a vertically integrated treatment of ice stream and ice shelf thermodynamics the extremely small vertical shear in ice stream and ice shelf flow simplifies the equations which govern their thermodynamic evolution complemented by the widely used shallow shelf approximation used to simplify the ice flow momentum balance a vertically integrated formulation of heat transfer presented here reduces the dimensionality of the thermodynamic problem from three to two plan view dimensions and thus significantly reduces the computational cost of treating ice stream and ice shelf thermodynamics in models for realistic conditions errors in ice stiffness parameter ice thickness and speed caused by the vertically integrated treatment of heat transfer are less than 5 of magnitudes of these values compared to the standard three dimensional thermomechanical computations in addition for the specific case of ice shelves with strong bottom melting the governing equation describing evolution of the vertically integrated ice stiffness parameter is derived which further reduces computational cost the presented error analysis and formulations of ice stream and ice shelf thermodynamics in terms of the vertically integrated temperature allow the thermodynamic effects on ice deformation to be easily incorporated into studies that traditionally disregard them

reconstruction of basal properties in ice sheets using iterative inverse methods inverse problems are used to estimate model parameters from observations many inverse problems are ill posed because they lack stability meaning it is not possible to find solutions that are stable with respect to small changes in input data regularization techniques are necessary to stabilize the problem for nonlinear inverse problems iterative inverse methods can be used as a regularization method these methods start with an initial estimate of the model parameters update the parameters to match observation in an iterative process that adjusts large scale spatial features first and use a stopping criterion to prevent the overfitting of data this criterion determines the smoothness of the solution and thus the degree of regularization here iterative inverse methods are implemented for the specific problem of reconstructing basal stickiness of an ice sheet by using the shallow shelf approximation as a forward model and synthetically derived surface velocities as input data the incomplete gauss newton ign method is introduced and compared to the commonly used steepest descent and nonlinear conjugate gradient methods two different stopping criteria the discrepancy principle and a recent improvement threshold are compared the ign method is favored because it is rapidly converging and it incorporates the discrepancy principle which leads to optimally resolved solutions

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

a variational approach to ice stream flow ice sheets are susceptible to the formation of ice streams or narrow bands of fast flowing ice whose high velocities are caused by rapid sliding at the contact between ice and the underlying bed based on recent geophysical work which has shown that the sliding motion of ice streams may be described by a coulomb friction law we investigate how the location of ice streams depends on the geometry of an ice sheet and on the mechanical properties of the underlying bed more generally this problem is relevant to the flow of thin films with coulomb or solid friction laws applied at their base by analogy with friction problems in elasticity we construct a variational formulation for the free boundary between ice streams where bed failure occurs and the surrounding ice ridges where there is little or no sliding this variational problem takes the form of a non coercive variational inequality and we show that solutions exist provided a force and moment balance condition is satisfied in that case solutions are also unique except under certain specialized circumstances which are unlikely to arise for a real ice sheet further we show how the variational formulation of the ice flow problem can be exploited to calculate numerical solutions and to simulate the effect of changing ice geometry and bed friction on the location and velocities of streaming flow lastly we study the effect of ice shelf buttressing on the flow of ice streams whose spatial extent is determined by the yield stress distribution of the bed in line with previous studies of ice shelf buttressing we find that the removal of an ice shelf can cause an ice stream feeding the ice shelf to speed up considerably which underlines the important role ice shelves may play in controlling the dynamics of marine ice sheets

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

igneous scapolite bearing associations in the chaine des puys massif central france and atakor hoggar algeria a scapolite amphibole clinopyroxene fe ti oxide apatite association has been found as megacrysts in tephra from the enval volvic volcanic line east of the chaine des puys massif central france and in the cinder cone of the segueika volcano atakor algeria in both kinds lavahosts are basanites although never seen together in a single xenolith a study of their inclusions indicates close genetic relationship between all 5 phases this association must be considered as a paragenesis that crystallized within a narrow pt range volcanological petrological and geochemical data suggest that these megacrysts are high pressure phenocrysts rather than mechanically desintegrated fragments derived from coarse rocks the composition of the amphibole suggests a pressure between 5 and 15 kb fe ti oxides imply a temperature close to 1 100 c

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

improved parameterization of seagrass blade dynamics and wave attenuation based on numerical and laboratory experiments seagrass blade dynamics were explored through numerical and laboratory experiments in order to improve parameterization of wave attenuation by submerged aquatic vegetation in the presence of a background current in the numerical model a single blade was modeled as a series of rigid plates attached by torsion springs for the laboratory model strips of low density polyethylene were placed in a recirculating wave flume a new form of the keulegan carpenter number based on the horizontal excursion of the blade tip was found to be an excellent predictor of drag coefficient an algebraic model for predicting wave attenuation was developed based on the following observations during the portion of the wave period when the fluid velocities are highest the blade motion is almost completely arrested and the vast majority of the turbulence production occurs during this time turbulence production when the blade is pronated is accurately predicted by the maximum fluid velocity over the wave period the relative contribution to the total turbulence production over the wave period is determined by the relative strength of the waves and the current therefore using a simple algebraic fit the total depth integrated time averaged turbulence production can be accurately predicted by two flow parameters the maximum fluid velocity over the wave period and a non dimensional number that compares the wave and current velocities by fitting the algebraic model to data from a particular site it can be used to efficiently estimate wave attenuation and drag coefficient in seagrass exposed to waves with a background current

recent changes in the flow of the ross ice shelf west antarctica abstract comparison of surface velocities measured during the ross ice shelf geophysical and glaciological survey riggs 1973 to 1978 and velocities measured via feature tracking between two moderate resolution imaging spectroradiometer modis mosaics compiled from 2003 4 and 2008 9 images reveals widespread slowing and minor areas of acceleration in the ross ice shelf ris over the approximately 30 year interval the largest changes 13 ma 2 occur near the whillans and mercer ice streams grounding line in the southernmost part of the ice shelf speed has increased over the interval up to 5 ma 2 between the macayeal ice stream grounding line and the shelf front and along the eastern shelf front changes in ice thickness computed using icesat laser altimetry are used together with a well tested model of the ice shelf to investigate underlying causes of change in the flow of the ice shelf over time the observed transients represent a combination of recent forcings and ongoing response to ice stream discharge variations over the past millennium while evidence of older events may be present the modern signal is dominated by shorter time scale events including the stagnation of kamb ice stream about 160 years ago recent changes in basal drag on the whillans ice stream ice plain and perhaps iceberg calving details in embayment geometry for example the shallow sea floor below crary ice rise modulate the spatial pattern of ice shelf response to boundary condition perturbations

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

a viscoelastic flowline model applied to tidal forcing of bindschadler ice stream west antarctica abstract the motion of bindschadler ice stream west antarctica is dominated by sliding over a nearly plastic bed according to analysis of kinematic gps data using a new viscoelastic flowline model inversions of time averaged velocity data with viscous ice flow models can be consistent with multiple sliding laws but propagation of velocity perturbations in a viscoelastic model can distinguish between sliding laws with different exponents we develop such a model and apply it to a time series of velocity for the tidally modulated flow of bindschadler ice stream formerly ice stream d observed velocity perturbations are found to be consistent with a flow law exponent m 8 which indicates basal motion with a relatively weak till bed lower exponents consistent with motion dominated by deformation within the ice over a hard or frozen bed are found to be unlikely this result suggests that bindschadler ice stream would respond rapidly and significantly to any future loss of buttressing from the ross ice shelf

