromboli in the last 50 years this event lasted only few minutes and was characterised by two violent explosions followed by gas and pyroclast emission in order to constrain models of the dynamics of the paroxystic event the viscosity of anhydrous and hydrous stromboli high potassium hk basaltic melts have been measured viscosity has been investigated in the low viscosity range with the falling sphere method at superliquidus temperatures 1423 to 1673 k and 0 5 gpa and in the high viscosity range with micropenetration near the glass transition temperature 723 to 1035 k at atmospheric pressure falling sphere experiments were performed in a piston cylinder apparatus with melts whose water content varies from nominally anhydrous 0 02 wt h 2 o to 4 16 wt h 2 o the combination of high and low viscosity data permits a general description of the viscosity as a function of temperature and water contentusing a modified tamman vogel fulcher equation using these new viscosity data an estimation of the flow regime and magma velocity is performed our data suggest that the ascent of magma from the 7 8 km deep reservoir to a shallower reservoir located at about 3 km of depth may occur within minutes moreover we infer a turbulent flow regime finally our estimates of the ascent velocity agree qualitatively with results from petrological studies e g bertagnini a metrich n landi p rosi m 2003 stromboli volcano aeolian archipelago italy an open window on the deep feeding system of a steady state basaltic volcano journal of geophysical research 108 2336 2350 which indicate a turbulent flow regime and rapid ascent velocities such to inhibit volatile loss induced crystallization we conclude that hazard evaluation at stromboli island should incorporate the likelihood of very rapid ascent of less evolved melts from depth

morphology genesis and distribution of nanometer scale pores in siliceous mudstones of the mississippian barnett shale research on mudrock attributes has increased dramatically since shale gas systems have become commercial hydrocarbon production targets one of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks our work on siliceous mudstones from the mississippian barnett shale of the fort worth basin texas shows that the pores in these rocks are dominantly nanometer in scale nanopores we used scanning electron microscopy to characterize barnett pores from a number of cores and have imaged pores as small as 5 nm key to our success in imaging these nanopores is the use of ar ion beam milling this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high magnification imaging nanopores are observed in three main modes of occurrence most pores are found in grains of organic matter as intraparticle pores many of these grains contain hundreds of pores intraparticle organic nanopores most commonly have irregular bubblelike elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm internal porosities of up to 20 2 have been measured for whole grains of organic matter based on point count data from scanning electron microscopy analysis these nanopores in the organic matter are the predominant pore type in the barnett mudstones and they are related to thermal maturation nanopores are also found in bedding parallel wispy organic rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter but this mode is not common although less abundant nanopores are also locally present in fine grained matrix areas unassociated with organic matter and as nano to microintercrystalline pores in pyrite framboids intraparticle organic nanopores and pyrite framboid intercrystalline pores contribute to gas storage in barnett mudstones we postulate that permeability pathways within the barnett mudstones are along bedding parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores

a general viscosity model of campi flegrei italy melts abstract viscosities of shoshonitic and latitic melts relevant to the campi flegrei caldera magmas have been experimentally determined at atmospheric pressure and 0 5 gpa temperatures between 840 k and 1870 k and h2o contents from 0 02 to 3 30 wt the concentric cylinder technique was employed at atmospheric pressure to determine viscosity of nominally anhydrous melts in the viscosity range of 101 5 103 pa s the micropenetration technique was used to determine the viscosity of hydrous and anhydrous melts at atmospheric pressure in the high viscosity range 1010 pa s falling sphere experiments were performed at 0 5 gpa in the low viscosity range from 100 35 to 102 79 pa s in order to obtain viscosity data of anhydrous and hydrous melts the combination of data obtained from the three different techniques adopted permits a general description of viscosity as a function of temperature and water content using the following modified vft equation l o g a b t c d t e e x p g w t where is the viscosity in pa s t the temperature in k w the h2o content in wt and a b c d e and g are the vft parameters this model reproduces the experimental data 95 measurements with a 1 standard deviation of 0 19 and 0 22 log units for shoshonite and latite respectively the proposed model has been applied also to a more evolved composition trachyte from the same area in order to create a general model applicable to the whole compositional range of campi flegrei products moreover speed data have been used to constrain the ascent velocity of latitic shoshonitic and trachytic melts within dikes using petrological data and volcanological information geometrical parameters of the eruptive fissure and depth of magma storage we estimate a time scale for the ascent of melt from 9 km to 4 km depth where deep and shallow reservoirs respectively are located in the order of few minutes such a rapid ascent should be taken into account for the hazard assessment in the campi flegrei area

triggering mechanism at the origin of paroxysms at stromboli aeolian archipelago italy the 5 april 2003 eruption 1 the 5 april 2003 paroxysm of stromboli occurred during an ongoing effusive episode that initiated on 28 december 2002 it mainly consisted in two a few seconds apart cannon like explosions followed by a vertical gas pyroclast jet the hk basaltic pumices that were generated mainly contain crystals that were drained back from the shallow parts of the plumbing system few euhedral to anhedral olivines and their melt inclusions testify to crystal fractionation and possible mixing between hk basaltic magmas variable in their volatile content extent of evolution and volume the most primitive volatile rich hk basalt term is sizable at micrometer scale only we propose that the triggering mechanism of the paroxysm was initiated at moderate pressures 240 mpa and was related to bubble driven ascent of magma blobs the latter rose through and interacted with overlying slightly more evolved melts and finally mingled with the shallow crystal rich magma just before the eruption