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

morphological characteristics and development of coastal nabkhas north east kuwait the coastal aeolian nabkhas in kuwait are mostly elongated with an average length of about 10 m an average width of 3 7 m and an average height of 0 65 m the horizontal component of the nabkha dune hc the mean of the length and width of nabkha dune developed around nitraria retusa was measured and compared with the height of the shrub a second polynomial function was found to best fit the set of data the fitting identified three trends of the development of the nabkhas initially as the height of shrubs h increases the hc increases linearly until it reaches approximately 10 m when h exceeds 2 m the effect of shrub height becomes ineffective in trapping more sediments that is reaching an equilibrium condition before a decrease in shrub height effectiveness occurs sediments of the nabkha crest and wings left and right sides are subjected to high energy winds and therefore they are generally coarser and better sorted than those of the tail or the nose minor differences are noted in the textural characteristics and the mineral composition of nabkha sediments and those of other aeolian landforms found in upwind nabkha field this indicates that the sediments were derived from the nearby sources

a laboratory model for melting erosion of a magma chamber roof and the generation of a rhythmic layering 1 a hot magma chamber can ascend by melting the roof rock a process which in turn affects the magma composition the disaggregated mineral particles which consisted the roof rock will descend in the magma chamber to form a sedimentary cumulate however the fluid dynamics leading to the formation of the sediment and how we can decipher them is unknown here we conducted a series of experiments modeling melting erosion of the roof with particle size consisting the roof rock as the parameter we find that there is a critical particle size below which the melting erosion occurs rhythmically melting erosion stops because the disaggregated particles are suspended in the magma chamber and suppress the vertical heat transfer the suspension then separates into an upper clear layer and a lower suspension layer eventually the heated stratified layers become unstable an overturn occurs and melting erosion resumes when the particles consist of two sizes such that at least one of them is smaller than the critical size a rhythmic erosion occurs particles are sorted during each erosion period and a size graded rhythmic layering is spontaneously generated we estimate that rhythmic layering can be generated from melting erosion in a basaltic magma chamber when the grain size of the roof rock is 0 6 mm assuming a vertical temperature difference of 10 c we suggest that rhythmic roof melting coupled with particle settling is one possible mechanism for generating the rhythmic layering which is commonly observed in solidified magma chambers

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

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

porous sidewall and sloping floor crystallization experiments using a reactive mush implications for the self channelization of residual melts in cumulates abstract tank crystallization experiments with aqueous solutions were used to explore interactions between a buoyant residual fluid and a porous cumulate at a sloping floor and a vertical wall the fluid flow path in a non reactive mush glass beads was used as a control for a vertical non reactive mush the rise of the buoyant solute depleted fluid was confined within it fluid flow paths were curved in the case of a sloping mushy floor with the upper part of the mush being principally a plume discharge zone and the lower part being a recharge zone more realistic reactive mushes were created by supercooling a nh 4 cl solution cooling against a vertical surface produced an initial mat of dendrites that were subsequently overgrown by a vertically grooved dendrite structure the grooves contained upwardly flowing residual melt while the ridges extended beyond the buoyant boundary layer into the thermally convecting solute rich part of the tank with time the grooves were covered over by a dendrite mat but remained as open channels within the mush focussing and accelerating the upflow of residual melt a buoyant boundary layer on the outside of the mush was not observed in the experiment with a sloping floor a thin mat of homogeneous mush was succeeded by a more organized deposit characterized by chimneys residual melt escaped as plumes through the chimneys chimneys formed because buoyant residual melt dissolved the mush as it rose and warmed chimney spacing increased and the number of chimneys decreased as the mush thickened dissection of the mush showed that individual chimney channels were inclined upslope so that melt residual from crystallization at the foot of the slope emerged from chimneys near the top of the slope formation of channels in cumulates would greatly increase fractionation efficiency by increasing the effective volume from which residual melts could be extracted and by increasing the rate at which residual melts could be transferred to the upper part of the chamber

mineralogical composition and potential sources of dust fallout deposits in kuwait northern arabian gulf abstract detailed mineralogical investigations of dust fallout deposits collected at eleven locations in kuwait during april 1979 march 1980 were carried out the mineralogy of five size fractions medium sand fine sand very fine sand coarse silt and clay was determined occurrence frequency and potential sources of the mineral grains that form the various size fractions are discussed and the gross mineralogical composition of the dust fallout calculated it was found that dust fallout deposited in kuwait is quartzitic calcareous sandy silt and is mostly derived from the dry mesopotamian flood plain in the middle and south of iraq the occurrence of sand size particles is attributed to pick up from local recent surface deposits

