crystals stirred up 1 direct numerical simulations of crystal settling in nondilute magmatic suspensions 1 this is the first paper in a two part series examining the fluid dynamics of crystal settling and flotation in the lunar magma ocean a key challenge in constraining solidification processes is determining the ability of individual crystals to decouple from vigorous thermal convection and settle out or float the goal of this paper is to develop a computational methodology capable of capturing the complex solid fluid interactions that determine settling and flotation in the second paper we use this computational approach to explore the conditions under which plagioclase feldspar would be able to buoyantly float and form the earliest crust on the moon the direct numerical method described in this paper relies on a fictitious domain approach and captures solid body motion in 2d and 3d with little overhead beyond single fluid calculations the two main innovations of our numerical implementation of a fictitious domain approach are an analytical quadrature scheme which increases accuracy and reduces computational expense and the derivation of a multibody collision scheme advantages of this approach over previous simulations of crystal bearing magmatic suspensions include the following 1 we fully resolve the two way interaction between fluid and solid phases implying that crystals are not only passively advected in an ambient flow field but are also actively driving flow and 2 we resolve the flow around each individual crystal without assuming specific settling speeds or drag coefficients we present several benchmark problems and convergence tests to validate our approach

k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

the dynamics of magma mixing in a rising magma batch the conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid dynamical experiments the results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number i mu 1 u g delta rho r 2 where 1 is the viscosity of the fluid u is the velocity g is the acceleration due to gravity is the density difference between the two fluids and r is the radius of the tube the parameter i represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls and the buoyancy forces which tend to keep the interface horizontal the experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube they show that overturning of the initial density stratification and mixing occur when i order 10 1 the two fluids remain stratified when i order 10 3 transitional states are observed when 10 3 i 10 1 these results are nearly independent of reynolds number re 1 rho 1 ur mu 1 and viscosity ratio mu 1 mu 2 in the range of 10 2 mu 1 mu 2 10 3 and re 1 300 applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch this mechanism can explain some occurrences of small volume mixed lava flows

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

rheology of bubble bearing magmas the rheology of bubble bearing suspensions is investigated through a series of three dimensional boundary integral calculations in which the effects of bubble deformation volume fraction and shear rate are considered the behaviour of bubbles in viscous flows is characterized by the capillary number ca the ratio of viscous shear stresses that promote deformation to surface tension stresses that resist bubble deformation estimates of ca in natural lava flows are highly variable reflecting variations in shear rate and melt viscosity in the low capillary number limit e g in carbonatite flows bubbles remain spherical and may contribute greater shear stress to the suspension than in high capillary number flows in which bubble deformation is significant at higher ca deformed bubbles become aligned in the direction of flow and as a result contribute less shear stress to the suspension calculations indicate that the effective shear viscosity of bubbly suspensions at least for ca 0 5 is a weakly increasing function of volume fraction and that suspensions of bubbles are shear thinning field observations and qualitative arguments however suggest that for sufficiently large ca ca greater than about 1 the effective shear viscosity may be less than that of the suspending liquid bubbles reach their quasi steady deformed shapes after strains of order one for shorter times the continuous deformation of the bubbles results in continual changes of rheological properties in particular for small strains the effective shear viscosity of the suspension may be less than that of the liquid phase even for small ca results of this study may help explain previous experimental theoretical and field based observations regarding the effects of bubbles on flow rheology

crystals stirred up 2 numerical insights into the formation of the earliest crust on the moon 1 this is the second paper in a two part series examining the fluid dynamics of crystal settling and flotation in the lunar magma ocean in the first paper we develop a direct numerical method for resolving the hydrodynamic interactions between crystals and their feedback on the flow field in magmatic liquid in this paper we use this computational technique to test the leading model for the formation of the earliest crust on the moon the anorthositic lithology of the lunar crust is thought to have been formed by the flotation of buoyant plagioclase crystals at a time when the lunar mantle was still wholly or largely molten this model is appealing from an observational point of view but its fluid dynamical validity is not obvious because 1 plagioclase probably started crystallizing very late i e when the magma ocean was already 80 solidified and 2 a significant portion of the shallow lunar crust consists of almost pure plagioclase 90 vol requiring very efficient plagioclase segregation the goal of this study is to better understand the fluid dynamical conditions that hinder or facilitate crystal settling or flotation our approach complements earlier studies by explicitly linking the petrological and fluid dynamical evolution and by focusing on the effect of increasing crystal fraction we find that crystal settling was probably possible throughout the entire solidification history of the lunar magma ocean as long as crystal sizes were sufficiently large r 1 mm and crystal fraction sufficiently low 13