ice stream basal conditions from block wise surface data inversion and simple regression models of ice stream flow application to bindschadler ice stream 1 widespread basal conditions controlling ice stream flows are still beyond the scope of direct observation thus knowledge of their magnitudes and variabilities comes from inversion of surface measurements ice velocities surface elevations and thicknesses we present a new approach to implement a widely accepted inverse method on regular 10 10 km blocks smaller than the whole domain to enhance the spatial resolution of calculated basal conditions inverted basal friction coefficients and calculated shear stress have sharp transitions and large variations in small areas overall the obtained basal shear stress is very small in regions of fast flowing ice the results of the inversion along with the surface variables are used to construct two simple regression models of bindschadler ice stream former ice stream d that reproduce 96 of observed velocity variations while highly idealized these regression models are sufficiently informative to be considered as parameterizations for ice stream flow in large scale ice sheet models which lack the spatial and temporal resolution necessary to simulate ice stream dynamics in detail

the geochemistry of phosphorus in granite rocks and the effect of aluminium phosphorus has been shown to have a profound influence on the evolution of silicate melt systems it also controls the geochemical behavior of rare earths uranium thorium strontium and other elements through the relationships between melt and accessory phosphates based on experimental data harrison and watson 1984 proposed a model of saturation behavior of apatite in crustal melts as a function of temperature and silica content subaluminous granites fit in well with such a model but departures are evident in the case of peraluminous granites a systematic study of the distribution of p2o5 in granitoids from all over the world shows that there are two modes to describe phosphorus enrichment in peraluminous granitic rocks 1 the hoyos type trend which is parallel to the one in harrison and watson s hw model but with higher p2o5 concentrations at the same silica contents and 2 the pedrobernardo type trend implying an increase in phosphorus in the most sio2 rich differentiates of strongly peraluminous low ca granites pedrobernardo type leucogranites are characterized by the presence of rare phosphates or p rich feld spars since they have excess p2o5 over cao to form normative apatite we here propose the term perphosphorous granites high phosphorus contents appear to be due to elevated apatite solubility in peraluminous granitic melts rather than to high amounts of restitic apatite both the hoyos and pedrobernardo type departures from the hw model can be corrected by using the expression p2o5corrected p2o5hw efactor where p2o5hw is the prediction according to the hw model and factor is asi 1 6429t asi being the al2o3 cao na2o k2o molar ratio we suggest that such departures could be caused by a decrease in the ca activity in the melt as a result of increased peraluminousity the pedrobernardo type trend is a limiting case in which ca activity in the melt is nearly zero and calcium instead of phosphorus becomes the essential structural constituent esc of the apatite when this happens phosphorus starts behaving as an incompatible element that is concentrated in late fluids high p2o5 contents are more characteristic of s than other granite types no significant departures from the hw model have been found for i a or m type granites in central iberia granites associated with u sn and w mineralizations usually have high phosphorus contents values of p2o50 5 wt at sio270 wt are good indicators of economic mineralization

medium pressure crystallization of a monchiquitic magma evidence from megacrysts of drever s block ubekendt ejland west greenland abstract megacrysts and polymineralic fragments of extraordinary diversity from a tertiary monchiquitic dyke of ubekendt ejland comprise three groups 1 cr diopside fassaitic diopside olivine fo90 5 81 5 cr al spinels ii fassaitic salite ferrisalite k ti pargasite ferropargasite apatite al ti magnetite iii scapolite hyalophane potassium feldspar nepheline analcime by comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in h2o and co2 at 5 11 kb group i megacrysts formed at temperatures of 1300 1150 c and group ii between 1150 800 c and f o 2 9 bar at the latter temperature high pco2 may have stabilized the scapolite in the more evolved liquid and k feldspar and nepheline began to crystallize at ca 800 c possibly together with the ferrisalite

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

sulfur bearing magmatic accessory minerals almost all magmas contain sulfide or sulfate bearing phases in most natural samples the sulfur bearing phase is a sulfide which typically is pyrrhotite fe1 x s or pyrite fes2 although chalcopyrite cufes2 pentlandite fe ni 9s8 sphalerite zns or molybdenite mos2 may be present as well sulfate minerals are rare at magmatic conditions and anhydrite caso4 is the most common other magmatic so4 bearing minerals include the sodalite group minerals hauyne simplified formula na ca 4 8 al6si6 o s 24 so4 cl 1 2 scapolite minerals silvialite ca na 4al6si6o24 so4 co3 and s bearing apatite ca5 po4 3 f cl oh barite baso4 has been mentioned in rare cases marchev 1991 sulfur bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals the crystallization of sulfide sulfate and s bearing minerals strongly depends on melt composition temperature and pressure and the s speciation in melt which in turn is strongly dependent on the prevailing oxygen fugacity baker and moretti 2011 this volume wilke et al 2011 this volume irrespectively of the low abundance of s bearing minerals the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates these minerals are critical for estimating the activity of various sulfur bearing species in magmas and can be used to constrain the oxygen fugacity and the s concentration in the melt e g pre eruptive sulfur concentration in melts the presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are s2 or s6 respectively typically one of these species is dominant but there are conditions where s2

the occurrence and distribution of mo and molybdenite in unaltered peralkaline rhyolites from pantelleria italy small hexagonal and triangular platelets of molybdenite mos2 5 to 25 m in diameter were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from pantelleria italy the mos2 occurs commonly in pantellerites peralkaline rhyolites rarely in pantelleritic trachytes and never in trachytes the occurrence of euhedral mos2 platelets in all phenocryst phases in matrix glass and even in some melt inclusions indicates that mos2 precipitated directly from the peralkaline melt despite mos2 saturation the melt glass contains greater than 95 of the mo in pantellerian rocks x ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less mo than corresponding matrix glasses the differences being in proportion to phenocryst abundances the mo contents increase with differentiation from trachytes 2 12 ppm to pantellerites 15 25 ppm and correlate positively with incompatible elements such as th y and nb the mo concentrations as determined by secondary ion mass spectrometry are essentially the same in matrix glasses and melt inclusions showing that mo did not partition strongly into a volatile fluid phase during outgassing the high mo contents of the pantellerites relative to metaluminous magmas with 1 5 ppm may be due to several factors 1 the enhanced stability of highly charged cations such as mo6 u4 and zr4 in peralkaline melts 2 the rarity of fe ti oxides and litanite into which mo might normally partition 3 reduced volatility of mo in low fo2 h2o poor 1 2 wt peralkaline magmas geochemical modeling indicates that the precipitation of mos2 can be explained simply by the drop in temperature during magmatic differentiation the occurrence of mos2 in pantellerites may result from their high mo concentrations and low redox state ni nio 2 5 relative to metaluminous magmas causing them to reach mos2 saturation at magmatic temperatures the apparent absence of mos2 microphenocrysts in more oxidized metaluminous rhyolites may indicate that mo is dissolved primarily as a hexavalent ion in those magmas

age of tertiary volcanic rocks on the west greenland continental margin volcanic evolution and event correlation to other parts of the north atlantic igneous province radiometric ages for undated parts of the volcanic succession and intrusions in west greenland were obtained by the 40ar 39ar incremental heating method acceptable crystallization ages were obtained for 27 samples combined with published results the new data provide a volcanic stratigraphy correlatable throughout the nuussuaq basin the thick onshore volcanic pile consists of four widespread formations 62 5 61 ma picrites vaigat formation 61 60 ma depleted basalts malig t formation and the hellefisk 1 well 60 58 ma less depleted basalts svartenhuk formation and 56 54 ma enriched basalts naqerloq formation two local successions comprise 53 5 ma alkali basalts erqua formation and 38 7 ma transitional basalts talerua member a central volcano developed on ubekendt ejland leading to the sarq ta qaq gabbro granophyre intrusion at 57 55 ma pre break up volcanism took place further south as early as 64 63 ma the offshore volcanic succession most probably comprises the known onshore succession plus some younger lavas the change in spreading direction near the paleocene eocene boundary took place west of the nuussuaq basin c 56 2 ma some tectonomagmatic events are correlatable across the entire north atlantic igneous province a quiescent 58 56 ma period correlates with similar periods in east greenland and the faroes and the naqerloq formation is coeval with the eocene basalts in east greenland the paleocene and eocene tholeiitic basalts are distinguishable chemically in early eocene time mantle typical of the iceland plume seems to have extended beneath the whole west greenland margin as well as the central east greenland margin

