multiphase petrography of volcanic rocks using element maps a method applied to mount st helens 1980 2005 quantitative textural analyses including crystal size distributions csds provide insights into crystallisation kinetics of magmatic systems investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases which must then be thresholded and or traced manually a laborious task and investigations are typically restricted to a single crystal phase a method is presented that uses energy dispersive x ray element maps to generate textural data each pixel is identified as a crystal phase glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase csd crystallinity and mineral mode data data processing is less time intensive for the operator but considerable instrument time is required to generate element maps the method is applied to 17 dacite samples from the 1980 1986 and 3 from the 2004 2005 eruptive periods of mount st helens volcano usa to provide quantitative insights into multiphase textural evolution all of the csds are curved and concave up in the standard csd plot with curvature increasing with plagioclase content to facilitate comparisons with previous studies csds for microlites 50 m length crystals are approximated as straight lines the line intercepts and slopes provide information on n 0 nucleation density and characteristic length or g the product of growth rate g and residence time respectively these parameters as well as the total groundmass crystallinity show distinct differences between explosive deposits from summer 1980 and post summer 1980 domes post summer 1980 microlite n 0 values are mostly at the lower end of the range of those measured for summer 1980 samples total groundmass crystallinities during summer 1980 are between 10 and 30 vol whereas post summer 1980 crystallinity increases to between 39 and 51 vol the range of n 0 values is similar to those previously published for mount st helens but g is consistently higher g of a may 1985 sample analysed in this study is approximately 2 m higher compared with previously published data for the same sample when processed using similar methodologies groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption increasing sharply after summer 1980 then increasing more gradually during the dome building phase of the eruption the effects of varying l the apparent crystal size and crystal aspect ratio on resultant csds are also investigated whilst relative temporal variations in csds for a given dataset can be investigated absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same

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

the cooling kinetics of plagioclase feldspar as revealed by electron microprobe mapping in this study we have used electron microprobe mapping to investigate plagioclase compositional evolution due to cooling kinetics we re analyzed five run products from a prior study iezzi et al 2011 crystallized by cooling a natural andesitic melt from 1300 to 800 c at 25 12 5 3 0 5 and 0 125 c min under atmospheric pressure and air redox state as the cooling rate decreases the texture of large plagioclases changes from skeletal to hollow to nearly equant in this study we use x ray map data to obtain a database of 12 275 quantitative chemical analyses the frequency of an rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate at 25 and 12 5 c min the distribution is single mode and narrow at 0 5 and 0 125 c min is single mode but very broad whereas at the intermediate cooling rate of 3 c min two distinct plagioclase populations are present this intermediate cooling rate is fast enough to cause departure from equilibrium for the crystallization of the an rich population but also sufficiently slow that an poor plagioclases nucleate from the residual melt we interpret our findings in the context of time temperature transformation ttt diagrams and infer the crystallization kinetics of plagioclase in the experiments compositional trends and our inferences regarding ttt systematics are consistent with two discrete nucleation events that produced separate populations of plagioclase i e an rich and an poor populations at 3 c min using plagioclase melt pairs as input data for the thermometric reaction between an and ab components we find that plagioclase mirrors very high near liquidus crystallization temperatures with increasing cooling rate these results have important implications for the estimate of post eruptive solidification conditions lava flows and intrusive bodies from centimeters to a few meters thick are characterized by a short solidification time and a significant thermal diffusion under such circumstances it is possible to crystallize plagioclases with variable and disequilibrium chemical compositions simply by cooling a homogeneous andesitic melt x ray element maps enrich the study of plagioclase compositional variations generated under conditions of rapid cooling

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 may 1980 mount st helens fallout deposit the 18 may 1980 eruption of mount st helens started with a lateral blast that fed a pyroclastic surge which then uplifted to form a co blast plume thirty minutes later plinian activity started at the vent and fluctuated in intensity for 9 h the resulting fallout deposit documented to 600 km from vent presents some striking features 1 displacement of the overall deposit to the north of the vent 2 a secondary thickness and mass maximum at 300 km from vent 3 a total grain size distribution dominated by fine ash 62 wt of the deposit 63 m and 4 individual grain size distributions that vary dramatically in the crosswind direction from strongly bimodal in the south to skewed unimodal in the north results from a new deconvolution of the individual grain size distributions show that they are a combination of a coarse subpopulation that decreases in size with distance from vent and a constant fine subpopulation with a mean of 15 m relative proportions of each subpopulation vary asymmetrically in the crosswind directions with the fine subpopulation preponderant toward the north and the coarse one dominating the south of the deposit both reach their absolute maxima in mass on the deposit axis componentry analyses of selected samples show that blast derived material is greatly enriched toward the north of the deposit these results indicate that the co blast plume dispersed fine grained material over great distances and dominated the fine subpopulation comparison with reanalysis data of atmospheric wind fields and satellite images of the spreading ash cloud suggests contrasting ash transport and depositional processes for the early co blast plume and the later vent derived plinian plumes the co blast plume is displaced to the north it had a high overshoot height and eastward dispersion via strong winds low in the stratosphere 10 15 km the plinian plumes were lower and dispersed most of the material to the southeast as the direction of high velocity winds shifted just before the late climactic plinian eruptive phase fine ash fine subpopulation was deposited continuously throughout the deposit with an increase of sedimentation rate 300 km from the vent where there is a secondary maximum in the deposit mass and thickness fine ash probably settled by a combination of enhanced sedimentation mechanisms including not only aggregation but also gravitational convective instabilities of cloud base hydrometeor formation and destruction and entrainment of small particles by larger ones finally we show that half of the deposit by mass in the medial area was deposited by the co blast plume and that a significant proportion of the mount st helens fallout deposit is nonjuvenile which has implications for the magmatic budget of this eruption

halogen and ar ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques abstract extension of ar ar methodology has been used to determine mineralisation ages from mica inclusions and to simultaneously evaluate the noble gas and halogen composition of inclusion fluids within irradiated quartz vein samples from five porphyry copper deposits samples have been collected from the potassic and propylitic zones of bingham canyon utah and four arizonan deposits silverbell globe miami pinto valley and ray data obtained using three noble gas extraction techniques laser ablation in vacuo crushing and stepped heating are compared with each other laser ablation provides a means for the analysis of individual fluid inclusions but is limited by blank levels and detection limits stepped heating and in vacuo crushing are bulk extraction techniques that preferentially release gases from solid and fluid inclusion phases respectively and can be used in combination to obtain accurate and meaningful ages of mineralisation ages obtained for porphyry copper deposits are as follows bingham canyon 37 1 0 5 ray 65 3 1 5 globe miami 61 7 3 4 pinto valley 63 2 8 0 and silverbell 55 8 1 8 the age of mineralisation at ray is of particular interest as it enables a reported discrepancy in k ar ages to be understood halogen data for the bingham canyon samples gives insight into the partitioning of the heavy halogens br and i between solid and liquid phases

the geochemistry of carbonate replacement pb zn ag mineralization in the lavrion district attica greece fluid inclusion stable isotope and rare earth element studies strata bound carbonate replacement pb zn ag deposits in the lavrion district greece are spatially related to a late miocene granodiorite intrusion 7 10 ma and various sills and dikes of mafic to felsic composition the plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a ca fe skarn carbonate replacement deposits occur predominantly in marbles upper and lower marble of the basal unit kaesariani schists and along a major detachment fault that separates the basal unit from the upper unit orebodies are mainly strata bound carbonate replacement although sulfides also occur in veins the mineralogy of carbonate replacement deposits is dominated by base metal sulfides and sulfosalts of ag bi sn sb as and pb particularly at plaka and kamariza carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite fluid inclusion studies of sphalerite fluorite calcite and quartz in carbonate replacement deposits suggest that they were deposited from 132 to 365 c from co2 poor low to high salinity fluids 1 20 wt nacl equiv carbon and oxygen isotope compositions of calcite 13c 15 6 to 1 5 and 18o 9 2 to 17 3 intergrown with sulfides reflect variable exchange of the ore bearing fluid with the upper and lower marbles and proximity to the plaka granodiorite post archean australian shale paas normalized rare earth and yttrium patterns of the upper and lower marbles and calcite intergrown with sulfides show positive eu and negative ce anomalies as well as y ho ratios between 40 and 80 normalized rare earth and yttrium patterns of fluorite also have positive eu and negative ce anomalies such anomalies for both the carbonates and fluorite reflect the high ph or high f o2 conditions of the late stage hydrothermal fluids and the likely derivation of calcium from marine carbonates precursors of the upper and lower marbles the range of sulfur isotope compositions for sulfides 34s 4 9 to 5 3 with one outlier of 9 4 in carbonate replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate sulfur isotope compositions of barite from carbonate replacement range from 34s 17 2 to 23 7 per mil and reflect miocene seawater sulfate values if a magmatic source of sulfur is assumed along with an average temperature of 250 c for the ore forming fluids as based on fluid inclusion studies sulfides in carbonate replacement deposits were deposited at values of log f o2 41 to 36 and a ph 5 8 to 9 1 however the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source the carbonate replacement deposits resemble manto type sulfide deposits in mexico central colorado south korea nevada and northern greece