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

barchan ripples under unidirectional water flows in the laboratory formation and planar morphology the purpose of the present study is to investigate the formation and planar shape of barchan ripples generated by laboratory unidirectional water currents and to compare the morphology with that of barchans formed by laboratory waves and flows in natural environments a thin veneer of sand as a sediment source was placed initially at the upstream part of a recirculating flume and the change in bed configurations by the flow was recorded by a video camera two types of formative process were observed one was that barchan ripples grew from barchanoid bedforms and the other was that they developed from a small aggregate of sand particles the barchan ripples in the present test had the same characteristics in the height width relation and in the migration speed as previously reported from the desert environment and wave flume studies an examination of the planar shape of the barchans led to the result that the barchan ripples had larger values of body length width compared with those of barchans formed by water waves in the laboratory and by airflow in natural deserts the horn length width of the barchan ripples was smaller than that of barchan dunes in deserts but larger than that of barchans in oscillatory laboratory flows copyright 2005 john wiley sons ltd

textural characteristics of dust fallout and potential effect on public health in kuwait city and suburbs the textural and mineralogical characteristics of the dust fallout along with the potential effect on human health were investigated at 12 locations in kuwait city and suburbs during march 2006 february 2007 mineralogically the dust comprises mostly calcite and quartz with grain size ranging from 1 to 25 m chemically elements such as ca fe mg and al are the dominant ones with the highest amount of average percentage concentration 12 89 for ca results showed that humans in the 12 sites are being exposed to different ranges of average percentage concentrations of minerals such as calcite 15 00 51 80 quartz 26 40 66 9 albite 6 30 17 30 dolomite 1 20 10 20 and gypsum 0 00 6 4 where site k3 recorded the lowest average concentrations for calcite dolomite and gypsum and the highest for quartz and albite the elements ca fe mg al k zn na cr and cd recorded ranges of 5 845 12 890 0 946 3 141 0 752 2 513 0 545 2 261 0 059 0 307 0 004 0 792 0 003 0 018 0 001 0 005 and 0 0004 respectively inhalation of such concentrations of calcite particles may cause coughing sneezing and nasal irritation and in cases of chronic exposure to excessive oral doses of calcite may produce alkalosis and hypercalcemia quartz can have potentially serious respiratory effects following long term exposure 1 year or more which is the case here in our study comparing with international and local standards it is highly likely that harmful and hazardous effects on the human body are being experienced by long term exposure to dust fallout in kuwait

an overview of the surface and near surface geology geomorphology and natural resources of kuwait abstract the surface and near surface geology geomorphology and natural resources of kuwait have been reviewed based on the published work the stratigraphy and structure of the near surface sediments of kuwait have been mainly controlled by the tertiary tectonic activity of the arabian shield the shield started separating from the african mainland by the close of the eocene and drifted northeastward towards the asian plate giving rise to the folding and thrusting of the zagros mountains of iran the tectonic activity superimposed a northwest southeast trend over an earlier north south structural trend and gave rise to parallel ridges and valleys trending northwest southeast through differential erosion it has been possible to identify 15 geomorphic zones and 11 morphostructures from analysis of the surface features of kuwait seven of these morphostructures can be attributed to tectonic processes and two to fluvial processes it has not been possible however to explain satisfactorily the origin of the jal az zor escarpment or the ad dalmaniyah cliff the whole of kuwait can be divided into seven drainage zones of these the internal drainage systems of the ar raudhatain umm al aish drainage system and the ad dibdibba drainage system are hydrogeologically most important the clastic deposits of kuwait group sediments often affected by low temperature carbonate calcite and dolomite and sulfate gypsum precipitation form the surface rock of kuwait these are covered by recent aeolian and residual deposits playa deposits in the enclosed basins and near the coast oolitic sediments beach sands and sabkha deposits the surface and near surface deposits of kuwait contain important reserves of construction sand and gravel as well as the only naturally occurring freshwater reserves of the country an understanding of the nature of these surface deposits is required for the management of environmental issues like sand encroachment land use planning and soil and groundwater pollution

challenges of calculating dunefield mobility over the 21st century attention has been directed towards both the impacts of future climate change on environmental systems and dunefield activity in the past but there has been relatively little consideration of potential dune mobility in a future and possibly warmer world this paper considers the use and limitations of four global circulation models gcms hadcm3 hadcm2 csiro mk2b and cgcm1 in combination with simple dune mobility indices to predict the activity of the kalahari dunefield it is clear that uncertainties surround gcm resolution and accuracy mobility index robustness for the calculation of intra annual dune mobility and data collection for mobility index calibration macro scale studies that look at large areas of the world over long time scales are well suited to gcm and mobility index use but dune mobility can be variable within a dunefield and it is the extreme sand transporting events occurring at high temporal resolutions that are the most important for short term studies to investigate intra annual changes in dune mobility over a specific dunefield techniques such as downscaling weather generators and probability curve fitting can help provide climate predictions for smaller areas over shorter time frames however these methods introduce uncertainty of their own and they often rely on the accuracy of original gcm predictions or the climate parameter relationships observed at present analysis of intra annual changes also requires mobility indices that can model monthly mobility patterns well although existing indices have only been used for calculating annual dune mobility potential when they are used for intra annual predictions the lack of lag response between precipitation decreases and the assumed vegetation dieback leads to an exaggerated amplitude pattern of dune mobility throughout the year calibration of dune mobility indices to dune mobility observed on the ground is therefore important but is hampered by a lack of observed measurements for individual months solutions are available to overcome some of the outlined problems but they can provide their own set of uncertainties which combine to further reduce the confidence given to future dune mobility predictions

thermodynamics of the mgo sio2 liquid system in earth s lowermost mantle from first principles abstract knowledge of the multi component thermodynamics and phase equilibria of silicate melts in earth s deep interior are key to understanding the thermal and chemical evolution of the planet yet the melting phase diagram of the lower mantle remains poorly constrained with large uncertainties in both eutectic composition and temperature we use results from first principles molecular dynamics of nine compositions along the mgo sio 2 binary to investigate the compositional dependence of liquid state thermodynamics applying our results to describe incongruent melting for the system at deep lower mantle pressures our phase diagram is bi eutectic throughout the lower mantle with no liquid immiscibility accounting for solid liquid partitioning of fe we find partial melts of basaltic and peridotitic lithologies to be gravitationally stable at the core mantle boundary while liquidus density contrasts predict that perovskite will sink and periclase will float in a crystallizing pyrolytic magma ocean

estimation of dust fallout rates in the northern arabian gulf the penetration of dust clouds over a sea surface is modelled as a sediment transport diffusion problem the model is used to determine the steady state concentration field of two dimensional dust clouds during offishore dust storms the rates of dust deposition into the sea can then be calculated from the obtained concentration fields the model was applied to offshore dust storms blowing over the arabian gulf yielding estimated dust sedimentation rates of 0 8 mm yr 1 this is substantially higher by several orders of magnitude than dust fallout estimates in most seas of the world the calculated fallout rates are in line with the results of existing field measurements

the 1995 grevena northern greece earthquake fault model constrained with tectonic observations and sar interferometry after the 1995 grevena ms 6 6 event in northern greece we mapped the earthquake fault break in detail the surface break is small 8 12 km long 4 cm slip compared to the moment release of the event however the morphologic and tectonic study of the active faults in the field and using the spot satellite imagery suggests that the earthquake ruptured part of a much larger fault system including interconnecting segments we used sar interferometry of the satellite ers 1 imagery to characterize the coseismic displacement field this shows a kidney shaped zone of subsidence reaching 30 cm flanked by an uplift zone reaching 5 cm we reproduce this field using dislocations in an elastic half space and our observations of the fault system this requires 1 m slip from 4 to 15 km depth on a main normal fault segment dipping nnw our preliminary model includes significant ne dipping scissors faulting at the eastern end of the rupture clearly seen in the interferograms

experiments on upward migration of a liquid rich layer in a granular medium implications for a crystalline magma chamber 1 we perform a series of experiments to investigate the situation in which a melt rich layer formed by a magma intrusion ascends through a crystalline magma chamber the initial condition is such that a heavier granular layer overlies a liquid layer the particles consisting the upper granular layer are in a jammed state and only the particles near the interface can move to form a dilated boundary layer the dilated layer detaches from the upper granular layer and forms downwelling plumes which drive a cellular convection within the liquid rich layer the convection erodes the upper granular layer and the liquid rich layer migrates upwards with time this upward migration of the liquid rich layer differs from the previously known mechanisms of liquid transport permeable flow in which the liquid migrates at the darcy velocity the stokes settling in which the individual particle settles and diapirs formed by the rayleigh taylor instability we find that the velocity of the upward migration of the liquid rich layer can be scaled by the volumetric flux of the liquid ascending through the narrow channel between the particles the upward migration of the liquid rich layer is faster than the darcy velocity in a mushy magma chamber whose crystals are in a jammed state neither the stokes settling nor the rayleigh taylor instability can occur we propose that the upward migration of the melt rich layer observed in our experiments can become an efficient mechanism of melt transport in a crystalline magma chamber

an experimental study on the effects of phenocrysts on convection in magmas abstract the effect of phenocrysts on convection in magma chambers is investigated experimentally using small heavy particles in convecting fluids the particles are initially uniformly distributed in a fluid which is either heated from below or cooled from above the system is allowed to evolve and temperature and particle concentration profiles are measured as functions of time when the concentration of particles is sufficiently small convection is basically unaffected by their presence when the concentration is above a critical value however the convective motion is considerably altered the effect of particles on the subsequent fluid behaviour is different in the cases of heating from below and cooling from above in the former case there are strong convective motions confined to a sedimentary layer of decreasing thickness beneath a clear layer which displays rather weak convective motions with time the destabilizing increase of temperature in the lower layer overcomes the stabilizing contribution to the bulk density due to the particles and the layer overturns quite suddenly in the situation of cooling from above a critical condition separates a case of continual overturn from a case of no overturn at all with the sedimentary layer falling unimpeded to the bottom theoretical analysis suggests that the critical value is determined primarily by the ratio of the contribution to the bulk density of the suspension due to particles to the change in fluid density due to the thermal effect the size distribution of the particles can also modify the fluid behaviour applying our general results to geological situations we suggest that the presence of relatively small concentrations of phenocrysts can critically influence the mode of convection in magmas

a model for episodic degassing of an andesitic magma intrusion 1 episodic magmatic degassing has been observed at numerous volcanoes especially those of intermediate composition it can span timescales from years to decades here we propose a physical model for the degassing of a shallow magma intrusion to explain this phenomenon the magma cools by convection which leads to melt crystallization volatile exsolution and magma overpressure when the pressure reaches a critical value wall rocks fracture and the exsolved gas escapes the intrusion then returns to the initial lithostatic pressure and a new cooling crystallization degassing cycle occurs a series of such cycles leads to episodic degassing the trend and timescale of the degassing process are mainly governed by magma cooling two degassing regimes are exhibited an early phase with a high frequency of gas pulses and a later phase with a lower gas pulse frequency the transition between these two regimes is caused by the viscosity increase when the magma crystallinity exceeds the crystal percolation threshold we find that the time to this transition is dependent on magma volume to a first approximation where observations are available from sustained geochemical surveillance the model provides constraints on key aspects of the subsurface magmatic system with estimation of the volume of an intrusion and tensile strength of the surrounding rocks it therefore represents a relevant tool for volcanic surveillance and hazard assessment

from landform to process morphology and formation of lake bed barchan dunes makgadikgadi botswana abstract a suite of crescentic landforms is visible from remotely sensed imagery within the ntwetwe panpan in the makgadikgadi basin botswana we investigate the most distinct of these landforms using morphometric measurements sedimentary data and optically stimulated luminescence osl signal analysis comparative analysis with previously published barchan morphological data sets suggest the ntwetwe features fall within the spectrum of morphometric parameters found in a range of barchan dunefields from around the world there is currently insufficient comparative morphometric data from sub aqueous dunefields to be able to distinguish the particular formative environment of the dune osl signal analyses however support the hypothesis of grove 1969 grove a t 1969 landforms and climatic change in the kalahari and ngamiland geographical journal 135 191 212 that the last deposition of the sediments within the ntwetwe forms was most likely aeolian in origin luminescence signal analysis is employed to investigate potential transport and bleaching environments of the sediments forming the features but results in this case do not shed further light on the formative conditions of these enigmatic landforms

modeling phase separation and phase change for magma ocean solidification dynamics just after accretion the earth s mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core the mineralogical observations and thermodynamics models of solid liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle solids were probably formed first at the bottom of the lower mantle or at mid mantle leaving two possible magma oceans a shallow one and an abyssal one near the bottom of the mantle the liquid phase might become denser than solids due to iron enrichment in the shallow magma ocean the crystallizing solid phase was denser and sank through the magma to settle and compact at depth to understand these complex dynamics we develop a two phase numerical code that can handle simultaneously convection in each phase and in the slurry and the compaction or decompaction of the two phases although our code can only run in a parameter range rayleigh number viscosity contrast between phases prandlt number far from what would be realistic we think it already provides a rich dynamics that illustrates what could have happened we show situations in which the crystallization front is gravitationally stable and situations were the newly formed solids are gravitationally unstable and can snow across the magma our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

aeolian processes and sand encroachment problems in kuwait abstract the surface of kuwait is carved in a calcretized clastic sequence of miocene pleistocene age mostly covered by a thin blanket of recent aeolian deposits fast rate of development and increase of human activities contributed to denudation of the vegetative cover and exposure of dry loose sediments to wind action at present kuwait is intensively subjected to aeolian actions manifested by frequent occurrences of sand and dust storms this dictated the need for regional understanding of the magnitude of the aeolian processes their behavior with man made establishments and their long and short term impacts a comparison between the nature and distribution of recent surface sediments in kuwait in 1979 and 1990 shows significant changes that reflect the work of aeolian processes during this period the direct impact of the enhancement of aeolian action is represented by the increase in the rate of sediment transport transportation of fine grained particles in suspension frequently occurs during dust storms usually initiated in southern iraq the particles migrate over kuwait as thick dust clouds and finally settle in the northern part of the arabian gulf bedload transport of sand or saltation frequently occurs during summer whenever wind speed reaches 5 4 m s the average annual sand drift rate in kuwait is about 20 m 3 m width 1 yr 1 the majority of the drift occurring during the period may august to the southeast bedform transport or sand dune movement is also recognized in kuwait sand dunes mostly occur as small barchans of about 3 m height the measured rate of movement of these barchans varies with height a barchan of average height usually moves about 20 m in nine monthes january august the remarkable high rate of sand transport and the increase in development activities in the desert areas are responsible for the enhancement of the sand encroachment problems in kuwait almost all installations roads and farms are significantly affected by the accumulation of considerable amounts of aeolian sand these problems have pronounced adverse environmental and economic impacts

phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite the campanian ignimbrite is a 4200 km trachyte phonolite pyroclastic deposit that erupted at 39 3 0 1ka within the campi flegrei west of naples italy here we test the hypothesis that campanian ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts 5 10 wt of crustal rock skarns and foid syenites during crystallization comparison of observed crystal and magma compositions with results of phase equilibria assimilation fractionation simulations melts is generally very good oxygen fugacity was approximately buffered along qfm 1 where qfm is the quartz fayalite magnetite buffer during isobaric fractionation at 0 15 gpa 6 km depth the parental melt reconstructed from melt inclusion and host clinopyroxene compositions is found to be basaltic trachyandesite liquid 51 1 wt sio2 9 3 wt mgo 3 wt h2o a significant feature of phase equilibria simulations is the existence of a pseudo invariant temperature 883 8c at which the fraction of melt remaining in the system decreases abruptly from 0 5 to50 1 crystallization at the pseudo invariant point leads to abrupt changes in the composition properties density dissolved water content and physical state viscosity volume fraction fluid of melt and magma a dramatic decrease in melt viscosity from 1700 pa s to 200 pa s coupled with a change in the volume fraction of water in magma from 0 1 to 0 8 and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger thermal models suggest a timescale of 200 kyr from the beginning of fractionation until eruption leading to an apparent rate of evolved magma generation of about 10 3 km year in situ crystallization and crystal settling in density stratified regions as well as in convectively mixed less evolved subjacent magma operate rapidly enough to match this apparent volumetric rate of evolved magma production

deformed barchans under alternating flows flume experiments and comparison with barchan dunes within proctor crater mars abstract it is generally considered that barchans isolated crescentic shaped dunes develop where wind is unidirectional and the available sand is insufficient to cover the entire dune field however bishop bishop m a 2001 seasonal variation of crescentic dune morphology and morphometry strzelecki simpson desert australia earth surface process and landforms 26 783 791 observed barchans that developed in areas where winds blow seasonally in opposite directions and described a peculiar deformation feature the rear slipface that is not found in ordinary barchans barchans under such bidirectional flows are poorly understood and it is necessary to study barchans that formed under many different flow conditions we conducted flume experiments to investigate the deformation of barchans under alternating water flow and observed new deformation features in addition to rear slipfaces we conclude that the deformation of barchans can be categorized into four types one of which shows morphologies similar to barchans within proctor crater mars the deformation type depends on the strength of the reverse flow relative to the forward flow and the absolute velocity of the forward flow comparison of our results with barchan dunes within proctor crater enable us to qualitatively estimate the wind strength and direction related to dune formation on mars these results are in agreement with those of fenton et al fenton l k toigo a d richardson m i 2005 aeolian processes in proctor crater on mars mesoscale modeling of dune forming winds journal of geophysical research 110 e6 e06005

morphologic characteristics and migration rate assessment of barchan dunes in the southeastern western desert of egypt abstract this work explores the morphologic characteristics of aeolian dune sand in the southeastern part of western desert of egypt it aims to assess the movement of barchan dunes and evaluate their environmental influence on the toshka project morphometric investigation of barchan dunes in the toshka area revealed that most barchans have high length width a c ratios fat to pudgy while one fifth of the studied barchans have lower a c ratios and so appear normal in their morphologic forms statistical analysis of the main parameters of barchan dunes in toshka and other desert regions in the kharga egypt kuwait southern morocco california and southern peru demonstrates that barchans of the toshka area are distinctive in their appearance they are characterized by distinct aspect with higher values of length and width and greater growth in height the high energy wind environment in addition to the large amount of drifting sand are principal factors responsible for the unique shape of toshka barchans the migration rate of barchan dunes in four chosen test locations within the central and western toshka area ranges from about 3 to 10 82 m year the calculated average migration rate of these dunes is about 6 m year in a ssw direction sand encroachment is more extensive in the central and western parts of the investigated toshka area risk evaluation of sand dune movements in the southeastern part of the western desert points to medium to high sand encroachment risk values these may represent serious hazards to the newly established toshka project threatening roads as well as cultivated lands in the area

geomorphological analysis of paleo drainage systems and their environmental implications in the desert of kuwait a well developed drainage network is carved in the hard calcretized and gypcretized gravelly sand of the pleistocene dibdibba formation in northern kuwait the present day aridity suggests that these drainages were developed during pluvial episodes that took place in the post pleistocene time and therefore are considered as paleo drainage detailed morphometrical analysis of the endoeric drainage systems was performed and the degree of correlation among their different morphometric parameters was investigated based on these parameters the studied drainage basins are statistically grouped into three groups such grouping was confirmed by discriminant analysis the categorical data analysis demonstrated the dependence of these groupings on the surface geology regional topography and local geomorphological settings infiltration measurements revealed that the drainage bedrocks have a low infiltration rate 20 cm h 1 whereas the drainage fill deposits have a relatively high infiltration rate 67 30 cm h 1 the impact of the drainage system pattern and morphometry on the hydrological conditions is discussed and potential near surface low salinity to freshwater aquifers is delineated the role of the studied drainage systems in the occurrence of mobile sand and sand dunes rainwater harvesting and land capability are discussed

barchans of minqin morphometry the barchans on the border of minqin oasis china were surveyed our data reconfirmed that the height of slipface is proportional to the width of horns and the baseline can be roughly described by parts of an ellipse and a parabola we attempted to study dune geomorphology using the theory of differential geometry the geometrical characteristics of the brink can be quantitatively determined by its curvature and torsion

sand movement patterns in the western desert of egypt an environmental concern wind action is the most dominant agent for erosion and deposition in the vast western desert of egypt analysis of wind data from seven meteorological stations distributed along the western desert reveals that this desert is characterized by high energy wind environments along the northern and southern edges and low energy wind environments throughout the rest of the desert accordingly sand drift potential follows the pattern of wind energy maximum sand drift potential was observed at the southern edge 571 vector units which equals 40 m3 m width year sand drift direction was observed towards the southeast except at the southern part of the desert where the trend of sand movement was towards southwest the major dune type recognized on satellite images was the simple linear type linear dunes are generally associated with bimodal wind regime rates of sand drift potential and sand dune migration were greatest at east of owinate region at the extreme southern part of the desert measurements of crescentic sand dune advance from two satellite images reveal a maximum advance rate of about 9 m year at the southern part of the desert dune movement creates potential hazard to the infrastructures in this open desert

width height and slope relationships and aerodynamic maintenance of barchans the allometric relationship between the height of barchan dunes and horn to horn widths is re examined the addition of new data from barchans in the namib desert and elsewhere confirms the relationship that the height of a barchan is approximately one tenth of the width of the dune height and width of the dune are closely linked to the three dimensional morphology of the dune in particular the relatively constant mean angles of around 11 at which the side slopes of the dune are maintained we suggest that the 3 d barchan form is directly controlled by aerodynamic processes constant feedback between three dimensional stoss face windward shape of the dune slipface height and horn to horn width and shear stress produces an equilibrium dune form

the relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers abstract in a cooling magma chamber magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body homogeneous fractionation and by mixing of the main magma with fractionated melt derived from low temperature mush zones boundary layer fractionation in this study the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics based mass balance model model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures 2 wt such as arc basalt and the magma chambers are located at depth 100 mpa because the melt derived from mush zones is enriched in alkalis and h 2 o crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature therefore homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active if magmatic differentiation proceeds primarily by boundary layer fractionation magmas can remain free of crystals for long periods during magmatic evolution

origin of crosscutting dissolution surfaces in magmatic plagioclase crosscutting dissolution surfaces dss are rare features observed in magmatic plagioclase crystals in this study crosscutting dss of dacitic plagioclase crystals from the shirahama group izu peninsula japan were investigated using a nomarski differential interference contrast imaging microscope and chemical compositional data crosscutting ds slopes were examined by tracing digitized images the following features of crosscutting dss are reported for the first time 1 traces of the crosscutting dss showed parabolic like shapes 2 crosscutting dss were found on select adjacent or opposite faces of the crystals suggesting a preferential dissolution of a limited area of the plagioclase 3 projecting points were observed with directional characteristics and 4 square or angular patterns were found in the less dissolved parts of the crystals the formation of crosscutting features is controlled by dissolution and crystal settling into hotter magma process velocities depend on the physical properties of the magma such that a description of crosscutting features can reveal valuable information about the magmatic environment and the dynamic behavior of crystals

magma differentiation rates from 226ra 230th and the size and power output of magma chambers we present a mathematical model for the evolution of the 226ra 230th activity ratio during simultaneous fractional crystallization and ageing of magma the model is applied to published data for four volcanic suites that are independently known to have evolved by fractional crystallization these are tholeiitic basalt from ardoukoba djibouti morb from the east pacific rise alkali basalt to mugearite from vestmannaeyjar iceland and basaltic andesites from miyakejima izu bonin arc in all cases 226ra 230th correlates with indices of fractional crystallization such as th and the data fall close to model curves of constant fractional crystallization rate the best fit rates vary from 2 to 6 10 4 yr 1 consequently the time required to generate moderately evolved magmas f 0 7 is of the order of 500 to 1500 yrs and closed magma chambers will have lifetimes of 1700 to 5000 yrs these rates and timescales are argued to depend principally on the specific power output i e power output per unit volume of the magma chambers that are the sites of fractional crystallization equating the heat flux at the epr to the heat flux from the sub axial magma chamber that evolves at a rate of ca 3 10 4 yr 1 implies that the magma body is a sill of ca 100 m thickness a value which coincides with independent estimates from seismology the similarity of the four inferred differentiation rates suggests that the specific power output of shallow magma chambers in a range of tectonic settings covers a similarly narrow range of ca 10 to 50 mw km 3 their differentiation rates are some two orders of magnitude slower than that of the basaltic makaopuhi lava lake hawaii that cooled to the atmosphere this is consistent with the two orders of magnitude difference in heat flux between makaopuhi and the east pacific rise 226ra 230th data for magma suites related by fractional crystallization allow the magma differentiation rate to be estimated and from this the thermal budget of the magma chamber addressed and where an independent measurement of heat flux exists to place constraints on the size of the magma chamber such results have the potential to constrain the likely timescale and size of future eruptions of evolved magmas

magnetic anisotropy as a tracer of crystal accumulation and transport middle banded series stillwater complex montana abstract fabric studies of layered mafic intrusions have led to improved understanding of the mechanical processes operating in large magma chambers including crystal accumulation and crystal mush deformation such studies however are typically limited by a tradeoff between breadth number of sites studied characteristic of field focused work and sensitivity ability to discern subtle fabric elements characteristic of laboratory fabric analyses magnetic anisotropy if analyzed in a systematic way and supported by single crystal and petrofabric measurements permits relatively rapid characterization of magmatic fabrics for large numbers of samples here we present the results of a study of remanence and susceptibility anisotropy from three transects through the middle banded series of the stillwater complex montana all three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of olivine bearing zone iii consistent with crystal mush compaction perhaps more importantly each transect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect the magnetic lineation directions which generally coincide with crystallographic preferred orientations of silicate minerals likely record a pre compaction fabric lineation directions differ from one transect to another implying that the process generating the lineation either slumping of a semiconsolidated crystal mush or magma transport acted on length scales of at most a few km these results demonstrate the sensitivity of magnetic anisotropy to petrofabric in mafic rocks

crystallization dynamics of granite magma chambers in the absence of regional stress multiphysics modeling with natural examples numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection during which wall rock contamination and magma mixing are possible followed by a long period of no convective cooling during which melt segregation occurs convection is driven by the negative density gradient generated in the upper cooling zone by melt to solid phase transformation convection breaks the upper mushy zone and drags the fragments downwards with descending rayleigh taylor fingers such fragments can be preserved as microgranular enclaves the descending rayleigh taylor fingers split low aspect ratio sill like magma chambers into nearly isolated convection cells if the magma is initially heterogeneous this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios and finally results in a pluton with marked lateral compositional variations easily misinterpreted as different intrusive batches convective heat loss quickly leads most of the magma chamber to critical crystallinity independently of the vertical coordinate so that a chamber wide three dimensional skeleton of crystals with uniform initial porosity c 0 4 0 5 is formed this configuration is gravitationally unstable therefore it spontaneously compacts towards an equilibrium vertical variation of porosity approaching atty s law in the absence of regional stress the upwards migration of the inter crystalline melts as a result of compaction is the most effective way of melt solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates granite magma chambers fractionated by these mechanisms will produce short range differentiation series from a composition slightly less silicic than the initial magma to high silica segregates in the presence of regional stress tectonic squeezing and shearing during the post convective stage can expel residual fluid more efficiently and lead to wide range granite differentiation series from rocks notably less silicic than the initial magma to high silica leucogranites

suspension in convective layers and style of differentiation of a terrestrial magma ocean recent physical theories for the formation of the earth suggest that about 4 5 b y ago the mantle of the earth was partially or completely molten fractional crystallization of this hypothetical magma ocean would result in a strong chemical stratification of the earth s mantle such a scenario is controversial from the geochemical point of view however it has been noted that the simple scenario of fractional crystallization could be avoidable in a convective magma ocean if crystals remain suspended in this paper the problem of suspension is developed with the help of an energetic approach convection must do some work against gravitational settling we distinguish three regimes of convective suspensions absolute or complete sedimentation occurs when the energy dissipation due to the settling exceeds the heat loss from the convective layer this is possible only in large scale systems like magma oceans and implies that cooling can proceed only together with sedimentation crystallization and a decrease in the liquidus temperature at a constant pressure a regime of partial differentiation occurs when the energy dissipation due to the settling is less than the total heat loss but larger than the power which can be spent by convection on the crystal reentrainment process the differentiation is not complete and a competition between the rate of cooling the rate of sedimentation and the rate of turbulent diffusion determines the degree of differentiation the third regime is an absolute suspension which could be sustained for an indefinitely long time in this case sedimentation starts only when the crystal fraction reaches the maximum packing value when the viscosity of the magma rapidly increases the power which can be spent by convection on reentrainment is equal to e gd cp of the total energy supply to the convective layer where e 15 gpa or 10 3 10 1 cm during crystallization of shallow layers the first regime fractional crystallization is unavoidable the estimates depend on various poorly constrained parameters and processes such as heat flux viscosity thermodynamical disequilibrium and highly variable viscosity convection for absolute suspension the crystal size must be at least e times less or 10 3 10 1 cm and 10 4 10 2 cm respectively if e 0 01 the partial differentiation occurs in a narrow one decade range between these two regimes the radius of about 1 cm must be considered as an absolute upper bound above which fractional differentiation is guaranteed these estimates for the critical crystal size are orders of magnitude lower than suggested previously and thus the problem of crystal sizes becomes a central one for magma oceans a necessary condition for reentrainment is the existence of local mechanisms the absence of such mechanisms to reentrain the particles from the bottom would mean that an absolute suspension is impossible even if the energetics allows it turbulence is considered as a possible important factor a simple model of convection predicts a strong turbulence provided the viscosity is less than 10 9 10 10 p rotation reduces this critical viscosity to 10 5 10 8 p but this is still sufficiently large and is reached only near the maximum packing crystal fraction power law or bingham rheology of partial melts can exclude any turbulence already at 20 30 of crystal fraction we also show that the energetic criterion for the absolute suspension with e 1 coincides with the condition that the particle concentration gradient suppresses the turbulence

the formation of high grade ignimbrites i experiments on high and low concentration transport systems containing sticky particles high grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature corresponding pyroclastic flow particles range from plastic to partially liquid and are able to aggregate or coalesce this contrasts with particles in pyroclastic flows producing unwelded ignimbrite which are capable of elastic grain interactions the low aspect ratio and great areal extent of high grade ignimbrites requires transport in a particulate state either by a high concentration mass flow facilitated by fluidizing gas reducing internal friction or by b expanded turbulent flow of low but downward increasing concentration this paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms gas fluidization experiments using polyethyleneglycole peg powders heated above minimum sintering tms and melting tm temperatures cover a wide range of fluidization velocities umf ua 0 6 ut but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash where particle volume concentration outside the bubbles is high 10 1 when the powders reach a critical temperature tm t tms defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high and low tm 30 wt peg powders when t tms of the lower tm powder this indicates that extended particulate transport at t tms is not possible at such high particle concentrations in the turbulent flow experiments liquid sprays of molten peg or water vertically injected into a high re 104 horizontal air flow form a low concentration 10 5 to 10 4 turbulent suspension current proximal formation of partially coalesced aggregates which settle faster than individual particles causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid particle sedimentation from turbulent suspensions as particles become finer downstream and coalescence efficiency decreases in response to cooling more distally formed aggregates become too small and rare to modify sedimentation rate decay from that of suspension flows containing solid particles the key difference between the two transport systems is particle concentration c since particle collision rate rcoll c2 collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency 1 ecoal 0 is larger than 10 3 low concentration suspensions on the other hand require much higher values of ecoal for significant aggregation to occur dilute pyroclastic flows will have higher particle volume fractions 10 3 than the experimental currents but then viscous pyroclasts should have lower coalescence efficiencies than peg droplets experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high grade ignimbrite sheets

the effect of wind and precipitation on vegetation and biogenic crust covers in the sde hallamish sand dunes vegetation and biogenic crust covers play an important role in sand dune stabilization yet there is a lack of high temporal and spatial resolution data on sand dune cover a field experiment aimed at measuring the dynamics of biogenic crust and vegetation in sand dunes was conducted at the sde hallamish sand dunes in the northwestern negev desert israel from july 2008 to august 2010 the climate of the sde hallamish sand dunes is arid the mean annual precipitation over the past 13 years is 61 mm and the dunes are linear and partially stable mainly due to the presence of biogenic crust and partially due to the presence of vegetation in july 2008 10 10 m plots on the four dune habitats crest interdune north slope and south slope were treated as follows i removal of vegetation and biogenic crust ii removal of biogenic crust only iii removal of vegetation only iv partial removal of biogenic crust and vegetation and v control plot the surface coverage of sand biogenic crust and vegetation was monitored on a monthly basis using a remote sensing technique especially developed for the sde hallamish sand dunes it was found that strong wind events with durations of several days accounted for the coverage changes in biogenic crust and vegetation the response to precipitation was much slower in addition no rehabilitation of biogenic crust and vegetation was observed within the experiment time period the changes in biogenic crust cover were not necessarily related to changes in dune dynamics since often an increase in biogenic crust cover is a result of wind erosion that exposes old crust that was buried under the sand wind hardly erodes biogenic crust at all due to its high durability to wind action the sde hallamish dunes seem to have become more active as a result of a prolonged drought during the past several years the field experiment reported here indicates that biogenic crust cover exhibits large seasonal variations that are not necessarily related to the growth of new crust but rather to the exposure of old buried crust

the role of laboratory experiments in volcanology experiment is central to scienti c methodology within science in general experiments are used for four primary purposes i e as a tool to explore novel phenomena and to provide systematic observations of processes to determine the values of key parameters to test hypotheses and theoretical models and to validate computational models which themselves can be considered as numerical experiments the study of volcanic eruptions and magmatic processes is becoming increasingly systematic quantitative and rigorous the drive is towards providing detailed quantitative descriptions and models of the ow phenomena in terms of fundamental physico chemical processes this is motivated in no small way by an increasing need for reliable and quantitative predictions of volcanic eruptions on which to base rational hazard and risk management decisions during volcanic crises a natural consequence of this development of the scienti c study of volcanic ow phenomena is that rigorous laboratory experimentation is becoming an increasingly important feature within volcanic research the contribution of experimental research to our understanding of volcanic processes is di cult to overstate the determination of the values of key parameters has traditionally taken place in the laboratory however more recently experiments have become much more widely applied to the problem of understanding complex dynamical processes and their underlying mechanisms without models experimental theoretical numerical we are limited to qualitative interpretations of eld observations and remote sensing field data are of course essential and provide the measure against which ultimately the applicability of observations from other sources must be assessed however there are several reasons why the scienti c study of volcanic phenomena cannot rely solely on eld data direct observations of eruption processes in the eld are limited to those parts of an eruption that are accessible this restricts detailed observations to a limited range of above surface processes moreover logistical constraints mean that the systematic collection of data will usually only be possible for volcanic eruptions near an established observatory and for activity for which there has been some forewarning as a result of these constraints direct eld based observations of natural volcanic eruption processes are generally incomplete and uncertain another source of natural data derives from observations made on volcanic deposits in principle processes can be inferred from the features

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