a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

a physical theory of snow gliding snow glide slip of the entire snowpack over the ground is not observed unless water reaches the ground interface in this paper snow gliding is approached from the point of view that a perfect s

mechanical properties of snow abstract the investigation of the mechanical properties of seasonal snow cover aims mostly at applications in avalanche release and avalanche control but also at no less important problems such as vehicle mobility in snow snow removal or construction on snow primary needs are 1 constitutive equations that is relations between the stress tensor and the motion and 2 fracture criteria which limit the region of validity of constitutive equations both can be tackled from the aspect of continiuum theories and structure theories with modern continiuum theories the characteristic nonlinear behavior of snow can be taken into account and also the strong dependence on stress and strain history when thermodynamics is introduced more insight into the deformation and fracture processes can be gained high initial deformation rates cause low dissipation elastic behavior and brittle fracture whereas when dissipative mechanisms can develop ductile fracture occurs the advantage of structural theories lies in the immediate physical insight into deformation mechanisms but the disadvantage is that only simple states of stresses acting macroscopically on a snow sample can be considered different approaches have been elaborated for low density snow the concept of chains a series of stress bearing grains or the neck growth model consideration of stress concentrations in bonds between grains and for high density snow the pore collapse model snow idealized as a material containing air voids structural constitutive equations were applied to the calculation of stress waves in snow recorded acoustic emissions indicating intergranular bond fractures can also be used for the construction of constitutive equations

retrograde p t t path for the voltri massif eclogites ligurian alps italy some tectonic implications the retrograde p t trajectory of the eclogitic fe ti gabbros from the ligurian alps is constrained by the appearance of mineral parageneses post dating the na clinopyroxene garnet eclogitic assemblage and indicating the following sequence of metamorphic events 1 amphibolitic stage edenite katophorite plagioclase an33 43 oxides in symplectitic aggregates 2 glaucophanic stage a porphyroblastic glaucophanic amphibole has overgrown the symplectite winchite also occurs as thin rims around glaucophane and both amphiboles are sometimes armoured by atoll garnets 3 albite amphibolite stage barroisite katophorite albite epidote oxides chlorite overprint the glaucophanic stage minerals 4 greenschist stage represented by actinolite albite epidote oxide paragenesis the metamorphic evolution is complex and the decompression path on a p t diagram is significantly different from those defined in the literature for the voltri eclogites the main features inferred from the p t path are the following 1 the pressure climax does not match the thermal climax the maximum temperature conditions are in fact achieved during the early stage of uplift 2 a decrease in temperature suggested by the appearance of glaucophane after the amphibolitic symplectite 3 successive uplift probably accompanied by an increase in temperature the complexity of the p t path drawn for the voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost sector of the ligurian alps

numerical modelling of arc continent collision application to taiwan two dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during arc continent collision continental margin subduction this plate has elasto plastic rheology with strain weakening and contains a volcanic arc with thinned and weakened lithosphere the overriding plate deforms due to the normal and tangential stresses applied along the interplate surface these boundary conditions represent the friction and pressure between the plates the latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy force generated by the subducting continental crust the modelling shows that continental margin subduction results in increasing compression and failure of the overriding plate which occurs along the surface dipping under the arc in either of two possible directions the failure mode is largely controlled by the two competitive factors the rigidity of the subducting plate and thickness gradient of the subducted continental crust a high rigidity favors failure along an ocean vergent fault which is followed by a subduction reversal while a high thickness gradient favors failure in the opposite direction which is followed by a fore arc block underthrusting beneath the arc both scenarios seem to have natural analogs we consider one of them the ongoing arc continent collision in taiwan and argue that this process occurs according to the second scenario corresponding to the fore arc underthrusting wavelet statistical analysis of the seismicity distribution to the south of taiwan has clearly displayed a shallow 0 40 km zone of high density coherently distributed seismicity beneath the luzon arc this zone interpreted as a lithospheric scale fault dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduction a self consistent combined mechanical gravity topography model is used to see whether failure of the overriding plate to the south of taiwan can be captured by this model by tuning diverent controlling parameters we did not succeed in obtaining realistic topography and gravity field in a model where failure of the overriding plate was not allowed introduction of this failure and underthrusting of the forearc block under the luzon arc allowed us to fit both topography and gravity data 2000 elsevier science b v all rights reserved

submarine hydrothermal activity along the mid kermadec arc new zealand large scale effects on venting 1 the 2 500 km kermadec tonga arc is the longest submarine arc on the planet here we report on the second of a series of cruises designed to investigate large scale controls on active hydrothermal venting on this arc the 2002 nzaplume ii cruise surveyed 12 submarine volcanic centers along 580 km of the middle kermadec arc mka extending a 1999 cruise that surveyed 260 km of the southern kermadec arc ska average spacing between volcanic centers increases northward from 30 km on backarc crust along the ska to 45 km on backarc crust along the southern mka to 58 km where the mka joins the kermadec ridge volcanic cones dominate in the backarc and calderas dominate the kermadec ridge the incidence of venting is higher along the mka 83 10 of 12 volcanic centers than the ska 67 8 of 12 but the relative intensity of venting as given by plume thickness areal extent and concentration of dissolved gases and ionic species is generally weaker in the mka this pattern may reflect subduction of the 17 km thick oceanic hikurangi plateau beneath the ska subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the ska conversely volcanic centers in the southern mka are starved of magma replenishment and so their hydrothermal systems are waning farther north where the mka centers merge with the kermadec ridge fewer but larger magma bodies accumulate in the thicker older crust ensuring more widely separated caldera dominated volcanic centers

halogen behaviour in subduction zones eclogite facies rocks from the western and central alps abstract we examined f cl br and i concentrations and distributions in eclogite facies rocks and minerals from the western and central alpine ophiolitic zone to determine halogen behaviour in subduction zones and to identify potential host phases that may be able to transport halogens to the deeper mantle analysis was carried out on a range of ophiolitic lithologies peridotites serpentinites metagabbros metabasalts and metasediments to assess the distribution of halogens within deeply subducted oceanic crust halogen abundances in individual mineral phases range from below detection 100 ppm to 1900 ppm for f 1 to 3000 ppm for cl 1 to 11 000 ppb for br and from

a subsolidus olivine water solubility equation for the earth s upper mantle the pressure and temperature sensitivity of the two most important point hydrous defects in mantle olivine involving si vacancies associated to trace amounts of titanium tichu pd or exclusively to si vacancies si was investigated at subsolidus conditions in a fluid saturated natural peridotite from 0 5 to 6 gpa approximately 20 200 km depth at 750 to 1050 c water contents in olivine were monitored in sandwich experiments with a fertile serpentine layer in the middle and olivine and pyroxene sensor layers at the border textures and mineral compositions provide evidence that olivine completely recrystallized during the weeklong experiments whereas pyroxenes displayed only partial equilibration a site specific water solubility law for olivine has been formulated based on the experiments reconciling previous contradictory results from low and high pressure experiments the site specific solubility laws permit to constrain water incorporation into olivine in the subducting slab and the mantle wedge as these are rare locations on earth where fluid present conditions exist chlorite dehydration in the hydrated slab is roughly parallel to the isopleth of 50 20 ppm wt h2o in olivine a value which is independent of the pressure and temperature trajectory followed by the slab hydrous defects are dominated by si under the relevant conditions for the mantle wedge affected by fluids coming from the slab dehydration slab adjacent low viscosity seismic low velocity channel p 3 gpa in cold subduction zones at 5 5 km from the slab surface the storage capacity of the mantle wedge at depths of 100 250 km ranges from 400 to 2 000 ppm wt h2o

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

tracking arc continent collision subduction zone processes from high pressure rocks in the southern urals subduction related high pressure metamorphic rocks in the southern urals confirm material flux was active within the subduction zone suprasubduction zone ophiolitic material was tectonically eroded in an intraoceanic setting metamorphosed under mantle conditions and exhumed with the entrance of the east european craton into the subduction zone the upper continental crust was removed at a shallow level whereas middle crustal material was subducted to mantle depths metamorphosed under eclogite facies conditions and interacted with the mantle wedge eclogite bearing rocks were exhumed to an upper mantle level and juxtaposed against material that was tectonically eroded from the fore arc region of the upper plate

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

petrology and geochemistry of the lower ocean crust formed at the east pacific rise and exposed at hess deep a synthesis and new results 1 the geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle how these melts mix and crystallize and the role of hydrothermal circulation in this portion of the crust unfortunately lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes at hess deep crust formed at the east pacific rise epr is exposed due to the propagation of the cocos nazca spreading center westward here we review our state of knowledge of the petrology of lower crustal material from hess deep and document new mineral major and trace element compositions amphibole plagioclase thermometry and plagioclase crystal size distributions samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive moderate cr high mg orthopyroxene and very calcic plagioclase because primitive mid ocean ridge basalts morbs are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle these apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction the occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal morb in the axial magma chamber amc but rather that some melts partially crystallize in isolation within the lower crust however evidence that primitive melts fed the amc along with steep fabrics in shallow gabbros from near the base of the dyke complex provides support for models in which crystallization within the amc followed by crystal subsidence is also an important process in lower crustal accretion more evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust amphibole plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high temperatures in the shallow gabbros tave 713 c and show that the temperature of amphibole formation was similar in deeper gabbros tave 722 c this thermometry also suggests that fracture and grain boundary permeability for seawater derived fluids was open over the same temperature interval

a potential method to confirm the previous existence of lawsonite in eclogite the mass imbalance of sr and lrees in multistage epidote ganghe dabie uhp terrane lawsonite is an important hydrous mineral that is stable at low temperature lt and high to ultrahigh pressure hp uhp conditions in subducted slabs the occurrence absence of lawsonite in eclogite is a significant constraint for the construction of the metamorphic tectonic and fluid melt evolution histories of an hp uhp terrane however lawsonite is very rarely preserved in natural eclogites and accurate judgment of its former existence is a significant challenge for petrologists at present whether lawsonite has ever existed in lawsonite absent eclogite is mainly judged by i pseudomorphs after lawsonite and ii phase equilibria modelling in this study major element and trace element distributions in multistage minerals were examined in the ganghe lawsonite absent uhp eclogite in the dabie uhp terrane eastern china this work demonstrates that the whole rock sr and light rare earth elements lrees are mainly dominated by epidote other minerals garnet omphacite quartz kyanite barroisite phengite and accessory minerals play a very limited role in the sr and lrees budgets two stages of epidote which have noticeably different sr and lrees contents were recognized in the eclogite i epidote porphyroblasts ep p core which are suspected to be the pseudomorphic mineral after lawsonite contain significantly high sr 7200 10 300 ppm and lrees 160 1300 ppm for la ii an earlier stage epidote ep in core occurs as inclusions in matrix omphacite or in omphacite inclusions in the suspected pseudomorphic minerals after lawsonite spmal this early epidote has significantly lower sr 990 1890 ppm average 1495 ppm n 17 and lrees contents 60 110 ppm for la average 91 ppm n 17 all of the existing early stage minerals predating the spmal have very low contents of sr and lrees and the total amounts of these elements in the early stage minerals do not balance those in the spmal this indicates that a missing ca al sr and lree rich mineral which was previously in equilibrium with the early stage minerals likely existed in the ganghe eclogite on the basis of the mineral geochemistry and phase equilibria modelling we confirm that the missing mineral cannot be anything but lawsonite this study indicates that examining the mass im balance of sr and lrees between multistage hp uhp epidote can be used as a potential method to confirm the previous existence of lawsonite in lawsonite absent eclogite

high pressure veins in eclogite from new caledonia and their significance for fluid migration in subduction zones abstract this paper describes the petrology and geochemistry of high pressure veins and associated metasomatic selvages that are hosted by eclogite in the pouebo eclogite melange of northern new caledonia isotopic and geochemical evidence indicates that the mafic host rock represents seafloor altered fractionated morb that underwent eclogite facies metamorphism in a subduction zone within the host rock are cm thick garnet quartz phengite veins that are enveloped by garnet poor omphacite rich selvages or bleach zones petrography thermometry oxygen isotope characteristics and mass balance calculations are used to show that the veins largely formed by fluid mediated mass transfer from the bleach zones during prograde metamorphism minerals in the veins are free of inclusions but the vein garnets preserve complex chemical zoning features that are not present in the host rock garnets vein garnets have mn and hree zoning patterns that are indicative of progressive garnet growth during prograde metamorphism whereas mg and ca contents reveal prominent sector zoning and fine scale intergrowth features we propose that the veins formed over a prolonged period during subduction by local circulation of fluid that was sourced from prograde dehydration of minerals in the host rock fluid circulation may have been driven by episodic microcracking sealing around garnet porphyroblasts which led to significant mass transfer and progressive vein growth mass balance calculations and phengite trace element compositions also require the additional of pelite derived components to the veins these components were probably introduced into the veins at conditions close to peak metamorphism via a relatively small external fluid flux this model for vein formation is consistent with previous studies that suggest fluid flow in deeply subducted oceanic crust is highly restricted in many cases the delay of fluid migration after hydrous mineral breakdown may provide an important source of water for arc magmas as free fluid is transported to deeper levels in subduction zones

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

the importance of blueschist eclogite dehydration reactions in subducting oceanic crust the metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure temperature p t paths with a model of metabasalt phase equilibria in steady state subduction zones with high rates of shear heating the upper parts of the subducting oceanic crust progress through the greenschist amphibolite granulite eclogite facies whereas lower parts of the subducting oceanic crust progress through the blueschist eclogite facies in steady state subduction zones with moderate rates of shear heating most of the subducting oceanic crust passes through the blueschist eclogite transition in steady state subduction zones with low rates of shear heating the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km for oceanic crust containing 1 2 wt h 2 o dehydration will not begin until the onset of eclogite or amphibolite facies metamorphism depending on the p t path for many subduction zones the most important dehydration reactions in the subducting oceanic crust occur at the blueschist eclogite facies transition associated with the breakdown of lawsonite or clinozoisite glaucophane and chlorite large amounts of h 2 o released by blueschist eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas

argon oxygen and boron isotopic evidence documenting 40 ar e accumulation in phengite during water rich high pressure subduction metasomatism of continental crust abstract the luliang shan area of the north qaidam high pressure hp to ultrahigh pressure uhp metamorphic terrane in northwestern china features thick garnet and phengite rich metasomatic selvages that formed around gneiss hosted mafic eclogite blocks during hp conditions here we present new 40 ar 39 ar 18 o and 11 b results from a previously studied 30 m 18 sample traverse that extends from the host gneiss into a representative eclogite block previous thermobarometry and new mica quartz oxygen isotope thermometry from the traverse reveal that the phengite rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure quartz and white mica 18 o data from the selvage cannot be explained by simple mixing of gneiss and eclogite and indicate a fluid rock ratio 1 during regional scale infiltration of high 18 o ca 14 fluids heavy 18 o overgrowths of metamorphic zircon over lighter 18 o detrital grains indicate that the gneiss was similarly affected starkly contrasting boron content and 11 b compositions for the host gneiss and the selvage also cannot be explained by local scale devolatilization of the gneiss to form the selvage instead the boron systematics are best attributed to two distinct phases of fluid infiltration 1 low boron selvage phengite with 11 b from 10 to 30 grew under hp conditions and 2 tourmaline and boron rich muscovite with generally positive 11 b crystallized in the host gneiss under subsequent lower pressure epidote amphibolite facies conditions as the luliang shan gneiss terrane was exhumed past shallower portions of the subduction channel consistent with observations made worldwide we were able to identify uptake of excess argon 40 ar e in phengite as a high pressure phenomenon phengite 40 ar 39 ar ages from massive eclogite exceed the ca 490 ma zircon u pb age of eclogite metamorphism by a factor of 1 5 however phengite ages from the more permeable schistose selvage were even older exceeding the time of eclogite formation by a factor of 1 7 in contrast lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 ar 39 ar ages that were younger than the time of hp metamorphism and consistent with regional cooling age patterns our observation of high 40 ar e concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at hp conditions runs contrary to widely held expectations conventional wisdom dictates that low phengite fluid partition coefficients for argon d phg fluid ar 10 3 to 10 5 coupled with the dry closed systems conditions that are widely reported to characterize hp metamorphism of continental crust explains why high concentrations of 40 ar e partitions are able to accumulate within phengite we alternatively propose that phengite fluid partition coefficients for argon increase linearly with pressure to values as high as 10 2 to allow phengites to accumulate large amounts of 40 ar e from aqueous fluids under hp to uhp conditions

glide avalanche response to an extreme rain on snow event snoqualmie pass washington usa rain on snow events trigger immediate and delayed avalanches as liquid water penetrates the snowpack we present results from an extreme rain on snow event that triggered a glide avalanche near snoqualmie pass washington usa snoqualmie pass recorded 463 cm of snowfall from 13 december 2008 to 6 january 2009 this period of snowfall was followed by a strong southwesterly tropical flow that resulted in an extreme rain on snow event sensors at snoqualmie pass recorded 285 mm of precipitation over a 52 hour period flooding slush flows landslides and avalanches resulted from the influx of precipitation snow heights decreased rapidly over the period with settlement rates approaching 80 mm h 1 liquid water infiltrated and flowed through the snowpack within a few hours of the arrival of rain yet many of the major avalanches occurred 12 30 or more hours after the onset of rain and water outflow a glide avalanche occurred 30 hours after the onset of rain and the establishment of drainage through the snowpack increasing glide rates correlate with periods of rapid snow settlement here glide rates approached 670 mm h 1 although glide and settlement rates increased during periods of intense precipitation glide failure occurred 8 hours after peak precipitation and outflow

translithospheric mantle diapirism geological evidence and numerical modelling of the kondyor zoned ultramafic complex russian far east we report new structural microstructural petrological and major and trace element data on ultramafic rocks from the kondyor zoned ultramafic complex in far east russia the ultramafic rocks are subdivided into three subconcentric lithologies from core to rim 1 a metasomatic domain where generally phlogopite rich dykes pervasively intrude dunite 2 a main dunite core 3 a pyroxenite rim the ultramafic rocks have nearly vertical contacts with the surrounding archaean basement gneisses quartzites and marbles and hornfelsed riphean sediments the hornfelsed sediments show a relatively steep 60 outward dipping layering which rapidly flattens to horizontal away from the inner contact although the riphean sediments define a dome like structure the inward shallow dipping foliation of the dunites indicates a synformal structure detailed petro structural investigations indicate that the kondyor dunites were deformed by solid state flow under asthenospheric mantle conditions the outward textural change from coarse to fine grained equigranular dunite and the outward increasing abundance of subgrains and recrystallized olivine grains suggest dynamic recrystallization while fluid circulation was channelized within the core metasomatic zone with a decreasing melt fraction from core to rim and also suggest that solid state deformation induced grain size reduction towards the cooling border of the kondyor massif based on their geochemistry the dunites are interpreted as mantle rocks strongly affected by reaction with melts similar to the jurassiccretaceous aldan shield lamproites rim pyroxenites were formed by a melt consuming peritectic reaction implying the existence of at least a small conductive thermal gradient around the dunite body while the latter was still at near solidus temperature conditions this suggests that the zoned structure of kondyor was initiated at mantle depths most probably within the subcontinental lithosphere upon cooling the lamproitic melts were progressively focused in the central part of the massif and drained into vein conduits where they reacted with the wall rock dunite two dimensional numerical modelling based on finite differences with a marker in cell technique incorporates temperature dependent rheologies for both molten and non molten host rocks the modelling consolidates the structural petrological and geochemical interpretations which show that the dunites represent the synformal flat lying apex of an asthenospheric mantle diapir triggered by fluid pressure channelized in the core which nearly reached the earths surface we conclude that translithospheric mantle diapirism is an important mode of mass transfer in the earth

talc magnesite and ti clinohumite bearing ultrahigh pressure meta mafic and ultramafic complex in the dabie mountains china

experimental constraints on recycling of potassium from subducted oceanic crust petrological experiments on oceanic crust samples characterize the recycling of potassium from mid ocean ridge basalts and sediments metasomatism could develop directly and continuously from subducted potassium bearing crust from shallow levels to a maximum depth of 300 kilometers phengite a potassium rich mica is the principal potassium host at subsolidus conditions it transports potassium and water to depths of up to 300 kilometers and could yield over the entire depth range potassium rich fluids or melts depending on the specific geotherm which are likely to constitute one of the primary metasomatic agents for generation of calc alkaline magmas

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

fate of the subducted farallon plate inferred from eclogite xenoliths in the colorado plateau we present the first finding of the high pressure mineral coesite in lawsonite bearing eclogite xenoliths from the colorado plateau united states the presence of coesite in these xenoliths supports the hypothesis that the eclogite formed in a low temperature high pressure environment such as envisaged inside subducted oceanic lithosphere ion microprobe u pb dating of micrometer scale zircons in the eclogites yields ages ranging from 81 ma to 33 ma the two extremes in age likely indicating the age of crystallization during subduction related metamorphism and the age of recrystallization by the host magmatic event respectively these observations conclusively demonstrate that certain eclogite xenoliths from the colorado plateau originated as fragments of the subducted farallon plate which had been residing in the upper mantle since the late cretaceous this is the first conclusive evidence that any eclogite xenoliths can be directly linked to a known subducted plate

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

structure petrology and metamorphic history of the nome group blueschist terrane salmon lake area seward peninsula alaska an 80 mi 2 area adjacent to salmon lake mapped at a scale of 1 36 680 includes a section of the nome group schist that is at least 2 000 m thick it is relatively intact with relict primary lithologic layering and not melange like or highly disrupted by faults the nome group metasediments include pelitic schist graphitic quartzite pure and impure marble calcareous schist chlorite albite schist chloritic quartz albite and mafic schists metaigneous rocks include fe ti rich metabasite and granitic to tonalitic orthogneiss the protoliths are precambrian to paleozoic in age and radiometric ages indicate that the high pressure metamorphism of the nome group is late jurassic blueschist facies assemblages occur in nome group metabasites and in most metasedimentary schists the blueschists are synkinematic they formed during an episode of widely distributed penetrative deformation and mesoscopic intrafolial isoclinal folding of consistent orientation the nome group of the salmon lake area underwent a prograde monocyclic polyfacial high p t metamorphism initial metamorphism was in the lower t regime of the lawsonite glaucophane field 300 c 7 kb and prograded with increasing t and p into the epidote almandine glaucophane field through an apparent p t maximum as indicated by the assemblage glaucophane omphacite garnet 420 c 9 10 kb the later stages of the metamorphism were characterized by increased t p likely during initial stages of decompression and the return of the normal geothermal gradient to the transition between the blueschist facies and the high p albite epiclote barroisite field 450 to 525 c 7 8 kb this high p t cycle was followed by an incipient static retrogressive metamorphism at low greenschist facies conditions during the later uplift of the nome group in the late cretaceous and early tertiary

water circulation and global mantle dynamics insight from numerical modeling we investigate water circulation and its dynamical effects on global scale mantle dynamics in numerical thermochemical mantle convection simulations both dehydration hydration processes and dehydration melting are included we also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed due to effective slab dehydration at shallow depths water still affects significantly the global dynamics by weakening the near surface oceanic crust and lithosphere enhancing the activity of surface plate motion compared to dry mantle case as a result including hydrous minerals the more viscous mantle is expected with several orders of magnitude compared to the dry mantle the average water content in the whole mantle is regulated by the dehydration hydration process the large scale thermochemical anomalies as is observed in the deep mantle is found when a large density contrast between basaltic material and ambient mantle is assumed 4 5 comparable to mineral physics measurements through this study the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection

the southern tyrrhenian subduction zone deep geometry magmatism and plio pleistocene evolution abstract we report on a high resolution vp vp vs and qp model of the southern tyrrhenian subduction zone obtained by the inversion of p and s wave arrival times and t values from intraslab seismicity the arcuate shape of the southern apennines calabrian arc sicilian maghrebides is perfectly mirrored by two rather continuous low and high vp bands lying beneath the belt system at ca 25 and 100 km respectively between 100 and 300 km two independent high vp slabs lie beneath the neapolitan region and the southern tyrrhenian sea separated by unperturbed mantle we suggest that the ca 150 km wide slab window beneath the southern apennines opened after a tear occurring within a composite subduction system formed by the apulian continental lithosphere and the ionian oceanic slab the abrupt slab rupture induced ultrafast southeastward retreat of the ionian slab and the 19 cm yr spreading of the back arc oceanic marsili basin between ca 2 1 and 1 6 ma ago the 25 km low vp zone beneath the arc denotes continental upper crustal rocks below the chain its striking continuity requires a unique orogenic wedge at 25 km depth below the southern apennines the calabrian arc and the sicilian maghrebides the alternative explanation would imply the ubiquitous occurrence of autochthonous lower plate rocks at 25 km depth i e a puzzling autochthonous continental calabria the ionian slab beneath calabria shows high vp high qp and low vp vs anomalies typical of old oceanic lithosphere intermediate depth seismicity is concentrated within its thin oceanic crust suggesting the occurrence of vigorous metamorphism the slab dehydration promotes the melting of the overlying mantle as testified by high vp vs and low qp anomalies between the slab and the aeolian magmatic arc