calculation of co 2 activities using scapolite equilibria constraints on the presence and composition of a fluid phase during high grade metamorphism thermodynamic and phase equilibrium data for scapolite have been used to calculate co2 activities aco2 and to evaluate the presence or absence of a fluid phase in high grade scapolite bearing meta anorthosite granulites calc silicates and mafix xenoliths the assemblage scapolite plagioclase garnet quartz may be used to calculate or limit aco2 by the reaction meionite quartz grossular anorthite co2 granulites from four high grade terranes grenville province canada sargut belt india furua complex tanzania bergen arcs norway yield aco2 0 4 1 with most 0 7 for scapolite bearing granulites from the furua complex in which aco2 0 9 calculated h2o activities ah2o based on phlogopite dehydration equilibria are uniformly low 0 1 0 2 the aco2 calculated for meta anorthosite from the grenville province ontario ranges from 0 2 to 0 8 for grenville meta anorthosite also containing epidote the ah2o calculated from clinozoisite dehydration ranges from 0 2 to 0 6 calc silicates from the grenville sargur and furua terranes mostly yield aco2 1 the calculated fluid activities are consistent with metamorphism 1 in the presence of a mixed co2 h2o fluid phase in which co2 is the dominant fluid species but other c o h s species are minor 2 in the absence of a bulk fluid phase fluid absent metamorphism or 3 in the presence of a fluid bearing melt phase the results for many granulites and grenville meta anorthosite are consistent with the presence of a co2 rich mixed co2 h2o fluid phase in contrast the relatively restricted and low values of aco2 for calc silicates require an h2o rich fluid or absence of a fluid phase during metamorphism the range of values for xenoliths are most consistent with absence of a fluid phase the primary implication of these results is that a co2 rich fluid accounts for the reduced ah2o in scapolite bearing granulites however scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase and the presence of scapolite is not a priori evidence of a co2 rich fluid phase in addition close association of scapolite free mafic granulites with scapolite bearing granulites having identical mineral compositions in the furua complex and the absence of scapolite from most granulite terranes implies that a co2 rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes

the volatile component of some pumice forming alkaline magmas from the azores and canary islands the abundances of pre eruptive magmatic volatile species in the system h o s may be determined by application of thermodynamic methods to phenocryst assemblages commonly found in volcanic rocks as demonstrated by rutherford and heming 1978 these methods are applied to alkaline pumice deposits of airfall and ignimbrite type from tenerife canary islands sao miguel and faial azores it is argued that reliable temperature and fo2 buffering mechanisms found in rhyolitic magmas appear not to operate in more alkaline liquids fh2o is estimated using biotite the high values found are shown to be compatible with the violently explosive nature of the magmas concerned fs2 is estimated from pyrrhotite composition fh2 fh2s fso2 fso3 are calculated from gas equilibria water fugacity may be very roughly estimated for non biotite bearing samples from data on the sulphur species abundances of these species are similar in alkaline and calc alkaline salic magmas volcanological implications relating to the release of volatiles during explosive eruptions are considered

surface water waves over a shallow canopy the dynamics of water waves passing over a vegetation canopy is modelled theoretically to simplify the geometry we examine a periodic array of vertical cylinders fixed on a slowly varying seabed the macroscale behaviour of wave attenuation is predicted based on microscale dynamics between plants interstitial turbulence is modelled by reynolds equations with a locally constant eddy viscosity determined by energy considerations using the asymptotic method of multiple scale expansions the slow evolution of waves is derived by considering the coupling with the small scale motion in the canopy after numerical solution of the canonical boundary value problem in a few unit cells predictions of macroscale effects such as wave attenuation are made and compared with laboratory experiments the counteracting effects of shoaling and dissipation are discussed for different vegetation densities

radar derived bed roughness characterization of institute and moller ice streams west antarctica and comparison with siple coast ice streams 1 subglacial bed conditions exert a significant control on ice stream behavior and evolution and can be characterized by determining bed roughness from fft analysis of radar imaged basal reflectors here we assess bed roughness across institute and moller ice streams west antarctica and compare our findings with bed roughness determined across the siple coast ice streams we find that variations in bed roughness are spatially organized and attribute this to the varying efficacy of subglacial erosion and deposition with rougher inland slow flowing regions largely manifesting preglacial topography and smoother downstream fast flowing regions evincing significant postglacial modification to the subglacial landscape the observed similarities between bed roughness characteristics of iis mis and the siple ice streams suggest that iis and mis are largely underlain by wet poorly consolidated sediments and may therefore be vulnerable to the types of dynamical instabilities experienced by the siple ice streams

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

sulfur partition coefficient between apatite and rhyolite the role of bulk s content experimental studies have been performed to constrain sulfur behavior during apatite crystallization and to determine sulfur partition coefficient between apatite and melt kd s apatite melt at oxidizing conditions crystallization experiments have been conducted with a hydrous rhyolitic melt and different bulk sulfur contents 0 15 to 2 wt s at 900 and 1 000 c 200 mpa and log f rm o 2 nno 3 6 the sulfur content in the glass increases with increasing amount of added s anhydrite crystallizes for s added 0 75 wt 0 10 and 0 13 wt so3 in glass at 900 and 1 000 c respectively the amount of anhydrite increases and the amount of apatite decreases with increasing amount of added sulfur the sulfur exchange reaction in apatite is influenced by the bulk composition of the melt e g p content however changing melt composition has only little effect on kd s apatite melt for the investigated rhyolitic composition the kd s apatite melt does not depend directly on temperature decreases from 14 2 to 2 7 with increasing s content in glass from so3 0 03 to 0 19 wt respectively and can be predicted from the following equation ln kd 0 0025 s in melt in ppm 2 9178 the combination of experimental data obtained for rhyolitic and andesitic melts reveals that the sulfur partition coefficient tends toward a value of 2 for high sulfur content in the glass 0 2 wt so3 using s in apatite as proxy for determining s content in melt is promising but additional experimental data are needed to clarify the individual effects of t f rm s 2 and p and ca content in the melt on s partitioning

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

scapolite and analcime monitors of magmatic fluid metasomatism in a major shear zone abstract scapolite and analcime are uncommon alteration minerals in syenite here we describe a syenite body in a fault zone that hosts these minerals and we elucidate the unusual chemical conditions that led to their formation the clarke head syenite is part of a regional late devonian early carboniferous back arc a type granitoid suite in the cobequid highlands of nova scotia canadian appalachians the syenite contains magmatic rutile and analcime magmatic k feldspar has been largely replaced by scapolite of marialite composition which shows a temporal transition from more cl rich to more carbonate rich varieties scapolite veins cut both the syenite and associated gabbro and diorite analcime i occurs as a late stage interstitial magmatic mineral analcime ii forms by alteration of albite lamellae in perthite replaced by scapolite analcime iii vugs that cross cut scapolite and analcime iv occurs in late veins cross cutting earlier analcime phases and scapolite the regional a type granite plutonism was na and halogen rich with widespread late magmatic albitization and f related mobility of ree minerals however only at clarke head is there extreme na and cl rich metasomatism indicated by scapolite and cl rich hastingsite with 1 wt cl content cl rich fluids were derived from regional halite evaporites at faulted basin margins the main circulating fluids were likely of magmatic origin based on the 18o of both replacement and vein scapolite as the nacl supply was reduced scapolite became more ca rich halogen rich fluids also mobilized ti and zr which were reprecipitated in hydrothermal rutile and zircon as the halogen activity diminished scapolite replacement of k feldspar released large amounts of k into the circulating fluids which converted ferromagnesian silicates in fault zones to secondary biotite

