inversion of aeromagnetic data using digital terrain models although draped magnetic surveys contain more information about the magnetization of the rocks near the surface of the earth than surveys at constant elevation allowance for the effects of the terrain is critical for their correct interpretation a new method for calculating the magnetic effect of the topography from a digital terrain model by integrating analytically in the vertical direction and then numerically in the horizontal plane is presented this method lends itself to the calculation of anomalies when the magnetization of the rocks varies with position and thus is well suited to the inversion of draped aeromagnetic surveys to obtain the apparent magnetization of the surficial rocks this inversion is achieved by repeated use of an approximate inverse function in the form of a two dimensional 2 d filter that is applied to gridded data an example using draped magnetic data collected over white island an active volcanic island of high relief shows that although the anomaly pattern is domina

the impact of pre existing gas on the ascent of explosively erupted magma most if not all magmas contain gas bubbles at depth before they erupt those bubbles play a crucial role in eruption dynamics by allowing magma to degas which causes the magma to accelerate as it ascends towards the surface there must be a limit to that acceleration however because gas bubbles cannot grow infinitely fast to explore that limit a series of experiments was undertaken to determine the maximum rate at which bubbly high silica rhyolite can decompress rhyolite melt that was hydrated at 150 mpa with 5 3 wt dissolved water and contained 7 to 18 vol bubbles can degas in equilibrium at 875 c when decompressed at rates up to 1 2 mpa s 1 from 150 to 78 mpa and up to 1 8 mpa s 1 when decompressed further to 42 mpa in contrast that same rhyolite cannot degas in equilibrium at 750 c if decompressed faster than 0 015 0 025 mpa s 1 when combined with other published experiments the maximum rate of decompression for equilibrium degassing is found to increase by a factor of ten for every 50 75 c increase in temperature when compared to predictions from conduit flow models that assume equilibrium degassing it is found that such models greatly over estimate the rate at which relatively cold rhyolite can decompress whereas that assumption is largely correct for hot rhyolite and thus for most other magmas all of which are less viscous than rhyolite in addition most bubbles that were 20 30 m in size at high pressure were lost from the population at low pressure that absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre eruptive bubbles even if small bubbles were originally present at depth

dynamics of giant volcanic ash clouds from supervolcanic eruptions 1 the largest explosive volcanic eruptions that have occurred on earth generated giant ash clouds from rising plumes that spread in the stratosphere around a height of neutral buoyancy with estimated supply rates that are in the range 1011 to 1013 m3 s these giant ash clouds are controlled by a balance between gravity and coriolis forces forming spinning bodies of nearly fixed proportions after a few hours and are initially insensitive to stratospheric winds in contrast volcanic plumes from eruptions with small to intermediate magnitude spread as inertial intrusions under the influence of stratospheric winds with the earth s rotation being unimportant in the largest eruptions the giant spinning ash clouds typically develop diameters greater than 600 km and up to 6000 km in the most powerful super eruptions thus explaining why areas of continental size can be covered with volcanic ash the radial expansion and spinning velocities are calculated at tens of metres per second and increase with eruption intensity higher spreading velocities carry larger ash grain sizes to a given radius so that grain size at a given distance from the source increases with eruption intensity consistent with geological observations

gravity terrain corrections calculated using digital elevation models corrections for terrain effects are required for virtually all gravity measurements acquired in mountainous areas as well as for high precision surveys even in areas of low relief terrain corrections are normally divided into two parts one part being the correction for terrain relatively close to the gravity station the inner zone correction and the other part being the correction for more distant say 2 km terrain the latter correction is normally calculated using a machine procedure that accesses a digital terrain data set the corrections for terrain very close to the gravity station are done manually using hammer s 1939 procedures or a similar method are guessed in the field or simply are neglected occasionally special correction procedures are used for the inner zone terrain corrections e g lafehr et al 1988 but such instances are uncommon

southward extension of the bangonghu nujiang suture evidence from early cretaceous intermediate and felsic magmatism in the gaoligong orogen china abstract it remains debated as to whether the gaoligong orogen between the tengchong and baoshan blocks in yunnan china is the southward extension of the tibetan bangonghu nujiang suture bns here we present new geochronological and petrochemical data for early cretaceous magmatic rocks of the gaoligong orogen the la icp ms u pb dating of zircons from diorites granodiorites and granites yielded weighted mean 206pb 238u ages of 120 118 128 122 and 109 128 ma respectively for the diorites the ehf t values of zircons fall in a range of 1 2 to 5 4 with tdmc ages of 744 979 ma for the granodiorites they fall in a range of 9 8 to 2 9 with tdmc ages of 888 1564 ma and for the granites the range is 8 0 to 1 1 with tdmc ages of 1101 1488 ma they are magnsian calc alkaline enriched in liles and lrees and have strong negative nb ta p and ti anomalies and high values of mg all these features are indicative of continental arc affinities the data suggest that the dioritic and granitic magmas were produced by the mixing of variable proportions of mantle and crust derived magmas at an active continental margin the ages and tectonic settings of the diorites granodiorites and granites are similar to rocks found within the bns considering the ophiolitic melange belt the middle jurassic basaltic magmatism and the marine facies stratigraphic features of the gaoligong orogen are very similar to features in the bns we conclude that the gaoligong orogen represents the suture between the tengchong and baoshan blocks and is the southward extension of the bns

