petrogenesis of the greenhills complex southland new zealand magmatic differentiation and cumulate formation at the roots of a permian island arc volcano a permian 265 ma intrusive complex which formed as a magmatic feeder reservoir to an immature island arc volcano is fortuitously exposed in southern new zealand known as the greenhills complex this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes cumulates which include dunite olivine clinopyroxenite olivine gabbro and hornblende gabbro norite are related products of parent magma fractionation both primary magmatic and secondary platinum group minerals occur within dunite at one locality using the composition of cumulus minerals mafic dykes and melt inclusions we have determined that the parent magmas of the complex were hydrous low k island arc tholeiites of ankaramitic affinities progressive magmatic differentiation of this parent magma generated fractionated melt of high alumina basalt composition which is now preserved only as dykes which cut the complex field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate rock formation recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface similar processes are inferred to account for volcanic rock compositions in other parts of this arc terrane and in modern island arc systems

igneous layering through oscillatory nucleation and crystal settling in well mixed magmas oscillatory or competitive nucleation about a binary or perhaps pseudo binary eutectic and ensuing crystal growth and settling is a commonly suggested means of producing layering in magmatic systems a quantitative model is presented of this outwardly relatively simple process of crystal nucleation growth and settling in an otherwise initially crystal free magma avrami style kinetics of crystallization in an always wellmixed body buried in conductive wall rock are coupled to a stokes like formulation of crystal settling in magma whose viscosity depends on temperature and crystallinity two dimensionless numbers se the settling number and av the avrami or kinetic number govern all the results av and se measure the relative importance of crystallization time and settling time respectively relative to the overall cooling time for any value of av which increases strongly with the maximum nucleation and growth rates and cooling time layering is possible only over a range or window of values of se both above and below this window a single layer crystalline below vitric above forms and within this window the number of layers increases systematically with increasing av and se grain size within any single layer generally coarsens upward because the characteristic settling and cooling times both depend on body thickness the lower limit of the settling window is also dependent on sheet thickness within the confines of this model and for nucleation and growth rates set by those observed in natural systems layering is unlikely in sheet like magmas thinner than about 100 m when the body is not always well mixed and crystallization is within inward propagating solidification fronts it is expected that this minimum body thickness will increase

draugen oil field haltenbanken province offshore norway the draugen oil field lies in block 6407 9 in the haltenbanken oil and gas province the field is located 150 km off the coast of norway and 200 km south of the arctic circle in water depths of 240 280 m the field was discovered in 1984 by well 6407 9 1 five more exploration appraisal wells and two dimensional seismic assisted in delineating the reservoir the field is hosted by a low relief north south trending anticline measuring some 20 x 6 km the reservoir lies at a depth of 1 600 m subsea and has an oil column of 40 m the main reservoir is the late jurassic rogn formation sandstones interpreted as a shallow marine sand bar the formation pinches out to the west and east and is capped by spekk formation shales a separate smaller accumulation has also been proved in middle jurassic gam formation sandstones in the western part of the field sand quality in both reservoirs is good to excellent field stoiip is estimated at 180 million sm sup 3 of oil expectation of recoverable reserves is 67 million sm sup 3 government approval for field development was given in december 1988 the field will be developed with a concrete gravity base structure and offshore loading more the initial development plan calls for six oil producers and six subsea water injectors the platform will be installed in the summer of 1993 with first oil shortly thereafter the planned plateau production rate is 14 300 sm sup 3 day dry oil pending a gas offtake solution for the haltenbanken region produced associated gas will initially be reinjected into a water bearing structure in the western part of the field less

gravity driven and in situ fractional crystallization processes in the centre hill complex abitibi subprovince canada evidence from bilaterally paired cyclic units abstract the formation of layers in mafic intrusions has been explained by various processes making it the subject of much controversy the concept that layering originates from gravitational settling of crystals has been superseded in recent years by models involving in situ fractional crystallization here we present evidence from the centre hill complex that both processes may be operative simultaneously within the same intrusion the centre hill complex is part of the munro lake sill an archean layered mafic intrusion emplaced in volcanic rocks of the abitibi subprovince the centre hill complex comprises the following lithostratigraphic units six lower cyclic units of peridotite and clinopyroxenite a middle unit of leucogabbro six upper cyclic units of branching textured gabbro btg and clotted textured gabbro ctg the uppermost of these units being overlain by a marginal zone of fine grained gabbro the cyclic units of peridotite clinopyroxenite and btg ctg are interpreted to have formed concurrently through fractional crystallization associated with periodic replenishment of magma to the chamber the units of peridotite and clinopyroxenite formed by gravitational accumulation of crystals that grew under the roof the cyclic units of btg and ctg formed along the upper margin of the sill by two different mechanisms 1 layers of btg crystallized in situ along an inward growing roof and 2 layers of ctg formed by accumulation of buoyant plagioclase crystals the layers of btg are characterized by branching pseudomorphs after fayalite up to 50 cm in length that extend away from the upper margin the original branching crystals are interpreted to have grown from stagnant intercumulus melt in a high thermal gradient resulting from the injection of new magma to the chamber

changing techniques and paradigms for the evaluation of magmatic processes the study of magmatic processes is at a juncture new continuum models for the complex physics of crystallization and melting with convection and crystal melt segregation are now available and the models are amenable to exploration by numerical methods these models which were largely developed in the fields of material science and mechanical engineering will allow petrologists to consider a wide range of transport phenomena crystal settling with reaction as fully multiphase flow combined thermal and compositional buoyancy with realistic material property variations open system and eruptive behavior and the appropriate system geometry and boundary conditions the traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal melt mush subject to reintrusion or mobilization percolative flow and reaction in this mush including contaminants may be as important as near liquidus processes in driving petrological diversity thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes melting of the crust following intrusion by basalt can be considered in light of two end members the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes although more commonly cartooned the large sill type may not be nearly as efficient as repeated diking in driving crustal melting magma mixing and mingling the greatest challenge for those studying magma dynamics will be to key the models to well constrained geological examples and to raise the sophistication of the content of these models in light of well defined tests that have geological significance

magmatic fractionation by compositional convection in a sheet like magma body constraints from the nosappumisaki intrusion northern japan the mechanism of fractionation in a sheet like magma body was investigated based on observations from the nosappumisaki intrusion northern japan this is one of a number of late cretaceous shoshonite sills in the nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing 50 vol cumulus crystals and a overlying middle zone free of such cumulus crystals these zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones it is inferred that massive crystal settling or flotation from a magma initially laden with 20 vol of crystals primocrysts in a sheet like magma body formed a cumulus pile and an overlying crystal free melt layer after the formation of the compound structure with partial freezing of the upper and lower marginal zones the internal part of the intrusion slowly solidified during this period the melt in the central part of the intrusive sheet underwent fractional crystallization this is shown by patterns of vertical variation in the composition of the interstitial melt the overall patterns are inverted mirrorimage s shaped and s shaped for incompatible and compatible elements respectively the incompatible element abundances show maxima near the bottom of the middle zone which corresponds to the final solidification horizon they show minima in the cumulate zone which suggests discharge of an evolved melt from the crystal pile fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer the melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile this conclusion is based on the following observations 1 there is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone 2 leucocratic pipe like structures which represent relict pathways for the evolved melt are developed in the cumulate zone these structures increase in diameter decrease in number density and increase in abundance of incompatible elements with height 3 cumulus augite in the cumulate zone has ca rich and al poor dissolved rims the extent of dissolution increases with stratigraphic height suggesting downwelling of an h2o rich melt from the overlying melt layer 4 there is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone 5 the initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical a mass balance model for compositional convection was constructed to quantify the melt transportation processes model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions the optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt the initially formed compound structure is a highly preferential environment for compositional convection

replenishment crystal accumulation and floor aggradation in the megacrystic kameruka suite australia detailed field evidence indicates that the kameruka suite plutons of the bega batholith eastern australia grew by crystal accumulation on the floor of a magma chamber depositional features in the plutons including mafic enclave channels asymmetric enclave pillows and exotic rafts load casts and flame structures and graded and trough cross beds indicate that the pluton built progressively upward the general eastward dip of depositional features in the main pluton implies a lower western and upper eastern contact consistent with a basal granite migmatite contact in the west and a sharp hornfelsic sidewall contact in the east mafic felsic and composite dykes most common near and below the basal western contact are interpreted as conduits for magma chamber replenishment and imply open system behaviour during pluton construction textural relations are also consistent with an open system cumulate origin typically centimetre scale grains of quartz plagioclase and megacrystic alkali feldspar form a touching framework with interstices filled with smaller biotite flakes and smaller intercumulus quartz and feldspar crystals alkali feldspar megacrysts vary from euhedral and unzoned to mantled and partially replaced by plagioclase to ovoid and completely pseudomorphed by quartz albite aggregates the common occurrence of mantled and pseudomorphed alkali feldspar in mafic enclaves and in hybrid tonalitic rocks forming the matrix to enclave swarms suggests that replacement or resorption of granitic primocrysts was associated with mafic replenishments the occurrence of all megacryst types at outcrop scale implies extended alkali feldspar crystallization in different parts of the chamber thorough stirring during or after periodic replenishment and final settling in a cumulate mush the bulk composition of the cumulate mush represented by granodiorite cannot represent the emplaced magma compositional variation can be modelled by variable degrees of crystal accumulation from a parental silica rich melt represented by the silicic dykes as dykes periodically fed the magma chamber crystals accumulated on the floor and more evolved melts probably erupted from its roof thus the average composition of the magma and the cumulus minerals may have remained relatively constant and the sublinear chemical trends that typify the kameruka suite simply reflect differing proportions of melt and cumulate material sublinear chemical trends can also be explained by a restite model however the distinctive ba light rare earth element and zr spikes at high silica can be explained only by a cumulate model which also explains why the low silica granites of the suite share the same chemical characteristics as the high silica granites

