time scales of hybridisation of magmatic enclaves in regular and chaotic flow fields petrologic and volcanologic implications this paper describes numerical models of advection diffusion between enclaves and host magmas applied with the aim of estimating time scales during which enclaves can be homogenised in particular advection was simulated using a numerical system consisting of regular and chaotic regions results indicate that the homogenisation time of enclaves in chaotic regions is several orders of magnitude faster than in regular regions for instance an enclave with a diameter of 100 cm may be homogenised in the chaotic region in 380 years assuming an advection velocity of 10 cm year whereas in the regular region it would require 6 5 105 years for complete homogenisation this implies that in the same magmatic system large differences in the degree of homogenisation may co exist generating magmatic masses with large spatial and temporal inhomogeneities

the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

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

interfacial turbulent mixing in stratified magma reservoirs abstract we analyse the interfacial turbulent mixing of molten magmas in a two layer stratified convecting magma chamber we examine situations where convection is driven by the heat and or compositional flux associated with the melting of a solid boundary the model applies to the case in which the rayleigh number is sufficiently large that the effect of a viscosity difference on turbulent entrainment is negligible and so the model predictions represent an upper bound for interfacial mixing our results suggest that the extent of mixing induced by the thermal buoyancy flux is relatively small however the compositional buoyancy fluxes associated with phase change may be much larger and the mixing they induce may significantly affect the evolution of the reservoir our analysis of the melting of the floor of a two layer stratified magma chamber shows that turbulent mixing at the interface between the two magmas may suppress the melting and decrease the final amount of melt produced when the system reaches thermal equilibrium it is also shown that although floor melting tends to decrease the density of the lower layer of magma somewhat counterintuitively turbulent interfacial mixing with the overlying layer of smaller density may inhibit large scale overturn and preserve the stratified state of the chamber in the case in which both the floor and roof of a chamber melt turbulent mixing at the interface between the newly formed light melt at the roof and the lower layer of dense magma induces a shift of the melting process from the floor to the roof for typical parameter values in a basaltic magma chamber we estimate that under conditions of very large rayleigh number the amount of crustal melt produced at the roof may be as high as 50 of that produced at the floor

variations in lava composition during the march 1981 eruption of mount etna and the implications of a compositional comparison with earlier historic eruptions the march 1981 eruption of mount etna occurred on the northern slopes of the volcano in an area previously inactive for more than 400 a a radial fissure system extending 7 5 km and producing 20 2 106 m3 of basic hawaiite lava opened at 2600 m a s l on 17th march and migrated downslope to 1125 m a s l where activity ceased on 23rd march gravity evidence points to the draining of a radial storage dyke located between sea level and 1 km a s l whole rock chemistry and petrography suggests that the magma in the dyke was a hybrid supporting crystal and compositional gradients developed as a result of progressively greater mixing of residual 1979 basic hawaiite magma with fresh basic hawaiite magma during permissive filling of the dyke around september 1980 these mixing gradients caused the eruption to display a significant contrast in the concentrations of al2o3 sr cao mgo and fe2o3 total between early high altitude and later low altitude products the fresh magma which entered the high level system around september 1980 ascended rapidly from a deep storage region at 16 24 km depth

evaporite accumulation during the messinian salinity crisis the suez rift case 1 the mediterranean basin may have not always been connected to the atlantic ocean during the messinian salinity crisis 5 96 5 33 myr the mediterranean sea reduced progressively its connection with the global ocean by a complex combination of tectonic and glacio eustatic processes during this period deep erosion occurred on the margin and on the continent furthermore a large quantity of evaporites was deposited in the basins 1500 m the way by which the evaporites accumulated in the various sub basins has not been precisely determined here we demonstrate by quantitative analysis that a combination of several mediterranean sea level drawdown events associated with limited atlantic sea inflow and continuous river discharge can explain the quantity of evaporites in the mediterranean basins and the observation of an intermediate lowstand at 500 m using a paleogeographical reconstruction and a numerical model we show that the evaporites of the gulf of suez gos are a consequence of the threshold which disconnected the gos red sea from the eastern mediterranean at the end of the miocene

some effects of viscosity on the dynamics of replenished magma chambers some aspects of the dynamical behavior of magma chambers replenished from below with hotter but denser magma have been modeled in a series of laboratory experiments in previously reported work the fluids used were aqueous solutions of comparable viscosity and thus the results should be applicable to basaltic magma chambers in which the magmas do not vary greatly in viscosity in that case the lower layer cools by convective heat transfer to the fluid above and crystallization causes the density of the residual liquid in the lower layer to decrease when the density becomes equal to that in the upper layer sudden overturning and intimate mixing take place the present paper reports experimental results that allow us to extend the application to systems in which there is a large viscosity ratio between the resident and the injected fluid for example to calcalkaline magmas where magma viscosity can vary by as much as 5 orders of magnitude the largest viscosity ratio in our experiments about 3000 was achieved using cold glycerine for the upper layer above a hot denser kno3 solution the most striking new feature with the very viscous upper layer is that now less dense fluid is released immediately and continuously from the interface and rises as plumes through the upper layer further crystallization occurs in the plumes and the crystals fall out but there is little mixing and a layer of depleted kno3 solution is eventually deposited at the top the transfer process between the layers is dominated by interfacial effects with the high viscosity upper layer acting as a nearly rigid lid that allows buoyant fluid to accumulate just below the interface and then rise in localized plumes across the interface into the viscous layer this physical picture is supported by a series of experiments in which the viscosity ratio is varied systematically the mixing behavior changes gradually between that described above for a large viscosity ratio and the sudden over turning characteristic of layers with comparable viscosity the importance of the viscosity ratio rather than just an increase in viscosity is confirmed by experiments in which both viscosities are increased by the same factor the overturning process is then slower but symmetrical other variations suggested by previous experiments are also described the release of gas by a chemical reaction to model the release of volatiles following an overturning event in a magma chamber the effect of a cold immiscible layer above the cooling crystallizing fluid the influence of two viscous layers with a density step between them and the constraining effects of a density with corresponding viscosity gradient in the upper region the experiments indicate that whatever the stratification whether it be in layers or continuous the form of the initial motion in the upper fluid is determined by the viscosity ratio between the two fluids immediately adjacent to the interface geological applications are not examined in detail in this paper but the experiments suggest that both sudden overturning characteristic of magmas of nearly equal viscosity and continuous release when the upper layer is much more viscous are viable mechanisms for magma mixing in the appropriate circumstances

hydroperoxo fe iii ooh and ferryl oxo fe iv o oxidative species involved in as iii oxidation catalyzed by pyrite under alkaline conditions abstract pyrite is an extensively distributed mineral that catalyzes oxidation of as iii to as v by dissolved oxygen do in natural environments however the catalytic mechanism of pyrite under alkaline conditions is still not clear in this study we observed increased oxidation rates of soluble as iii with increasing phs under alkaline conditions 7 5 11 0 esr analysis and quenching experiments rule out contributions of oh o2 and 1o2 to as iii oxidation and indicate that fe iii ooh and or h2o2 are indeed the primary oxidative species at phs 9 0 and 11 0 and fe iv o also contributes at ph 11 0 selective oxidation of a series of organic substrates to the corresponding oxidation products confirms the presence of fe iii ooh and or fe iv o in the pyrite system in alkaline solutions or organic solvents although the addition of glucose apparently increases the production of h2o2 at phs 9 0 and 11 0 such an increase does not apparently accelerate the oxidation rate of as iii implying that h2o2 is not the predominant species in the oxidation of as iii this study provides new insight into the oxidative species in the pyrite do system under alkaline conditions

geophysical survey of the intra caldera icefield of mt veniaminof alaska mt veniaminof is a large active stratovolcano located on the alaska peninsula 56 28 n 1598 w we present results of the first geophysical survey of the icefield that fills much of the 10 km 8 km caldera that was most recently modified during the last major eruption roughly 3700 bp the subglacial topography and ice volume are derived from an 8 mhz radio echo sounding survey conducted in july 2005 prominent internal reflectors are assumed to be isochronal ash acid deposits related to local eruptions accumulation rates and basal melt rates are calculated using a nye one dimensional steady state accumulation model applied at a location that approximates an ice divide and calibrated by matching internal reflectors with published records of recent local volcanic eruptions the model yields order of magnitude estimates of the accumulation rate of 4 m yr 1 water equivalent and 2m yr 1 of basal melt the subsequent geothermal flux of 19 w m 2 is similar to active hydrothermal vents in volcanic lakes we suggest that these values represent an upper limit for the geothermal flux within the ice covered regions of the main caldera we also analyze likely subglacial meltwater flow paths to examine the implications of recent eruption activity at an active intra caldera cinder cone two lava producing eruptions from the cinder cone in 1983 84 and 1993 94 melted roughly 0 17 km 3 of ice the lack of significant deformation of the internal stratigraphy to the south and east of the melt hole suggests that any subglacial drainage in those directions was entirely within subglacial deposits we suggest that the more likely drainage route was northwest into a large outlet glacier

relationships between deformation and mixing processes in lava flows a case study from salina aeolian islands tyrrhenian sea the massive unit of a lava flow from porri volcano salina aeolian islands displays many unusual structures related to the physical interaction between two different magmas the magma a represents approximately 80 of the exposed lava surface it has a crystal content of 51 vol and a dacitic glass composition sio2 63 64 wt the magma b has a basaltic andesite glass composition sio2 54 55 wt and a crystal content of approximately 18 vol it occurs as pillow like enclaves banding boudin like and rolling structures which are hosted in magma a structural analysis suggests that banding and boudin like structures are the result of the deformation of enclaves at different shear strain the linear correlation between strain and stratigraphic height of the measured elements indicates a single mode of deformation we deduce that the component b deformed according to a simple shear model glass analyses of the a b boundary indicate that a and b liquids mix together at high shear strain whereas only mingling occurs at low shear strain this suggests that the amount of deformation i e forced convection plays an important role in the formation of hybrid magmas high shear strain may induce stretching shearing and rolling of fluids which promote both forced convection and dynamical diffusion processes these processes allow mixing of magmas with large differences in their physical properties

study of effect of magma pocket on mixing of two magmas with different viscosities and densities by analogue experiments article i nfo we conducted analogue experiments to examine the effect of a magma pocket on magma mixing processes at both high and low reynolds numbers our idea is that when mafic and silicic magmas are simultaneously injected into a magma pocket gravitationally unstable layers may be formed at both high and low reynolds numbers under certain conditions the rayleigh taylor instability occurs and mixing of the two magmas is promoted in the pocket even at a low reynolds number this shows that gravitational force overcomes viscous force and the annular flow of the magmas in the pocket is disrupted if the viscous force is large the annular flow is maintained and it passes through the pocket at a critical condition two liquids with different densities and viscosities become unstable while mixing in the pocket this condition is represented by a dimensionless parameter p p u g ab where is the viscosity u is the flow velocity g is the acceleration due to gravity is the density difference between the two liquids and a and b are the base dimensions of the apparatus when pb0 1 the two liquids are gravitationally unstable and they mix in the pocket however when pn0 1 the annular flow of the two liquids is maintained and they do not mix with each other the critical condition is also dependent on the flux of the liquids and the height of the pocket and is applicable to the present experimental conditions the presence of the magma pocket is one of the factors responsible for the disruption of the annular flow leading to the mixing of the two magmas we conclude that magmas can mix thoroughly if they flow through several magma pockets

a bimodal volcanic plutonic system the zarembo island extrusive suite and the burnett inlet intrusive complex the zarembo island volcanic rocks and the burnett inlet plutonic complex in central southeastern alaska were investigated to determine if they are genetically related the zarembo island volcanic suite consists of basalt andesite and rhyolite lava flows which exhibit features that suggest simultaneous eruptions of mafic and felsic lavas five kilometres to the southeast the broadly layered burnett inlet plutonic complex consists of gabbro diorite and granite plutons that also show characteristics of contemporaneous mafic and felsic magmatism these bimodal volcanic and plutonic rocks are similar in age ranging from 18 5 to 21 5 ma both suites show a gap in silica concentration between 60 and 65 wt and have similar major trace and rare earth element composition both suites also show igneous layering either as interlayered basalt and rhyolite flows or as alternating gabbro and granite sheets additionally both groups contain magma mingling and mixing textures including mafic enclaves in felsic members and quartz xenocrysts rimmed by clinopyroxene in enclaves these characteristics suggest that the burnett inlet intrusive complex and the zarembo island volcanic suite represent an eroded shallow level plutonic center and its eruptive cover the style of volcanism and the bimodal nature of magmatism suggest that igneous activity occurred during crustal extension and thinning that accompanied strike slip tectonic motion in southeastern alaska during the tertiary the volcanic plutonic rock associations now exposed at the surface indicate that at least 7 of post 20 ma crustal tilting has affected the region and can help to explain aberrant paleomagnetic poles in mid cretaceous intrusions of the cordillera coast belt

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

magma mixing in mantle xenolith bearing calc alkalic ejecta ichinomegata volcano northeastern japan abstract the latest pleistocene ejecta from the ichinomegata volcano ne japan composed of three maars show a wide compositional variation ranging from basalt 51 sio 2 to dacite 65 sio 2 these have the same phenocryst assemblage of olivine augite hornblende biotite quartz plagioclase and titanomagnetite upper mantle peridotite xenoliths are found not only in basalts but also in andesite whole rock major chemistry and modal compositions indicate that the chemical and modal variations have been caused by the mixing of three magmatic components primitive high alumina basalt sio 2 51 wt mgo 8 5 wt with olivine fo 88 and augite phenocrysts differentiated basaltic andesite sio 2 54 wt mgo 5 wt which has the same but slightly fe rich phenocryst phases as the primitive basalt and porphyritic dacite sio 2 65 wt mgo 2 0 wt with phenocrysts of hornblende biotite quartz plagioclase and titanomagnetite the upper mantle peridotite bearing andesite may not be derived directly from the upper mantle and its peridotite xenoliths may have been carried by the primitive basaltic magma the estimated density for three magmatic end components requires a high flow velocity of basaltic magma and a small volume 5 m 3 of the dacite magma body to facilitate breaking of the stratification in the dacite magma body this is consistent with the retention of peridotite xenoliths in the mixed magma and the preserved disequilibrium petrographic features

magmatic emulsion texture formed by mixing during extrusion rauðafell composite complex breiðdalur volcano eastern iceland the rau afell composite complex is part of the neogene brei dalur volcano eastern iceland and is composed of a composite feeder dyke a vent structure and a composite flow the two end members of the composite complex are rhyolite and basalt and both are rich in plagioclase macrocrysts bytownite in basalt and oligoclase in rhyolite the rhyolite also includes ferroaugite macrocrysts the mixed rocks are separated in three textural groups related to mixing proportions when the basaltic end member is dominant a hybrid texture with a homogeneous matrix is observed and the only evidence of mixing is the presence of antecrysts of both end members when the basaltic end member represents c 65 to 30 of the mixture we observe emulsion textures composed of finely co mingled basalt and rhyolite the difference between these two textural expressions of mixing is due to effects of diffusion the third texture shows mafic enclaves suspended in a rhyolitic matrix in these rocks the proportion of the basaltic end member is inferior to 30 implying that the basalt froze solid in contact with the rhyolite zoning of plagioclase shows that the mixing processes are driven initially by highly efficient micro mingling the emulsification is possibly a result of compositional gradient stresses korteweg stress between miscible basalt and rhyolite this is followed by chemical diffusion hybridisation and tend to protect antecrysts from reaction with the primitive magmas when antecrysts originated in the evolved magma they undergo dissolution due to thermal disequilibrium during mingling and chemical disequilibrium during hybridisation we argue that such mixing processes are important in producing intermediate rocks in iceland and elsewhere that shows only the chemical attributes of an origin by mixing the preservation of emulsion textures is rare and highly dependent on cooling history

magma mixing in alkaline magmas an example from tenerife canary islands abstract the central region of tenerife canary islands offers a wide variety of alkaline rocks of intermediate composition between basalts and more differentiated magmas a recent less than 0 1 ma old eruption on the southwestern slopes of teide volcano produced separate felsic sio 2 59 and mafic sio 2 47 rocks as well as abundant hybrid inclusions sio 2 50 57 that show a wide range of geochemical and mineralogical variation between the end members mineralogical evidence reverse zoning skeletal and patchy feldspars incompatible assemblages mineral disequilibrium and geochemical data characteristic trends of major and trace elements ree ratios major and trace mass balance calculations support the generation of a mixing process we present a model in which basaltic magma entered a trachyte magma chamber which resulted in a stratified chamber of trachytic above more mafic hawaiitic melts the formation of hybrid magmas between these two end members is believed to involve some chemical diffusion across the interface between the two magmas before mechanical mixing a lowering of temperature due to the cooling effect of the trachytic magma and consequently a decrease in density by crystallization of plagioclase permitted the basaltic magma to penetrate through the trachyte which caused the eruption this natural example is in good agreement with the results obtained in experimental work on interdiffusion of silicate melts

improved modelling of the messinian salinity crisis and conceptual implications abstract this paper presents the last developments of a simple oceanographic modelling of the water and salt budgets of the mediterranean sea during the late miocene salinity crisis the messinian mediterranean is treated as analogous to the present one i e divided into two main basins separated by a sill at shallow or intermediate depth when the supply of marine water from the atlantic is progressively reduced both basins undergo a rise in salinity until saturation is reached this is when the true evaporitic sedimentation begins before the level drawdown the partition of the mediterranean into two mains basins causes a shift in the evaporitic sedimentation from the distal eastern basin to the proximal western basin so that the evaporitic deposits are not fully contemporaneous in the western and the eastern basin the drawdown is limited by the equilibrium of the evaporation and chemical activity of the brines at the surface against the surface area and evaporation attempts at adjusting the model both to an accurate stratigraphic frame and to a rough budget of the evaporites shows that the upper evaporites and brackish water lago mare series must have been deposited as secondary deposits after the closure of the atlantic passages was completed the present evaporitic potentialities of the mediterranean sea remains quite as strong as during the miocene so that climatic change cannot be inferred from the msc itself

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

magma mixing in a squeezed conduit abstract the mixing of mafic and silicic magmas in a squeezed conduit is simulated by fluid dynamics experiments using glue and dilute glycerin in a squeezed vinyl tube lighter and more viscous glue initially stably overlies dilute glycerin in a tube as the tube is squeezed downward by a roller the glue and dilute glycerin axisymmetrically circulate in the tube the stable density stratification overturns due to the circulative motion because of viscosity contrast between the two liquids the convective motion becomes unsteady and chaotic which leads to efficient mixing of two liquids even when the tube is squeezed very slowly it is suggested that magma mixing in a squeezed conduit may explain some occurrences of natural mixed lavas with nearly homogeneous groundmass

exponential decay of concentration variance during magma mixing robustness of a volcanic chronometer and implications for the homogenization of chemical heterogeneities in magmatic systems abstract the mixing of magmas is a fundamental process in the earth system causing extreme compositional variations in igneous rocks this process can develop with different intensities both in space and time making the interpretation of compositional patterns in igneous rocks a petrological challenge as a time dependent process magma mixing has been suggested to preserve information about the time elapsed between the injection of a new magma into sub volcanic magma chambers and eruptions this allowed the use of magma mixing as an additional volcanological tool to infer the mixing to eruption timescales in spite of the potential of magma mixing processes to provide information about the timing of volcanic eruptions its statistical robustness is not yet established this represents a prerequisite to apply reliably this conceptual model here new chaotic magma mixing experiments were performed at different times using natural melts the degree of reproducibility of experimental results was tested repeating one experiment at the same starting conditions and comparing the compositional variability we further tested the robustness of the statistical analysis by randomly removing from the analysed dataset a progressively increasing number of samples results highlight the robustness of the method to derive empirical relationships linking the efficiency of chemical exchanges and mixing time these empirical relationships remain valid by removing up to 80 of the analytical determinations experimental results were applied to constrain the homogenization time of chemical heterogeneities in natural magmatic system during mixing the calculations show that when the mixing dynamics generate millimetre thick filaments homogenization timescales of the order of a few minutes are to be expected

emplacement of a silicic lava dome through a crater glacier mount st helens 2004 06 the process of lava dome emplacement through a glacier was observed for the first time after mount st helens reawakened in september 2004 the glacier that had grown in the crater since the cataclysmic 1980 eruption was split in two by the new lava dome the two parts of the glacier were successively squeezed against the crater wall photography photogrammetry and geodetic measurements document glacier deformation of an extreme variety with strain rates of extraordinary magnitude as compared to normal alpine glaciers unlike normal temperate glaciers the crater glacier shows no evidence of either speed up at the beginning of the ablation season or diurnal speed fluctuations during the ablation season thus there is evidently no slip of the glacier over its bed the most reasonable explanation for this anomaly is that meltwater penetrating the glacier is captured by a thick layer of coarse rubble at the bed and then enters the volcano s groundwater system rather than flowing through a drainage network along the bed

a proposed new type of arsenian pyrite composition nanostructure and geological significance this report describes a new form of arsenian pyrite called as3 pyrite in which as substitutes for fe fe as s2 in contrast to the more common form of arsenian pyrite as1 pyrite in which as1 substitutes for s fe as s 2 as3 pyrite has been observed as colloformic overgrowths on as free pyrite in a hydrothermal gold deposit at yanacocha peru xps analyses of the as3 pyrite confirm that as is present largely as as3 empa analyses show that as3 pyrite incorporates up to 3 05 at of as and 0 53 at 0 1 at 0 27 at 0 22 at 0 08 at and 0 04 at of pb au cu zn ni and co respectively incorporation of as3 in the pyrite could be written like as3 y au 1 y 2 fe2 where au and vacancy help to maintain the excess charge hrtem observations reveal a sharp boundary between as free pyrite and the first overgrowth of as3 pyrite 20 40 nm thick and co linear lattice fringes indicating epitaxial growth of as3 pyrite on as free pyrite overgrowths of as3 pyrite onto as free pyrite can be divided into three groups on the basis of crystal size 8 20 nm 100 300 nm and 400 900 nm and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals the yanacocha deposit in which as3 pyrite was found formed under relatively oxidizing conditions in which the dominant form of dissolved as in the stability field of pyrite is as3 in contrast reducing conditions are typical of most environments that host as1 pyrite as3 pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments including high sulfidation epithermal deposits and shallow groundwater systems where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of as3 and toxic heavy metals in oxidizing natural systems

dusty and honeycomb plagioclase indicators of processes in the uchino stratified magma chamber izu peninsula japan abstract two types of melt inclusion bearing plagioclase coexist in the uchino basalt izu peninsula japan 1 dusty plagioclase with an ab rich core mantled by a dusty zone consisting of more an rich plagioclase and fine melt inclusions and 2 honeycomb plagioclase with a central core containing larger melt inclusions the dusty plagioclase probably forms through partial dissolution of ab rich plagioclase as dusty zones truncate the concentric zonal structure of the ab rich inner core the honeycomb plagioclase is formed by skeletal growth under supercooling conditions this is indicated by the euhedral geometry of the boundary between the inner inclusion free and the outer inclusion bearing zones of the honeycomb plagioclase which has a concentric inner core bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims the core rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry respectively this suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization these features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber

life time of a stratified magma chamber recorded in ultramafic xenoliths from ichinomegata volcano northeastern japan variations in major element chemistry and modal compositions of the mantle xenolith bearing calc alkalic ejecta from ichinomegata volcano are inferred to be due to mixing of three magmatic end members basalt i sio2 51 wt mgo 8 5 wt basalt ii sio2 54 wt mgo 5 wt and dacite sio2 65 wt mgo2 wt ultramafic xenoliths are found in mafic mixtures of dacite basalt i and dacite basalt ii the thermal histories of the xenoliths in both mixtures are compared with each other chemical compositions of olivine and orthopyroxene in xenoliths suggest that xenoliths in basalt i were equilibrated at about 800 c while those in basalt ii were also equilibrated originally at about 800 c but were subsequently annealed at about 1000 c for more than 102 3 years prior to the eruption

experimental simulation of magma mixing at high pressure abstract magma mixing features are observed in many plutonic and volcanic environments they result from the juxtaposition of two chemically contrasted magmas usually not only during the replenishment of a magmatic reservoir but also syn eruptively within the conduit despite its ubiquity only a few experimental studies have explored mixing between magmas existing data have been mostly acquired at atmospheric pressure and high shear rates 10 1 s 1 which differ from those accompanying magma mixing in reservoirs to fill this gap we performed high pressure mixing experiments at strain rates ranging from 4 10 4 to 1 10 3 s 1 layers of a synthetic crystal free haplotonalite and a natural partially molten basalt were juxtaposed in a paterson apparatus at 300 mpa and deformed between 900 and 1200 c the experiments shed light on the first stages of magma mixing and illustrate the role and behaviour of crystals either pre existing or newly grown experiments evidence a rheological threshold for mafic material disruption which sets in abruptly as its melt fraction exceeds 50 which in the experiments occurs in the narrow temperature interval 1160 1170 c below this threshold plagioclase crystals in the mafic magma form a rigid touching network and all the deformations are accommodated by the less viscous felsic layer above it the crystal network collapses allowing typical mingling mixing features to appear altogether such as enclaves melt filaments and single xenocrysts isolated into the felsic end member coexisting with newly grown phases plagioclase and pyroxene whose compositions spread out over considerable ranges the pre existing fabric of the mafic magma is only slightly affected by deformation altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage

time scale of magma mixing between basalt and dacite estimated for the saga futagoyama volcanic rocks in northwest kyushu southwest japan abstract a composite lava extrusion of the saga futagoyama volcanic rocks consisting of a central unit of the dacite lava flanked by the andesite lava containing phenocrysts of quartz and plagioclase is exposed in northwest kyushu southwest japan field and microscopic observations indicate that the andesite and the dacite simultaneously emerged at the surface disequilibrium relationships between plagioclase and bulk rock compositions suggest that the andesite lava is a hybrid of more mafic and more silicic magmas the whole rock chemistry of the composite lava suggests that the contemporaneous eruption had two end members of basalt and dacite magmas the time scale of magma mixing between basalt and dacite is estimated using reaction coronas around quartz and dissolution textures of plagioclases in the andesite the typical reaction duration was estimated at 1 4 h to 12 days using reaction coronas around quartz and at 2 8 h to 1 day using dissolution textures of plagioclases these estimates indicate that the mixing essentially occurred during ascent in a conduit the concentration of the andesite compositions at sio2 59 wt requires other processes to homogenize mixed magma such as phenocryst stirring

diffusive fractionation of trace elements by chaotic mixing of magmas abstract recent research on magma mixing systems has shown that the mixing process is governed by chaotic dynamics and that this process is responsible for the generation of fractal structures that propagate within the magmatic mass from the meter to the micrometer length scale laser ablation icp ms trace element analyses have been performed on rock samples with evidence of chaotic mixing phenomena results indicate that trace elements with similar values of diffusion coefficient display good correlations in inter elemental plots whereas as the difference between diffusion coefficients increases the correlation is progressively lost in addition a large variability of ree patterns is observed with the remarkable feature of the presence of positive and negative eu anomalies occurring at short length scale of the order of few mm given the chaotic nature of magma mixing structures the mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme by considering several trace elements with variable diffusivities simulations indicate that such a model explains with good approximation the variable correlations among trace elements observed in natural samples in addition the same patterns of ree observed in natural samples including the occurrence of positive and negative eu anomalies at short length scale are observed indicating that a chaotic advection diffusion dynamic system is a suitable model to explain natural data results presented in this contribution indicate that at the micrometric length scale small volumes of magmas are strongly influenced by the coupled action of chemical diffusion and chaotic flow fields and hence they do not represent magmas de facto present in the magmatic system because their compositions may have experienced a diffusive fractionation process these results may have important petrogenetic implications for instance if such melts were trapped as melt inclusions they would provide misleading information about melt compositions it is suggested that the approach of studying the degree of correlation among trace elements may be a possible method to test if melt inclusion compositions commonly used as petrogenetic indicators display evidence of such a diffusive fractionation process

arsenic incorporation into authigenic pyrite bengal basin sediment bangladesh abstract sediment from two deep boreholes 400 m approximately 90 km apart in southern bangladesh was analyzed by x ray absorption spectroscopy xas total chemical analyses chemical extractions and electron probe microanalysis to establish the importance of authigenic pyrite as a sink for arsenic in the bengal basin authigenic framboidal and massive pyrite median values 1500 and 3200 ppm as respectively is the principal arsenic residence in sediment from both boreholes although pyrite is dominant ferric oxyhydroxides and secondary iron phases contain a large fraction of the sediment bound arsenic between approximately 20 and 100 m which is the depth range of wells containing the greatest amount of dissolved arsenic the lack of pyrite in this interval is attributed to rapid sediment deposition and a low sulfur flux from riverine and atmospheric sources the ability of deeper aquifers 150 m to produce ground water with low dissolved arsenic in southern bangladesh reflects adequate sulfur supplies and sufficient time to redistribute the arsenic into pyrite during diagenesis

the influence of magma ascent path on the texture mineralogy and formation of hornblende reaction rims abstract although hornblende reaction rims are widely used as a tool for evaluating magma ascent during volcanic eruptions very few studies constrain the manner in which they form this study investigates the influence of magma ascent path on the formation of hornblende reaction rims to do this we conducted isothermal 840 c decompression experiments using dacite pumice samples erupted in december 1989 from redoubt volcano alaska experiments were first held within the hornblende stability field determined through phase equilibria experiments for 3 to 5 days before being decompressed to different pressures ranging from 100 to 2 mpa for 1 to 30 days before being quenched decompression was performed in either multiple equal steps constant rate or in one single step results from multi step experiments show that reaction rims form preferentially at pressures from 10 to 40 mpa and that this favorable pressure range narrows and decreases with increased decompression duration hornblendes in multi step experiments are tightly enclosed by fine grained reaction rims composed of plagioclase and orthopyroxene crystals with high aspect ratios ranging from 1 to 12 for single step experiments reaction rims also form preferentially within a narrow pressure range 60 70 mpa where they consist of medium grained plagioclase titanomagnetite and orthopyroxene crystals with aspect ratios ranging from 1 to 6 that broadly surround subrounded hornblendes hornblendes from single step experiments dropped to lower pressures

replenishment of magma chambers by light inputs magma chambers particularly those of basaltic composition are often replenished by an influx of magma whose density is less than that of the resident magma this paper describes the fundamental fluid mechanics involved in the replenishment by light inputs if denotes the uniform density of the resident magma and that of the input the situation is described by the reduced gravity g g the volume flux q and the viscosities of the resident and input magmas e and i respectively the nondimensional reynolds numbers ree g q3 1 5 e and rei g q3 1 5 i and chamber geometry then completely specify the system for sufficiently low values of the two reynolds numbers each less than approximately 10 the input rises as a laminar conduit for larger values of the reynolds numbers the conduit may break down and exhibit either a varicose or a meander instability and entrain some resident magma at still larger reynolds numbers the flow will become quite unsteady and finally turbulent the values of the reynolds numbers at which these transitions occur have been documented by a series of experiments with water glycerine and corn syrup if the input rises as a turbulent plume significant entrainment of the resident magma can take place the final spatial distribution of the mixed magma depends on the geometry of the chamber if the chamber is much wider than it is high the mixed magma forms a compositionally stratified region between the roof and a sharp front above uncontaminated magma in the other geometrical extreme the input magma is mixed with almost all of the resident magma if the density of the resident magma is already stratified the input plume may penetrate only part way into the chamber even though its initial density is less than that of the lowest density resident magma the plume will then intrude horizontally and form a hybrid layer at an intermediate depth this provides a mechanism for preventing even primitive basaltic magmas of minimum density from erupting at the surface by conducting an experiment using aqueous solutions we show that entrainment can lead to crystallization of the magma in the input plume by making it locally supersaturated all these effects are discussed and illustrated by photographs of laboratory experiments

mediterranean sea level variations during the messinian salinity crisis 1 the mediterranean basin has not always been connected to the atlantic ocean during the messinian salinity crisis msc the mediterranean sea became progressively isolated by a complex combination of tectonic and glacio eustatic processes when isolated the mediterranean water level depends on the hydrological flux and is expected to vary significantly the amplitude and number of large water level fluctuations in the isolated mediterranean is still controversial despite numerous geological investigations the observation of 3 5 surfaces of erosion in the nile delta eastern basin provides new elements for understanding the dynamics of the msc our model demonstrates that numerous water level falls of short duration may explain the preservation of a discontinuous river profile at 500 m and 1500 m in the western basin as well as the existence of deep surfaces of erosion in the eastern basin

a model of reverse differentiation at dikii greben volcano kamchatka progressive basic magma vesiculation in a silicic magma chamber dikii greben volcano is the largest modern volcano with silicic rocks in the kurile kamchatka island arc it consists of many domes and lava flows of rhyodacite dacite and andesite which were erupted in a reverse differentiation sequence non equilibrium phenocryst assemblages quartz mg rich olivine an rich an poor plagioclase etc abundance of chilled mafic pillows in the dacites and andesites and linear variations of rock compositions in binary plots are considered as mineralogical textural and geochemical evidence for mixing mafic pillows in volcanics have a lower density because of high porosity and contain the same non equilibrium phenocryst assemblages as the host rocks their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water rich basaltic magma at depth they are considered as supercooled vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling the lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption the rhyodacitic and basic end members of the mixing series cannot be linked by low p fractionation though high p amphibole rich fractionation is not excluded

role of magma mixing during the recent activity of la fossa di vulcano aeolian islands italy abstract relatively homogeneous trachytes have been erupted for approximately 3800 years at la fossa di vulcano from the punte nere eruptive cycle up to the palizzi cycle the products varied little while after the palizzi cycle 1600 1000 a b p to the latest eruption 1888 1890 ad a spectrum of compositions with rhyolite dominating characterized the erupted products a stratigraphic sequence starting with the palizzi lava flow has been studied focussing the attention on lavas and volcanic bombs to define the role that magma mixing processes have played in the recent history of la fossa di vulcano textural and chemical analyses of whole rocks glass groundmass and mineral phases indicate that only the breadcrust bombs erupted during the 1888 1890 show evidence of mixing between trachytic and rhyolitic end members interestingly in the deposits of the same eruption trachytic bombs also occur the lava flows erupted before 1888 1890 display general features suggesting that they entrained crystals and lava fragments during magma ascent during the 1888 1890 eruption the trachytic bombs were erupted before the breadcrust bombs which have a more evolved and hybrid composition these characteristics together with the change of the nature of the products after the palizzi cycle require a complex volcanological model for the recent history of la fossa di vulcano

modelling of the erosion in the rhone valley during the messinian crisis france abstract a geometric reconstruction of the present day messinian rhone valley shows two slope breaks after taking into account the post messinian tectonics and messinian isostatic compensation due to deep valley incision and mediterranean sea unloading these two slope breaks were still visible on the reconstructed original messinian rhone profile one is located on the cevennes fault system and the other one is downstream of st marie the slope of the ante messinian profile was approximately 0 05 the messinian rhone profile was simulated using a numerical erosional model based on mass conservation the messinian profile cannot be explained only by variations of erosional processes such as lithology or changes in discharge and one has to consider that the messinian crisis happened in two phases the profile of the rhone was in steady state just after the first messinian phase of lowering the amplitude of the sea level lowering is around 600 m for the first phase whereas the amplitude of the second one is 1300 1700 m the isostatic deformation during the messinian crisis due to bedrock erosion and sea unloading produced important changes in the mediterranean dynamic

oxidation reactivity of as iii containing pyrites differences between structurally incorporated and adsorbed as iii abstract natural pyrites contain high contents of adsorbed and structurally incorporated arsenic as which may in turn affect the oxidation rates of pyrite as well as the transformation and immobilization behaviors of as contaminants however the speciation and distribution of as on the oxidation reactivity of pyrites is still not clear yet in this study the oxidation reactivity of three different pyrites namely pure pyrite py free as iii adsorbed pyrite py as iii ad and as iii structurally incorporated pyrite py as iii in including py 0 02 and py 0 04 will be compared in py as iii in as iii replaces s i and changes the relative ratio of fe s the xps results reveal that fe iii s ii site densities on the pyrite surface increase with incorporated as indicating that py as iii in with more as carries more surface defects the oxidation rates of pyrites are accelerated at phs 7 0 and 9 0 by both structurally incorporated and adsorbed as iii which is more significant in structurally incorporated as iii at ph 2 5 structurally incorporated as iii promotes the oxidation of pyrite whereas adsorbed as iii inhibits its oxidation the tafel plots and the electrochemical impedance spectroscopy eis analysis further support that as in py as iii in decreases the charge transfer resistance on the pyrite surface and facilitates electron transfer between dissolved oxygen do and the pyrite surface thus enhancing the oxidation rates of pyrites variations of h2o2 in py as iii in in the full ph range also indicate that as increases the two electron transfer from pyrite to do resulting in the formation of more h2o2 at ph 2 5 the generated oh from h2o2 is the predominant species in pyrite oxidation and at phs 7 0 and 9 0 h2o2 fe iv and oh all contribute to pyrite oxidation compared with py as iii in as in py as iii ad is more readily to be mobilized with more as v being detected at ph 2 5 subsequent decreases of as iii and as v suggest re immobilization of the released as via the adsorption or co precipitation pathways solid characterizations further reveal that dissolved or adsorbed as iii significantly inhibit the development of lepidocrocite goethite and magnetite whereas structurally incorporated as iii does not triggering differences in the speciation of secondary fe minerals the present study reveals that the speciation and distribution of as iii significantly affects the chemical and electronic properties of different pyrites thus affecting the corresponding oxidation reactivity of different pyrites

kinematic significance of morphological structures generated by mixing of magmas a case study from salina island southern italy morphological features of mixing mingling structures of heterogeneous juveniles from the 13 ka old upper pollara eruption salina island southern italy are studied these heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion results show that the upper pollara rocks were produced by mixing 24 49 of andesite and 76 51 of rhyolite multifractal analysis of the mixing structures is performed and the mixing intensity i e the degree of interaction between the two end member magmas has been deduced from the analysis of the singularity spectrum results show that narrower is the multifractal spectrum the higher is the degree of homogenization of magmas reynolds number re during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and re results show that re is between ca 500 and 7000 there is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and re in agreement with the observation that the higher is the percentage of the mafic lower viscosity magma the higher is the turbulence of the mixing system in addition our analysis reveals an unexpected inverse relationship between the calculated re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit it is also suggested that mixing occurred in a shear layer type or pipe type flow

geothermal activity in the subglacial katla caldera iceland 1999 2005 studied with radar altimetry the katla caldera is located under the myrdalsjokull ice cap and is one of the most hazardous volcanoes in iceland due to major jokulhlaups that accompany eruptions subglacial geothermal activity is manifested in several 10 50 m deep depressions ice cauldrons within and at the caldera rim and the total geothermal heat output is of the order of a few hundred megawatts a short lived but powerful pulse in geothermal heat output took place in 1999 probably including a minor subglacial eruption when new ice cauldrons formed in three places and an unexpected jokulhlaup occurred following these events a comprehensive monitoring program was set up for katla including ice surface elevation profiling from aircraft to monitor variations in geothermal heat and detect signs of subglacial water accumulation a radar altimeter coupled with a kinematic gps is used achieving an absolute elevation accuracy of 3 m and internal consistency of 1 2 m profiles across the caldera are flown twice a year an annual accumulation ablation cycle in surface elevation with amplitude of 5 10 m is observed by removing this cycle from the data changes due to subglacial geothermal activity are obtained after the events in 1999 a decline in geothermal activity was observed in 2001 03 some ice cauldrons expanded and deepened by 10 15 m indicating renewed increase in geothermal activity this trend is also apparent for 2003 05 the increase in geothermal power amounts to a few tens of megawatts it is likely that the increased thermal output is related to increased seismicity and caused by magma inflow

anisotropy of magnetic susceptibility of silicic rocks from quarries in the vicinity of sao marcos rio grande do sul south brazil implications for emplacement mechanisms abstract the parana etendeka large igneous province includes acid volcanic rocks that can be found throughout its extension several aspects concerning those rocks remain controversial including their mechanism of emplacement and location of their eruptive sources opening of several quarries of dimension stone near the city of sao marcos rio grande do sul brazil offers a unique opportunity to study in detail the acid products here we present the results of a study of the anisotropy of magnetic susceptibility ams completed in some rocks that had been interpreted as the roots of volcanic conduits our results and reexamination of the textural features of the rocks lead to a reinterpretation that suggests that these rocks were emplaced subaerially and involved assimilation and remelting of clastic components of previous products due to the inferred conditions of emplacement it is unlikely that the eruptive vents are located far from the area of study therefore ruling out the long travelled nature of these products

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

longitudinal evolution of the suez rift structure egypt abstract a three dimensional study of the structure of the suez rift has been carried out using field and subsurface data in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift as in most intracontinental rifts the structure of the gulf of suez area is governed by normal faults and tilted blocks whose crests constitute the main target of exploratory wells the fault pattern consists of two major sets of trends 1 1 longitudinal faults parallel to the rift axis and created in an extensional regime where 3 was trending ene wsw and 2 2 transverse faults with a n s to nne ssw dominant trend the transverse faults are inherited passive discontinuities while most of the longitudinal faults were created during neogene times in a purely extensional regime both sets were simultaneously active producing a zigzag pattern and rhombic shaped blocks the transverse faults can show horizontal strike slip components and act as relays between major normal faults although the suez rift appears as a simple narrow elongated trough dominated by two almost symmetrical shoulders its internal structure is asymmetrical cross profiles show that all the major blocks are tilted in the same direction however the tilt direction changes twice along the rift to the north and to the south of the rift the blocks are tilted eastward while in the central part they are tilted westward to the north the change of dip is accompan ed by a graben type twist zone without transverse faulting at least in the neogene series to the south the change of dip is accompanied by a more complex structure involving both a major transverse fault and a horst type twist zone in this latter case the transverse fault does not cut through the entire rift balanced cross sections established from subsurface data show that the tilt angle and the amount of extension increase from north to south while the width of the blocks decreases indicating a pole of opening close to the northern end of the gulf minimum values for the amount of opening range from 5 km in the north to about 20 km in the south

experimental mixing of hydrous magmas deformation experiments involving hydrous magmas of different compositions basalt and haplotonalite have been performed in a paterson press at 300 mpa in the temperature range 600 c 1020 c with water saturated melts during 2 4 h prior to deformation the two end member magmas were annealed at either 950 c or 1000 c yielding magmas with crystal contents in the range 31 53 wt and 2 sets of viscosity contrasts under the experimental conditions investigated i e moderate shear rates 950 c in the temperature range 950 985 c a few mixing and mingling textures occur though both end members essentially retain their physical integrity it is only at or above 1000 c that a dramatic jump in mingling efficiency happens corresponding to a crystal fraction of 45 vol textures include entrainment of mafic crystals into the felsic magma mafic felsic banding enclave formation and diffusion induced interface the latter only over limited distances 300 m due to the short run durations in the most strained parcels of interacting magmas complex mixing mingling textures were produced similar to those observed in volcanic and plutonic rocks in arc settings the experiments show that mixing between hydrous felsic and mafic magmas takes place at around 1000 c a temperature which is almost 200 c lower than mixing under dry conditions magma mixing is commonly invoked as a trigger for volcanic eruptions our experiments suggest that such eruptions can be driven by small 15 c temperature fluctuation in the reservoir our results also suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result in stratification between end members rather than in a homogeneous mixture

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

analysis and simulation of magma mixing processes in 3d abstract magma mixing structures from the lava flow of lesbos greece are analyzed in three dimensions using a technique that starting from the serial sections of rock cubes allows the reconstruction of the spatial distribution of magmas inside rocks two main kinds of coexisting structures are observed i active regions ar in which magmas mix intimately generating wide contact surfaces and ii coherent regions cr of more mafic magma that have a globular shape and do not show large deformations the intensity of mingling is quantified by calculating both the interfacial area ia between interacting magmas and the fractal dimension of the reconstructed structures results show that the fractal dimension is linearly correlated with the logarithm of interfacial area allowing discrimination among different intensities of mingling the process of mingling of magmas is simulated using a three dimensional chaotic dynamical system consisting of stretching and folding processes the intensity of mingling is measured by calculating the interfacial area between interacting magmas and the fractal dimension as for natural magma mixing structures results suggest that as in the natural case the fractal dimension is linearly correlated with the logarithm of the interfacial area allowing to conclude that magma mixing can be regarded as a chaotic process since chemical exchange and physical dispersion of one magma inside another by stretching and folding are closely related we performed coupled numerical simulations of chaotic advection and chemical diffusion in three dimensions our analysis reveals the occurrence in the same system of active mixing regions and coherent regions analogous to those observed in nature we will show that the dynamic processes are able to generate magmas with wide spatial heterogeneity related to the occurrence of magmatic enclaves inside host rocks in both plutonic and volcanic environments

an experimental study on the formation of composite intrusions from zoned magma chambers abstract we present a model which accounts for the common but paradoxical arrangement of composite intrusions i e silicic core and mafic margins on the basis of analogue experiments using gelatin and aqueous solutions the present model involves simultaneous flow out of the upper and lower magmas from a longitudinal crack along the chamber wall experimental results suggest that the mafic magma from the lower layer leaks from the side wall of the chamber and travels faster than the silicic magma because of its lower viscosity so that the mafic magma reaches the tip of the crack first once the mafic magma reaches the crack tip then the rate of dyke propagation becomes determined by the viscosity of the less viscous mafic magma and so it can advance rapidly the viscous silicic magma can flow efficiently into the center of the dyke being lubricated by the mafic magma margins this model accounts for the common arrangement of composite intrusions and gives an efficient mechanism of flow of viscous silicic magmas

the interplay between crystallization replenishment and hybridization in large felsic magma chambers abstract while hybridized granitoid magmas are readily identifiable the mechanisms of hybridization in large crustal magma chambers are so not clearly understood characteristic features of hybrid granitoids are 1 both the granitoid and included enclaves are commonly hybrids as shown by mineralogy geochemistry and isotopes 2 mixing seen in zoned plutons and synplutonic dykes and enclaves occurred early 3 zoned plagioclase phenocrysts commonly show very complex life histories of growth and dissolution 4 mafic end members in hybrids are commonly fractionated magmas and 5 stratification in subvolcanic granitoid magma chambers is not uncommon and stratification has been identified in some deeper level plutons hybridization must overcome the tendency to form a stable stratification of dense mafic magma underlying less dense felsic magma experimental work with magma analogues and theoretical considerations reveal very severe thermal rheological and dynamical limitations on mixing only very similar composition temperature magmas are likely to mix to homogeneity and only moderately silicic hybrids are likely to be produced however impossibly silicic hybrids do exist synchronous interactive fractional crystallization and hybridization may provide a mechanism for hybridization of magmas in the following manner a mafic magma intrudes into the base of a stratified felsic magma and is cooled against it crystallization of the upper boundary layer of the mafic magma yields an eventually buoyant residual melt that overturns and mixes with an adjacent stratum of the felsic magma chamber subsequently melt released by crystallization pf this now hybrid zone mixes with adjacent more felsic zones thus a suite of hybrid magmas are progressively formed density inhibitions are overcome by the generation of relatively low density residual melts as crystallization proceeds later injections are preserved as dykes and enclaves composed of hybrid magma in this process only physically adjacent and dynamically thermally similar magmas directly interact and so may mix to homogeneity finally not simply felsic and mafic endmembers mix but a whole suite of intermediate endmembers participate ranging from relatively mafic through to felsic pairs of magmas direct mixing between the primary magmas only occurs at the beginning

thermal and rheological controls on the formation of mafic enclaves or banded pumice magma mixing can occur in a fluid manner to produce banded pumice or in a brittle manner to form enclaves we propose that the critical control on mixing style is a competition between developing networks of crystals in the intruding magma that impart a strength to the magma and melting and disrupting those networks in the host x ray computed tomography analysis demonstrates that banded pumice from the 1915 mt lassen eruption lacks crystal networks in contrast rhyodacite hosts with mafic enclaves from chaos crags contain well developed networks of large crystals we present a one dimensional conductive cooling model that predicts mixing style either ductile or brittle as a function of magma compositions temperatures and the size of the intruding dike our model relies on three assumptions 1 mixing is initiated by the injection of a hot dike into a cooler magma body with a yield strength 2 when magma crystallinity exceeds a critical value 13 vol plagioclase the magma develops a yield strength and 3 when total crystallinity exceeds 40 vol the magma has a penetrative crystal network and is effectively solid importantly because the two magmas are of different compositions their crystallinities and viscosities do not have the same variations with temperature as the intruding magma cools it crystallizes from the outside in while simultaneously host magma temperature near the intruder rises mixing of the two magmas begins when the host magma is heated sufficiently to 1 disrupt the crystal network and 2 initiate convection if the shear stress exerted by the convecting host magma on the dike is greater than the yield strength of the dike margin and dike crystallinity does not exceed 40 then fluid mixing occurs otherwise enclaves form by brittle deformation of the dike application of the model to magma compositions representative of lassen and chaos crags shows that emplacement of dikes 1 m thick should produce enclaves whereas thicker dikes should generate fluid mixing and form banded pumice within days to weeks of emplacement similar relationships apply to other modeled magmatic systems including pinatubo unzen and ksudach shtuybel volcanoes for all studied systems the absolute size of the intruding dike not just its proportion relative to the host influences mixing style

chaotic advection fractals and diffusion during mixing of magmas evidence from lava flows structures of magma mixing from three different lava flows have been analyzed and the degree of mingling has been quantified by measuring the contact perimeter between magmas and the fractal dimension of structures in each lava flow the values of these parameters suggest that the magma mixing structures were produced by chaotic dynamics induced by stretching and folding processes between the interacting magmas the mingling of magmas has been simulated using a chaotic dynamical system consisting of repeated stretching and folding processes the simulation shows the same patterns of variation of contact perimeter and fractal dimension as those observed in natural structures and indicates that magma interaction processes acted with different intensities in the three lava flows in response to different magmatic interaction regimes since physical dispersion of one magma inside another through stretching and folding processes and chemical exchanges are closely related we performed coupled numerical simulations of chaotic advection and chemical diffusion the results show a good agreement between the computed and natural structures in particular the occurrence in the same system of well and poorly mixed regions it is shown that magma interaction processes are able to generate magmatic masses having wide spatial heterogenity at many length scales this occurrence can account for the presence of magmatic enclaves inside host rocks showing a variable degree of hybridization in both plutonic and volcanic environments

the space and time complexity of chaotic mixing of silicate melts implications for igneous petrology abstract we present new experimental results on the study of the space and time modulation of compositional fields during chaotic mixing between mafic and felsic silicate melts the experimental strategy was planned using numerical simulations performed using the experimental geometry these mixing experiments were performed using a recently developed experimental apparatus which is capable of mixing high viscosity silicate melts at high temperatures and under precisely controlled conditions of fluid dynamics and strain the compositional variability produced by the mixing process was investigated both along linear analytical transects and on high resolution 2d x ray maps covering the mixing patterns our results indicate that chaotic flow fields represent very powerful dynamics to blend silicate melts even under laminar fluid dynamic conditions reynolds number ca 10 7 and for dissimilar melts with high viscosity ratios on the order of 103 the repetition of stretching and folding processes between the two melts induced a strong increase of contact interfaces thus favoring efficient chemical exchanges as a result the initial mafic composition is no longer detectable in the mixing system after ca 2 h i e the duration of the experiment a further important result is the observation of highly non linear patterns in inter elemental plots produced by the onset of diffusive fractionation processes this is contrary to common thinking that magma mixing should always produce linear trends between pairs of chemical elements a new measure the concentration variance is proposed to quantify chemical element mobility during the mixing process this measure is statistically robust and can be quantitatively used to measure chemical element mobility independently of the geometry in which the compositional variation i e transects areas etc is embedded or the local strain history of the mingling our results highlight concentration variance as a robust probe of the as yet poorly understood processes involved in the common petrological process of magma mixing

volcano glacier interactions on composite cones and lahar generation nevado del ruiz colombia case study the catastrophic lahars triggered by the 13 november 1985 eruption of the ice clad nevado del ruiz volcano colombia demonstrate that the interaction of hot pyroclasts with snow and ice can release 30 50 million m 3 of meltwater in 30 90 minutes the 1985 eruption caused a 16 loss in area and a 9 loss in volume of snow firn and ice turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0 5 1 6 mm s 1 laboratory experiments suggest that turbulent fluidized pyroclastic density currents exert mechanical and thermal scour thereby efficiently transferring heat from hot pyroclasts to snow ice cap loss at nevado del ruiz continued between 1985 and 2000 representing a 52 decline in area and a 30 fall in volume ice 60 190 m thick caps the east and southeast summit plateau whereas an ice field 30 m thick and devoid of snow is retreating on the north northeast and west edges this asymmetrical distribution of ice reflects combined long term effects of the 1985 eruption and of the post 1985 ice cap retreat should volcanic activity resume steep sided glaciers can fail and pyroclastic flows and surges can sweep the snowpack and generate mixed avalanches and lahars although the potential source of meltwater has decreased since 1985 extensive debris at the ice cap margins can be incorporated to future lahars