storage conditions of the 29 ka rhyolitic guangoche white pumice sequence los azufres volcanic field central mexico abstract rhyolitic magmas in occasions can be related to explosive eruptions in active volcanoes that may not exhibit precursory signals making them very dangerous for the surrounding communities to understand the pre eruptive conditions of such magmas it is important to decipher the mechanisms capable to produce these explosive events in this study we analyzed the pre eruptive conditions of guangoche stratovolcano located to the southwest of the los azufres volcanic field in the central part of the trans mexican volcanic belt during the late pleistocene guangoche generated three plinian subplinian eruptions and some effusive eruptions all of them involving high silica rhyolitic magmas one of these plinian events vei 5 occurred 29 ka and deposited the white pyroclastic sequence wps this eruption ejected 0 7 km3 dre volume of rhyolitic magma and deposited a pumice fallout followed by three pumice rich pyroclastic density currents petrography combined with mineral composition melt inclusions and water saturated phase equilibria experiments were used to determine the evolution of the magma storage of the wps event prior to the eruption the magma was a crystal poor 14 vol rhyolite containing the following mineral assemblage biotite fe ti oxides amphibole sanidine and quartz set in a rhyolitic matrix 77 3 wt sio2 with low ba 121 16 7 ppm eu 0 15 0 02 ppm and sr 37 7 ppm based on fe ti oxythermometers and phase equilibria experiments we determine that the magma was stored between 75 and 100 mpa 3 4 km depth at a temperature of 769 8 c moreover h2o co2 dissolved in quartz melt inclusions and the textural features of the mineral assemblage of the wps products provided insights of the presence of a quartzo feldspathic crystal mush located at minimum depths between 4 3 and 9 5 km 98 218 mpa large sanidine quartz plagioclase and amphibole phenocrysts and mineral clots in the wps rocks represent assimilated parts from this mush by a melt extraction process probably triggered by the arrival of a relatively hotter magma at the base of the crystal mush which in turn caused reheating and partial melting of the quartzo feldspathic crystal mush generating the 29 ka rhyolitic wps magma

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 3 application to the peach spring tuff arizona california nevada usa establishing the depths of magma accumulation is critical to understanding how magmas evolve and erupt but developing methods to constrain these pressures is challenging we apply the new rhyolite melts phase equilibria geobarometer based on the equilibrium between melt quartz and two feldspars to matrix glass compositions from peach spring tuff arizona california nevada usa high silica rhyolite we compare the results to those from amphibole geothermobarometry projection of glass compositions onto the haplogranitic ternary and glass sio2 geobarometry quartz 2 feldspar rhyolite melts pressures span a relatively small range 185 230 mpa consistent with nearly homogeneous crystal compositions and are similar to estimates based on projection onto the haplogranitic ternary 250 50 mpa and on glass sio2 255 275 mpa amphibole geothermobarometry gives much wider pressure ranges temperature independent 65 300 mpa temperature dependent 75 295 mpa amphibole only 80 950 mpa average anderson and smith am mineral 80 549 559 1995 blundy and holland contrib miner petrol 104 208 224 1990 or holland and blundy contrib miner petrol 116 433 447 1994 thermometer a b pressures are most similar to phase equilibria results 220 210 190 mpa respectively crystallization temperatures determined previously with rhyolite melts 742 c zr in sphene 769 20 c and zircon saturation 770 780 c geothermometry are similar but temperatures from amphibole geothermometry 450 955 c are notably different the average anderson and smith holland and blundy 1994 thermometer b 710 c temperature is most consistent with previous estimates the rhyolite melts geobarometer effectively culls glass compositions affected by alteration or analytical issues peach spring glass compositions that yield pressure estimates reveal a tight range of plausible na2o and k2o contents suggesting that low na2o and high k2o contents of many peach spring samples are due to alteration use of altered whole pumice compositions in rhyolite melts simulations is likely the cause of the incorrect crystallization sequence reported previously for peach spring compositions using the rhyolite melts geobarometer we estimate a more realistic composition for peach spring tuff high silica rhyolite and the calculated composition finds close matches with some analyzed rocks and results in the expected sequence of crystallization

estimation of crystallization pressure of granite intrusions abstract a numerical method is presented to estimate the crystallization pressure of granite intrusions based on two polynomial equations obtained by an analysis of the existing haplogranite ternary phase diagram and associated dataset the results indicate that the pressure is correlated respectively with normative quartz qtz content and with normative albite ab plus orthoclase or contents of granitic rocks as follows 1 p 0 2426 qtz 3 26 392 qtz 2 980 74 qtz 12563 2 p 0 2426 ab or 3 46 397 ab or 2 2981 3 ab or 64224 r 2 0 9943 where p is pressure in mpa and r denotes correlation coefficient it is noted that the procedure of normalizing the sum of cipw norm quartz albite orthoclase contents to 100 is required before using eqs 1 and 2 the difference in pressure calculations between these two equations is 16 mpa for the range of normative quartz contents from 15 to 40 wt an example of how to use these equations to estimate the crystallization pressure of a granite intrusion is also provided to show the validity and convenience of this method the uncertainty of such pressure estimation is not well known although it must fall into the uncertainty range of the existing experimental work on pressure constraints the simplicity of this empirical method is appreciable although its applicability to natural granitoids needs further test more experimental work is required to constrain the effects of components such as cao feo mgo f cl co2 on the granite phase equilibria these equations however can be used for estimating crystallization pressures of water saturated and quartz oversaturated granitic systems

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 1 principles procedures and evaluation of the method constraining the pressure of crystallization of magmas is an important but elusive task in this work we present a method to derive crystallization pressures for rocks that preserve glass compositions either glass inclusions or matrix glass representative of equilibration between melt quartz and 1 or 2 feldspars the method relies on the well known shift of the quartz feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure the critical realization for development of the method is the fact that melt quartz and feldspars need to be in equilibrium at the liquidus for the melt composition the method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite melts over a range of pressures and searching for the pressure at which the expected assemblage quartz 1 feldspar or quartz 2 feldspars is found at the liquidus we evaluate errors resulting from uncertainties in glass composition using a series of monte carlo simulations for a quartz hosted glass inclusion composition from the bishop tuff which reveal errors on the order of 20 45 mpa for the quartz 2 feldspars constraint and on the order of 25 100 mpa for the quartz 1 feldspar constraint we suggest actual errors are closer to the lower bounds of these ranges we investigate the effect of fluid saturation in two ways 1 by applying our procedure over a range of water contents for three glass compositions we show that the effect of fluid saturation is more important at higher pressures 300 mpa than at lower pressures 100 mpa but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant h2o concentrations 3 wt and 2 by performing the same type of pressure determinations with a preliminary version of rhyolite melts that includes a h2o co2 mixed fluid phase we use a range of h2o and co2 concentrations for two compositions characteristic of early erupted and late erupted bishop tuff glass inclusions and demonstrate that calculated pressures are largely independent of co2 concentration for co2 1 000 ppm at least for relatively high h2o contents as expected in most natural magmas such that co2 concentration can be effectively neglected for application of our method finally we demonstrate that pressures calculated using the rhyolite melts geobarometer compare well with those resulting from h2o co2 glass inclusion barometry and al in hornblende barometry for an array of natural systems for which data have been compiled from the literature the agreement is best for quartz hosted glass inclusions while matrix glass yields systematically lower rhyolite melts pressures suggestive of melt evolution during eruptive decompression

crystallization stages of the bishop tuff magma body recorded in crystal textures in pumice clasts the bishop tuff is a giant silicic ignimbrite erupted at 0 76 ma in eastern california usa five pumice clasts from the late erupted bishop tuff aeolian buttes were studied in an effort to better understand the pre and syn eruptive history of the bishop magma body and place constraints on the timescales of its existence this study complements and expands on a previous study that focused on early erupted bishop tuff pumice clasts bulk densities of pumice clasts were measured using an immersion method and phenocryst crystal contents were determined using a sieving and winnowing procedure x ray tomography was used to obtain qualitative and quantitative textural information particularly crystal size distributions csds we have determined csds for crystals ranging in size from approx 10 to approx 1000 micro m for three groups of mineral phases magnetite ilmenite pyroxene biotite quartz feldspar similar to early erupted pumice late erupted pumice bulk density and crystal contents are positively correlated and comparison of crystal fraction vs size trends suggests that the proportion of large crystals is the primary control on crystallinity porosity is negatively correlated with crystal content which is difficult to reconcile with closed system crystallization magnetite and pyroxene biotite size distributions are fractal in nature often attributedmore to fragmentation however crystals are mostly whole and euhedral such that an alternative mechanism is necessary to explain these distributions quartz feldspar size distributions are kinked with a shallow sloped log linear section describing large crystals 140 micro m and a steep sloped log linear section describing small crystals 140 micro m we interpret these two crystal populations as resulting from a shift in crystallization regime we suggest that the shallow sloped section describes a pre eruptive quartz feldspar growth dominated regime whereas the steep sloped section represents a population that grew during a nucleation dominated regime that began as a result of decompression at the onset of eruption timescales of quartz growth calculated from the slopes of these two segments of the size distributions indicate that the pre eruptive crystal population grew on timescales on the order of millennia and may describe the timescale of crystallization of the bishop magma body the syn eruptive population gives timescales of 1 2 years but possibly much less and probably marks the onset of eruptive decompression less

repetitive duality of rhyolite compositions timescales and storage and extraction conditions for pleistocene caldera forming eruptions hokkaido japan

does draping aeromagnetic data reduce terrain induced effects contrary to intuition draped aeromagnetic surveys when compared to typical level surveys amplify rather than reduce the problem of magnetic terrain anomalies calculations of the total magnetic field of various simple magnetic topographies on level and draped surfaces support this conclusion in cases where draped surfaces are lower than level surfaces the draped profiles exhibit steeper gradients and deeper polarity lows over topography than do the level profiles on the other hand where draped surfaces are higher than level surfaces all anomalies are attenuated so that magnetic terrain effects might be reduced relative to subsurface sources depending upon the magnetization of each the difference in magnetic behavior between level and draped data can be explained by a contribution of a vertical derivative component in the draped case that is absent in the level case the contribution is most significant near topographic features because both the observation surface and the topographic surface

a glimpse into augustine volcano s pleistocene past insight from the petrology of a massive rhyolite deposit abstract activity at augustine volcano alaska has been characterized by intermediate composition domes flows and tephras during the holocene erosive lahars and pyroclastic flows associated with the 2006 eruption revealed large exposures of voluminous rhyolite pumice fall beneath glacial tills the massive rhyolite deposit is evidence of anomalously large prehistoric eruptions the rhyolite is petrologically and mineralogically different from more recent eruptive products with abundant amphibole calcic amphiboles and cummingtonite and quartz three texturally and chemically distinct lithologies are present fe ti oxide equilibria suggest temperatures of 765 c and oxygen fugacities of nno 1 5 melt inclusions indicate that magma representing the stratigraphically lowest lithology was crystallizing at 260 mpa with a contemporary mixed h2o co2 fluid phase becoming progressively more h2o rich magma forming the other lithologies crystallized in the presence of this h2o dominated fluid as demonstrated by the presence of cummingtonite and little to no co2 in melt inclusions amphibole and quartz feldspar melt equilibria models yield results indicating a range of crystallization pressures from 400 mpa to 175 mpa apatites and melt inclusions have lower chlorine contents than more recently erupted material at augustine suggesting that the composition of augustine s volatile phase has changed over time reheating textures in melt inclusions and phenocrysts as well as the presence of xenocrysts of olivine and clinopyroxene containing mafic melt inclusions signify the likelihood of mixing and or mingling of magmas as an eruption trigger the unique qualities of this pleistocene rhyolite and the potential hazard of a similarly large eruption in modern times indicate that further study is warranted

the bishop tuff new insights from eruptive stratigraphy the 0 76 ma bishop tuff from long valley caldera in eastern california consists of a widespread fall deposit and voluminous partly welded ignimbrite the fall deposit f exposed over an easterly sector below and adjacent to the ignimbrite is divided into nine units f1 f9 with no significant time breaks except possibly between f8 and f9 maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for f1 f8 as ca 90 hrs the ignimbrite ig is divided into chronologically and or geographically distinct packages of material earlier packages ig1 were emplaced mostly eastward are wholly intraplinian coeval with fall units f2 f8 lack phenocrystic pyroxenes and contain few or no glass mountain derived rhyolite lithic fragments earlier packages ig2 were erupted mostly to the north and east are at least partly intraplinian interbedded with fall unit f9 to the east contain pyroxenes and have lithic fractions rich in g

implications of equilibrium and disequilibrium among crystal phases in the bishop tuff eruption of the bishop tuff magma preserved equilibrium of exchange components and element concentrations among magnetite ilmenite biotite apatite zircon and liquid orthopyroxene and clinopyroxene were not in exchange equilibrium with the other mgfe bearing phases but they appear to have been in equilibrium among themselves internally consistent temperatures recorded by the feti oxide ti in quartz and 18 o quartz magnetite thermometers coupled with evidence for magmatic corrosion of quartz and sanidine indicate that an initially low t 700 c near h 2 o saturated high sio 2 rhyolite magma was heated up to 800 c and its crystal cargo partially melted by recharge of hotter melt from below oxygen fugacity and compositions of biotite ilmenite magnetite and silicate liquid initially adjusted by internal rearrangement of components and conservation of oxygen partial melting of feldspars liberated sr and ba back into the melt mixing during recharge eventually re introduced compatible elements e g mg ba sr as well as foreign crystals of euhedral ortho and clinopyroxene which evidently never totally re equilibrated with the rhyolite liquid introduction of co 2 and accompanying reduction in the a h 2 o during recharge raised crystallization temperatures of quartz and sanidine in the rhyolite sufficient to allow marginal regrowth of these phases with enhanced contents ti ba and sr

diverse late stage crystallization and storage conditions in melt domains from the youngest toba tuff revealed by age and compositional heterogeneity in the last increment of accessory phase growth the chemical record contained within the final increment of growth on crystals is utilized to reveal the dynamics and timescales of magma assembly and storage before eruption of the cataclysmic 2800 km3 youngest toba tuff ytt indonesia in situ u th disequilibrium dates and trace element concentrations were obtained via secondary ionization mass spectrometry sims on unsectioned and unpolished faces of individual zircon and allanite crystals the six high silica 73 wt sio2 pumices from which crystals were derived are among the more evolved and lower crystallinity 25 wt pumices from the ytt eruption and likely represent the melt dominated portion of the magma system discrete sims measurement cycles were coupled with statistical treatments to detect zircon and allanite surface zoning domains at the 1 m scale coupled r melts and accessory phase saturation modeling indicates that at the granite ternary minimum or eutectoid conditions that define this portion of the ytt zircon and allanite crystallization is dependent on and proportionate to major phase crystallization and is more limited than at pre eutectoid conditions a lower proportion of near eruption zircon surface ages in the comparatively cool and wet ytt relative to other hotter and drier voluminous silicic eruptions could reflect the influence of eutectoid storage conditions on magmatic responses to remobilization related magmatic recharge

a new variable magnetization terrain correction method for aeromagnetic data terrain effects in aeromagnetic data are produced by rugged magnetic topography these effects mimic the shape of topography and can often be so large that they obscure anomalies of interest thus it is desirable to remove terrain effects from aeromagnetic data in order to isolate the anomalies to be investigated however removal of aeromagnetic terrain effects has been a longstanding problem previously developed methods have succeeded only in certain specific geologic situations i present a new aeromagnetic terrain correction method that is superior to the previously developed methods for the general case this method takes into account the highly variable magnetic properties of rocks and can remove terrain effects whether the sources of interest are shallow or deep the new method is based on the assumption that magnetic sources of interest are often geometrically unrelated to terrain it finds the magnetization that gives a magnetic field residual with minimum correlation to terrain effects for a

fitful and protracted magma assembly leading to a giant eruption youngest toba tuff indonesia the paroxysmal eruption of the 74 ka youngest toba tuff ytt of northern sumatra produced an extraordinary 2800 km3 of nonwelded to densely welded ignimbrite and coignimbrite ashfall we report insights into the duration of ytt magma assembly obtained from ion microprobe u th and u pb dates including continuous age spectra over 50 of final zircon growth for pumices and a welded tuff spanning the compositional range of the ytt a relatively large subpopulation of zircon crystals nucleated before the penultimate caldera related eruption at 501 ka but most zircons yielded interior dates 100 300 ka thereafter zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of 100 to 500 ka final zircon growth is evident as thin rim plateaus that are in th u chemical equilibrium with hosts and that give crystallization ages within tens of ka of eruption the longevity and chemical characteristics of the ytt zircons as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge is especially favored at the cool and wet eutectoid conditions that characterize at least half of the ytt wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals repeated magma recharge may have contributed to the development of compositional zoning in the ytt but considered together with limited allanite quartz and other mineral dating and geospeedometry regular perturbations to the magma reservoir over 400 ka did not lead to eruption until 74 ka ago

the geochemical and petrological characteristics of prenatal caldera volcano a case of the newly formed small dacitic caldera hijiori northeast japan evaluating the magma depth and its physical properties is critical to conduct a better geophysical assessment of magma chambers of caldera volcanoes that may potentially cause future volcanic hazards to understand pre eruptive conditions of a magma chamber before its first appearance at the surface this paper describes the case of hijiori caldera volcano in northeastern japan which emerged approximately 12 000 years ago at a place where no volcano ever existed we estimated the depth density bulk modulus vesicularity crystal content and bulk h 2 o content of the magma chamber using petrographic interpretations bulk and microchemical compositions and thermodynamic calculations the chemical mass balance calculations and thermodynamic modeling of the erupted magmas indicate that the upper portion of the hijiori magmatic plumbing system was located at depths between 2 and 4 km and had the following characteristics 1 pre eruptive temperature about 780 circ c 2 bulk magma composition 66 1 5 wt sio 2 3 bulk magmatic h 2 o approximately 2 5 wt and variable characteristics that depend on depth 4 crystal content le 57 vol 5 bulk modulus of magma 0 1 0 8 gpa 6 magma density 1 8 2 3 g cm3 and 7 amount of excess magmatic h 2 o 11 32 vol or 48 81 mol the range of melt water contents found in quartz hosted melt inclusions 2 9 wt suggests the range of depth phenocrysts growth to be wide 2 sim 13 km our data suggest the presence of a vertically elongated magma chamber whose top is nearly solidified but highly vesiculated this chamber has probably grown and re mobilized by repeated injections of a small amount of hot dacitic magma originated from the depth

magma storage and evolution of the most recent effusive and explosive eruptions from yellowstone caldera between 70 and 175 ka over 350 km3 of high silica rhyolite magma erupted both effusively and explosively from within the yellowstone caldera phenocrysts in all studied lavas and tuffs are remarkably homogenous at the crystal eruption and caldera scale and yield quilf temperatures of 750 25 c phase equilibrium experiments replicate the observed phenocryst assemblage at those temperatures and suggest that the magmas were all stored in the upper crust quartz hosted glass inclusions contain 1 0 2 5 h2o and 50 600 ppm co2 but some units are relatively rich in co2 300 600 ppm and some are co2 poor 50 200 ppm the co2 rich magmas were stored at 90 150 mpa and contained a fluid that was 60 75 mol co2 co2 poor magmas were stored at 50 70 mpa with a more h2o rich fluid x text co 2 40 60 storage pressures and volatiles do not correlate with eruption age volume or style trace element contents in glass inclusions and host matrix glass preserve a systematic evolution produced by crystal fractionation estimated to range from 36 12 to 52 12 wt because the erupted products contain 10 wt crystals crystal poor melts likely separated from evolving crystal rich mushes prior to eruption in the tuffs of bluff point and cold mountain creek matrix glass is less evolved than most inclusions which may indicate that more primitive rhyolite was injected into the reservoir just before those eruptions the presence and dissolution of granophyre in one flow may record evidence for heating prior to eruption and also demonstrate that the yellowstone magmatic system may undergo rapid changes the variations in depth suggest the magmas were sourced from multiple chambers that follow similar evolutionary paths in the upper crust

experimental constraints on rhyolite melts and the late bishop tuff magma body thermodynamic models are vital tools to evaluate magma crystallization and storage conditions before their results can be used independently however they must be verified with controlled experimental data here we use a set of hydrothermal experiments on the late erupted bishop tuff lbt magma to evaluate the rhyolite melts thermodynamic model a modified calibration of the original melts model optimized for crystallization of silicic magmas experimental results that are well captured by rhyolite melts include a relatively narrow temperature range separating the crystallization of the first felsic mineral and the onset of the ternary minimum quartz plus two feldspars and extensive crystallization over a narrow temperature range once the ternary minimum is reached the model overestimates temperatures by 40 c a known limitation of rhyolite melts at pressures below 110 mpa model and experiments differ in the first felsic phase suggesting that caution should be exercised when applying the model to very low pressures our results indicate that for quartz sanidine plagioclase magnetite and ilmenite to crystallize in equilibrium from lbt magma magma must have been stored at 740 c even when a substantial amount of co2 occurs in the coexisting fluid such temperatures are in conflict with the hotter temperatures retrieved from magnetite ilmenite compositions 785 c for the sample used in the experiments consistent with other recent studies we suggest that the fe ti oxide phases in the late bishop tuff magma body are not in equilibrium with the other minerals and thus the retrieved temperature and oxygen fugacity do not reflect pre eruptive storage conditions

an early cretaceous carbonate replacement origin for the xinqiao stratabound massive sulfide deposit middle lower yangtze metallogenic belt china stratabound massive sulfide deposits are widespread along the middle lower yangtze metallogenic belt mlymb and serve as an important copper producer in china two contrasting genetic models have been proposed interpreting the stratabound massive sulfide deposits as a carboniferous sedex protore overprinted by cretaceous magmatic hydrothermal system or an early cretaceous carbonate replacement deposit these two contrasting models have been applied to the xinqiao stratabound cu au sulfide deposit which is dominated by massive sulfide ores hosted in marine carbonates of the carboniferous chuanshan and huanglong formations with minor cu au skarn ores localized in the contact zone between the cretaceous diorite jitou stock and the carboniferous carbonate rocks new sims zircon u pb dating suggests that the jitou stock formed at 138 5 1 1 ma 2 mswd 0 6 pyrite re os dating yields an imprecise date of 142 47 ma 2 mswd 7 8 the geochronological data thus constrain the mineralization of the xinqiao deposit at early cretaceous fluid inclusions in prograde skarn diopside have homogenization temperatures of 450 600 c and calculated salinities of 13 58 wt nacl equiv quartz from the stratabound ores and pyrite quartz vein networks beneath the stratabound ores have homogenization temperatures of 290 360 and 200 300 c with calculated salinities of 5 12 and 2 10 wt nacl equiv respectively quartz from the skarn ores and veins beneath the stratabound ores have 18o values of 12 32 0 55 2 sd n 22 and 15 57 1 92 2 sd n 60 respectively corresponding to calculated 18o values of 6 22 1 59 2 and 6 81 2 76 2 for the equilibrated ore forming fluids the fluid inclusion and oxygen isotope data thus support a magmatic hydrothermal origin rather than a sedex system for the stratabound ores with the hydrothermal fluids most likely being derived from the jitou stock or associated concealed intrusion results from this study have broad implications for the genesis and exploration of other stratabound massive sulfide deposits along the mlymb

oxygen isotope study of the long valley magma system california isotope thermometry and convection in large silicic magma bodies products of voluminous pyroclastic eruptions with eruptive draw down of several kilometers provide a snap shot view of batholith scale magma chambers and quench pre eruptive isotopic fractionations i e temperatures between minerals we report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite pyroxene and zircon from individual pumice clasts of ignimbrite and fall units of caldera forming 0 76 ma bishop tuff bt pre caldera glass mountain 2 1 0 78 ma and post caldera rhyolites 0 65 0 04 ma to characterize the long lived batholith scale magma chamber beneath long valley caldera in california values of 18o show a subtle 1 decrease from the oldest glass mountain lavas to the youngest post caldera rhyolites older glass mountain lavas exhibit larger 1 variability of 18o quartz the youngest domes of glass mountain are similar to bt in 18o quartz values and reflect convective homogenization during formation of bt magma chamber surrounded by extremely heterogeneous country rocks ranging from 2 to 29 oxygen isotope thermometry of bt confirms a temperature gradient between late 815 c and early 715 c bt the 18o quartz values of early and late bt are 8 33 and 8 21 consistent with a constant 18o melt 7 8 0 1 and 100 c temperature difference zircon melt saturation equilibria gives a similar temperature range values of 18o quartz for different stratigraphic units of bt and in pumice clasts ranging in pre eruptive depths from 6 to 11 km based on melt inclusions and document vertical and lateral homogeneity of 18o melt worldwide five other large volume rhyolites lava creek lower bandelier fish canyon cerro galan and toba exhibit equal 18o melt values of earlier and later erupted portions in each of the these climactic caldera forming eruptions we interpret the large scale 18o homogeneity of bt and other large magma chambers as evidence of their longevity 105 years and convection however remaining isotopic zoning in some quartz phenocrysts trace element gradients in feldspars and quartz and zircon crystal size distributions are more consistent with far shorter timescales 102 104 years we propose a sidewall crystallization model that promotes convective homogenization roofward accumulation of more evolved and stagnant volatile rich liquid and develops compositional and temperature gradients in pre climactic magma chamber crystal melt gas bubbles mush near chamber walls of variable 18o gets periodically remobilized in response to chamber refill by new hotter magmas one such episode of chamber refill by high ti sr ba zr and volatile richer magma happened 103 104 years prior to the 0 76 ma caldera collapse that caused magma mixing at the base mush thawing near the roof and walls and downward settling of phenocrysts into this hybrid melt

oxygen isotope thermometry reveals high magmatic temperatures and short residence times in yellowstone and other hot dry rhyolites compared to cold wet systems the eruption and storage temperatures of rhyolitic magmas are critical factors for understanding the mechanisms of their eruption and petrogenesis temperatures are particularly important when comparing the magmatic histories of hot dry rhyolites from the yellowstone snake river plain ysrp and iceland to cold wet rhyolites such as the bishop tuff here we employ mineral pair oxygen isotope fractionations for estimating rhyolite temperatures independent of pressure and other compositional factors we report high precision oxygen isotope analyses of quartz pyroxene magnetite and zircon that we use to estimate crystallization and storage temperatures temperatures for ysrp and icelandic rhyolites are highest for quartz magnetite and quartz clinopyroxene 950 c with lower quartz zircon 850 c temperatures that are similar to estimates of zircon saturation the magnitude and pattern of these temperatures is consistent with crystallization from near liquidus rhyolites in contrast oxygen isotope temperatures calculated for the bishop and other cold wet type tuffs define low 760 c temperatures for all three mineral pairs consistent with prolonged mineral residence at near solidus conditions preservation of a down temperature crystallization sequence of hot magnetite and clinopyroxene with colder zircon in hot dry ysrp and icelandic rhyolites suggest 1000 yr magma residence where magnetite does not have sufficient time to diffusively equilibrate oxygen in a lower temperature melt this is consistent with recently determined high precision u pb crystallization ages zircons from the same units indicating magma generation shortly before eruption

micro analytical perspectives on the bishop tuff and its magma chamber new in situ major and trace element analytical data are presented for crystals sanidine plagioclase biotite orthopyroxene clinopyroxene and matrix glasses from juvenile materials representing the full bishop tuff sequence from the earliest fall unit f1 to the latest ignimbrite package ig2nc these data are combined with published information to investigate the nature and zonation of the pre eruptive bishop magma chamber our data confirm that this magma chamber was a single unitary body that was thermally and compositionally zoned the zonation was largely established prior to the growth of crystals and also prior to mixing in the lower parts of the chamber induced by late stage intrusion of a magma of contrasting composition and slightly higher temperature the bright rim magma sparse mixed swirly and dacitic pumices show enrichments in ba sr and ti that identify these pumices as possible representatives of the bright rim magma a model revised from previously published work for the pre eruptive magma chamber comprises three main parts 1 an upper volumetrically dominant 2 3 relatively unzoned region that was the source of the earlier eastern erupted ignimbrite units and their coeval fall units 2 a volumetrically minor transition zone that shows evidence for minor degrees of mixing and was the dominant source for the latest eastern erupted part of ig1eb sherwin subunit and the earlier part of the northern erupted ignimbrite ig 2na 3 a lower volumetrically subordinate 1 3 region that was affected by mixing with the bright rim invasive magma in the lead up to the eruption and fed later northern erupted units ingress of the bright rim magma introduced orthopyroxene and bright rimmed zircon crystals and induced partial resorption then overgrowth of rims enriched in ti sr and ba on sanidine and quartz and development of zoning in clinopyroxene based on pumice proportions and associated crystal and glass chemistries through the eruptive sequence we infer that the roof and floor of the magma chamber were stepped down to the north such that the transition zone magma formed the floor of the southern part of the melt dominant chamber and the roof of the northern part our data reinforce the previous concept of a single compositionally and thermally zoned bishop magma chamber and additionally support a temporally constrained role for pre eruptive magma mixing and the introduction of melts and minerals with contrasting compositions to the resident bishop magma

zoned quartz phenocrysts from the rhyolitic bishop tuff cathodoluminescence cl reveals growth zones in quartz phenocrysts from the rhyolitic bishop tuff melt inclusions occur in various zones and record the evolving melt composition during zonal growth the zones form an oscillatory pattern between bright and dark cl quartz there are three recognizable patterns of cl zoning in these crystals 1 weakly zoned cores and bright cl rims 2 weakly zoned cores and dark cl rims and 3 no cl intensity difference from core to rim dark cl quartz generally occurs at crystal edges contains most of the melt inclusions and is interpreted as fast growing zones that occur along recognizable crystal edges edge zones are thicker than the same zone on adjacent faces consistent with relatively fast growth of these zones in each successive zone these edge zones decrease in size toward the rim while the zones along the crystal faces increase some of the melt inclusions have bright cl quartz locally associated with them this is interpreted as the postentrapment crystallization of slow growing quartz in the melt inclusions many crystals display zone discordance from the weakly zoned interiors to the rims most of the discordant surfaces are rational and probably are primary growth features pumice clasts from the southern vents are largely compositionally and texturally distinct from those from the northern vents and this distinction is also evident in the quartz cl the crystals that have bright cl rims are all associated with the late erupted northern part of the bishop tuff melt inclusion compositions and cl zoning patterns suggest a common origin for early and middle erupted quartz and the interior zones of late erupted quartz however the bright cl rim on the late erupted quartz indicates an additional stage of crystallization in late erupted magma melt inclusions in individual early erupted crystals have small variations in ba whereas inclusions in late erupted crystals markedly increase in ba toward the rim which is opposite to the normal zoning of sequentially trapped melts expected during closed system crystallization differentiation the quartz zoning features are consistent with the hypothesis of crystal settling in evolving magma that erupted late from northern vents

early silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system the case of the xuehuading panshanchong pluton south china block

magnetic models of crystalline terrane accounting for the effect of topography igneous rocks commonly have large magnetic susceptibilities so that high topographic relief in crystalline terrane can produce significant anomalies in aeromagnetic surveys topographic anomalies are particularly significant in relatively undeformed volcanic terrane because young volcanic rocks generally have large natural remanent magnetizations as well as large susceptibilities these anomalies commonly appear in aeromagnetic surveys as a complex pattern of high amplitude short wavelength magnetic features that tend to obscure anomalies caused by deeper geologic sources we have facilitated geologic interpretation of an aeromagnetic survey of the oregon cascade range by calculating the magnetic field caused by a three dimensional 3 d topographic model maps of the calculated field are compared with observed aeromagnetic data both visually and with a numerical technique that produces a contour map of correlation coefficients for the model these comparisons allow quick recognition of anomalies caused

magnetic field changes on white island new zealand and the value of magnetic changes for eruption forecasting for about 15 years after their commencement in 1968 magnetic measurements in the crater of white island gave a good indication of the state of activity and the likelihood of an eruption episode commencing during this period magnetic thermal ground level and chemical measurements were all generally consistent with a model in which these surface changes were caused by increasing temperatures under the donald mound area in the central sub crater in more recent times there has not been such a relationship between the changes around the remnants of donald mound and the volcanic activity which has become localised in the west sub crater it appears that changes under the crater especially lowering of the ground surface and the water table at the west end of the crater have meant that eruptions are no longer preceded by heating under donald mound the west sub crater is not easily accessible for repeat surface surveys one way in which we might be able to monitor future volcanic activity at white island would be by repeated aeromagnetic surveys as the magnetic changes are so large that changes of hundreds of nanoteslas are likely to be observed at practical flying heights

role of hypsometry and planform in basin hydrologic response the effects of variations of drainage basin morphometry and relief characteristics on flood peak magnitude and time to peak are investigated using simulated stream networks the networks are produced by three models headward growth systematic capture and minimum power relaxation translational and kinematic wave flood routing were used to generate synthetic hydrographs peak discharge and time to peak are predictable to a high degree by five different sets of morphometric relief parameters in order of decreasing order of importance in predictive ability the parameters characterize basin size relative relief basin concavity and basin shape both simulated and natural stream networks exhibit strong dependence of planimetric morphometry upon basin concavity the effect of this dependency is to increase the effect of basin concavity upon flood hydrographs

evaluating the construction and evolution of upper crustal magma reservoirs with coupled u pb zircon geochronology and thermal modeling a case study from the mt capanne pluton elba italy abstract evaluating mechanisms and rates for magma transport and emplacement in the upper crust is important in order to predict the thermal and rheological state of the crust and understand the relationship between plutonism and volcanism u pb geochronology on zircon is commonly used to constrain magma emplacement and storage time in the crust but interpreting complex zircon age populations in terms of in situ crystallization versus crystallization at a deeper level is not trivial this study focuses on the mt capanne pluton in elba italy a well documented example of arc related laccolith emplaced in the upper continental crust previous studies proposed that the mt capanne intrusion was accreted in less than 10 000 yr by distinct and mappable magma pulses here we couple high precision id tims u pb zircon geochronology with numerical thermal simulations to evaluate emplacement rates test different emplacement models inform zircon age interpretations and evaluate the potential for melt storage during construction of the mt capanne pluton our results require that the mt capanne intrusion was built in at least 250 000 yr by multiple magma injections a variety of emplacement scenarios show that melt was preserved for