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

trace element evidence for plagioclase recycling in calc alkaline magmas abstract trace element zoning in plutonic plagioclase feldspars from the val fredda complex southern adamello massif italy has been measured using secondary ion mass spectrometry sims the studied samples range from hornblende gabbro through diorite to granitoid with an overall range in plagioclase composition from an96 to an15 individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures there is equally extensive zoning in trace elements notably sr and ba broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition using a relationship betweenxan and the distribution coefficientsdsr anddba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone the calculated variations in ba and sr are greater than can be attributed to changes in pressure or temperature alone indicating that compositional variation in the coexisting melt was the principal cause of zoning the ba sr melt trend for each crystal describes a two stage crystallisation history the calcic cores of all crystals show melt trends of increasing sr and ba which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas in contrast the melt trends recorded by the rims vary from sample to sample but essentially show decreasing sr and ba consistent with fractionation of sodic plagioclase alkali feldspar and or biotite the plagioclase cores are thought to have crystallised in a common magma chamber at 6 10 kbar pressure while the rims crystallised at the emplacement level 1 5 kbar from interstitial melt whose composition is determined by that of the host magma the data suggest that early formed calcic plagioclase was recycled by the magma during fractionation vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re entrain cumulus crystals into more evolved magmas the occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale

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

shrimp u pb zircon geochronology of the strzelin gneiss sw poland evidence for a neoproterozoic thermal event in the fore sudetic block central european variscides zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central european variscides a shrimp u pb zircon geochronological study of the strzelin gneiss in the fore sudetic block sw poland indicates the presence of 1 inherited zircon cores of palaeo to mesoproterozoic 206pb 238u ages between ca 2 000 and 1 240 ma and 2 zoned rims of neoproterozoic age with two distinct means of 600 7 and 568 7 ma the proterozoic age range of the cores suggests that different precambrian crustal elements were the source for the protolith of the gneiss a likely scenario is the erosion of various proterozoic granites and gneisses sedimentation after 1 240 ma and partial resistance of the original components to subsequent metamorphic dissolution and or anatectic resorption in neoproterozoic times the zoned zircon rims of both of the younger neoproterozoic ages are indistinguishable in the cathodoluminescence images the data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca 600 and 568 ma in general the new results confirm earlier assumptions of the proterozoic age of the gneiss protoliths and indicate their similarity to orthogneisses in the east sudetes tectonic domain e g the velke vrbno and desna gneisses the neoproterozoic dates are different from the age of 504 3 reported earlier for the go ciecice gneiss from a neighbouring locality in the strzelin massif the new data strongly indicate a moravo silesian bruno vistulian affinity for the strzelin gneiss and support the hypothesis that the strzelin massif lies within the tectonic boundary zone between the west and east sudetes domains which represents the northern continuation of the moldanubian thrust

an investigation of disequilibrium growth processes of plagioclase in the system anorthite albite water by methods of numerical simulation compositional zoning of plagioclase is useful as a recorder of dynamic geological conditions if the mechanisms of crystal growth are known although the present lack of quantitative information on specific kinetic processes limits their accuracy numerical simulations of phenoycryst plagioclase growth are useful both for identifying the most influential kinetic processes for example diffusion that should receive priority in experimental measurements and for designing informative growth experiments the interaction of kinetic processes at a crystal face is so complex that the overall result cannot be assessed intuitively a primary purpose of these papers is to explore this interaction in the plagioclase system as quantitatively as data permit

dynamics of a confined lava flow on kilauea volcano hawaii this paper presents a new method of analysing lava flow deposits which allows the velocity discharge rate and rheological properties of channelled moving lavas to be calculated the theory is applied to a lava flow which was erupted on kilauea in july 1974 this flow came from a line of fissures on the edge of the caldera and was confined to a pre existing gully within 50 m of leaving the vent the lava drained onto the floor of the caldera when the activity stopped but left wall and floor deposits which showed that the lava banked up as it flowed around each of the bends field surveys established the radius of curvature of each bend and the associated lava levels and these data together with related field and laboratory measurements are used to study the rheology of the lava the results show the flow to have been fast moving but still laminar with a mean velocity of just over 8 m s 1 the lava had a low or negligible yield strength and viscosities in the range 85 140 pa s an extension of the basic method is considered and the possibility of supercritical flow discussed

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

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

40ar 39ar mineral age record of polyphase tectonothermal evolution in the southern mauritanide orogen southeastern senegal 40 ar 39 ar incremental release mineral ages have been determined for several tectonostratigraphic units in southernmost portions of the maiuritanide orogen in senegal here middle proterozoic crystalline basement rocks of the west african craton are unconformably overlain by an upper proterozoic ca 1100 ma continental sedimentary succession madina kouta group transitional into volcanic and intercalated deep marine sediments tennesse group western portions of the volcano sedimentary unit were variably deformed and metamorphosed prior to deposition of an unconformably overlying upper precambrian lower cambrian sedimentary succession mali group muscovite within cleavage domains in the low grade metamorphic terrane records 40 ar 39 ar plateau dates of ca 655 665 ma and are interpreted to closely date the pan african i phase of orogenic activity the mali group itself was locally folded during a second phase of pan african tectonothermal activity which also affected a western calc alkaline volcanic plutonic sequence niokolo koba group igneous hornblende from several granites within this sequence displays internally discordant 40 ar 39 ar release spectra which record variably developed plateaus of ca 640 660 ma these are interpreted to approximately date initial postmagmatic cooling through 500 c and are consistent with a previously published rb sr whole rock iso chron crystallization age of 683 18 ma for one of the plutons associated calc alkaline volcanic and volcaniclastic rocks are more penetratively deformed muscovite that is aligned along a regional cleavage records 40 ar 39 ar plateau ages of ca 550 560 ma which are interpreted to closely date the pan african ii orogenesis widespread deposition of a molassic sequence youkoukoun formation occurred in the middle cambrian early ordovician this was disconformably overlain by upper ordovician upper devonian sedimentary rocks bove basin succession which are locally folded more significant late paleozoic tectonothermal activity is recorded in north westernmost portions of the study area where the calc alkaline upper proterozoic granites locally display penetrative mylonitic fabrics development of these fabrics is interpreted to be closely dated by the 40 ar 39 ar 270 to 280 ma plateau ages recorded by dynamically recrystallized muscovite developed along fluxion foliations these results allow demarcation of a late paleozoic tectonic front and permit correlations with genetically related events in southeastern portions of the appalachian orogen

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

numerical simulations of crystallization processes of plagioclase in complex melts the origin of major and oscillatory zoning in plagioclase numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents the simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased and that the water content in the melt has little influence on this result comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees c can be rapidly imposed on plutonic phenocrysts large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals the observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates even at large undercooling there is no significant increase in temperature at the interface caused by the release of heat of crystallization

plagioclase residence times at two island arc volcanoes kameni islands santorini and soufriere st vincent determined by sr diffusion systematics the diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase melt partition coefficient trace element diffusion will subsequently act to modify this initial profile and given enough time produce a profile that is in equilibrium with the anorthite variations within the crystal we argue that the trace element partition coefficient da b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal liquid partition coefficients and that the equilibrium profile of the crystal can be calculated the time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase strontium plagioclase melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions a one dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir the model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the kameni island dacites santorini and from the 1979 soufriere andesite st vincent three out of eight plagioclase phenocrysts have sr concentration profiles that are not in diffusive equilibrium for these the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase maximum crystal residence times range from 100 to 450 years this is in good agreement with estimates from crystal size distribution and from ra th disequilibrium studies for the kameni islands for soufriere however such short residence times are incompatible with u th mineral errorchron data that suggest residence times of 40 ka in a thermally buffered magma reservoir to reconcile these apparently different ages we invoke a more complicated magmatic history for soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption

dissolution origin and syn entrapment compositional change of melt inclusion in plagioclase abstract in the system di ab an h 2 o melt inclusions in plagioclase were reproducibly formed in a series of partial dissolution experiments and the compositional differences from the surrounding matrix melt were evaluated at 200 mpa and 1050 1150 c a seed crystal of an 59 was partially dissolved into the matrix melt which was in equilibrium with more calcic plagioclase than the seed the reaction proceeded inward from the crystal surface by a dissolution and recrystallization process to form a reaction zone composed of melt channels and recrystallized calcic plagioclase the melt channels were choked necked off by the recrystallized plagioclase at the crystal surface to create melt inclusions the experimentally formed inclusions having square and zigzag shapes up to a few tens of micrometers in size are similar in texture to some common types of natural melt inclusion since the partial dissolution of phenocrysts can occur during various magma mixing processes it should be taken into consideration as one of the inclusion formation mechanisms when trying to interpret information from natural melt inclusions especially for those in hydrous arc magmas the chemical composition of the reproduced melt inclusions deviates from the matrix melt basically along the plagioclase liquidus isotherm the molar ratio of ca ca na and the mgo concentration in the synthesized melt inclusion are up to 17 and 61 relative lower than those in the matrix melt respectively this compositional difference is caused by a combination of factors 1 supersaturation with dissolved plagioclase component 2 insufficient diffusive homogenization in the melt channels due to rapid dissolution of plagioclase and 3 the choking off effect on the basis of the experimental results a model for diffusion in a growing melt channel in dissolving plagioclase was constructed which revealed that the concentration of incompatible elements with low diffusivities can be significantly lower in the melt inclusions than that in the matrix melt these results demonstrate the importance of petrography for interpretation of inclusion data and of more experimental studies on melt inclusion origin

boundary layer crystallization in a basaltic magma chamber evidence from rishiri volcano northern japan the evolution of basaltic magma by mushy boundary layer processes introduction in a magma chamber is documented for an alkali basalt kutsugata the evolution of magma chambers is of major petrologic lava from rishiri volcano northern japan on the basis of zoning interest and has been studied extensively experimental patterns of plagioclase phenocrysts and their spatial distribution in and theoretical studies that have investigated the cooling the lava plagioclase phenocrysts with complex zoning patterns can of magma chambers have demonstrated that a magma be divided into four types type 1 is characterized by an extremely chamber solidifies through formation of a mushy boundan rich core an71 90 which is commonly corroded and filled with ary layer composed of crystals and interstitial melt along sodic plagioclase type 2 has an ab rich core surrounded by a calcic the chamber walls e g brandeis jaupart 1986 turner mantle characteristic of partial dissolution type 3 is characterized by et al 1986 marsh 1988 tait jaupart 1992 layered igneous complexes e g morse 1969 shirley 1987 an ab rich core that is commonly surrounded by a reversely zoned mcbirney 1996 provide useful information on how slightly calcic mantle type 4 has an ab rich core without a calcic mafic magma chambers evolve and consolidate but they mantle the an rich cores of the type 1 plagioclase are too calcic do not offer much insight into the instantaneous state of to have crystallized from a liquid represented by any whole rock a chamber because those intrusions integrate processes composition at the estimated temperatures of the kutsugata magma that occurred over long solidification times mcbirney system the an rich cores are inferred to have been formed in a noyes 1979 tait 1988 volcanic products such as mushy boundary layer along the wall of a magma chamber where lava flows and tephra deposits can sample the inthe magma was relatively cool and rich in water because of significant stantaneous state of a magma chamber there are a few olivine fractionation and possibly because of addition of water studies along this line of approach tait 1988 de silva expelled from the chilled margin and or groundwater from the 1989 nakada et al 1994 these studies focused on surrounding crust the ab rich cores of types 2 4 plagioclase crystal aggregates in volcanic products and concluded phenocrysts are interpreted to have crystallized in the main magma that the crystal formation in a mushy boundary layer body of the reservoir the types 2 and 3 ab rich cores were brought plays a dominant role in magmatic evolution in a magma to the mushy layer by convection the temporal and spatial variations reservoir however these studies were mostly on silicic of zoning patterns of plagioclase phenocrysts in basaltic lava flows magmas and there are significant differences in physical give useful information on the evolution of mafic magma chambers and chemical features between silicic and mafic magma chambers in this paper the magmatic evolution of an alkali basalt kutsugata lava from rishiri volcano northern japan is studied the kutsugata lava is divided into three lava flows on the basis of whole rock chemical

trace elements in anorthite megacrysts from the kurile island arc a window to across arc geochemical variations in magma compositions abstract anorthite megacrysts from twelve volcanic centers of the kurile island arc were analyzed by the ion microprobe for concentrations of li be b f mg p cl k ti fe co rb sr y zr nb cs ba la ce pr nd sm eu and pb these 1 2 cm megacrysts are mainly found as early phenocrysts in basalts but are also present in andesites rhyolites and hybrid rocks from almost every studied volcanic center both subaerial and submarine electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned whereas rims often result from overgrowth of sodium richer plagioclase in equilibrium with its host silica richer melt the cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a window into an earlier basaltic stage of magmatic evolution petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second after olivine liquidus phase in high al basalts regardless of varying k2o contents across the arc given the narrow compositional range of anorthite megacrysts the across arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc the concentration of k in megacrysts increases linearly by a factor of 11 ba 13 sr eu 4 be ti p 3 li 2 ree 5 2 whereas concentrations of pb b and halogens and their ratios to hfse either remain constant or decrease with deepening of the benioff zone from 110 to 220 km we used ion microprobe derived plagioclase basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values the reconstructed concentrations of several key trace elements and their correlation with benioff zone depth are consistent with whole rock basalt data but all give systematically more primitive compositions therefore anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing they can be used to assess delicate variations in primary basaltic magma chemistry as a function of benioff zone depth we also report new results for sr nd and pb isotopes for anorthite bearing rocks and other rocks from the same volcanoes across arc zoning with respect to trace element concentrations ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes trace element transfer by fluids variation in degree of partial melting and progressive change of the melting source

crystallization and resorption in plutonic plagioclase implications on the evolution of granodiorite magma gesiniec granodiorite strzelin crystalline massif sw poland abstract granodiorite from the gesiniec intrusion strzelin crystalline massif sw poland contains complexly zoned plagioclases five chemically and structurally distinct zones can be correlated among crystals cores 25 35 an inner mantles 40 45 an outer mantles 40 25 an resorption zones 35 50 an and rims 35 30 an good structural and chemical major and trace elements correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale plagioclase being the liquidus phase records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes crystallization began with the formation of inner mantles the paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma normally zoned inner mantles were formed under increasing undercooling compositional trends in mantles suggest closed system crystallization the major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase the injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast kinetically controlled growth of complex multiple oscillatory zonation within resorption zones the ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles the cores the decompression range is estimated at a minimum of 2 kbar emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases melt inclusions crystallized completely during post magmatic cooling the zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur crystals probably did not travel between different regimes resorption occurred but as single albeit complex episodes good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects

crystallization processes in the rocky hill granodiorite pluton california an interpretation based on compositional zoning of plagioclase the rocky hill pluton was chosen for study of plagioclase zoning because it has an apparently simple geometry and one stage emplacement history documented by putman and alfors 1969 the pluton is divided texturally into a hypidiomorphic granular rim fades and porphyritic core facies both comprising zoned plagioclase phenocrysts averaging 2 4 mm in diameter major features of compositional zoning profiles of plagioclase were found to be consistent within a sample showing resorbed core regions followed by a sharp drop of ca 10 an to a plateau region and ending with normal zoning the width of the plateau region is narrow in samples near the pluton rim but increases systematically toward the pluton core and the outer part of the plateau develops reverse zoning in the core facies a review of the effects of pressure water content of the melt and temperature on equilibrium and disequilibrium crystallization processes concludes that only variations of temperature and water content during disequilibrium crystallization of plagioclase phenocrysts can produce the observed zoning patterns experimental data suggest that the trend toward finer grain size toward the interior of the pluton may be due to increasing water content rather than due to increasing cooling rate the compositional zoning of plagioclase and textural variations in the pluton can be explained by a model in which the pluton crystallized by migration of a partly crystalline mush zone inward from the edge of the plutonic chamber while convection within the residual magma 1 redistributed water excluded from the mush zone and 2 slowed the cooling rate of the mush zone the reverse zoning of plagioclase growing in the convecting magma changed to normal zoning after capture in the mush zone because the cooling rate increased and water was removed comparison of the results from our study of rocky hill with published descriptions of plutons and experimental and theoretical studies of crystallization processes leads to the conclusions that 1 convection is an active process during the crystallization of many plutons causing redistribution of mainly water and heat in silicic ones but differentiating other components in intermediate ones 2 most of the crystallization of granitic rocks may occur in a mush zone or static boundary layer between the solid boundary and convecting inner magma body 3 water content may be an important factor controlling grain size 4 reverse zoning of plagioclase may be more typical of plagioclase grown under slowly cooled conditions than normal zoning and 5 variations of plagioclase zoning in plutons can provide useful information on crystallization processes

cellular plagioclase intergrowths as a result of crystal magma mixing in the proterozoic aland rapakivi batholith sw finland finely cellular plagioclase intergrowths have been studied in xenocrystic andesine an32 and andesine mantled k feldspars within mafic magmatic enclaves in a quartz feldspar porphyry from the proterozoic subvolcanic hammarudda complex aland rapakivi batholith sw finland the cellular intergrowths usually occur as 0 2 2 0 mm mantles around xenocrysts but also as entirely cellular grains and are built up of a network of two distinct phases one relatively na rich an31 and one relatively ca rich an50 the grains are also covered by a thin 0 08 0 12 mm continuous normally zoned rim outside the cellular mantle small inclusions 0 01 0 05 mm of fe mg minerals are concentrated in the ca rich part of the network compositionally the na rich phase of the network is close to the inner non cellular andesine of the xenocrysts however it has a lower or and a slightly lower an content the ca rich phase has the same composition as the inner part of the normally zoned rim which outwards grades into lower an contents that overlap the an content of the matrix plagioclases the cellular network was developed after the andesine xenocrysts or andesine mantled k feldspars were engulfed in mafic magmatic enclaves during a mixing event the xenocrysts became heated to a temperature just below the liquidus of the mafic magma dissolution of the xenocrysts developed a spongy cellular texture which was penetrated by enclave magma ca rich plagioclase crystallized in the cells in equilibrium with the enclave magma trapping fe mg rich melt as the enclaves cooled the outermost thin rim and matrix plagioclases crystallized from the mafic melt these processes operated in fairly large enclaves as the one studied here which has a diameter of 70cm smaller enclaves on the other hand were cooled more rapidly to temperatures close to the solidus of the enclave magma and consequently had no time to dissolve the xenoxrysts

partition coefficients for iron between plagioclase and basalt as a function of oxygen fugacity implications for archean and lunar anorthosites abstract experimental studies were undertaken to determine d for feot between plagioclase and melt as a function of oxygen fugacity for a basaltic composition that occurs as matrices for plagioclase megacrysts that are similar to those in calcic anorthosites at liquidus conditions the d for feot between calcic plagioclase and tholeiitic basalt changes very little from 0 030 to 0 044 between the very low o 2 of the iron wustite buffer and that of the quartz fayalite magnetite qfm buffer at fugacities above qfm the value for d increases rapidly to 0 14 at the magnetite hematite buffer and 0 36 in air the increase in d results from the fact that nearly all of the fe is in the fe2 state at o 2 below qfm and the fe 3 fe 2 ratio in the melt increases rapidly above qfm causing more fe to enter the plagioclase which accepts fe3 more readily than fe2 there is essentially no change in values of d over cooling rates that vary by two orders of magnitude the d for feot of 0 030 at iron wustite is in good agreement with earlier studies at low o2 terrestrial calcic plagioclase matrix pairs display ds of about 0 043 in excellent agreement with the experimental value of 0 044 at qfm which is the accepted fugacity for terrestrial tholeiitic basalts application of the ds for feot and other components to terrestrial calcic anorthosites indicates that the anorthosites were in equilibrium with nearby coeval tholeiitic basalts at qfm as was expected lunar anorthosites however have anomalously low contents of feot as well as mgo and would be in equilibrium with melts that are several times lower in these components than expected the low values of feot and mgo in the plagioclase of lunar anorthosites are a result of subsolidus reequilibration with pyroxene olivine and other fe mg bearing phases during high temperature metamorphism as is the case in terrestrial anorthosites that have undergone granulite grade metamorphism the ds for k2o mgo and tio2 were determined as 0 21 0 044 and 0 033 respectively and show no variation with o 2 the value for k2o is in good agreement with natural assemblages the values for mgo and tio2 however are lower in natural assemblages but the high degree of mobility of mg and the very low contents of tio2 in plagioclase makes the reliability of the natural values rather low also the d for tio2 may be affected by rate of cooling attempts to determine the d for cr2o3 as a function of o 2 were made difficult by the low levels of cr2o3 300 to 500 ppm in the melt when saturation with a cr spinel occurred because the cr concentrations in plagioclase are nearly 100 times less than those in the melt it was not possible to obtain reliable analyses for plagioclase these uncertainties constrain the d for cr to be less than 0 07 at the qfm buffer

minor and trace element zoning in plagioclase implications for magma chamber processes at parinacota volcano northern chile textural and compositional zoning in plagioclase phenocrysts in a sample from parinacota volcano chile was investigated using backscattered electron images and electron microprobe analysis of major and trace elements large 2 mm oscillatory zoned crystals type i with resorption surfaces of moderate an discontinuities 10 an and decreasing trace element contents sr mg ti towards the rim reflect melt differentiation and turbulent convection in the main magma body early recharge with a low sr mafic magma is seen in the core small scale sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years smaller crystals type ii with low trace element an ratio reflect the influence of an h2o rich melt probably from a differentiated boundary layer closed system in situ crystallisation mafic magma recharge and the role of a water rich differentiated boundary layer can be distinguished from the an trace element relationships crystals apparently move relatively freely between different parts and regimes in the magma chamber evidence for convective crystal dispersion high sr type ii crystals indicate an earlier input of sr rich mafic magma recharge of two distinct mafic magma types is thus identified high sr and low sr which must have been present at increasing recharge rates with time in the plumbing system throughout the volcano s history