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

the crystallization of lava lakes thermal convection driven by the kinetic undercooling of solidification can produce compositional and textural variations in an initially uniform lava flow that is cooled predominantly from above we show that such kinetic undercooling is sufficient to drive convection at high rayleigh numbers even in shallow lava lakes we then employ a theoretical model that includes the effects of convection at high rayleigh number to make predictions of the complete evolution of the lava we show that predictions of the rate of growth of solid crust at the surface of the lake do not differ much from predictions made by ignoring effects due to convection however convective motions do shorten the time for complete solidification because when they are driven by kinetic undercooling there is internal crystallization in addition to that occurring near the cooled upper boundary of the lava this important effect which is absent from purely conductive models predicts stratification and zonation of the lava in agreement with field observations we also examine the effect of taking into account the heat transfer to the country rock below the lava lake and determine how the evolution of the lava lake varies with the viscosity of the lava

particle dynamics and aggregation of crystals in a magma chamber with application to kilauea iki olivines abstract crystals settling in relative proximity to each other will settle faster than their respective stokes velocities u s due to their mutual interactions pairs of equant spheres within 100 diameters of each other are accelerated to 1 55 u s modification of han and lawler s han m lawler d f 1991 interactions of two settling spheres settling rates and collision efficiency journal of hydraulic engineering 117 10 1269 1289 equations to include inertia shows that the velocities of a pair of spheres differ and change during descent allowing them to come into contact in quiescent settling experiments with olivine shaped clay prisms the pairs of prisms usually settled faster than their u s the clay prisms aggregated 80 of the time with the pairs of prisms rotating and tumbling as they fall coming into and out of contact size variations in kilauea iki olivines control aggregation during quiescent settling the settling velocities of individual crystals are variable increasing when they are incorporated into aggregates a suspension of olivine crystals more than 0 01 vol in a magma will settle slower than the average stokes velocities except when a buoyant phase is present addition of a buoyant phase such as co 2 bubbles can increase the settling velocity of the olivines when the total concentration olivine plus bubbles e is e 35

preeruption conditions and timing of dacite andesite magma mixing in the 2 2 ka eruption at mount rainier analytical field and experimental evidence demonstrate that the mount rainier tephra layer c 2 2 ka preserves a magma mixing event between an andesitic magma whole rock sio 2 content of 57 60 wt and a dacitic magma whole rock sio 2 content of 65 1 wt the end member andesite a mix of an injected and chamber andesite and dacite can be characterized on the basis of the homogeneity of the matrix glass and phenocryst rim compositions many pumices however contain mixtures of the end members the end member dacite contains a microlite free matrix glass with 74 77 wt sio 2 orthopyroxene rims of mg 57 64 clinopyroxene rims of mg 66 74 and plagioclase rim anorthite contents of an 45 65 the temperature and oxygen fugacity from fe ti oxide compositions are 930 10 c and 0 5 0 75 log units above nno the mixed andesite contains mg 73 84 orthopyroxene rims mg 73 78 clinopyroxene rims an 78 84 plagioclase rims and mg 67 74 amphibole rims the temperature from fe ti oxides hornblendeplagioclase and two pyroxene geothermometry is 1060 15 c and the oxygen fugacity is approximately one log unit above nno for the injected andesite the chamber andesite is estimated to be a magma with a 64 65 wt sio 2 melt at 980 c and a nno oxygen fugacity we conclude that the andesitic and dacitic magmas are from separate magma storage regions at 7 km and 2 4 km due to differences in the bimodal whole rock matrix glass and phenocryst compositions and the presence or absence of stable hornblende the time involved from the mixing event through the eruption is limited to a period of 4 5 days based on fe ti oxide reequilibration phenocryst growth rates and hornblende breakdown the eruption sequence is interpreted as having been initiated by an injection of the 1060 15 c andesitic magma into the 980 c 7 km andesite storage region the mixed andesitic magma then intersected a shallow 2 4 km dacitic storage system on its way toward the surface the eruption became more dacitic over time and the final products some show evidence of partial reequilibration between the andesite and dacite

magma dynamics at the base of an evolving mafic magma chamber incompatible element evidence from the partridge river intrusion duluth complex minnesota usa abstract a characteristic feature of the partridge river intrusion of the keweenawan duluth complex is the approximately fivefold to ninefold increase in the concentrations of incompatible elements in the lower zone compared with cumulates stratigraphically higher the concentrations of incompatible elements decrease from the lower zone upward to steady state values which is ascribed to variations in the proportions of trapped liquid rather than variable degrees of fractional crystallization of a single parental magma the calculated average composition of trapped liquid using our algorithm is similar to typical keweenawan low alumina high ti p basalts associated with the duluth complex but is different from the leading edge ferrodioritic liquid quenched in the chilled margin of the intrusion this difference suggests that the chilled margin does not represent the original parental magma composition from which the whole intrusion solidified and that the enrichment of incompatible elements may be related to the local flotation of magmatic suspensions to test the latter hypothesis numerically we have used heat mass transfer models assuming a sheet like magma chamber to calculate the parameters of the model that best reproduce the observed distribution of incompatible elements in a mush zone at the base of the partridge river intrusion the results indicate that a mush zone enriched in the incompatible elements is produced if the velocity of movement of the lower solidification front into the magma body was less than the floating velocity of the bulk crystal mush the dynamic parameters that best reproduce the observed distribution of incompatible elements include a magma emplacement pressure of 2 kbar critical crystallinities of 50 68 in the mush zone from which the liquid is being expelled and an emplacement temperature of 1160 c for the initial magma

multistage magma mixing revealed in phenocryst zoning of the yunokuchi pumice akagi volcano japan detailed study of the texture and chemical zoning of phenocrysts in the compositional and thermal histories at the sites a plinian fall deposit yunokuchi pumice from akagi volcano where they grew only recently have such studies been japan permits reconstruction of the subvolcanic magma chamber undertaken focusing on magma chambers as the sites hypersthene phenocrysts are complexly zoned some have spongy of phenocryst formation tait 1988 de silva 1989 cellular textures whereas others exhibit simple reverse zoning augite umino iwano 1992 fodor et al 1993 nakada et phenocrysts have texturally similar cores but al and ti vs mgal 1994 the magma chambers depicted by these number zoning patterns are correlatable with hypersthene cores studies are mainly composed of phenocryst poor magma most plagioclase phenocrysts have a uniform calcic core enclosed surrounded by a comparatively thin crystallizing boundby a normally zoned rim others have variable an contents and ary layer phenocrysts are interpreted to have crysmottled textures these characteristics and melt glass compositions tallized either within the main body of magma or in suggest a complex multistage mixing of low t 940 960 c situ within the boundary layer highly silicic and high t 980 1060 c less silicic magmas combination of geophysical and petrological data stratigraphic variations of the phenocryst types show that the earlier together with fluid dynamical simulations of magmatic erupted magma inherited phenocrysts from more high t magma processes beneath mid oceanic ridges has shown that a components we propose the presence of an inhomogeneous mushy mush of crystals and interstitial melt 25 55 vol chamber filled with a highly silicic magma which was recharged crystals probably makes up the majority of the magma periodically by less silicic magmas the large viscosity contrast and chamber and only a small volume exists as a meltdensity balance of the new magma relative to the mush controlled enriched lens e g sinton detrick 1992 nicolas et al the efficiency of mixing and the position of emplacement of the high1993 barth et al 1994 likewise it is highly probable t magma in the mush tapping the chamber first expelled the that magma chambers beneath island arc volcanoes fluidal mixed magma followed by progressive erosion of the mush largely consist of a mush of crystals and interstitial melt as the eruption continued recent studies on arc volcanic products have presented evidence for the existence of such mushy magma chambers these include an estimation of very high crystallinity

evolution of double diffusion convection in a felsic magma chamber abstract the onset of double diffusion convection ddc is modeled in a two dimensional case in respect to magma chambers the viscosity model for the melt takes into account the effects of temperature and concentration of the dissolved component h 2 o the upper boundary of the convecting magma chamber is assumed to be anhydrous and at constant temperature whereas the lower boundary is treated as being hydrous permeable with a temperature greater than that within the upper boundary the case of positive compositional and thermal buoyancy of melt is studied assuming a h 2 o diffusion coefficient small in comparison with thermal diffusivity the ddc has been modeled using a system of equations solved by the finite difference method on a square grid the convective pattern evolution has been studied for fixed boundary conditions as well as for cooling and degassing due to the higher viscosity in the upper zone the upper boundary layer is thicker than the lower one the variation of water concentration in this zone of the convective cell can be significant in nature the high gradient of water concentration can be responsible for the observed variations of water content in minerals crystallized from a granite melt e g biotite because of a high lewis number 100 temperature variations in the magma chamber decay much faster than the water concentration in this case the intensive convection can continue at a constant temperature due to the non zero water content in the chamber in principle the effect can be applied to the formation of magmatic bodies if the cooling and degassing system reaches a uniform temperature distribution prior to the crystallization temperature water content throughout the body may still remain variable

mechanism of crystal redistribution in a sheet like magma body constraints from the nosappumisaki and other shoshonite intrusions in the nemuro peninsula northern japan processes of crystal separation in a magma heavily laden with crystals without phase change are investigated from observations on frozen magma systems nosappumisaki and other shoshonite intrusions in the nemuro peninsula japan for which the origin of the crystals and the initial conditions are well constrained the nosappumisaki intrusion is 120 m in thickness and extends for more than 1 5 km it exhibits a wide range of lithological variation principally as a result of crystal redistribution after intrusion crystals in each lithology can be clearly divided into two kinds according to their composition and texture those present before the intrusion of the magma phenocrysts and those that crystallized in situ after intrusion from the vertical change in mode and size of phenocrysts it is shown that 1 augite phenocrysts were rapidly deposited with little overgrowth after intrusion by significant coagulation or clustering on a time scale of more than a few years and 2 plagioclase phenocrysts definitely denser than the melt but concentrated in the upper level floated by counter flow of massive deposition of augite phenocrysts these results indicate that in a magma heavily laden with crystals of a few millimeters in size 20 vol crystal crystal and crystal melt interaction play an important role in the separation of crystals from the host melt

convection and crystallization in magma cooled from above calculations are presented for the cooling from above of melts in the di an system in which the kinetics of crystallization are incorporated and play a dominant role the results indicate that even with no initial superheat whatsoever convection plays an important role in the cooling of magma chambers and allows substantial internal cooling crystallization and differentiation the calculations show in agreement with observations that in magma bodies hundreds of meters thick crystallization occurs predominantly in the interior or at the floor even though heat is lost only from the roof the ratio of the final thickness of the layer formed at the floor to that formed at the roof increases as the overall size of the chamber increases owing to the effects of convection

construction of a pluton evidence from an exposed cross section of the searchlight pluton eldorado mountains nevada a top to bottom cross section of the searchlight pluton is exposed in a large tilt block in the colorado river extensional corridor of southern nevada hornblende barometry and geologic relations indicate that the pluton was about 10 km thick extending from approximately 3 to 13 km depth the pluton is stratified with about 2 km of fine grained quartz monzonite below the roof 2 km of granite in the center and 6 km of coarser more mafic quartz monzonite at the bottom the lower unit has a pronounced magmatic foliation that was subhorizontal prior to tilting contacts between the units are gradational over a few centimeters to about 20 m geometry field relations and elemental and isotopic data suggest that the three units mark the terminal stages of evolution of magma that filled a very thick magma chamber the upper unit formed as a solidification front that migrated downward from the roof and the middle unit granite and the lower unit represent complementary segregated melt and crystal rich cumulate thus the dominant part of the pluton appears to have solidified in monotonic fashion from more or less uniform magma that was compositionally similar to the upper unit the basal quartz monzonite and the lower part of the granite enclose widely scattered synplutonic hornblende gabbro and diorite pods that range from centimeter to kilometer scale these rocks are commonly fine grained and formed as quenched mafic melts they are isotopically distinct from the main sequence and represent discrete injections into the magma chamber the mafic magmas were contaminated by but did not strongly affect the main sequence magma the nd and sr isotopic compositions of the main sequence magma are intermediate between those of regional crust and enriched mantle derived basalts like those represented by the mafic pods this suggests that the principal magma was a hybrid with about 60 mantle component and thus the pluton represents both addition to and internal reorganization of the crust

comparison of lake and land tephra records from the 2015 eruption of calbuco volcano chile tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies however their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit to assess the reliability of data from lake cores we compare tephra deposited on land during the 2015 eruption of calbuco volcano in chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits in general the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar as anticipated however cores sampled close to here within 300 m of fluvial inflows were affected by sediment deposition from the lake s catchment they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading cores 850 m away from the inlet were not affected we consider our results in the context of the particle settling regime as well as each lake s location bathymetry and catchment area we find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles we conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux as well as lake features such as bathymetry and catchment area

abrupt change in magma liquidus temperature because of volatile loss or magma mixing effects on nucleation crystal growth and thermal history of the magma the liquidus temperature of magma that reaches shallow levels introduction during its ascent may change abruptly as a result of the release of on its way from the source region into the earth s volatiles or through magma mixing or a combination of both crust or even onto the earth s surface magma travelling immediately after this abrupt change of the liquidus temperature through the mantle and crust evolves and one of the occurs a disequilibrium is observed in the melt and here it is shown goals in petrology is to characterize the various processes that the melt rapidly re equilibrates to its thermodynamic equilibrium that occur during this passage of special interest in the a quantitative model for cooling and crystallization of a simple framework of this study are shallow level processes such two component model melt is used to investigate such re equilibration as magma crust interaction magma mixing deprocesses the relaxation time defined as the time required for the volatilization and depressurization that change the geosystem to re equilibrate after varying degrees of liquidus temperature chemical signature of the melt its crystal content its perturbations is found to be 3 or significantly less than the liquidus temperature its flow behaviour and its erupttime it takes to crystallize 40 of the melt regardless of the ability whereas crustal contamination of magma can be amplitude of the perturbation associated with the re equilibration traced through for example trace element and isotope process is an increase in crystal fraction that can easily reach studies e g wilson 1989 and is discussed extensively 10 depending on the amplitude of the perturbation during the in the other papers of this special volume cooling compensation of the liquidus temperature perturbation the temdepressurization and possibly magma mixing strongly perature of the melt remains nearly constant or may even increase slightly depending on the latent heat budget possibly heat of mixing affect the thermal history of magma rising in the mantle and heat absorbed during volatile exsolution which suggests and crust for example any pressure decrease in volatilecrystallization of the melt without cooling saturated systems not uncommon in natural systems especially for more evolved compositions will lead to volatile exsolution and consequent increased liquidus temperature and change in the melting range one of the major consequences of such behaviour is isothermal crystallization which was recognized almost 40 years ago by tuttle bowen 1958 pp 67 and 68 and the

boundary layer fractionation constrained by differential information from the kutsugata lava flow rishiri volcano japan detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber kutsugata lava a suitable target of such study is a quaternary alkali basalt 51 3 53 2 wt in sio2 from rishiri volcano northern japan despite the narrow range in the whole rock compositional variation chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition in the north lava 51 3 51 9 wt in sio2 the less differentiated portion of the kutsugata lava most crystals which include low ni olivine and plagioclase were derived from the mushy boundary layer the main part of the magma body was principally aphyric 30 vol by such mechanisms as compaction and compositional convection this fractionated melt was mixed with the overlying main magma causing differentiation of the kutsugata magma the average temperature of the extracted melt is 1010 c significantly lower than 1100 c estimated for the main magma a quantitative model of magmatic differentiation which includes thermal and compositional evolution of a mushy boundary layer can successfully reproduce the observed compositional trends of the north lava

settling driven convection a mechanism of sedimentation from stratified fluids convection driven by sediment particles may play an important role in sedimentation from the base of buoyant hypopycnal plumes for example fluvial plumes in stratified estuaries and lakes black smokers on the ocean floor volcanic clouds and coastal currents in addition to the well known double diffusive convection mechanism another mode of convective instability development is by settling across the density interface we performed laboratory experiments to investigate this fingering convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed a simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal plume formation at a flat heated plate this model which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid is in good agreement with measurements of finger size and instability wavelength from visualization experiments since fingering at the density interface drives larger scale convection in the fluid below a mass balance model of the lower layer assuming strong mixing i e uniform sediment concentration is successfully applied to predict sediment concentration in the water column and deposition at the bed strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities as convection proceeds the sediment concentrations in the two layers approach each other and convection will die out using the model equations we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column

post emplacement melt flow induced by thermal stresses implications for differentiation in sills abstract we present the first steps of a new explanation model for differentiation in sills using a combination of geochemical data and field observations numerical modeling and dimensional analysis geochemical data from a saucer shaped dolerite sill intruded into the karoo basin south africa reveal a differentiation process which causes d shaped profiles the geometry name is based on the variation in whole rock mg number mg mg mg fe from floor to roof in a sill the d shaped geochemical profiles represent sheet intrusions with the most primitive composition i e high mg in its center and progressively more evolved composition i e low mg towards the upper and lower margins the differentiation is reversed compared to the normal differentiation produced by fractional crystallization c shaped profiles c shaped profiles are believed to be formed by segregation of crystals from the magma we propose that the opposite the d shaped profile may result from melt segregation from the crystal mush this is achieved by porous melt flow through a consolidated crystal network after the main phase of emplacement and before complete solidification we show that a significant flow is feasible under natural occurring conditions an underpressure of magnitude 108 pa develops at the cooling margins due to volume reduction of the crystallizing porous melt the resulting pressure gradient is the driving force for the melt flow towards cooling margins considered in this work as a result the margins will be enriched in the incompatible elements associated with the melt phase while the center will be depleted we show that the amount of flow is primarily a function of viscosity of the melt and permeability of the crystal network which in turn is a transient phenomenon dependent on a number of parameters diagrams have been constructed to evaluate the feasibility of substantial melt extraction in terms of these poorly constrained parameters data from the golden valley sill and many other natural occurrences of d and i shaped geochemical profiles show a reasonable agreement with our predictions of melt flow potential and are thus well explained by the presented model we conclude that in order to fully understand differentiation processes occurring in sheet intrusions we need to account for post emplacement segregation of melt from crystals and not only segregation of crystals from melt

crystal mush compaction in the cohassett flood basalt flow hanford washington abstract the 40 m thick central part of the cohassett flow which was probably emplaced by inflation of an earlier more ti rich flow exhibits textural evidence of crystal mush compaction during solidification although most of the crystallization started in the roof zone as water circulated through fractures in the crust plumes of dense crystal mush must have dripped from the roof to the floor of the sheet thus displacing upward the colonnade entablature boundary the mush initially consisted of loosely stacked plagioclase crystals and liquid surrounding rapidly grown pyroxene oikocrysts containing radial chadacrysts of plagioclase upon compaction in the floor zone the liquid between the loosely stacked plagioclase crystals was expelled as the crystals packed tightly together to form sub parallel grains wrapped around the pyroxene oikocrysts which recrystallized into granular aggregates of augite and pigeonite stacks of sinking plagioclase crystals and rising liquid accumulated on the upper and lower sides respectively of horizontal plagioclase phenocrysts this asymmetric texture clearly indicates the relative motion of solids and liquid the degree of compaction as determined from variations in the amount of interstitial liquid and quantitative measures of textural anisotropy accounts for the chemical variation through the central part of the flow with compatible elements being concentrated in the lower part of the flow and incompatible elements concentrated in the upper part modeling of the compaction process using the iridium melt infiltration and reaction program of boudreau a 2003 iridium a program to model reaction of silicate liquid infiltrating a porous solid assemblage computers and geosciences 29 423 429 shows that the observed chemical and compaction dilation profiles are to be expected in a 40 m thick sheet of basaltic magma solidifying on the earth s surface compaction of crystal mush should be even more likely in slowly cooled intrusive bodies

in situ crystallization observed in the osumi granodiorite batholith abstract the osumi granodiorite is a 14 ma batholith emplaced in accreted sedimentary rocks in the forearc of southwest japan it has a chemical and mineralogical zonal structure with zones which become progressively more felsic inward feo and mgo decrease while sio2 k2o and rb increase from its margin to the interior in accord with this alkali feldspar and quartz become more abundant in inner zones and biotite and plagioclase vary oppositely the mineral assemblage however remains unvaried major bulk rock compositions form a linear variation trend which is found to be almost identical with the average trend of japanese granitic rocks these petrographic and chemical characteristics are well explained by in situ crystallization model in which the liquid returning from the solidification zone to the chamber has the granitic minimum liquid composition the composition of the liquid in the chamber and that of the cumulate lie on the straight line that connects the granitic minimum and the initial chamber composition

geochemical reversals within the lower 100 m of the palisades sill new jersey transects through the lower part of the palisades sill were made at fort lee and alpine new jersey in order to characterize the petrologic signature of previously proposed reversals in the normal tholeiitic differentiation trend petrographic and geochemical data include 1 modal and grain size analyses 2 bulk rock major and trace element concentrations by dcp aes and 3 augite orthopyroxene magnetite and olivine compositions by electron microprobe analysis anomalous horizons defined by increased bulk rock mg cr ni and co concentrations and abrupt modal and grain size changes occur at 10 m the well known olivine zone 27 m 45 m and 95m above the basal contact thermal models coupled with estimates of the emplacement rate and total magma volume indicate that the olivine zone oz is an early stage feature related to the emplacement of initial magma into the palisades chamber stoke s law calculations indicate that the settling velocity of average sized olivine crystals in a high titanium quartz normative htq magma is too slow for significant gravity settling to have occurred prior to the solidification of the basal 20 m of the sill it is suggested that the oz resulted from the emplacement of a heterogeneous initial magma from a compositionally stratified sub palisades storage chamber located within the upper crust however heterogeneity may have been derived directly from the mantle or during rapid ascent geochemical models indicate that the oz contains accumulated olivine that is not in cotectic or constant proportions with the other cumulus phases suggesting a mechanical sorting process magma chamber recharge is proposed to have occurred at the 27 m and 45 m levels when a slightly more primitive htq magma was injected into the palisades sill chamber zones of elevated mg and cr 6 to 10 m thick at these two horizons may indicate the thickness of the hybrid magma formed by the mixing of these two compositions geochemical models indicate that the rocks at these levels have accumulated excess orthopyroxene relative to samples from the rest of the sill normal faulting in the fort lee area at the 95 m level has caused repetition of the stratigraphic section and hence the sharp reversal observed at this level

magma evolution recorded in plagioclase zoning in 1991 pinatubo eruption products plagioclase the most abundant phenocryst at mount pinatubo displays varying textures and compositions within the 1991 eruption products in june 7 14 dome forming andesite plagioclase phenocrysts show prominent rims with higher mgo 0 04 0 06 wt jo fe20j 0 6 0 8 wt o and k20 at given an than the interiors the compositions of the rims are identical to those of microlites which are abundant in the groundmass glass white dacitic pumice the most voluminous product of the june 15 eruption contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites the interiors of phenocrysts from the dacitic pumice and the dome forming andesite are remarkably similar in terms of textures and compositions they show oscillatory zoning mostly an35 6o low mgo 0 02 wtojo and fe20j 0 10 0 30 wt jo and similar k20 at given an this similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt the oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids a portion of the felsic magma 800 c mixed with a mafic melt 1000 1100 c to become an andesitic magma that extruded to form the june 7 14 dome all plagioclase phenocrysts in the andesite were derived from the felsic magma the mixing caused destabilization of phenocrysts forming sieve textures dusty zones and partial resorption extrusion of the mixed magma resulted in overgrowths on once resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass in the unmixed remaining portion of the felsic magma some plagioclase underwent partial resorption but did not develop overgrowths this lack of overgrowths and the absence of micro lites in the groundmass glass of the june 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption supporting the linkage between the two

the formation of plagioclase chains during convective transfer in basaltic magma the basaltic rock in the lower part of the thick holyoke lava flow in connecticut and massachusetts has been shown to have a remarkable texture with crystals of feldspar linked together in a continuous three dimensional network of chains1 heating experiments have revealed that this network persists to temperatures where the rock is 75 liquid2 and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow3 4 despite the texture s importance to our understanding of how such basalt flows form the origin of the texture has remained uncertain1 2 3 here we show that although the network is present in the lower third of the flow it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush convection of this type has been postulated for intrusive magma chambers but corroborative field evidence has been equivocal especially in lava lakes and flows preservation of the roof generated texture in the lower part of a thick flood basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet

magma dynamics crystallization and chemical differentiation of the 1959 kilauea iki lava lake hawaii revisited abstract using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers we identify the main physical processes at work during the solidification of the 1959 kilauea iki lava lakes in turn we investigate their quantitative influence on the crystallization and chemical differentiation of the magma and on the development of the internal structure of the lava lake in contrast to previous studies vigorous stirring in the magma driven predominately by the descent of dense crystal laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor is predicted to have been an inevitable consequence of very strong cooling at the roof and floor the flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma producing minor compositional stratification at the roof and thermal stratification at the floor the efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals which ultimately settled to the floor additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing in addition to convection in the magma circulation driven by the convection of buoyant interstitial melt in highly permeable crystal melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification in the floor zone mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof we hypothesize that the highly evolved pipe like vertical olivine rich bodies vorbs bull volcanol 43 1980 675 observed in the floor zone of the lake are fossil chimneys in the roof zone buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or leaked into the overlying highly permeable cumulate displacing less buoyant interstitial melt downward the results from rayleigh fractionation type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front the remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements although cooling was concentrated at the roof of the lake the floor zone is found to be thicker than the roof zone implying that it grew more quickly the large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

hawaiian basalt and icelandic rhyolite indicators of differentiation and partial melting in spite of the voluminous basaltic volcanism on the island of hawaii rhyolite is not produced iceland on the other hand exhibits common rhyolitic volcanism amounting to some 10 12 of its surface rocks this contrast is investigated using the fundamental igneous processes exhibited by sheet like hawaiian lava lakes and shonkin sag laccolith in montana highly differentiated residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes only when a large initial phenocryst population is present from which a thick basal cumulate can rapidly form is it possible to supply highly differentiated melt into the active i e eruptable portion of the magma chamber although there is protracted control of differentiation at hawaii by settling of olivine further differentiation occurs within the solidification fronts only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front 7 mgo and 51 5 sio2 crystal size distributions csds for hawaii and shonkin sag are used to demonstrate the inferred physical and chemical processes of solidification including the kinetics of crystallization

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

tephra dispersal from myojinsho japan during its shallow submarine eruption of 1952 1953 a new and detailed bathymetric map of the myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952 1953 eruption especially how the silicic pyroclasts both primary and reworked enlarged the volcano and were dispersed into the surrounding marine environment myojinsho 420 km south of tokyo along the izu ogasawara arc was the site of approximately 1000 phreatomagmatic explosions during the 12 5 month eruption these explosions shattered growing dacite domes producing dense clasts that immediately sank into the sea minor amounts of pumice floated on the sea surface after some of these events the myojinsho cone has slopes of almost precisely 21 in the depth range 300 700 m we interpret this to be the result of angle of repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions many of these gravity flows resulted from explosions and associated dome collapse but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1 2 km of the volcano s summit entering water as deep as 400 m because the fall velocity of single particles decreased by a factor of 30 in passing from air into the sea we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra gravitational instabilities within these tephra choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained upon reaching the sea floor many of these currents probably continued to move downslope along myojinsho s submarine slopes fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption ocean currents carried this tephra to distal areas where it presumably forms a pyroclastic component of deep sea sediment

mechanisms of differentiation in shallow mafic alkaline intrusions as illustrated in the big bend area western texas abstract syenitic bodies are a common feature in alkaline sills and laccoliths that range in composition from syenogabbroic to syenodioritic the syenitic bodies are generally accepted to be the result of in situ differentiation such bodies are usually called segregations but relatively little discussion is given to the actual ways they may form subhorizontal sheets or layers are most common although rounded ocelli and vertically elongated cylindroidal forms are also common these are all systematically arranged in the intrusions such features are found in at least ten sills in the big bend area of texas and are reported as well in montana utah australia sakhalin scotland and new zealand detailed chemical and mineralogical analyses demonstrate crystal fractionation very convincingly for the generation of syenitic liquids that form the bodies the analyses also allow the calculation of viscosities cooling time and mineral settling rates in the sill studied most thoroughly these factors indicate or are compatible with proposed mechanisms for the formation of syenite bodies in many intrusions there is a bimodal distribution of syenite compositions one type is relatively plagioclase rich and the other type is plagioclase poor mass balance calculations on several intrusions show that a residual liquid of plagioclase rich composition will form after about 30 of the parent magma has crystallized whereas a residual liquid of plagioclase poor composition will form only after about 50 of the original magma has crystallized the several mechanisms by which plagioclase rich residual liquids are aggregated into the different forms of syenite bodies include localized crystal settling and sagging of a crystaline framework on a scale of tens of centimeters to a meter within an upper solidification front formation of cylindroidal columns of syenitic differentiate in a crystal mush within a lower solidification front and diapiric rise of such masses after the amount of magma crystallized reaches 50 or more rupture of a rigid crystalline framework occurs through the processes of contraction upon cooling shearing caused by deformation and expansion of bubbles in the interstitial residual liquid while plagioclase poor differentiate flows into spaces created in these processes important for the effectiveness of these processes is the buildup of volatiles in the residual liquids which results from crystallization of anhydrous minerals that causes second boiling and lowering of the viscosity of the liquids

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

a refined technique for measuring crystal size distributions in thin section log normal size distributions are of the form n noe l where n number density l crystal length and is a constant a method for measuring three dimensional log normal crystal or grain size distributions csds in thin section has been deduced from computer experiments in which 2d sections were cut through assemblages of 3d solids the size ranges and distributions studied were appropriate for igneous microphenocryst to megacryst populations conversion from 2d to 3d is based on an exact correction for spheres of uniform diameter cumulate numbers of polygons with length l n2d are converted to n3d by the equation ln n3d ln n2d l s ln l s the number density is then obtained as n dn dl the parameters s and correct the measured lengths and no no number density at l 0 respectively and are functions of crystal shape the parameter is a weak function of the degree of spatial orientation of the crystals highly symmetrical shapes such as cubes octahedra and elongated prisms can be accurately measured when randomly oriented however rectangular solids with a b c cannot be accurately measured because they produce bimodal length distributions in cross section strongly oriented textures trachytic or lineated can be accurately measured regardless of crystal shape new csd data from alkaline rocks and a kimberlite give examples of csds modified by megacryst retention xenocryst addition phenocryst accumulation and groundmass nucleation

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

assimilation by lunar mare basalts melting of crustal material and dissolution of anorthite we discuss techniques for calculating the amount of crustal assimilation possible in lunar magma chambers and dikes based on thermal energy balances kinetic rates and simple fluid mechanical constraints assuming parent magmas of picritic compositions we demonstrate the limits on the capacity of such magmas to melt and dissolve wall rock of anorthitic troctolitic noritic and kreep quartz monzodiorite compositions significant melting of the plagioclase rich crustal lithologies requires turbulent convection in the assimilating magma and an efficient method of mixing in the relatively buoyant and viscous new melt even when this occurs the major element chemistry of the picritic magmas will change by less than 1 2 wt diffusion coefficients measured for al2o3 from an iron free basalt and an orange glass composition are 10 12 m2 s 1 at 1340 c and 10 11 m2 s 1 at 1390 c these rates are too slow to allow dissolution of plagioclase to significantly affect magma compositions picritic magmas can melt significant quantities of kreep which suggests that their trace element chemistry may still be affected by assimilation processes however mixing viscous melts of kreep composition with the fluid picritic magmas could be prohibitively difficult we conclude that only a small part of the total major element chemical variation in the mare basalt and volcanic glass collection is due to assimilation fractional crystallization processes near the lunar surface instead most of the chemical variation in the lunar basalts and volcanic glasses must result from assimilation at deeper levels or from having distinct source regions in a heterogeneous lunar mantle

magmatic processes under mid ocean ridges a detailed mineralogic study of lavas from east pacific rise 9 30 n 10 30 n and 11 20 n 1 detailed petrologic study has been made for lavas from the northern east pacific rise epr 9 30 n 11 20 n and 10 30 n 9 30 n and 11 20 n have robust magma supply and a shallow melt lens while 10 30 n has nonrobust magma supply and no shallow melt lens lavas from all three localities are sparsely phyric and glassy containing plagioclase olivine pyroxene typically the lavas contain several to many up to seven distinct chemical groups of plagioclase that are not always distinct texturally the lavas may also contain up to three chemically distinct groups of olivine and two groups of pyroxene the lavas contain both individual crystals and groups comprising reticulate and dendritic clots that we interpret to represent bits of crystal networks forming in mushy zones of an axial magma chamber 21 of the 23 samples studied in detail have diverse crystal compositions and require mixing which most likely occurs when mostly liquid magma passes through mushy zones we find no significant systematic differences between robust and nonrobust segments in terms of their crystal content proportion of texturally distinct xenocrysts crystal size aspect ratio roundness modal abundance magma residence time number of diverse mineral chemical groups and characteristics of mixing like the total range of composition of disequilibrium minerals present and the magnitude of the chemical gaps between disequilibrium and equilibrium compositions this unexpected and remarkable similarity suggests that the presence or absence of a seismically imaged shallow melt lens has essentially no effect on the mineralogy of erupted lavas we conclude that the fundamentally important magmatic process under mid ocean ridges from slow to fast is the formation of crystal networks and their subsequent compaction the seismically detected shallow melt lenses likely contain highly evolved magma formed by expelled interstitial melt during crystal network compaction and play very little role in crustal accretion our conclusion implies that the thick 3 km low velocity zone near the base of the crust produces the thin 30 m shallow melt lenses not the other way around

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

mechanisms of pluton emplacement and structural evolution of a 2 1 ga juvenile continental crust the birimian of southwestern niger abstract the 2 1 ga birimian terranes of southwestern niger consist of a granite greenstone association the structures of the greenstone belts result from the interference between pluton emplacement and regional transcurrent deformation sinistral n s strike slip faults ne sw trending schistosity the regional cleavage trend is observed to change in the vicinity of elliptical plutons cleavage trajectories bend around their contacts resulting in triple junctions at the pluton extremities and narrow aureoles of intense penetrative schistosity and high grade amphibolitefacies metamorphism are developed around them structural features of plutons suggest syntectonic emplacement and age data indicate that all plutons were emplaced between 2 16 and 2 10 ga the plutons are assembled in two distinct types of batholith elongated either parallel or perpendicular to the inferred regional shortening direction a batholith parallel to the inferred regional shortening direction dolbel batholith consists of km sized subcircular alkali granite plutons except for the southernmost one which is of a larger size and exhibits an elliptical shape with a nw se short axis parallel to the shortening direction crosscutting relationships indicate a progressively northward emplacement trend for those plutons internal fabrics generally correspond to layering and magmatic flow fabrics and accessorily to schistosity and shear zones which represent late stage plastic deformation restricted to the pluton margin batholiths which are perpendicular to the inferred regional shortening direction are composed of large elliptical plutons 500 to 5000 km 2 in area consisting mainly of granodiorite and covering 70 of the region the plutons never show crosscutting relationships but are moulded against each other their internal fabric corresponds to a solid state foliation it either dips towards the centre of the pluton funnel or teardrop shaped intrusions or is subhorizontal in the centre and vertical at the margin dome shaped intrusions the foliation and the geometry of the largest pluton tera pluton were acquired during pluton ascent and inflation whereas the geometry of later shear zones indicates that deformation in the pluton was then passively acquired at the final emplacement level due to regional deformation the contrast in both the orientation and structural features of the batholiths reflects distinct emplacement mechanisms the small alkali granite plutons resulted from magma injection within nw se fractures parallel to the inferred regional shortening direction whereas the granodiorite plutons are assumed to be emplaced as diapirs forceful intrusions with long axes perpendicular to the shortening direction these two intrusion mechanisms for plutons emplaced during the same deformation event and at the same crustal level are considered to reflect a local change in the rheological behaviour of the crustal segment studied in connection with the thermal effect induced by the large granodioritic plutons this vertical tectonic regime appears to be typical of palaeoproterozoic juvenile continental crust areas and related to an interplay between emplacement of tremendous volumes of granodioritic magma and a regional transcurrent deformation

addressing complexity in laboratory experiments the scaling of dilute multiphase flows in magmatic systems the kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments we present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles crystal bubble or pyroclast and the continuous phase liquid or gas we consider three canonical igneous systems magma chambers volcanic plumes and pyroclastic surges and we provide estimates of the proposed scaling relations for published experiments on those systems dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average eulerian properties and instantaneous lagrangian variables the eulerian properties of the mixtures yield the reynolds number re which indicates the level of unsteadiness in the continuous phase the lagrangian acceleration of particles is a function of the viscous drag and gravity forces and from these two forces are derived the stokes number st and the stability number rt two dimensionless numbers that describe the dynamic behavior of the particles within the mixture the compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition ren10 4 and for scaling st and rt among the particle dynamic regimes present in surges and plumes some deserve special attention such as the role of mesoscale structures on transport and sedimentary processes or the consequences of the transition to turbulence on particle gathering and dispersal the compilation of seven experimental studies relevant to magma bodies indicates that in the laminar regime crystals mostly follow the motion of the melt and thus the physical state of the system can be approximated as single phase in the transition to turbulence magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients this heterogeneity has a strong potential for crystals sorting in conclusion the re st rt framework demonstrates

fractionation and magma mixing within intruding dike swarm evidence from the miocene shitara otoge igneous complex central japan abstract the analysis of intrusion pattern and petrological character of the central dike swarm in the miocene igneous complex of the shitara district central japan clarified that magma mixing between a strongly differentiated magma and a less differentiated magma occurred within a dike swarm the dike rocks have a wide compositional variation ranging from 5 5 to 0 7 wt mgo they are divided into p1 and p2 types the p2 type rocks provide many lines of evidence for magma mixing such as reversely zoned phenocrysts bimodal composition distribution and dissolution texture whereas p1 type rocks do not phenocryst compositions of p2 type suggest that the magma mixing occurred between a less fractionated phenocryst poor magma and a strongly fractionated crystal rich magma concentration ratios among incompatible elements show that the mixing end components were derived from a similar parental magma common to p1 type by fractional crystallization in a near closed system the dikes with evidence for the magma mixing p2 type are distributed only in the southern marginal part of the dike swarm whereas p1 type dikes do not show any such localization the distribution and the intrusion direction of the dikes indicate a nearly horizontal outward flow of magmas in the southern part of the dike swarm and accompanied magma mixing in the dike during intrusion the fractionated end component is inferred to be a product of crystal fractionation within small and ephemeral magma pockets in the dike swarm magma mixing is thought to have occurred when a newly intruded dike ruptured the magma reservoir the frequency of magma mixing was controlled mainly by competition between the lifetime of ephemeral magma reservoir and frequency of dike intrusions the condition of magma mixing was satisfied only in the southern part of the dike swarm affected by the preceding volcanic activities

convection and crystal settling in sills it has been advocated that convective and crystal settling processes play significant and perhaps crucial roles in magmatic differentiation the fluid dynamics of magma chambers have been extensively studied in recent years both theoretically and experimentally but there is disagreement over the nature and scale of the convection over its bearing on fractionation and possibly over whether it occurs at all the differential distribution of modal olivine with height in differentiated alkaline basic sills provides critical evidence to resolve this controversy at least for small to medium large magma chambers our own and others published data for such sills show that irrespective of overall olivine content modal olivine contents tend to increase in a roughly symmetrical manner inwards from the upper and lower margins of the sill i e the distribution patterns are more often approximately d shaped rather than the classic s shape generally ascribed to gravity settling we concur with the majority of other authors that this is an original feature of the filling process which has survived more or less unchanged since emplacement we therefore conclude that the magmas have not undergone turbulent convection and that gravity settling has usually played only a minor modifying role since the intrusion of these sills we offer a possible explanation for the apparent contradiction between fluid dynamical theory and the petrological evidence by suggesting that such sills rarely fill by the rapid injection of a single pulse of magma rather they form from a series of pulses or a continuous pulsed influx over a protracted interval during which marginal cooling severely limits the potential for thermal convection

crystallization calculations for a binary melt cooling at constant rates of heat removal implications for the crystallization of magma bodies abstract we report on thermodynamic non equilibrium crystallization calculations for a unit volume of a binary melt subject to a constant prescribed rate of heat loss crystallization histories and crystal size distributions for both melt components were calculated by accounting for the nucleation and growth of crystals the crystal sizes were found to decrease with increasing rates of heat loss the crystallization time defined as the time to crystallize 99 of the unit volume also decreased strongly with increasing rates of heat loss up to a critical rate the critical rate was found to be somewhat smaller than the heat loss rate for the beginning of glass formation at larger than critical rates crystallization time increased again and for rates larger than the glass formation rate crystallization time became infinite the residual melt composition was found to increasingly deviate from the equilibrium composition with increasing rates of heat loss but as long as the loss rate was less than the critical rate the crystallization path reverted to the eutectic composition during the final crystallization for supercritical rates no such reversion was observed we compared the critical rate with estimates of the rates of heat loss in magmatic intrusions based on the stefan solution for a freezing half space it was found that rates of heat loss should be supercritical at distances of up to 0 5 m from the margin of an intrusion in this region non equilibrium effects are expected to dominate and the texture of the crystallized rock should be characterized by small crystals and by glass the glass and the crystals should be of non equilibrium composition non equilibrium effects should be negligible only at distances of more than 5 m from the margin where the rates of heat loss are less than 10 2 times critical at these distances the crystallized rock should have an equigranular texture and an equilibrium composition

the role of diffusion controlled oscillatory nucleation in the formation of line rock in pegmatite plite dikes the george ashley block gab located in the pala pegmatite introduction district san diego county california is a composite pegmatite modal or phase layering is a phenomenon that is seen aplite dike of 8 m thickness displaying striking mineralogical layering in plutonic igneous rocks of all compositions from highin the aplite portion of the dike referred to as line rock rhythmic temperature low viscosity mafic complexes to low temlayering is characterized by garnet rich bands alternating with perature high viscosity felsic complexes the sizes of albite quartz muscovite rich bands cumulus textures are notably such plutonic bodies can range from the large layered absent from the layered portion of the dike elongated quartz intrusive complexes such as the 3000 m thick skaergaard megacrysts are oriented perpendicular to the garnet rich layers and complex down to dikes a few meters wide there is poikilitically include garnet albite and muscovite calculated however no consensus as to the origin of the layering crystal free magma viscosity with 3 h2o is 10 pa s and the modal layering that results from the gravitational settling calculated settling velocity for garnet is 0 51 cm year conductive of crystals from a magma is a model often invoked to cooling calculations based on emplacement of a 650 c dike into explain the layering in mafic magmatic systems this is 150 c fractured gabbroic country rock at 1 5 kbar and accounting more problematic in granitic systems because of the for latent heat of crystallization demonstrate that the line rock higher viscosities of the melts and the smaller density portion of the dike cools to 550 c in about 1 year crystal difference between crystals and melt size distribution studies also suggest very rapid nucleation and this paper presents a temperature crystallization crystallization diffusion controlled gel crystallization experiments model for the george ashley composite pegmatite aplite yield textures virtually identical to those observed in the layered dike which displays well developed rhythmic layering aplite including rhythmic banding colloform layering and band line rock in the aplite portion of the dike kleck 1996 discontinuities thus observed textures and calculated magmatic proposed that the layering in the george ashley block parameters suggest that mineralogical layering in the gab results gab a float slide block from this dike could be exfrom an in situ diffusion controlled process of oscillatory nucleation plained by the gravitational settling of garnet and albite and crystallization we propose that any event that promotes from the melt and that many of the textural features strong undercooling has the potential to initiate rapid heterogeneous which characterize the line rock such as offsets in the nucleation and oscillatory crystal growth leading to the development layers formed as a result of soft sediment deformation of a layer of excluded components in front of the crystallization of a crystal liquid mush the purpose of this investigation front and the formation of line rock was to evaluate crystal settling as a possible mechanism

the petrology and geochemistry of the aniakchak caldera forming ignimbrite aleutian arc alaska aniakchak caldera alaska produced a compositionally heterogeneous ignimbrite 3400 years ago which changes from rhyodacitic at the base to andesitic at the top of the eruptive sequence interpretations of compositionally heterogeneous ignimbrites typically include either in situ fractional crystallization of mafic magma and generation of a stratified magma body or replenishment of a silicic magma chamber by mafic inputs another possibility silicic replenishment of a more mafic chamber exists geochemical characteristics of the caldera forming rhyodacite and several late precaldera rhyodacites indicate independent origins for each within a maximum of 5000 years prior to caldera formation isotopic considerations preclude derivation of the caldera forming rhyodacite from the caldera forming andesite however the caldera forming rhyodacite can be explained as the residual liquid of a mostly crystallized basalt with addition of crustal material the aniakchak andesite probably formed in a shallow chamber by successive mixing events involving small volumes of basalt and rhyodacite together with contamination the pre caldera rhyodacites represent erupted portions of intruding silicic magma whereas another portion homogenized with the resident mafic magma the caldera forming event reflects a large influx of rhyodacite which erupted before significant mixing occurred and also triggered draining of much of the andesitic magma from the chamber

cooling and crystallization in sheet like magma bodies revisited abstract in order to characterize the interaction of convection and crystallization during the solidification of magmatic sills a one dimensional model for the solidification of magmatic sills cooling from above has been used the model is based on a stefan type solution for a mushy zone and takes into account the inward moving solidification front whose speed is exclusively controlled by crystal growth below the solidification front the magma cools by convection it may eventually cool below its liquidus and start to partially crystallize a kinetic model has been implemented to account for this crystallization the model calculations lead to three main conclusions 1 crystal growth is not the only factor controlling the advance of magmatic solidification fronts the advance appears to be mainly controlled by nucleation processes and only to a small extent by crystal growth 2 the longevity and vigor of convection inside a magma chamber is strongly dependent on the kinetics of crystallization of magma inside the chamber 3 immediately after the initiation of cooling the melt becomes laced with nuclei

anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust abstract the sept iles layered intrusion quebec canada is dominated by a basal layered series made up of troctolites and gabbros and by anorthosites occurring 1 at the roof of the magma chamber 100 500 m thick and 2 as cm to m size blocks in gabbros of the layered series anorthosite rocks are made up of plagioclase with minor clinopyroxene olivine and fe ti oxide minerals plagioclase displays a very restricted range of compositions for major elements an 68 an 60 trace elements sr 1023 1071 ppm ba 132 172 ppm and sr isotopic ratios 87 sr 86 sr i 0 70356 0 70379 this compositional range is identical to that observed in troctolites the most primitive cumulates of the layered series whereas plagioclase in layered gabbros is more evolved an 60 an 38 the origin of sept iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts the density of the feo rich tholeiitic basalt parent magma first increased from 2 70 to 2 75 g cm 3 during early fractionation of troctolites and then decreased continuously to 2 16 g cm 3 with fractionation of fe ti oxide bearing gabbros plagioclase an 69 an 60 was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite with further differentiation plagioclase 60 became negatively buoyant and anorthosite stopped forming blocks of anorthosite autoliths even fell downward to the basal cumulate pile the presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and or minor plagioclase nucleation within the main magma body dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid the processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust with gabbro noritic rocks at the base and anorthositic rocks at the top we propose that the lunar crust mainly crystallized bottom up this basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian suite without kreep contamination while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust

testing the validity of the petrological hypothesis no phenocrysts no post emplacement differentiation an extravagant hypothesis no phenocrysts no post emplacement differentiation has been put forward by marsh in a series of papers for the development of mafic ultramafic intrusions this hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks to explain this the hypothesis postulates that phenocryst free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts producing compositionally uniform magmatic bodies layered well differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions rather than to slow magma cooling and fractional crystallization as conventional models imply such phenocryst laden magma pulses are supposed to be derived from an underlying magmatic mush column however structureless undifferentiated mafic ultramafic bodies simply do not exist in nature all wellstudied mafic ultramafic bodies with or without residual granitic rocks that crystallized from parental magmas of non eutectic composition tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences e g ol opx opx pl opx pl cpx mineral compositions e g an in plag en in cpx and compatible incompatible major and trace element geochemistry e g mg number cr rare earth elements this is especially evident in layered intrusions that represent the key evidence against the hypothesis essentially by denying the ability of magma to differentiate in intrusive bodies the hypothesis forbids magmatic differentiation in any sub chamber related to the entire magmatic mush column as a result magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions the hypothesis is thus contradictory baseless and fundamentally flawed it should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of bowen and amply confirmed over almost a century by subsequent studies of layered intrusions the classical model was is and will most probably remain the best explanation for the origin of differentiated magmatic bodies

crystal size distribution systematics and the determination of magma storage times the 1959 eruption of kilauea volcano hawaii abstract the theory of crystal size distribution csd theory is based on a steady state population balance that monitors the flux of crystals growing into and out of specificed size categories in precipitating solutions the conservation equation describing this balance permits crystal growth and nucleation rates to be determined directly from crystal size distribution data in this investigation it is shown that csd analysis can alternatively be used to calculate the residence time of crystals in the system or in volcanologic terms the magma storage time prior to eruption if the characteristic crystal growth rate can be independently determined or estimated the crystal size data needed for storage time determinations are easily obtained from thin sections of glassy eruption samples and the mathematical calculations are relatively simple analysis of the errors inherent in this new technique predicts storage estimates accurate to within an order of magnitude

thermal and compositional evolution of a cooling magma chamber by boundary layer fractionation model and its application for primary magma estimation a boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented the model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body the magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer the consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space the model is used to evaluate a primary magma estimation from volcanic rock series it is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation this can cause significant errors in estimating the conditions at which the primary magma was generated

a new view of the dynamics stability and longevity of volcanic clouds abstract powerful explosive volcanic eruptions inject ash high into the atmosphere which spreads to form umbrella clouds identifying key physical processes governing the dynamics stability and longevity of umbrella clouds is central to assessing volcanic hazards as well as the nature of volcanic forcings on climate here we present a series of laboratory experiments producing turbulent particle laden jets and subsequent axisymmetric intrusive gravity currents into a stratified environment our experiments reproduce many of the main dynamical regimes observed during the formation of an explosive volcanic column and highlight new dynamics for the umbrella cloud theoretical predictions of column collapse from a simple model of a turbulent jet are in good agreement with experimental observations as well as previous studies depending on the flow intensity the strength of the initial environmental density stratification and the particle concentration at the source resulting umbrella clouds can however evolve through a series of new regimes as a result of the dynamics of particle sedimentation within these flows as well as from their bases using scaling theory we show that during cloud spreading internal sedimentation drives the growth and intermittent overturn of thin gravitationally unstable particle boundary layers pbls as particle rich plumes this pbl driven convection can have remarkable effects ranging from progressive dilution of clouds to their catastrophic overturn and collapse in natural eruptions whether the dynamics of pbls play a major role in particle sedimentation depends on the grain size distribution inside the cloud and on eruption column height in general particles larger than 60 m 1 mm are expected to settle individually whereas finer particles will accumulate pbls resulting in the formation of armless mammatus clouds or dangerous gravity currents at much larger distances from the volcanic vent than ever considered before such dynamics is apparent in observations of numerous modern eruptions and is inferred from the deposits of historic and prehistoric eruptions for where there exist appropriate data consideration of the consequences of these phenomena for problems such as volcanic hazards to humans and climate change may thus be very important in the assessment of future eruptions

hawaiian magma reservoir processes as inferred from the petrology of gabbro xenoliths in basalt kahoolawe island gabbro xenoliths in a tholeiitic lava of kahoolawe island hawaii a 1 3 1 4 ma shield volcano are 1 3 cm in size and comprised of plagioclase clinopyroxene and orthopyroxene gabbro textures while intergranular and in part subophitic are open due to 28 48 vol of vesicular basalt occupying xenolith space vesicles in and around the xenoliths are lined or filled with rhyolitic glass segregation vesicles the host is evolved tholeiite mgo 6 1 wt with phenocrysts microphenocrysts and glomerocrysts of olivine clinopyroxene orthopyroxene and plagioclase and megacrysts 1 cm of plagioclase the sr isotope ratio of one xenolith is 0 70489 the host basalt ratio is 0 70460 xenolith isotope composition grain resorption and clinopyroxene fs12 5 15wo38 35 5 orthopyroxene fs19 5 24wo4 1 and plagioclase an68 65or0 8 1 2 compositions suggest that these gabbros crystallized from kahoolawe tholeiitic magma of essentially the same composition as the host basalt but pre dating the magma represented by the host based on the absence of intergranular fe ti oxide phases from the pl cpx opx assemblages and the open vuggy textures we envision crystallization on a reservoir roof at temperatures 1100 c entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush suspension zone occurred during convection associated with replenishment of the magma reservoir these open textured gabbro xenoliths are therefore not fragments of preexisting coarse grained bodies such as sills or segregation veins rhyolitic glass in vesicles represents a gas effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths

shear induced pressure changes and seepage phenomena in a deforming porous layer i summary we present the results of an analytical investigation into the deformation behaviour of a threelayer permeable material that provides solutions of the stress equilibrium equations and the equation of continuity coupled with darcy s law two distinct regimes are investigated the first is the one in which the externally applied fluid pressures are significant the second is the limit in which these pressure increases are insignificant the typical time constant for the process is derived in the latter case the long term behaviour of the fluid pressure in the three layers is entirely determined by the applied shear stress rate and a stable steady state appears when there is an external fluid pressure increase no steady state settles and the leading term in the long time expansion is proportional to the time t the instability that is created in this way needs examination as the hydraulic failure that is associated with it will take place at the position where the pressure increase is greatest note that the flow that follows from the pressure gradient is greatest in the vicinity of the top layer which is presumed to be more permeable k 1 k 2 10 3 all this holds for slow processes where the time constant is long compared to the consolidation time the stress state in the material in the vicinity of the granular layer becomes therefore more anisotropic which may cause the surrounding material to fail the problem of melt extraction in a flattening and deforming plume head at the base of the lithosphere was investigated for a fixed loading rate of 10 15 s 1 e xcess pressures and flow rates are of the order 10 7 to 10 9 pa and ca 1 0 13 ms 1 respectively increasing with increasing rate of shear evaluation of the ratio of hydrostatic pressure to isotropic stress points to hydraulic fracturing as a potentially important aspect of the long term deformation behaviour

what can crystal size distributions and olivine compositions tell us about magma solidification processes inside kilauea iki lava lake hawaii abstract lava lakes offer the opportunity to investigate magma solidification and can be considered as a proxy for small magma chambers here we present olivine compositions and crystal size distributions csds from scoria and drill core samples from kilauea iki lava lake which formed during the 1959 eruption of kilauea volcano hawaii three chemically distinct olivine populations were distinguished in the basis of their forsterite fo content 1 a high fo population fo86 90 2 an intermediate fo population fo78 82 and 3 a minor low fo population fo74 78 populations 1 and 2 both have deformed and undeformed crystals the third population may be the result of rejuvenation olivine in the lower 60 m of lake has a less fo rich composition and more crystals are deformed the csd analysis yields estimates of the average olivine residence time 1 60 years the shape of the olivine csds is fairly uniform with respect to depth curved csds are considered to be evidence of hybrid populations partly or totally involving crystal or magma mixing the turndown at the smallest sizes of most foundered crust and lake csds may be the result of coarsening making this process active both before and after eruption our csd modelling does not support significant crystal settling and overall convection in the lava lake although small advective currents are known to have occurred the olivine vertical stratification cannot be an original feature which is consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases it is also possible that independent basal feeding of the lake during the eruption may be needed to explain fully features of the chemical and mineralogical stratification

basalt convection and plagioclase retention a model for the generation of high alumina arc basalt recent investigations of crystal settling in convecting magma have suggested that crystals may be spatially sorted according to both size and mineralogy for a basalt magma simultaneously crystallizing plagioclase clinopyroxene and olivine a possible result is the selective removal of mafic minerals and the preferential retention of plagioclase crystals along the intrusion margin this represents a possible mechanism for generating plagioclase phyric magmas of high alumina basalt composition in this study a combined basalt crystallization and convection model has been developed to assess the extent to which crystallization and convection leads to mineralogic sorting and preferential plagioclase retention in a primitive mantle derived calc alkaline basalt intrusion the model assumes a simple rectangular chamber geometry steady state non turbulent single cell convection and spatially uniform crystallization the results indicate that over a narrow range of convective velocities 1 crystal sorting

multiple merensky reef of the bushveld complex south africa the merensky reef mr of the bushveld complex is known for containing the lion s share of the world s reserves of platinum group elements pge it is commonly described as a single relatively thin layer or package of chromite and sulphide bearing orthopyroxenite hosted by almost barren norite and or anorthosite here we present a drill core section that contains however four separate mr orthopyroxenite layers hosted by mottled anorthosites each of these orthopyroxenites is distinguished through occurrence absence of marginal chromitite seams grain size petrographic composition and pge abundance we interpret them as multiple undercutting mr that occurs as sill like apophyses extending laterally from a pothole margin into the footwall the undercutting mr is attributed to in situ crystallization within sheet like melt filled cavities at potholes margins which are produced through thermal and or chemical erosion of the footwall rocks by replenishing magmas this finding indicates that the mr forming event involved several distinct pulses of replenishing magmas whose crystallization products were mostly eroded away in the stratiform portions but remained locally preserved in apophyses within footwall rocks the mineralized mr formed a few meters below a temporary chamber floor is indicative of strong thermal compositional convection in the basal layer of the bushveld s chamber the convection was able to deliver large volumes of fresh magma for the scavenging of pge by sulphide and chromite growing within the sheet like cavities of the mr potholes

a melt viscosity scale for preeruptive magmas a simplified method to estimate preeruptive melt viscosity by using only melt sio2 content groundmass sio2 content is proposed for sub alkaline magmas melt viscosity is controlled by many magmatic properties e g melt composition melt water content and temperature however these properties are linked by phase equilibrium in preeruptive magmas in this study the magmatic properties were investigated by compiling data of phase equilibria experiments performed under preeruptive conditions negative correlations are found between melt sio2 contents and liquidus temperatures and between liquidus temperatures and melt water contents both increasing melt sio2 content and decreasing liquidus temperature have the effect of increasing the melt viscosity producing a linear positive correlation between logarithmic values of melt viscosity and linear values of melt sio2 content for a specific melt sio2 content an increase in liquidus temperature causes a decrease in melt viscosity whereas a decrease in water content causes an increase in melt viscosity as a result of this opposing effect the melt viscosity is strongly correlated with the melt sio2 content the correlation coefficient of 1 based on this relationship an empirical equation predicting logarithmic values of preeruptive melt viscosity is proposed as a linear function of melt sio2 content referred to as the melt viscosity scale the equation reproduces melt viscosities for compiled experimental melts and natural melts with root mean square deviation of 0 4 and 0 5 log units respectively this method provides order of magnitude estimates for preeruptive melt viscosity its strength is in being applicable to examples for which a full dataset of preeruptive magmatic properties is lacking or has large uncertainties e g slowly cooled lavas the simplicity of this method enables us to easily and promptly estimate preeruptive melt viscosity combined with rheological models for multiphase magmas the present method can be applied widely and thus greatly increase the number of case studies which include evidence based estimates of preeruptive magma viscosity

k feldspar megacryst accumulations formed by mechanical instabilities in magma chamber margins asha pluton nw argentina abstract the asha pluton the oldest unit of the san blas intrusive complex early carboniferous exhibits impressive examples of magmatic structures formed by accumulation of k feldspar megacrysts enclaves and schlieren almost all recognized structures are meter scale vertically elongate bodies of variable shapes defined as fingers trails drips and blobs they preferentially developed near the external margin of the asha pluton and generally are superimposed by chamber wide magmatic fabrics they mostly have circular or sub circular transverse sections with an internal fabric defined by margin parallel inward dipping concentric foliation and steeply plunging lineation at upper parts and flat foliation at lower parts the concentration of megacrysts usually grades from upper sections where they appear in a proportion similar to the host granite to highly packed accumulations of k feldspar along with grouped flattened enclaves at lower ends these features suggest an origin by downward localized multiphase magmatic flow narrowing and log jamming and gravitational sinking of grouped crystals and enclaves with compaction and filter pressing as main mechanisms of melt removal crystal size distribution analysis supports field observations arguing for a mechanical origin of accumulations the magmatic structures of the asha pluton represent mechanical instabilities generated by thermal and compositional convection probably owing to cooling and crystallization near the pluton margins during early stages of construction of the intrusive complex

a magma ocean on vesta core formation and petrogenesis of eucrites and diogenites abstract available evidence strongly suggests that the hed howardite eucrite diogenite meteorites are samples of asteroid 4 vesta abundances of the moderately siderophile elements ni co mo w and p in the hed mantle indicate that the parent body may have been completely molten during its early history during cooling of a chondritic composition magma ocean equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0 80 when the convective system freezes and melts segregate from crystals by gravitational forces the extruded liquids are similar in composition to main group and stannern trend eucrites and the last pyroxenes to precipitate out of this ocean before convective lockup span the compositional range of the diogenites subsequent fractional crystallization of a main group eucrite liquid which has been isolated as a body of magma produces the nuevo laredo trend and the cumulate eucrites the predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites thus eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with vesta

contrasting stratified plutons exposed in tilt blocks eldorado mountains colorado river rift nv usa abstract roof to floor exposures of mid miocene plutons in tilt blocks south of las vegas nv reveal distinct but strongly contrasting magma chamber statigraphy the searchlight and aztec wash plutons are well exposed stratified intrusions that show a similar broad range in composition from 45 75 wt sio 2 homogeneous granites that comprise about one third of each intrusion are virtually identical in texture and elemental and isotopic chemistry mafic rocks that are present in both plutons document basaltic input into felsic magma chambers isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination whereas more felsic rocks are hybrids that are either juvenile basaltic magma crustal melt mixtures or products of anatexis of ancient crust young mesozoic or miocene mafic intraplate despite general similarities the two plutons differ markedly in dimensions and lithologic stratigraphy the searchlight pluton is much thicker 10 vs 3 km and has thick quartz monzonite zones at its roof and floor that are absent in the aztec wash pluton isotopic and elemental data from searchlight pluton suggest that the upper and lower zones are cogenetic with the granite we interpret the finer grained slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate in contrast the upper part of the aztec wash pluton is granite and a heterogeneous mafic rich injection zone with distinct isotopic chemistry forms the lower two thirds of the intrusion similar mafic rocks are relatively sparse in searchlight pluton and do not appear to have played a central role in construction of the pluton large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the aztec wash pluton but absent in the searchlight pluton thus although both intrusions were filled by similar magmas and both developed internal stratification the two intrusions evolved very differently the distinctions may be attributable to scale and resulting longevity and or to subtle differences in tectonic setting

vertical density currents a review of their potential role in the deposition and interpretation of deep sea ash layers marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology recent experiments and field data from the 1991 pinatubo eruption challenge tacit assumptions that following atmospheric fallout on the ocean surface marine sedimentation of ash grade material occurs by stokes law settling of individual grains laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates further experiments described herein involving sustained particle fluxes entering stratified environments demonstrate that ash particles can decouple from the transporting fluid whose vertical motions are limited by the stable density gradient in the ocean vertical density currents generated by ash loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1 3 orders of magnitude faster than possible by stokesian settling greatly reducing their residence time sea bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column plus suspension fallout from the polydisperse turbid layer delivered by convective plumes yielding normal distribution grading the turbid layer may continue laterally as a nepheloid current under the influence of bottom slopes and or geostrophic forces either mechanism possibly modifying the grain size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations consequently and regardless of any subsequent reworking deep sea ash isopachs may not directly reflect an eruptive signal and the ash layers may not directly record the order rate or location of arrival of ash particles at the ocean surface

constraints on magma degassing beneath the far southeast porphyry cu au deposit philippines k silicate alteration in the far southeast fse porphyry cu au introduction deposit formed at 1 4 ma concentric to dikes of quartz diorite magmas become saturated with volatile components as porphyry at 2 km paleodepth the hydrothermal system consisted they ascend in the crust and either erupt or crystallize of magmatic hypersaline liquid and vapor at 550 c and 50 at depth saturated magmas that remain at depth exsolve mpa lithostatic pressure advanced argillic alteration formed at an aqueous fluid that forms a hydrothermal system on the same time over the deposit at 1 km paleodepth from acidic separation from the magma chamber burnham 1979 condensates of the vapor at 1 3 ma k silicate alteration was although part of the exsolved fluid a low density vapor overprinted by 350 c magmatic liquid 5 wt nacl equiv may discharge to the surface from passively degassing at hydrostatic pressure sericite alteration and much of the cu au volcanoes hedenquist 1995 part will remain at depth mineralization formed at this later stage evolution of the magmatic and this dense residual magmatic liquid may be asfluid composition was simulated with a magma chamber cryssociated with hydrothermal mineralization in the portallization model homogeneous crystallization during early stage phyry ore environment hedenquist lowenstern convection is assumed whereas at 50 vol crystals the chamber 1994 becomes stagnant and crystallizes from rim to core over a narrow in this paper we use evidence from a porphyry ore crystallization interval the model calculation based on a magma deposit in the philippines to determine the composition chamber with 2 km thickness at 6 km depth 150 mpa and of the fluid that exsolved from the deeper parent magma 800 c saturated melt composition at 30 vol crystals 5 the evolving composition of the magmatic fluid at the wt h2o 0 2 wt cl and dd of 40 can reproduce the depth of the ore body can then be used to place constraints chemical and isotopic compositions of the early and late magmatic on the nature of the evolved magma chamber far below fluids the most critical factor controlling the compositional evolution the porphyry ore body this approach differs from that of the model hydrothermal system is the transition from convective of other studies where a portion of the parent magma to stagnant magma chamber crystallization there is also a sharp chamber is available to study directly e g at yerington decrease in the rate of fluid exsolution associated with this transition nevada dilles 1987 which can account for the thermal collapse of the fse porphyry the far southeast fse porphyry cu au deposit system from k silicate to sericite alteration and overlying lepanto epithermal cu au ore body are

are arc type rocks the products of magma crystallisation observations from a simple oceanic arc volcano raoul island kermadec arc sw pacific raoul island is the emergent summit of a large intra oceanic strato volcano in what is globally one of the simplest of subduction settings in this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high mg numbers and high ni and cr expected of primitive mantle derived melts the lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene olivine orthopyroxene and spinel phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition indicating a decoupling of melt and crystal components in the system a consistent model for the raoul magmatic system is that primitive high mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low mg magmas by fractional crystallisation within the lower crust low mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone these processes are essentially those that characterise continental subduction related magmatic systems but differ only in the absence of an evolved crustal component

the origin of basic ultrabasic sills with s d and i shaped compositional profiles by in situ crystallization of a single input of phenocryst poor parental magma an attempt is made to develop an in situ crystallization model based on the concept of soret fractionation to explain the origin of commonly observed s d and i shaped compositional profiles in sills formed from a single pulse of phenocryst poor parental magma the model envisages that the various compositional profiles observed in sills can be interpreted in terms of different combinations of four principal units basal zone and layered series forming the floor sequence and top zone and upper border series constituting the roof sequence the basal and top zones represent mirror images of the layered and upper border series respectively and therefore are referred to as basal and top reversals it is proposed that the formation of basal and top reversals takes place through the non equilibrium soret differentiation of liquid boundary layers which form as a consequence of the temperature gradient imposed by the cold country rock in contrast the layered and upper border series originate predominantly through the crystal liquid boundary layers developing in equilibrium conditions the model permits the production of s d and i shaped compositional profiles from the same magma composition all that is necessary to produce a specific shape of compositional profile is an appropriate temperature gradient imposed by the cold country rock on the liquid boundary layers of a parental magma of a given composition

an improved quantitative measure of the tendency for volcanic ash plumes to form in water implications for the deposition of marine ash beds laboratory experiments and numerical simulations have shown that volcanic ash particles immersed in water can either settle slowly and individually or rapidly and collectively as particle laden plumes the ratio of timescales for individual and collective settling in the form of analytical expressions provides a dimensionless quantitative measure of the tendency for such plumes to grow and persist which has important implications for determining particle residence times and deposition rates however existing measures in the literature assume that collective settling obeys stokes law and is therefore controlled by the balance between gravitational forces and viscous drag despite plume development actually being controlled by the balance between gravitational forces and inertial drag even in the absence of turbulence during early times this paper presents a new measure for plume onset which takes into account the inertial drag controlled rather than viscous drag controlled nature of plume growth and descent a parameter study comprising a set of numerical simulations of small scale volcanic ash particle settling experiments highlights the effectiveness of the new measure and by comparison with an existing measure in the literature also demonstrates that the timescale of collective settling is grossly under estimated when assuming that plume development is slowed by viscous drag furthermore the formulation of the new measure means that the tendency for plumes to form can be estimated from the thickness and concentration of the final deposit the magnitude and duration of particle flux across the water s surface do not need to be known the measure therefore permits the residence times of particles in a large body of water to be more accurately and practically determined and allows the improved interpretation of layers of volcaniclastic material deposited at the seabed

upper crustal structure of newberry volcano from p wave tomography and finite difference waveform modeling 1 seismic tomography combined with waveform modeling constrains the dimensions and melt content of a magma body in the upper crust at newberry volcano we obtain a p wave tomographic image by combining travel time data collected in 2008 on a line of densely spaced seismometers with active source data collected in the 1980s the tomographic analysis resolves a high velocity intrusive ring complex surrounding a low velocity caldera fill zone at depths above 3 km and a broader high velocity intrusive complex surrounding a central low velocity anomaly at greater depths 3 6 km this second upper crustal low velocity anomaly is poorly resolved and resolution tests indicate that an unrealistically large low velocity body representing 60 km3of melt could be consistent with the available travel times the 2008 data exhibit low amplitude first arrivals and an anomalous secondary p wave phase originating beneath the caldera two dimensional finite difference waveform modeling through the tomographic velocity model does not reproduce these observations to reproduce these phases we predict waveforms for models that include synthetic low velocity bodies and test possible magma chamber geometries and properties three classes of models produce a transmittedp phase consistent with the travel time and amplitude of the observed secondary phase and also match the observed lower amplitude first arrivals these models represent a graded mush region a crystal suspension region and a melt sill above a thin mush region the three possible magma chamber models comprise a much narrower range of melt volumes 1 6 8 0 km3 than could be constrained by travel time tomography alone

k ar geochronology of a middle miocene submarine volcano plutonic complex in southwest japan a volcano plutonic complex in the susa area southwest japan consists of the yamashima andesites the koyama gabbros and syn plutonic porphyrite dykes derived from a common basaltic andesite magma the complex is closely associated with middle miocene turbidite deposits the yamashima andesites are composed mainly of basaltic andesite feeder dykes massive submarine lavas with hyaloclastites and their reworked deposits the lavas and deposits immediately overlie turbidite deposits indicating submarine volcanic activity the koyama gabbros formed hornfels by contact metamorphism of the surrounding turbidites and andesites highly purified clinopyroxene and plagioclase mineral separates from the yamashima andesites were dated by a k ar method using an ultra low blank k analysis procedure ages obtained from duplicate analyses are 16 5 1 5 15 2 1 4 15 8 1 7 and 16 5 2 0 ma for clinopyroxene and 14 2 0 8 15 2 0 9 and 15 6 0 9 ma for plagioclase the clinopyroxene and plagioclase data define a mineral isochron age of 14 7 0 9 1 ma with an initial 40 ar 36 ar ratio of 297 3 2 4 1 suggesting that clinopyroxene has no excess argon and can be reliably dated by k ar most of the groundmass ages are considerably younger 12 1 14 6 than the isochron age perhaps due to argon loss during alteration the gabbros give ages of 14 2 0 3 and 14 1 0 3 ma for biotite and 13 7 0 3 and 13 7 0 7 ma for green hornblende the porphyrite dyke yields an age of 12 5 0 3 ma for the groundmass and the pelitic hornfels gives a biotite age of 14 8 0 3 ma our new k ar ages together with previous studies show that a series of geological events took place in the susa area between 16 and 13 ma conglomerates and sandstones were deposited in the beginning of marine transgression subsequent abrupt deepening led to deposition of a thick black shale unit turbidite deposits and large scale submarine channel fill deposits coeval igneous activity formed the volcano plutonic complex the magmato tectonic event was synchronous with the opening of the japan sea and the associated clockwise rotation of the southwest japan arc sliver recording a unique tectonic setting

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

evidence of magma mixing in the daly gap of alkaline suites a case study from the enclaves of pantelleria italy the island of pantelleria consists of trachytes pantellerites and minor mildly alkaline basalts rocks of intermediate composition falling in the so called daly gap such as mugearites benmoreites and mafic trachytes occur only in the form of enclaves in trachytes and pantellerites inside the main caldera of the island caldera cinque denti which collapsed during the green tuff ignimbrite eruption at 50 ka the enclaves include volcanic subvolcanic and intrusive rock types the enclaves in host trachyte contain traces of glass devitrified glass occurs within enclaves in host pantellerites minerals in the enclaves show regular compositional variations with whole rock silica content glass present in the medium grained samples is interpreted to be the result of incipient melting the major and trace element compositions of the enclaves show regular and linear variations between an evolved mafic magma hawaiite and a felsic end member similar to the green tuff trachyte fractional crystallization modelling of compatible and incompatible trace elements v ni zr la sm lu nb y th does not reproduce the observed trends rocks of intermediate composition within the daly gap can be explained only by magma mixing between an already differentiated mafic magma hawaiite and an anorthoclase rich trachytic melt in the lower and higher parts respectively of a stratified magmatic chamber medium grained enclaves are interpreted as the result of fragmentation of solidified mixing layers in the roof of the magma chamber during the eruption of the green tuff when the collapse of the caldera took place diffusion calculations suggest a residence time of 5 days for the enclaves in their host magmas

lacertoid footprints from the upper triassic santa maria formation of southern brazil the triassic of southern brazil is well known for its rich tetrapod body fossils however trace fossils such as tetrapod trackways and trails were discovered only recently from the predebon outcrop upper part of the alemoa member santa maria formation providing new information on small sized animals the trackways can be identified as rhynchosauroides retroversipes isp nov rhynchosauroides isp rhynchosauroides isp and drag marks produced by swimming animals of lacertoid affinity the preservation of the tracks and trackways was influenced by the water level whilst they were being made and subsequent subaerial exposure the trackmaker of r retroversipes isp nov corresponds to a lacertoid quadruped tetrapod with primitive autopodia a sprawling gait and possibly a long tail whilst the pes was directed postero laterally the trackmaker may have been a climbing animal with adaptations that include curved claws and reversion of the pes on the ground this animal would have had a slow lumbering locomotion although it could attain higher speeds over short distances using a bipedal gait these trackways can be attributed to lacertoid reptiles possibly sphenodontids whose skeletons are encountered in the caturrita formation which overlies the santa maria formation

on some fundamentals of igneous petrology the age old process of crystal fractionation leading to the diversity of the igneous rocks and earth itself is an exceedingly well understood chemical process in magmatism and physical chemistry but the broader physical aspects of this and related processes have proven elusive on many fronts especially in its relation to the spatial variations in rock composition texture and macroscopic features like layering magmatic systems be they volcanic dikes sills or plutons are generally analyzed with a problem at hand and an end result in mind the processes invoked to solve these problems which are most often purely chemical are often unique to each problem with few if any general principles emerging that are central to understanding the wider perspective of magmatic processes and problems an attempt is made at the outset to provide a list of inviolate magmatic first principles that are relevant to analyzing most magmatic problems these involve initial conditions critical crystallinity solidification fronts transport and emplacement fluxes phenocrysts xenocrysts primocrysts crystal size layering and crystal sorting thermal convection magmatic processes are physical along with these principles two reference magmatic systems are suggested where the initial conditions and outcome are unequivocal the sudbury impact melt sheet and the hawaiian lava lakes sudbury formed in 5 min by superheated magma crystallized to a near uniform sequence while the tiny lava lakes formed of crystal laden slurries form a highly differentiated layered sequence the major difference is in the initial conditions of formation especially the nature of the input materials the challenge is to construct and analyze magmatic systems i e magma chambers sills dikes and lavas using these reference end members and the suggested principles the hawaiian 500 000 year volcanic record exhibits what can be expected as input materials namely a highly varied output of magma of an overall composition reflecting the abundance of entrained olivine primocrysts the provenance of these crystals is varied and within any single sample the population may be highly heterogeneous in composition from crystal to crystal yet the overall pattern of chemical fractionation is exceedingly regular and well defined if similar inputs go to form large intrusions these systems will undoubtedly be dominated by crystal rich slurries which provide a vast set of physical processes promoting exotic layering and at the same time given the effects of annealing and continued crystal growth a final chemical record adhering to all the time honored effects of crystal fractionation the long assumed initial condition of instantaneously emplaced crystal free magmas cannot reasonably produce the observed rock records

petrology of gabbroic xenoliths in 1960 kilauea basalt crystalline remnants of prior 1955 magmatism the 1960 kapoho lavas of kilauea s east rift zone contain 1 10 cm xenoliths of olivine gabbro olivine gabbro norite and gabbro norite textures are poikilitic ol sp cpx in pl and intergranular cpx pl ol opx poikilitic xenoliths which we interpret as cumulates have the most primitive mineral compositions fo82 5 cpx mg 86 5 and an80 5 many granular xenoliths ol and noritic gabbro contain abundant vesicular glass that gives them intersertal hyaloophitic and overall open textures to suggest that they represent mush and crust of a magma crystallization environment their phase compositions are more evolved fo80 70 cpx mg 82 75 and an73 63 than those of the poikilitic xenoliths associated glass is basaltic but evolved mgo 5 wt tio2 3 7 5 8 wt the gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various kilauea lavas to one another feo mgo crystal liquid partitioning is consistent with the poikilitic ol gabbro assemblage forming as a crystallization product from kilauea summit magma with 8 wt mgo that was parental to evolved lavas on the east rift zone for example least squares calculations link summit magmas to early 1955 rift zone lavas 5 wt mgo through 28 34 crystallization of the ol sp cpx pl that comprise the poikilitic ol gabbros the other ol gabbro assemblages and the olivine gabbro norite assemblages crystallized from evolved liquids such as represented by the early 1955 and late 1955 lavas 6 5 wt mgo of the east rift zone the eruption of 1960 kapoho magmas then scoured the rift zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift zone magmas

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

magma mixing and petrogenesis of the 13 november 1985 eruptive products at nevado del ruiz colombia abstract magma erupted on november 13 1985 at nevado del ruiz colombia was compositionally diverse with bulk compositions ranging from andesite 59 sio2 and 6 mgo to dacite 65 sio2 and 3 mgo the textural heterogeneity of juvenile products is emphasized by clasts of black scoria gray scoria black scoria with white pumiceous inclusions banded scoria banded pumices and white pumices numerous phenocrysts are out of equilibrium in the andesitic component including magnesian olivine sodic plagioclase and hypersthene plagioclase compositions are bimodal such features and textural heterogeneities are interpreted to result from transfers of phenocrysts by a mechanical mixing process mainly from the felsic component to the mafic one three mineralogical associations have been observed dacitic association andesine an 35 50 hypersthene en 68 70 edenite augite phlogopite magnetite and ilmenite andesitic association labradorite an 55 65 bronzite en 75 78 pargasite augite phlogopite magnetite and ilmenite mafic association magnesian olivine fo 79 87 and bronzite en 78 83 basic magma was not erupted but is thought to be responsible for triggering the eruption it could have been injected into a shallow dacitic reservoir ph2o about 1 kbar destabilized it and mechanical mixing could have started in the magma chamber or through a conduit just before eruption the basic magma evolved completely to andesite as suggested by the experiments of kouchi and sunagawa in 1983 while some original dacitic magma was preserved as complete homogenization was not attained the resulting products are texturally heterogeneous such a genesis explains the mineralogical disequilibria

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself