gas evolution in eruptive conduits combining insights from high temperature and pressure decompression experiments with steady state flow modeling abstract in this paper we examine the consequences of bubble nucleation mechanism on eruptive degassing of rhyolite magma we use the results of published high temperature and pressure decompression experiments as input to a modified version of conflow the numerical model of mastin and ghiorso 2000 u s g s open file rep 00 209 53 pp and mastin 2002 geochem geophys geosyst 3 10 1029 2001gc000192 for steady two phase flow in vertical conduits synthesis of the available experimental data shows that heterogeneous nucleation is triggered at p 120 150 mpa and leads to disequilibrium degassing at extreme h2o supersaturation in this latter case nucleation is an ongoing process controlled by changing supersaturation conditions exponential bubble size distributions are often produced with number densities of 106 109 bubbles cm3 our numerical analysis adopts an end member approach that specifically compares equilibrium degassing with delayed disequilibrium degassing characteristic of homogeneously nucleating systems the disequilibrium simulations show that delaying nucleation until p 150 mpa restricts degassing to within 1500 m of the surface fragmentation occurs at similar porosity in both the disequilibrium and equilibrium modes 80 vol but at the distinct depths of 500 m and 2300 m respectively the vesiculation delay leads to higher pressures at equivalent depths in the conduit and the mass flux and exit pressure are each higher by a factor of 2 0 residual water contents in the melt reaching the vent are between 0 5 and 1 0 wt roughly twice that of the equilibrium model

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

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

toward an understanding of disequilibrium dihedral angles in mafic rocks the median dihedral angle at clinopyroxene plagioclase plagioclase junctions in mafic rocks cpp is generally lower than equilibrium 109 plus minus 2 observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of cpp is incorrect in several important respects firstly the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three grain junctions and hence cpp is formed during solidification the igneous process sub solidus textural modification in most dolerites and gabbros previously thought to be the dominant control on cpp is insignificant cpp is governed by mass transport constraints the inhibiting effects of small pore size on crystallization and variation in relative growth rates of pyroxene and plagioclase during rapid cooling pyroxene preferentially fills wider pores while the narrower pores remain melt filled resulting in an initial value of cpp of 78 rather than 60 which would be expected if all melt filled pores were filled with pyroxene lower cooling rates create a higher initial cpp due to changes in relative growth rates of the two minerals at the nascent three grain junction low cpp associated with cuspate clinopyroxene grains at triple junctions can also be diagnostic of infiltration of previously melt free rocks by late stage evolved liquids the metasomatic process modification of cpp by sub solidus textural equilibration the metamorphic process is only important for fine grained mafic rocks such as chilled margins and intra plutonic chill zones in coarse grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts associated with rounding of chadacrysts

the fidelity of melt inclusions as records of melt composition a series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1 150 c 1 0 gpa to investigate diffusive fractionation during melt inclusion formation additionally p diffusion in a basaltic melt was measured at 1 0 gpa melt inclusions and melts within a few 100 microns of plagioclase melt interfaces were analyzed for comparison with melt compositions far from the crystals melt inclusions and melt compositions in the boundary layer close to the crystal melt interface were similar but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals the compositional profiles of s cl p fe and al in the boundary layers were successfully reproduced by a two step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation 10 9 10 8 m s 1 the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient but sulfur with a diffusivity similar to major elements and co2 was somewhat enriched in the boundary layer melt and phosphorus with its low diffusion coefficient similar to other high field strength elements and rare earth elements was significantly enriched thus the concentrations of sulfur and phosphorus in melt inclusions may over estimate their values in the bulk melt and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt

partial melting and recrystallization of granite and their application to deep disposal of radioactive waste part 1 rationale and partial melting abstract a high temperature deep borehole disposal scheme for nuclear waste is outlined this requires that granite host rock can be partially melted by the heat from the waste then completely recrystallized both on the time scale of the cooling of the waste experimental studies of non equilibrium partial melting of a typical s type crustal granite were carried out under the conditions likely to arise during implementation of this scheme p 0 15 gpa t 700 850 c the melting relations were determined under h2o saturated and h2o undersaturated conditions melting begins just under 700 c and the amount of melt increases with temperature and especially h2o content up to saturation such that with no added h2o only 5 of melt is produced even at 850 c but the addition of 1 h2o will generate 40 of melt 50 c lower at 800 c the partial melts formed at high degrees of melting e g h2o saturated t 800 c approximate to equilibrium melting but the smaller amounts of melt produced at lower temperatures under h2o undersaturated conditions clearly do not on the basis of these experiments it is concluded from the point of view of partial melting that an s type granite similar to the one studied would be a suitable host rock for the proposed high temperature very deep disposal scheme for radioactive waste

an experimental study of dissolution and precipitation of forsterite in a thermal gradient implications for cellular growth of olivine phenocrysts in basalt and melt inclusion formation the morphology of crystals in magmas strongly depends on the temperature regime of the system in particular the degree of undercooling and the cooling rate to simulate low degrees of undercooling we developed a new experimental setup based on thermal migration in which large cylinders of forsterite single crystals immersed in haplobasaltic melt were subjected to a temperature gradient as forsterite solubility is sensitive to temperature the forsterite on the high temperature side undergoes dissolution and the dissolved components are transported toward the low temperature side where a layer of newly grown forsterite forms up to 340 m thick after 101 h a striking feature is that the precipitation process does not produce a planar front of forsterite advancing at the expense of liquid the growth front shows a fingered outline in planar section with solid lobes separated by glass tubes that are perpendicular to the growth front we ascribe this texture to cellular growth a type of growth that had not been experimentally produced so far in silicate systems we find that the development of cellular growth requires low degrees of undercooling a few c and large crystal liquid interfaces 1 mm across or more and that it occurs at a growth rate of the order of 10 9 m s we found natural occurrences of cellular growth on the rims of olivines from basanites but otherwise cellular textures are poorly documented in natural volcanic rocks melt inclusions were produced in our experiments showing that they can form in olivine at relatively slow rates of growth 10 9 m s or lower

magnetic and petrologic characterization of synthetic martian basalts and implications for the surface magnetization of mars 1 a suite of synthetic martian basalts is generated with the objective of providing fundamental material properties data for use in modeling and interpretation of mission data we systematically evaluate the effects of major element composition oxygen fugacity fo2 and cooling rate on phase chemistry and magnetic mineralogy grain size and intensity of remanent magnetization the range of experimental compositions and fo2 are chosen to bracket the range expected in the martian crust our results should therefore span the range of possible mineralogies textures and magnetic properties in rapidly cooled mars crustal materials two starting compositions are used for the sample synthesis 1 an fe rich al poor composition patterned after snc basaltic meteorites and 2 a composition based on thermal emission spectrometer tes data with a much lower fe al ratio the resulting magnetic phase in samples generated at the quartz fayalite magnetite qfm buffer is a spinel structured oxide with varying amounts of cr ti mg and al compositional differences depend on bulk composition cooling rate differences in crystallization sequence and the kinetics of silicate mineral nucleation and growth oxide abundance and magnetic intensity are most strongly influenced by fo2 with more subtle composition and cooling rate effects moderately oxidizing qfm conditions result in an intense magnetization 2 3 10 5 am2 kg 1 to 1 4 10 2 am2 kg 1 especially in the meteorite derived basalts however an increase of magnetic grain size into the multidomain range meteorite type and or low unblocking temperatures resulting from increased cr substitution tes type may affect the long term stability of the remanence in qfm samples

experimental investigation of ostwald ripening rates of forsterite in the haplobasaltic system abstract the rate of ostwald ripening of forsterite is experimentally measured in a haplobasaltic melt sio 2 al 2 o 3 cao mgo the goal of the experiments is to determine if the rate of ostwald ripening is sufficient to affect textural development in crystallizing magmas of a basaltic composition with preequilibrated charges experiments were run isothermally so that all textural changes could be ascribed to ostwald ripening an increase of crystal size from 3 5 m to 14 m in average has been observed during 10 days glass compositions were identical in all charges indicating that the observed increase in average crystal size is not the results of chemical disequilibrium but the results of ostwald ripening due to the constraints on experimental duration the rate limiting process i e diffusion control or reaction control could not be determined our experimental results however demonstrate that the rate of ostwald ripening in basaltic rocks is sufficient to be of fundamental importance in the development of textures in igneous rocks these results strongly suggest that ostwald ripening as well as nucleation and crystal growth should be considered in analyses of textures in igneous rocks

origin of crystal poor differentiated magmas insights from thermal gradient experiments crystal poor differentiated magmas are commonly erupted from shallow thermally zoned magma chambers in order to constrain the origin of these magmas we have experimentally investigated crystallization differentiation and crystal melt separation in presence of a thermal gradient experiments have been designed taking advantage of the innate temperature gradient of the piston cylinder apparatus and carried out on a phonolitic system at 0 3 gpa and temperature ranging from 1 050 to 800 c crystallization degree and melt composition in experimental products vary as a function of the temperature gradient in particular melt composition differentiates from tephri phonolite starting material to phonolite moving from the hotter glassy zone t 1 050 c towards the cooler heterogeneously crystallized zone t 900 c of the charge the heterogeneously crystallized zone is made up of 1 a crystal rich mushy region crystallinity 30 vol 2 a rigid crystal framework crystallinity 80 vol and 3 glassy belts of phonolitic glass at the top thermal gradient experiments picture crystallization differentiation and crystal melt separation processes occurring in a thermally zoned environment and reveal that relatively large volumes of crystal poor melt glassy belts can originate as a consequence of the instability and collapse of the rigid crystal framework analogously in thermally zoned magma chambers the development and collapse of a solidification front may represent the controlling mechanism originating large volumes of crystal poor differentiated magmas

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

capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the juan fernandez islands olivine crystals in basalt contain helium that can be extracted for isotopic analysis helium bearing olivine phenocrysts in picritic tholeiites from the juan fernandez islands se pacific crystallized from moderately differentiated liquids none are xenocrysts of mantle peridotite the helium occurs in cavities or bubbles in inclusions best seen at high magnification in the olivine the olivine grew around spinel sulfide and bubbles that preferentially nucleated on crystal surfaces inclusions within these phases range from large cavities to tiny faceted pits arranged in rows many crystals contain curving trains of inclusions along annealed features the inclusions formed during mixing between cooler differentiated magma and hotter olivinecharged magma which accelerated vesiculation bubbles nucleated on the olivine as they do on ice when stirred in carbonated water mixing also induced thermal stress fracturing like the cracking of ice dropped into water cracking irregular extinction and subgrain formation occurred when faceted crystals collided with each other or with conduit walls boundary layer melts and bubbles were drawn quickly into the fractures thus few inclusions contain equilibrium proportions of minerals and vapor mantle derived helium clearly permeated into shallow storage reservoirs including rift zones where magmatic differentiation mixing and vapor exsolution were fairly extensive

first principles investigations on the elasticity and phase stability of grossular garnet 1 in order to understand the fate of subducted continental materials by means of mechanisms such as tectonic erosion and sediment subduction it is important to understand the high pressure stability and elastic properties of grossular garnet we study these questions using the first principles computation method grossular garnet was found to dissociate into an assemblage of casio3 ca perovskite pv and al2o3 corundum cor at about 23 4 gpa accompanied by remarkable jumps of compressional wave 8 0 shear wave 11 6 bulk sound 5 7 velocities and density 12 1 although pv with the grossular garnet composition was suggested experimentally after the decomposition reaction this phase was found to be less stable than an assemblage of capv and cor at pressures higher than 20 gpa this indicates that observed pv with the grossular garnet composition is metastable but can be obtained because of the slow kinetics of the garnet decomposition reaction our results imply that the dissociation of grossular garnet as well as that of jadeite included in continental materials subducted into the deep mantle increases the complexity of the 660 km discontinuity and can explain seismically observed multiple reflections we have also investigated the anisotropy of elastic velocities of grossular garnet capv and cor and found that capv is the most anisotropic mineral this implies that capv could produce seismic anisotropy in the uppermost lower mantle

crystallization of the little three layered pegmatite aplite dike ramona district california subhorizontally layered pegmatite aplite bodies are characterized by fine grained sodic to granitic aplite that is usually juxtaposed abruptly above by much coarser grained commonly graphic potassic pegmatite although well studied there currently is little concensus as to how such dikes form the little three dike near ramona california is representative of such zoned bodies in this and other regions and contains discontinuous miarolitic pockets near the base of the graphic pegmatite zone tourmaline garnet biotite and muscovite show no changes in major or minor element compositions indicative of progressive magmatic fractionation until the immediate vicinity of the main miarolitic zone where they record abrupt and extreme enrichments in li f and mn there is no correlation of chemical changes in the dike with the appearance of small miarolitic vugs well below the main miarolitic zone nor is there any indication that the aplite graphic pegmatite or miarolitic pockets represent separate magma injections the chemistries of tourmaline garnet and micas however preclude conventional models of rayleigh fractionation or traditional zone refining textural features and modeled cooling histories indicate that the dike cooled quickly and might have solidified partially or totally to glass before crystallization commenced geothermometry based on the compositions of coexisting plagioclase and homogeneous nonperthitic k feldspar indicates inward crystallization of the dike from 400 435 c at the margins to 350 390 c within 20 30 cm of the pocket horizon then a sharp decrease to 240 275 c in the pockets where k feldspar is perthitic we interpret the feldspar geothermometry except perhaps in the miarolitic cavities to reflect the temperatures at crystallization fronts that advanced into the pegmatite first from the foot wall and eventually joined by a similar front downward from the hanging wall crystallization down from the hanging wall may have commenced after 70 80 of the foot wall aplite had crystallized the very abrupt increases of li mn and f in tourmaline and garnet near the miarolitic zone appear to be explained best by the process of constitutional zone refining in which a fluxed crystallization front sweeps an incompatible element enriched boundary layer through a solid or semi solid after these two highly fluxed boundary layers merged near the main miarolitic zone compositional evolution could have proceeded by crystal melt fractionation

olivine growth rates in a tholeiitic basalt an experimental study of melt inclusions in plagioclase abstract basaltic glass inclusions trapped in plagioclase phenocrysts an 84 are remnant of their parent magmatic liquid they can be used as natural reactors for the experimental investigation of olivine growth rate as a function of temperature the growth of one olivine nucleus can be observed at constant temperature supercooling from 15 to 150 c have been investigated growth habits vary from equant to feather in qualitative agreement with previous studies growth rates vary from 10 m s 1 to 6 10 7 m s 1 they vary with the direction the growth process planar or dendritic and the degree of supercooling chemical analysis of crystal overgrowth and the remaining liquid composition enables a mass balance calculation which confirms the rates determined optically the small number of results obtained so far does not permit to draw inferences on the growth mechanisms

equilibration scales in silicic to intermediate magmas implications for experimental studies experimental phase equilibrium studies are increasingly being used for the determination of intensive variables p t fh2o fo2 in silicic to intermediate magmas in contrast silicic igneous bodies are now perceived as open periodically recharged systems involving only limited chemical equilibration thus the use of laboratory determined crystal liquid equilibrium data needs clarification here we review the field petrological and geochemical evidence concerning states and scales of chemical equilibrium in silicic magma bodies it is concluded that total chemical equilibrium is generally not the rule however a subsystem in local equilibrium the reactive magma can be identified equilibration scales in silicic magmas are rate limited either by diffusive flux in crystals dicl regime or by diffusive flux in the melt md regime the recognition that equilibrium in magmas is limited to a reactive subsystem requires phase equilibrium studies to be chemically scaled experiments either of total or partial equilibrium type should aim at a close reproduction of equilibrium states specific to natural systems the laboratory reconstruction of the natural equilibrium states guarantees a precise determination of the pre eruptive parameters and a reliable application of the experimental data to active volcanic systems

experiments on the kinetics of partial melting of a leucogranite at 200 mpa h 2 o and 690 800 c compositional variability of melts during the onset of h 2 o saturated crustal anatexis we have experimentally investigated the kinetics of melting of an aplitic leucogranite quartz sodic plagioclase of ab90 k feldspar traces of biotite at 690 740 and 800 c all at 200 mpa h2o leucogranite cylinders 3 5 mm in diameter and 7 mm in length were run in the presence of excess h2o using cold seal pressure vessels for 11 2 925 h at 690 and 740 c and any experimental time and 800 c and short run times silicate glass melt at run conditions occurs as interconnected films along most of the mineral boundaries and in fractures with the predominant volume occurring along quartz feldspars boundaries and quartz plagioclase k feldspar triple junctions glass film thickness is roughly constant throughout a given experimental charge and increases with experimental temperature and run duration the results indicate that h2o saturated partial melting of a quartzo feldspathic protolith will produce an interconnected melt phase even at very low degrees 5 vol of partial melting crystal grain boundaries are therefore completely occluded with melt films even at the lowest degrees of partial melting resulting in a change in the mechanism of mass transport through the rock from advection of aqueous vapor to diffusion through silicate melt at 690 and 740 c the compositions of glasses are homogeneous and at both temperatures close to but not on the h2o saturated 200 mpa haplogranite eutectic glass compositions do not change with run duration at 800 c glasses are heterogeneous and plot away from the minimum although their molar ratios asi mol al2o3 cao na2o k2o and al na are constant throughout the entire charge at any experimental time glass compositions within individual 800 c experiments form linear trends in wt normative quartz albite orthoclase space the linear trends are oriented perpendicular to the 200 mpa h2o haplogranite cotectic line reflecting nearly constant albite orthoclase ratio versus variable quartz feldspar ratio and have endpoints between the 800 c isotherms on the quartz and feldspar liquidus surfaces with increasing experimental duration the trends migrate from the potassic side of the minimum toward the bulk rock composition located on the sodic side due to more rapid and complete dissolution of k feldspar relative to plagioclase the results indicate that partial melting at or slightly above the solidus 690 740 c is interface reaction controlled and produces disequilibrium melts of near minimum composition that persist metastably for up to at least 3 months relict feldspars show no change in composition or texture and equilibration between melt and feldspars might take from a few to tens of millions of years partial melting at temperatures well above the solidus 800 c produces heterogeneous disequilibrium liquids whose compositions are determined by the diffusive transport properties of the melt and local equilibrium with neighboring mineral phases feldspars recrystallize and change composition rapidly partial melting and equilibration between liquids and feldspars might take from a few to tens of years h2o saturated conditions at these temperatures well above the solidus

thermal interaction between lava lobes pahoehoe lava may be emplaced as lobes which flow on top of each other a cross section was investigated through two lava lobes from the active pu u o o vent kilauea hawaii consisting of a 6 6 cm thick top lobe and the upper part of a 3 2 cm thick underlying bottom lobe the top lobe cooled at the contact with air and has a quenched glassy margin which is almost 1 cm thick the underlying lobe although originally also in contact with air is entirely crystalline suggesting that the bottom lobe experienced crystallization during reheating by the top lobe based on published data experimental results on reheating show that crystallization of basalt glass requires temperatures above 920 c and knowing that the pu u o o melt is initially at about 1 160 c the minimum temperature of the bottom lobe should have been 700 c prior to emplacement of the top lobe crystal size distribution csd theory demonstrates an oscillation of grain size which is superposed on an overall increase in size with depth cooling rates estimated from simple modeling and growth rates derived from csd suggest a thermal interaction of the bottom lobe on the top lobe to about 2 cm from the contact

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

oxygen isotope heterogeneity of arc magma recorded in plagioclase from the 2010 merapi eruption central java indonesia chemical and isotopic compositions of magmatic crystals provide important information to distinguish between deep juvenile and crustal contributions in this work high resolution multicollector secondary ion mass spectrometry data reveal strong variations of 18o values in three plagioclase crystals 800 1700 m from two representative basaltic andesite samples of the 2010 merapi eruption central java indonesia the 18o values from 4 6 to 7 9 are interpreted to reflect oxygen isotope heterogeneity in the melt composition during plagioclase growth the lowest 18o values 4 6 6 6 are found in anorthite rich cores an82 97 whereas higher 18o values 5 7 to 7 9 are found in anorthite poorer zones an33 86 typically in crystal rims combining these new plagioclase 18o data with 18o of calc silicate crustal xenoliths erupted between 1994 1998 the composition of glass inclusions hosted by the anorthite rich plagioclase an82 92 available experimental data and the results of thermodynamic modelling using the magma chamber simulator code we conclude that the abundant anorthite rich cores crystallized from a mantle derived hydrous basaltic to basaltic trachyandesite melt that recharged a deeper 200 600 mpa magma storage zone whereas lower anorthite zones crystallized at shallower levels 100 200 mpa the oxygen isotope variations in the plagioclase are explained by a two stage model of interaction of the hydrous mafic mantle derived magma 1 with old crustal rocks depleted in 18o due to high temperature alteration that yielded the low 18o values in the anorthite rich cores at deep levels 13 20 km and later 2 with 18o enriched carbonate material that yielded the high 18o values in anorthite poorer zones at shallow levels 4 5 9 km thermodynamic modelling is consistent with 18 wt assimilation of crustal calc silicate material at 925 950 c and 100 200 mpa by the 2010 merapi basaltic andesite magma prior to eruption timescales for plagioclase phenocryst growth and residence in the magmatic plumbing system are 34 years the combined data thus reveal efficient magma recharge and crustal assimilation processes that characterize the open system magma storage and transport systems associated with the 2010 merapi eruption

diversity of eruptive styles inferred from the microlites of mt pelee andesite martinique lesser antilles abstract we have studied the compositional and textural characteristics of plagioclase microlites from several dome forming eruptions dome block and ash flows and surges and plinian fallouts of the recent period of activity of mt pelee in order to infer magma ascent dynamics dome forming and plinian samples display a wide range of plagioclase microlite compositions from an 35 to an 90 and an 55 to an 90 respectively microlites with compositions an 50 60 crystallized from a mafic melt that intruded the andesitic reservoir in proportion of 10 vol during magma ascent in the conduit the fraction number density and morphology of the decompression induced microlites compositions 50 60 suggest that the plinian magmas experienced a nucleation dominated crystallization regime with high undercooling in contrast magmas from domes and block and ash flows were dominated by a growth regime with low undercooling and the surges were influenced by both regimes according to these crystallization regimes and new experimental data on h 2 o content of the matrix glasses we propose magma relative ascent rates and fragmentation pressures for the different eruptive styles observed at mt pelee that may explain why surges are explosive events and why they may or may not be followed by a plinian event

syneruptive mixing degassing and crystallization at redoubt volcano eruption of december 1989 to may 1990 abstract the 1989 1990 eruption of redoubt volcano included initial energetic vertical venting of pumiceous tephra followed by shedding of dense pyroclastic flows from successive unstable domes the suite of quenched magmatic materials thus produced coupled with detailed seismic observations provides a record of establishment and subsequent evolution of an active magma column in the upper crust the initial eruptive event involved a dacite magma with rhyolite melt and an andesite magma with dacite melt the dacite magma contains reaction rim free hornblende high cl concentrations 0 12 0 02 wt of melt and is microlite free all evidence for rapid ascent from depth in contrast the andesite component is microlite rich though this characteristic may have arisen by mixing between the two magmas rather than by slow ascent abundant dense juvenile lava like clasts with low h2o 0 1 0 3 wt were ejected in the initial eruption event implying that the magma column had reached the near surface and its upper portion had undergone degassing and bubble collapse prior to explosive disruption later eruptive events contain evidence of progressive blending of the two magmatic components as well as mixing between fresh reservoir derived and old conduit resident magma magma apparently ascended to the surface from the 6 10 km deep source chamber in a series of pulses rather than by steady flow or gradual emptying of a shallowly intruded body increased shallow long period seismicity which may indicate subsurface degassing from the flowing magma column occurred 5 days before each major eruptive event petrologic evidence indicates that each eruption contained some magma that ascended quickly from the chamber with time the proportion of new magma in eruptive batches may have decreased however even the final dome contains magma that left the chamber no more than a few days before extrusion together the pulses apparently record passage of an andesite plume through redoubt s dacite magma reservoir

sub micron scale trace element distributions in natural zircons of known provenance implications for ti in zircon thermometry nanosims measurements of ti p y ce and hf in zircon separates from the youngest toba tuff the bishop tuff the quottoon igneous complex the sierra nevada batholith and an adirondack migmatite show that micron scale oscillatory zoning of ti is common the zircons we have studied typically display banded concentration gradients having between 1 and 7 peaks in ti abundance over length scales of 10 20 m the beam diameter commonly used for sims trace element zircon analyses with amplitudes of up to 4 5 ppm ti baseline values are 1 4 ppm and widths measured at half height of between 0 2 and 4 m spatial correlations between concentrations of ti and other trace elements p y and ce are also common but variable in character ranging from oscillatory co variation of ti p y and ce to cases where only a subset of peaks for a given element is spatially correlated with peaks in ti there are also longer length scale generally positive correlations among concentrations of ti p y and ce i e gradients on which narrower peaks are superimposed in contrast hf concentrations are either uncorrelated or inversely correlated with these longer length scale variations in ti concentrations the wide range in ti concentrations over distances of less than 1 m and the various correlations between ti concentrations and those of the other analyzed elements suggest that on the micron scale and at temperatures between 700 and 800 c zircon liquid ti partitioning is not controlled by bulk or lattice equilibrium treating our nanosims ti ion maps as though they were conventional sims analyses i e generating an average ti concentration for each map we evaluate the hypothesis that while micron scale variations in ti concentrations might be kinetically controlled when averaged over 100 400 m2 such variations capture the thermal state of the growing zircon using these average ti concentrations independent petrologic pressure and temperature constraints estimates of silica and titania activities based on phase assemblages as well as calculations using rhyolite melts we show that crystallization temperatures predicted by ti in zircon geothermometry generally do not agree with the independently constrained temperatures for the samples

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

geochemical modeling of low melt fraction anatexis in a peraluminous system the pena negra complex central spain abstract a study was made of the chemical fractionation associated with four cases of anatectic segregation of low melt fraction cordieritic granites from migmatized meta greywackes the aims of the study were to 1 reveal the fractionation patterns of major and trace elements 2 compare the major element chemistry of leucogranites and the quantitative behavior of source minerals during anatexis inferred by mass balance adjustment with available experimental data for peraluminous systems and 3 discuss the behavior of trace elements in crustal melting by comparing the chemically determined composition of leucogranites with the results of three fractionation models two of these assume a perfect diffusive behavior of trace elements within residual solids but they use a different set of distribution coefficients the third assumes a perfect nondiffusive behavior in relation to their source rocks the leucogranites are strongly depleted in li transition elements and light rare earth elements but enriched in k2o sio2 and ba the k rb ratio in leucogranites is higher than in sources and reaches values typical of high grade metamorphism mass balance analysis using the anatexis mixing model shows that the chemistry of cordierite leucogranites is compatible with its having originated by closed system water undersaturated anatexis on previously migmatized meta greywackes leaving a residue enriched in cordierite plus biotite and exhausted in k feldspar biotite melts congruently unless important amounts of sillimanite were also present in the source compared with experimental melts obtained from sources with the same chemical composition but with a different femic mineralogy biotite sillimanite instead of cordierite biotite the pena negra leucogranites are richer in k2o and mgo with a lower fe fe mg ratio the differences in magnesium are believed to result from the changes in the mineral assemblage of the source rocks however the differences in potassium contents are still not well understood the behavior of trace elements during anatexis cannot be modeled by trace element fractionation equations because of the sluggish diffusion kinetics of trace elements at the temperature of anatexis making the melting rate of minerals greater than the diffusion rate of trace elements within minerals the behavior of trace elements essentially consists in following that of the minerals containing them

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

experimental constraints on plagioclase crystallization during h2o and h2o co2 saturated magma decompression experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions interpretation of natural samples may be complicated however by both the decompression path and the composition of exsolving volatiles which affect the time evolution of crystal textures here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of mount st helens rhyodacite three families of experiments were employed to simulate varied ph2o t trajectories single step h2o saturated decompression ssd continuous h2o saturated decompression cd continuous h2o co2 saturated decompression quantitative textural data crystal abundance number density and size are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products these are the first experiments to quantify feldspar nucleation and growth rates during h2o co2 saturated decompression we find that reducing the initial melt water content through addition of co2 increases nucleation rates relative to the pure water case an effect most pronounced at low dp dt moreover these early formed textural distinctions persist at the lowest pressures examined suggesting that deep h2o co2 fluids could leave a lasting textural fingerprint on erupted magmas crystals formed prior to decompression during the annealing process also modulate sample textures and growth on pre existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions the phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption finally quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive effusive transition at mount st helens this comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the may 18 plinian eruption such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories

origin of olivine in kimberlite phenocryst or impostor abstract kimberlite hosts two populations of olivine that are distinguished on the basis of grain size and morphology the populations are commonly described genetically as xenocrysts and phenocrysts olivine xenocrysts or macrocrysts are thought to derive from disaggregation of mantle xenoliths whereas the smaller euhedral olivine crystals are presumed entirely cognate to the kimberlite melt recent studies of zoning patterns of euhedral olivine in kimberlite have however cast doubt on the actual origins of the smaller olivine crystals here we elucidate the nature and origins of the textural and chemical zonation that characterize both populations of olivine macrocrysts olivine i and euhedral crystals olivine ii specifically we show that both olivine i and olivine ii feature chemically distinct overgrowths resulting from heterogeneous crystallization onto pre existing olivine xenocrysts our analysis limits the total volume of olivine crystallized during transport to 5 in contrast to previous estimates of 25 the reduced extent of olivine crystallization allows for closer reconciliation of crystallized olivine compositions and estimates of mg s for primitive kimberlite melts it also places constraints on processes involving orthopyroxene assimilation by kimberlite melt if olivine crystallization and orthopyroxene assimilation are coupled then orthopyroxene assimilation is limited to 7 larger masses of orthopyroxene assimilation i e 25 are possible only if kimberlite magmas originate at super liquidus 100 oc conditions and sub equal amounts of olivine crystallization occurs

formation of spinifex texture in komatiites an experimental study the formation of platy olivine spinifex the texture that characterizes komatiite lavas has long been enigmatic a major problem is that the dendritic morphology of the olivine resembles that of crystals grown in laboratory experiments at high cooling rates 50 c h but at the position where these textures form up to several meters below the komatiite flow top the cooling rate cannot have been greater than 1 5 c h we performed experiments that demonstrate that the platy habit of spinifex olivine or pyroxene is a consequence of slow cooling of ultramafic magma in a thermal gradient 7 35 c cm the charges were cooled at rates between 2 and 1428 c h and even at the low cooling rates the thermal gradient led to constrained growth and the development of preferentially oriented dendritic crystals with orphologies like those in natural platy spinifex textured lavas under these conditions olivine starts to crystallize at temperatures well below the equilibrium liquidus temperature 37 c dt 56 c depending on the composition of the starting material when the cooling rate is high the thermal gradient has a negligible effect on the texture and the crystals have a random orientation like that in the upper parts of komatiite flows

transcrystalline melt migration in clinopyroxene glass inclusions in clinopyroxene phenocrysts from la sommata vulcano island aeolian arc were reheated and submitted to a sustained thermal gradient each remelted inclusion undergoes a transient textural and chemical reequilibration and concomitantly begins to migrate along a crystallographic direction at a small angle with the thermal gradient the completion of morphological evolution requires a characteristic time that is governed by chemical diffusion chemical reequilibration results in the formation of a colored halo that delineates the former location and shape of the inclusion after it has migrated away transcrystalline migration proceeds by dissolution of the host clinopyroxene ahead and precipitation astern its rate is not limited by fick s law but by the crystal melt interface kinetics clinopyroxene dissolution and growth are slower than for olivine in similar conditions but obey the same analytical law which can be transposed to equally or more sluggish melting or crystallization events in nature when a gas bubble is initially present it responds to elastic forces by quickly shifting toward the cold end of the inclusion where it soon becomes engulfed as an isolated fluid inclusion in the reprecipitated crystal this study confirms that transcrystalline melt migration beside its possible implications for small scale melt segregation and fluid inclusion generation in the earth s mantle provides an experimental access to interfacial kinetic laws in near equilibrium conditions

a practical petrological method for the determination of volume proportions of magma chamber refilling abstract a method is described for determining the proportion of the volume of a magma chamber refilled by new pulses of hot primitive melts this method is based on the changes is crystal contents and in phenocryst zoning resulting from temperature equilibration study of phenocryst zoning and the composition of interstitial glasses from various chilled magmatic inclusions and mafic bands in hybrid rocks supports the idea that equilibration of temperatures between two magmas is reached before their physical mixing if the magmas are genetically related for example magma fractionate and initial hot melt this leads to equilibration of the compositions of residual melts and phenocrysts rims intervals of reverse zoning in minerals resulting from temperature increases and intervals of normal zoning caused by cooling along with the changes in modal mineral contents enable estimation of the amounts of released and adsorbed heat and therefore assessment of the proportion of the volume of each magma application of the method to some hybrid volcanic series of the kurile kamchatka island arc shows that the proportion of mafic magma intruded into a siliceous magma chamber reservoir never exceeds 10 of the volume of the host magma based upon thermal considerations however the observed degree of compositional mingling requres 25 50 of the mafic end member it is shown that this contradiction is related to flotation of vesiculated magmatic inclusions towards the chamber roof where they become concentrated near the exit and are further mingled with the host during the eruption forced convection

the effect of crystallization time on plagioclase grain shape in dolerites the average aspect ratio of plagioclase measured in thin section varies systematically through dolerite sills with a symmetrical m shaped profile observed in sills thinner than 200 m thicker sills show the same marginal reversal at the base but average aspect ratios appear to continue increasing towards the top creating an s shaped profile a distorted m shaped profile is visible in the stratigraphic variation of plagioclase average aspect ratios in the prehistoric makaopuhi lava lake with the centre of the m in the olivine enriched horizon close to the base of the lake higher overall values of average aspect ratio are observed in thin sills compared to thicker sills demonstrating that the plagioclase in more slowly crystallized bodies is more equant than that in more rapidly crystallized bodies there is a strong correlation between the plagioclase average aspect ratio in the central parts of the sills and the crystallization time calculated using a simple one dimensional thermal model assuming conductive cooling the cause of the marginal reversals in average aspect ratio is not well understood but may result from early grain impingement in the developing crystal mushy layer

application of cathodoluminescence to magmatic quartz in a tin granite case study from the schellerhau granite complex eastern erzgebirge germany a model of the cooling history of tin bearing granitic magma forming the schellerhau granites eastern erzgebirge germany is shown on the basis of quartz textures similar grain size similar grain habit and correlatable growth textures of phenocrysts in different granite varieties give proof of a common crystallization history before the melts of the schellerhau granite varieties were intruded four nucleation events occurred during crystallization in different crustal levels between about 20 and 1 km depth the parental melt of the schellerhau granites is interpreted to have contained 2 5 wt h2o originally the water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt and characterizes highly evolved residual melts that enable the formation of tin deposits this study contributes to a better understanding of the development and behaviour of fractionated tin bearing granitic melts and links quartz cathodoluminescence cl with microanalytical studies

an experimental study of ostwald ripening of olivine and plagioclase in silicate melts implications for the growth and size of crystals in magmas we carried out an experimental study to characterize the kinetics of ostwald ripening in the forsterite basalt system and in the plagioclase an65 andesite system eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution csd with time t the experiments were performed in a 1 atmosphere quench furnace at 1 250 c for plagioclase and 1 300 c for olivine very contrasted coarsening kinetics were observed in the two series in the plagioclase series the mean grain size increased as log t from 3 m to only 8 7 m in 336 h the kinetic law in log t means that ostwald ripening was rate limited by surface nucleation at plagioclase liquid interfaces in the olivine series the mean grain size increased as t1 3 from 3 m to 23 2 m in 496 h a kinetic law in t1 3 is expected when ostwald ripening is rate limited either by diffusion in the liquid or by grain growth dissolution controlled by a screw dislocation mechanism the shape of olivine csds in particular their positive skewness indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism not by diffusion as the degrees of undercooling t or supersaturation involved in ostwald ripening are essentially 1 c the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers we extrapolated our experimental data to geological time scales to estimate the effect of ostwald ripening on the size of crystals in magmas in the case of plagioclase ostwald ripening is only efficient for mean grain sizes of a few microns to 20 m even for a time scale of 105 years it can however result in a significant decrease of the number of small crystals per unit volume and contribute to the development of convex upwards csds for olivine the mean grain size increases from 2 3 m to 70 m in 1 year and 700 m in 103 years a mean grain size of 3 mm is reached in 105 years accordingly the rate of grain size dependent processes such as compaction of olivine rich cumulates or melt extraction from partially molten peridotites may significantly be enhanced by textural coarsening

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

dendritic crystallization a single process for all the textures of olivine in basalts the olivine macrocrysts found in oceanites picrites and magnesian basalts erupted at hotspot volcanoes are generally interpreted either as phenocrysts crystallized from the magma or as xenocrysts extracted from a deforming cumulate to constrain the origin of these crystals we studied their texture and composition at piton de la fournaise volcano la reunion we show that macrocrysts are organized and subdivided into parallel units this suggests a crystallization by dendritic growth and ripening rather than by a complex combination of paired nucleation crystal aggregation or synneusis dendritic growth is also evidenced by the occurrence of hollow faces p rich zones melt and cr spinel inclusions formed from the accumulation of slow diffusing impurities p cr al in the liquid at the contact with rapid growing olivine we suggest that early dendritic crystallization may even cause branch misorientations and lattice mismatches yielding subgrain boundaries dislocation lamellae and to a certain extent undulose extinction which have all been formerly interpreted in terms of plastic intracrystalline deformation we interpret olivine macrocrysts as phenocrysts crystallized under a strong degree of undercooling t 60 c and derived from a harrisitic mush formed on the cold walls of the magma reservoir given the growth shapes indicated by p zoning patterns and external faces the olivine macrocrysts which consist of groups of several subcrystals have grown in suspension within the liquid and were neither aggregated into a dense cumulate nor corroded shocked or deformed before or during their transport to the surface the major consequence of our study is that most olivine macrocrysts are not xenocrysts and very few of them if any have experienced intracrystalline deformation the importance of deforming creeping cumulate bodies thought to accommodate the spreading of basaltic volcanoes in la reunion and hawaii may hence have been overestimated

crystal growth mechanisms in miarolitic cavities in the lake george ring complex and vicinity colorado the crystal peak area of the pikes peak batholith near lake george in central colorado is world renowned for its crystals of amazonite the blue green variety of microcline and smoky quartz such crystals collected from individual microlitic pegmatites have a remarkably small variation in crystal size within each pegmatite and the shapes of plots of their crystal size distributions csds are invariably lognormal or close to lognormal in all cases there observations are explained by a crystal growth mechanism that was governed initially by surface controlled kinetics during which crystals tended to grow larger in proportion to their size thereby establishing lognormal csds surface controlled growth was followed by longer periods of supply controlled growth during which growth rate was predominantly size independent consequently preserving the lognormal shapes of the csds and the small size variation the change from surface to supply controlled growth kinetics may have resulted from an increasing demand for nutrients that exceeded diffusion limitations of the system the proposed model for crystal growth in this locality appears to be common in the geologic record and can be used with other information such as isotopic data to deduce physico chemical conditions during crystal formation

on the structure of mare basalt lava flows from textural analysis of the lapaz icefield and northwest africa 032 lunar meteorites quantitative textural data for northwest africa nwa 032 and the lapaz lap mare basalt meteorites lap 02205 lap 02224 lap 02226 and lap 02436 provide constraints on their crystallization and mineral growth histories in conjunction with whole rock and mineral chemistry textural analysis provides powerful evidence for meteorite pairing petrographic observations and crystal size distribution csd measurements of nwa 032 indicate a mixed population of slowly cooled phenocrysts and faster cooled matrix lapaz basalt crystal populations are consistent with a single phase of nucleation and growth spatial distribution patterns sdp of minerals in the meteorites highlight the importance of clumping and formation of clustered crystal frameworks in their melts succeeded by continued nucleation and growth of crystals this process resulted in increasingly poor sorting during competition for growth as the melt crystallized based on csd and sdp data we suggest a potential lava flow geometry model to explain the different crystal populations for nwa 032 and the lapaz basalts this model involves crystallization of early formed phenocrysts at hypabyssal depths in the lunar crust followed by eruption and flow differentiation on the lunar surface lava flow differentiation would allow for formation of a cumulate base and facilitate variable cooling within the stratigraphy explaining the varied textures and modal mineralogies of mare basalt meteorites the model may also provide insight into the relative relationships of some apollo mare basalt suites shallow level crystal fractionation processes and the nature of mare basalt volcanism over lunar history

melting kinetics of a plagioclase feldspar partial melting experiments on plagioclase feldspar have been carried out to investigate textures and kinetics of the melting a labradorite single crystal was heated at one atmosphere pressure and temperatures within its melting interval as a function of time so called honeycomb fingerprint or sieve textures were produced except for the runs just below the liquidus the melting was initiated by heterogeneous nucleation of melt at the surface and or interior cracks and possively dislocations of the crystal the pattern of the melt is dendritic with a few m arm spacing after the melt develops throughout the crystal the volumes of melt and residual crystal become larger and smaller respectively without changing the arm spacings the melt is homogeneous and has the approximate temperature dependent liquidus composition irrespective of the time there are compositional gradients in the residual crystal after short periods of melting the an content of the crystals increases with increasing time until it finally reaches equilibrium with the melt after several thousands minutes of heating it is concluded that the enlargement of the melt the main process of the melting is controlled by diffusion in the crystal the fact that partial melts have the composition of the equilibrium liquidus even from the first several minutes strongly suggests that the local equilibrium at the crystal liquid interface is satisfied during the melting some of the honeycomb fingerprint and sieve textures found in xenoliths and phenocrysts of sodic plagioclase in volcanic rocks would be caused by heating events such as magma mixing during which temperatures of magmas were temporarily higher than the solidus of some of the minerals

kinetics of crystallization of igneous rocks the geochemistry of igneous rocks is discussed with the primary objectives of bringing together the theories underlying the kinetics of crystallization of igneous rocks and illustrating the use of these theories in understanding experimental and observational data the primary purpose of the chapter is to introduce current thinking about the kinetics of igneous rocks and to provide a basis for understanding other work a basic assumption made in the discussion is that the rate of any chemical reaction including the crystallization of igneous rocks is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium as such an understanding of the processes operating in igneous rocks requires an understanding of how deviation from equilibrium affects the rates and mechanisms of the processes occurring during crystallization these processes are detailed with special emphasis given to nucleation and crystal growth jmt

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

a numerical experiment of crystallization for a binary eutectic system with application to igneous textures numerical experiments of crystallization in a binary eutectic system were carried out to understand the origin of textural variation in igneous rocks in this study the number density of crystals was intensively examined as a parameter of texture the constant cooling rate experiment no latent heat release yields a higher number density of crystals by 1 order of magnitude than the constant heat loss experiment full latent heat release differences in growth mechanism diffusion limited with capillary effect versus reaction limited without capillary effect result in the different dependence of cooling rate on number density of crystals for diffusion limited growth with the capillary effect for spherical crystals the cooling rate dependence of number density of olivine clinopyroxene plagioclase and opaque minerals seems to be consistent with the experimental data with superliquidus initial condition for plagioclase experimental data with subliquidus initial temperature and the observational data are not consistent with the present numerical results for both growth processes suggesting that the plagioclase growth is governed by a different mechanism of nucleation and growth from homogeneous nucleation and both diffusion limited growth with capillary effect and simple reaction limited growth without capillary effect assuming the conductive cooling of a dike it is predicted that for the diffusion limited growth the number density of crystals is proportional to y 3 where y is distance from the cooling boundary this is in good agreement with some of the observational data by gray 1978 except for plagioclase the consistency among numerical natural and experimental data highly suggests that at the nucleation stage with superliquidus initial condition crystals grow by the diffusion limited process taking the spherical shape for olivine pyroxene plagioclase and some oxides the difference between volcanic and plutonic textures is caused by the difference of cooling rate by 2 orders of magnitude the variation from intergranular ophitic to poikilitic textures results from the difference in cooling rate dependence on number density between the plagioclase and the other minerals the formation of porphyritic texture must satisfy the limited condition the very small efficiency of latent heat production nearly constant cooling rate small values of interfacial energy of the first precipitating phase and smaller value of interfacial energy of the second phase

effect of stirring on crystallization kinetics of basalt texture and element partitioning isothermal crystallization experiments on basalt have been carried out using an infrared heating furnace to investigate the effect of stirring when stirring was not applied static experiment the results agreed well with previous experiments but when stirring was applied and a flow of reynolds number 10 3 4 was present dynamic experiment considerably different results were obtained especially in respect to the nucleation rate and the morphology of crystals at t 25 c essentially similar results were obtained on the nucleation rates and morphologies of crystals in both static and dynamic experiments however at supercoolings larger than 45 c nucleation density increased drastically in dynamic experiments reaching up to ten times as large as that in static experiments crystals of plagioclase and clinopyroxene were small and adapted acicular morphology regardless of t in dynamic experiments and hyalopilitic textures were formed a ttt diagram shows that the nucleation incubation time is shorter in dynamic experiments than in static experiments no compositional difference in major elements was found in plagioclase and clinopyroxene produced in both static and dynamic experiments however minor element concentrations e g mg in plagioclase and ti al in clinopyroxene were found to increase with both t and flow velocity all these results imply that although chemical diffusion in the melts did not play an important role in the dynamic experiments interface kinetics were important it is suggested that hyalopilitic texture commonly seen in natural basalt is mainly due to flow in magma

polymerization of silicate and aluminate tetrahedra in glasses melts and aqueous solutions v the polymeric structure of silica in albite and anorthite composition glass and the devitrification of amorphous anorthite abstract 29si mas nmr experiments have been carried out to determine the silica species distribution q distribution in albite naalsi3o8 and anorthite caal2si2o8 composition glasses designated albite and anorthite glass our results indicate that the q distribution of albite glass contains all five possible silica species and shows a tendency towards high q3 and q4 concentrations whereas anorthite glass does not contain q4 and has a high q0 concentration rationalizations are made in terms of the observed q distributions to explain differences in devitrification behavior of these two glasses 27al mas nmr data for these glasses suggest that differences in devitrification behavior between these two glasses should be ascribed to small growth rates rather than small nucleation rates of crystalline albite from albite glass

eruption dynamics inferred from microlite crystallization experiments application to plinian and dome forming eruptions of mt pelee martinique lesser antilles decompression experiments have been conducted to simulate syn eruptive crystallization in volcanic conduits to infer magma ascent rates and conditions during dome forming and plinian eruptions of silicic arc volcanoes the experiments were carried out on starting material with the composition of mt pele e rhyolitic interstitial melt 76 wt sio2 and consisted of three consecutive steps hydration decompression and annealing hydration saturated and undersaturated was performed at 8508c and 200mpa and was followed by isothermal decompression either linearly or stepwise to a final pressure pf of 30 or 5 10mpa decompression rates range from 0 003 to 25mpamin 1 decompression durations of 15 min to 40 days two samples were cooled by 258c and 508c during a 3 day step at pf subsequent to decompression the samples were held for up to 15 days at pf the experiments generated three types of crystals as products of pre syn and post decompression crystallization the experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre decompression crystals on the decompression induced crystallization the effects of pre decompression crystals decompression rate undercooling pf and terminal cooling have been determined on plagioclase nucleation growth morphology and composition the main results suggest a positive correlation between decompression rate and the number density of plagioclase crystals nucleated at pf and highlight the effect of pre decompression crystals in further decompression induced crystallization the relations between the decompression conditions and the plagioclase characteristics have been used to infer mt pele e eruption dynamics suggesting that 1 plinian magmas ascend from the reservoir within less than 1h 1 10 m s 2 dome and block and ash flow magmas ascend within more than 2 5 days giving time for syn decompression crystallization around pre existent microlites 3 dome magmas provide evidence for long stagnation and cooling at low pressure and 4 surge magmas ascend without significant crystallization within less than 4 days and massively nucleate plagioclase at very low pressure the extent and violence of dome destruction may depend on the size or age of the dome with large or old domes favouring mildly explosive block and ash flows whereas small or young protodomes may generate highly explosive surges

numerical study of nucleation and growth of bubbles in viscous magmas the nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity as well as diffusivity interfacial tension and decompression rate the numerical solutions show two regimes in the nucleation and growth process a diffusion controlled regime and a viscosity controlled regime mainly depending on the decompression rate initial saturation pressure and viscosity the basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components in basaltic eruptions the vesiculation is essentially controlled by diffusion and the viscosity controlled regime is limited to very high decompression rate and very small water content when andesitic magma saturated by water at 10 mpa is decompressed through the propagation of rarefraction wave induced by a landslide as took place in the mount st helens 1980 eruption the vesiculation is controlled by the viscosity up to 100 m depth on the other hand in a rhyolitic magma for the same situation vesiculation is controlled by the viscosity over the whole depth of the magma column in the viscosity controlled regime the vesicularity may be 90 or less as seen in silicic pumice whereas in the diffusion controlledmore regime the vesicularity equals or exceeds 98 such as in reticulite in hawaiian basalt an observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the sio2 content can be attributed approximately to the dependence of diffusivity of viscosity on sio2 content and temperature assuming the apparent correlation between sio2 content and temperature of magma less

cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity scanning electron microscopy cathodoluminescence imaging sem cl and secondary ion mass spectroscopy ion microprobe analysis sims are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning two contrasting crystal morphologies have been studied porphyritic euhedral bipyramidal quartz crystals in dacite from bonne nuit bay jersey uk show fine scale growth zoning in sem cl zones are narrow 1 40 m wide planar parallel to crystal faces and overgrow more broadly zoned rounded cores these crystals are interpreted to have formed at small degrees of undercooling by diffusion controlled growth quiescent magma conditions are indicated by the preservation of an al rich diffusive boundary layer which has not been removed by melt shearing during convection in contrast rounded and embayed porphyritic quartz grains in a quartz dacite from the esperanza mine e1 salvador northern chile show broad internal sem cl zones with irregular boundaries some of which are truncated by grain margins indicating dissolution crystal rounding may occur by dissolution or melting due to thermal or thermal chemical disequilibrium with surrounding melt and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures strings of amphibole inclusions adjacent to quartz rims several episodes of rounding can be recognised in some xenocrysts ion microprobe sims analysis of quartz from both samples shows that a1 contents are higher in areas of bright cl emission previous studies of cl in quartz have suggested that several factors control luminescence although a1 substitution for si in sio4 tetrahedra may be partly responsible for sem cl zoning recorded in volcanic quartz additional mechanisms including the effect of growth rate on defect density probably also play a major role

crystallization of the shap granite evidence from zoned k feldspar megacrysts textural chemical and sr isotopic studies of feldspars from the subsolvus shap granite northern england demonstrate that a number of magma mixing events have dominated the evolution of this pluton k feldspar megacrysts are phenocrysts formed in the magma chamber they contain a number of ba rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas diorite enclaves represent the relicts of these magmas and these also contain k feldspar megacrysts which show evidence of major dissolution they are xenocrysts picked up from the host granite and incorporated in the basic magma increasing h 2 o contents during fractional crystallization caused a late switch from growth of megacrysts to finer grained k feldspars in the matrix the chemically and isotopically zoned k feldspar megacrysts preserve an exceptional record of the evolution of the magma and the zones also had a significant influence on the development of exsolution microtextures during cooling

partial melting and recrystallization of granite and their application to deep disposal of radioactive waste part 2 recrystallization abstract a previously published scheme for the high temperature disposal of nuclear waste in deep boreholes requires that granite host rock partially melted by the heat from the waste can be completely recrystallized on the time scale of the cooling of the waste experimental studies of dynamic recrystallization of a typical s type crustal granite were therefore undertaken under the pressure 0 15 gpa and other conditions likely to arise during implementation of this scheme linear cooling experiments at 1 c h 0 1 c h and 1 c day were carried out over the temperature range 800 560 c these demonstrate that the partially melted granite can be fully recrystallized when cooled at 0 1 c h or slower crystal growth rates for the main phases formed during cooling are linearly proportional to total h 2 o content and decrease from alkali feldspar through biotite to quartz recrystallizing the partially melted granite at 0 1 c h compared with 1 c h increases grain size by a factor of 2 5 on the basis of partial melting experiments reported previously by us lithos 2003 this issue for the same granite and the results presented here it is concluded that an s type granite similar to the one studied would be a suitable host rock for the proposed high temperature very deep disposal scheme for radioactive waste

the effect of diffusive re equilibration time on trace element partitioning between alkali feldspar and trachytic melts abstract we present new experimental data on major and trace element partition coefficients between alkali feldspar and trachytic melt experiments were conducted at 500 mpa 870 890 c to investigate through short disequilibrium and long near equilibrium experiments the influence of diffusive re equilibration on trace element partitioning during crystallization our data show that ba and sr behave compatibly and their partition coefficients are influenced by re equilibration time orthoclase or content growth rate and cation order disorder high field strength elements hfse and rare earth elements except eu are strongly incompatible but alkali feldspar efficiently fractionates light lree from heavy rare earth elements hree our crystallization experiments reveal a strong influence of disequilibrium crystal growth on the partitioning of ba and sr in particular short duration experiments show that rapid alkali feldspar crystal growth after nucleation promotes disordered growth and less selectivity in the partitioning of compatible trace elements that easily enter the crystal lattice e g ba and sr this produces partition coefficients of compatible elements higher than those obtained through long duration experiments in which growth is slower and more selective on approach to equilibrium decreasing growth rate and timescales of diffusive re equilibration facilitates the incorporation of compatible elements within the crystal lattice through ordered growth on the basis of the charge and size of the octahedral site resulting in lower partition coefficients of ba and sr therefore our results indicate that besides crystal chemical effects i e lattice strain the substitution mechanisms during growth with different degrees of ordering of ba and sr in alkali feldspar are strongly influenced by diffusive re equilibration and crystal growth kinetics this implies that partition coefficients of ba and sr can be used to establish timescales of crystallization processes under pre and syn eruptive conditions the application of our results to alkali feldspar in rocks from campi flegrei constrain the magma residence time at subliquidus conditions in a reservoir to a maximum of 6 days under disequilibrium conditions and to a minimum of 9 days upon approaching near equilibrium conditions

igneous microstructures from kinetic models of crystallization during crystallization a variety of competing kinetic processes determine the evolution of the igneous microstructure yet the relative contribution of each process remains elusive to this end a stochastic algorithm is developed to yield a detailed spatial representation of the igneous microstructure during progressive crystallization this algorithm is used to test a variety of kinetic models for nucleation and crystal growth that yield realistic igneous microstructures the algorithm itself relies on a theoretical model of crystallization the avrami method and can therefore be internally validated in addition to being verified against natural microstructures using crystal size distributions csds the most realistic simulated igneous microstructure in which the csds remain approximately log linear throughout the crystallization interval is produced using a kinetic model involving exponential nucleation rate and constant growth rate an extension of the algorithm is used to simulate multiply saturated mineral growth emulating basalt crystallization through simultaneous nucleation and growth of plagioclase and clinopyroxene a fundamentally important feature of this procedure is the numerical production of detailed 3d representations of the microstructure that can be interrogated to study many physical and chemical processes for example the critical crystallinity necessary for the onset of a finite yield strength and the interstitial melt flow within the igneous microstructure depend on the percolation of the solid and melt phases respectively bounds on the percolation thresholds the critical crystallinity at which the phase of interest is connected or contiguous such that it extends across the full microstructure from one face to the opposite face are determined for the simulated igneous microstructure and are comparable with previously published results for the percolation threshold of both the melt phase and solid network the simulated igneous microstructure can also be used as input into other physical models to ascertain the physical properties of partially molten magmas that are otherwise difficult to estimate by experiment in a real sense these computed microstructures are equivalent to 3d microtomographic images of partially molten basalt within a solidification front

textural and geochemical constraints on andesitic plug emplacement prior to the 2004 2010 vulcanian explosions at galeras volcano colombia hazardous sequences of vulcanian explosions are thought to result from the repeated emplacement and destruction of degassed highly crystalline magma plugs in the shallow conduit of arc volcanoes the processes governing the timing and magnitude of these explosions are thought to be related to magma ascent rate and efficiency of degassing and crystallisation we study a rare suite of time constrained ballistic bombs from the 2004 2010 period of activity of galeras volcano to reconstruct magma plug architecture prior to six individual vulcanian explosions we find that each plug was vertically stratified with respect to crystallinity vesicularity and melt volatile content melt composition and viscosity we interpret this structure as resulting from multiple bubble nucleation events and degassing driven crystallisation during multi step ascent of the magma forming the plug followed by spatially variable crystallisation within the plug under contrasting conditions of effective undercooling created by degassing we propose that the shallow conduit evolved from more open degassing conditions during 2004 2008 to more closed conditions during 2009 2010 this resulted in explosions becoming smaller and less frequent over time during 2004 2008 then larger and more frequent over time during 2009 2010 this evolution was controlled by changing average ascent rates and is recorded by systematic changes in plagioclase microlite textures our results suggest that small volume vulcanian explosions 105 m3 should generally be associated with longer repose times hundreds of days and produce ballistics characterised by small numbers of large prismatic plagioclase microlites larger volume vulcanian explosions 106 m3 should be associated with shorter repose times tens of days and produce ballistics characterised by high numbers of small more tabular plagioclase microlites

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

an origin of small scale fluctuations in orthopyroxene composition in the lower and critical zones of the bushveld complex south africa abstract numerous small and apparently haphazard reversals in mg fe ratio of orthopyroxene grains have been reported by e n cameron in the lower and lower critical zones of the bushveld complex a hypothesis for their origin is presented by extending the suggestion of cameron that modal variation in layered complexes may be attributed to pressure fluctuation an increase in pressure will cause the magma to be supercooled thus speeding up crystallization rates this results in a greater proportion of interstitial liquid being trapped in the cumulate pile on crystallization this liquid adds relatively fe rich orthopyroxene to the cumulus of mg rich phase homogenization produces minerals whose compositions are displaced from the cumulus composition towards the low temperature end member by an amount proportional to the percent of interstitial liquid decreases in pressure produce the opposite effect and may explain the origin of pure adcumulate rocks quantitative data are presented from the lower zone critical zone contact and from the duluth complex to demonstrate this effect

the flash melting of chondrules an experimental investigation into the melting history and physical nature of chondrule precursors abstract constraints placed on chondrule formation have largely been generated from experiments which use a long duration below liquidus isothermal melting minutes to hours rather than a short duration above liquidus flash melting event seconds to minutes in this paper we examine how a short duration superliquidus heat pulse can produce chondrule textures by incompletely melting material with a type of flash melting we show that the maximum temperature limit of chondrule formation was approximately 2100 c almost 400 c higher than previously constrained previous experiments also have not studied the effect of variations in precursor grain size on the formation of chondrule textures for this reason we simultaneously investigate the effect of variations in the grain size of a starting composition on the formation of chondrule textures we show how mgo rich type ia chondrules and other fine grained chondrules could only have been formed from the incomplete melting of a rather uniformly grain sized precursor of less than 63 m because fine grained mgo rich chondrules have the some of the highest chondrule liquidus temperatures we proposed that these types of textures define a minimum melting temperature for chondrule formation

crystallization history of the 1984 mauna loa lava flow during a 3 week eruption in 1984 mauna loa produced vent lavas that increased in crystallinity from 1 to 30 and 27 km long flows that increased in crystallinity as they moved downstream we examined the crystallization history of these lavas using crystal size distribution csd analysis to study the rates of crystallization viscosity increase and latent heating typical average growth and nucleation rates were 5 10 9 cm s 1 and 5 cm 3 s 1 for microphenocrysts 20 to 500 m size crystals nucleated in the rift zone and 5 10 8 cm s 1 and 5 104 cm 3 s 1 for microlites 1 to 20 m size crystals nucleated in the channel these crystallization rates are high compared with those found in other csd studies of igneous rocks probably due to highly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel growth and nucleation rates decreased with time at the vent and with distance downstream the maximum downstream total crystallinity measured is 39 25 microlites 14 microphenocrysts in a quenched sample 14 km from the vent growth and nucleation rates cannot be calculated for postemplacement samples but they place upper limits of 53 58 on the amount of crystallization in the channel 9 20 km from the vent crystallization could have been mostly responsible for the 105 fold downstream increase in apparent viscosity although degassing and increasing incorporation of solid lava fragments also contributed another effect of crystallization on the lava flow was the sizeable latent heating 0 01 j g 1 s 1 over the first half of the flow length if the crystallinity of downstream quench samples is representative of the hot fluid core which may have been counteracted by entrainment of cooler material measurements of crystallization are shown to be crucial in the study of lava flow emplacement dynamics

textural and micro petrological variations in the eruptive products of the 2006 dome forming eruption of merapi volcano indonesia implications for sub surface processes the interplay between magma ascent degassing and changing magmatic properties are widely recognized as critical factors controlling the style of silicic volcanic eruptions microlite textures in samples from the prolonged dome forming eruption of merapi in 2006 provide a record of changing magmatic ascent conditions and shallow conduit processes throughout the eruption analysis of microlite textural parameters including measurements of areal number density na mean microlite size crystal aspect ratio and groundmass crystallinity combined with the monitoring record and field observations indicate that magma ascent paths change between continuous ascent at varying rates from a deeper magma storage region to ascent being temporarily stalled at shallow depths in the latter stages of the eruption supporting the idea of an ephemeral shallow magma storage region at merapi plagioclase microlite compositions show evidence of decompression induced degassing often displaying rims of anorthoclase and more k rich alkali feldspar sanidine anorthite contents also support the textural data of later erupted magma being temporarily stalled at shallow depths crystal size distributions csds are interpreted to show that both growth dominated and nucleation dominated crystallisation regimes existed during the 2006 eruption resulting from changing conditions of undercooling t during variable magma ascent paths by contrast microlite textural analysis and feldspar microlite compositions of samples from the fast growing lava dome of the second phase of the 2010 eruption prior to the cataclysmic events on 5 november indicate faster ascent rates a crystallisation regime more strongly dominated by nucleation due to high t and interaction of the 2010 magma with more hotter magma from depth

morphology versus upb systematics in zircon a high resolution isotopic study of a zircon population from a variscan dike in the central alps upb isotopic measurements on individual zircon crystals combined with morphological analyses permit the identification of three distinct components within the zircon population of the saedelhorn diorite a variscan dike from the western gotthard central alps switzerland i 94 of the grains in the zircon population are elongate crystals with pronounced skeletal morphology indicative of rapid growth from a supercooled melt ii 5 of the population consist of turbid mostly subhedral zircons frequently showing d type morphology classification according to pupin and turco 1 and elevated uranium contents compared to the skeletal variety single crystal and multi grain upb isotopic data of group i and ii zircons define an intrusion age of 293 5 4 m y for the dike iii rare transparent zircon crystals 1 of the zircon population yield apparent upb ages in the range of 370 490 m y and display morphological and isotopic characteristics closely resembling those of a caledonian orthogneiss intruded by the dike this implies presence of assimilated wall rock components in the macroscopically homogeneous dike sample a comparison of the data obtained by conventional analysis of multi grain zircon fractions and those obtained by grain by grain analysis demonstrates that age resolution is considerably improved by single crystal upb dating furthermore quantitative identification of zircon components assimilated by the ascending magma along its path to the present level of exposure is feasible by the latter technique since it is likely that such zircon grains are common to a broad variety of magma types valuable information on age and composition of crustal layers not accessible to direct observation may readily become available by application of precise micro analytical techniques low initial143nd 144nd end 2 7 at the time of intrusion of the saedelhorn dike requires the magma to be derived partially or totally from a crustal source for this crustal precursor a model age of 1050 m y tdm is obtained indicating that proterozoic crust was involved in the petrogenesis of the variscan intrusives of the gotthard area

crystal nucleation and growth produced by continuous decompression of pinatubo magma high temperature decompression experiments demonstrate that crystal textures preserve a record of the style and rate of magmatic ascent to reinforce this link we performed a suite of isothermal decompression experiments using starting material from the climactic 1991 pinatubo eruption we decompressed experiments from 220 mpa to final quench pressures of 75 or 30 mpa using continuous decompression rates of 100 30 10 3 1 and 0 3 mpa h 1 amphibole clinopyroxene and plagioclase crystallized during the experiments with plagioclase microlites dominating the assemblage total microlite number densities range from 107 6 0 4 up to 108 2 0 2 cm 3 with plagioclase accounting for up to 65 of the total number plagioclase microlite area increased systematically from 19 8 to 937 487 m2 with increasing experiment duration our textures provide time integrated records of crystal kinetics average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions 107 5 cm 3 h 1 and 10 1 4 1 m2 h 1 respectively and more than an order of magnitude lower in the slowest experiments 105 5 cm 3 h 1 and 0 8 0 2 m2 h 1 respectively both nucleation and growth rates are highest at high degrees of disequilibrium we find that peak supersaturation dependent instantaneous rates are generally more than an order of magnitude faster than average rates we use those instantaneous nucleation and growth rates to introduce an iterative model to evaluate the effects of different decompression rates decompression paths continuous single step or multistep and the presence of phenocrysts on final crystallinity and microlite size distribution

origin of podiform chromite deposits by magma mingling abstract i have oversaturated the immiscible system sio2 al2o3 feo k2o with fecr2o4 to study the fractionation of chromite between conjugate siliceous and fayalitic melts although both melts are oversaturated with chromite chromite only nucleates in the fayalite rich melt where the crystal melt interfacial energy sl is lowest in less than three days at 1150 c all excess chromium in the bulk composition is concentrated as cumulus chromite in the fayalitic melt pools producing nodular cumulate textures as observed in podiform chromite ore when these results are applied to the lithospheric mantle podiform chromite deposits may form when droplets of olivine normative high pressure low viscosity melt mingle with siliceous low pressure more viscous melt in shallow magma conduits it is inferred from the experimental analogue that chromite only nucleates in the melt where sl is lowest which in the mantle would be the high pressure melt the low pressure melt only serves as the diffusive chromium reservoir the richest chromite ores form when the volume ratio of the melt nucleating chromite is small relative to the ambient melt

podogenic columnar calcite from the oolite group lower carboniferous south wales abstract calcrete palaeosols in the oolite group have microspar hardpan layers overlying clay rich nodular layers which in turn overlie the degraded host oolite within and immediately below hardpan layers there are extensive sheet like and fan developments of columnar calcite with individual crystals up to 30 cm long crystals range from those with straight extinction and normal growth zonation through feathery crystals to those showing sweeping extinction the calcretes accumulated within a seasonal climate with hardpan accumulation probably occurring through evaporation during the dry season the columnar calcite precipitated from more dilute solutions with maximum growth probably occurring during the wet season precipitation may have been partly or wholly bacterially mediated or induced by the reduction in p co 2 of fluids as they migrated out of the extremely restricted microporosity of the hardpan columnar fabrics reflect low supersaturations relative to those at which the hardpan accreted inhibition of crystal nucleation and of growth on x axis parallel faces and an excess of co 2 2 availability over ca 2 growth and nucleation may have been inhibited by the presence of particular organic compounds or clays the range in crystal form is related to the degree of poisoning of crystal growth faces by organic compounds

glass forming ability and crystallisation behaviour of sub alkaline silicate melts abstract the intrinsic solidification behaviour and glass forming ability gfa of natural sub alkaline silicate melts have been experimentally quantified via cooling induced solidification approach gfa is measured by the critical cooling rate rc the rate at which a melt solidifies 2 area of crystals cooling rates of 9000 1800 180 60 7 and 1 c h have been run between 1300 c super liquidus region and 800 c quenching temperature at air f o 2 and ambient p for six silicate melts with compositions ranging from basalt b to rhyolite r i e b 100 b 80 r 20 b 60 r 40 b 40 r 60 b 20 r 80 and r 100 and water contents comprised between 53 b 100 and 384 b 20 r 80 ppm the ranges of cooling rates and chemical compositions used in this study are the broadest ever investigated in the earth sciences the phase proportions area were determined by image analysis on about 500 back scattered electron images collected over different magnifications phases are glass clinopyroxene cpx spinel sp and plagioclase plg sp is ubiquitous with abundance of few area and nucleates earlier than silicate crystals cpx solidifies in all runs except in r 100 and its abundance follows asymmetric broad gaussian like trends with tails towards low rates as a function of cooling rate moving from b 100 to b 40 r 60 these trends conserve their shape but shift progressively to lower cooling rates and mineral abundances plg crystallises only at low cooling rates and in sio 2 poor compositions run products with low amounts of crystals 5 area clearly show that cpx preferentially nucleates on surfaces of sp whereas a significant crystallisation of cpx 5 area is observed with decreasing cooling rate and with changing composition from b 100 to b 20 r 80 the crystallisation of silicate crystals is related to the chemical diffusivity of components in the melt also the initial crystallisation of plg occurs preferentially on cpx in general the amount of crystals decreases as the cooling rate increases however in some cases the amount of crystals remains constant or even decreases for b 80 r 20 with decreasing cooling rate rc values change over 5 orders of magnitude being 100 b 20 r 80 b 40 r 60 b 60 r 40 and b 80 r 20 and b 100 respectively the variation of rc can be modelled through nbo t nonbridging oxygen per tetrahedron parameter by the following equation rc a 1 e nbo t b c where a b and c are fitting parameters equal to 9214 0 297 and 0 040 respectively similarly to other glass forming liquids network metallic and molecular systems rc for natural sub alkaline silicate melts is inversely related to the reduced glass transition parameter trg trg tg tm and can be quantified with the equation rc a trg b where a and b are 1 19 10 4 and 28 7 respectively these results may be used to retrieve the solidification conditions of aphyric degassed and oxidised lavas in addition our data provide general constrains on the crystallisation kinetics of natural crystal bearing silicate melts erupted on earth e g lavas with phenocrysts the relationship between crystal content and cooling rate suggests that the solidification path induced by degassing can be also complex and nonlinear the growth of crystals with size up to 1 mm from a nearly anhydrous superheated silicate melt indicates that variable cooling conditions of lavas have to be accounted to discriminate among minerals formed before during and after eruptions moreover our results can be used to design glass ceramics from naturally available easy to find low cost starting materials

kinetics of crystal evolution as a probe to magmatism at stromboli aeolian archipelago italy abstract the kinetics of hydrated basaltic melts erupted during the present activity at stromboli have been studied to estimate the growth and dissolution rate of plagioclase specifically a high k basalt composition pst 9 has been studied to investigate magma and eruption dynamics at stromboli volcano by combining crystallization kinetics of plagioclase and csd measurements on natural samples from literature armienti et al 2007 fornaciai et al 2009 a series of water saturated decompression experiments over a range of final water pressure pf 75 5 mpa at constant temperature 1075 c show that plagioclase is systematically present from 50 to 5 mpa at water saturated conditions moreover these experiments show that anorthite an content decreases with decreasing p h 2 o reaching the same composition as the natural plagioclase in stromboli scoria at pressure below 20 mpa and that the plagioclase crystal fraction increases as the experimental conditions tend to lower final pressure plagioclase growth rate gl is observed to increase with undercooling for the pf investigated during decompression experiments except for the 75 mpa pf serie that only has two samples with the presence of plagioclase crystals the values of gl vary from 10 7 to 10 8 cm s for pf from 75 to 25 mpa while at pf from 10 to 5 mpa growth rates are approximately of 10 6 cm s a series of dissolution experiments at atmospheric pressure and over a range of temperature has been done for plagioclase t range of 1220 1240 c dissolution rate g for plagioclase 10 7 cm s tends to be slightly higher at higher temperature in the range of 1220 1240 c and appears to be time independent for the experimental durations investigated 10 30 h these trends could be related to development of a diffusion limited boundary layer adjacent to the dissolving crystal by comparison of the experimental data on plagioclase composition growth rates and dissolution in stromboli basalt it is possible to place the reservoir of the crystal rich magma in the upper part from 400 m to the surface of the volcanic conduit kinetic data of the plagioclase the most important phase of the shallow magmatic system of stromboli show that the magmatic processes are quite dynamic and in a relatively short time hours or several days the system can change considerably furthermore the results from this work combined with observations on natural samples help to improve our knowledge of the magma plumbing system of interactions between resident magma and new magmas the dynamics of volcanic activity of stromboli and the time scales of magmatic processes that change in a few hours to 1 month

sector zoned phlogopites in igneous rocks sector zoned micas are described in three occurrences of igneous rocks basal sectors 001 and lateral sectors 010 are well defined analyses of probably isochronous growth points indicate that there are consistent chemical composition differences between the sectors the 010 sector being richer in si and poorer in ti fe al and ba relative to the 001 sector within each sector type fe fe mg increases from core to rim these differences vary in amplitude from one occurrence to the other but are systematic for micas with extremely different tetrahedral cations possible factors influencing this type of crystallization are briefly reviewed growth rate geometry of protosites and bulk composition of the liquid

kinetics of convective crystal dissolution and melting with applications to methane hydrate dissolution and dissociation in seawater abstract large quantities of methane hydrate are present in marine sediment when methane hydrate is exposed or released to seawater it dissolves in seawater or dissociates into methane gas and water there was some confusion in the literature about the kinetics of these processes it is critical to realize that dissolution and dissociation are two different processes dissolution is due to instability in the presence of seawater similar to dissolution of nacl in water and is controlled by mass transfer dissociation is due to inherent instability similar to melting of ice with or without water although presence of warm water may increase the dissociation rate dissociation of methane hydrate into gas and water is similar to ice melting and is controlled by heat transfer hence dissolution is relatively slow and dissociation is rapid in this work we extend previous theory on convective crystal dissolution and melting to greater reynolds numbers we carry out laboratory experiments on the dissolution and descent of nacl kcl nabr and kbr in water to verify the applicability of our theory we then apply our models as well as previous ones to estimate methane hydrate dissolution and dissociation rates for several cases including dissolution of exposed methane hydrate floor dissolution and dissociation of hydrate as it rises through seawater the results show i convective dissolution rate of exposed hydrate floor is of the order 0 07 m yr ii convective dissolution rate of a rising hydrate crystal is 0 2 0 3 m s and a crystal of 5 mm radius is able to survive the rise through an 1800 m seawater column and iii convective dissociation rate is high and depends on the difference between the ambient water temperature and the equilibrium dissociation temperature of hydrate starting from a depth when hydrate just reaches dissociation instability a hydrate sphere of 5 mm radius would survive only a 47 m water column because hydrate is unstable in the surface ocean and would undergo rapid dissociation only very large hydrate chunks greater than about 0 09 m radius would be able to survive a 530 m surface water column

estimation of flow duration of basaltic magma in fissures abstract we suggest a relationship between the thickness of chilled margins often observed in basaltic dikes and magma flow duration for this purpose a new thermal model of cooling basaltic dikes is proposed which takes into account flow duration in a fissure before solidification of the magma the thermal problem is solved by adding a boundary condition serving as a substitute for magma flow to the classical initial conditions for dikes of fourier s equation we assume a bound temperature t o at the center of the dike as long as magma flows at the beginning of the cooling process the new model gives the same results as the classical one and consequently the same chilled margin thicknesses but later on when the classical model enforces temperature decreases our temperatures increase due to the heat brought by magma flow the chilled margin is reheated and partially or fully destroyed petrological observations on the chilled margins of dikes from salagou and bedarieux south of france and their country rock support this theoretical study the chilled margins are always thinner than 10 mm glass includes rare true phenocrysts mainly olivine and fe ti oxides and branching quench crystallites of clinopyroxene the conditions of formation and evolution of the chilled margins have been specified from experimental data cooling rate 800 c h 1 glass transition temperature t m 675 c and glass deformation temperature t d 705 c the initial chilled margins predicted by the two models are always thicker than those actually observed there is evidence for reduction due to reheating by devitrification and softening in our new model the chilled margin thickness which remains is directly dependent on the flow duration for example for a dike of salagou 1 m wide and a chilled margin of 5 mm a flow duration of six days has been computed at bedarieux for a 1 m wide dike and a thicker chilled margin 8 5 mm flow duration was 4 6 days these values must be regarded as upper limits because the effects of the latent heat on the temperature distribution during reheating have not been taken into account the metamorphism of the country rock pelites and bauxites has been investigated temperatures estimated from classical geothermometers are in agreement with those computed by our model and support its validity

crystal size distributions and scaling laws in the quantification of igneous textures this study demonstrates the value of the combined use of scaling of the chemical evolution of cooling magmas have investigated the detailed relationships between temanalysis and crystal size distribution csd measurements in the interpretation of igneous textures and outlines applications of these perature phase abundance and phase composition the reaction series bowen 1922 and the cipw norm cross methods to other fundamentally important problems theoretical calculations and measurements of natural samples are used to et al 1903 have been supplemented by modern thermodynamic models that predict the compositions and abundcharacterize the relationship between igneous texture and cooling history the total number nt and mean length l of crystals ances of minerals in equilibrium with melt of a given composition and temperature e g ariskin et al 1993 in a sample are correlated through a scaling relationship of the form n t l this proportionality depends on the mineralogy ghiorso sack 1995 although there are a few areas of uncertainty the chemistry of igneous petrogenesis is of the rock and so a modal normalization factor c is introduced the csd slope s and intercept ln n are uniquely related to fairly well understood but chemical models cannot address the central probeach other through the equation ln n 4ln s ln c where s l this overall relationship allows the texture of a rock lem of igneous crystallization which is the physical process of generating a set of crystals from magma two to be related to the local duration of cooling through an arbitrary crystal growth model quantification of the link between the texture magmas with the same composition will normally produce the same minerals in the same proportions however of a rock and its cooling history makes it possible to predict spatial variations in texture we show an example of this method using depending on their thermal histories these two magmas can solidify into rocks with drastically different physical the sudbury igneous complex canada additional applications include relating the textures of volcanic rocks to spatial and temporal appearances e g basalt and gabbro the set of minerals is the same but what distinguishes them is the abundance variations in the magma chamber and extracting kinetic parameters from suites of comagmatic rocks and size of the constituent crystals the fine grained rocks in the chilled margins of most basic intrusions cooled rapidly and the coarse grained rocks in the interiors of plutons cooled slowly this has

crystallization kinetics and rheology of leucite bearing tephriphonolite magmas from the colli albani volcano italy abstract we have investigated the rheology of liquid and crystal bearing tephriphonolite magmas from the colli albani volcanic district high 1124 1569 c and low 690 800 c temperature anhydrous liquid viscosities were determined by a combination of concentric cylinder 101 0 to 103 6 pa s and micropenetration 109 2 to 1012 1 pa s viscometry comparison with literature data reveals that at high temperatures viscosity seems to be related to the melts degree of polymerization nbo t while at low temperatures the dependency is not linear with values of viscosity higher than expected subliquidus isothermal crystallization experiments and viscosity determinations were carried out at high temperature 1150 1240 c in air using a concentric cylinder apparatus at constant shear strain rate 0 1 s 1 the overall crystal fraction varies between 0 06 at 1240 c leucite and 0 34 at 1150 c leucite 0 32 plagioclase 0 02 with a direct linear increase of crystal content with decreasing temperature which parallels the viscosity increase the inspection of products quenched at the end of the crystallization stage defined when viscosity reaches a constant value reveals strong evidence of leucite clustering after the first segment of the experiment performed at a constant shear rate a second stage of experiments at variable shear rate was performed comprised of an up ramp 0 1 s 1 0 9 s 1 and a down ramp 0 9 s 1 0 1 s 1 segment at the end of the down ramp leucite crystals appear sub spherical and unclustered for the same applied shear rate the viscosity values of the up ramp are not recovered within the experimental time scale indicating strain and strain rate dependent rheology for these suspensions while the down ramp viscosity data are shown to be in perfect agreement with literature models discrepancies between the up ramp data and pre existing predicting models have been observed we suggest that this complex behavior is related to the clustering of leucite crystals during the crystallization process and their breakage during the higher shear rate viscosity measurements the kinetics of crystallization in terms of nucleation and growth rates seems to be strongly affected by the degree of undercooling and dynamic stirring conditions we conclude that any modeling of tephriphonolite magma flow either in conduit or subaerial should take into account the effects of strain strain rate and internal textures i e clustering

crystallization kinetics in continuous decompression experiments implications for interpreting natural magma ascent processes decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma these experiments differ from previous studies in that they employ a continuous rather than stepwise pressure time trajectory four series of experiments were performed at rates of 0 5 10mpa h corresponding to ascent rates of 0 007 0 14 m s experiments quenched along each decompression path allow snapshot style monitoring of progressive crystallization as expected rates of plagioclase nucleation and growth depend strongly on decompression rate regardless of decompression rate approximately equal volumes of feldspar crystallize during a given decompression interval even when the observed crystallinity is lower than the equilibrium crystallinity apparently a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existing crystals other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks feldspar microlites do not maintain chemical equilibrium with melt they are more anorthite rich than equilibrium plagioclase suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static phase equilibrium experiments although decompression rate is an important element controlling final textures in volcanic rocks no single compositional or textural parameter uniquely records this rate of the criteria examined microlite number density and morphology are the best indicators of magma decompression rate based on a small number of comparisons between continuous and multi step decompression style at the same time integrated decompression rate 1mpa h decompression path evidently influences crystal texture with stepwise decompression yielding textures correlative with faster decompression than indicated by the time integrated decompression rate thus consideration of magma ascent style in addition to rate will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes

cooling rate induced differentiation in anhydrous and hydrous basalts at 500 mpa implications for the storage and transport of magmas in dikes firb miur development of innovative technologies for the environmental protection from natural events pd and ac trigs project sixth framework programme of the european commission and to the new and emerging science and technology pathfinder sm

kinetics of cooling and decompression induced crystallization in hydrous mafic intermediate magmas article i nfo the crystallization of a hydrous basaltic andesite magma was investigated using cooling and decompression experiments these experiments were designed so that the driving force for crystallization of feldspar the undercooling is equal whether in the direction of cooling or decompression after initial equilibration steps at conditions t 1 025 c and ph2o 150 mpa charges were either cooled or decompressed near instantaneously to final conditions corresponding to undercoolings t 5 2 154 c with dwell times of 12 24 and 48 h at the final p t t his single step experimental strategy ssd for decompression experiments ssc for cooling allowed derivation of kinetic parameters such as nucleation and growth rates as a function of undercooling even though our setup was primarily designed to investigate feldspar crystallization we report kinetic and geochemical data for all phases present glass feldspar clinopyroxene olivine and amphibole at low to intermediate undercoolings the solidification behavior of mafic intermediate magmas is very similar texturally and compositionally whether the crystallization mechanism is cooling or decom pression at high degrees of undercooling feldspar and clinopyroxene nucleation rates are nevertheless higher in ssd compared to ssc runs perhaps due to slower diffusivities or larger nucleation barriers in the cooling runs minerals transition from interface limited growth to diffusion limited growth with increasing undercooling while nucleation regimes shift from homogeneous to heterogeneous crystal morphologies are highly variable but can be used qualitatively to assess the extent of undercooling relative to more silicic melts with similar crystal contents feldspar formed in andesitic melts nucleate at a lower rate but grow to larger sizes published by elsevier b v

vapor undersaturated experiments with macusani glass h 2 o at 200 mpa and the internal differentiation of granitic pegmatites vapor undersaturated fractional crystallization experiments with macusani glass macusanite a peraluminous rhyolite obsidian at 200 mpa yield mineralogical fabrics and zonation and melt fractionation trends that closely resemble those found in zoned granitic pegmatites and other granitoids of comparable composition typically peraluminous li be ta rich deposits the zonation from the edge of charges inward is characterized by 1 fine grained sodic feldspar quartz border zones 2 a fringe of very coarse grained graphic quartz feldspar intergrowths that flair radially toward melt and terminate with nearly monophase k feldspar 3 cores of very coarse grained nearly monominerallic quartz or virgilite lialsi5o12 mica and 4 late stage fine grained albite mica intergrowths that are deposited from alkaline na rich interstitial melt at vapor saturation similar experimental products have been observed in compositionally simpler less evolved systems liquid lines of descent from initially h2o undersaturated runs are marked by a decrease in sio2 and increases in na k b p f h2o and a variety of trace lithophile cations these trends are believed to be governed by three factors 1 disequilibrium growth of feldspars quartz via metastable supersaturation 2 fractionation of melt toward sio2 depleted na rich compositions due to increases in b p and f and 3 changes in nucleation and growth rates mostly as a function of the h2o content of melt x w m in contrast experiments that are cooled below the liquidus from the field of melt aqueous vapor london et al 1988 fail to replicate pegmatitic characteristics in most respects on the basis of these and other experiments we suggest that the formation of pegmatite fabrics stems primarily from fractional crystallization in volatile rich melts and that enrichments in normally trace lithophile elements result from melt differentiation trends toward increasingly alkaline silica depleted compositions although vapor saturation at near solidus and subsolidus conditions may promote extensive recrystallization an aqueous vapor phase does not appear to be necessary for the generation of most of the salient characteristics of pegmatites