how late are k feldspar megacrysts in granites abstract various petrologists have suggested that k feldspar megacrysts grow in granites that are extensively crystallized even at subsolidus conditions however experimental evidence indicates that though k feldspar nucleates relatively late in the crystallization history abundant liquid is available for development of large crystals a great deal of evidence involving many different factors favours a magmatic phenocrystic origin for k feldspar megacrysts in granites namely simple twinning oscillatory zoning euhedral plagioclase inclusions and concentric crystallographically controlled arrangements of inclusions in addition abundant evidence has been presented of 1 mechanical accumulation of k feldspar megacrysts in granites 2 alignment of megacrysts and megacryst concentrations in magmatic flow foliations 3 involvement of megacrysts in zones of magma mixing in granite plutons and 4 occurrence of megacrysts in some volcanic rocks implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains

viscosity of himalayan leucogranites implications for mechanisms of granitic magma ascent the viscosity of the gangotri himalayan leucogranite has been experimentally determined between 80 o and 1100oc 300 and 800 mpa for meltwater contents of 3 98 and 6 66 wt the melt viscosity is independent of pressure and shows an arrhenian behavior relative to temperature within the range of conditions investigated we present an empirical relation that can be used to determine leucogranite magma viscosities knowing their meltwater content and temperature this relation together with phase equilibria experiments constrain the viscosity of the itimalayan leucogranit s to be around 104 5 pas during their eraplacement these viscosities and the widths of dikes belonging to the feeder system are consistent with the theoretical relationship relating these two parameters and show that the precursor magma of the leucogranite was at near liquidus conditions when emplaced within host rocks with preintmsion temperatures around 350oc calculated terminal asa nt rates for the magma in the dikes are around 1 m s magma cmber assembly time is on ths basis estimated to be less than 100 years for a volume of 150 icm3 in addition the dynamical regime of the magma flow in the dikes was essentially laminar thus allowing preservation of any chemical heterogeneity acquired in the source these results constrain the viscosity of melts f rmed during the first steps of crustal anatexis those involving muscovite breakdown to be alsoround 104 5 pas thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement

granitic melt viscosity and silicic magma dynamics in contrasting tectonic settings models for granitic melt viscosities reveal the influences of h 2 o temperature and tectonics on melt segregation ascent and emplacement low t magmas are wetter than those formed at high t so log 10 melt viscosity pa s is 3 2 6 3 for leucogranite to tonalite only 10 1000 times more viscous than basalts there should be no correlation between critical dyke width to prevent freezing and tectonic setting granitic magma ascent rates in dykes vary by

quantum magmatism magmatic compositional gaps generated by melt crystal dynamics compositional gaps are common in volcanic series worldwide the pervasive generation of compositional gaps influences the mechanical and thermal properties of the crust and holds clues on how our planet differentiates we have explored potential mechanisms to generate these gaps using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs we show that gaps are inherent to crystal fractionation for all compositions as crystal liquid separation takes place most efficiently within a crystallinity window of 50 70 vol crystals the probability of melt extraction from a crystal residue in a cooling magma chamber is highest in this crystallinity window due to 1 enhanced melt segregation in the absence of chamber wide convection 2 buffering by latent heat of crystallization and 3 diminished chamber wall thermal gradients this mechanical control of igneous distillation is likely to have played a dominant role in the formation of the compositionally layered earth9s crust by allowing multiple and overlapping intrusive episodes of relatively discrete or quantized composition that become more silicic upward

numerical simulations of parasitic folding in multilayers we use the finite element method to simulate slow viscous newtonian flow in two dimensions without gravity and to model asymmetric sand z shaped and symmetric m shaped parasitic folds during multilayer folding during multilayer folding the matrix between stiffer layers shows a deformation close to pure shear in the hinge area and a combination of pure and simple shear in the limb areas thinner layers placed between thicker layers develop symmetric parasitic folds in the hinge and eventually asymmetric parasitic folds in the limbs of the larger fold our results verify numerically the theory that asymmetric parasitic folds develop from symmetric buckle folds that are sheared by the hingeward relative displacement of the thick layers in the limbs of the first order fold to develop asymmetric shapes the amplitudes of the parasitic folds must exceed a critical value before the first order fold begins to amplify otherwise the parasitic folds are unfolded during flattening that takes place in the limb area between the thick layers more than five thin layers are necessary to generate distinct asymmetric parasitic folds for the applied model setting more layers generate higher amplification rates in the thin layers and hence higher amplitudes 2006 elsevier ltd all rights reserved

quantifying magma segregation in dykes abstract the dynamics of magma flow is highly affected by the presence of a crystalline load during magma ascent it has been demonstrated that crystal melt segregation constitutes a viable mechanism for magmatic differentiation moreover crystal melt segregation during magma transport has important implications not only in terms of magma rheology but also in terms of differentiation of the continental crust however the influences of the crystal volume percentage of their geometry their size and their density on crystal melt segregation are still not well constrained to address these issues we performed a parametric study using 2d direct numerical simulations which model the ascension of a crystal bearing magma in a vertical dyke using these models we have characterised the amount of segregation as a function of different physical properties including the density contrast between crystals and the melt phase the size of the crystals ac and their aspect ratio r results show that small values of r do not affect the segregation in this case the amount of segregation depends upon four parameters segregation is highest when and ac are large and lowest for large pressure gradient pd and or large values of dyke width wd these four parameters can be combined into a single one the snumber which can be used to quantify the amount of segregation occurring during magma ascent based on systematic numerical modelling and dimensional analysis we provide a first order scaling law which allows quantification of the segregation for an arbitrary snumber and encompassing a wide range of typical parameters encountered in terrestrial magmatic systems although developed in a simplified system this study has strong implications regarding our understanding of crystal segregation processes during magma transport our first order scaling law allows to immediately determine the amount of crystal melt segregation occurring in any given magmatic dyke system

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

middle ordovician organic matter assemblages and their effect on ordovician derived oils two distinct middle ordovician oil prone organic matter assemblages occur in thermally immature to marginally mature petroleum source rocks from iowa the substantially more oil prone assemblage a generates the unique ordovician oil fingerprint which has been associated with the organic walled microfossil gloeocapsamorpha prisca the second assemblage b generates a more ordinary signature the two assemblages which are mixed or interbedded in many ordovician sediments of north america explain the variations in oils derived from ordovician source rocks these mixtures also aid in interpretation of the chemistry of g prisca core samples examined are from three wells in iowa penetrating the primarily carbonate sequence from the guttenberg member of the decorah formation down to the st peter sandstone microscopy rock eval pyrolysis and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages solvent extracts from rocks containing predominantly assemblage a have the diagnostic ordovician gas chromatographic signatures with an odd preference in c11 c19 normal alkanes low amounts of heavier normal alkanes and virtual absence of isoprenoids including pristane and phytane although assemblage a occurs only in very thin beds it is potentially so oil generative that thicknesses in inches may be significant extracts from r cks containing predominantly assemblage b show reduced odd preference a full profile of normal alkanes and conspicuous occurrence of isoprenoids including pristane and phytane these samples from iowa are representative of middle ordovician source rocks found in most paleozoic basins of north america as well as baltic europe and australia therefore the conclusions drawn from these middle ordovician rock samples from iowa provide a basis to explain variations in ordovician derived oils found in many other areas

granite rheology magma flow and melt migration granite magma rheology has long been studied by rock physicists petrologists and experimentalists new data on viscosity of dry and h 2 o bearing melts and glasses in a wide range of temperatures and h 2 o content changed drastically the general view on the arrhenian behaviour of granitic melts recent studies on the deformation of partially molten granite rocks at up to 50 vol of melting show the failure of the rcmp rheologically critical melt percentage concept for granites we present new direct measurements of melt migration from a combination of centrifuge experiments falling sphere viscometry of partially molten granites assisted by a centrifugal field and viscoelastic behaviour of partially molten granites in torsion oscillation apparatus the results suggest that the extrapolation of the einstein roscoe equation for the viscosity of partially molten rocks does not give adequate results for granites with 50 vol of melt phase rheology of granite magmas is very sensitive to chemical composition and viscosity of the melt phase adding h 2 o to partially molten granites results in a decrease in the viscosity of the melt phase as well as dissolution of quartz crystals in turn this produces more felsic and viscous melt thus these factors smear out the contrast between partially and completely molten granites and between anhydrous and h 2 o bearing magmas at moderate h 2 o contents 1 to 2 wt

experimental constraints on magma ascent rate for the crater flat volcanic zone hawaiite phase equilibria and isothermal constant rate decompression experiments conducted on a hawaiite from the crater flat volcanic zone near yucca mountain nevada provide insight into the dynamics of magma transport for this magmatic province h 2 o contents up to 4 6 wt in melt inclusions the lack of plagioclase phenocrysts and the presence of amphibole all suggest that phenocrysts and melt in the crater flat volcanic zone were equilibrated at near water saturated conditions during ascent comparison of decompression induced crystallization of plagioclase microlites in experiments with natural tephra implies that magma ascent rates for the region are 0 04 m s if the magmas are water saturated estimates of minimum ascent rates increase if the magmas are water undersaturated a two stage process is needed to generate reaction rims found on natural amphibole suggesting that small batches of magma were briefly stored 3 5 days at depths as shallow as 800 m prior to eruption

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

new interpretation for the main sediment source of the rapidly deposited sediment drifts on the northern slope of the south china sea abstract the slope of the northern south china sea scs is one of the world s most active areas of modern marine sedimentary processes a rapidly deposited sediment drift rdsd was identified on the slope near dongsha island the sources of this body have been studied and discussed by several authors but no consensus has been achieved based on multi channel seismic profiles interpretation and data analysis of long term observation of bottom currents we identified that although the amounts of sediments input from the pearl and hanjing rivers and from kao ping and tseng wen rivers are large considering the current situation of northern scs none of the pearl river the hanjiang river the kao ping river and the tseng wen river can maintain the deposition pattern of the northern scs margin as the single source especially to maintain the supper high sedimentation rate at the rdsd area the miocene drape formation mdf calmly draped over an unconformity after rifting stage resulting in a relatively constant thickness of about 2000 m throughout the overall northern scs continental margin and followed by a standard configuration of a clinoform the pliocene formation and another clinoform of about another 700 m in total thickness due to dongsha uplift since 3 ma roughly 1 2 of mdf and the n2 formation in the dongsha area measures 150 km by 200 km were denudated resulting in an average annual denudation rate of 60 106 t y if these sediments were deposited on a slope region within an area measuring 100 km by 100 km the sedimentation rate could reach 250 cm ky this is clearly an impressive new sediment source that has never been discussed before moreover the rdsd is near the dongsha uplift and on the adjacent slope we believe that this new sediment source should be the main contributor for the rdsd development

short eruption window revealed by absolute crystal growth rates in a granitic magma giant volcanic eruptions occur when large volumes of magma accumulate in crustal reservoirs and do not cool and crystallize to form a solid pluton of rock within the crust geochronological dating of a pluton from elba italy shows that the magma solidified in the crust within just 10 40 thousand years of being injected into the crust implying that the time window for an eruption was short

numerical modelling of magma transport in dykes the rheology and dynamics of an ascending pure melt in a dyke have been extensively studied in the past from field observations it is apparent that most dykes actually contain a crystalline load the presence of a crystalline load modifies the effective rheology of such a system and thus the flow behaviour indeed the higher density and viscosity of each crystal compared to the melt cause a decrease of the ascent velocity and modify the shape of the velocity profile from a typical poiseuille flow to a bingham type flow a common feature observed in the field is the arrangement of crystals parallel or at a very low angle to the edge of the dyke such a structural arrangement is often interpreted as the result of magma flow which caused the crystals to rotate and align within the flow direction but this process remains unclear another issue related to the introduction of a crystalline load concerns the possibility for crystals to be segregated from a viscous granitic melt phase during magma ascent the implications of such a process on magmatic differentiation have not previously been considered nor has such a process been previously investigated via numerical models in this study we examine the flow dynamics of a crystal bearing granitic melt ascending in a dyke via numerical models in our models both the crystal and melt phases are represented as highly viscous fluids in a stokes regime our results reveal that the presence of crystals in the melt modifies the magma velocity profile across the dyke furthermore we observe that whilst crystals continually rotate in the shear flow over one period of revolution their major axis has a high probability to be aligned parallel to the flow direction moreover some experiments showed that the melt phase can effectively be squeezed out from a crystal rich magma when subjected to a given pressure gradient range this demonstrates that crystal melt segregation in dykes during granitic magma ascent constitutes a viable mechanism for magmatic differentiation

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

a model for the rheology of particle bearing suspensions and partially molten rocks an edited version of this paper was published by agu copyright 2009 american geophysical union

modelling of melt segregation processes by high temperature centrifuging of partially molten granites i melt extraction by compaction and deformation the present study deals with the experimental modelling of two different mechanisms of crystal melt segregation in crustal rocks 1 the buoyancy driven compaction of the crystal melt matrix and 2 melt filtering in a partially crystalline matrix due to differential stresses these two segregation mechanisms have differing relative efficiencies in the deformation of crustal rocks and result in different texture scales depending on melt fraction melt viscosity and tectonic stresses a centrifuge furnace has been used in the present study for the modelling of melt migration in partially molten granitic rocks samples of beauvoir granite massif central france with a grain size of 0 16 0 5 mm and dimensions of diameter 5 mm length 16 mm were used these samples had been pre fused at temperatures of 1000 1075 c yielding an initial average melt fraction of 45 50 volume per cent the centrifuging of partially melted samples during 6 hr at an acceleration of 1000g g is gravity results in a linear vertical distribution of melt over the length of the sample without the development of a compaction layer the gradient of the melt fraction melt migrates to the top of samples correlates with temperature 1075 c 7 volume per cent mm 1 1050 c 4 volume per cent mm 1 1000 c 1 5 volume per cent mm 1 the calculated rate of melt migration varies from 3x10 5 cm s 1 1075 c to 2x10 6 cm s 1 1000 c differential stresses of 0 7 1 4 mpa have been generated in the centrifuge by putting a piston weight 1 02 2 05 g diameter 4 5 mm on the top of the partially melted sample which is then centrifuged at 1000g the rate of melt squeezing from the sample in this case is about two orders of magnitude higher than that observed without the piston after centrifuging for 6 hr a compaction layer below the piston is formed with a thickness of 2 5 mm and a crystal fraction of 70 65 volume per cent further centrifuging 15 hr does not result in any increase of the compaction layer thickness or volume percentage of crystals in it the comparison of the two segregation mechanisms confirms the much greater efficiency of differential stress induced melt segregation and accumulation in veins and pockets than the compaction mechanism

chilled margins in igneous rocks abstract when hot magma contacts cold solid rock a chilled margin forms because the heat flux from the magma cannot balance the very large initial conductive flux in the rock however the conductive flux decreases with time and can eventually be exceeded by the heat flux from the magma in these circumstances the chilled margin begins to remelt and can even disappear this process is modelled quantitatively for the case of a constant heat flux from the magma the results are applied to the turbulent steady flow of komatiite magma in dykes and as lavas the initial chilled margin with cold rock only survives for a few tens of seconds and so does not inhibit thermal erosion significantly the theory can also be applied to magmatic intrusions where the heat flux varies with time results are presented for the remelting of a chilled margin formed at the roof of a basaltic sill for sills with thicknesses from 10 to 1000 m the remelting of initial chilled margins from a few tens of centimetres to a few tens of metres is possible three kinds of contact are predicted at the margins of intrusions first contacts are developed where the chilled margin grows continually without any meltback second meltback can occur but does not completely redissolve the initial chilled margin this causes an internal contact to form within the intrusion third meltback is complete and the country rock can then be melted and assimilated the marginal rocks of such intrusions need not be fine grained

gas driven filter pressing in magmas most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less the buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization and this process of gas driven filter pressing assists the crystallization differentiation of magmas for gas driven filter pressing to be effective the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt these conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs gas driven filter pressing thereby adds fractionated melt to magma bodies gas driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons this process could also account for the production of voluminous crystal poor rhyolites

magma ascent by porous flow porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle recent work has identified the importance of matrix deformation and compaction in this process we present finite difference calculations based on a generalized form of darcy s law that includes matrix deformation in two dimensional cartesian and cylindrical coordinates we emphasize the existence of solitary wave solutions called magmons these waves are regions of locally high porosity that ascend through regions of low uniform porosity they differ from diapirs where the liquid and matrix ascend together the one dimensional waves that were previously reported are found to be unstable in two dimensions breaking down to form two dimensional waves of circular cross section both the development of the instability and the form of the two dimensional waves are supported by theoretical analysis numerical experiments in which two dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves the conservation can be very good in other cases larger waves appear to grow at the expense of smaller waves although the quantification of the relevant permeability and rheological parameters remains uncertain geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year in our preliminary assessment of the relevance of these results to igneous processes we find that magmons are unlikely to be important in regions of broad upwelling such as beneath spreading centers here the transport of liquid adjusts to match the supply by melting and compaction processes are not important if liquid is supplied from below to a stable partially molten region of the asthenosphere it is expected to ascend in magmons this may occur beneath oceanic volcanic centers or in subcontinental mantle it is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity if higher level processes do not obscure this influence the waveform of a magmon ascends faster than the liquid within the magmon so new liquid is taken in from above while original liquid is lost from below consequently magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface

granular flow and viscous fluctuations in low bagnold number granitic magmas recent thermal and fluid dynamical models have shown that density driven ascent of isoviscous granitic melt through the crust in narrow dykes is geologically instantaneous provided the role of suspended crystals is ignored however it is well known that solids in suspension can avect significantly the rheology and hence flow properties of magmas leading to the subjective criticism that granite ascent and emplacement rates based on simple dyke transport models may be unrealistic by estimating the magnitude of shear stresses developed during continuous flow in a 6 m wide granitic dyke we show yield strengths in excess of 10 4 pascals are required to stop flow entirely if the pressure gradient driving the flow is high using granular flow theory we show how fluctuations in particle velocity in the shear field can be used to predict the rheological behaviour of magmas with crystal contents solidosity in the range 0 1 specimen calculations for the solidosity and fluctuation on particle velocity for ascending granitic magmas show that the energy associated with these irregular movements is much greater than that expected on purely brownian grounds we find no clear correlation between viscosity i and the yield strength and while it is possible that magma has curious unexpected bonding properties magmatic suspensions where the grain size is on the scale of centimetres e g some coarsely porphyritic magmas low estimated bagnold numbers c 10 8 render bingham flow models inappropriate self organization of the flow into a crystal poor margin and crystal rich core flow differentiation by diffusion of particles towards the region of minimal shear results in the familiar plug type flow profiles traditionally associated with an inherent yield strength shear enhanced diffusion is an inevitable consequence of granular flow that leads to flow differentiation providing a simple mechanistic explanation for high concentrations of pheoncrysts in the cores of porphyritic dykes bagnold effect deviation from simple newtonian flow is unlikely to be significant until the magma has stopped ascending and emplacement begins

the geochemical signature of a felsic injection complex in the continental crust opinaca subprovince quebec abstract the archean opinaca subprovince of the superior province in northern quebec is an injection complex consisting of lower granulite facies metasedimentary terrane that was pervasively injected with a large volume of leucogranitic melt which forms innumerable thin dykes and veins the typical microstructure of most leucogranites can be divided into two groups 1 one has an open framework constructed of k feldspar crystals in which the interstices are filled with albite rich plagioclase and quartz and 2 a microstructure defined by a framework of albite rich plagioclase crystals in which the interstices are filled by quartz and in some cases small pink garnets however pegmatitic and aplitic textures also occur the leucogranites that form the injection complex are all sio 2 rich above 70 wt and have low feo t mgo tio 2 contents compared to granitic melts obtained from partial melting experiments the mafic minerals were fractionated from the magma before its emplacement into the injection complex as leucogranite the leucogranites form a coherent trend on major element diagrams that is best explained by the removal of a cumulate dominated by k feldspar from a granitic initial melt composition the cumulate rocks are enriched in k 2 o whereas the residual melts are richer in sio 2 and na 2 o based on these results leucogranites with a k feldspar framework are interpreted as cumulate rocks this indicates that the early crystallising calcic plagioclase had already been fractionated from the magma before its injection into the complex the leucogranites have rare earth element ree patterns typical of feldspar accumulation the residual melts extracted after the k feldspar framework developed are those that have the framework of albite rich plagioclase these have ree patterns typical of highly evolved melts from which accessory minerals have also been fractionated the highly evolved nature of the leucogranites that were injected into granulite facies host rocks in the opinaca subprovince has several important implications for crustal evolution 1 the large volume of evolved melts emplaced at depth released a significant amount of h 2 o into the adjacent crust thus injection complexes should be considered as a potentially important source of h 2 o in the middle crust that is capable of rehydrating the granulite terranes into which they are injected 2 a corollary of the evolved nature of the leucogranite in the injection complex is that there should be evidence of accumulation of the early fractionated material still deeper in the crust 3 these findings are natural examples that corroborate predictions from numerical modelling which suggest that the whole process from melting to crystallisation of leucogranite can be confined to deep levels of the crust below or close to the solidus this has important impact on our general view of the transfer of leucogranites in the continental crust which is the main mechanism of crustal differentiation

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

modelling of melt segregation processes by high temperature centrifuging of partially molten granites ii rayleigh taylor instability and sedimentation summary the present experimental study deals with the laboratory modelling of two different mechanisms of gravitational percolation in partially melted rocks 1 diapiric percolation of heavy material and 2 the sedimentation of heavy particles these two mechanisms of mass transport in partially melted rocks result in different scales of the segregation process in the melt crystal matrix a centrifuge furnace was used to simulate the percolation of the heavy particle layer through the partially molten granite at temperatures of up to 1000 c samples of beauvoir granite massif central france grain size 0 16 0 5 mm with an initial degree of partial melting 45 per cent were used as a matrix a layer of pt powder suspended in a melt of the same composition as the partially melted matrix was placed on the top of the granite sample after centrifuging for various times up to 2 x lo4 s x ray images of samples were obtained and the evolution of the percolation process of heavy suspension in the partially molten granite was monitored from the pt particle distribution the diapiric or finger regime of percolation starts when the growth rate of a raleigh taylor instability of the heavy layer is faster than the stokes sedimentation velocity of individual particles in the upper layer this relationship is a complex function of the size and initial concentration of heavy particles as well as the ratio of particle to crystal size the permeability of the matrix and the heterogeneity scale in the partially melted matrix at small concentrations several per cent and at large concentrations where close packing of heavy particles results in an anomalous viscosity increase in the upper heavy layer stokes sedimentation is dominant in the vertical percolation of the heavy material the sinking velocity of the diapir decreases when the size of heavy particles in it becomes comparable with the size of crystals in the partially melted granite in this situation the vertical sinking of the diapir is not stable and the horizontal instability of the vertical mass transport starts to become important mass transport via diapiric percolation results in more efficient crystal melt segregation of partially melted rocks the percolation of individual particles provides only local melt crystal flow on a scale comparable with the heavy particle size the diapiric percolation provides a much larger scale of partial melt segregation with a length scale comparable with the diapir size

rheological properties of magma from the 1538 eruption of monte nuovo phlegrean fields italy an experimental study abstract the rheology of natural partially crystallized samples from monte nuovo 1538 ad eruption have been investigated using a high temperature high pressure internally heated pressure vessel within the temperature interval of investigation the samples exhibited newtonian rheology at deformation strain rates up to 10 4 s 1 at higher strain rates a decrease of viscosity with increasing strain rate shear thinning eventually terminating in a sudden viscosity drop stress weakening was observed stress weakening occurred at higher applied strain rates and stresses with increasing temperature the microstructural analyses of the samples revealed that stress weakening resulted from strain localization which occurred in brittle and viscous regimes at the lowest and highest temperatures investigated respectively the different rheological behaviors of the investigated material is likely responsible for the changes in the dynamics of magma ascent within the volcanic conduit during the eruption of monte nuovo volcano we infer that the occurrence of syn eruptive decompression driven crystallization observed in previous studies induced strong variations in the rheological properties of magma which in turn was responsible for the changes of eruptive style observed during the monte nuovo eruption

the effect of particle shape on suspension viscosity and implications for magmatic flows 1 the rheology of crystal bearing magma and lava depends on both the shape and volume fraction of the suspended crystals we present the results of analogue rheometric experiments on monodisperse suspensions of solid particles in a newtonian liquid in which particle volume fraction and aspect ratio rp are varied systematically we find that the effect of on viscosity is well captured by the maron pierce model and that this model is valid across the range of particle aspect ratios investigated 0 04 rp 22 i e strongly oblate to strongly prolate when the maximum packing fraction m is treated as a fitting parameter the value of m derived from fitting to our experimental data depends strongly and systematically on particle aspect ratio hence m represents an effective proxy for the influence of particle shape on suspension rheology we present a simple relationship for m rp which allows the viscosity of a suspension to be calculated as a function of and rp only we investigate the impact of accounting for crystal shape when modelling volcanic flows by simulating the eruption of magma carrying crystals of different aspect ratio and conclude that the effect of crystal shape should not be neglected

granitic magma ascent and emplacement neither diapirism nor neutral buoyancy it is probable that granitic magma ascent does not result from the intrin sic properties of the magmas within the uppermost crust neither the reduced viscosity nor the density contrast between magma and surroundings are them selves sufficient to induce either low inertia flow diapirism or fracture induced magma propagation dyking igneous diapirism is intrinsically restricted to the lower ductile crust dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels a neutral buoyancy level in the crust at which magma ascent should stall is never observed this is demon strated by coeval emplacement of magmas with different compositions and densi ties and the negative gravity anomalies measured over many granitic plutons we suggest that deformation through strain partitioning is necessary to magma ascent pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties as a result of its intermittent character deformation both local and regional induces magma pulses and this may have important consequences for the chemical homogeneity of intruded magmas ascent of granitic leucogranitic to tonalitic magma in the continental lithosphere is the most potent means of mass transfer and chemical segregation between the lower and upper crust granitic magmatism contributes to crustal recycling and to a lesser extent continental growth in rapidly accreted crust granitic rocks can occupy up to one third of the volume of the middle crust meissner 1986 together with shear zones they are the most conspicuous effects of plate convergence and collision in orogenic zones consequently they are a key element in understanding the evolution of earth s mechanical workings as most felsic magmas are emplaced in a liquid or near liquid state at temperatures over 800 a major heat source must be involved in their genesis the existence of such a heat source also has consequences for the deformation regime of the crust com monly a large temperature rise in the lithosphere will also increase the capacity of crustal material to respond to stress by deformation such a thermal anomaly will contribute to the dissipation of energy and thus buffer the temperature effects the stress regime will be imposed externally by the tectonic processes operating at the time as a reaction to a significant increase in crustal heat flow the rheological capacity of rocks to sustain stresses decreases and this activates deformation therefore most departures from a normal heat distribution in the lithosphere will trigger deformation e g formation of extensional rift zones or shear zones in convergent situations

formation of igneous layering in granodiorite by gravity flow a field microstructure and geochemical study of the tuolumne intrusive suite at sawmill canyon california layered granites and granodiorites occur in many plutons they occur together with non layered granodiorites at the margin of the tuolumne intrusive suite one of the largest late cretaceous intrusions in the sierra nevada results from field observations quantitative microstructural analysis and whole rock compositions from the non layered kuna crest half dome and cathedral peak granodiorites and layered granodiorites from the sawmill canyon area are compared to determine which processes contributed to the formation of the layering the non layered granodiorites have foliated but homogeneous microstructures which are attributed to the orientation of crystals with high aspect ratios when the magma flowed before solidification their compositions are consistent with magma mixing and fractional crystallization the microstructure in the layered granodiorite is heterogeneous and shows normal gradation of the dense primocryst minerals magnetite hornblende and titanite at the base and inverse size gradation with large crystals of the less dense primocryst phases plagioclase and biotite at the top and a systematic change in the preferred orientation of minerals from base to top macroscopic structures in the layers indicate erosion and the action of traction currents when the layers formed and the sinking of the dense minerals and rise of interstitial melt immediately afterwards the layered rocks have similar grain size and mineral compositions to the non layered granodiorites and thus could be derived from the same magmas as the kuna crest half dome and cathedral peak non layered granodiorites the major and trace element variations in the layered rocks are very different from those in the non layered rocks and indicate a strong segregation of crystals during formation of the layers however the separation process did not fractionate the main rare earth element bearing phases from one another the macrostructure and microstructural features in the layers resemble those from hyper concentrated sediment gravity flows the layering is interpreted to have formed from gravity flows of melt and crystals initiated by syn magmatic tectonic deformation and seismic shaking two types of layering are identified one called normal layering represents material derived from a single granodiorite magma and deposited from a relatively low energy gravity flow whereas the other called melanocratic layering incorporated mafic minerals eroded from the substrate had the felsic tops of layers eroded off by subsequent flows incorporated crystals from more than one magma source and formed from gravity flows that had higher energy

the rheological transition in plagioclase bearing magmas 1 volcanological processes such as melt segregation ascent and eruption are directly dependent on the rheological behavior of magmatic suspensions an increase of the crystal fraction of the suspension leads to the formation of a solid particle network which abruptly increases magma viscosity the crystal fraction at which this rheological transition occurs depends on parameters such as the strain rate and the size shape and sorting of particles to determine the influence of the crystal shape on the rheological transition suspensions of plagioclase a representative anisometric crystal have been investigated at high temperatures and pressures synthetic suspensions with crystal fractions s ranging from 0 38 to 0 75 were deformed both in compression and torsion in a paterson apparatus at 300 mpa 900 c and 800 c and for strain rates between 1 0 10 5 and 1 0 10 3 s 1 all suspensions exhibit a non newtonian shear thinning rheological behavior the experimental results coupled with existing data and models at low crystal fractions s 0 3 allow several rheological domains to be identified from steady state flow to strain weakening each characterized by a specific microstructure in particular a progressive evolution from a pervasive to a strain partitioning fabric is found when increasing the crystal fraction our results highlight the influence of both the strain rate and the shape of crystals on the rheological behavior of magmas during crystallization magmatic suspensions of anisometric minerals such as plagioclase would develop a solid particle network earlier s 0 3 than suspensions of isometric minerals s 0 5 our study shows that localization of strain early in the crystallization history of mushy zones in the magma chamber near the conduit margins and at the base of lava flows would facilitate the mobilization the transfer and the final emplacement at the surface of highly viscous feldspar rich magmas

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

transient two dimensional dynamics in the upper conduit of a rhyolitic eruption a comparison of closure models for the granular stress to elucidate the role of particle collisions in redistributing momentum after fragmentation a numerical study was performed comparing the behavior of an inviscid or collision less granular material with a granular material whose viscosity and pressure were modeled using kinetic theory the granular viscosity calculation is sensitive to particle size and three particle sizes were considered 0 0002 m 0 002 m and 0 02 m a critical volume fraction of gas 0 75 identifies the onset of fragmentation dividing the region of rhyolitic magma with dispersed bubbles from the region of turbulent gas with dispersed particles the transient simulations can be divided into two dynamic regimes an initial shock followed by a transition to steady state during the initial shock phase treatment of the granular pressure and viscosity led to greater particle velocities relative to the inviscid calculation due to the development of higher gas pressure at fragmentation however as steady state is approached the viscosity slows the particle phase relative to the inviscid counterpart for particles greater than a millimeter the modeled sub millimeter particle velocity was insensitive to the treatment of the granular pressure and viscosity centimeter scale particles have a much higher granular viscosity up to 10 1 pa s and the kinetic theory calculation is thus relevant in these conditions after reaching steady state the differential velocity between gas and particles at the conduit exit correlates with particle size for both the inviscid and granular viscosity calculations negligible differential velocity develops for sub millimeter particles but centimeter scale particles exit the conduit with 5 differential velocity between phases

rheology of magmas with bimodal crystal size and shape distributions insights from analog experiments 1 magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts as often indicated by volcanic rock textures in this study we present two different experiments that investigate the flow behavior of these bidisperse systems in the first experiments rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller prolate particles microlites and larger equant particles phenocrysts in a bubble free newtonian liquid silicate melt our data show that increasing the relative proportion of prolate microlites to equant phenocrysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude consequently the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles we propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models in a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup the resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in well organized strings with sphere depleted shear bands in high shear regions these observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio the implications for magma flow are discussed and rheological results and textural observations are compared with observations on natural samples

potential causes for the non newtonian rheology of crystal bearing magmas experimental studies indicate that crystal bearing magma exhibits non newtonian behavior at high strain rates and solid fractions we use a zero dimensional 0 d inversion model to reevaluate rheological parameters and shear heating effects from laboratory data on crystal bearing magma the results indicate non newtonian behavior with power law coefficients of up to n 13 5 it has been speculated that finite strain effects shear heating power law melt rheology or plasticity are responsible for this non newtonian behavior we use 2 d direct numerical crystal scale simulations to study the relative importance of these mechanisms these simulations demonstrate that shear heating has little effect on aggregate bulk rheologies finite strain effects result in both strain weakening and hardening but the resulting power law coefficient is modest maximum n 1 3 for simulations with spherical crystals the strain weakening and hardening behavior is related to rearrangement of crystals rather than strain rate related weakening finite strain effects were insignificant in a numerical simulation with naturally shaped crystals strain partitioning into the melt phase may induce microscopic stresses that are adequate to provoke a nonlinear viscous response in the melt large differential stresses and low effective stresses revealed by the simulations are sufficient to cause crystals to fail plastically numerical experiments that account for plastic failure show large power law coefficients n 50 in some simulations we conclude that this effect is the dominant cause of the strong nonlinear viscous response of crystal bearing magmas observed in laboratory experiments