normalized undrained shear strength of clay shales this paper examines the possibility of normalizing the undrained shear strength behavior of clay shales shales are formed in sedimentary basins by diagenetic processes which turn young clay sediments into compacted and lithified shales cementation at grain contacts affects many properties of shales including the apparent preconsolidation and shear strength based on a study of 25 different types of materials the normalized undrained shear strength of clay shales was found to correlate satisfactorily with overconsolidation ratio following the shansep stress history and normalized soil engineering properties normalization procedure for clays the normalized undrained shear strength correlation appears valid regardless of the cause of the apparent preconsolidation correlations of the apparent preconsolidation stress with unconfined compressive strength and compressional wave velocity are proposed to predict the normalized undrained shear strength of clay shales

microchemical and sr isotopic investigation of zoned k feldspar megacrysts insights into the petrogenesis of a granitic system and disequilibrium crystal growth k feldspar megacrysts kfm are used to investigate the magmatic evolution of the 7ma monte capanne mc monzogranite elba italy dissolution and regrowth of kfm during magma mixing or mingling events produce indented resorption surfaces associated with high ba contents diffusion calculations demonstrate that kfm chemical zoning is primary core to rim variations in ba rb sr li and p support magma mixing i e high ba and p and low rb sr at rims but more complex variations require other mechanisms in particular we show that disequilibrium growth related to variations in diffusion rates in the melt may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber initial 87 sr 86 sr ratios isr obtained by microdrilling decrease from core to rim inner core analyses define a mixing trend extending towards a high isr rb sr melt component whereas the outer cores and rims display a more restricted range of isr but a larger range of rb sr lower isr at the rim of one megacryst suggests mixing with high k calc alkaline mantle derived volcanics of similar age on capraia trace element and isotopic profiles suggest 1 early megacryst growth in magmas contaminated by crust and refreshed by high isr silicic melts as seen in the inner cores and 2 later recharge with mafic magmas as seen in the outer cores followed by 3 crystal fractionation with possible interaction with hydrothermal fluids as seen in the rim the model is compatible with the field occurrence of mafic enclaves and xenoliths

mafic magmatic enclaves and mafic rocks associated with some granitoids of the central sierra nevada batholith california nature origin and relations with the hosts abstract the calc alkaline granitoids of the central sierra nevada batholith are associated with abundant mafic rocks these include both country rock xenoliths and mafic magmatic enclaves mme that commonly have fine grained and less commonly cumulate textures scarce composite enclaves consist of either xenoliths enclosed in mme or of mme enclosed in other mme with different grain size and texture enclaves are often enclosed in mafic aggregates and form meter size polygenic swarms mostly in the margins of normally zoned plutons enclaves may locally divert schlieren layering mafic dikes which also occur in swarms are undisturbed composite or largely hybridized in central sierra nevada with the exception of xenoliths that completely differ from the other rocks host granitoids mafic aggregates mme and some composite dikes exhibit a bulk compositional diversity and at the same time important mineralogical and geochemical including isotopic similarities mme and host granitoids display distinct major and trace element compositions however strong correlations between mme host granitoid pairs indicate interactions and parallel evolution of mme and enclosing granitoid in each pluton identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution mafic dikes have broadly the same major and trace element compositions as the mme although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids the composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred respectively at depth during ascent and emplacement and after emplacement the occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas the sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes unusual mme that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement mme and host granitoids are considered cogenetic because both are hybrid rocks that were produced by the mixing of the same two components in different proportions the felsic component was produced by partial melting of preexisting crustal materials whereas the dominant mafic component was probably derived from the upper mantle however in the lack of a clear mantle signature the origin of the mafic component remains questionable

contrasting zircon hf and o isotopes in the two episodes of neoproterozoic granitoids in south china implications for growth and reworking of continental crust abstract the genetic links among rift magmatism crustal growth and water rock interaction are an important issue about mass and heat transfer between mantle and crust during supercontinent breakup a combined study of hf and o isotopes in zircons from neoproterozoic granitoids in south china provides evidence for growth and reworking of juvenile and ancient crusts with different styles of water rock interactions along rift tectonic zones two generations of the granitoids show contrasting features in both zircon hf and o isotope compositions indicating their distinct petrogenesis the 825 ma granitoids exhibit negative ehf t values of 3 4 0 8 to 1 6 0 8 with old model hf ages of 1 81 0 07 to 1 92 0 10 ga and high 18o values of 8 7 to 10 4 these indicate that the source material of granitoid magmas was derived from melting of paleoproterozoic basement that has the hf isotope signature similar to the enriched mantle but experienced chemical weathering process before anatexis reworking of ancient crust is demonstrated to occur at 825 ma in the orogenic collapse zone with overprinting of subsolidus hydrothermal alteration during magma emplacement in contrast the 760 750 ma bimodal intrusives are characterized by positive ehf t values of 3 5 0 8 to 9 9 0 8 with young model hf ages of 0 94 0 06 to 1 18 0 06 ga and both low and high 18o values of 4 2 to 6 2 relative to 5 3 0 3 for the normal mantle zircon prompt reworking of juvenile crust is suggested to occur at 750 ma in the rifted tectonic zone with occurrence of supersolidus hydrothermal alteration and local low 18o magmatism during supercontinent breakup contributions of the depleted mantle to their magma sources are contrasting in the two episodes of magmatism in association with breakup of the supercontinent rodinia while the change in melt source from the crust to the mantle keeps pace with the advance from continental rifting to supercontinent breakup significant transport of both heat and material from the depleted mantle to the continental crust only occurred along the active rifting zone in either case the growth and reworking of continental crust are episodically associated with rift magmatism

crystallization and hybridism in microgranitoid enclave magmas microstructural evidence microstructural evidence indicates that many microgranitoid enclaves mafic inclusions in granitoids crystallize from globules of magma that have undergone previous magma mixing hybridism the evidence includes 1 rounded xenocrysts ocelli of quartz rimmed with fine grained aggregates of early formed minerals especially mafic minerals 2 k feldspar megacrysts commonly rimmed with plagioclase rapakivi structure and 3 corroded cores dendritic overgrowths and calcic spikes in plagioclase initial quenching of a magma globule in felsic magma involves rapid temperature equilibration which causes a high rate of nucleation of minerals precipitating from the enclave magma at the time followed by a period of slow cooling during which the nucleation and growth rates of later crystallizing minerals are similar to the much slower rates prevailing in the granitic melt this may result in markedly poikilitic microstructures involving inclusion of the earlier formed finer grained minerals in quartz and or k feldspar which are common in microgranitoid enclaves

petrogenesis of the late yanshanian mantle derived intrusions in southeastern china response to the geodynamics of paleo pacific plate subduction abstract several late yanshanian syenitic and gabbroic rocks from the coastal area of southeastern se china have been studied to determine zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings the systematic la icp ms zircon u pb dating results indicate that they can be divided into two main stages the early stage 141 118 ma and the late stage 98 86 ma which temporally correspond to the regional lower and upper volcanic series respectively the early stage syenitic and gabbroic rocks are accompanied by vast rhyolitic and dacitic volcanic rocks with minor granites their parental magma was derived from an enriched mantle metasomatized by subducted sedimentary materials the late stage syenitic rocks mainly occur as the central intrusion of the caldera with ages close to the accompanied volcanics forming a caldera related volcanic intrusive ring complex they were produced by magma mixing between depleted asthenosphere melts and subduction related enriched mantle melts it is suggested that the late yanshanian tectonic settings of se china transformed from a compressional environment to an extensional environment corresponding to the transformation from forward to rollback subduction of the paleo pacific plate at approximately 110 ma

origin of cretaceous aluminous and peralkaline a type granitoids in northeastern fujian coastal region of southeastern china abstract in this work we report the geochemical whole rock sr nd and hf isotopes and zircon u pb age and hf o isotopic compositions for six granitic intrusions in northeastern fujian coastal region of southeastern china to elucidate the petrogenesis of aluminous and peralkaline a type granites in this region zircon u pb dating of these rocks including porphyritic syenites porphyritic granites alkali feldspar granites and arfvedsonite granites yielded late cretaceous ages of 94 98 ma mineral assemblage and geochemical features suggested that the arfvedsonite granites were peralkaline a type but the porphyritic syenites porphyritic granites and alkali feldspar granites were aluminous a type geochemical data whole rock nd hf isotopes end t 5 5 to 4 0 ehf t 3 6 to 0 and the zircon hf o isotopes ehf t 3 0 to 1 4 18o 5 4 6 0 indicated that the porphyritic syenites were produced by partial melting of crustal rocks in the lower crust the porphyritic granites and alkali feldspar granites had similar geochemical features and zircon hf o isotopic composition with the porphyritic syenites suggesting that they were formed by feldspar dominant crystal fractionation of deep crust derived syenitic magma under shallow crustal level compared with the aluminous a type granites the peralkaline a type granites had lower abundance of al2o3 mgo cao ba and sr higher abundance of sio2 and hfse as well as higher ratio of total feo mgo ga al and rb sr however the whole rock nd hf and zircon hf o isotopic compositions of the peralkaline and aluminous a type granites were similar it could be inferred that the peralkaline a type granites were formed by crystal fractionation of aluminous a type granitic magmas since the studied a type granitic intrusions were coeval with the bimodal volcanic rocks and the extensional basins in the coastal region of southeastern china they were likely produced in an extensional setting possibly during lithospheric thinning that resulted from the subduction of the paleo pacific plate beneath southeastern china

carbonate assimilation at merapi volcano java indonesia insights from crystal isotope stratigraphy recent basaltic andesite lavas from merapi volcano contain abundant complexly zoned plagioclase phenocrysts analysed here for their petrographic textures major element composition and sr isotope composition anorthite an content in individual crystals can vary by as much as 55 mol an

crystal recycling in the steady state system of the active stromboli volcano a 2 5 ka story inferred from in situ sr isotope and trace element data in situ sr isotope data by microdrilling coupled with major and trace element analyses have been performed on plagioclase and clinopyroxene from seven samples collected during the 2002 2003 eruptive crisis at stromboli volcano aeolian islands italy on 28 december 2002 the persistent moderate explosive activity was broken by an effusive event lasting about 7 months a more violent explosion paroxysm occurred on 5 april 2003 two magma types were erupted namely a volatile poor and highly porphyritic magma hp magma poured out as scoria or lava and a volatile rich phenocryst poor magma lp magma found as pumice lp magma differs from the hp magma also for its slightly less evolved chemistry the groundmass composition and the lower sr isotope ratios micro sr isotope data show the presence of zoned minerals in strong isotope disequilibrium as previously found in products erupted in 1984 1985 and 1996 ad with 87sr 86sr values generally decreasing from cores to rims of minerals only some outer rims testify for equilibrium with the host groundmass the internal mineral zones with high sr isotope ratios 0 70665 0 70618 are interpreted as antecrysts crystallised during the previous activity and recycled in the present day system since the opening shoshonitic activity of the recent period which occurred at about 2 5 ka ago this result has implications for the dynamics of the present day plumbing system of stromboli at intermediate pressure about 2 3 km depth and allows us to propose a model whereby an hp magma reservoir is directly interconnected at the bottom with a cumulate crystal much reservoir efficient mixing between residing hp and input lp magmas can occur in this reservoir due to more similar rheological characteristics of the two magmas than in the conduit where crystallisation is enhanced by degassing antecrysts and possibly melts re enter in the hp magma reservoir both from the bottom recycled by ascending lp magmas crossing the crystal mush and from the top recycled by descending degassed and dense hp magma residual of the periodic strombolian explosions at the surface the isotope variation measured in the groundmasses allows calculating the proportion of the lp magma entering the shallow hp magma reservoir at 20 from this proportion we estimate that the total volume of lp magma input during 2002 2003 closely matches the magma volume erupted in the effusive event suggesting a steady state system at broadly constant volume the comparison with estimates of the lp magma volume ejected by the paroxysm indicates that the lp magma amount directly reaching the surface during the 5 april paroxysm is minimal with respect to that entering the system

submarine geomorphology of northeast baffin bay and its implications for local paleo ice sheet dynamics abstract high resolution bathymetric data from close to the northwest greenland coast in northern melville bay northeast baffin bay reveal a range of glacial and nonglacial landforms the glacial landforms include crag and tails mega scale glacial lineations msgl rock drumlins and roche moutonnees that indicate paleo ice sheet dynamics the nonglacial landforms include steep ridges that are interpreted as volcanic dykes neoproterozoic thule dyke swarms and paleoproterozoic melville bugt dyke swarms these dykes are glacially overprinted and may have channeled ice and subglacial meltwater flow through narrow subglacial cavities some of the glacial landforms eroded into bedrock indicate a southward paleo ice stream orientation while other glacial landforms including the sedimentary depositional landforms indicate a westward paleo ice stream orientation the glacial landforms were likely produced during at least two epochs and under changing thickness of the ice streams the glacial landforms eroded into the bedrock are likely older than the sedimentary glacial landforms and were likely produced by a thin south southeastward ice stream that allowed cavity formation the westward oriented glacial landforms were likely produced by a thick ice stream that was probably active during the last glacial maximum lgm the different orientations of the glacial landforms indicate a reorganization of the ice streams that may have occurred during retreat and subsequent readvance this reorganization likely coincided with a migration of the ice divide from south to north

magma recharge processes of the yandangshan volcanic plutonic caldera complex in the coastal se china constraint from inter grain variation of sr isotope of plagioclase abstract in situ sr isotope of plagioclase can be used to elucidate the details of magmatic processes here we report in situ sr isotopic compositions of plagioclase from the intra caldera porphyritic quartz syenite and mafic microgranular enclave of the yandangshan caldera se china plagioclase phenocrysts from porphyritic quartz syenite show consistent initial 87sr 86sr ratios 0 7088 0 7091 plagioclases in groundmass of mafic microgranular enclave have low initial 87sr 86sr ratios 0 7080 0 7081 in contrast the plagioclase phenocrysts of mafic microgranular enclave show relatively variable initial 87sr 86sr ratios of 0 7081 0 7088 which exhibit distinct inter grain variation with high values similar to the plagioclase phenocrysts in porphyritic quartz syenite and low values similar to the plagioclase in groundmass however the initial 87sr 86sr ratios of individual plagioclases from phenocrysts and groundmass show insignificant intra grain variation furthermore initial 87sr 86sr ratios 0 7080 0 7088 of plagioclase from phenocrysts and groundmass of mafic microgranular enclave are within and lower than the whole rock initial 87sr 86sr ratios 0 7082 0 7088 while the values of plagioclase phenocrysts from the porphyritic quartz syenite are within and higher than the whole rock initial 87sr 86sr ratios 0 7086 0 7089 therefore we propose that the inter grain sr isotope variation of plagioclase is related to magma recharge events which induce chemical and physical exchanges between the recharge magma and the host magma and the remobilization of crystal mush in magma reservoir furthermore our study shows that whole rock sr isotopic compositions could not represent the end members and they may be a result of the assemblage of rock forming minerals such as plagioclase with different initial sr isotopic ratios

water transfer during magma mixing events insights into crystal mush rejuvenation and melt extraction processes many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal rich mushes to investigate water transfer processes in such systems following thermal equilibration we conducted 24 h experiments to establish the petrological evolution of a water undersaturated 4 wt h 2 o in the interstitial melt quartz bearing dacite crystal mush 0 5 0 8 in crystal fraction intruded by a water saturated 6 wt h 2 o initially crystal free andesite magma at 950 c and 4 kbar 12 km depth our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content specifically mafic samples dramatically crystallize during water escape into the felsic end members and consequent increase in liquidus temperature conversely the addition of water to the felsic mush reduces the liquidus temperature leading to an increase in melt fraction the experiments provide insights into how volatiles contribute to crystal mush rejuvenation i e increase of melt fraction however h 2 o diffusion alone is not sufficient to promote melt extraction from short and long lived mushes in the earth s crust

the mixing of magmas in plutonic and volcanic environments analogies and differences abstract magma mixing processes have been widely recognized in both the plutonic and volcanic environments but the quality and quantity of information that can be extracted from the two environments are substantially different understanding the advantages and limits associated with the study of plutonic and volcanic rocks is essential to establish precise methodological approaches to build the most complete conceptual model by merging information from these two complementary igneous environments in this work we review magmatic interaction processes in the plutonic and volcanic environments by considering several aspects of these geological phenomena in particular we first briefly report on the structural and geochemical evidence for magma mixing in both plutonic and volcanic rocks with the aim to provide a general picture of this natural phenomenon successively we discuss some recent results about magma mixing achieved using the concepts from chaos theory and discuss their potential impact on magma differentiation finally we attempt to build a general picture of this igneous process by merging present day information from both the plutonic and volcanic environments it emerges from the general picture that the time spent by the magmatic system in the molten or partially molten state is the crucial factor for the preservation of the fingerprints of magma mixing in the two environments we propose a conceptual model that may be useful to understand what kind of information we can obtain from volcanic and plutonic rocks and ultimately to maximize our knowledge about magma mixing

dynamics of magma mixing and degassing recorded in plagioclase at stromboli aeolian archipelago italy crystal rich materials scoriae and lava flows emitted during the 1985 2000 activity of stromboli were taken into consideration for systematic study of bulk rock matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase the most abundant mineral phase over the considered time period bulk rock composition remained fairly constant in both major sio2 49 2 50 9 wt and k2o 1 96 2 18 wt and trace elements the quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol plagioclase ranges in composition between an62 and an88 and is characterized by alternating 10 100 m thick bytownitic and labradoritic concentric layers although the dominant and representative plagioclase of scoriae is an68 the labradoritic layers an62 70 show small scale 1 5 m oscillatory zoning are free of inclusions and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass in contrast the bytownitic layers an70 an88 are patchy zoned show sieve structure with abundant micrometric glass inclusions and voids and are attributed to rapid crystal growth

the magma mixing origin of mantled feldspars the key to mantled feldspar genesis is epitaxial nucleation of plagioclase on k feldspar or k feldspar on plagioclase once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures the most common mantling is plagioclase on k feldspar which occurs in both volcanic and plutonic environments in the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non cellular shell of plagioclase in the plutonic environment early stages of plagioclase overgrowth also tend to be dendritic although with coarser grained characteristics dendritic morphology is thus a common denominator in rapakivi genesis since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma such magma mixing yields a heterogeneous system at first one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on k feldspar mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals

magma evolution and open system processes at shiveluch volcano insights from phenocryst zoning phenocryst zoning patterns are used to identify open system magmatic processes in the products of the 2001 eruption of shiveluch volcano kamchatka the lavas and pumices studied are hornblende plagioclase andesites with average pre eruptive temperatures of 840c and fo2 of 1 5 2 1 log units above nickel nickel oxide nno plagioclase zoning includes oscillatory and patchy zonation and sieve textures hornblendes are commonly unzoned but some show simple multiple or patchy zoning apatite microphenocrysts display normal and reverse zoning of sulphur the textural similarity of patchy hornblende and plagioclase together with ba sr systematics in patchy plagioclase indicate that the cores of these crystals were derived from cumulate material plagioclase liquid equilibria suggest that the patchy texture develops by resorption during h2o undersaturated decompression when h2o saturated crystallization recommences at lower pressure reduced ph2o results in lower xan in plagioclase causing more al rich hornblende to crystallize plagioclase cores with diffuse oscillatory zoning and unzoned hornblende crystals probably represent a population of crystals resident in the magma chamber for long periods of time in contrast oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization increasing fe al in oscillatory zoned rims suggests oxidation as a result of degassing of h2o during decompression a general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization we present evidence that the host andesite has mixed with both more felsic and more mafic magmas olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt sieve textured plagioclase resulted from mixing of a more felsic magma with the host andesite the mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma which implies efficient thermal quenching and thus small volumes of intruding magma mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents we suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber producing incremental variations in whole rock compositions

textural analysis of magmatic enclaves from the kameni islands santorini greece abstract the kameni islands have been the focus of historic volcanic activity on santorini greece the islands comprise a series of dacite lavas the erupted compositions of which have remained approximately constant for the last 2200 years the dacite lava flows of nea kameni contain a variety of mafic enclaves distinguished by differences in texture composition and mechanical behaviour the mafic enclaves represent chilled fragments of mafic magma that crystallised rapidly on contact with the cooler more silicic host magma the absence of chilled margins on the enclaves suggests that the replenishing magma initially formed a dense layer at the base of the chamber groundmass textures are interpreted as an indication of the degree of undercooling between the enclaves and the host groundmass textural information obtained from the enclaves has been used to determine the relative volumes of replenishing magmas injected into the host magma before eruption which indicates that the volume of erupted magma is directly proportional to the volume of replenishing magma emplaced in the chamber prior to eruption which thus may act as a potential eruption trigger the variety of replenishing magmas implies the existence of a complex conduit and chamber system beneath santorini volcano andesitic enclaves with linear crystal size distributions are thought to have originated as an aphyric andesitic melt expelled from a crystal mush by filter pressing disequilibrium phenocryst assemblages in the lavas and phenocryst bearing enclaves provide evidence for the entrainment and recycling of older phenocryst populations

magma mingling as indicated by texture and sr ba ratios of plagioclase phenocrysts from unzen volcano sw japan abstract textural and geochemical characteristics of plagioclase phenocrysts from the eruptive products of mount unzen sw japan record repeated intrusions of basaltic magma into a dacitic host magma chamber over the lifetime of the volcano an important aspect of this mixing style is the exchange of phenocrysts between the intruding basalt magma and host dacite magma and the effect that this wide reaching mixing style has on the mineralogical diversity of the erupted products plagioclase phenocrysts that originally crystallized from the host dacite magma are identified by oscillatory zoning patterns low an content cores an45 to an60 and low sr ba ratios host derived plagioclase phenocrysts are engulfed during intrusion of basaltic magma evidenced by their presence in basaltic to andesitic enclaves in response to changes in temperature and composition of the surrounding melt the engulfed plagioclases develop resorption zones which are composed of a densely packed network of micron sized glass inclusions and high an content plagioclase an72 an92 with high sr ba ratios that match those of plagioclase microphenocrysts inherent to the enclave forming magma over time host derived plagioclase phenocrysts that were once engulfed during replenishment events are recycled back to the host as enclaves disaggregate e g clynne m a 1989 the disaggregation of quenched magmatic inclusions contributes to chemical diversity in silicic lavas of lassen peak california bull new mexico bureau of mines and mineral resources 131 54 an eruption of andesite lava with no enclaves something particularly unique for unzen occurred in 1663 similar to enclaves all plagioclase phenocrysts in this lava flow are surrounded by resorption zones suggesting that the 1663 lava may represent a magma that was erupted after thoroughly mixing with the intruding basaltic using experimentally calibrated crystallization rates we estimate that phenocrysts exist in the unzen chamber a minimum of 0 5 3 months between the time of their encounter with a basaltic intrusion and eruption

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

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

processes and sources during late variscan dioritic tonalitic magmatism insights from plagioclase chemistry gesiniec intrusion ne bohemian massif poland the ge siniec intrusion strzelin massif east sudetes 307 290 ma is composed predominantly of dioritic to tonalitic rocks with sr sr ratios ranging from 0 7069 to 0 7080 and end1 4 3 1 to 4 2 emplaced as post collisional magmas following thevariscan orogeny in situ sr isotope and trace element analyses of plagioclase were carried out on five diorite tonalite samples with variable whole rock sr sr compositions to constrain magma sources and elucidate magma chamber processes plagioclase is characterized by complex zoning patterns such as patchy zoning asymmetrical zoning and strong resorption between an rich cores and more albitic rims the range of sr isotopic compositions recorded in plagioclase is 0 7069 0 7091 greater than that of the whole rocks no change in isotopic composition is observed across resorbed core rim boundaries as would be expected if the resorption was caused by magma mixing two samples are close to sr isotopic equilibrium between plagioclase and the whole rock three samples are isotopically heterogeneous and exhibit the largest variations in sr isotope ratios within high an plagioclase cores associated with only slight resorption and almost no change in an content consequently we interpret the core resorption to be due to decompression during emplacement of phenocryst bearing magmas in the upper crust rather than to magma mixing whereas we interpret the isotopic heterogeneity in the high an plagioclase cores to be due to open system processes in the lower crust the trace element and sr isotopic compositions of the high an plagioclase cores provide an insight into the lower crustal processes that took place prior to decompression we show that different diorite tonalite types cannot be related to each other by assimilation fractional crystallization processes and consequently they probably evolved as separate magma batches processes recorded in plagioclase core compositions include contamination by high sr sr crustal material and interaction with water rich magma the studied samples represent magmas that probably formed either by continuous differentiation and crustal contamination of originally more mafic magmas or by remelting of 1 3 2 1 ga basaltic materials in the lower crust this study emphasizes the importance of integrating textural and in situ data to elucidate the processes that contribute to the formation of texturally and compositionally complex plagioclase crystals in plutonic rocks

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

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

triggering explosive eruptions the case for silicic magma recharge at huaynaputina southern peru recharge by mafic magma is established as a trigger of major explosive volcanic eruptions however recharge by silicic magma can also induce perched resident magma to erupt the evidence of such recharge may be more cryptic than for mafic recharge and requires detailed petrologic and geochemical study to identify we present a study of plagioclase petrology supported by other data that implicates silicic silicic magma interaction during the eruption and evolution of the a d 1600 eruption of huaynaputina one of the largest historical eruptions recharge by silicic magma is probably more common than recognized and should be considered an important eruption trigger

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

claystone porosity and mechanical behavior vs geologic burial stress abstract a dataset was assembled from studies on 35 preserved claystone cores from different locations around the world and burial depths from

in situ sr isotopic analysis by laser ablation we demonstrate that the sr isotopic compositions of geological materials can be measured in situ through laser ablation using an icp multiple collector double focusing magnetic sector mass spectrometer mc icpms this provides rapid texturally sensitive high precision 0 004 sr isotopic analysis without the need for chemical preparation of samples the system used employs a q switched nd yag infrared laser the laser ablated material is carried by ar flow to an icp torch the resulting ions are extracted from the plasma and focused with d c quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of faraday collectors a feldspar megacryst with independently known 87sr86sr of 0 703117 13 and a modern gastropod shell with independently known 87sr86sr of 0 709170 10 were used evaluate the method both have 2000 ppm sr laser power was adjusted to give an average 88sr of 2 10 11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry tims within run precisions in 87sr86sr of 4 10 5 compare favorably with the precision from tims thirteen analyses of the cameroon feldspar megacryst yielded a weighted average 87sr86sr of 0 703106 22 in perfect agreement with the tims measurement similarly 22 analyses of the modern gastropod shell gave a weighted average of 0 709182 22 identical to the known value no memory effect was observed when switching from the cameroon feldspar sample run for several hours to the modern shell material the power of this technique is illustrated with a preliminary study of systematic differences in 87sr86sr of 0 0004 between two feldspar crystals of a long valley basalt one feldspar has identical 87sr86sr to the host basalt confirming the lack of any matrix effect with this method

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

assessment of β the compression coefficient of mudstones and its relationship with detailed lithology abstract the mechanical compaction of soils and fine grained sediments is commonly described in soil mechanics by a relationship between void ratio and effective stress the relationship is defined by two parameters derived from the slope of the line relating void ratio and log effective stress and e the void ratio at a chosen effective stress e g 100 kpa and e are strongly correlated and are related to the atterburg liquid limit which is itself related in a general way to the grain size and mineralogy of the sediment in general finer grained sediments have higher initial void ratios and compact faster than coarser sediments porosity depth trends for mudstones in sedimentary basins are therefore strongly influenced by lithology steps or changes in the slope of porosity depth trends can be caused by lithological change rather than changes in the pressure regime a method is presented to evaluate for mechanically compacted mudstones using porosities determined from the density log fdc and based on the relationship between effective stress and void ratio developed in soil mechanics the method allows one to obtain rapidly approximate values for materials for which it is essentially impossible to obtain compaction parameters experimentally calculated values for mudstones in wells 8 3 2 and 30 2 1 on the norwegian margin vary between 0 2 and 0 65 the values correlate positively with the clay particle content of the mudstones which we have determined for 50 mudstone samples from the same wells the strong link between mudstone lithology and suggests that a downhole log may be a useful way to visualize downhole variations in mudstone lithology the log also shows how lithology affects the rate of porosity loss with depth or effective stress for example porosity in mudstones within both wells 8 3 2 and 30 2 1 is essentially constant over 700 m sections of normally pressured mudstones the log and grain size data indicate that this reflects changes in the lithological character and of the muds and not as commonly inferred a change in the pressure regime

intra grain sr isotope evidence for crystal recycling and multiple magma reservoirs in the recent activity of stromboli volcano southern italy over the last several hundred years stromboli has been characterized by steady state strombolian activity the volcanic products are dominated by degassed and highly porphyritic hp magma black scoria bombs lapilli and lava flows of basaltic shoshonitic composition periodically about one to three events per year more energetic explosive eruptions also eject light coloured volatile rich pumices with low phenocryst content lp magma that have more mafic compositions than the hp magma an in situ major and trace element and sr isotope microanalysis study is presented on four samples chosen to characterize the different modes of activity at stromboli a lava flow 1985 1986 effusive event a scoria bomb from the normal present day activity of stromboli april 1984 and a scoria and coeval pumice sample from a recent more explosive eruption september 1996 plagioclase an

preferential eruption of andesitic magmas through recharge filtering at subduction zones andesite lavas erupt in greater abundance than the individual magmas that mix to produce them textural and geochemical analyses of andesites from mount hood oregon reveal that injection and mixing of iron and magnesium rich magma in the magma chamber can initiate the volcanic eruption andesitic volcanic rocks are common in subduction zones and are argued to play an important role in the formation and evolution of the continental crust at convergent margins1 2 3 4 andesite formation is dominated by mixing between iron and magnesium rich mafic magmas and silica rich felsic magmas1 2 4 the abundance of andesites in many subduction zones suggests they erupt in preference to the magmas that mix to produce them4 however the reasons for this remain unclear here we use textural and geochemical analyses of andesites from mount hood oregon to show that eruptions are closely linked with episodes of mafic recharge the intrusion of mafic magma into a shallow felsic magma reservoir the felsic and mafic magmas involved rarely erupt by themselves probably because the former are too viscous and the latter too dense mafic recharge overcomes these barriers to eruption and as it also promotes efficient mixing results in preferential eruption of mixed andesitic magmas the abundance of andesites therefore relates to local crustal conditions and the ability of magmas to erupt we suggest that volcanoes such as mount hood that erupt homogeneous andesitic compositions through time are those that are the most reliant on mafic recharge to initiate eruptions

zircon trace element constrains on the link between volcanism and plutonism in se china abstract trace elements in igneous zircon crystals are variable within single crystals or amongst populations of crystals and are particularly sensitive to changes in melt composition in this study we present zircon trace element data from both volcanic and plutonic rocks of the yandangshan caldera complex from sechina the erupted rhyolitic rocks are interpreted to represent the most differentiated magma whereas the intruded porphyritic quartz syenites are thought to be representative of the residual crystal mush the zircon crystals of both lithologies are subdivided into two types based on their internal structures type a with a distinct oscillatory growth zoning and type b crystals are bright in cathodoluminescence cl bright type b zircon occurs as rims of type a zircon or as individual grains compared to type a type b zircons commonly show lower hf u th and y concentrations and higher ti and zr hf and eu eu ratios the zircon trace element compositions of the volcanic samples generally overlap those of the intracaldera porphyritic quartz syenite and mafic microgranular enclaves that contain both type a and type b zircons despite a general overlap zircon grains from the volcanic rocks show relatively higher hf contents and y dy ratios lower ti contents and th u ratios and larger negative eu anomalies than the plutonic samples we suggest that the inter and intragrain compositional variability of zircon crystals can be used to track the evolution of magmatic systems we show that in our case the volcanic and plutonic rocks crystallized continuously during the magmatic evolution in a common magma system and that the magmas of these rocks may be linked by fractional crystallization furthermore the type b zircon grains are inferred to have crystalized in relatively hot and less evolved magma indicating a magma recharge event consequently the geochemical variability of igneous zircon crystals can be used to infer on the magmatic evolution of single volcanic system which may also be applicable in other geodynamic environments

volume changes during diagenesis abstract the literature pertaining to volume change during diagenesis of clastic sediments is reviewed with respect to the problem of calculating pre and syn compaction thicknesses of sediments for basin reconstruction and stragraphic correlation studies four major mechanisms for volume change are identified mechanical compaction mechanical dissolution chemical dissolution and phase change the first two of these are found to be strongly dependent on the effective stress whilst the latter two show at least a pressure dependence quantification of the relationships between porosity and depth of burial of a sediment seems to be possible only for specific examples of the first of these processes at present this quantification is dealt with in the accompanying related publication

tracing magma mixing in granite genesis in situ u pb dating and hf isotope analysis of zircons in situ zircon u pb and hf isotopic data have been determined for mafic microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china in order to constrain the sources and petrogenesis of granites the zircon u pb age of the enclaves 120 1 ma is identical to that of the host monzogranite 120 1 ma establishing that the mafic and felsic magmas were coeval the hf isotopic composition of the enclaves ehf t 4 5 to 6 2 is distinct from the host monzogranite ehf t 15 1 to 25 4 indicating that both depleted mantle and crustal sources contributed to their origin the depleted mantle component was not previously revealed by geochemical and nd and sr isotopic studies showing that zircon hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information some wall rock contamination is indicated by inherited zircons with considerably older u pb ages and low initial hf isotopic compositions hafnium isotopic variations in early cretaceous zircons rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data

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

geological perspectives on consolidation of clay rich marine sediments 1 experimental consolidation of uncemented clay rich marine sediments provides information concerning their stress history a main finding is that some of the well known behavior of soft sediment deformation in geotechnical applications cannot validly be extrapolated to sediments that have been subjected to higher stresses and longer times of geologic conditions this study confirms that the yield stress of the uncemented sediment accurately reflects its previous maximum consolidation state furthermore we have identified a new phase of post yield strain that is associated with higher values of the modified compression index the slope of the porosity versus the logarithm of effective vertical stress than that of elastic deformation but with much lower values than that for primary consolidation this post yield behavior is a linear non elastic deformation and is termed tertiary consolidation yield stress appears independent of creep time or strain rate whereas the tertiary primary consolidation transition is sensitive to these parameters during post yield creep secondary consolidation the slope of the porosity versus the logarithm of time curve or the secondary consolidation index is generally assumed constant however this is not valid for claystones at effective vertical stresses above about 1 mpa where the secondary consolidation index increases with stress at a given effective vertical stress the secondary consolidation index also increases with creep times greater than about 105 s 28 h

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

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

experimental and textural constraints on mafic enclave formation in volcanic rocks abstract we have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at southwest trident volcano alaska andesite enclaves are present throughout the four dacite lava flows produced by the eruption and resemble mafic enclaves commonly found in other volcanic rocks our experiments replicate the pressure temperature time path taken by enclave forming andesite magma as it is engulfed in dacite during magma mixing pressure and temperature information for the andesite and dacite are from coombs et al contrib mineral petrol 140 2000 99 118 the andesite was annealed at 1000 c and then cooled to 890 c at rates of 110 c h 1 10 c h 1 and 2 c h 1 once cooled to 890 c andesite was held at this lower temperature from times ranging from 1 to 40 h the andesite that was cooled at the slower rates of 2 c h 1 and 10 c h 1 most resembles enclave groundmass texturally and compositionally based on simple thermal calculations these rates are more consistent with cooling of the andesite groundmass below an andesite dacite interface than with cooling of enclave sized spheres if enclaves do crystallize as spheres post crystallization disaggregation must occur calculations using the melts algorithm ghiorso and sack contrib mineral petrol 119 1995 197 212 show that for incoming andesite to become less dense than the dacite 34 volume of its groundmass must crystallize to undergo 18 volume vesiculation these values are similar to those determined for southwest trident enclaves thus such crystallization may lead to flotation of enclaves and be a viable mechanism for enclave formation and dispersal the residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments decompression experiments on southwest trident dacite suggest an average ascent rate for the eruption of 2 3 mpa h 1 an andesite experiment that was cooled and then decompressed at this rate contains melt that matches that of the natural enclaves it is apparent that decompression ascent induced crystallization occurs in enclaves but not in the form of microlites as happens in the dacite host due either to insufficient residence time at chamber temperatures or to the pre existing microphenocrysts which act as sites for new growth

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