origin of sulphur in the quaternary perpotassic melts of italy evidence from hauyne sulphur isotope data abstract hauyne na ca 4 8 alsio 4 6 so 4 s 1 2 is a rare mineral which occurs in ejecta and lavas of perpotassic volcanoes of the roman comagmatic region italy as a high temperature sulphur bearing phase it is a valuable indicator of the sulphur characteristics in the melt therefore sulphur isotope determinations were carried out for the first time on samples from some italian volcanoes on average 34 s values 1 1 to 10 8 are above the normal range 0 to 3 for igneous materials and can be classified into two groups phenocrysts in lavas avg 34 s 4 2 and metasomatic crystals in ejecta avg 34 s 8 3 such data are compatible with the hypothesis that hydrous fluids released from subducted sediments were responsible for the high volatile and lile supply to the mantle region where the magma generating process occurred

tertiary and quaternary volcanism in the massif central france summary continental alkaline volcanics of the french massif central range in age from 65 ma to 3450 years bp and can be divided by geography and age into 20 separate areas there is no correlation between the age of an area and the geographic locality two magma series are present a silica undersaturated basanite tephrite phonolite series and an alkali basalt trachyandesite trachyte rhyolite series the evolved members of which are silica saturated rare nephelinites are also found primitive magmas of both main series may be derived from 2 20 partial melting of a mantle enriched with light rare earth elements with variable amounts of garnet in the residue nephelinites were formed from a source which was isotopically and chemically distinct fractional crystallization of observed phases accounts for the diversity of intermediate and evolved products amphibole fractionation in basalts at depth causes the trend towards silica saturation while alkali feldspar fractionation dominates the final stages of crystallization removal of small quantities of sphene and zircon explains trace element depletions in trachytes phonolites and rhyolites up to 30 crustal contamination has occurred in the evolved magmas but contamination is generally absent from their basaltic parents sr nd and pb isotope data indicate that the most probable contaminant is undepleted meta sedimentary granulite facies lower crust variations in sr and nd isotope ratios also constrain petrogenetic hypotheses such as amphibole fractionation and magma mixing mineralogical disequilibrium compositional banding and emulsification textures indicate extensive magma mixing some flows are homogeneous hybrid lavas in which only the unusual composition and rare mineral disequilibria indicate the mode of formation

the limited growth of vegetated shear layers 1 in contrast to free shear layers which grow continuously downstream shear layers generated by submerged vegetation grow only to a finite thickness because these shear layers are characterized by coherent vortex structures and rapid vertical mixing their thickness controls exchange between the vegetation and the overlying water experiments conducted in a laboratory flume show that the growth of these obstructed shear layers is arrested once the production of shear layer scale turbulent kinetic energy ske is balanced by dissipation of ske within the canopy this equilibrium condition along with a mixing length closure scheme was used in a one dimensional numerical model to predict the mean velocity profiles of the experimental shear layers the agreement between model and experiment is very good but field application of the model is limited by a lack of description of the drag coefficient in a submerged canopy

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

the origin of k feldspar megacrysts hosted in alkaline potassic rocks from central italy a track for low pressure processes in mafic magmas abstract in situ sr isotope and microchemical studies were used to determine the provenance of k feldspar megacrysts hosted in mafic alkaline potassic ultrapotassic rocks and in differentiated rocks from two nearby volcanic apparatus in central italy at monte cimino volcanic complex mafic leucite free ultrapotassic megacryst bearing rocks of olivine latitic composition are associated with evolved latite and trachyte here latites and trachytes straddle the sub alkaline field age corrected 87sr 86sr values sri of the analysed cimino olivine latites vary from 0 71330 and 0 71578 and strongly increase at constant mg value latite and trachyte have lower sri than olivine latites ranging between 0 71331 and 0 71361 sri of k feldspar megacrysts from olivine latites are between 0 71352 and 0 71397 but core and rim 87sr 86sr ratios within individual megacryst are indistinguishable in all the mafic rocks the megacrysts are not in isotopic equilibrium with the hosts k feldspar megacrysts from both the latite and trachyte have similar sr isotope compositions sri 0 71357 0 71401 to those in the olivine latites however sri of megacryst in the trachyte vary significantly from core to rim sri from 0 71401 to 0 71383 as with the olivine latites the k feldspar megacrysts are not in isotopic equilibrium with bulk rock compositions of the latite or trachyte at vico volcano megacryst bearing rocks are mafic leucite free potassic rocks mafic leucite bearing ultrapotassic rocks and old trachytic rocks the mafic leucite bearing and leucite free rocks are a tephri phonolite and an olivine latite respectively a megacryst in vico trachyte is isotopically homogeneous sri core 0 71129 rim 0 71128 and in equilibrium with the host rock sri bulk rock 0 71125 sri of megacryst from tephri phonolite is clearly not in isotopic equilibrium with its host sri bulk rock 0 71158 and it increases from core sri 0 71063 to rim sri 0 71077 a megacryst in vico olivine latite is isotopically homogeneous sri core 0 71066 rim 0 71065 but not in equilibrium with the host rock sri bulk rock 0 71013 the sr isotope microdrilling technique reveals that cimino megacrysts were crystallised in a cimino trachytic magma and were subsequently incorporated by mixing mingling processes in the latitic and olivine latitic melts a model invoking the presence of a mafic sub alkaline magma which was mixed with the olivine latite is proposed to justify the lack of simple geochemical mixing relation between cimino trachytes and olivine latites this magmatological model is able to explain the geochemical characteristics of cimino olivine latites otherwise ascribed to mantle heterogeneity the similarity of core sri of megacrysts hosted in vico tephri phonolite and olivine latite suggests that the k feldspar megacrysts are co genetic isotopic equilibrium between megacryst and vico host trachyte indicates that the trachyte is the parent of this megacryst on the contrary the megacrysts hosted in tephri phonolite and olivine latite do not derive from the old trachytic magma because no diffusion process may explain the core to rim sr isotope increase of the xenocryst hosted in the tephri phonolite the megacrysts hosted in the vico mafic rocks might derive from a trachytic melt similar in composition to the old vico trachytes

the age of carbonatites kimberlites and lamprophyres from southern west greenland recurrent alkaline magmatism during 2500 million years abstract alkaline rocks were intruded into the precambrian basement in southern west greenland during at least five separate episodes the tupertalik carbonatite intrusion was emplaced around or before 2650 m y it is now metamorphosed in granulite facies but is recognisable by its trace element content lamprophyre dykes were intruded at ca 1800 m y and again at ca 1200 m y at ca 600 m y the sarfartoq carbonatite intrusion and extensive kimberlite dykes were emplaced in the holsteiborg stromfjord region at 225 115 m y the qaqarssuk carbonatite complex was emplaced in the sukkertoppen region and numerous lamprophyre and kimberlite dykes were emplaced between fishenasset and ivigtut all these episodes are correlatable with contemporaneous alkaline activity in canada and scandinavia and substantiate the periodicity of carbonatite emplacement the mesozoic suite of intrusions is related to rifting prior to continental break up and the formation of the north atlantic ocean the eocambrian suite of 600 m y is thought to have formed under similar conditions during continental break up and the formation of the iapetus ocean

a regional strategy for geothermal exploration with emphasis on gravity and magnetotellurics abstract as part of the resource evaluation and exploration program conducted by los alamos scientific laboratory for the national hot dry rock hdr geothermal program a regional magnetotelluric mt survey of new mexico and arizona is being performed the mt lines are being located in areas where the results of analysis of residual gravity anomaly maps of arizona and new mexico integrated with other geologic and geophysical studies indicate the greatest potential for hdr resources the residual gravity anomalies are derived by applying the concept of predicting gravity anomalies from topography this can be accomplished by employing reductions similar to those used in some isostatic investigations in which a regional topographic surface is used as the bouguer reduction datum the datum is derived by comparison of various harmonics of bouguer anomalies and elevations of stations topography can be used to predict bouguer anomalies because of isostatic compensation the resultant anomalies can be considered high frequency residual anomalies or isostatic anomalies corrected for regional compensation such maps have been produced for arizona new mexico west texas and chihuahua mexico the main objective of the mt project is to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and or upper mantle beneath the colorado plateau and the adjacent basin and range province and rio grande rift the mt survey consists of 200 sites along several long profiles with site spacing of 15 20 km pre existing available mt data are being integrated with the new data after the data are processed a one dimensional inversion is applied to the sounding curve and used as a starting point for 2 d modeling such a project and ultimate map will be of major value in studying the regional geophysics and tectonics of the southwest united states as they now apply to hdr resources in particular and geothermal resources in general electrical conductivity anomalies of large areal extent are of particular interest in geothermal exploration correlation analysis of large conductive anomalies with other geophysical geological and geotectonic data is being performed preliminary analysis of the data has suggested several major regions of anomalously shallow high electrical conductivity among these is the aquarius area of northwest arizona which is the site of a longwavelength residual anomaly low which when modeled and correlated with other geophysical data can be shown to be possibly related to low density and high temperature in the crust at depths of 20 km or less preliminary analysis of mt data indicates the possible existence of a mid crustal high electrical conductivity anomaly in this same region

scapolite phenocrysts in a latite dome northwest arizona u s a abstract a latite dome in northwest arizona contains a rare occurrence of primary so 4 rich scapolite phenocrysts the total phenocryst assemblage consists of plagioclase an 20 an 33 hornblende biotite and scapolite me 68 microphenocrysts include allanite and oxidized low ti magnetite electron microprobe analyses show that the scapolite contains about 1 74 wt s which indicates an atomic s s c of 0 58 although scapolite occurs in xenoliths in volcanic rocks and diatremes as well as a metamorphic mineral in granulites its occurrence as a primary igneous mineral is extremely rare ca rich scapolite has been crystallized experimentally by others from melts with a wide range of sio 2 cao and na 2 o contents at temperatures above 825 c and pressures ranging from 3 to 15 kbar comparison of scapolite from this latite with synthetic scapolite crystallized from nepheline syenite melt suggests that the arizona phenocrysts crystallized under conditions of 850 to 900 c 3 6 kbar total pressure and unusually high co 2 and so 2 the rarity of scapolite as a phenocryst mineral suggests that high partial pressures of co 2 and so 2 are rare in the magmatic environment

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

sulfide globules in mid ocean ridge basalts morb and the effect of oxygen abundance in feso liquids on the ability of those liquids to partition metals from morb and komatiite magmas the coexistence of sulfide liquids or more accurately fe s o liquids with silicate liquids in a variety of magma systems is well known as is the ability of immiscible fe s o liquids to segregate ni cu and platinum group metals into orebodies evidence for association of fe s o liquids with magmas that form mid ocean ridge basalts morb comes from sulfide globules in basalt glass from siqueiros fracture zone east pacific rise three fracture zones in the indian ocean lau basin and 23 previously published oceanic localities approximately 40 of the sulfur originally present in magma at siqueiros fracture zone exsolves as a sulfide liquid during fractional crystallization virtually all of the cu and 20 of the ni originally in the magma is extracted by the sulfide liquid significant variations in the ability of fe s o liquids to partition ni cu exist among morb and other mafic magmas such as komatiites and layered mafic intrusions nernst partition coefficients sulfide to silicate melt dsl are 2 3 times higher for ni in morb than in komatiite and 5 times higher for cu reasons for this variation are not entirely clear but a possible contributing factor is that fe s o liquids in morb appear to have a higher oxygen content than those in komatiites that fe s o liquids associated with morb magma are oxygen rich is suggested by low observed values at siqueiros fracture zone for the ni fe exchange coefficient kd3 1 21 4 56 kd3 feni olivine feni sulfide it has been previously shown that natural magmatic fe s o liquids are rich in o2 o s o is apparently always greater than 0 49 and that kd3 is inversely correlated with o s o kd3 values below 12 would result from fe s o liquids with o s o 0 49 kd3 value of 20 would be impossible values from 12 to 20 would be unlikely because they would require an fe s o liquid poor in o2 this argument for low kd3 in magmas is strengthened by the low kd3 values of morb whose globules are inarguably of magmatic origin kd3 in komatiites is higher than in morb indicating a lower oxygen fugacity in komatiite magma and varies from 5 to 20 with values of 20 coming from highly metamorphosed komatiites fleet and macrae argued on the basis of experimental results that kd3 values of 20 could not have resulted from magmatic processes but might have come from metamorphism under oxygen deficient conditions

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

modelling waving crops using large eddy simulation comparison with experiments and a linear stability analysis in order to investigate the possibility of modelling plant motion at the landscape scale an equation for crop plant motion forced by an instantaneous velocity field is introduced in a large eddy simulation les airflow model previously validated over homogeneous and heterogeneous canopies the canopy is simply represented as a poroelastic continuous medium which is similar in its discrete form to an infinite row of identical oscillating stems only one linear mode of plant vibration is considered two way coupling between plant motion and the wind flow is insured through the drag force term the coupled model is validated on the basis of a comparison with measured movements of an alfalfa crop canopy it is also compared with the outputs of a linear stability analysis the model is shown to reproduce the well known phenomenon of honami which is typical of wave like crop motions on windy days the wavelength of the main coherent waving patches extracted using a bi orthogonal decomposition bod of the crop velocity fields is in agreement with that deduced from video recordings the main spatial and temporal characteristics of these waving patches exhibit the same variation with mean wind velocity as that observed with the measurements however they differ from the coherent eddy structures of the wind flow at canopy top so that coherent waving patches cannot be seen as direct signatures of coherent eddy structures finally it is shown that the impact of crop motion on the wind dynamics is negligible for current wind speed values no lock in mechanism of coherent eddy structures on plant motion is observed in contradiction with the linear stability analysis this discrepancy may be attributed to the presence of a nonlinear saturation mechanism in les 2010 cambridge university press

the effects of transverse bed topography variations in ice flow models 1 a suite of ice flow models is subject to a performance test designed to investigate treatment of flow over variable basal topography using transfer functions developed by gudmundsson 2003 and numerical models of various complexity this study demonstrates that a widely used flowband model has strong limitations associated with its underlying assumptions and thus should be applied only in specific geometrical settings its performance can be significantly improved by using the laterally averaged basal topography instead of centerline topography in applications where spatial variability of flow fields is important hybrid ice flow models can be a viable alternative to flowband models in addition analysis of horizontal distributions of the various ice flow characteristics e g surface elevation velocity and horizontal stress components shows that field observations that focus on assessing these parameters only along the centerline of ice flow could be misleading these results also suggest that such spatial variability needs to be taken into account when designing field surveys of ice flow

similarity of organized patterns in driving and basal stresses of antarctic and greenland ice sheets beneath extensive areas of basal sliding the rate of ice transport from the interior of ice sheets to their margins and hence the rate with which it contributes to sea level is determined by the balance of driving stress basal resistance and ice internal deformation using recent high resolution observations of the antarctic and greenland ice sheets we compute driving stress and ice deformation velocities inferring basal traction by inverse techniques the results reveal broad scale organization in 5 20 km band like patterns in both the driving and basal shear stresses located in zones with substantial basal sliding both ice sheets experience basal sliding over areas substantially larger than previously recognized the likely cause of the spatial patterns is the development of a band like structure in the basal shear stress distribution that is the results of pattern forming instabilities related to subglacial water the similarity of patterns on the greenland and antarctic ice sheets suggests that the flow of ice sheets is controlled by the same fundamental processes operating at their base which control ice sheet sliding and are highly variable on relatively short spatial and temporal scales with poor predictability this has far reaching implications for understanding of the current and projection of the future ice sheets evolution

magmatic anhydrite sulfide assemblages in the plumbing system of the siberian traps we report the first discovery of magmatic anhydrite sulfide assemblages in a subvolcanic intrusion associated with the siberian traps the 34s values of anhydrite and coexisting sulfide crystals analyzed by ion probe are 18 22 and 9 11 respectively more than 50 of the total sulfur in the intrusion is estimated to derive from marine evaporites in the footwall strata the contaminated magma was highly oxidized and able to dissolve up to one order of magnitude more sulfur than pure mantle derived basaltic magma such contaminated magma if erupted would have released far more so2 into the atmosphere than is generally appreciated

anhydrite in the 1989 1990 lavas and xenoliths from redoubt volcano alaska abstract the eruption of redoubt volcano in alaska produced a moderate sulfur emission estimated at 1 10 tons so 2 but relatively small volume of lava 0 11 km with pre eruption estimates of 840 950 c and f o 2 1 5 to 2 0 log units above nno swanson s e nye c j miller t p avery v f 1994 magma mixing in the 1989 1990 eruption of redoubt volcano part ii evidence from mineral and glass chemistry journal of volcanology and geothermal research 62 453 468 petrologic estimates of sulfur production sigurdsson h devine j d davis a n 1985 the petrologic estimation of volcanic degassing jokull 35 1 8 from this eruption gerlach t westrich h r casadevall t j finnegan d l 1994 vapor saturation and accumulation in magmas of the 1989 1990 eruption of redoubt volcano alaska journal of volcanology and geothermal research 62 317 337 are considerably less than the measured sulfur emission leading workers to propose the existence of a pre eruption vapor phase to explain the excess sulfur initial examination of the 1989 1990 redoubt eruptive products reported anhydrite nye c j swanson s e avery v f miller t p 1994 geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry journal of volcanology and geothermal research 62 429 452 in interstitial glass from some cognate gabbroic xenoliths but anhydrite was not noted in any of the andesites a boeing 747 encountered the ash plume from the initial eruptive phase on december 15 1989 and provided ash samples that reportedly contained gypsum bayhurst g k wohletz k h mason a s 1994 a method for characterizing volcanic ash from the december 15 1989 eruption of redoubt volcano alaska u s geological survey bulletin 2047 13 17 however the identification was based on eds analyses on a sem and the mineral could have been anhydrite reexamination of the 1989 1990 redoubt lavas and xenoliths revealed the presence of rare fine grained 10s of microns anhydrite crystals in the lavas and larger 10s to 100s of microns crystals in the xenoliths the anhydrite forms elongate crystals with sharp contacts against the groundmass glass grains show rounded terminations suggesting breakdown of the anhydrite perhaps during eruption the anhydrite is a sparse phase in the lavas but occurs in lavas and cognate xenoliths erupted throughout the eruption the presence and breakdown of anhydrite may explain the excess sulfur associated with the redoubt eruption a careful search of other freshly erupted andesitic lavas solidified from hydrous oxidizing magmas should reveal other examples of anhydrite and cause a rethinking of the source of sulfur in volcanic emissions

petrogenesis of monte vulture volcano italy inferences from mineral chemistry major and trace element data the paper presents major and trace element data and mineral compositions for a series of foiditic tephritic to phonolitic rocks coming from monte vulture southern italy and investigates their origin evolution and relationship with the other centres of the roman province

igneous sulfides in the fish canyon tuff and the role of sulfur in calc alkaline magmas pyrrhotite is a common inclusion in several phenocrystal phases from the fish canyon tuff a voluminous homogeneous ash flow erupted from the central san juan field the fugacities of various gaseous species have been calculated using the composition of the pyrrhotite iron titanium oxides and biotite sulfur and oxygen fugacities fall very near the sulfur condensation curve implying that liquid sulfur may have been present in the parent magma high sulfur fugacities during melt formation can cause high oxygen fugacity in calc alkaline magmas due to the breakdown of pyrite at high temperatures during cooling of such magmas separation of so 2 rich gases causes a reduction in oxygen fugacity significant amounts of sulfur are available during degassing and represent a major source of sulfur for related ore deposits the ultimate source of the high sulfur and oxygen activities is probably pyrite and magnetite deposited during the waning stages of hydrothermal circulation at active oceanic ridges other ash flows and calc alkalic volcanic rocks should be carefully examined for similar sulfide phases

the experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt abstract a first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions experiments were performed in an internally heated gas pressure vessel at 500 mpa and temperatures between 840 and 1000 c for 120 168 h three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by mc icp ms i kinetic experiments using 57 fe enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h ii isotope exchange experiments using mixtures of hydrous peralkaline rhyolitic glass powder 4 wt h 2 o and natural pyrrhotites fe 1 x s as starting materials and iii crystallisation experiments in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt consistently showed that pyrrhotite preferentially incorporates light iron no temperature dependence of the fractionation factor was found between 840 and 1000 c within experimental and analytical precision an average fractionation factor of 56 fe 54 fe pyrrhotite melt 0 35 0 04 2se n 13 was determined for this temperature range predictions of fe isotope fractionation between fes and ferric iron dominated silicate minerals are consistent with our experimental results indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt consequently the fractionation factor determined in this study is representative for the specific fe 2 fe ratio of our peralkaline rhyolitic melt of 0 38 0 02 at higher fe 2 fe ratios a smaller fractionation factor is expected further investigation on fe isotope fractionation between other mineral phases and silicate melts is needed but the presented experimental results already suggest that even at high temperatures resolvable variations in the fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

can seismic observations of bed conditions on ice streams help constrain parameters in ice flow models we investigate correlations between seismically derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface to bed inversion using a stokes ice flow model using high resolution measurements along several seismic profiles on pine island glacier pig we find no significant correlation at kilometer scale between acoustic impedance and either retrieved basal slipperiness or basal drag however there is a stronger correlation when comparing average values along the individual profiles we hypothesize that the correlation appears at the length scales over which basal variations are important to large scale ice sheet flow although the seismic technique is sensitive to the material properties of the bed at present there is no clear way of incorporating high resolution seismic measurements of bed properties on ice streams into ice flow models we conclude that more theoretical work needs to be done before constraints on mechanical conditions at the ice bed interface from acoustic impedance measurements can be of direct use to ice sheet models

high resolution reflectance spectra of mars in the 2 3 μm region evidence for the mineral scapolite spatially resolved reflectance spectra of mars in the 2 2 to 2 4 m spectral region were obtained in august 1988 using the nasa 3 m infrared telescope facility the spectra show weak absorption features due to martian atmospheric carbon monoxide and a surface mineral both co and the mineral absorptions are composed of overlapping narrow features but in many locations such as hellas chryse eden and moab the mineral absorptions are quite strong at least 3 times stronger than at the most absorbing wavelengths of co near 2 33 m therefore co complicates the analysis of the surface mineral but does not always overwhelm its signature model removal of the martian atmospheric co has been performed and the remaining absorption bands are identified as scapolite relatively strong absorptions that match bands in the spectrum of scapolite and have little or no co absorption interference are seen near 2 41 2 39 and 2 29 m absorption also occurs at the scapolite bands at 2 36 and 2 33 m but the analysis is complicated by uncertainty in the atmospheric co removal at these wavelengths weaker scapolite bands are seen at 2 44 and 2 23 m where there is virtually no atmospheric interference the scapolite bands observed on mars are due to hco3 and hso4 ions in the scapolite structure the bicarbonate and bisulfate contents appear to vary with location the scapolite in hellas is more bisulfate rich relative to that in the chryse moab eden area other locations contain little arabia syrtis major hellespontica and isidis or no scapolite e g margaritifer ausonia and erythraeum the calculated abundances are unconstrained because the amounts of hco3 and hso4 in the martian scapolites as well as their grain sizes are not known if the scapolites contain about 3 wt of each near the maximum possible the scapolite abundances probably range from about 5 wt scapolite at eden and hellas 3 5 at chryse moab and oxia palus 2 3 at arabia syrtis major and isidis to less than 2 at hellespontica syrtis minor and margaritifer assuming a relatively large grain size of 50 100 m if the characteristic grain sizes are smaller or the hco3 and hso4 contents are lower the scapolite abundances required to match the observed band depths would be higher the mineral bands are apparent in many of the mars spectra measured so it appears to be widely but not uniformly distributed the newly observed fine structure also varies greatly in both depth and spectral detail with location on mars thus there appears to be regional variations in composition the mineral phases appear to reflect local or regional geology and are not primarily contained in the homogeneous globally redistributed aeolian dust higher spectral resolution martian spectra in the 2 3 m region as well as at 3 9 m are needed to confirm the scapolite identification and to constrain its abundance

long waves through emergent coastal vegetation we study the effects of emergent coastal forests on the propagation of long surface waves of small amplitude the forest is idealized by an array of vertical cylinders simple models are employed to represent bed friction and to simulate turbulence generated by flow through the tree trunks a multi scale homogenization analysis similar to that for seepage flows is carried out to deduce the effective equations on the macro scale the effective coefficients are calculated by numerically solving the micro scale problem in a unit cell surrounding one or several cylinders analytical and numerical solutions for wave attenuation on the macro scale for different bathymetries and coastal forest configurations are presented for a transient incident wave analytical results are discussed for the damping of a leading tsunami for comparison series of laboratory data for periodic and transient incident waves are also presented good agreement is found even though some of the measured waves are short or nonlinear

sulfur partitioning between apatite and melt and effect of sulfur on apatite solubility at oxidizing conditions the effect of sulfur on phosphorus solubility in rhyolitic melt and the sulfur distribution between apatite anhydrite melt and fluid have been determined at 200 mpa and 800 1 100 c via apatite crystallization and dissolution experiments the presence of a small amount of sulfur in the system 0 5 wt s under oxidizing conditions increases the solubility of phosphorus in the melt probably due to changing calcium activity in the melt as a result of the formation of ca s complexing cations apatite solubility geothermometers tend to overestimate temperature in ca poor s bearing system at oxidizing conditions in crystallization experiments the sulfur content in apatite decreases with decreasing temperature and also with decreasing sulfur content of the melt the sulfur partition coefficient between apatite and rhyolitic melt increases with decreasing temperature kdsapatite melt 4 5 14 2 at t 1 100 900 c under sulfur undersaturated conditions no anhydrite the sulfur partition coefficient is lower in anhydrite saturated melt 8 at 800 c than in anhydrite undersaturated melt suggesting that kdsapatite melt depends not only on the temperature but also on the sulfur content of the melt these first results indicate that the sulfur content in apatite can be used to track the evolution of sulfur content in a magmatic system at oxidizing conditions

an evaluation of the solubility of sulfide sulfur in silicate melts from experimental data and natural samples abstract an evaluation of the solubility of sulfur in silicate melts from experimental data obtained under a wide variety of experimental conditions has been undertaken under reducing conditions fo2 below qfm sulfur is present as sulfide ii oxidation state and precipitates as pyrrhotite or an fe s o sulfide phase when the sulfur concentration in the silicate melt is sufficiently high the basic solubility reaction is 1 2 s 2 g o 2 silicate melt 1 2 o 2 g s 2 silicate melt for silicate melts with high feo contents 10 wt feo sulfur dissolves as an fe3so2 or 2feo fes species when saturated with pyrrhotite fes sulfide melt 2feo silicate melt fe 3 so 2 silicate melt log k log x s 21ogx feo 0 8 at 1200 c 1 bar under saturated and unsaturated conditions 1 2 s 2 g 3feo silicate melt fe 3 so 2 silicate melt 1 2 o 2 g log k log x s 1 2 1og o 2 3 log x feo 1 2 log s 2 4 1 at 1200 c 1 bar log k also shows a positive pressure dependence 0 1 per kbar for silicate melts with low feo contents fes sulfide melt o 2 silicate melt feo silicate melt s 2 silicate melt log k log x s log x feo 5 8 9007ndash 1000 c 1 6 kbar for silicate melts with intermediate feo contents 1 10 wt feo the behavior of sulfur is more complex with sulfur probably present as an fes species fes sulfide melt fes silicate melt log k log x s 1 05 3000 t k 850 1200 c 1 bar 2 2 kbar the effects of temperature pressure melt composition and other experimental conditions have been assessed the ability to predict the solubility of sulfur in silicate melts has implications for the formation of ore deposits associated with igneous systems application of the results to natural systems shows that some samples have a concentration of sulfur higher than that which could have been dissolved in the silicate melt indicating that the magma carried excess sulfur and or sulfur has been added after crystallization of the magma

effects of fo2 fs2 temperature and melt composition on fe ni exchange between olivine and sulfide liquid implications for natural olivine sulfide assemblages the apparent equilibrium constant for the exchange of fe and ni between coexisting olivine and sulfide liquid kd xnis xfes liquid xnisi12o2 xfesi12o2 olivine xi mole fraction has been measured at controlled oxygen and sulfur fugacities fo2 10 8 1 to 10 10 and fs2 10 0 9 to 10 1 7 over the temperature range 1200 to 1385 c with 5 to 37 wt ni and 7 to 18 wt cu in the sulfide liquid at log fo2 of 8 7 0 1 and log fs2 of 0 9 to 1 7 kd is relatively insensitive to sulfur fugacity but comparison with previous results shows that kd increases at very low sulfur fugacities kd values show an increase with the nickel content of the sulfide liquid but this effect is more complex than found previously and is greatest at log fo2 of 8 1 lessens with decreasing fo2 and kd becomes independent of melt ni content at log fo2 9 5 the origin of this variation in kd with fo2 and fs2 is most likely the result of nonideal mixing of fe and ni species in the sulfide liquid such behavior causes activity coefficients to change with either melt oxygen content or metal sulfur ratio effects that are well documented for metal rich sulfide melts application of these experimental results to natural samples shows that the relatively large dispersion that exists in kd values from different olivine sulfide saturated rock suites can be interpreted as arising from variations in fo2 fs2 and the nickel content of the sulfide liquid estimates of fo2 based on kd and sulfide melt composition in natural samples yields a range from fayalite magnetite quartz fmq 1 to fmq 2 or lower which is in good agreement with previous values determined for oceanic basalts that use glass ferric ferrous ratios anomalously high kd values recorded in some suites such as disko island probably reflect low fs2 during sulfide saturation which is consistent with indications of low fo2 for those samples it is concluded that the variation in kd values from natural samples reflects olivine sulfide melt equilibrium at conditions within the t fo2 fs2 range of terrestrial mafic magmas

experimental constraints on pre eruptive conditions of phonolitic magma from the caldera forming el abrigo eruption tenerife canary islands abstract we have performed phase equilibrium experiments to determine the pre eruptive conditions of the largest caldera forming eruption 20 km 3 of dre that has occurred on tenerife canary islands the abrigo ignimbrite is phonolitic and was erupted during the last caldera forming episode ca 190 ka prior to the construction of the teide pico viejo active central complex the abrigo phonolite contains low amounts of phenocrysts ca 7 vol mainly sanidine biotite clinopyroxene and magnetite with minor amounts of titanite apatite and hauyne we have investigated pressures from 50 to 250 mpa temperatures of 700 900 c water contents in the melt from 1 5 to ca 10 wt and oxygen fugacity f o 2 from 1 log unit above the ni nio solid buffer to quartz magnetite fayalite comparison of the natural and experimental phase proportions and compositions indicates that the phonolite at the roof of the abrigo magma reservoir was at 130 50 mpa corresponding to ca 4 5 km below the surface 825 25 c with 3 1 wt dissolved h 2 o and f o 2 at the ni nio buffer 1 log unit this shows that the magma that produced the largest ignimbrite on tenerife was stored at relatively shallow depths but was water undersaturated and its eruption was probably triggered by input of fresh mafic magma comparison of the abrigo phase equilibrium results with those obtained in other phonolitic magmas e g eifel kerguelen somma vesuvius shows that the major element and probably fluid composition h 2 o cl and s may have a strong effect on the presence or absence of phases and their compositions extrapolating phase equilibria results obtained on a specific phonolite to constrain the pre eruptive conditions of other phonolites should be done with care

ice sheet velocity mapping a combined interferometric and speckle tracking approach the first and second radarsat antarctic mapping missions amm 1 and 2 have now acquired interferometric synthetic aperture radar sar overmuch of the ice sheet the radarsat 24 day repeat cycle is nearly ideal for measuring slow ice motion e g 1 but application of sar interferometry is limited in faster moving areas with a 1day repeat period ers 1 2 tandem sar data are much better matched to fast motion but are not always available fortunately several authors have demonstrated the ability to measure velocity in fast moving areas by tracking sar speckle from image to image which works well even in the absence of visible features while these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements they serve well in areas where there are no data suitable for conventional interferometry this paper describes algorithms i have developed for merging interferometric and speckle tracking data from multiple swaths to form a single seamless mosaic of velocity at each point in the mosaic all the available data are combined to produce estimates of the velocity and the associated error this technique is demonstrated using radarsat data collected over lambert glacier antarctica during amm 1 and 2

cl br of scapolite as a fluid tracer in the earth s crust insights into fluid sources in the mary kathleen fold belt mt isa inlier australia a combination of analytical methods including trace element analysis of br in scapolite by la icp ms was employed to unravel the fluid rock interaction history of the mary kathleen fold belt of northern australia halogen ratios in the metamorphic and hydrothermally derived scapolite from a range of rock types record interaction between the host rocks and magmatic hydrothermal fluids derived from granite plutons and regional metamorphism the results show that halite dissolution supplied at best only minor chlorine to fluids in the fold belt chlorine bromine ratios in metamorphic scapolite indicate that fluids were dominantly derived from basinal brines formed from sub aerial evaporation of seawater beyond the point of halite saturation this bittern fluid infiltrated the under lying sedimentary sequences prior to regional metamorphism zoned scapolite in a major late meta morphic mineralized shear zone records three discrete pulses of magmatic and metamorphic fluid and it is suggested that fluid mixing may have assisted mineralization along and around this shear zone as a crucial prerequisite for halogen fluid tracer studies using scapolite we find in our samples that cl and br do not fractionate when incorporated in scapolite furthermore unaltered rims of heavily retrogressed scapolite show indistinguishable cl br signatures compared with fresh grains from the same sample indicating retrograde metamorphism did not significantly affect cl and br signatures in scapolite group minerals

shear induced instabilities of flows through submerged vegetation we consider the instabilities of flows through a submerged canopy and show how the full governing equations of the fluid structure interactions can be reduced to a compact framework that captures many key features of vegetative flow first by modelling the canopy as a collection of homogeneous elastic beams we predict the steady configuration of the plants in response to a unidirectional flow this treatment couples the beam equations in the canopy to the fluid momentum equations subsequently a linear stability analysis suggests new insights into the development of instabilities at the surface of the vegetative region in particular we show that shear at the top of the canopy is a dominant factor in determining the onset of instabilities known as monami based on numerical and asymptotic analysis of the quadratic eigenvalue problem the system is shown to be stable if the canopy is sufficiently sparse

sulfur evolution of oxidized arc magmas as recorded in apatite from a porphyry copper batholith uniformly sulfur rich cores abruptly zoned to sulfur poor rims 1 to 0 2 wt so3 in apatite from the yerington batholith nevada indicate that early magma that is crystal poor oxidizing and sulfate rich evolved to sulfate poor magma via crystallization of anhydrite a mineral observed in magmas from pinatubo and el chichon we predict that the characteristic zonation to sulfur poor rims of apatite in the yerington batholith is common in other oxidized hydrous calc alkaline magmas and can be used to track cryptic anhydrite saturation as well as to monitor sulfur evolution sulfate rich arc magmas such as yerington magmas may crystallize to produce hydrothermal fluids rich in chlorine copper and sulfur and porphyry copper ores

high ti amphibole as a petrogenetic indicator of magma chemistry evidence for mildly alkalic hybrid melts during evolution of variscan basic ultrabasic magmatism of central iberia central iberian variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous high k calc alkaline ultrabasic to intermediate rock series despite their overall calc alkaline affinity the mafic ultramafic members contain high ti amphibole oikocrysts rimmed by lower ti amphibole cummingtonite and high ti amphibole replacing early phlogopite to understand the factors controlling the saturation of high ti amphibole in the parental magmas clinopyroxene melt phlogopite melt and amphibole melt relationships are reviewed this analysis reveals that for melts with intermediate compositions the affinity of tio2 for amphibole rises in alkalic magmas accordingly mildly alkalic trachytoid to subalkaline medium to high k andesite and dacite compositions are estimated for interstitial high ti amphibole saturated melts amphibole ce pb ratios reveal a mantle crust hybrid nature for interstitial melts with subalkaline trachytoid compositions the hydrous character of the variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of na2o with respect to silica during amphibole crystallization

mixing layers and coherent structures in vegetated aquatic flows 1 to date flow through submerged aquatic vegetation has largely been viewed as perturbed boundary layer flow with vegetative drag treated as an extension of bed drag however recent studies of terrestrial canopies demonstrate that the flow structure within and just above an unconfined canopy more strongly resembles a mixing layer than a boundary layer this paper presents laboratory measurements obtained from a scaled seagrass model that demonstrate the applicability of the mixing layer analogy to aquatic systems specifically all vertical profiles of mean velocity contained an inflection point which makes the flow susceptible to kelvin helmholtz instability this instability leads to the generation of large coherent vortices within the mixing layer observed in the model at frequencies between 0 01 and 0 11 hz which dominate the vertical transport of momentum through the layer the downstream advection of these vortices is shown to cause the progressive coherent waving of aquatic vegetation known as the monami when the monami is present the turbulent vertical transport of momentum is enhanced with turbulent stresses penetrating an additional 30 of the plant height into the canopy

microphenocrystic pyrrhotite from dacite rocks of satsuma iwojima southwest kyushu japan and the solubility of sulfur in dacite magma microphenocrystic pyrrhotites were observed in the glassy groundmass of two dacite rocks from satsuma iwojima southwest kyushu japan it suggests that the dacite magma was saturated with respect to pyrrhotite at the time of eruption and thus the sulfur contents in the groundmass can be taken as the solubility of sulfur in the dacite magma the solubility of sulfur in the dacite rocks thus calculated is 65 to 72 ppm sulfur at the estimated conditions of t 900 50 c f text o text 2 10 12 pm 1 and f text s text 2 10 0 6 pm 0 6 atm

application of control methods for modelling the flow of pine island glacier west antarctica the distribution of basal traction on a transect along pine island glacier west antarctica is estimated by inverting observed surface velocities using a control method and a simple numerical stream flow model this model calculates the horizontal flow along a transect based on the assumptions that the horizontal flow is independent of ice depth and that the driving stresses are balanced by resistive forces at the glacier bed and margin and by gradients in longitudinal stress basal traction is assumed to be linearly related to the basal velocity for the lateral shear traction a parameterization based on an inversion of glen s flow law is used the applicationof the control method allowsus to calculate the set of model parameters e g the basal friction coefficient that gives the best fit betweenmodelled andobserved surface velocities themodel is used to investigate the stress regime of pine island glacier in particular to estimate the importance of basal lateral and longitudinal stresses relative to each other in the flat region just behind the grounding line basal drag lateral drag and the longitudinal stress gradient are the same order of magnitude in the steep region up glacier from the grounding line the driving stresses are highest and balanced predominantly by basal resistive stresses further upstream in the trunk of the glacier lateral and basal drag predominate

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock