reconciling heterogeneity in crystal zoning data an application of shared characteristic diagrams at chaos crags lassen volcanic center california isotope trace element and textural crystal zoning patterns record heterogeneity in magmatic systems not resolved by whole rock analyses these zoning data are used to infer crystal residence times magma mixing and other magmatic processes in many magmatic systems we present the shared characteristic diagram scd as an organizational framework for crystal zoning data that compares information from different phases and chemical tracers in a common framework an example from chaos crags in the cascade arc produces three main results 1 anorthite zoning profiles in plagioclase have fewer shared characteristics in mafic inclusions than in the host rhyodacite 2 single crystal 87sr 86sr data from previous studies tepley et al 1999 are consistent with more shared history between crystals than in anorthite profiles this difference reflects a more homogeneous distribution of 87sr 86sr than the intensive parameters controlling plagioclase composition 3 the chaos crags system exhibits a layer of heterogeneity in crystal populations that is not represented in whole rock analyses that indicate only simple binary mixing the inconsistency between 87sr 86sr and anorthite zoning data highlights decoupling between compositionally controlled and temperature water controlled zoning in plagioclase from chaos crags

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

a cross correlation method for chemical profiles in minerals with an application to zircons of the kilgore tuff usa magmatic crystals are characterised by chemical zonation patterns that reflect the thermal and chemical conditions within magma reservoirs in which they grew crystals that exhibit similar patterns of zonation are often interpreted to have experienced similar conditions of growth these patterns of zonation may represent continuous processes such as cooling or more instantaneous events such as magma injection and provide an insight into the structure and evolution of a magmatic system both temporally and spatially we have developed an algorithm that is objectively able to quantify the similarity within and between suites of magmatic crystals from different samples significantly the algorithm is able to identify correlation that occurs between the interiors of two crystals but does not extend to the rim which provides an opportunity to understand the long term evolution of magmatic systems we develop and explain the mathematical basis for our algorithm and introduce its application using cathodoluminescence images of zircons from the kilgore tuff usa the results allow us to correlate samples from two different outcrops that are found over 80 km apart

high resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images new constraints on oscillatory zoning oscillatory zoning in plagioclase is investigated at small scale 10 m using accumulated backscattered electron bse images as high resolution imaging method combined with electron microprobe quantitative analysis gray value profiles across these images can be calibrated for an content with a resolution of 0 5 mol an applied to oscillatory zoned crystals this new application of bse imaging allows better characterization of zoning patterns along a profile and quantification of wavelength amplitude and shape of the oscillations we also obtain high resolution information on the morphology of growth zones boundaries this approach allows us to better classify the different types of oscillations and concentric zoning dissolution is more frequent than usually recognized major resorption surfaces crosscut several growth zones irregular 5 10 m saw tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small scale oscillations 1 3 m with straight boundaries this suggests at least two mechanisms for the formation of these zoning patterns faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics

wavelet based correlation wbc of zoned crystal populations and magma mixing magma mixing is a common process and yet the rates kinematics and numbers of events are difficult to establish one expression of mixing is the major trace element and isotopic zoning in crystals which provides a sequential but non monotonic record of the creation and dissipation of volumes of distinct chemical potential we demonstrate a wavelet based correlation wbc technique that uses this zoning for the recognition of the minimum number of mixing or open system events and the criteria for identifying populations of crystals that have previously shared a mixing event when combined with field observations of the spatial distribution of crystal populations wbc provides a statistical link between the time varying thermodynamic and fluid dynamic history of the magmatic system wbc can also be used as a data mining utility to reveal open system events where outcrop is sparse an analysis of zoned plagioclase from the tuolumne intrusive suite provides a proof of principle for wbc 1 2002 elsevier science b v all rights reserved

timescales of magma storage and migration recorded by olivine crystals in basalts of the march april 2010 eruption at eyjafjallajokull volcano iceland the early eruptive phase of the 2010 eruption at the fimmvor uhals pass east of eyjafjallajokull volcano produced poorly evolved basalts with mildly alkaline affinity and benmoreitic tephra were emitted during the second explosive phase from the summit vent of the volcano in this study textural features and chemical zoning preserved in olivine crystals of the early erupted basalts have been used to define the timescales of differentiation processes and magma ascent before the eruption these lavas contain a mineral assemblage constituted by olivine fo70 88 and plagioclase an57 83 in similar proportions with scarce clinopyroxene and opaque oxides olivine occurs as euhedral or embayed crystals characterized by different core compositions and zoning patterns three main olivine populations have been found namely crystals with 1 wide fo88 cores with normal zoning toward narrow rims p1 2 fo81 cores with either no zoning or slight reverse zoning patterns toward the rims p2 3 fo77 cores with reverse zoning at the rims p3 the olivine reverse zoning indicates that these poorly evolved magmas experienced mixing processes in addition to limited fractional crystallization at different levels of the plumbing system timescales of transfer dynamics before the eruption have been estimated through fe mg diffusion modeling on these olivine populations the olivine melt equilibration through diffusion was triggered by interaction of magmas differing in their evolutionary degree p1 and p2 crystals recorded a first event of interaction in a 22 km deep reservoir that took place about one month before the emission of the analyzed products only part of p2 crystals records reverse zoning due to interaction with more basic magma bearing p1 crystals which consequently develop normal zoning suggesting fast timescales of magma mixing that prevented the complete homogenization a second mixing event which is evident in the p3 olivines occurred at shallower levels 5 6 km of depth 15 days before the emplacement and can be considered the triggering mechanism leading to the eruption at the fimmvor uhals pass integration of our timescales with seismic data relative to the hypocenter migration indicate rates of magma ascent throughout the deep plumbing system of 0 01 m s this study provides evidence that magmas emitted at eyjafjallajokull volcano and more in general at similar other volcanic systems in ocean ridge settings can undergo complex processes during their storage and transport in the crust chiefly due to the presence of a multilevel plumbing system

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

composite trough evolution of selected west pacific extratropical cyclones abstract the observed vertical structures of the trough axes for 27 extratropical cyclones are presented this study is motivated by results from a simple theoretical model two observing times during the cyclone life cycle are shown prior to development and during the mature but still amplifying stage prior to development upper and lower troughs are present and separate each has little or no tilt the upper one is typically prominent down to 4 km elevation and the separation between the lower and the upper features varies depending on where the approaching upper trough happens to be at the observing time at the mature stage upper and lower features are connected a uniform tilt typically develops through the entire troposphere the tilt is typically due west with height and the tilt may have a preferred slope an empirical orthogonal function eof analysis finds that two modes account for more than 97 of the variance the equivalent barotropic eof has the most variance by far though the frac

observations of plagioclase zoning using interference imaging abstract interference imaging techniques laser interference microscopy in transmitted light and nomarski differential interference contrast in reflected light have been used to study different types of zoning in magmatic plagioclase crystals samples studied come from a wide range of rock types and geological environments analysis of zoning profiles for individual unit zones shows that a saw tooth pattern in which an initially high an decreases as growth proceeds is the most common type of zoning configuration we recognize 2 major types of zoning type 1 small zones usually less than 10 microns thick with compositional amplitudes up to 10 an and type 2 zones up to50 microns thick with large compositional amplitudes of up to 35 an detailed analysis of zoning patterns shows that oscillatory zoning is not actually oscillatory in the sense of a sine wave it is discontinuous unit zones are characteristically bounded by discontinuities this common pattern is consistent with alternating periods of growth and periods of dissolution reaction and lack of growth or possibly very slow growth our observations firmly establish that dissolution and magmatic reaction involving both growth and dissolution are common during growth of the magmatic plagioclase which we have studied type 1 zonation appears consistent with perturbations of local conditions in a diffusion controlled boundary layer around the growing crystal type 2 zoning is probably due to large scale conditions faceted planar growth surfaces appear to be the rule in magmatic crystallization although in some instances rounded or irregular convolute zonation can occur the repetitive nature of many of the features we have observed is consistent with and suggestive of some form of self organization

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

osmium isotope constraints on lower crustal recycling and pluton preservation at lassen volcanic center ca osmium isotope compositions of intermediate to silicic composition calc alkaline volcanic rocks from the lassen volcanic region of the cascade arc are significantly more radiogenic qos 23 to 224 than typical mantle these evolved arc rocks in the lassen region have unradiogenic sr nd and pb isotope compositions which overlap with those of contemporaneous mafic lavas crystal fractionation of mafic to intermediate composition magmas produces re os ratios that are high enough to evolve to very radiogenic os isotope compositions in only a few million years providing a potential fingerprint for detecting the involvement of such young relatively mafic crust in magmatic systems however the sr nd and pb isotope compositions will remain constant over such short time intervals due to relatively low parent daughter enrichment during magmatic evolution the radiogenic os isotope compositions in typically evolved lassen rocks are interpreted to reflect significant interaction with lower crustal material that has radiogenic os isotope compositions beneath this section of the cascade arc large amounts of such high qos lower crust may have formed and been isolated from mash zone mixing and homogenization processes during the pliocene or late miocene the results from this study indicate that os isotopes may provide a unique glimpse into lower crustal processes such as recycling in primitive orogenic arcs 9 2002 elsevier science b v all rights reserved

time scales of crystal mixing in magma mushes magma mixing is widely recognized as a means of producing compositional diversity and preconditioning magmas for eruption however the processes and associated time scales that produce the commonly observed expressions of magma mixing are poorly understood especially under crystal rich conditions here we introduce and exemplify a parameterized method to predict the characteristic mixing time of crystals in a crystal rich magma mush that is subject to open system reintrusion events our approach includes novel numerical simulations that resolve multiphase particle fluid interactions it also quantifies the crystal mixing by calculating both the local and system wide progressive loss of the spatial correlation of individual crystals throughout the mixing region both inertial and viscous time scales for bulk mixing are introduced estimated mixing times are compared to natural examples and the time for basaltic mush systems to become well mixed can be on the order of 10 days

crustal differentiation processes at krakatau volcano indonesia anak krakatau is a basaltic andesite cone that has grown following the famous caldera forming 1883 eruption of krakatau it breached sea level in 1927 and since the 1950s has been growing at an average rate of similar to 8 cm a week we present new major and trace element data combined with whole rock delta o 18 sr and nd isotope data for 1883 1993 and 2002 krakatau eruptive products and the surrounding crust bombs erupted from anak krakatau during 2002 contain frothy metasedimentary and plutonic xenoliths that show variable degrees of thermal metamorphism plastic deformation and partial melting contact metamorphic minerals such as cordierite and tridymite in metasedimentary xenoliths are consistent with high temperature metamorphism and incorporation at mid to upper crustal depth energy constrained assimilation and fractional crystallization modelling of whole rock data suggests that the anak krakatau magmas have a genetic relationship with the 1883 eruption products the geochemical impact of crustal contaminants on whole rock compositions is apparently small and we conclude that low levels of assimilation of a quartzo feldspathic sediment are recorded in anak krakatau magmas plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning however and are also isotopically variable with a total range in sr 87 sr 86 of 0 center dot 7043 0 center dot 7048 as determined by in situ laser ablation inductively coupled plasma mass spectrometry this suggests that although shallow crustal assimilation appears to have had a limited effect on whole rock chemistry a complex late stage differentiation history is recorded within the magma s cargo of crystals and xenoliths

calcic cores of plagioclase phenocrysts in andesite from karymsky volcano evidence for rapid introduction by basaltic replenishment calcic cores in plagioclase of karymsky andesite of the 1996 2000 eruptive cycle texturally and compositionally both trace and major elements mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle these observations support the view that simultaneous eruption of andesite and basalt at karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir homogeneity of andesitic output occurred within two months this suggests to us that blending of injected basalt into reservoir magma was thorough and rapid

enclaves in the cadillac mountain granite coastal maine samples of hybrid magma from the base of the chamber the cadillac mountain intrusive complex is dominated by the introduction cadillac mountain granite and a 2 3 km thick section of interlayered enclaves occur in nearly all granites and have recently gabbroic dioritic and granitic rocks which occurs near the base of been the focus of many studies see reviews by vernon the granite the layered rocks record hundreds of injections of basaltic 1983 and didier barbarin 1991 although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma magmatic enclaves ranging in different sources there has been a growing consensus composition from 55 to 78 wt sio2 are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks the most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments their compositions can be best explained by periodic vernon 1984 1990 these can appropriately be termed replenishment mixing and fractional crystallization of basaltic magmatic enclaves their compositions are commonly magma at the base of the chamber the intermediate to silicic intermediate but can vary from basaltic to highly silicic enclaves formed by hybridization between the evolved basaltic magma magmatic enclaves occur in all types of granite and are and resident silicic magma there is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite instead alkaline i type granites in spite of partial reequilibration hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have nd isotopic commagmas at the base of the chamber enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics suggesting support their formation from mantle derived magmas they were derived from a discrete batch of hybrid magma enclaves holden et al 1987 were probably dispersed into a localized portion of the granitic the compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton and they can only layer rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma there is ample petrographic and chemical evidence in many enclaves for hybridization mixing of liquids and

fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy implications for crystal transfer and magma mingling in s type granites microsampling of cm scale feldspar crystals within an s type granite from the lachlan fold belt of southeastern australia has revealed complex internal sr and nd isotopic variations the observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle like magmas the latter stages of which are now represented in modified form by microgranular enclaves similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation feldspar rims have higher 87sr 86sri and lower nd i than adjacent whole rock analyses but match those of mineral separates from the surrounding enclave matrix this suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87sr 86sr low nd i component such as metasedimentary wall rocks isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma our results reveal major isotopic heterogeneities on a single crystal and also inter mineral scale in a pluton which shows well constrained evidence for magma mingling these results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale

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

case studies of plagioclase growth and residence times in island arc lavas from tonga and the lesser antilles and a model to reconcile discordant age information abstract the time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved however crystals and liquids can have different ages leading to apparently contradictory information from different dating methods two case studies are presented from island arc lavas that highlight such dichotomies plagioclase crystal size distributions and u th ra isotope data are presented for lavas from late and metis shoal in tonga and from soufriere st vincent in the lesser antilles in all three lavas mineral data yield apparent u th ages of 30 50 kyr yet 226 ra excesses are preserved in both the minerals and the whole rocks the crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations furthermore a cumulate from soufriere yields the same u th age as its host lava on 226 ra ba versus time evolution diagrams curves for melts in equilibrium with plagioclase provide apparent ages of 1700 3300 yr for tonga but for soufriere they are displaced below that of the whole rock our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age the apparent u th ages reflect mixtures of old cores and young rims and indicate a volume integrated age of 10s of kyr for the time for initial crystal growth crystal liquid segregation the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption the 226 ra excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma because crystal growth segregation and entrainment can all appear to occur in 100s to a few thousand years at most the bulk of the u th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches plagioclase growth rates of 3 9 10 13 cm s 1 would reconcile the combined ra th and crystal size distribution data magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times indicating complex phenocryst liquid behaviour beneath island arc volcanoes

isotopic responses to basaltic injections into silicic magma chambers a whole rock and microsampling study of macrorhythmic units in the pleasant bay layered gabbro diorite complex maine usa the pleasant bay layered gabbro diorite complex 420 ma formed via repeated injections of mafic magma into a felsic magma chamber it is dominated by repeating sequences macrorhythmic units with chilled gabbroic bases which may grade upward into medium grained gabbro diorite and granite each unit represents an injection of mafic magma into the chamber followed by differentiation increases in sri and decreases in endi with stratigraphic height indicate open system isotopic behaviour and exchange between the mafic and felsic magmas isotopic variations of whole rock samples in individual macrorhythmic units do not conform to bulk mixing or afc models between potential parental magmas sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important that some of the rocks contain feldspar xenocrysts and that the rocks are isotopically heterogeneous on an intercrystal scale xenocryst abundance increases with stratigraphic height suggesting that crystal exchange occurred in situ the lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes representing the parental melts and suggest rapid in situ diffusional exchange following emplacement of individual mafic replenishments

the analysis of zoning in magmatic crystals with emphasis on olivine many of the observed features of zoning in magmatic phenocrysts may be due to the orientation of the section rather than inherent properties of the crystals an ideal section for the studying of zoning in magmatic crystals has two characteristics it goes through the center of the crystal and is perpendicular to one or more crystal faces using a model zoned olivine crystal it is possible to construct accurate zoning profiles for different types of section centered symmetrical and skewed the probability of obtaining a random section which passes within x of the center of a crystal is shown to be p 0 0x while the probability that a random section will be within a degrees of perpendicular to a given plane is p sin a a systematic approach to the study of zoned crystals is outlined in particular it is suggested that composition be plotted against distance cubed in order to correct for the volume versus size problem a method of determining if a given section goes through or near the center of a zoned crystal is also presented the reasoning in this work applies to other types of magmatic crystals such as pyroxenes and plagioclase

heat flow and fluid flow regime in the western nankai accretionary prism the thermal structure of the western nankai subduction zone has been studied through surface heat flow measurements with conventional geothermal probes and estimation of heat flow from depths of gas hydrate bottom simulating reflectors temperature and thermal conductivity measurements at odp site 808 located at the toe of the nankai accretionary prism added important information on any variation in heat flow with depth and thus on thermal effects of pore fluid flow in the prism surface heat flow is generally higher than 130 mw m 2 on the floor of the nankai trough such high heat flow cannot be explained simply by conductive cooling of the subducting shikoku basin lithosphere recent closely spaced probe measurements revealed that the heat flow on the trough floor shows local variation with particularly high values near the deformation front the heat flow distribution suggests that warm fluid flows seaward possibly along the decollement and locally rises toward the seafloor along some channels resulting in both the regional and local high heat flow anomalies contrary to this inference no active localized fluid flow was detected at site 808 the temperatures measured at six depths down to 347 m below seafloor mbsf just below the frontal thrust can be well explained by conductive heat flow of 125 130 mw m2 no geochemical anomaly associated with present day pore fluid movement was observed in cores recovered from the frontal thrust or the decollement on the other hand the high heat flow at site 808 requires some heat source below 350 mbsf there is a possibility that the fluid flow in the nankai accretionary prism is localized and transient and thus was not detected at this single drill site

magma mixing recharge and eruption histories recorded in plagioclase phenocrysts from el chichon volcano mexico operative throughout the 200 ky history of the el chichon magma consistent core to rim decreases of sr sr ratios and coincident system increases in sr concentrations in plagioclase phenocrysts of varying size 1 cm to 2 mm are reported from samples of the 1982 and pre 1982 200 ka eruptions of el chichon volcano maximum sr sr ratios of 0 7054 significantly higher than the wholerock isotopic ratios 0 7040 0 7045 are found in the cores

open system dynamics and mixing in magma mushes magma in earth s crust is in a crystal rich mushy state yet must be fluidized before eruptions numerical simulations show that rapid injection of new magma into a reservoir creates a mixing bowl of fluid and crystals that are able to erupt

non stationary entrainment and tunneling eruptions a dynamic link between eruption processes and magma mixing the products of many intermediate arc volcanoes and batholiths manifest compositional complexity with mafic and mixed material occurring early and through out a period of eruption and pluton assembly we present a vesiculation model suggesting that the progressive and repeated vesiculation of volatile rich magmas yields a super linear buoyancy acceleration this style of non stationary entrainment provides a mechanism for compositional reversal the intimate co occurrence of mixed and un mixed magmas and rapid chaotic crystal and melt dispersal under some circumstances entrainment can cease entirely producing eruptions with compositional tunneling where nearly unmixed mafic or newly intruded silicic magma exits first

sr isotope disequilibrium during differentiation of the bandelier tuff constraints on the crystallization of a large rhyolitic magma chamber the 1 61 ma otowi member of the bandelier tuff jemez mountains volcanic field new mexico is a caldera forming high silica rhyolitic ignimbrite with a precursor fallout deposit the otowi member has high rb sr ratios the 87 rb 86 sr ratios from sanidines and glasses range from 14 to 570 most sanidines from glomerocrysts in the ignimbrite have 87 sr 86 sr i 0 7052 0 7056 whereas the ratios from glasses from glomerocrysts range from 0 7052 to 0 7079 quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic at 87 sr 86 sr i 0 7105 0 7113 despite having much lower rb sr ratios than glomerocryst glasses these relations require that the inclusion glasses are more contaminated with proterozoic country rock than are glomerocryst glasses textural isotopic and trace element data support a model in which crystals grow in a boundary layer with the most inclusion rich quartz grains closest to the magma country rock contact phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace disseminated throughout the magma prior to eruption an important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

volcanic expression of bimodal magmatism the cranberry island cadillac mountain complex coastal maine the cranberry island series is a thick succession of silurian volcanic breccias ash flow tuffs lava flows and hypabyssal intrusions comprising the southern margin of mount desert island and the cranberry islands of southeastern maine the rocks of the series preserve remarkable textures indicative of the simultaneous eruption of mafic and felsic magmas during both explosive and effusive events the cadillac mountain intrusive complex is an approximately 20 km wide composite layered mafic to felsic plutonic body that dominates mount desert island immediately north of the outcrop of the cranberry island series the rocks of the cranberry island series and those of the cadillac mountain intrusive complex are correlative in age and similar in composition the age of the cranberry island series based on new u pb geochronology of zircon is 424 ma pm 1 ma the cadillac mountain granite the somesville granite and the southwest harbor granite all parts of the cadillac mountain intrusive complex are

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

dynamics of magma mixing in partially crystallized magma chambers textural and petrological constraints from the basal complex of the austurhorn intrusion se iceland the tertiary austurhorn intrusive complex in se iceland represents an exhumed magma chamber that has recorded an extensive history of magma mixing and mingling the basal part of the intrusion consists predominantly of granophyres that have been intensively and repeatedly intruded by more mafic magma this association of granophyres basic and hybrid rocks at austurhorn is referred to in the literature as a net veined complex but field relations suggest a much more complex emplacement history here we present petrological and physical constraints on the various processes that resulted in magma mixing and mingling and the formation of different generations of hybrid rocks at austurhorn the complexity of the mixing and mingling processes increases towards the inferred centre of the intrusion where chaotic hybrid rocks dominate the exposed lithology complex cross cutting relations between different hybrid generations strongly suggest multiple magma injection and reheating events in the basal part of the shallow magma chamber model calculations employing distribution coefficients based on rare earth element concentrations reveal that early stage hybrid magma generations formed by pure endmember mixing between felsic and mafic magma with about 10 mafic fraction in the hybrids with repeated injections of mafic magma into the base of the magma chamber the intruding magma interacted to a greater extent with pre existing hybrids this led to the formation of hybrid magma compositions that are shifted towards the mafic endmember over time with up to 30 of the mafic fraction in the hybrids these mixing processes are recorded in the zonation patterns of clinopyroxene and plagioclase phenocrysts the latter have been divided into four main groups by cross correlation analysis melt viscosity calculations were performed to constrain the possible conditions of magma mixing and the results indicate that the interaction of the contrasting magmas most probably occurred at temperatures of approximately 1000 c up to 1120 c this suggests that the initiation of effective magma mixing requires local superheating of the felsic magmas thereby confining the process to areas of localized substantial mafic magma injection

magma mixing and crustal recycling recorded in ternary feldspar from compositionally zoned peralkaline ignimbrite a gran canaria canary islands miocene ignimbrite a on gran canaria contains three compositional endmember fiamme types two rhyolites and one trachyte each of which crystallized distinct feldspar various textural and compositional criteria are interpreted as reflecting a complex scenario within the magma chamber in which the crystals formed about 25 30 of the feldspar phenocrysts contain evidence for magma mixing in the form of 1 partial to severe dissolution resorption rims 2 distinct zones of drastically different compositions and 3 overgrowth textures on formerly resorbed crystals four major types of zoning in the oligoclase to anorthoclase feldspars of ignimbrite a include a normal and a reversely zoned type and two complexly zoned types the feldspars with normal and reverse zonation show only minor compositional amplitudes between individual zones ab or 4 whereas the complexly zoned types show compositional differences between zones of up to 18 mol ab and 20 mol or and are commonly associated with an internal dissolution surface complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movements within the magma and across compositional boundaries between magma batches a multiple step cycle model involving growth and transport of a crystal into another magma batch and its return to the original host magma is suggested by the data moreover feldspars from one rhyolite compositional group are found to be substantially elevated in 18o suggesting an input of a high 18o component to this rhyolite the other endmember rhyolite appears to be related to the endmember trachyte by mainly crystal fractionation of anorthoclase feldspar this observation is consistent with trace element and rare earth element concentrations for the magma endmembers and their feldspars where contamination led to a depletion in incompatible trace elements and light rare earth elements in the contaminated rhyolite and its feldspar phenocrysts we suggest that the combination of textural and compositional variation in ternary feldspar of peralkaline rhyolitic systems is well suited to reconstruct dynamic processes such as magma mixing and contamination in evolving rhyolitic magma chambers

insights into magmatic evolution and recharge history in capraia volcano italy from chemical and isotopic zoning in plagioclase phenocrysts plagioclase phenocrysts in dacites from the high k calc alkaline capraia volcano were investigated for major trace element and sr isotope variations in order to gain better insight into the proposed open system behaviour of the volcano repeated dissolution zones in plagioclases from the early erupted monte castello mc and later monte rucitello mr dacitic units are associated with a sharp increase in an content and variation in isotopic compositions 87sr 86sr 0 70872 0 71004 which argue for repeated magma mixing in particular petrographic and mineralogical evidence suggest that recharge with hot basaltic magma occurred repeatedly during the lifetime of the capraia volcanic system we attempted to correlate shared crystallization and resorption histories using chemical mol an fe and sr isotopic zoning in plagioclase in this correlation framework the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber or compositional zoning within a single magma chamber a minor role of crustal contamination is inferred instead the intra crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the mc plagioclase growth history the dominant radiogenic component is similar to lamproites occurring in tuscany recharge with this component occurred throughout the genesis of the mr dacites but was restricted to the early growth history of the mc dacites the mr plagioclases are inferred to have crystallized from separate more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir decrease in the an content after most dissolution zones in all phenocrysts and low temperature estimates from zircon saturation thermometry 800 c also emphasize the importance of crystal fractionation during the evolution of the volcano in agreement with the occurrence of a significant amount of rhyolitic material thus it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity

organization of oscillatory zoning in zircon analysis scaling geochemistry and model of a zircon from kipawa quebec canada abstract the character of oscillatory zoning within a zircon crystal from the syenite kipawa complex quebec varies with scale of observation analysis of an scanning electron microscopy sem back scatter gray scale traverse at a resolution of one pixel 2 43 m revealed 145 zones over 5130 m whereas a detailed high resolution one pixel 0 195 m section near the crystal rim revealed 225 zones over 795 m in order to mathematically characterize the zoning pattern wavelet fourier and nonlinear analysis techniques were used on profiles of the sem gray scale data and a series constructed was from the zone widths results demonstrate that the zircon oscillatory zoning preserves nonlinear and periodic components secondary ion mass spectrometry electron microprobe and sem analyses of trace elements show the sem back scatter bright zones are enriched in u th and rare earth elements ree in comparison to the darker zones ree patterns are sharply heavy ree enriched and have negative eu anomalies and prominent positive ce anomalies we model the oscillatory zoning including a measure of its chemical variation by use of a periodically forced nonlinear system results of this data driven model are quantitatively similar to the natural data we envisage that the small scale oscillatory zoning was the result of a nonlinear feedback process wherein the crystal growth modified the adjacent melt which in turn affected the crystal composition the large scale harmonic zones likely reflect changes in the bulk geochemistry of the system from which the zircon grew

magma recharge contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks we illustrate a rapid and effective method for determining variations in 87sr 86sr across single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spectrometer the speed of analysis proves to be a particular advantage permitting representative populations of crystals to be characterized or enabling reconnaissance analysis of rocks to test for mineral scale isotopic heterogeneity our results from magmatic crystals in lavas from el chichon volcano confirm trends previously obtained by the more time consuming microdrilling method for the first time with confidence the laser data allow us to associate abrupt changes in 87sr 86sr with distinct petrographic discontinuities such as dissolution surfaces in the crystal because the isotopic variations within crystals would be expected to equilibrate diffusively with time the width over which abrupt changes in 87sr 86sr occur can also be used to constrain timescales at which the crystal has been held at magmatic temperatures

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

magmatic interactions as recorded in plagioclase phenocrysts of chaos crags lassen volcanic center california the silicic lava domes of chaos crags in lassen volcanic national and mingling of this material into the silicic host these processes park contain a suite of variably quenched hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions the inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement sr sr ratios 0 7037 0 7038 of the inclusions are distinctly lower than those of the host rhyodacite

magma evolution revealed by anorthite rich plagioclase cumulate xenoliths from the ontong java plateau insights into lip magma dynamics and melt evolution abstract petrologic studies of igneous rocks often rely on whole rock analyses alone to deduce source and melting histories whole rock compositions represent a mixture of the processes responsible for the final texture and composition of the rock but do not emphasize the evolutionary processes that the rock may have experienced we examine the major minor and trace element compositions of plagioclase phenocrysts and plagioclase rich cumulate xenoliths from a sequence of kwaimbaita pillow basalt and massive flows recovered from ocean drilling program odp site 1183 on the expansive 122 ma ontong java plateau ojp and from subaerial outcrops on the island of malaita solomon islands these data are used to reconstruct equilibrium liquid compositions to understand magma dynamics magma evolution and the distribution basalt types across the ojp results reveal that crystals frequently are out of equilibrium with their host basalt two distinct compositional zones are present in plagioclase phenocrysts and xenolith crystals an80 86 and an65 79 equilibrium magma compositions derived from each of these zones are distinct in relative terms the following distinctions are made an80 86 sr y and eu richer ba and lree depleted higher sr ti lower ba sr and la y an65 79 sr y and eu poorer ba and lree enriched lower sr ti higher ba sr and la y we observed several an65 79 parent magmas with higher la y lower sr and higher lree than the majority of an65 79 zones which suggests that they grew from highly fractionated magmas the an80 86 zones of both xenolith crystals and phenocrysts appear to have crystallized relatively early and are attributed to growth in magmas more primitive than parent magmas of an65 79 zones or the host magma using melts modeling we demonstrate a significant role for h2o is not necessary to form an rich crystals from known ojp magma types these types of zones can instead form when relatively primitive ojp magmas ascend and partially crystallize at low pressures crystal mush layers are rheological elements of multiply saturated solidification fronts and conditions for extreme fractionation are met deep with thermally and mechanically insulated mush solidification fronts conditions in a homogenous magma body alone cannot generate the range of magma compositions and textures recorded in ojp plagioclase crystals we suggest that the ojp magma chamber system consisted of crystal mush dominated regions and liquid dominated regions in a laterally and vertically extensive system of interconnected dikes and sills solidification front disruption was common during magma recharge and transport which freed debris to mix with ojp magmas the volume of crystalline debris from disrupted solidification fronts was small relative to the overall magma volume which leads to little change of bulk magma chemistry this can explain the dominance of the kwaimbaita basalt type across the ojp evidence of solidification front disruption is best preserved as allochthonous crystals both more and less primitive than bulk magma chemistry both of which were observed in ojp basalts coincident with a zone of neutral buoyancy the shallow ojp magma chamber system existed at a depth of 0 7 km the ojp magma chamber system was thinner near the plateau margins which provided less density filtration of magmas this allowed more diverse magma types to reach the surface along the margins it is envisaged that as the ojp formed the zone of neutral buoyancy slowly migrated upward leading to slow yet pervasive assimilation of overlying seawater altered basalt at the base of the lava pile

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

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods