constraints on the nature and evolution of the magma plumbing system of mt etna volcano 1991 2008 from a combined thermodynamic and kinetic modelling of the compositional record of minerals maren kahl sumit chakraborty massimo pompilio and fidel costa school of earth and environment university of leeds leeds ls2 9jt uk institut fur geologie mineralogie und geophysik ruhr universit at bochum bochum 44780 germany istituto nazionale di geofisica e vulcanologia sezione di pisa pisa 56126 italy and earth observatory of singapore nanyang technological university singapore 639798 singapore

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

accuracy of timescales retrieved from diffusion modeling in olivine a 3d perspective diffusion modeling in olivine is a useful tool to resolve the timescales of various magmatic processes practical olivine geospeedometry applications employ 1d chemical transects across sections that are randomly sampled from a given 3d crystal population but the accuracy and precision with which timescales can be retrieved from this procedure are not well constrained here we use numerical 3d diffusion models of fe mg to evaluate and quantify the uncertainties associated with their 1d counterparts the 3d diffusion models were built using both simple and realistic olivine morphologies and incorporate diffusion anisotropy as well as different zoning styles the 3d model crystals were sectioned along ideal or random planes which were used to perform 1d models and timescale comparisons results show that the timescales retrieved from 1d profiles are highly inaccurate and can vary by factors of 0 1 25 if diffusion anisotropy is not taken into account even when anisotropy is corrected for timescales can still vary between 0 2 10 times the true 3d diffusion time due to crystal shape and sectioning effects simple grain selection procedures are described to reduce the misfit between calculated and actual diffusion times and achieve an accuracy and precision of 5 and 15 25 relative respectively provided that the grains are carefully selected about 20 concentration profiles and associated 1d models suffice to achieve this accuracy

rapid assembly and rejuvenation of a large silicic magmatic system insights from mineral diffusive profiles in the kidnappers and rocky hill deposits new zealand abstract the timescales over which magmas in large silicic systems are reactivated assembled and stored remains a fundamental question in volcanology to address this question we study timescales from fe mg interdiffusion in orthopyroxenes and ti diffusion in quartz from the caldera forming 1200 km3 kidnappers and 200 km3 rocky hill eruptions from the mangakino volcanic centre taupo volcanic zone new zealand the two eruptions came from the same source area have indistinguishable 40ar 39ar ages 1 0 ma and zircon u pb age spectra but their respective deposits are separated by a short period of erosion compositions of pumice glass and mineral species in the collective eruption deposits define multiple melt dominant bodies but indicate that these shared a common magmatic mush zone diffusion timescales from both eruptions are used to build on chemical and textural crystal signatures and interpret both the crystal growth histories and the timing of magma accumulation fe mg interdiffusion profiles in orthopyroxenes imply that the three melt dominant bodies established through extraction of melt and crystals from the common source were generated within 600 years and with peak accumulation rates within 100 years of each eruption in addition a less evolved melt interacted with the kidnappers magma beginning 30 years prior to and peaking within 3 years of the eruption this interaction did not directly trigger the eruption but may have primed the magmatic system orthopyroxene crystals with the same zoning patterns from the kidnappers and rocky hill pumices yield consistently different diffusion timescales suggesting a time break between the eruptions of 20 years from core rim zones to 10 years outer rim zones diffusion of ti in quartz reveals similarly short timescales and magmatic residence times of 200 km 3 eruption our data support concepts of large silicic systems being stored as long lived crystal mushes with eruptible melts generated over extraordinarily short timescales prior to eruption

years to weeks of seismic unrest and magmatic intrusions precede monogenetic eruptions seismic deformation and gas activity unrest typically precedes volcanic eruptions tracking the changes of this activity with monitoring data makes it increasingly possible to successfully forecast eruptions from stratovolcanoes however this is not the case for monogenetic volcanoes eruptions from these volcanoes tend to be small but are particularly difficult to anticipate since they occur at unexpected locations and there is very limited instrumental monitoring data many monogenetic volcanic fields occur in high density populated areas and or tourist destinations and thus even a small eruption can have a major economic and societal impact we have gathered the available instrumental data for unrest and combined it with new historical accounts of seismicity our occurrences are mainly from high magmatic flux oceanic islands canary islands iceland papua new guinea mexico and japan we find that seismic activity may start one or two years before eruption but it intensifies at approximately two or three months and one or two weeks the petrological and geochemical characteristics of the deposits show that multiple magma batches interacted in a subvolcanic reservoir and multiple intrusions occurred on a similar time scales to the seismicity we propose a general model for these eruptions where early dike intrusions in the crust do not erupt e g stalled intrusions and make small plumbing systems but they probably are key in creating a thermal and rheological pathway for later dikes to be able to reach the surface these observations provide a conceptual framework for better anticipating monogenetic eruptions in similar settings and magmatic fluxes and should lead to improved strategies for mitigation of their associated hazards and risks

linking petrology and seismology at an active volcano many active volcanoes exhibit changes in seismicity ground deformation and gas emissions which in some instances arise from magma movement in the crust before eruption an enduring challenge in volcano monitoring is interpreting signs of unrest in terms of the causal subterranean magmatic processes we examined over 300 zoned orthopyroxene crystals from the 1980 1986 eruption of mount st helens that record pulsatory intrusions of new magma and volatiles into an existing larger reservoir before the eruption occurred diffusion chronometry applied to orthopyroxene crystal rims shows that episodes of magma intrusion correlate temporally with recorded seismicity providing evidence that some seismic events are related to magma intrusion these time scales are commensurate with monitoring signals at restless volcanoes thus improving our ability to forecast volcanic eruptions by using petrology

tof sims and electron microprobe investigations of zoned magmatic orthopyroxenes first results of trace and minor element analysis with implications for diffusion modeling zoned phenocrysts in volcanic rocks potentially provide an archive of magmatic processes as a crystal grows and comes into contact with different melt batches the chemical and textural signature of this journey is recorded within its crystal lattice the timescale of some magmatic processes can be investigated through the relaxation of chemical gradients across crystal growth zones through the application of diffusion modeling techniques one of the current limitations to diffusion modeling is the spatial and analytical resolution of the chemical profile that conventional techniques such as electron probe microanalyzer epma dynamic secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms can achieve here for the first time we present time of flight tof sims tof sims data for zoning of orthopyroxene crystals from the may 1982 eruption of mount st helens volcano u s a and cross calibrate these data between backscattered electron images and epma tof sims has the advantage of being able to achieve micrometer to nanoscale spatial resolution of major elements as well as analyses of light elements such as li and trace and minor elements na k and ni at concentrations that cannot be achieved by epma provided that convolution overlap effects and polyatomic mass inferences are carefully considered with tof sims analyses we identified zoning of li on a spatial scale ca 5 10 m that would be inaccessible to most other conventional analytical techniques preliminary results indicate that li a fast diffusing element may be introduced to the crystals in the minutes hours or days prior to eruption and may provide insights into pre eruptive magmatic processes thus tof sims has the potential to be a powerful tool for obtaining minor and trace element profiles across compositional interfaces within crystals at high spatial resolution

timescales of mixing and mobilisation in the bishop tuff magma body perspectives from diffusion chronometry we present two feldspar thermometry and diffusion chronometry from sanidine orthopyroxene and quartz from multiple samples of the bishop tuff california to constrain the temperature stratification within the pre eruptive magma body and the timescales of magma mixing prior to its evacuation two feldspar thermometry yields estimates that agree well with previous fe ti oxide thermometry and gives a 80 c temperature difference between the earlier and later erupted regions of the magma chamber using the thermometry results we model diffusion of ti in quartz and ba and sr in sanidine as well as fe mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these crystal phases diffusion profiles of ti in quartz and fe mg in orthopyroxene both yield timescales of 150 years for the formation of overgrowth rims in contrast both ba and sr diffusion in sanidine yield nominal timescales 1 2 orders of magnitude longer than these two methods the main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for ba and sr diffusion modelling i e growth zoning exists utilising the divergent diffusion behaviour of ba and sr we place constraints on the initial width of the interface and can refine our initial conditions considerably bringing ba and sr data into alignment and yielding timescales closer to 500 years the majority of which are then within uncertainty of timescales modelled from ti diffusion in quartz care must be thus taken when using ba in sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the bishop tuff magma chamber at least 500 years prior to eruption timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1 2 orders of magnitude or that the minerals concerned did not experience a common history despite being extracted from the same single pumice clasts introduction of the magma initiating crystallisation of the contrasting rims on sanidine quartz orthopyroxene and zircon was prolonged and may be a marker of other processes that initiated the bishop tuff eruption rather than the trigger itself

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

timescales of magmatic processes at ruapehu volcano from diffusion chronometry and their comparison to monitoring data abstract recent eruptions from mt ruapehu have been difficult to predict despite the presence of a multi parametric monitoring network as a result it is necessary to assess precursory signals prior to an eruption and align those to magmatic processes at depth fortuitously scoria from all historical ruapehu eruptions contains pyroxene crystals that are strongly reversely zoned in the form of a thin 2 to 3 m outermost rim these crystals therefore preserved changes in the magmatic system soon before their eruption we used experimentally determined diffusion coefficients to assess the timescales of magma magma interaction and compared those to the monitoring record four of the five eruptions analysed 1969 1971 1977 1995 gave diffusion timescales 3 to 5 months before their eruption with an increased number of crystals recording timescales within 1 month of eruption pyroxene crystals from the 1996 eruption record events that occurred prior to and during the 1995 eruption suggesting that the bulk of the 1996 crystals was derived from the 1995 magma these diffusion timescales do not compare well to a change in any monitoring signal before historical eruptions however an examination of recent seismicity 2005 2013 since a significant upgrade both in number of stations and type of seismometers showed that two phreatic eruptions in 2006 and 2007 were preceded by a seismic swarm from 5 to 15 km depth 3 to 5 months before each eruption consistent with the diffusion timescales based on this correlation deep seismic swarms likely indicate a period of pressurisation in the magmatic system which may lead to gas rich phreatic eruptions

time scales and temperatures of crystal storage in magma reservoirs implications for magma reservoir dynamics the thermal and therefore physical state of magma bodies within the crust controls the processes and time scales required to mobilize magmas before eruptions which in turn are critical to hazard a

moment tensors of ten witwatersrand mine tremors ground motions recorded both underground and on the surface in two of the south african gold mining districts were inverted to determine complete moment tensors for 10 mining induced tremors in the magnitude range 1 9 to 3 3 the resulting moment tensors fall into two separate categories seven of the events involve substantial coseismic volumetric reduction v together with normal faulting entailing shear deformation ad where the summation is over fault planes of areaa and average slipd for these events the ratio v ad ranges from 0 58 to 0 92 with an average value of 0 71 for the remaining three events v is not significantly different from zero these events are largely double couple sources involving normal faulting surprisingly the two types of source mechanism appear to be very distinct in that there is not a continuous distribution of the source mix from v 0 to v ad presumably the coseismic closure indicates substantial interaction between a mine stope and adjacent shear failure in the surrounding rock under the influence of an ambient stress for which the maximum principal stress is oriented vertically

multifractal investigation of continuous seismic signal recorded at el hierro volcano canary islands during the 2011 2012 pre and eruptive phases abstract the multifractal detrended fluctuation analysis mf dfa is an effective method that allows detecting multifractality in non stationary signals we applied the mf dfa to the continuous seismic signal recorded at el hierro volcano canary islands which was affected by a submarine monogenetic eruption in october 2011 we investigated the multifractal properties of the continuous seismic signal before the onset of the eruption and after we analysed three frames of the signal one measured before the onset of eruption that occurred on october 10 2011 and two after but corresponding to two distinct eruptive episodes the second one started on november 22 2011 and lasting until late february 2012 the results obtained show a striking difference in the width of the multifractal spectrum which is generally used to quantify the multifractal degree of a signal the multifractal spectra of the signal frames recorded during the eruptive episodes are almost identical and much narrower than that of the signal frame measured before the onset of the eruption such difference indicates that the seismic signal recorded during the unrest reflects mostly the fracturing of the host rock under the overpressure exerted by the intruding magma while that corresponding to the eruptive phases was mostly influenced by the flow of magma through the plumbing system even some fracturing remains not being possible to distinguish among the two eruptive episodes in terms of rock fracture mechanics

experimental constraints on plagioclase crystallization during h2o and h2o co2 saturated magma decompression experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions interpretation of natural samples may be complicated however by both the decompression path and the composition of exsolving volatiles which affect the time evolution of crystal textures here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of mount st helens rhyodacite three families of experiments were employed to simulate varied ph2o t trajectories single step h2o saturated decompression ssd continuous h2o saturated decompression cd continuous h2o co2 saturated decompression quantitative textural data crystal abundance number density and size are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products these are the first experiments to quantify feldspar nucleation and growth rates during h2o co2 saturated decompression we find that reducing the initial melt water content through addition of co2 increases nucleation rates relative to the pure water case an effect most pronounced at low dp dt moreover these early formed textural distinctions persist at the lowest pressures examined suggesting that deep h2o co2 fluids could leave a lasting textural fingerprint on erupted magmas crystals formed prior to decompression during the annealing process also modulate sample textures and growth on pre existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions the phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption finally quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive effusive transition at mount st helens this comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the may 18 plinian eruption such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories

discerning crystal growth from diffusion profiles in zoned olivine by in situ mg fe isotopic analyses mineral zoning is used in diffusion based geospeedometry to determine magmatic timescales progress in this field has been hampered by the challenge to discern mineral zoning produced by diffusion from concentration gradients inherited from crystal growth a zoned olivine phenocryst from kilauea iki lava lake hawaii was selected for this study to evaluate the potential of mg and fe isotopes for distinguishing these two processes microdrilling of the phenocryst 300 m drill holes followed by mc icpms analysis of the powders revealed negatively coupled mg and fe isotopic fractionations 26mg from 0 1 to 0 2 and 56fe from 1 2 to 0 2 from core to rim which can only be explained by mg fe exchange between melt and olivine the data can be explained with ratios of diffusivities of mg and fe isotopes in olivine scaling as d2 d1 m1 m2 with mg 0 16 and fe 0 27 la mc icpms and mc sims fe isotopic measurements are developed and are demonstrated to yield accurate 56fe measurements within precisions of 0 2 1 sd at spatial resolutions of 50 m 56fe and 26mg stay constant with fo in the rim late stage overgrowth whereas in the core original phenocryst 56fe steeply trends toward lighter compositions and 26mg trends toward heavier compositions with higher fo a plot of 56fe vs fo immediately distinguishes growth controlled from diffusion controlled zoning in these two regions the results are consistent with the idea that large isotopic fractionation accompanies chemical diffusion in crystals whereas fractional crystallization induces little or no isotopic fractionation the cooling timescale inferred from the chemical isotope zoning profiles is consistent with the documented cooling history of the lava lake in the absence of geologic context in situ stable isotopic measurements may now be used to interpret the nature of mineral zoning stable isotope measurements by la mc icpms and mc sims can be used as standard petrologic tools to identify samples for diffusion based geospeedometry

evidence for extensile crack initiation in point source time dependent moment tensor solutions abstract a time dependent moment tensor inversion technique is applied to 20 seismic events recorded ahead of an advancing tunnel excavation at the underground research laboratory url in canada the data are recorded on a 16 sensor triaxial accelerometer array and the events are chosen from a section of the tunnel such that the event sensor paths are unaffected by the presence of the tunnel factorization of the moment tensor rate functions for each event and subsequent decomposition of the subevents into isotropic and deviatoric components indicate that the events are of three types the first group shows implosional shear failure source mechanisms these are associated with the observed collapse of excavation holes drilled at the side of the tunnel and provide confidence that the method is producing realistic results the second group of events occurs in the region ahead of the face and show tensile shear failure mechanisms these are interpreted in terms of crack initiation ahead of the face and may explain why the tunnel undergoes notching and failure in the floor and roof when the modeled stresses are below the measured rock strength the third group of events shows predominant shear failure throughout their rupture history these are interpreted in terms of failure on pre existing crack surfaces this study highlights the value of using source mechanism studies of associated seismicity to understand the processes that may be taking place in environments where the stress field is complex both spatially and temporally

a process based model of pre eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes abstract using long term data sets the eruptive size timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns the usaid usgs volcano disaster assistance program vdap utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size style and onset times as well as changes within ongoing eruptions from millions of seismic event observations we show that seismic data a proxy for rate dependent strain changes in the magmatic system combined with a process based conceptual geologic model enables eruption forecasting especially for stratovolcanoes dormant 20 years we show that this model often works for some open systems ones that sufficiently reseal to plug the conduit and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun this model is divided into four seismicity and geologic stages we describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage stage 1 deep intrusion and seismicity occurring at 10 to 40 km depth below the volcanic edifice stage 2 distal vt seismicity induced at distances from 2 to 30 km laterally from the summit by magma intruding into and inflating the upper crustal magma storage region stage 3 vent clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity stage 4 repetitive event seismicity associated with the final ascent of magma from 2 km to the surface we present 36 examples of this seismic progression from 26 volcanoes we believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions

re examination of crystal ages in recent mount st helens lavas implications for magma reservoir processes u series data for recent mount st helens lavas suggest that crystallization preceded eruption by more than 0 5 ka but are complicated by possible evidence of crystal recycling and or addition of radium to the liquid after crystallization we report new ion and electron microprobe trace and major element data for plagioclase and pyroxene in these recent mount st helens lavas and use these data to reassess 226ra 230th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates revised 226ra 230th model crystallization ages are 2 4 ka for plagioclase with the exception of the 1982 dacite and 0 15 5 7 ka for pyroxene in contrast to previous interpretations no late stage addition of ra to the liquid after precipitation of the minerals is required the variability of ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re equilibration of xenocrysts with a new host liquid ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases ra th ages have not been significantly modified by ra diffusion into or out of the crystals high 226ra ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal liquid chemical equilibrium one possibility is that of growth entrapment of surface enrichments during rapid crystallization which could lead to less discrimination between ra and ba than predicted by calculated bulk partition coefficients 226ra 230th crystal ages for the castle creek andesite and basalt that are younger than 230th 238u ages of the same crystals could be explained by mixing of crystals into melts with different 230th 232th ratios by combinations of older and younger crystal growth within the same magma or for the basalt by diffusion of ra into crystals at high magmatic temperatures average plagioclase ages in most of the samples of 2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano this observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch

textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of mount st helens usa this study presents major and trace element chemistry of plagioclase phenocrysts from the 1980 eruptions of mount st helens volcano despite the considerable variation in textures and composition of plagioclase phenocrysts distinct segments have been cross correlated between crystals the variation of sr and ba concentration in the melt as calculated from the concentration in the phenocrysts using partition coefficients suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector in both of these melts sr and ba are correlated despite the abundance of plagioclase in the 1980 dacites we propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower sr and higher ba as documented in the rims of the phenocrysts although the cores of the phenocrysts crystallised at relatively shallow depths they preserve the sr and ba of the deep seated melts as they ascended from a deeper region further magma ascent resulted in microlite nucleation which is responsible for a similar shift to even lower sr concentration as observed in the groundmass of post 18 may 1980 samples

silica replaced fossils through the phanerozoic a systematic survey of 1863 papers on macrobenthic assemblages reveals that an average of 21 of published paleozoic papers concern silicified fossils but that average drops to just 4 for post paleozoic papers during the paleozoic silicified fossil occurrences do not significantly correlate with the amount of shelf chert outcrop area time duration of geologic intervals or carbonate rock volume this substantial drop in numbers of silicified fossils coincides temporally with increased importance of aragonite faunas following the end permian extinctions however qualitative measurements of secular changes in abundance and diversity of siliceous sponges are consistent not only with the post paleozoic decline in fossil silicification but also with fluctuations in the amount of silicified fossils throughout the paleozoic the facies distribution of silicified fossils in the permian of west texas also suggests that the distribution of silicified fossils may reflect the occurrence of siliceous sponges the decline in silicified fossils after the permian may be related to a concomitant rise in offshore bedded chert deposition and movement of the locus of biogenic silica formation from nearshore to offshore regions beginning in the triassic rather than with the expansion of diatoms in the cretaceous

monitoring the 1980 1982 eruptions of mount st helens compositions and abundances of glass the mount st helens eruptive sequence of 1980 through 1982 reflects the tapping of successively less water rich more highly crystallized and more viscous highly phyric dacitic magmas these changes reflect both syn and preeruption processes the decreasing water content points to a continued decline in the volume and intensity of explosive pyroclastic activity this decreasing water content appears to be composed of a long term trend established during a long period of repose about 130 years imposed on short term trends established during short periods about 7 to 100 days of repose between eruptions in the present eruptive cycle the last two eruptive cycles of this volcano the t a d 1800 and w cycles about a d 1500 exhibited similar trends these changes are inferred from a combination of petrographic bulk chemical and electron and ion microprobe analyses of matrix and melt inclusion glasses

dynamic plumbing system beneath volcanoes revealed by kinetic modeling and the connection to monitoring data an example from mt etna abstract our ability to monitor volcanoes using seismic signals ground deformation gas fluxes or other ground and satellite based observations as well as our understanding of melt reservoirs that feed eruptions have evolved tremendously in recent years the complex plumbing systems that are thought to feed eruptions are however difficult to relate to the monitoring signals here we show that the record preserved in compositional zoning of erupted minerals may be used to reconstruct sections of the plumbing system kinetic modeling of such zoning can yield information on the residence time of magma in different segments of the plumbing systems this allows a more nuanced evaluation of the link between observed monitoring signals or eruption styles and the magmatic processes and movement of batches of melts at depth the approach is illustrated through a study of the compositional zoning recorded in olivine crystals from the 1991 1993 se flank eruption products of mt etna sicily the zoning patterns in crystals reveal that the plumbing system of the volcano consisted of at least three different magmatic environments between which magma was transported and mixed in the year or two preceding the start of eruption quantification of this history indicates that two main pathways of melt migration and three timescales dominated the dynamics of the system combination of this information with the timing of observation of various monitoring signals allows a reconstruction of the dynamic evolution of this section of the plumbing system during the early stages of the 1991 1993 eruption it is seen for example how the migration of melt through the same sections of the plumbing system can cause pre eruptive triggering enhance strombolian activity and through the ensuing eruption cleanse and flush the plumbing system different kinds of mixing occur simultaneously at different sections of the plumbing system on different timescales a few days up to two years

dissecting a volcano eruption forecasting is a central goal in volcanology in recent years eruption forecasts have achieved great success due to the increased monitoring of active volcanoes however understanding the physical processes responsible for volcanic unrest remains a challenge in the january issue of american mineralogist viccaro et al 2016 linked signals of seismic unrest to magma mixing events responsible for the 2010 eruption at eyjafjallajokull in iceland their study represents a multi disciplinary effort in which integration of petrological and geophysical observations leads to a better understanding of how volcanoes work by providing a look into eyjafjallajokull s magmatic plumbing system and estimates of its magmatic ascent rates this information is key to interpreting monitoring data and successfully forecasting eruptions

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

origin and evolution of silicic magmas at ocean islands perspectives from a zoned fall deposit on ascension island south atlantic ascension island in the south atlantic is a composite ocean island volcano with a wide variety of eruptive styles and magmatic compositions evident in its 1 million year subaerial history in this paper new observations of a unique zoned fall deposit on the island are presented the deposit gradationally changes from trachytic pumice at the base through to trachy basaltic andesite scoria at the top of the deposit the key features of the eruptive deposits are described and are coupled with whole rock xrf data major and trace element analyses of phenocrysts groundmass glass and melt inclusions from samples of the compositionally zoned fall deposit to analyse the processes leading up to and driving the explosive eruption closed system crystal fractionation is the dominant control on compositional zonation with the fractionating assemblage dominated by plagioclase feldspar and olivine this fractionation from the trachy basaltic andesite magma occurred at pressures of 250 mpa there is no evidence for multiple stages of evolution involving changing magmatic conditions or the addition of new magmatic pulses preserved within the crystal cargo volatile concentrations range from 0 5 to 4 0 wt h2o and progressively increase in the more evolved units suggesting crystal fractionation concentrated volatiles into the melt phase eventually causing internal overpressure of the system and eruption of the single compositionally zoned magma body melt inclusion data combined with fe ti oxide modelling suggests that the oxygen fugacity of ascension island magmas is not affected by degree of evolution which concentrates h2o into the liquid phase and thus the two systems are decoupled on ascension similar to that observed in iceland this detailed study of the zoned fall deposit on ascension island highlights the relatively closed system evolution of felsic magmas at ascension island in contrast to many other ocean islands such as tenerife and iceland

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

evaluation of the effects of 3d diffusion crystal geometry and initial conditions on retrieved time scales from fe mg zoning in natural oriented orthopyroxene crystals abstract volcano petrologists and geochemists increasingly use time scale determinations of magmatic processes from modeling the chemical zoning patterns in crystals most determinations are done using one dimensional traverses across a two dimensional crystal section however crystals are three dimensional objects with complex shapes and diffusion and re equilibration occurs in multiple dimensions given that we can mainly study the crystals in two dimensional petrographic thin sections the determined time scales could be in error if multiple dimensional and geometrical effects are not identified and accounted for here we report the results of a numerical study where we investigate the role of multiple dimensions geometry and initial conditions of fe mg diffusion in an orthopyroxene crystal with the view towards proper determinations of time scales from modeling natural crystals we found that merging diffusion fronts i e diffusion from multiple directions causes additional diffusion that has the greatest influence close to the crystal s corners i e where two crystal faces meet and with longer times the affected area widens we also found that the one dimensional traverses that can lead to the most accurate calculated time scales from natural crystals are along the b crystallographic axis on the ab plane when model inputs concentration and zoning geometry are taken as measured rather than inferred from other observations more specifically accurate time scales are obtained if the compositional traverses are highly symmetrical and contain a concentration plateau measured through the crystal center on the other hand for two dimensional models the ab and ac planes are better suited if the initial pre diffusion concentration and zoning geometry inputs are known or can be estimated although these are a priory unknown and thus may be difficult to use in practical terms we also found that under certain conditions a combined one dimensional and two dimensional model performed on the ab section can reveal the initial pre diffusional concentration and thus offer a unique opportunity to recover lost petrologic information the influence of three dimensional diffusion on the one dimensional two dimensional model estimates is the combined result of the crystal shape in particular the presence of facets and somewhat of the aspect ratio our study focuses on fe mg in orthopyroxene but many of the effects we report are also applicable to other minerals and elements

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

nanoscale characterisation of crystal zoning abstract advances in analytical techniques are fundamental to the enhanced understandings of many geological processes zoned volcanic crystals have been analysed by low 5 kv field emission gun electron probe micro analyser feg epma and nanosims to obtain sub micrometre chemical profiles and compared to time of flight sims tof sims and high 15 20 kv epma profiles plagioclase and orthopyroxene crystals have been analysed by feg epma at accelerating voltages of 5 kv providing a spatial resolution step size of 350 nm the resolution of the lowest energy x ray for orthopyroxene crystals using a 30 nm beam and ca 750 nm for plagioclase crystals which at low voltages are unstable and require a 500 nm defocused beam step sizes are comparable in size to interaction volumes analytical protocols are detailed that permit quantitative major and minor element compositions to be acquired at similar precision and accuracy as traditional epma analyses at 15 20 kv nanosims analysis of the same crystals provides a greater spatial resolution of up to 200 nm and allows the measurement of li also the nanosims profiles however cannot currently be quantified the ability to analyse crystals at sub micrometre scales is demonstrated by the good agreement between nanosims feg epma conventional epma and tof sims data feg epma nanosims and tof sims techniques have broad applications within the earth sciences in petrologic studies for example these methods have the ability to analyse small crystals in experimental charges and provide chemical profiles of crystal zoning at a spatial resolution of ca 200 300 nm such profiles are important in crystal forensics and diffusion chronometry studies the implications for the latter application are that timescales of volcanic processes that occur in the days years immediately prior to the eruption can now be studied

aluminum diffusion in diopside using the27al p γ 28si nuclear reaction preliminary results abstract we have made a first determination of the diffusion coefficient d of al in diopside via a nuclear microanalysis technique proton energy scanning using the 992 kev resonance of 27 al p 28 si we find d 0 32 0 07 10 16 cm 2 s at t 1180 c which is the mean value of two experiments

linking fluid dynamics and olivine crystal scale zoning during simulated magma intrusion the compositional zoning styles of natural crystals produced during magma intrusion can be used to investigate the structure of magmatic plumbing systems and its relation to expressions of volcanic unrest seismic deformation volatiles however magma intrusion is a progressive dynamic process and yields non monotonic heterogeneities in physio chemical variables such as complex spatial variations in temperature and liquid composition with time such changes in variables are difficult to incorporate in models of crystal zoning in natural systems here we take another approach by integrating the results of a numerical multiphase simulation of melt arrival in an olivine rich reservoir with models of chemical re equilibration of olivine we evaluate the diversity of chemical zoning styles and the inferred time scales using fe mg diffusion in olivine for a limited range of system geometries and time composition temperature values although our models are still a large simplification of the processes that may occur in natural systems we find several time dependent and systematic relations between variables that can be used to better interpret natural data the proportions of zoned and unzoned crystals the zoning length scales and the calculated diffusion times from the crystals vary with time and the initial position of the crystal in the reservoir these relationships can be used for example to better constrain the plumbing structure and dynamics of mafic eruptions from monogenetic volcanoes by detailed studies of changes in the zoning of the crystal cargo with eruptive sequence moreover we also find that the time scales obtained from modeling of crystals at a single temperature and boundary condition tend to be shorter about 25 than the residence time which could also be tested in natural studies by combining crystal time scale records with monitoring datasets

magma transport and storage at mt etna italy a review of geodetic and petrological data for the 2002 03 2004 and 2006 eruptions abstract a detailed reconstruction of magma movements within the plumbing system of mt etna volcano has been made by reviewing the eruptions occurring during the october 2002 december 2006 period the availability of continuous gps data allowed detecting at least ten different ground deformation stages highlighting deflationary and inflationary episodes as well as the occurrence of a shallow dike intrusion these data have been coupled with the available petrological datasets including major trace elements and sr nd pb isotope compositions for the volcanic rocks erupted in the 2002 2006 period we identified two main magmatic reservoirs located at different depths along the plumbing system of the volcano the former is located at a depth of 7 km bsl and fed the 2001 and 2002 03 eruptions while the latter located from 3 5 to 5 5 km bsl fed the 2004 05 and 2006 eruptions petrological characteristics of emitted products have been correlated with the inflation vs deflation cycles related to the identified sources providing evidence for changes through time of the evolutionary degree of the erupted magmas along with variations in their geochemical feature finally we suggest that a modification of the deep plumbing system of the volcano might have occurred during the 2002 03 eruption as a consequence of the major seaward motion of the eastern flank of the volcano