the behavior of trace elements during the chemical evolution of the h 2 o b and f rich granite pegmatite hydrothermal system at ehrenfriedersdorf germany a sxrf study of melt and fluid inclusions detailed melt and fluid inclusion studies in quartz hosts from the variscan ehrenfriedersdorf complex revealed that ongoing fractional crystallization of the highly evolved h2o b and f rich granite magma produced a pegmatite melt which started to separate into two immiscible phases at about 720 c 100 mpa with cooling and further chemical evolution the immiscibilty field expanded two conjugate melts a peraluminous one and a peralkaline one coexisted down to temperatures of about 490 c additionally high salinity brine exsolved throughout the pegmatitic stage along with low density vapor towards lower temperatures a hydrothermal system gradually developed boiling processes occurred between 450 and 400 c increasing the salinities of hydrothermal fluids at this stage below the late hydrothermal stage is dominated by low salinity fluids using a combination of synchrotron radiation induced x ray fluorescence analysis and raman spectroscopy the concentration of trace elements mn fe zn as sb rb cs sr zr nb ta ag sn ta w rare earth elements ree and cu was determined in 52 melt and 8 fluid inclusions that are representative of distinct stages from 720 c down to 380 c homogenization temperatures and water contents of both melt and fluid inclusions are used to estimate trapping temperatures thus revealing the evolutionary stage during the process trace elements are partitioned in different proportions between the two pegmatite melts high salinity brines and exsolving vapors concentrations are strongly shifted by co ncomitant crystallization and precipitation of ore forming minerals for example pegmatite melts at the initial stage 700 c have about 1 600 ppm of sn concentrations in both melts decrease towards lower temperatures due to the crystallization of cassiterite between 650 and 550 c tin is preferentially fractionated into the peralkaline melt by a factor of 2 3 while the last pegmatite melts are low in sn 64 ppm at 500 c early hydrothermal fluids become again enriched with about 800 ppm of sn at the boiling stage a sudden drop in late hydrothermal fluids 23 ppm of sn at 370 c results from precipitation of another cassiterite generation between 400 and 370 c zinc concentrations in peraluminous melts are low some tens of parts per million and are not correlated with temperature in coexisting peralkaline melts and high t brines they are higher by a factor of 2 3 zinc continuously increases in hydrothermal fluids 3 000 ppm at 400 c where the precipitation of sphalerite starts the main removal of zn from the fluid system occurs at lower temperatures similarly melt and fluid inclusion concentrations of many other trace elements directly reflect the crystallization and precipitation history of minerals at distinctive temperatures or temperature windows

porphyry and epithermal deposits in greece an overview new discoveries and mineralogical constraints on their genesis abstract greece hosts a variety of magmatic hydrothermal ore deposits prospects with porphyry and epithermal styles playing a major role in its total gold endowment these deposit types are mainly clustered in two areas the rhodope and attico cycladic massifs and formed from about 33 ma to the pleistocene as a result of back arc extension in the aegean sea metamorphic core complex formation and contemporaneous post subduction and arc magmatism in the serbo macedonian massif porphyry cu au deposits include skouries fisoka tsikara vathi and gerakario causative intrusives are oligocene to miocene granodiorites to monzonites gold and pge mineralization is associated with potassic alteration of the intrusives in the eastern rhodope massif and the ne aegean islands porphyry prospects occur at pagoni rachi konos hill myli melitena west thrace fakos sardes and kaspakas limnos island and stypsi lesvos island mineralization is associated with oligocene to miocene subvolcanics of calc alkaline to shoshonitic affinity feature of these prospects which they share in common with several porphyry au only systems is their shallow depth of emplacement the presence of potassic sodic calcic and or phyllic alteration a strong epithermal overprint their low cu content an extreme re enrichment the multistage introduction of au the presence of banded quartz veinlets and the local presence of tourmaline new discoveries of porphyry style mineralization at king arthur st philippos and aisymi increases the gold potential in west thrace high intermediate sulfidation epithermal au ag polymetallic deposits prospects overprint and or occur laterally from porphyry style mineralization where they are spatially associated with lithocaps of advanced argillic alteration high intermediate sulfidation au ag epithermal mineralization at perama hill mavrokoryfi and pefka in west thrace and at pterounda mesotopos and megala therma on lesvos island is controlled by steeply dipping extensional faults within volcanic rocks without any obvious genetic relationship to spatially related porphyry style mineralization polymetallic epithermal deposits and prospects contain critical and energy critical metals e g te se bi sb in ge and ga which may be considered as by products in the attico cycladic area porphyry mo w mineralization occurs as sheeted quartz veins and stockworks cutting a potassic to sericitic altered miocene granodiorite stock in the lavrion district bonanza grade au and or ag rich veins with epithermal affinities crosscut metamorphic rocks at lavrion and on syros tinos antiparos and anafi islands milos island is characterized by shallow submarine volcanic hosted is hs epithermal au ag te and base metal deposits antimony as ag au deposits prospects on chios samos and kos islands in the eastern aegean sea indicate the potential for carlin style mineralization in greece several factors played a role to the metal endowment of the aegean porphyry epithermal systems magma fertility in the source regions depth of emplacement of causative intrusives relative contribution of mantle versus crustal material redox state of subduction related magmas and physico chemical fluid conditions at the site of ore deposition

an integrated method for reconstructing regional palaeoclimates australia 18 000 yr b p abstract archaeological research on human behavioural adaptations to changes in climate environment requires palaeoclimatic reconstruction on a regional scale most often neither proxy data nor numerical modelling alone can adequately meet this requirement in an effort to satisfy this need the author suggests a method based seasonal circulation patterns and calculations of changes in evaporation rates over land and ocean in this paper circulation models for australia at 18 000 yr b p are based on output from ncar s community climate model and evaporation rate changes are calculated for twelve regions within australia as well as eight offshore winter and summer palaeoclimates for each of these areas are retrodicted on the basis of these changes and these retrodictions are tested against the available proxy data while there are clear discrepancies between the data and retrodictions for the southern part of the continent the method performs well in the north this method has proven useful in reconciling apparently contradictory data and in allowing retrodictions on the seasonal level and for providing palaeoclimatic estimates areas where data are scarce

is there a link between faulting and magmatism in the south central aegean sea a distinct spatial relationship between surface faulting magmatic intrusions and volcanic activity exists in the aegean continental crust in this paper we provide detailed structural observations from key onshore areas as well as compilations of lineament maps and earthquake locations with focal plane solutions from offshore areas to support such a relationship although pluton emplacement was associated with low angle extensional detachments the nne to ne trending strike slip faults also played an important role in localizing the middle miocene plutonism providing ready pathways to deeper magma batches and controlling the late stage emplacement and deformation of granites in the upper crust additionally the linear arrangements of volcanic centres from the quaternary volcanoes along the active south aegean volcanic arc are controlled primarily by ne trending faults and secondarily by nw trending faults these volcanic features are located at several extensional settings which are associated with the main ne trending faults such as i in the extensional steps or relay zones between strike slip and oblique normal fault segments ii at the overlap zones between oblique normal faults associated with an extensional strike slip duplex and iii at the tip zone of a ne trending divergent dextral strike slip zone the ne trend of volcano tectonic features such as volcanic cone alignments concentration of eruptive centres hydrothermal activity and fractures indicates the significant role of tectonics in controlling fluid and magma pathways in the aegean upper crust furthermore microseismicity and focal mechanisms of earthquakes in the area confirm the activity and present kinematics of these ne trending faults

3d subduction dynamics a first order parameter of the transition from copper to gold rich deposits in the eastern mediterranean region the natural variability of geometry and dynamics of subduction zones leads to a variety of mantle and crustal processes that may influence the genesis of ore deposits in the overriding plate these complex interactions cannot be fully represented by two dimensional 2d models but require that the spatial and temporal evolution of ore deposits be examined in a detailed 3d tectonic framework we compare and discuss the geodynamic and metallogenic evolution of the eastern mediterranean subduction zone since the late cretaceous by integrating a newly compiled metallogenic database with a recent kinematic reconstruction model the resulting paleotec tonic maps identify 1 a late cretaceous cu rich metallogenic period with large deposits formed above a linear and stable subduction zone that produced large amounts of potentially fertile magmas in a narrow magmatic arc and 2 a late eocene miocene pb zn followed by au rich metallogenic period with generally smaller deposits spread over a wide back arc basin that was opened due to subduction retreat and lateral slab tearing supported by high resolution numerical modeling the proposed dynamic arc and back arc metallogenic models emphasize 1 the influence of 3d slab dynamics and associated asthenospheric flow on ore distribution in space and time and 2 the importance of exhuming metamorphic domes in extensional back arc setting to focus metal bearing fluid circulations these models differ from collision and post collision models and may be valid on the western and eastern terminations of the whole tethyan orogenic belt i e mediterranean and southeast asia where fast 3d subduction dynamics has influenced the geodynamic and metallogenic evolution of the overriding plate

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

late quaternary landscape evolution in the keep river region northwestern australia abstract this paper evaluates the late quaternary chronostratigraphic context of archaeological sites in the keep river region northern territory australia cosmogenic dating luminescence dating and sediment characterisation reveal sedimentary processes commencing from erosion of the escarpment and plateaux source through temporary storage in sand sheets to final deposition in alluvial floodplains erosion of the sandstone plateaux 5 mm ka 1 and escarpment faces probably 50 100 mm ka 1 provide the main sediment source for the adjacent sand sheets which have evolved over the past 100 000 years as the product of ongoing cycles of accumulation and denudation the rate of sediment accumulation is lowest near the escarpments on the low energy sediment limited sand sheets 1 and greatest near the main streams 400 mm ka 1 that have more numerous sediment sources collectively luminescence ages indicate an apparent increase in sediment accumulation rate in the sand sheets from 100 mm ka 1 in the late pleistocene to over 200 mm ka 1 in the holocene this most likely reflects enhanced monsoonal activity following postglacial marine transgression palaeosol horizons in the creek profile distinguished by sediment mottling mark potentially significant palaeoenvironmental and palaeoclimatic changes during the quaternary

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

mnd microlite number density water exsolution rate meter abstract microlites in effusive or pyroclastic rocks are possible indicators of water exsolution in particular the microlite number density mnd is considered to be a function of the rate of water exsolution from melt in this paper we have constructed a mnd water exsolution rate meter based on the recent results of theory experiments and the natural observation of crystallization kinetics the mnd method accounts for the effects of melt composition and water content on the diffusivity of crytallizing components in melt by using this meter we can estimate the water exsolution rate at the microlite nucleation depth from a mnd value provided the crystal phase plagioclase or clinopyroxene is known we applied the meter to the case of the 1991 1995 dome eruption at unzen and the 1986b subplinian eruption at izu oshima we obtained the water exsolution rates in the range of 6 1 10 6 to 2 8 10 5 wt s approximately at 70 mpa for unzen plagioclase mnd 1014 to 1015 m 3 and 1 1 10 3 to 1 1 10 1 wt s for izu oshima pyroxene mnd 1015 to 1017 m3 under the assumption of equilibrium vesiculation and steady state flow the corresponding decompression rate and ascent velocity are calculated in the range of 240 to 1100 pa s and 0 014 to 0 068 m s for unzen and 1 2 104 to 1 3 106 pa s and 1 3 to 133 m s for izu oshima this contrast in the ascent velocity at the microlite nucleation depth is closely related to the reason why unzen and izu oshima revealed the different eruption styles namely dome growth and explosive eruption respectively

controls on tellurium in base precious and telluride minerals in the panormos bay ag au te deposits tinos island cyclades greece tellurium bearing base metal assemblages dominate the eight stages of mineralization in quartz calcite adularia illite epithermal au ag te veins at panormos bay tinos island cyclades tellurium bearing sulfides and sulfosalts both preceded and followed the main telluride forming event stage v the complex stage v telluride assemblage was characterized by a sequence of early ag middle cu and late au bearing tellurides there is textural evidence for epitaxial growth of the tellurides phase separation of carbon dioxide would have caused h2te g to partition into a gaseous phase and resulted in ph neutralization due to rapid up vapor transport tellurium bearing vapors probably supersaturated condensed as droplets and were resorbed into the ore forming solution these mechanisms enhanced local te enrichment in the mineralizing liquid above equilibrium of te as h2te g i e log f h2te g values of 0 5 to 1 5 and resulted in epitaxial growth of stage v tellurides epitaxy in the panormos bay case can be simulated by the stranski krastanov crystal growth model for supersaturation ratios with s values of 1 and 4 we predict an overall increase of the nucleation rate of tellurium and subsequent telluride precipitation

an oligocene extrusion wedge of blueschist facies nappes on evia aegean sea greece implications for the early exhumation of high pressure rocks we show that the styra nappe of the cycladic blueschist unit on evia constitutes a wedge of high pressure rocks extruded during early stages of orogeny the nappe pile on evia was assembled during d 2 top to the ssw directed thrusting in restored oligocene coordinates which emplaced the styra and ochi nappes of the cycladic blueschist unit above the almyropotamos nappe between c 33 and 21 ma peak metamorphism of the almyropotamos nappe at c 23 ma occurred at lower metamorphic pressure showing exhumation of the styra nappe during underthrusting and burial of the almyropotamos nappe this exhumation was largely accomplished by the d 2 top to the nne displacing mt ochi normal sense shear zone normal shearing commenced at c 33 ma under peak high pressure metamorphism in the styra and ochi nappes fission track dating indicates slow cooling after d 2 in the styra nappe subsequently the former thrust contact between the almyropotamos nappe and the styra nappe was reactivated as a d 3 top to the ese extensional shear zone and this extensional phase led to the formation of a number of middle to late miocene graben our main conclusion is that there is strong evidence for an oligocene extrusion wedge accomplishing the early exhumation of the styra nappe which demonstrates the importance of extrusion wedges for the initial exhumation of the cycladic blueschist unit

crystallization kinetics in continuous decompression experiments implications for interpreting natural magma ascent processes decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma these experiments differ from previous studies in that they employ a continuous rather than stepwise pressure time trajectory four series of experiments were performed at rates of 0 5 10mpa h corresponding to ascent rates of 0 007 0 14 m s experiments quenched along each decompression path allow snapshot style monitoring of progressive crystallization as expected rates of plagioclase nucleation and growth depend strongly on decompression rate regardless of decompression rate approximately equal volumes of feldspar crystallize during a given decompression interval even when the observed crystallinity is lower than the equilibrium crystallinity apparently a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existing crystals other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks feldspar microlites do not maintain chemical equilibrium with melt they are more anorthite rich than equilibrium plagioclase suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static phase equilibrium experiments although decompression rate is an important element controlling final textures in volcanic rocks no single compositional or textural parameter uniquely records this rate of the criteria examined microlite number density and morphology are the best indicators of magma decompression rate based on a small number of comparisons between continuous and multi step decompression style at the same time integrated decompression rate 1mpa h decompression path evidently influences crystal texture with stepwise decompression yielding textures correlative with faster decompression than indicated by the time integrated decompression rate thus consideration of magma ascent style in addition to rate will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes

evolution of crystal sizes in the series of dissolution and precipitation events in open magma systems article i nfo we propose a model that describes the evolution of crystal sizes and crystal size distributions csd of igneous phenocrysts in a sequence of dissolution and crystallization events this model is based on the assumption that crystal dissolution is rate limited by diffusion in melt while crystal growth is controlled by the slower kinetic of new nucleation and growth as a result the dissolution rate is inversely proportional to crystal size coming into effect through the curvature of the crystal s surface but the growth rate does not depend on the crystal size closed form analytical solution of equation for csd is obtained we apply results of modeling to quartz and zircon two prime minerals in silicic igneous systems that are widely used in geochemical and isotopic investigations the time series of multiple solution reprecipitation episodes generate concave downward csds and this result fits well with experimental and natural observations on the abundant concave down csd in silicic igneous rocks we suggest that maturation of crystal populations with sizes above several micrometers can not be caused by a size effect on the solubility of the crystals ostwald ripening but is rather driven by thermal oscillations in experiments and in nature the model predicts that mean crystal size increases with time proportionally to t 0 20 which is very close to the published experimental results for quartz maturation with the exponent of 0 19 0 22 our proposed model gives an opportunity to use natural csds for interpretation of pre eruptive magma history when solubilities and diffusion data are available for constituent elements of the dissolving mineral in particular we present time estimates for maturing zircon populations in large volume ignimbrites and estimate that it takes 100 1000 yrs to mature an initially exponential csd to a lognormal csd

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

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

crystallization and textural evolution of a closed system magma chamber insights from a crystal size distribution study of the lilloise layered intrusion east greenland the recognition that postcumulus processes significantly modify primary textures in layered mafic intrusions has thrown into question many early observations on which classical crystallization theories are based petrographic observations combined with quantitative textural analysis of samples from various stratigraphic levels of the lilloise intrusion east greenland demonstrate that postcumulus textural modification of cumulates formed during the solidification of a closed system magma chamber may be detected crystal size distribution csd measurements of lilloise cumulates and the resulting csd profiles are compared to predicted theoretical closed system csd profiles similarities between the measured csd profiles and published predicted csd profiles support lilloise magma evolving in a closed system chamber and indicate that primary crystallization processes can be distinguished using quantitative textural techniques textural coarsening driven by syn magmatic deformation is suggested to be the dominant postcumulus process affecting csd plot morphology csd slope values and profiles plot shapes remain relatively constant for a given liquidus mineral particularly olivine and clinopyroxene so that the number of phases on the liquidus at any one time affects mineral modal abundances as a result csds generally exhibit overall smaller grainsizes and progressively lower nucleation densities at higher levels in the intrusion

i type plutonism in a continental back arc setting miocene granitoids and monzonites from the central aegean sea greece miocene extension in the aegean sea was accompanied by the intrusion of granitoid and monzonitic magmas at upper crustal levels with increasing distance from the trench the metaluminous to slightly peraluminous i type intrusives show a systematic regional variation in their compositions medium to high k granodiorites in the wsw high k low silica granites in the central region and high k high silica granites monzonites monzogranites and composite lamprophyric monzonitic dikes in the ene the granodioritic and low silica granitic plutons comprise similar ranges of sio2 63 5 71 6 wt and initial nd sr and o isotopic values end i 7 0 to 9 8 87sr 86sr i 0 7092 0 7115 18o 9 7 11 5 but display significant differences in their chemical compositions compared with the granodiorites the low silica granites have higher abundances of large ion lithophile light rare earth and some high field strength elements initial isotopic signatures of the high silica granites in the ene end i 9 0 to 9 6 87sr 86sr i 0 7128 0 7144 18o 10 8 12 0 are slightly more crust like than those of the low silica granites in chemical variation diagrams samples from the high silica granites continue the trends defined by those from the low silica granites it is suggested that all granitoid magmas originated by dehydration melting of metaluminous older crustal sources most probably metagreywackes u pb systematics of zircon populations in conjunction with zr sio2 systematics and zircon saturation temperatures suggest a high proportion of restitic or assimilated crustal material in these magmas monzonitic to monzogranitic rocks show chemical and isotopic characteristics end i 0 8 to 3 6 87sr 86sr i 0 7059 0 7071 18o 9 4 10 6 that are significantly different from those of the granitoids a small portion of an older crustal component in these magmas is indicated by u pb systematics of zircon populations lamprophyric dikes display chemical and isotopic signatures end i 0 5 87sr 86sr i 0 7044 18o 8 9 that are compatible with an origin from either a mantle source previously enriched by slab derived hydrous fluids melts as suggested by negative nb ta anomalies or a new k rich basaltic underplate abundant composite dikes indicate the coexistence of and mingling mixing between lamprophyric melts and monzonitic magmas

in search of regional palaeoclimates australia 18 000 yr bp abstract this paper continues the investigation of regional palaeoclimates in australia at 18 000 yr b p using the method set out in hubbard 1995 initial research showed that while the palaeodata were on the whole in keeping with climates retrodicted for the northern part of the continent there was substantial disagreement between the data and retrodictions in the south of australia while several possible reasons for this disagreement are considered in this paper it is the author s conclusion that the 18 000 yr b p run of the community climate model placed the southern boundary of the mid latitude belt of high pressure south of its actual position new seasonal circulations based on a northward position of this belt are outlined in this paper and using the method given in hubbard ibid new climate retrodictions are given for each of 12 australian regions these new retrodictions are almost entirely in keeping with the published data review suggesting that the community climate model 0 run pressure results were incorrect the revised seasonal circulation patterns can be used therefore to reconstruct likely palaeoclimates of regions that currently have few or no palaeoclimatic data

combining csd and isotopic microanalysis magma supply and mixing processes at stromboli volcano aeolian islands italy integrating isotopic microanalysis with other analytical techniques creates powerful new methodologies for understanding the evolution of rock samples at the sub grain scale here we present crystal size distribution csd data for a 26 000 year old sample from stromboli volcano and accompanying isotopic microanalysis of the phenocrysts a technique called the icsd plot is introduced which given stated assumptions allows the integration of both sets of data to generate timelines of isotopic evolution through the volcanic system the combined approach is powerful allowing investigation of the magma supply mixing crystallisation and contamination processes prior to eruption of a volcanic sample for stromboli volcano the combined analysis suggests that the change in magma type following a cone collapse took roughly five years to complete similar to the timescale of changes seen in recent decades

timing slip rate displacement and cooling history of the mykonos detachment footwall cyclades greece and implications for the opening of the aegean sea basin we constrain the slip and cooling history of the mykonos detachment footwall using thermochronometry a u pb zircon age of 13 5 0 3 ma dates intrusion of the mykonos monzogranite 40 ar 39 ar hornblende and biotite ages from the monzogranite are 12 7 0 6 ma and 10 9 0 6 ma whereas zircon and apatite fission track ages range from 13 0 8 ma to 10 7 0 8 ma and 12 5 2 2 ma to 10 5 1 8 ma u th he ages range from 13 6 0 6 ma to 9 0 0 7 ma for zircon and 11 1 0 5 ma to 8 9 0 4 ma for apatite the ages in part overlap within 2 errors and together with the long apatite fission track lengths 14 m support rapid cooling at rates 100 c ma 1 the low temperature thermochronometric ages decrease east northeastwards in the direction of hanging wall transport on the mykonos detachment age distance relationships show that the mykonos detachment slipped at an average rate of 6 0 9 2 2 4 km ma 1 causing c 30 km of offset and c 12 km of exhumation this result indicates that miocene low angle normal faulting was not important for the exhumation of the cycladic blueschist unit the opening of the aegean sea basin in the miocene was controlled by a few large magnitude low angle normal faults

the kallianos au ag te mineralization evia island greece a detachment related distal hydrothermal deposit of the attico cycladic metallogenetic massif the kallianos au ag te deposit in evia island greece comprises sixteen syntaxial veins that intersect the schists and marbles of the cycladic blueschist unit the veins are filled with quartz i and quartz ii contain two hypogene ore stages stages i and ii and are surrounded by infrequent muscovite chlorite alteration zones a magmatic origin is deduced for the kallianos au ag te mineralization based on stable s c si o and h and radiogenic pb sr and rb isotopes and fluid chemistry the veins were deposited under pressures of 220 to 145 bars and temperatures of 260 to 120 c from almost neutral h2te bearing hydrothermal fluids with a salinity of 2 2 to 15 8 wt nacl equivalent the telluride mineralization related to stage ii was deposited at 180 c and evolved from au through bi to ag bearing tellurides the vein minerals precipitated by simple cooling of almost non reactive fluids that retained their initial magmatic signature taking into account the magmatic origin of the kallianos deposit we examine two proximal areas with granodioritic and leucogranitic intrusions similar to those associated with base and precious metal mineralization at lavrion 8 3 to 7 1 ma and tinos island 15 to 13 ma as potential sources for the kallianos fluids rb sr geochronology dating of the distal kallianos au ag te veins gave an age range of 8 1 to 7 6 ma implying that an overlapping and channelized hydrothermal system was active at the northern part of the attico cycladic metallogenetic massif utilizing the cbu detachment zones as fluid pathways

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

subvolcanic plumbing systems imaged through crystal size distributions the configuration of subvolcanic magma storage regions exercises a fundamental control on eruptive style and hazard such regions can be imaged remotely using seismic geodetic or magnetotelluric methods although these are far from routine and rarely unambiguous the textures of erupted volcanic rocks as quantified through crystal size distributions csd provide space and time integrated information on subvolcanic plumbing systems although these data cannot be used readily for reconstruction of key parameters such as conduit geometry or magma chamber depth here we develop a numerical approach to interpretation of csd in products of steady eruptions based on crystallization kinetics and hydrodynamic flow simulation to image subvolcanic plumbing systems the method requires knowledge of magma properties crystal growth kinetics measured experimentally and discharge rate measured observationally the method is applicable to steady state eruptive regimes distributions of pressure temperature crystal content and conduit cross section area with depth are obtained from a csd from a sample erupted from mount st helens volcano usa values of average conduit diameter 30 m and magma chamber depth 14 km below the summit are in good agreement with independent estimates

multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data implications for timescales of magma chamber processes associated with the 1915 eruption of lassen peak ca products of the 1915 lassen peak eruption reveal evidence for a magma recharge magma mixing event that may have catalyzed the eruption and from which four compositional members were identified light dacite black dacite andesitic inclusion and dark andesite crystal size distribution textural and in situ chemical major and trace element and sr isotope data for plagioclase from these compositional products define three crystal populations that have distinct origins phenocrysts long axis40 5 mm that typically have core an contents between 34 and 36 mol microphenocrysts long axis between 0 1 and 0 5 mm that have core an contents of 66 69 and microlites long axis50 1mm with variable an core contents from 64 to 52 phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling based on textural compositional and isotopic data for the magma represented by the dacitic component magma recharge was not an important process until just prior to the 1915 eruption average residence times for phenocrysts are in the range of centuries to millennia microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition high thermal contrast between the two end member magmas led to relatively high degrees of undercooling which resulted in faster crystal growth rates and acicular and swallowtail crystal habits some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution precipitation seen in both crystal textures and rim compositions average microphenocryst residence times are of the order of months microlites may have formed in response to decompression and or syn eruptive degassing as magma ascended from the chamber through the volcanic conduit chemical distinctions in plagioclase microlite an contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions we suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing as a consequence melt and microlites among these three products have similar compositions the results of this study highlight the integrated use of crystal size distribution textural and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies

environmental history of northwest australia based on phytolith analysis at carpenter s gap 1 examination of a phytolith assemblage from the archaeological site of carpenter s gap 1 provides an hitherto unrecognised source of vegetation history for the tropical savannah region of north western australia two predominant mechanisms contributed to the formation of the phytolith assemblage firstly the introduction of phytoliths in plant materials brought in by humans and secondly the introduction of phytoliths contained in faecal pellets deposited by animals separating the effects of both mechanisms enables local vegetation patterns and by inference climatic conditions to be reconstructed the period ca 40 000 years bp was probably wetter than today allowing the southerly expansion of palms beyond their present day distribution grassland compositional changes occurred by ca 33 000 years bp probably resulting from a combination of lowered rainfall decreased temperatures and possibly aboriginal firing activities a reduction in palm at a similar time followed by its complete disappearance in association with the loss of ulmaceae prior to the last glacial maximum lgm indicates a continued trend in decreasing water availability an abundance of cyperaceae sponge spicules and diatoms during the lgm when considered in conjunction with the other evidence possibly represents altered human behaviour in response to increased aridity

verification of ideal semi logarithmic lognormal or fractal crystal size distributions from 2d datasets crystal size distributions can follow any function provided the total crystal content is less than 100 however many theories suggest that they approximate a number of ideal distribution models the most widely adopted model for igneous rocks produces a straight line on a classic csd diagram of ln population density versus size lognormal by size distributions have been proposed for igneous and metamorphic rocks and materials that crystallise at low temperatures lognormal models are difficult to prove with simple linear frequency histograms or on a classic csd diagram instead it is better to use a normalized cumulative distribution function diagram which gives a straight line for lognormal distributions similarly fractal size distributions can best be verified simply by using a bi logarithmic diagram statistical measures can be used to invalidate a null hypothesis but with much caution for real data especially those derived from two dimensional measurements and with a limited range in sizes these methods are applied to csds of plagioclase in three igneous rock samples to illustrate the pitfalls of model fitting a simple nomenclature is proposed for ideal csd shapes

subduction and exhumation related structures in the cycladic blueschists insights from south evia island aegean region greece 1 detailed geological mapping structural investigation and amphibole chemistry analyses in southern evia aegean sea greece allow us to place new constraints on the internal structural architecture and tectonic evolution of the cycladic blueschists we show that the early deformation history was related to ese directed thrusting resulting in the stacking of the styra and ochi nappes which constitute the cycladic blueschist unit in evia these early thrust movements initiated just before and proceeded at peak conditions of the eocene high pressure metamorphism subsequent constrictional deformation gave rise to e w trending upright folding accomplished at the early exhumation stage the main ductile stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite this phase was characterized by localization of ductile deformation into a series of major tens of meters thick ene directed shear zones which cut up section in their transport direction and restack the early thrust and fold sequence locally bringing the structurally lower styra nappe over the higher ochi nappe it is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the oligocene ene ward extrusion of the blueschists brittle ductile ne dipping normal faulting of post early miocene age was probably responsible for the final exhumation of rocks

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

effects of magma storage and ascent on the kinetics of crystal growth the size distributions of crystals of olivine plagioclase and oxides of the 1991 93 eruption at mt etna italy are analyzed the simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas three distinct episodes are detectable a storage of the magma in a deep reservoir characterized by nearly constant and low nucleation and growth rates near to equilibrium an ascent phase with an ever increasing nucleation rate related to volatile exsolution and finally a quenching phase in addition to geochemical and geophysical evidence the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of etnean lavas are due to mixing of different populations this strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates

rheological controls on the eruption potential and style of an andesite volcano a case study from mt ruapehu new zealand abstract the evolving magma rheology of three recent ruapehu eruptions 1969 1977 and 1995 is estimated using a combination of thermodynamic models and rheological calculations supported by textural observations of the erupted scoria we use a well established thermodynamic model to determine the composition of these representative ruapehu magmas from 300mpa to 30mpa the outputs of the model provide the changing crystal and bubble content in a closed system assuming no gas loss as well as the fractionating melt compositions we calculate the melt viscosity and the effect of bubbles and crystals to quantify the rheology of the magma during ascent under assumed equilibrium conditions the moderately high phenocryst content of ruapehu scoria 30 means that only a small amount of additional microlite crystallisation 5 would result in a yield strength which may lead the magma to stall however if the strain rates are high enough more crystallisation would be possible without developing a yield stress this suggests that microlite rich magmas are almost certain to stall unless they encounter significant fluid addition from a source such as a hydrothermal system groundwater or surface water i e ruapehu s crater lake ruapehu magmas are initially h 2 o undersaturated and as a consequence crystallisation and bubble growth were suppressed until the magma achieved saturation at 100 to 50 mpa from this analysis we suggest that ruapehu magmas are more likely to erupt compared to magmas of a similar composition that are h 2 o saturated this partly explains the regular albeit small volume eruptions at ruapehu and the propensity for phreatomagmatic eruptions when the magma water ratio is low

granite intrusion in a metamorphic core complex the example of the mykonos laccolith cyclades greece abstract the aegean domain is a well suited place to study the formation of metamorphic core complex mcc and to investigate the role of syn tectonic granites on their development in the northern cyclades the mykonos delos rhenia mcc is characterized by the intrusion of a kilometer scale late miocene pluton of i type granitoids within a migmatitic gneiss dome new combined ams anisotropy of magnetic susceptibility and microstructural studies on the mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers the mykonos granitoids form a laccolith like intrusion with a n70 e long axis the laccolith is strongly asymmetric with an outlying root zone to the sw and a major body mainly developed to the ne the laccolith construction is due to successive pulses of more or less differentiated magma that intruded the cycladic blueschist unit the attitude of stretching markers suggests an important about 60 vertical axis local rotation phenomenon in the cycladic upper crust during the exhumation of the mykonos mcc structural data suggest a four stage evolution of the mykonos mcc i a first stage characterized by flat shearing toward the n ne and by the formation of a domal structure in migmatitic paragneisses with multi scale generation of folds with axes either perpendicular or parallel to the regional stretching as a result of the interplay between regional n20 e directed extension and ew shortening ii a second stage marked by the emplacement of the mykonos laccolith at 13 5 0 3 ma at the top of the migmatitic paragneisses iii the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions and iv the late stage marked by an acceleration of the exhumation of the mykonos mcc this exhumation was accommodated by important rotations of upper crustal blocks during the end of the exhumation processes around 10 ma deformation localized at the top of the laccolith in semi ductile conditions and then in brittle conditions in the major detachment plane our study shows that the cycladic plutonism event had no role on the initiation of the mcc however the geometry of the mykonos intrusion supports that the magmas are sucked into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the mcc development this acceleration was marked by a very fast exhumation of the laccolith after its emplacement

alpine high pressure metamorphism at the almyropotamos window southern evia greece the alpine orogenic belt of the hellenides has been strongly reworked by ductile and brittle extensional tectonics extensional structures have affected the central aegean region and obliterated much of the original orogenic architecture since at least early miocene times in the area of almyropotamos on the island of evia flanking the western part of the aegean a unique remnant compressional nappe stack involving tertiary metamorphic rocks has been preserved this nappe sequence comprises a high pressure rock unit on top of a lower grade unit the upper unit south evia blueschist belt is thought to be the westward continuation of the cycladic blueschist belt metamor phosed at high pressure conditions during late cretaceous eocene times the underlying unit the almyropotamos unit is a continental margin sequence covered by a flysch and containing lutetian nummulites indicating that this unit accumulated sediments until at least late eocene times in the present study we analyse the petrology of the almyropotamos nappe stack and define the p t conditions of each of the different rock units exposed there the presence of glaucophane law sonite rimmed by epidote and jadeite 70 mol suggest that peak p t conditions in the south evia blueschist belt reached approximately 10 12 kbar and 350 450 c unlike previous studies which estimated that the underlying almyropotamos unit reached only greenschist facies conditions glau cophane relics and si rich phengites were found by us in this unit these indicate that high pressure metamorphism and crustal thickening in this part of the aegean lasted until at least the late eocene or early oligocene we note that in this respect the architecture of southern evia resembles that of north ern greece olympos ossa our structural data indicate that rock units in the almyropotamos area record different folding phases with the south evia blueschist belt having a more complex fold his tory than the underlying almyropotamos unit the entire nappe stack shares large scale folds which are e w trending and locally overturned to the south and which may represent at present coordi nates n s contraction and nappe transport

the lavrion pb zn fe cu ag detachment related district attica greece structural control on hydrothermal flow and element transfer deposition abstract the impact of lithological heterogeneities on deformation fluid flow and ore deposition is discussed based on the example of the lavrion low angle detachment partly accommodating gravitational collapse of the hellenides orogenic belt in greece the lavrion peninsula is characterised by a multiphase pb zn fe cu ag ore system with a probable pre concentration before subduction followed by progressive remobilisation and deposition coeval with the development of a low angle ductile to brittle shear zone the mylonitic marble below the detachment shear zone is composed of white layers of pure marble alternating with blue layers containing impurities sio2 al2o3 carbonaceous material ductile mylonitic deformation is more pervasive in the less competent impure blue marble we propose that localised deformation in the impure marble is associated with fluid circulation and dolomitisation which in turn causes an increase in competence of these layers mineralised cataclastic zones crosscutting the mylonitic fabric are preferentially localised in the more competent dolomitic layers oxygen and carbon isotopic signatures of marble invaded by carbonate replacement deposits during ductile to ductile brittle deformation are consistent with decarbonation coeval with the invasion of magmatic fluids mineralised cataclastic zones reflecting brittle deformation evolve from low 13c to low 18o signatures interpreted as local interaction with carbonaceous material that trends toward the contribution of a surface derived fluid these features indicate that the lavrion area records a complex deposition history influenced by the evolution of fluid reservoirs induced by the thermal and mechanical evolution of the marble nappe stack ore remobilisation and deposition associated with the activity of the low angle detachment is i firstly related to the intrusion of the plaka granodiorite leading to porphyry type and carbonate replacement mineralisation during ductile brittle deformation and ii then marked by progressive penetration of surface derived fluids guided by strain localisation in the more competent levels leading to epithermal mineralisation associated with brittle deformation

origin of a barite sulfide ore deposit in the mykonos intrusion cyclades trace element isotopic fluid inclusion and raman spectroscopy evidence abstract the polymetallic mykonos vein system in the cyclades greece consists of 15 tension gashes filled with barite quartz pyrite sphalerite chalcopyrite and galena in ca 13 5 ma i type mykonos monzogranite zones of silica and chlorite muscovite alteration are associated with the veins and overprint pervasive silicification phyllic and argillic alteration that affected large parts of the monzogranite the mineralization cements breccias and consists of an early barite silica pyrite sphalerite chalcopyrite assemblage followed by later argentiferous galena a combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt nacl equivalent at temperatures of 225 to 370 c under a hydrostatic pressure of 100 bars the mineralizing fluids boiled and were saturated in h 2 s and so 2 calculated 18 o h2o and d h2o initial 87 sr 86 sr isotope compositions and the trace and rees elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the mykonos intrusion and subsequently mixed with miocene seawater heterogeneities in the calculated 34 s so4 2 and 34 s h2s compositions of the ore fluids indicate two distinct sources for sulfur namely of magmatic and seawater origin and precipitation due to reduction of the so 4 2 during fluid mixing the physicochemical conditions of the fluids were ph 5 0 to 6 2 log f s2 13 8 to 12 5 log f o2 31 9 to 30 9 log f h2s g 1 9 to 1 7 log f te2 7 9 and log so 4 2 aq h 2 s aq 2 6 to 5 5 we propose that retrograde mesothermal hydrothermal alteration of the mykonos monzogranite released barium and silica from the alkali feldspars barite was precipitated due to mixing of so 4 2 rich miocene seawater with the ascending ba rich magmatic fluid venting upwards in the pluton

an origin for harrisitic and granular olivine in the rum layered suite nw scotland a crystal size distribution study dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low results from crystal size distribution csd measurements on harrisitic olivines from rum scotland combined with estimated olivine growth rates suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long may have exceptionally short crystal growth times several hours to several hundreds of days this together with very low calculated nucleation densities for harrisitic olivine supports the interpretation of harrisite being a disequilibrium texture developed in response to supersaturation of the magma in olivine we propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the rum magma chamber floor beneath cooler resident magma it is envisaged that the picrite sheets were largely free of suspended olivine crystals coupled with the olivine enriched composition of the melt and the increasing cooling rate this allowed homogeneous nucleation of olivine to set in at deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei the enhanced supersaturation caused rapid growth of olivine once nucleation began with skeletal and dendritic shapes it is suggested that the observed interlayered sequences of harrisite and cumulus peridotite found throughout the rum layered suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling

timing of tectonic emplacement of the ophiolites and terrane paleogeography in the hellenides the timing of tectonic emplacement of the ophiolites is analyzed in the four oceanic terranes of the hellenides h2 h4 h6 h8 the criteria for this analysis are based on a the post emplacement sedimentary cover or intrusive rocks b the syn emplacement tectonostratigraphic formations and c the youngest rocks involved in the structure of the autochthon and the allochthon unit in each case the timing becomes younger towards the more external tectonic units of the hellenides with i late eocene oligocene age in the external ophiolite belt of the pindos cyclades oceanic terrane h2 ii late jurassic early cretaceous age in the internal ophiolite belt of the vardar axios oceanic terrane h4 iii post liassic pre late jurassic age in the ophiolites of lesvos circum rhodope oceanic terrane h6 and iv pre late jurassic age in the ophiolites of volvi eastern rhodope terrane h8 an ophiolite obduction model can be applied with the ophiolitic nappes always emplaced on top of pre alpine continental terranes with mesozoic shallow water carbonate platforms the geometry of the continental terranes drifting during the mesozoic within the tethys ocean controls the number and dimensions of the tethyan oceanic basins where a continental terrane dies out the two adjacent oceanic basins merge into one larger basin this seems to be the case of the pelagonian terrane h3 which is terminated north of skopje where the pindos oceanic basin h2 merges with the vardar axios oceanic basin h4

an integrated zircon geochronological and geochemical investigation into the miocene plutonic evolution of the cyclades aegean sea greece part 1 geochronology we use 369 individual u pb zircon ages from 14 granitoid samples collected on five islands in the cyclades in the aegean sea greece for constraining the crystallisation history of i and s type plutons above the retreating hellenic subduction zone miocene magmatism in the cyclades extended over a time span from 17 to 11 ma the ages for s type granites are systematically 2 million years older than those for i type granites considering plutons individually the zircon data define age spectra ranging from simple and unimodal to complex and multimodal seven of the 14 investigated samples yield more than one distinct zircon crystallisation age with one i type granodiorite sample from mykonos island representing the most complex case with three resolvable age peaks two samples from s type granites on ikaria appear to have crystallised zircon over 2 3 million years whereas for the majority of individual samples with multiple zircon age populations the calculated ages deviate by 1 1 5 million years we interpret our age data to reflect a protracted history involving initial partial melting at deeper lithospheric levels followed by crystallisation and cooling at shallower crustal levels our study corroborates published research arguing that pluton construction is due to incremental emplacement of multiple magma pulses over a few million years assuming that multiple age peaks of our 14 samples can indeed serve to quantify time spans for magmatic emplacement our data suggest that aegean plutons were constructed over a few million years our tectonic interpretation of the u pb ages is that the s type granites resulted from partial melting and migmatisation of the lower crust possibly starting at 23 ma the i type granites and associated mafic melts are interpreted to reflect the magmatic arc stage in the cyclades starting at 15 ma

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

viscosity measurements of subliquidus magmas alkali olivine basalt from the higashi matsuura district southwest japan abstract we carried out viscosity measurements and sampling of a crystal suspension derived from alkali olivine basalt from the matsuura district sw japan at subliquidus temperatures from 1230 c to 1140 c under 1 atm with nno oxygen buffered conditions viscosity increased from 31 to 1235 pa s with a decrease in temperature from 1230 to 1140 c on cooling olivine first appeared at 1210 c followed by plagioclase at 1170 c the crystal content of the sample attained 31 vol at 1140 c plagioclase 22 olivine 9 non newtonian behaviors including thixotropy and shear thinning were pronounced in the presence of tabular plagioclase crystals the cause of such behavior is discussed in relation to shear induced changes in melt crystal textures relative viscosities r s m where s and m are the viscosities of the suspension and the melt respectively were obtained by calculating melt viscosities from the melt composition and temperature at 1 atm using the equation proposed by giordano and dingwell giordano d dingwell d b 2003 non arrhenian multicomponent melt viscosity a model earth and planetary science letters 208 337 349 the obtained relative viscosities are generally consistent with the einstein roscoe relation which represents r for suspensions that contain equant and equigranular crystals even though the crystal suspension analyzed in the present experiments contained tabular plagioclase and granular olivine of various grain sizes this consistency is attributed to the fact that the effect of crystal shape was counterbalanced by the effect of the dispersion of crystal size the applicability of the einstein roscoe equation with respect to crystal shape is discussed on the basis of the present experimental results our experiments and those of sato sato h 2005 viscosity measurement of subliquidus magmas 1707 basalt of fuji volcano journal of mineralogical and petrological sciences 100 133 142 show that the relationship between relative viscosity and crystal fraction is consistent with the einstein roscoe relationship for axial ratios that are smaller than the critical value of 4 6 5 but discrepancies occur for higher ratios

the role of oxides in the shallow vesiculation of ascending magmas abstract despite their generally low volume fraction fe ti oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation once nucleated bubbles respond fast to pressure changes fostering rapid expansion and explosive behavior yet oxide microlite quantifications are often absent from data of explosive products we used new re analyzed and published data to build a compilation of oxide number densities onds and vesicle number densities vnds of trachytic and calc alkaline products four eruptive styles were selected 1 vulcanian explosions from soufriere hills volcano montserrat lascar volcano chile and kilian volcano france 2 blasts from mt pelee volcano lesser antilles mount st helens usa and merapi volcano indonesia 3 a sub plinian explosion from merapi volcano and 4 lava dome effusions with intermittent collapse from soufriere hills and mt pelee volcanoes natural samples were separated into two groups according to the dominant texture of the products of each event 1 vesicular pumice clasts from explosions with a strong vertical component and 2 dense clasts with diktytaxitic textures from dome collapse event and lateral blast group 1 samples are either distributed alongside the 1 1 trend between vnd and ond that spans from 1015 to 1017 m 3 or have a constant vnd of 1016 16 5 m 3 regardless of ond a large proportion of oxides 55 100 touch vesicles a more variable proportion of vesicles 16 72 are in contact with oxides because of syn explosive growth and coalescence group 2 samples have onds in the same broad range as group 1 samples we also used new and published data to build a compilation of onds and vnds of five series of experimental decompression of rhyolitic and phonolitic melts in samples with ond vnd most bubbles are in contact with more than one oxide and 64 88 of the oxides are in contact with bubbles such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated when vnd ond nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models which we used to calculate syn explosive decompression rates from vnds at merapi soufriere hills and kilian these rates and textural evidence suggest that the decompression front accompanying these vulcanian and sub plinian explosions is responsible for syn explosive bubble nucleation we calculated the average pre explosive ascent rates necessary to yield the observed onds at soufriere hills and merapi volcanoes the resulting rates 0 005 20 m s overlap considerably with the range of critical ascent rate inferred for the effusive explosive transition supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities focusing on the older eruption of kilian for which no observation is available we infer that pre explosive ascent rates of 7 10 3 m s were necessary for explosive behavior to occur

extensional faulting on tinos island aegean sea greece how many detachments zircon and apatite fission track zft and aft and u th he 40ar 39ar hornblende and u pb zircon ages from the granites of tinos island in the aegean sea greece suggest together with published zft data that there are three extensional detachments on tinos the tinos granites crosscut the tinos detachment cooling of the granites was controlled by the livadi detachment which occurs structurally above the tinos detachment our u pb zircon age is 14 6 0 2 ma and two 40ar 39ar hornblende ages are 14 4 0 4 and 13 7 0 4 ma zft and aft ages go from 14 4 1 2 to 12 2 1 0 ma and 12 8 2 4 to 11 9 2 0 ma u th he ages are from 10 4 0 2 to 9 9 0 2 ma zircon and 11 9 0 5 to 10 0 0 3 ma apatite all ages decrease northeastward in the direction of hanging wall transport on the livadi detachment and age distance relationships yield a slip rate of 2 6 3 3 1 0 km ma 1 this rate is smaller than a published slip rate of 6 5 km ma 1 for the vari detachment which is another detachment structurally above the tinos detachment because of the different rates and because published zft ages from the footwall of the vari detachment are 10 ma we propose that the vari detachment has to be distinguished from the older livadi detachment we discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the vari detachment cut down into the footwall of the livadi detachment successively exhuming deeper structural units the thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation

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

footwall mineralization during late miocene extension along the west cycladic detachment system lavrion greece the west cycladic detachment system wcds has been mapped from the western cycladic islands to lavrion mainland greece where several top to ssw low angle normal faults at different structural levels are observed near the detachment horizon hydrothermal fluid flow originating from miocene intrusions lead to high temperature carbonate strata bound pb ag zn ore deposits zircon u th he cooling ages from the mineralized footwall are 7 1 0 6 to 7 9 0 6 ma carbon and o isotope analyses of the carbonate host indicate high water to rock ratios during brittle deformation induced metasomatism and that this interaction unrelated to proximity of the intrusions plays the dominant role in the mineralization the late miocene geology of lavrion is strikingly similar to serifos island on the se termination of the wcds both characterized by strong localization of detachment faulting and the interaction of brittle deformation with the injection of fluids related to granitoid intrusions

physicochemical controls on bismuth mineralization an example from moutoulas serifos island cyclades greece the 11 6 to 9 5 ma serifos pluton intruded schists and marbles of the cycladic blueschist unit causing thermal metamorphism the development of magnetite ca exo and endo skarns and the formation of low temperature vein and carbonate replacement ores potentially the most important ores occur in the moutoulas prospect where the mineralization in retrograde skarn and quartz veins culminated with the deposition of native bismuth a combination of fluid inclusion microthermometry and isotope geothermometry suggests that the moutoulas mineralization formed at a hydrostatic pressure of 100 bars from moderate to low temperature 190 250 c and low salinity 1 3 5 6 wt nacl equivalent fluids the calculated 34 s h 2 s compositions are consistent with the ore fluids having been derived from the serifos pluton bismuth mineralization is interpreted to have occurred as a result of wall rock interaction and mixing of a bi bearing ore fluid with meteoric waters native bismuth and bismuthinite deposited at 200 c near neutral ph 6 5 low f s 2 f o 2

the geological signature of a slab tear below the aegean in this paper we explore the geology of the aegean region in the miocene to identify the geological signature of a first order slab tear observed in all tomographic models from 15 to 8 ma the tear is associated spatially and timely with a fast clockwise rotation of the external hellenides alkaline volcanism high temperature metamorphic domes with a predominance of north dipping detachments and south westward migration of granitoid intrusions these features suggest a warmer geodynamic environment during the rotation and the impact of a hot mantle flow associated with the tear the 8 ma duration between the first high temperature metamorphic domes in the centre of the archipelago and the beginning of the fast rotation may correspond to the time needed for the slab to bend stretch and finally tear

quantifying the building blocks of igneous rocks are clustered crystal frameworks the foundation most phenocryst populations in volcanic rocks and those preserved in shallow level igneous intrusions are clustered variously referred to as clots clumps or glomerocrysts these clusters of crystals are the building blocks that accumulate to form the high porosity touching crystal frameworks from which igneous cumulates form examination of touching crystal frameworks in olivine komatiite cumulates and experimental charges and plagioclase dominant crystal populations holyoke flood basalt connecticut usa reveal complex high porosity clustered crystal arrangements olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 x 10 10 mm s to less than 3 years for a growth rate of 5 x 10 10 mm s based on crystal size distributions the origin of crystal clusters is likely to involve either or a combination of heterogeneous nucleation remobilization of cumulate mushes or crystals sticking together during settling and or flow the spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non touching crystal populations and further improve our understanding of crystal packing arrangements and cluster size distributions

desert pavement development on the lake shorelines of lake eyre south south australia abstract to the southwest of lake eyre south south australia silcrete boulders exposed by the erosion of the surrounding fine sediments undergo mechanical weathering to form desert pavement successive palaeoshorelines of lake eyre have exposed an age related sequence of different stages in the weathering of the boulders this study investigates desert pavement development in this saline environment in addition it attempts to develop a model for the development of desert pavement following exposure of the silcrete boulders based on palaeo lake shorelines dated from previous studies seven stages can be recognised corresponding to stages of soil and pavement development prior to stage one is the actual exposure of the boulder as the result of erosion by wave action at the lake shoreline or by erosion as the lake level falls during desiccation at stage 1 the upper surface of the boulder breaks up through mechanical weathering salt weathering while the rest of the boulder is still buried at stage 2 the surface fragments fall to the edge of the stone and expose more of the stone which continues to break up there is no soil development in stages 1 and 2 by stage 3 most of the stone is exposed and broken up making a mini hill at this stage soil development begins with the accumulation of sandy soil between the rock fragments at stage 4 the stones form small cones and the soil is more developed it is sandy with a typical of colour 10 yr 6 6 at stage 5 the stones forming the small cone are completely fragmented stone fragments at the centre are very angular but smoother at the edges of the mini hill as the result of weathering etching by chemical processes soil texture is silty sand and soil colour is 7 5 yr 6 6 at stage 6 the surface is nearly flat the soil is sandy silt and soil colour is between 7 5 yr 5 6 and 7 5 yr 5 8 stage 7a is the gibber plain phase composed of small well rounded stones as a result of continued etching of the edges of the fragmented stones the soil is silty and the soil colour is between 5 yr 5 6 and 5 yr 5 8 stage 7b is also gibber plain with small well rounded stone fragments but where the soil has been replaced by crystalline gypsum this sequence differs from sequences described in other areas especially on alluvial fan or terrace surfaces this may be partly due to the different origin of the clasts as pre weathered silcrete boulders and partly due to the importance of chemical weathering by etching in this salt rich environment

intrusion related gold deposits in the tanami and kurundi kurinelli goldfields northern territory australia constraints from la icpms analysis of fluid inclusions abstract gold deposits in the tanami and kurundi kurinelli goldfields of the northern territory australia have geological and geochemical characteristics that distinguish them from the typical vein hosted orogenic gold deposits that occur in other parts of australia this study used cathodoluminescence cl and laser ablation inductively coupled plasma mass spectrometry la icpms analyses of fluid inclusions to investigate the source of mineralising fluids from both the highly mineralised tanami goldfield and weakly mineralised kurundi kurinelli goldfields in northern australia cathodoluminescence showed three generations of quartz in fractured and mineralised veins from the groundrush deposit in the tanami goldfield but the remaining deposits had uniform dark cl images the la icpms trace element data indicate an evolution from b as enriched magmatic fluid to a highly sr ba enriched fluid which results from interaction with sedimentary rocks the fluids from both regions have chlorinity normalised zn and pb concentrations at the magmatic end of the metamorphic magmatic mixing trend for these elements this is in agreement with previous studies showing that gold mineralisation is coeval with magmatism in the tanami region the relatively high salinity of the fluids in both the tanami and kurundi kurinelli goldfields and their trace element contents indicates that the deposits in both regions are intrusion related gold deposits the higher gold endowment of the tanami region may result from the multiple gold mineralisation events in this region whereas fluid flow associated with mineralisation in the kurundi kurinelli goldfields appears to be of limited extent

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

crystal size distribution of plagioclase and amphibole from soufriere hills volcano montserrat evidence for dynamic crystallization textural coarsening cycles the crystal size distributions csds of plagioclase and amphibole were determined from andesites of the soufriere hills volcano montserrat plagioclase occurs as separate crystals and as chadocrysts in large amphibole oikocrysts the chadocrysts represent an earlier stage of textural development preserved by growth of the oikocryst seventeen rock and eight chadocryst plagioclase csds are considered together as a series of samples of textural development all are curved concave up and coincident differing only in their maximum crystal size three amphibole csds have a similar shape and behaviour but at a different position from the plagioclase csds a dynamic model is proposed for the origin of textures in these rocks crystallization of plagioclase started following emplacement of andesite magma at a depth of at least 5km a steep straight csd developed by nucleation and growth this process was interrupted by the injection of mafic magma into the chamber or convective overturn of hotter magma the magma temperature rose until it was buffered initially by plagioclase solution and later by crystallization during this period textural coarsening ostwald ripening of plagioclase and amphibole occurred small crystals dissolved simultaneously with the growth of large crystals the csd became less steep and extended to larger crystal sizes early stages of this process are preserved in coarsened amphibole oikocrysts repetitions of this cycle generated the observed family of csds textural coarsening followed the communicating neighbours model hence each crystal has its own unique growth solution history without appealing to mixing of magmas that crystallized in different environments

estimates of sea surface temperature in the coral sea at the last glacial maximum the climap 1981 reconstruction of the coral sea found relatively little cooling 2 c in the low latitudes 10 s but a warming off australia at about 25 s the small low latitude changes are of interest because terrestrial pollen and snowline data from the new guinea highlands imply that surface temperatures may have been 6 to 9 c colder at the last glacial maximum lgm the purpose of this paper is to evaluate these conclusions on the basis of additional core sites new oxygen isotope stratigraphy and new sea surface temperature sst estimates using the modern analog technique mat in the northern coral sea planktonic foraminifer assemblages consist of tropical subtropical species that show little change over the past 20 kyr quantitative estimates of sst using the modern analog technique mat confirm the climap 1981 conclusion that little or no temperature change occurred in this tropical region at the lgm thus reinforcing the conflict with terrestrial evidence in the southern region 25 s two cores indicate that foraminifer faunas became more subtropical at the lgm the mat estimates for the lgm are 3 to 4 c colder than modern producing a steeper thermal gradient in the southern coral sea these data remove the warm sst anomaly along the eastern coast of australia and indicate that during the lgm cool high latitude waters were displaced northward along the coast of australia into the southern coral sea

the genesis of epithermal au ag te mineralization panormos bay tinos island cyclades greece the panormos bay au ag te vein system tinos island greece which is hosted in mesozoic marbles comprises 30 subparallel steeply dipping quartz veins that extend for at least 500 m in places gold telluride mineralization occurs in hydraulic breccias and alteration halos which are the products of co 2 effervescence of the ore fluid and wall rock interaction respectively seventy ore and gangue minerals have been identified zones of chlorite talc and muscovite albite tourmaline alteration up to 50 cm wide are associated with precious metal bearing milky and clear quartz veins the au ag te mineralization is developed in stage v of eight hydrothermal stages stage v consists of three substages early middle and late that are characterized by ag cu and au bearing tellurides respectively hessite sylvanite altaite native tellurium stutzite cuprian cervelleite and an unnamed cu bearing precious metal sulfotelluride ag au cu 9 te 2 s 3 characterize the early substage whereas melonite rickardite vulcanite weissite and native tellurium are present in the middle substage the late substage contains rickardite kostovite krennerite petzite and calaverite unexploited au ag te mineralization at panormos bay is though to be genetically related to a fluorine and boron bearing per aluminous leucogranite tinos leucogranite a combination of fluid inclusion microthermometry and sylvanite geothermometry suggests that the hydrothermal mineralization was deposited under hydrostatic pressures at a minimum depth of 1 km from moderate temperature 155 320 c low to moderate salinity 0 2 13 2 wt nacl equiv effervescing co 2 bearing fluids that contained appreciable amounts of cacl 2 and mgcl 2 calculated isotope compositions of 18 o 3 3 to 5 1 per mil and d of 73 to 62 per mil for waters in equilibrium with muscovite quartz and talc are consistent with the ore fluids being derived from the tinos leucogranite 18 o 4 2 and d 71 and their subsequent mixing with a more dilute low temperature miocenic meteoric hydrothermal fluid calculated 13 c fluid 2 0 to 0 3 and 34 s h 2 s 10 5 to 0 8 compositions of the ore fluids indicate an igneous source of carbon and sulfur which were highly exchanged with a metasedimentary source physicochemical conditions of the ore fluids were t 300 to 200 c ph 4 6 to 6 5 f s 2 10 10 9 to 10 15 f o 2 10 31 5 to 10 41 f te 2 10 7 7 to 10 10 7 f co 2 10 0 2 to 10 0 8 and f h 2 s 10 1 7 to 10 1 8

the silicon isotope composition of granites because weathering results in preferential incorporation of the lighter si isotopes into clay minerals the si in magmas derived from meta sedimentary lithologies enriched in such phases should also be isotopically light relative to melts derived from igneous sources with the advent of new high precision techniques it should therefore be possible to resolve such differences hence si isotopes have to potential to distinguish between and characterise different magmatic source regions to explore this here we report the si isotopic compositions of a suite of phanerozoic i s and a type granites and mineral separates as well as samples of associated country rock the whole rock data fall between 30si 0 40 and 0 11 0 04 95 s e this range is more limited than that defined by previous low precision measurements of granites but is significantly broader than that displayed by more recent high precision measurements of high si extrusive igneous rocks as predicted s type granites are on average isotopically lighter than i and a type samples and negative correlations with alumina saturation index asi values and initial 87sr 86sr provide evidence that si isotope variations in granites can be explained by variable incorporation of an enriched sedimentary component however the relatively small isotopic perturbations and apparent degree of overlap between granite affinities suggest that si isotopes are not as sensitive to sedimentary input relative to more established proxies such as for example o isotopes the average si isotope value of all granites analysed in this study is 30si 0 23 0 15 2 s d which is entirely consistent with a recently published estimate for the continental crust based on an andesitic bulk composition and the well defined relationship between si isotopes and magmatic fractionation

textural constraints on the kinetics of crystallization of igneous rocks natural systems provide time controlled experiments on scales unreachable in the laboratory and with complexities unapproachable by computer experiments since temperature undercooling cannot be directly scaled with time in situ crystallization experiments provide information unavailable by other means the kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems e g uhlmann 1982 james 1982 but predictive models for nucleation behavior in particular remain inadequate part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process crystal growth measurements are more accessible experimentally and for this reason crystal growth is better understood e g dowty 1980 kirkpatrick 1981 james 1982 baronnet 1984 however predictive models of crystal growth e g lasaga 1982 require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization information that is currently limited the cse parameterization of the position of ymax as a function of t tl and t suggested by dearnley 1983 provides a potentially useful model that requires further testing similarly application of the adiabatic nucleation model of meyer 1986 to simple systems suggests that the position of tmax relative to tl has an approximately constant value of 0 55 0 60 exceptions are low dsf minerals such as albite and quartz additionally while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models experimental growth rate data is confined to simple systems and conditions of moderately large undercooling while growth rate estimates in complex natural systems suggest that under most conditions undercoolings remain very small quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time e g ikeda 1977 which is in turn related to effective undercooling a function of both total dike width and distance from the dike margin by analogy minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies reflecting a systematic relationship between growth rate and cooling rate effective undercooling for plagioclase in basaltic systems while the data are not available for other mineral systems there is no reason to believe that similar patterns would not exist for other silicate minerals i e the similarity in dendritic olivine growth rates estimated by fowler et al 1989 using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments analysis of measured crystal size frequencies provides a means of extracting effective bulk kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change

the lavrion pb zn ag rich vein and breccia detachment related deposits greece involvement of evaporated seawater and meteoric fluids during postorogenic exhumation