ammonium in granites and its petrogenetic significance abstract ammonium is present as a trace constituent in all granites with an average concentration of 45 ppm nh4 equivalent to 35 ppm of elemental n it shows wide variations related to petrography and region but the only significant correlation between ammonium and other geochemical parameters is that it is most abundant in peraluminous granites and least abundant in peralkaline granites these variations can be related to a the amount of sedimentary material in the magmatic source region and b redox conditions in the source region the ammonium content of granitic magmas can also be modified by fractionation or contamination hydrothermal alteration has a major effect on the ammonium content of granitic rocks and variation due to this cause may exceed the magmatic variation most hydrothermally altered granites are enriched in ammonium as a result of the transfer of ammonium from sedimentary country rocks by the hydrothermal fluids

decrypting silicic magma plug fragmentation at azufral crater lake northern andes insights from fine to extremely fine ash morpho chemistry azufral sw colombia is a dangerous silicic volcano hosting a crater lake which serves as an excellent example of an incipient plug disruption through phreatomagmatism we studied the youngest succession of dilute pyroclastic density currents pdcs onlapping the north eastern crater rim scanning electron microscopy coupled with energy dispersive x ray spectroscopy was used to carry out an automated single particle analysis of fine to extremely fine ash we were able to obtain fast and accurate chemical analysis and imaging of 15 098 particles within the 250 63 m and the 63 32 m size ranges the 2d form and roughness parameters were determined for 4895 juvenile glassy particles and validated by 3d micro x ray computer tomography there are two end members of high group 1 and low group 2 roughness juvenile glassy particles group 1 comprises high roughness glass particles with solidity values as low as 0 34 in 2d and 0 33 in 3d and convexity values as low as 0 33 in 2d and 0 26 in 3d group 2 comprises low roughness glass particles with 2d solidity values 0 79 and 3d solidity values typically 0 58 in this group 2d convexity values are 0 68 and 3d convexity values are 0 71 both end members are mostly discriminated by the 2d concavity index 0 14 to 0 77 in group 1 vs 0 05 0 35 in group 2 the remaining group 3 comprises particles of intermediate roughness values in this study we show how an incipient plug developed over a short repose time might be subjected to only a few cycles of vesicle nucleation collapse and densification retaining the characteristics of juvenile glass each glassy juvenile ash type defined by a particular morphology roughness and microtexture can be linked to a density stratified conduit model in azufral the capping and conduit lining dense regions and the permeable zones of the incipient plug likely cracked the newly formed cracks could allow hydraulic forcing caused by external water and induce phreatomagmatic interaction this interaction favoured the fine fragmentation of the plug while enhancing ongoing magmatic processes finally the variations of bulk componentry provided clues on dilute pyroclastic density current transport and physical fractionation processes by secondary fragmentation elutriation and interaction with the crater rim

a scaling growth model for bubbles in basaltic lava flows abstract pahoehoe an and massive lavas from mount etna show common statistical properties from one sample to another which are independent of scale size over certain ranges the gas vesicle distribution shows two scale invariant regimes with number density n v v b 1 where v is the volume and empirically b 0 for small bubbles and b 1 for medium to large bubbles we introduce a bubble growth model which explains the b 1 range by a strongly non linear cascading growth regime dominated by a quasi steady state coalescence process the small bubble region is dominated by diffusion its role is to supply small bubbles to the coalescence regime the presence of measured dissolved gas in the matrix glass is consistent with the notion that bubbles generally grow in quasi steady state conditions the basic model assumptions are quite robust with respect to the action of a wide variety of processes since we only require that the dynamics are scaled over the relevant range of scales and that during the coalescence process bubble volumes are approximately conserved the model also predicts a decaying coalescence regime with b 1 associated with a depletion of the gas source or alternatively a loss of large vesicles through the surface of the flow our model thus explains the empirical evidence pointing to the coexistence of two different growth mechanisms in subsurface lava flows but acting over distinct ranges of scale with non linear coalescence as the primary growth process the total vesicularity of each sample can then be well estimated from the partial vesicularity of each growth regime without any outlier problems

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

the role of magma mixing in triggering the current eruption at the soufriere hills volcano montserrat west indies the andesite lava currently erupting at the soufriere hills volcano montserrat contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite the andesite phenocrysts have a range of textures and zonation patterns which suggest that non uniform reheating of the magma occurred shortly before the current eruption reheating resulted in remobilisation of the resident magma and may have induced eruption

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

rheological change and degassing during a trachytic vulcanian eruption at kilian volcano chaine des puys france magma ascent during silicic dome forming eruptions is characterized by significant changes in magma viscosity permeability and gas overpressure in the conduit these changes depend on a set of parameters such as ascent rate outgassing and crystallization efficiency and magma viscosity which in turn may influence the prevailing conditions for effusive versus explosive activity here we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive explosive transitions during vulcanian phases of the 9 4 ka eruption of kilian volcano chaine des puys france our results suggest that effusive activity at the onset of vulcanian episodes at kilian volcano was promoted by i rapid ascent of initially crystal poor and volatile rich trachytic magma ii a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization and iii efficient degassing outgassing in a crystal rich magma at shallow depths trachytic magma repeatedly replenished the upper conduit and variations in the amount of decompression and cooling caused vertical textural stratification leading to variable degrees of crystallization and outgassing outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles fractures and tuffisite veins fostering the formation of cristobalite in the carapace and talus regions build up of overpressure was likely caused by closing of pore space bubbles and fractures in the dome through a combination of pore collapse cristobalite formation sintering in tuffisite veins and limited pre fragmentation coalescence in the dome or underlying hot vesicular magma sealing of the carapace may have caused a transition from open to closed system degassing and to renewed explosive activity we generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity rapid microlite growth and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents we show that trachytes may erupt with a similar style e g vulcanian but at significantly higher ascent rates than their andesitic dacitic and rhyolitic counterparts this suggests that the periodicity of effusive explosive transitions at trachytic volcanoes may differ from that observed at the well monitored andesitic dacitic and rhyolitic volcanoes which has implications for hazard assessment associated with trachytic eruptions

a comprehensive study of pumice formation and dispersal the cretaio tephra of ischia italy the cretaio tephra is a small volume 0 02 km3 pumice deposit on the island of ischia in italy it was erupted during historical time and is associated with a tuff ring dome complex extruded along a n s trending normal fault system bordering the east side of the mt epomeo resurgent block 14c ages of the deposit average 1860 a b p which is in agreement with the potsherds found in paleosols underlying and overlying the deposit the eruption sequence consists of 7 members most of these members are pumice fall deposits one is an ash and pumice fallout and another is a surge bed the tephra have a trachytic phonolitic composition all chemical characteristics including major and trace elements and sr isotope composition are homogeneous throughout the sequence the vesicularity of pumice clasts does not show any systematic change the erupted magma had the same amount of volatiles the same flux and vesicle growth behavior throughout the eruption sequence grain size analyses of 90 tephra samples representative of all the members at variable distance from the supposed vent have been processed using the sequential fragmentation transport theory in order to define the effects of eruptive fragmentation and subsequent transport on the particle sizes distribution four significant subpopulations have been identified and related to transport mechanisms

gas transport and bubble collapse in rhyolitic magma an experimental approach a series of experiments was conducted to test concepts of porous flow degassing of rhyolitic magma during ascent and of the subsequent collapse of vesicles in degassed magma to form obsidian dense synthetically hydrated natural glasses were pressurized under water saturated conditions and then decompressed to achieve a range of porosities in the presence of a tracer vapor d2o rapid isotopic exchange indicative of vapor transport rather than of simple diffusion occurred at a porosity 60 vol in accord with earlier gas permeability measurements on cold natural samples in another series of experiments natural and synthetic pumices vesiculated by degassing to atmospheric pressure rapidly collapsed to dense glass on repressurization to the modest pressures prevailing in lava flows no relict bubble textures remained these results support the hypothesis that effusive eruptions result from the syneruptive escape of gas from permeable magmatic foam and that a process analogous to welding yields dense lavas when such foams are extruded

the glass transition in hydrous granitic melts abstract the influence of water on the glass transition temperature of hydrous granitic melts is estimated on the basis of a variety of available sources of data that provide phenomenological data for the liquid glass transition and for which the relaxation timescale can be accurately estimated these sources include a dilatometric micropenetration viscosity determinations b temperature dependent spectroscopic investigations of hydrous melts and c densities of synthetic fluid inclusions in glasses all available evidence on the glass transition temperature of hydrous granitic melts when corrected for equivalent timescales yields a single consistent trend of decreasing tg with increasing water content that has powerful applications in the rheology and kinetics of processes involving late stage silicic intrusives and eruptive products the brittle ductile transition in hydrous silicic liquids can be accurately predicted using this trend combination of the presently derived relationship for the influence of water on the glass transition of granitic melts with presently available data for the high temperature viscosities of such systems permits the estimation of the temperature dependence of the viscosity of hydrous melts over the entire range of temperature relevant to granite petrogenesis such a comparison reveals that the viscosity temperature relationships of hydrous granitic melts are not arrhenian as is assumed by most existing calculational schemes for the prediction of hydrous melt viscosities the increasing deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian with increasing water content is broadly consistent with similar trends accompanying the addition of excess alkalies to metaluminous granitic melt base compositions in detail however the influence of water on the deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian is not as strong as would be predicted from a molar comparison of the effects of h2o vs alkali oxide or alkaline earth oxide added to these melts the answer to this apparent discrepancy may lie in the incomplete dissociation of water in the melt structure

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

the age and nature of the basement of southern britain evidence from sr and pb isotopes in granites the isotopic study of granitic intrusions in southern britain provides an indication of the age and geochemical nature of the south british basement which is otherwise generally obscured from observation by sedimentary cover this paper reports pb isotopic compositions of feldspars in granites from sw england lundy mountsorrel and shap in addition to age and initial 87 sr 86 sr data on two of the granites a review of published rb sr age and isotope data for southern britain and consideration of the new pb data using the plumbotectonics model of doe zartman 1979 indicate that the basement to southern britain has an age of 800 ma with an upper limit of 1200 ma the granites studied in this work have substantial components of crustally derived sr and pb

morphometric analysis of relic landslides using detailed landslide distribution maps implications for forecasting travel distance of future landslides abstract this study analyzed the morphometry of relic landslides in four mountainous areas in japan using detailed landslide maps issued by the national research institute for earth science and disaster prevention of japan the mobility of 338 relic landslides was evaluated based on the h l ratio i e the equivalent coefficient of friction the h l ratio strongly correlates with the initial slope tan r r 2 0 78 0 88 the h l ratio and tan r of recent disaster causing landslides within the past few decades were also measured in each investigated area the data of recent landslides correspond to the 95 lower prediction limit of the tan r h l regression line for relic landslides this result implies that morphometric analysis of relic landslides around an unstable mass allows for forecasting of travel distance of the unstable mass

a fragmentation criterion for highly viscous bubbly magmas estimated from shock tube experiments abstract when a highly viscous bubbly magma is sufficiently decompressed layer by layer fracturing propagates through the magma at a certain speed fragmentation speed on the basis of a recent shock tube theory by koyaguchi and mitani koyaguchi t mitani n k 2005 a theoretical model for fragmentation of viscous bubbly magmas in shock tubes journal of geophysical research 110 b10 b10202 doi 10 1029 2004jb003513 gas overpressures at the fragmentation surface are estimated from experimental data on fragmentation speed in shock tube experiments for natural volcanic rocks with various porosities the results show that gas overpressure at the fragmentation surface increases as initial sample pressure increases and sample porosity decreases we propose a new fragmentation criterion to explain the relationship between the gas overpressure at the fragmentation surface the initial pressure and the porosity our criterion is based on the idea that total fragmentation of highly viscous bubbly magmas occurs when the tensile stress at the midpoint between bubbles exceeds a critical value we obtain satisfactory agreement between our simulation and experiment when we assume that the critical value is inversely proportional to the square root of bubble wall thickness this fragmentation criterion suggests that long micro cracks or equivalent flaws e g irregular shaped bubbles that reach the midpoints between bubbles are a dominant factor to determine the bulk strength of the bubbly magma

the measurement of δ15n in crustal rocks by static vacuum mass spectrometry application to the origin of the ammonium in the cornubian batholith southwest england the release of ammoniacal nitrogen from muscovite biotite orthoclase microcline and a whole rock granite by vacuum pyrolysis has been investigated under the heating regimes employed it was found that micas lose most of their nitrogen below the temperature at which the powders begin to sinter ca 1100 c the peak release of n occurring in either the 900 c or 1000 c heating step it is possible that the loss of ammonium is linked to the loss of h2o and perhaps f for the feldspars there was insignificant release of n below the melting point of the minerals about 1150 c suggesting that diffusion of ammonium or ammonia is the principal control for both micas and feldspars there was significant kinetic fractionation of the nitrogen isotopes during stepped heating from these results a technique has been devised for the routine determination of 15n and ammonium concentration in whole rock granites by static vacuum mass spectrometry the ammonium concentrations and 15n values have been determined in whole rock samples from the cornubian batholith southwest england the ammonium concentrations were in the range 8 to 187 ppm and the 15n values were in the range 5 1 to 10 2 most of the granite samples had 15n values between 8 4 and 10 2 and are within the range that have been obtained from amphibolite facies metasediments these granites are also typified by high 87sr86srinit 0 7130 to 0 7166 and high normative corundum 2 31 to 2 55 taken together the data suggest that the ammonium present within these granites is of biological origin and that it was incorporated via the anatexis of metasediments which originally contained organic material the samples were collected over a distance of 120 km and the limited range in 15n suggests either 1 1 the 15n values within the metasedimentary protolith were rather homogeneous or 2 2 the nitrogen isotopes were homogenised during the incorporation of ammonium into the granite magmas two of the granites had lower 15n values of 5 1 and 7 0 these granites also have lower ammonium concentrations 8 and 15 ppm lower 87sr86srinit 0 7094 0 7095 and lower normative corundum 1 44 and 1 63 the preferred explanation is that they were derived from or have had a contribution from a second crustal source containing sedimentary or metasedimentary material with a lower proportion of pelitic to other constituents

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

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

breadcrust bombs as indicators of vulcanian eruption dynamics at guagua pichincha volcano ecuador vulcanian eruptions are common at many volcanoes around the world vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits breadcrust bombs characteristic of vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre eruption magma ascent the morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from guagua pichincha volcano ecuador thus allow us to constrain the physical processes responsible for vulcanian eruption sequences of this volcano morphologically breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks thick rinds fracture to create deep widely spaced cracks that form large rectangular domains of surface crust in contrast thin rinds form polygonal networks of closely spaced shallow cracks rind thickness in turn is inversely correlated with matrix glass water content in the rind assuming that all rinds cooled at the same rate this correlation suggests increasing bubble nucleation delay times with decreasing pre fragmentation water content of the melt a critical bubble nucleation threshold of 0 4 0 9 wt water exists below which bubble nucleation does not occur and resultant bombs are dense at pre fragmentation melt h2o contents of 0 9 wt only glassy rinds are dense and bomb interiors vesiculate after fragmentation for matrix glass h2o contents of 1 4 wt rinds are thin and vesicular instead of thick and non vesicular a maximum measured h2o content of 3 1 wt establishes the maxi mum pressure 63 mpa and depth 2 5 km of magma that may have been tapped during a single eruptive event more common h2o contents of 1 5 wt suggest that most eruptions involved evacuation of 1 5 km of the conduit as we expect that substantial overpressures existed in the conduit prior to eruption these depth estimates based on magmastatic pressure are maxima moreover the presence of measurable co2 17 ppm in quenched glass of highly degassed magma is inconsistent with simple models of either open or closed system degassing and leads us instead to suggest re equilibration of the melt with gas derived from a deeper magmatic source together these observations suggest a model for the repeated vulcanian eruptions that includes 1 evacuation of the shallow conduit during an individual eruption 2 depressurization of magma remaining in the conduit accompanied by open system degassing through permeable bubble networks 3 rapid conduit re filling and 4 dome formation prior to the subsequent explosion an important part of this process is densification of upper conduit magma to allow repressur ization between explosions at a critical overpressure trapped pressurized gas fragments the nascent impermeable cap to repeat the process

constraints on cooling rates and permeabilities of pumice in an explosive eruption jet from colour and magnetic mineralogy abstract in explosive volcanic eruptions vesicular magma droplets produced by fragmentation are propelled into the atmosphere where they are chilled to form pumices the thermal history of droplets and the permeability of their internal bubble networks determine how much they are deformed in the eruption jet and hence what information pumices record about the state of the magma at fragmentation we study these aspects of the minoan plinian eruption of santorini volcano by quantifying the rate of oxidation reactions that took place when air entered the hot magma fragments in our experiments white minoan pumices were heated for minutes to hours between 600 and 850 c either in air or in an atmosphere with an oxygen fugacity at the ni nio buffer pumices were unchanged by heating at ni nio those heated in air often became pink to dark pink depending on heating time and their curie temperatures as determined by magnetic susceptibility measurements increased we use oxidation rates deduced from these experiments in conjunction with calculations of the rate of conductive cooling and of the rate at which air can enter a pumice to constrain the conditions experienced by pumices during the eruption natural minoan pumices less than about 5 cm in radius are white whereas larger ones often have white rims and pink interiors with curie temperatures higher than those of white material we infer that small pumices were cooled before being oxidized and that oxidation of the interiors of large clasts mostly took place during flight at temperatures within a few tens of degrees of magmatic values white rims of large pumices despite being permeable were cooled before oxidation could occur permeability developed in the liquid state but did not develop early enough with respect to cooling or was not large enough to allow extreme oxidation we give measurements of pumice permeabilities that should be close to magmatic values

fragmentation of magma during plinian volcanic eruptions the ratio of the volume of vesicles gas to that of glass liquid in pumice clasts v g v l reflects the degassing and dynamic history experienced by a magma during an explosive eruption v g v l in pumices from a large number of plinian eruption deposits is shown here to vary by two orders of magnitude even between pumices at a given level in a deposit these variations in v g v l do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities despite large ranges in these variables gas volume ratios of pumices do however vary systematically with magma viscosity estimated at the point of fragmentation and we infer that pumices do not quench at the level of fragmentation but undergo some post fragmentary evolution on the timescale of plinian eruptions pumices with viscosities 109 pa s can expand after fragmentation as long as their bubbles retain gas at a rate inversely proportional to their viscosity once the bubbles connect to form a permeable network and lose their gas expansion halts and pumices with viscosities 105 pa s can collapse under the action of surface tension textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place the magnitudes of expansion and collapse therefore depend critically on the timing of bubble connectivity relative to the final moment of quenching we propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching after accounting for these effects we derive new information on the fragmentation process from two characteristics of pumices the most important is a relatively constant minimum value of v g v l of 1 78 64 vol vesicularity in all samples with viscosities 105 pa s this value is independent of magma composition and thus reflects a property of the eruptive mechanism the other characteristic is that highly expanded pumices 85 vol vesicularities are common which argues against overpressure in bubbles as a mechanism for fragmenting magma we suggest that magma fragments when it reaches a vesicularity of 64 vol but only if sheared sufficiently strongly the intensity of shear varies as a function of velocity in the conduit which is related to overpressure in the chamber so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes

vesiculation of basaltic magma during eruption vesicle size distributions in vent lavas from the pu9u9o9o kupaianaha eruption of kilauea volcano are used to estimate nucleation and growth rates of h 2 o rich gas bubbles in basaltic magma nearing the earth9s surface 120 m depth by using well constrained estimates for the depth of volatile exsolution and magma ascent rate nucleation rates of 35 9 events cm 3 s 1 and growth rates of 3 2 x 10 4 cm s are determined directly from size distribution data the results are consistent with diffusion controlled growth as predicted by a parabolic growth law this empirical approach is not subject to the limitations inherent in classical nucleation and growth theory and provides the first direct measurement of vesiculation kinetics in natural settings in addition perturbations in the measured size distributions are used to examine bubble escape accumulation and coalescence prior to the eruption of magma

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

stable isotope o h s c and n systematics of quartz vein systems in the turbidite hosted central and north deborah gold deposits of the bendigo gold field central victoria australia constraints on the origin of ore forming fluids the central and north deborah gold deposits are located in one of 15 mining centers of the bendigo gold field central victoria australia a major world class gold province developed in a turbidite terrane gold mineralization occurs primarily within domes and reverse fault systems in multiply deformed lower ordovician metaturbidites deformation and thrusting occurred between 450 and 420 ma coevally with regional metamorphism at zeolite to greenschist facies in the central and north deborah deposits six stages of quartz veining each with characteristic geometry and mineral paragenesis developed sequentially stages 1 2 and 4 are gold bearing whereas stages 3 5 and 6 are barren gold related quartz veins were deposited at temperatures of 350 25 c based on oxygen isotope geothermometry of silicate phases quartz muscovite and sulfur isotope compositions of coexisting sulfide mineral pairs sphalerite galena this result is consistent with temperatures inferred from fluid inclusions the 18o values of gold bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15 9 to 17 2 per mil and 11 6 to 12 8 per mil respectively the d values of hydrothermal muscovites are also uniform at 68 to 55 per mil ore fluids have calculated 18o values of 8 to 11 per mil and d values of 37 to 17 per mil pyrite of synsedimentary diagenetic origin in slate and sandstone displays sulfur isotope compositions of 23 5 to 8 0 per mil and 0 1 to 11 9 per mil respectively sulfides from gold bearing quartz veins have a range of 7 4 to 8 1 per mil median ca 2 5 the relatively uniform vein pyrites indicate a large uniform sulfur source most likely from desulfidation reactions of sulfide minerals interaction of the ore forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values carbon isotope compositions of carbonates range from 5 1 to 9 0 per mil median ca 6 5 nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2 84 to 4 49 per mil respectively collectively the data define a band in n content vegh rsus 15n coordinate space from higher n and lower 15n in the phanerozoic metasedimenary hosted central and north deborah gold deposits to lower n and higher 15n in predominantly meta igneous hosted neoarchean gold provinces collectively the h o s c and n isotope compositions of ore related hydrothermal minerals indicate that the dilute aqueous carbonic and n bearing ore forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition this genetic model for the central and north deborah gold deposits is consistent with other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes in contrast these data are not consistent with either mantle derived fluids or magmatic fluids nor do they support a deeply circulated meteoric surface water model for vein formation

magma fragmentation speed an experimental determination abstract the propagation speed of a fragmentation front combined with the ascent velocity of magma is in all likelihood a controlling factor in the dynamics of explosive volcanic eruptions direct measurement of the fragmentation speed in natural systems appears to be impossible at present fortunately laboratory experiments can provide information on the propagation speed of the fragmentation front here we present the results of fragmentation speed determinations using a so called fragmentation bomb these are to the best of our knowledge the first in situ fragmentation speed determinations performed on magma natural magma samples merapi basaltic andesite mount st helens dacite and unzen dacite have been investigated in the temperature range of 20 950 c and at pressures up to 25 mpa two techniques have been employed firstly in experiments at 20 c dynamic pressure transducers were placed above and below the magma samples and the fragmentation speed of the magma sample was derived from an analysis of the decompression curves secondly at elevated temperatures an alternative technique was introduced and successfully employed this involved the severing via fragmentation of conducting wires placed within the samples at various heights fragmentation speeds are very low falling in the range of 2 70 m s and increasing with an increase in the magnitude of the decompression step responsible for the fragmentation the first high temperature determination seems consistent with low temperature results implications for explosive volcanism are discussed briefly

ammonium geochemistry in mineral exploration a comparison of results from the american cordilleras and the southwest pacific abstract ammonium concentrations in the host rocks to circum pacific epithermal precious metal and exhalative base metal deposits vary from 0 to 25 000 ppm high ammonium concentrations in mineralizing fluids are probably derived from the breakdown of organic matter in sediments or sedimentary rocks within the hydrothermal system shales are particularly good ammonium source rocks ammonium substitutes for k and other alkali elements in crystal lattices host rocks with k bearing minerals therefore have the highest potential for the development of ammonium haloes both surface and subsurface rocks can contain high concentrations of ammonium and substantial haloes have been found associated with precious metal veins particularly in the hanging wall such haloes are well developed at mines in mexico bolivia and peru and ammonium geochemistry thus provides a potentially useful exploration tool especially for vein type epithermal precious metal deposits

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

measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes abstract this paper outlines methods for determining a bubble size distribution bsd and the moments of the bsd function in vesiculated clasts produced by volcanic eruptions it reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes the analysis is based on a quantitative morphological stereological method for 2 dimensional imaging of cross sections of samples one method determines with some assumptions the complete shape of the bsd function from the chord lengths cut by bubbles the other determines the 1st 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area the surface area per unit volume and the volume fraction of bubbles comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former though the results coincide in absolute and relative magnitudes results of the analysis for vesiculated rocks from eleven subplinian to plinian eruptions show some interesting systematic correlations both between moments of the bsd and between a moment and the eruption column height or the sio2 content of magma these correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas this suggests that bubble coalescence does not predominate in sub plinian to plinian explosive eruptions the moment moment correlations put constraints on the style of the nucleation and growth process of bubbles the scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization which leads to an intermediate growth law between the diffusional growth r m t 2 3 at a constant depressurization rate and the ostwald ripening r m t 1 3 under a constant pressure where rm and t are the mean radius of bubble and the effective time of diffusion respectively occurred in the eruptions it is emphasized that the bsd in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit

heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure abstract the nucleation of h 2 o 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 800 c and either 50 or 125 mpa and then decompressed by 20 91 mpa at temperatures that ranged between 550 and 700 c and held at the lower pressures for 10 720 s before being quenched bubbles nucleated in number densities n b that vary between 3 10 7 and 2 10 8 cm 3 blocky shaped magnetite or the ends of needle shaped hematite acted as sites for nucleation but only if a minimum super saturation was exceeded which increases with increasing melt viscosity bubbles did not nucleate along the lengths of hematite needles nor on plagioclase both the beginning and ending times of the nucleation event increases with increasing melt viscosity using nucleation theory predictions neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature water diffusivity or viscosity instead the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor resulting from the increasing polymerization of the melt at lower temperatures and water contents because only 10 8 bubbles cm 3 nucleate on magnetite in the rhyolite melt used almost regardless of experimental conditions results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions

abrupt transitions during sustained explosive eruptions examples from the 1912 eruption of novarupta alaska plinian ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes we focus here on the processes leading to the termination of the second and third of these three episodes early erupted pumice from both episodes show a very similar range in bulk vesicularity but the modal values markedly decrease and the vesicularity range widens toward the end of episode iii clasts erupted at the end of each episode represent textural extremes at the end of episode ii clasts have very thin glass walls and a predominance of large bubbles whereas at the end of episode iii clasts have thick interstices and more small bubbles quantitatively all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 m vesicle diameter and cumulative number densities ranging from 107 109 cm 3 patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles compared to experimental data for bubble growth in silicic melts the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma the most distinct clast populations occurred toward the end of plinian activity preceding effusive dome growth distributions skewed toward small sizes thick walls and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing the data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes ii and iii had very different causes through episode iii the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open system outgassing as a precursor to effusive dome growth no such trend is visible in the episode ii clast assemblages we suggest that external changes involving failure of the conduit vent walls are more likely to have effected the break in explosive activity at 45 h

3d particle size distributions from 2d observations stereology for natural applications abstract we have developed a general formulation for stereological analysis of particle distributions which is applicable to any particle size or size distribution not limited to log normal unimodal etc we have applied numerical techniques to define intersection probability distributions for any shapes previously only known for spheres and quantified the errors involved in using spherical coefficients for various non spherical particles this stereological technique is based on knowing the probability distribution of random cross sections through various particles so that small circle cross sections can be subtracted from an observed population to provide the 3d size distribution of particles the results indicate that the most important parameter controlling calculated size distribution is particle aspect ratio for a distribution of particles with a specific aspect ratio or range of aspect ratios variations of particle form spherical vs cubic rectangular vs elliptical do not alter the results so the technique can be applied to a range of particle shapes applications can be made in a petrology volcanology and other fields only a few of which can also be treated using expensive x ray techniques

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

from vulcanian explosions to sustained explosive eruptions the role of diffusive mass transfer in conduit flow dynamics abstract magmatic explosive eruptions are influenced by mass transfer processes of gas diffusion into bubbles caused by decompression melnik and sparks melnik o e sparks r s j 2002 modelling of conduit flow dynamic during explosive activity at soufriere hills volcano montserrat in druitt t h kokelaar b p eds the eruption of soufriere hills volcano montserrat from 1995 to 1999 geological society london memoirs 21 307 317 proposed two end member cases corresponding to complete equilibrium and complete disequilibrium in the first case diffusion is fast enough to maintain the system near equilibrium and a long lived explosive eruption develops in the latter case pre existing bubbles expand under conditions of explosive eruption and decompression but diffusive gas transfer is negligible this leads to a much shorter eruption here we develop this model to consider the role of mass transfer by investigating transient flows at the start of an explosive eruption triggered by a sudden decompression the simulations reveal a spectrum of behaviours from sustained to short lived highly non equilibrium vulcanian style explosions lasting a few tens of seconds through longer lasting eruptions that can be sustained for tens of minutes and finally to eruptions that can last hours or even days behaviour is controlled by a mass transfer parameter which equals n 2 3 d where n is the bubble number density and d is the diffusivity the parameter is expected to vary between 10 5 and 1 s 1 in nature and reflects a time scale for efficient diffusion the spectrum of model behaviours is consistent with variations in styles of explosive eruptions of silicic volcanoes in the initial stages peak discharges occur over 10 20 s and then decline to low discharges if a critical bubble overpressure is assumed to be the criterion for fragmentation then fragmentation may stop and start several times in the declining period causing several pulses of high intensity discharge for the cases of strong disequilibria the fluxes can decrease to negligible values where other processes such as gas escape through permeable magma prevents explosive conditions becoming re established so that explosive activity stops and dome growth can start for cases closer to the equilibrium the eruption can evolve towards a quasi steady sustained flow never declining sufficiently for gas escape to become dominant

on the second nucleation of bubbles in magmas under sudden decompression abstract i conducted numerical experiments to evaluate the condition for the second nucleation of bubbles when a sudden change in the decompression rate occurs in magmas with preexisting bubbles i took into account homogeneous nucleation and diffusive bubble growth results from numerical experiments with variable number densities and sizes of preexisting bubbles were able to provide the limiting condition for the decompression rate above which the second nucleation effectively takes place the limiting condition as a function of number density and size of preexisting bubbles can be divided into two regimes that are controlled by the mass ratio of dissolved volatiles to exsolved volatiles as preexisting bubbles at the initial state in the case with a realistically high mass ratio namely with a relatively small preexisting bubble volume the detectable second nucleation requires decompression rates that are higher by a factor of about 10 than those that are hypothetically required to form the preexisting bubbles by consulting a simple scaling argument we derive the expression for the limiting decompression rate as a function of the number density and size of preexisting bubbles the graphical display of the expression allows us to evaluate whether the second nucleation will effectively take place for practical cases with a given number density and size of preexisting bubbles in natural pumice and scoria the preexisting bubbles may be observed as larger pheno bubbles that provide buoyancy at the storage regions which is needed to trigger the eruption such pheno bubbles are isolated in smaller matrix bubbles that are formed by the rapid decompression that takes place during the ascent of magma to the surface

migration of biogenic hydrocarbons into granites a review of hydrocarbons in british plutons abstract the occurrence of hydrocarbons bitumens senso lato in granite plutons and other basement rocks is not per se evidence of an abiogenic origin for the hydrocarbons on the contrary plutons are thermal foci and often highly fractured and therefore are hosts to the migration of fluids from outside the plutons including water and biogenic hydrocarbons hydrocarbons are associated with several british granites in all cases they can be attributed to biogenic sedimentary source rocks the occurrences are particularly related to hydrothermal mineralized veins in granites including the cornish hercynian granites the mountsorrel granodiorite and the weardale granite

the distribution of ammonium in granites from south west england the ammonium contents of granites pegmatites and hydrothermally altered rocks from sw england have been measured ammonium levels in the granites are generally high compared with those from other regions averaging 36ppm and they differ markedly between intrusions the pegmatites show higher ammonium contents than any other igneous rocks which have yet been investigated ammonium contents are strongly enriched in the hydrothermally altered rocks including greisens and kaolinized granites there is a good correlation between the average ammonium content of the intrusions in sw england and their initial 87 sr 86 sr ratios and peraluminosity this relationship supports the hypothesis that the ammonium in the granites is derived from a sedimentary source either in the magmatic source region or via contamination of the magma

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

vesiculation and fragmentation history in a submarine scoria cone forming eruption an example from nishiizu izu peninsula japan an uplifted 50 m thick half dissected submarine emplaced below wave base scoria cone occurs as dipping beds in coastal outcrops at nishiizu on the izu peninsula in japan concentrically outward dipping weakly stratified ungraded framework supported thin to very thick beds consist of brown coarse tuff to scoria lapilli tuff with outsized fluidal bombs throughout accessory lithic clasts chiefly occur in the lowermost visible beds scoria bombs have quenched margins weak bread crust textures and their vesicle number densities decrease inward which is indicative of fast surface cooling composite textures in the scoria bombs indicate recycling and agglutination of quenched and semi molten pyroclasts at the submarine vent in contrast to weak concentric gradations in vesicle size distribution in the bombs lapilli have asymmetrical gradients in vesicle size distribution indicating that they are fragments of coarser quenched lumps three grain size modes characterise the nishiizu brown scoria with coarse magma lumps ejected during magmatic fragmentation and quench jointed upon contact with seawater to be subsequently fragmented into lapilli and coarse ash by various styles of fragmentation where seawater plays a critical role the cone was constructed by slow moving fallout fed granular flow creep fed directly by suspension settling focused at the crater rim but extending onto the cone flanks with only minor resedimentation by granular flows nishiizu deposits yield an exceptional record of eruption and sedimentation dynamics during submarine cone building activity and in this study we compare their vesiculation and fragmentation mechanisms with those of potential subaerial analogues

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

pre explosive conduit conditions of the 1997 vulcanian explosions at soufriere hills volcano montserrat i pressure and vesicularity distributions an authoritative case of vulcanian eruptive dynamics is the series of 88 explosions that occurred between august and october 1997 at soufriere hills volcano on montserrat island the state of the magmatic column just before a vulcanian explosion is still poorly understood but conditions the eruptive style this study establishes such a pre explosive stratigraphy by 1 documenting the textures covering the range of the 1997 products 2 quantitative analysis of h2o content in interstitial glass measured by karl fischer titration and 3 combining these data with a simple model linking pre and post explosive vesicularities the model shows that syn explosive degassing affects greatly the way porosity evolves by decompression during an explosion the stratigraphy reconstruction shows a three part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested a dense and strongly degassed plug caps the column it is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 mpa at higher pressure up to 80 mpa lies a more homogeneous zone of relatively dense 10 20 vol magma which was emplaced under partly open system degassing this conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse but rather as stages the transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the vulcanian jets at the beginning of the explosions this contrasts with the nearly constant vesicularities of the deeper part of the pre explosive magma column which are propitious to a general short lived disruption

the geochemistry metasomatism and petrogenesis of the granites of the english lake district new geochemical data for the carrock threlkeld ennerdale shap skiddaw and eskdale granites of the lake district are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions the caledonian granites of the lake district have more associated hydrothermal activity and mineralization than their equivalents north of the iapetus suture in scotland pervasive high temperature metasomatism which affects the shap skiddaw and eskdale intrusions is accompanied by remobilization of the large ionic lithophile elements k rb sr u and li and b although high field strength elements including the rare earth elements remain unaffected except near greisen and mineral veins the threlkeld intrusion appears to have suffered loss of a ba and sr during a low temperature event however there is no consistent relationship between granite composition hydrothermal activity and mineralization which could be used to support the granite cupola model for mineralization in the province rather the episodic mineralization which affects the lake district may be related more to the broad geothermal field consequent upon the emplacement of the deeper lake district batholith identified from geophysical data despite the variable metasomatic perturbations of their primary chemistry three distinct groups of intrusions can be recognized on geochemical grounds the carrock granophyre is a product of tholeiitic fractionation and the threlkeld and ennerdale bodies are more typical calc alkaline intrusions whereas the eskdale shap and skiddaw plutons show a progressive evolution towards more complex geochemical patterns suggesting decreasing hornblende but increasing plagioclase control of fractionating processes the culmination of this trend may have been the emplacement of a large high heat production granite beneath the lake district at the end of the caledonian orogeny the suggestion made on geophysical grounds that the eskdale intrusion is a cupola of the lake district batholith cannot be simply reconciled with geochemical modelling none of the lake district granites has geochemical or isotopic characteristics which are convincingly s type i e formed by partial melting of a sedimentary protolith and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate

degassing at the soufriere hills volcano montserrat recorded in matrix glass compositions compositions of matrix glasses from the current eruption of soufriere hills volcano indicate that decompression driven crystallization results in 20 70 wt groundmass crystallization during eruption and variable degassing variations in crystallinity and volatile contents water and chlorine of matrix glasses are attributed to variations in extrusion rates and residence times in the lava dome residual water contents in pumice clasts 0 2 0 6 wt indicate minimum pressures of 1 1 3 7 mpa in 1997 vulcanian explosions residual water contents of 1 6 wt in a ballistic block ejected in sub plinian explosive activity on 17 september 1996 imply larger pressure drops similar to20 mpa variable residual water contents in dome samples are consistent with pressure variations of up to 9 mpa in the lava dome interior large variation in chlorine contents between lava blocks compared with explosion pumice clasts indicates that shallow level processes dominate degassing low melt chlorine contents of dome samples cannot be explained by open or closed system degassing especially when crystallization is taken into account instead heterogeneous chlorine leaching by circulation of groundwater vapour in the dome is proposed variable and elevated matrix glass deltad values can also be attributed to interaction with isotopically heavy surface waters hcl emission during the current eruption can be accounted for by cl loss from the melt consistent with the melt being undersaturated in chlorine

magmatic and hydrothermal ammonium in granites of the tuscan magmatic province italy abstract the granites of the tuscan magmatic province have total ammonium contents that vary from 0 to 636 ppm in the three largest intrusions of elba giglio and montecristo the granites have not been affected by hydrothermal alteration and have low ammonium contents ranging up to 50 ppm there are detectable differences between the separate intrusions and also within the biggest intrusion granitic samples from the smaller outcrops and boreholes on the tuscan mainland are nearly all hydrothermally altered and these rocks show much higher ammonium contents ranging up to 636 ppm in the hydrothermally altered rocks the ammonium contents vary considerably over short distances and where they have been most closely sampled in the s2 borehole of castel di pietra there is a clear correlation between the ammonium content and the intensity of sulphide mineralization the major conclusion of the study is that the amount of ammonium present in the granites is determined much more by post magmatic hydrothermal alteration than by magmatic variation the mean ammonium contents of the fresh and hydrothermally altered granites are 17 and 95 ppm respectively

experimental study of bubble coalescence in rhyolitic and phonolitic melts abstract we have experimentally studied the process of bubble coalescence in rhyolite and phonolite melts of natural composition the experiments involved decompression of water saturated melts equilibrated at pressures and temperatures from 100 to 150 mpa and 775 to 840 c in vertically oriented rapid quench capable cold seal pressure vessels one type of experiments rhyolite mcr 100 120 150 and phonolite lsp 120 series approximates a static bubble coalescence case where we held the decompressed samples for 5 seconds to 4320 minutes 3 days before quenching the second type rhyolite lpc 100 series replicates an expanding bubble coalescence environment where we continually decompressed the experiments at a rate of 0 5 mpa s examining samples quenched at ending pressures between 10 and 80 mpa our static case mcr 100 120 and 150 and lsp 120 results show significant increases in the modal bubble sizes and in the sizes of the largest bubbles corresponding to measurable broadening in the size distributions their bubble number densities nv decrease as a function of hold time at their final pressures pf and can be fit well by power law functions our expanding case experiments lpc 100 show a significant drop in nv during the duration of the experiments that can be fit by an exponential equation as nv vs either time or pf average estimates of bulk coalescence rates indicate a 1 order of magnitude drop in nv for static case rhyolites in a 2 3 day period and 2 orders of magnitude for phonolites within a 3 day period despite a 2 order of magnitude difference in viscosity coalescene in the phonolite is only slightly faster than the rhyolite the expanding case experiments show a 1 order of magnitude drop in nv over 180 seconds thus nv s decrease 4 orders of magnitude faster in expanding vs static bubbly rhyolite melts our results imply that significant bubble coalescence can occur in rhyolite magmas at relatively fast 20 m s ascent rates in the conduit thus bubble interconnectivity leading to high permeability is possible during ascent bubble coalescence may occur during second boiling in magma bodies that are stalled in the crust the timescales over which this occurs is much faster than the estimated rise rates for bubbles to reach the top of the magma chamber

the effect of magma flow on nucleation of gas bubbles in a volcanic conduit abstract we solve the dynamics of magma ascent and the kinetics of bubble nucleation and growth simultaneously which allow us to predict bubble sizes and number densities under ascent conditions as magma rises toward the surface the pressure decreases and eventually becomes less than the solubility pressure when the degree of supersaturation becomes great enough bubbles nucleate nucleation will stop as the concentration of volatiles in the melt decreases due to growth of existing bubbles and hence the degree of supersaturation decreases we show that a second nucleation event may occur just below the fragmentation level near that level the degree of supersaturation continuously increases as the magma is rapidly decompressed as a result nucleation will not stop until fragmentation occurs this second nucleation event should be taken into account when interpreting bubble size distribution measurements made on natural pumices the bubbles of the second nucleation event have high internal gas pressures up to 2 mpa greater than the liquid pressure suggesting that the second nucleation event may enhance fragmentation of magma we apply the model to the calculation protocol defined at the volcanic eruption mechanism modeling workshop durham 2002 we found that as a result of disequilibrium degassing fragmentation occurs higher in the conduit than under equilibrium degassing

characterisation of the 1997 vulcanian explosions of soufriere hills volcano montserrat by video analysis the activity of convention at montserrat soufriere hills volcano montserrat during the period 1995 1999 included numerous violent explosions two major cycles of vulcanian explosions occurred in 1997 a first of 13 explosions between 4 and 12 august and a second of 75 between 22 september and 21 october the explosions were short lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows and buoyant plumes ascended 3 15 km into the atmosphere each explosion discharged on average 3 105 m3 dense rock equivalent dre of magma draining the conduit to depths of 1 2 km the paper focuses on the first few seconds of three explosions of the 75 that occurred in september october 1997 6 october 1997 at 17 50 7 october 1997 at 16 02 and 9 october 1997 at 12 32 physical parameters such as exit velocities magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum dominated finger jets visible on videos during the initial stages of each explosion the model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters each explosion reveals a progressive increase in eruptive intensity with time jet exit velocities increasing from 40 m s 1 at the beginning of the explosion up to 140 m s 1 after a few seconds modelling suggests that the first magma to exit was largely degassed whereas that discharged after a few seconds contained up to 2 wt water magma overpressures up to 10 mpa are estimated to have existed in the conduit immediately prior to each explosion progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre eruptive gradients in water content and magma pressure in the upper reaches probably 100 500 m of the conduit fountain collapse occurred during the first 10 20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant such high eruptive densities were due to the presence of partially degassed magma in the conduit

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

bubble nucleation in magmas a dominantly heterogeneous process abstract bubble nucleation is at the heart of magma ascent and degassing nucleation kinetics control how easily a magma exsolves its volatiles and thereby its explosivity close to the surface homogeneous nucleation i e without assistance from crystal substrates or other phases often requires attainment of large supersaturation pressures p n 100 mpa the change in pressure required for onset of nucleation while heterogeneous nucleation on pre existing crystals can occur at significantly lower values p n p n given experimental constraints on pressure conditions at the onset of nucleation decompression rate meters e g toramaru 2006 also relate the number density of bubbles in pyroclasts a readily obtained textural characteristic to rates of pressure change dp dt during ascent all these treatments require an explicit or implicit decision as to whether magma degassing was dominated by homogeneous or heterogeneous nucleation which is guided by the choice of surface tension values this contribution exploits the current textural datasets available for eruptions involving a variety of magma compositions to test the assumptions that homogeneous and or heterogeneous nucleation dominate bubble vesiculation in natural melts the comparison between pyroclast textures and those obtained via decompression experiments in the laboratory indicates that supersaturation pressures and decompression rates required for homogeneous nucleation are unrealistically high for natural magmas i review the arguments for and against homogenous nucleation and defend the alternate view that heterogeneous nucleation may dominate in all magma types including rhyolites in order to solve this central debate our focus needs to be shifted towards understanding the pre decompression phase equilibria and textural characteristics of magnetite in addition to the state of the melt structure

experimental constraints on degassing and permeability in volcanic conduit flow this study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions degassing and permeability in magmas we studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature from the data collected we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium equilibrium bubble growth occurs when the decompression rate is slower than 0 1 mpa s 1 while higher rates cause porosity to deviate rapidly from equilibrium defining the first disequilibrium regime of degassing if the deviation is large enough a critical threshold of super saturation is reached and bubble growth accelerates defining the second disequilibrium regime we studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions under these open conditions we observed that bubbles start to coalesce at 43 vol porosity regardless of decompression rate coalescence profoundly affects bubble texture and size distributions and induces the melt to become permeable we determined coalescence to occur on a time scale 180 s independent of decompression rate we parameterized and incorporated our experimental results into a 1d conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust compared to previous models that assume equilibrium degassing of the melt during ascent the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by 25 the acceleration at high porosities 50 vol by a factor 5 and the associated decompression rate by an order of magnitude the integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape

decompression experiments identify kinetic controls on explosive silicic eruptions 1 eruption intensity is largely controlled by decompression induced release of water rich gas dissolved in magma it is not simply the amount of gas that dictates how forcefully magma is propelled upwards during an eruption but also the rate of degassing which is partly a function of the supersaturation pressure pcritical triggering gas bubble nucleation high temperature and pressure decompression experiments using rhyolite and dacite melt reveal compositionally dependent differences in the pcritical of degassing that may explain why rhyolites have fueled some of the most explosive eruptions on record

textural studies of vesicles in volcanic rocks an integrated methodology abstract vesicles in volcanic rocks are frozen records of degassing processes in magmas for this reason their sizes spatial arrangements numbers and shapes can be linked to physical processes that drive magma ascent and eruption although numerous techniques have been derived to describe vesicle textures there is no standard approach for collecting analyzing and interpreting vesicular samples here we describe a methodology for techniques that encompass the entire data acquisition process from sample collection to quantitative analysis of vesicle size and number carefully chosen samples from the lower mean and higher density vesicularity endmembers are characterized using image nesting strategies we show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes a new program foams was designed to perform the quantification stage from vesicle measurement to distribution plots altogether this approach allows substantial reduction of image acquisition and processing time while preserving enough user control to ensure the validity of obtained results we present three cameo investigations on basaltic lava flows scoria deposits and pumice layers to show that this methodology can be used to quantify a wide range of vesicle textures which preserve information on a wide range of eruptive conditions

ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the european variscan belt the content and isotopic composition of ammonium nh4 were measured by using kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the european variscides i e the erzgebirge and the zone of erbendorf vohenstrauss in germany the studied erzgebirge schists represent an early paleozoic passive margin sequence that reappears in different deep subducted metamorphic units this allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism ammonium is found to be progressively depleted accompanied by a shift in 15n from the low grade units 2 kbar 300 c with 638 124 ppm and 15n 2 2 0 6 to the garnet phyllite unit 9 kbar 470 c with 621 190 ppm and 15n 3 5 0 9 the mica schist eclogite unit 12 kbar 550 c with 394 113 ppm and 15n 3 9 0 8 and to the gneiss eclogite unit 12 kbar 730 c with 99 32 ppm and 15n 7 7 2 0 using equilibrium models for rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release only for the mica schists this isotope fractionation can be explained by the loss of molecular nitrogen in comparison with the early paleozoic schists the proterozoic ortho and paragneisses of the erzgebirge gneiss unit 6 8 kbar 650 c contain significantly lower amounts of ammonium 70 ppm and reach relatively low 15n values 2 5 to 3 6 the paragneisses from the zone of erbendorf vohenstrauss zev sampled from the ktb pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies 7 kbar 650 700 c the metagraywackes have nh4 contents of 80 ppm and 15n of approximately 6 gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in nh4 250 ppm and 15n up to 12 which suggest late fluid rock interactions with a nitrogen and 15n enriched fluid

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

bubble growth in rhyolitic melts experimental and numerical investigation bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically rhyolitic melts were hydrated at 150 mpa 780 850 c to uniform water content of 5 5 5 3 wt the pressure was then dropped and held constant at 15 145 mpa upon the drop bubbles nucleated and were allowed to grow for various periods of time before final rapid quenching of the samples the size and number density of bubbles in the quenched glasses were recorded where number densities were low and run duration short bubble sizes were in accord with the growth model of scriven 1959 for solitary bubbles however most results did not fit this simple model because of interaction between neighboring bubbles hence the growth model of proussevitch et al 1993 which accounts for finite separation between bubbles was further developed and used to simulate bubble growth

coupled sm nd and u pb systematics in late caledonian granites and the basement under northern britain the british late caledonian granites display a close correspondence between inherited u pb systematics in zircons and initial end indicating that both were controlled by the age of a major component of continental crust the data confirm and refine previous models of a basement comprising young crust in the south which includes arc and oceanic crust scrapings and progressively older continental crust northwestwards across scotland

magma storage region processes inferred from geochemistry of fe ti oxides in andesitic magma soufriere hills volcano montserrat w i analyses of fe ti oxides help constrain models of magma storage region processes for the soufriere hills volcano montserrat w i and provide clear evidence of the nature of transient heating events in the magma storage region to constrain timescales of magma heating and remobilization the tio2 zoning patterns in a time series of natural titanomagnetites were compared with those produced in controlled phase equilibrium experiments on the andesite most samples of andesite erupted from 1995 to 2002 contain titanomagnetite crystals with uniform core compositions tio278 wt many crystals are characterized by rimward increases in tio2 interpreted to be ti diffusion gradients caused by heating of the andesite by invading basaltic magma some andesites erupted during periods of the highest observed mass eruption rate however contain titanomagnetite with uniformly low tio2 contents from core to rim the observation that no ti diffusion gradients and no elevated core tio2 contents occur in the vast majority of titanomagnetite grains in magma batches that were erupted more than 2 years after the onset of the present eruption strongly suggests first that heating of the batches of andesite occurred just before eruption and second that injection of basaltic magma has continued throughout the eruption heat but little mass may be transferred from the invading basalt to the andesite in the magma storage region by injection of dikes or formation of sills ponding of basaltic magma at the base of a pre existing andesitic magma storage region is the simplest explanation consistent with observations the fe ti oxide data strongly suggest that an internal conduit within the andesitic magma storage region carries magma from the zone of heating to the overlying conduit which carries the magma through the upper arc crust in this model the magma chamber is being emptied from the bottom at the contact between pre existing andesite and newly intruded basalt

the fragmentation threshold of pyroclastic rocks in response to rapid decompression porous magma may fragment explosively this occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles this occurs at a critical pressure difference between the bubbles and the surrounding magma in this study we have investigated this pressure threshold necessary for the fragmentation of magma here we present the first comprehensive high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity permeability crystallinity and chemical composition we exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated experimentally we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample our results show that the fragmentation threshold is strongly dependent on porosity high porosity samples fragment at lower pressure differentials than low porosity samples the fragmentation threshold is inversely proportional to the porosity of the other factors permeability likely affects the fragmentation threshold at high porosity values whereas chemical composition crystallinity and bubble size distribution appear to have minor effects the relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma

kinetics of heterogeneous bubble nucleation in rhyolitic melts implications for the number density of bubbles in volcanic conduits and for pumice textures we performed decompression experiments to simulate the ascent of a phenocryst bearing rhyolitic magma in a volcanic conduit the starting materials were bubble free rhyolites water saturated at 200 mpa 800 c under oxidizing conditions they contained 6 0 wt dissolved h2o and a dense population of hematite crystals 8 7 2 105 mm 3 pressure was decreased from the saturation value to a final value ranging from 99 to 20 mpa at constant temperature 800 c the rate of decompression was either 1 000 or 27 8 kpa s in all experiments we observed a single event of heterogeneous bubble nucleation beginning at a pressure pn equal to 63 3 mpa in the 1 000 kpa s series and to 69 1 mpa in the 27 8 kpa s series below pn the degree of water supersaturation in the liquid rapidly decreased to a few 0 1 wt the nucleation rate dropped and the bubble number density bnd stabilized to a value strongly sensitive to decompression rate 80 mm 3 at 27 8 kpa s vs 5 900 mm 3 at 1 000 kpa s this behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite h2o system and in numerical simulations of vesiculation in ascending magmas similar degrees of water supersaturation were measured at 27 8 and 1 000 kpa s implying that a faster decompression rate does not result in a larger departure from equilibrium our experimental results imply that bnds in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate left left text d p text d t right right and the number density of phenocrysts especially the number density of magnetite microphenocrysts 1 100 mm 3 which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation very low bnds 1 mm 3 are predicted in the case of effusive eruptions left left text d p text d t right right 0 1 kpa s high bnds up to 107 mm 3 and bimodal bubble size distributions are expected in the case of explosive eruptions 1 a relatively small number density of bubbles 1 100 mm 3 will first nucleate in the lower part of the conduit left left text d p text d t right right 10 kpa s either at high pressure on magnetite or at lower pressure on quartz and feldspar or by homogeneous nucleation in the liquid and 2 then extreme decompression rates near the fragmentation level left left text d p text d t right right 103 kpa s will trigger a major nucleation event leading to the multitude of small bubbles typically a few micrometers to a few tens of micrometers in diameter which characterizes most silicic pumices

vesicle connectivity in pyroclasts and implications for the fluidisation of fountain collapse pyroclastic flows montserrat west indies abstract pyroclastic flows are noted for their highly mobile behaviour being able to flow on slopes as low as a few degrees and this has been attributed to fluidisation or partial fluidisation by escaping gases the three types of pyroclastic flows generated during the eruption of soufriere hills volcano on montserrat dome collapse fountain collapse and surge derived differ in their apparent mobility suggesting different degrees of fluidisation and friction reduction in the flowing granular materials several possible sources of fluidising gas operate in pyroclastic flows the purpose of this paper is to assess the feasibility of one mechanism the release during flow of pressurised gases trapped in the vesicles of juvenile clasts measurements with a helium pycnometer show that all vesicles in dense lava blocks from dome collapse pyroclastic flows are connected so that the mechanism is probably not viable for such flows on the other hand pumices from fountain collapse pyroclastic flows from soufriere hills and other volcanoes contain up to 10 vol of isolated vesicles this is confirmed by sem imagery which shows that soufriere hills pumices contain two vesicle populations with a bimodal size distribution the large vesicles are mostly interconnected whereas about two thirds of the smaller population

the ammonium content of caledonian granites the caledonian granites of the british isles have nh 4 contents in the range of 0 167 parts per million high contents 60 ppm are confined to muscovite rich granites occurring principally in a restricted area namely se ireland and the isle of man their magmas are inferred to have formed by the partial melting of a sedimentary protolith rich in organic matter there is no apparent correlation between nh 4 content and initial 87 sr 86 sr ratios

expansion and quenching of vesicular magma fragments in plinian eruptions the conditions of pumice generation in plinian eruptions are studied a physical model describes the behavior of gas bubbles in a magma fragment which is carried upward in a volcanic conduit and an atmospheric eruption column the effects of pressure release and cooling are calculated for a range of eruption conditions the magma fragment expands in the conduit and stops expanding soon after leaving the vent when a thin viscous rind forms against the cold mixture of magmatic gas and air this rind prevents further volume changes pumice vesicularity is a function of the decompression rate in the conduit which depends on ascent velocity and fragmentation depth it is also sensitive to the cooling rate in the atmospheric column which depends on vent radius and mass discharge rate different fragments follow different trajectories in the column and are subjected to different cooling rates this generates a range of vesicularities which reflects the eruptive conditions all else being equal pumice vesicularity increases as magma viscosity decreases these predictions are consistent with observations pumices provide quantitative constraints on conduit flow conditions and mass discharge rate these concepts are applied to two plinian eruptions vesicularity values for the bishop tuff long valley caldera require a mass discharge rate between 108 and 109 kg s 1 vesicularity variations during plinian phase 1 of the minoan eruption santorini are explained by the conduit radius increasing from about 30 m to 120 m both cases require large decompression rates in the eruption conduit suggesting that flow pressures were close to lithostatic values

constraints on cooling and degassing of pumice during plinian volcanic eruptions based on model calculations during explosive volcanic eruptions pumice clasts are transported into the atmosphere and their thermal and degassing histories determine how much volatiles are lost syneruptively this process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice in the model we investigate the impact of various parameters e g mass eruption rate biot number eruption temperature and geometry on the cooling and degassing of pumice and find that the biot number and the eruption temperature are the most influential during typical risetimes of pumices inside eruption columns 200 300 s those smaller than 0 5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume in the case of larger pumices the ratio of the cooling timescale r p 2 where rp is the pumice radius and is the thermal diffusivity to the degassing timescale w 2 d 0 where w is the vesicle wall thickness and d 0 is the species diffusivity controls degassing if this ratio is larger than 50 degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice for a ratio of less than 0 1 no volatiles escape the pumice typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing cl and h 2 o the transition from 0 1 to 50 will be in the pumice size range of 1 10 cm here volatile concentration is a strong function of position inside the pumice suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass a number which is frequently used to estimate the total volatile input of volcanic eruptions this effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward d 80

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

bubble growth during decompression of magma experimental and theoretical investigation a model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt the model accounts for the interplay of three dynamic processes decompression rate of the magma deformation of the viscous melt around the growing bubble and diffusion of volatiles into the bubble generally these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically for a better understanding of the physics of the processes we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance diffusion or linear decompression rate we show that the solutions are a function of time and two dimensionless numbers which are the ratios of either the diffusive or viscous time scales over the decompression time scale the conditions for each growth regime are provided as a function of the two governing dimensionless parameters analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts the model solutions including the division to the growth regimes as function of the two parameters provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure supersaturation and gas volume fraction the model results are in agreement with the conditions of plinian explosive eruption e g mount st helens 18 may 1980 where high gas overpressure is expected the conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions mostly in equilibrium degassing and partly in overpressure conditions we show that in a situation of quasi static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble insensitive to the diffusivity profile

simulating bubble number density of rhyolitic pumices from plinian eruptions constraints from fast decompression experiments decompression experiments of a crystal free rhyolitic liquid with 6 6 wt h2o were carried out at a pressure range from 250 mpa to 30 75 mpa in order to characterize effects of magma ascent rate and temperature on bubble nucleation kinetics especially on the bubble number density bnd the number of bubbles produced per unit volume of liquid a first series of experiments at 800 c and fast decompression rates 10 90 mpa s produced huge bnds 2 1014 m 3 at 10 mpa s 2 1015 m 3 at 90 mpa s comparable to those in natural silicic pumices from plinian eruptions 1015 1016 m 3 a second series of experiments at 700 c and 1 mpa s produced bnds 9 1012 m 3 close to those observed at 800 c and 1 mpa s 6 1012 m 3 showing that temperature has an insignificant effect on bnds at a given decompression rate our study strengthens the theory that the bnds are good markers of the decompression rate of magmas in volcanic conduits irrespective of temperature huge number densities of small bubbles in natural silicic pumices from plinian eruptions imply that a major nucleation event occurs just below the fragmentation level at which the decompression rate of ascending magmas is a maximum 1 mpa s