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

experimental and theoretical studies of the stabilities of talc antigorite and phase a at high pressures with applications to subduction processes we have experimentally determined the equilibrium talc enstatite quartz coesite h2o to 40 kbar in the system mgosio2h2o msh using both synthetic and nearly pure mg end member natural talc and other synthetic starting materials for the other solid phases at 40 kbar the equilibrium dehydration boundary lies 150 c higher than that calculated using data from the existing internally consistent thermochemical data bases the reason for this discrepancy lies in the erroneous compressibility data of talc in the data bases we have retrieved the compressibility of talc from the experimental phase equilibrium data and have also calculated sereral other equilibria in the msh system involving talc antigorite and the dense hydrous magnesium silicate dhms commonly referred to as phase a comparison of these equilibria with selected thermal profiles at the leading edge of young and old subducting oceanic slabs along with the dehydration condition of basaltic amphibole and solidus of mantle peridotite provides an explanation for the observed heights of the volcanic fronts above subducting oceanic lithosphere further it is found that in cold oceanic slabs 50 ma with subduction velocity of 10 cm y antigorite will transform to the dhms phase a through a vapor conserved reaction at a depth of 200 km phase a will then serve as a carrier of water into the deeper mantle

guided waves at subduction zones dependencies on slab geometry receiver locations and earthquake sources summary we investigate the geometry of deep subduction zone waveguides depth 100 km the wavefield characteristics for up dip profiles are described and compared with data recorded at the chile peru subduction zone observed distorted p onsets at stations in northern chile near 21 s can be matched by 2 d finite difference simulations of a thin low velocity layer lvl atop the slab in an iasp91 velocity model the replacement of the lvl by simple random velocity undulations in the slab in the same model cannot explain the observations varying slab geometries are investigated and the distribution of guided wave onsets originating in deep waveguides is predicted relative to the slab surface further double couple source position and orientation is explored and found to be closely limited by the guided wave observations sources situated above the layer and at distances more than 2 layer widths below the subducted moho are not suitable for the remaining favourable source locations a strong link between pulse shapes and fault plane dip angle is evident we conclude that up dip guided wave observations at subduction zones follow a simple pattern given by slab geometry and modified by source position the resulting onsets are shaped by layer thickness and velocity contrast and further influenced by the shape of the slab surface

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

is phase a relevant to the earth s mantle abstract dense hydrous magnesium silicates dhms such as phase a mg 7 si 2 o 8 oh 6 are potential hosts for water in the earth s mantle previous work has delineated the stability field of phase a pha coexisting with clinoenstatite cen and forsterite fo in the system mgo sio 2 h 2 o msh in this study the effects of the presence of magnesite mc diopside di and pyrope py on the stability of coexisting pha cen fo have been determined in msh the breakdown reaction pha cen fo h 2 o was bracketed between 720 and 740 c at 7 gpa between 800 and 820 c at 8 gpa and between 860 and 880 c at 9 gpa at 10 gpa pha cen transforms to a clinohumite ch bearing assemblage between 920 and 940 c which in turn dehydrates between 940 and 960 c to fo v at higher pressures pha becomes unstable relative to phase e another dhms kanzaki 1991 addition of carbonate to msh changes the breakdown reaction to pha cen mc fo v where v is an h 2 o co 2 fluid at 8 gpa this reaction occurs between 800 and 850 c indistinguishable from the position of the co 2 free reaction in msh in the presence of diopside in the cao mgo sio 2 h 2 o system the reaction pha en ss fo di ss h 2 o occurs between 700 and 730 c at 8 gpa where en ss and di ss are ca poor and ca rich pyroxene solid solutions respectively in the pyrope containing assemblage in the mgo al 2 o 3 sio 2 h 2 o system the pha bearing assemblage reacts to form ch between 740 and 760 c at 8 gpa the ch bearing assemblage dehydrates to fo py v between 800 and 830 c at 8 gpa in the system mgo feo sio 2 h 2 o with molar mg mg fe 0 90 the reaction pha cen ss ch v occurs at ss ch fo v reaction occurs between 850 and 900 c in no case did the presence of additional components such as cao feo al 2 o 3 or co 2 or the additional phases diopside magnesite or pyrope stabilize the hydrous phases pha or ch to significantly higher temperatures therefore pha will not be a stable phase in lherzolite assemblages under normal geothermal gradients in the mantle in the interior of subducting slabs temperatures may be sufficiently low that assemblages containing pha may be stable to pressures greater than 10 gpa allowing pha to transport h 2 o to the deeper mantle it is more likely that the higher pressure phase e will be stabilized instead

full depth avalanche occurrences caused by snow gliding coquihalla british columbia canada snow glide is the translational slip of the entire snowpack over a sloping ground surface and it is thought that rapid rates of snow glide precede full depth avalanches the nature of avalanches that release at the ground makes them difficult to predict and difficult to control using explosives on slope instrumentation comprised of stainless steel glide shoes was used to measure rates of snow glide for two winters on a bedrock slope adjacent to the coquihalla highway cascade mountains british columbia canada climate data and avalanche occurrences were recorded by the british columbia ministry oftransportation and highways our results show that the supply of free water to the snow ground interface by rain or snow melt is the most important influence on full depth avalanche release full depth avalanche release responds to rainfall and snowmelt events within 12 24 hours occasionally full depth avalanches occur unexpectedly during clear cold periods snowmelt by radiation is thought to contribute enough meltwater during these cold periods to induce higher rates of snow glide and full depth avalanche release the results also indicate that snow glide alone is not a reliable indicator for full depth avalanche release

generation of felsic crust in the archean a geodynamic modeling perspective abstract as a consequence of secular cooling of the earth there is generally no modern analog to assist in understanding the tectonic style that may have operated in the archean higher mantle temperatures and higher radiogenic heat production in the archean earth would have impacted the thickness and composition of the crust for this reason well constrained numerical modeling based on the fragmentary evidence preserved in the geological record is the most appropriate tool to evaluate hypotheses of archean crust formation the main lithology of archean gray gneiss complexes is the sodic tonalite trondhjemite granodiorite ttg suite melting of hydrated basalt at garnet amphibolite granulite or eclogite facies conditions is considered to be the dominant process that generated the archean ttgs taking into account geochemical signatures of possible mantle contributions to some ttgs models proposed for the formation of archean crust include subduction melting at the bottom of thickened continental crust and fractional crystallization of mantle derived melts under water saturated conditions we evaluated these hypotheses using a 2d coupled petrological thermomechanical tectono magmatic numerical model with initial conditions appropriate to the eoarchean mesoarchean based on the result of our experiments we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust 1 delamination and dripping of the lower mafic crust into the mantle 2 local thickening of the crust and 3 small scale crustal overturns in the context of a stagnant deformable lid tectono magmatic geodynamic regime that is terminated by short lived subduction we identify two distinct types of continental crust the first type is a pristine granite greenstone like crust with dome and keel geometry formed over delaminating upwelling mantle which is mostly subjected to vertical tectonics processes by contrast the second type is a reworked accreted crust comprising strongly deformed granite greenstone and subduction related sequences and subjected to both strong horizontal shortening and vertical tectonics processes thus our study has identified a possible spatial and temporal transition from pristine granite greenstone like crust with dome and keel geometry to reworked accreted crust forming more felsic gneiss terranes in the archean we suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the archean geological record

uranium and 10 be enrichments by fluids in andean arc magmas volcanic rocks from subduction settings frequently contain 10be enrichments that have been attributed to sediment subduction1 6 and are often characterized by 238u 230th ratios greater than unity in contrast to mid ocean ridge and ocean island basalts7 8 here we report such an excess of 238u over 230th in historic lavas from the southern volcanic zone of chile 33 42 s the greatest uranium excess is found south of 39 s where 10be 9 be ratios are highest and the trench is filled with sediment these features suggest that both u and 10be enrichments result from preferential partitioning of u and be into a fluid phase during dehydration of the recently subducted sediments the presence of strong 238u 230th disequilibria in basaltic magmas of the southern volcanic zone suggests that the timescale for dehydration melting and eruption in this arc is probably less than 20 000 yr

melting experiments on lherzolite klb 1 under hydrous conditions and generation of high magnesian andesitic melts the partial melting of natural lherzolite klb 1 at 1 gpa was studied for both h2o undersaturated and saturated conditions the quenched glasses obtained at 1000 and 1050 c are andesitic in composition having 54 4 60 3 wt sio2 whereas partial melts formed at temperatures above 1100 c have basaltic compositions this suggests that high magnesian andesites can be formed by direct partial melting of the hydrous upper mantle only at relatively low temperatures these andesitic melts together with those obtained at 1100 c contain more than 3 wt h2o and have compositions characteristic of hydrous melts high sio2 and al2o3 and low feo mgo and cao the results of melting studies on klb 1 show a strong increase in the degree of melting when h2o is added to lherzolite at a given temperature

electrical conductivity of stishovite as a function of water content abstract the electrical conductivity of stishovite with various al2o3 and h2o contents was measured at 12 gpa of pressure p and temperatures t up to 1900 k in a kawai type multi anvil apparatus starting materials were pre synthesized at 12 gpa and 1673 k from various mixtures of sio2 al2o3 and al oh 3 the synthesized stishovite aggregates contained various h2o concentrations up to 0 25 wt the conductivity of relatively dry stishovite was almost constant independently of al content whereas the conductivity significantly increased with increasing h2o content in stishovite all electrical conductivity data fit the formula for electrical conductivity 0 c w exp h 0 c w 1 3 kt where 0 is the pre exponential term cw is the h2o concentration h0 is the activation enthalpy at very low h2o concentration and k is the boltzmann constant the activation enthalpy decreased from 1 22 to 0 90 ev with increasing h2o content from 0 01 to 0 22 wt a nearly linear correlation of the conductivity values on the h2o content suggests that the dominant mechanism of charge transport in stishovite is proton conduction although electrical conductivity of hydrous stishovite is higher than that of garnet in the subducted oceanic crust small amount of hydrous stishovite is insufficient to raise conductivity on the other hand hydrous stishovite can contribute to the high conductivity occasionally observed at the mantle transition zone if the subducted archean continental crusts with tonalite trondhjemite granodiorite ttg composition were accumulated above the 660 km seismic discontinuity

impact of arc continent collision on the conditions of burial and exhumation of uhp lt rocks experimental and numerical modelling abstract a 2 d physical and finite element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere the experimental technique allowed us to model the whole subduction collision process from oceanic subduction to deep subduction of the continental crust with the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process the results obtained by both techniques are coherent and mutually complementary they show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction the conditions for such a failure the weakening of this plate are prepared during oceanic subduction the weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates and due to back arc spreading in oceanic subduction zones with a compressional regime no back arc opening thick and strong back arc lithosphere the weakest zone is volcanic arc area when weakening becomes sufficient during subduction the lithosphere fails in this area the failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore arc almost half of the presently active subduction zones are characterised by a tensional subduction regime with back arc spreading in such subduction zones the weakest place is not the volcanic arc but the back arc spreading centre when a subduction regime changes from tensional to compressional failure occurs in the vicinity of the extinct spreading centre this process can occur during oceanic subduction again along either a trench vergent or trenchward dipping fault but the formation of a trench verging fault is most likely in this latter case which is a principal subject of our study the failure is followed by partial subduction of the arc plate complete subduction occurs during arc continent collision subduction of the continental margin when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle the arc itself can be subducted completely or be partially or completely scraped off and accreted a deeply subducted material including continental margin is preserved at relatively low temperatures between the lithospheric mantle and the cold subducted arc plate to about 150 km depth subduction of the arc plate is a major phenomenon which affects all processes associated with continental subduction from deep burial and hp lt metamorphism to exhumation of subducted material does this process occur in nature future investigations will allow us to answer this question in this paper we analyse the conditions of emplacement of a very young oceanic lithosphere samail ophiolite on the continental crust in oman in the late cretaceous and argue that this lithosphere formed in a back arc basin it reached and overthrust the arabian continent after complete subduction of the arc plate

overcoming the stauchwall viscoelastic stress redistribution and the start of full depth gliding snow avalanches 1 when a full depth tensile crack opens in the mountain snowcover internal forces are transferred from the fracture crown to the stauchwall the stauchwall is located at the lower limit of a gliding zone and must carry the weight of the snowcover the stauchwall can fail leading to full depth snow avalanches or it can withstand the stress redistribution the snowcover often finds a new static equilibrium despite the initial crack we present a model describing how the snowcover reacts to the sudden transfer of the forces from the crown to the stauchwall our goal is to find the conditions for failure and the start of full depth avalanches the model balances the inertial forces of the gliding snowcover with the viscoelastic response of the stauchwall we compute stresses strain rates and deformations during the stress redistribution and show that a new equilibrium state is not found directly but depends on the viscoelastic properties of the snow which are density and temperature dependent during the stress redistribution the stauchwall encounters stresses and strain rates that can be much higher than at the final equilibrium state because of the excess strain rates the stauchwall can fail in brittle compression before reaching the new equilibrium snow viscosity and the length of the gliding snow region are the two critical parameters governing the transition from stable snowpack gliding to avalanche flow the model reveals why the formation of gliding snow avalanches is height invariant and how technical measures to prevent snowpack glide can be optimized to improve avalanche mitigation

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

subduction initiation during collision induced subduction transference numerical modeling and implications for the tethyan evolution the collision induced subduction transference is a composite dynamic process including both the terrane collision accretion and the subduction initiation si at the neighboring passive margin thi

triaxial tests on snow at low strain rate part i experimental device a deformation controlled triaxial apparatus has been developed in order too investigate the mechanical behaviour of alpine snow cylindrical specimens 58 mm in diameter 126 mm in height can be axially deformed by applying tensile or compressive strain rates between 2 4 10 7 s 1 and 2 4 10 2 s 1 compressed air is used to laterally load the snow specimens with pressures of up to 40 kpa the volumetric deformation is found by carefully encapsulating the snow samples in an airtight mylar foil and measuring the expelled pore air during sample deformation the multidimensional stress strain behaviour can therefore be determined and constitutive theories of snow advanced the device is located run the cold rooms of the weissfluhjoch research station of the swiss federal institute for snow anti avalanche research seine 180 tests have been performed over the last four winters the density of the snow specimens varied between 190 and 435 kg m 3 part i of this two part paper discusses the development of the apparatus and the applied testing methodology qualitative results are provided in part ii a detailed analysis of the results is presented and the viscoclastic properties of snow quantified

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

na rich character of metasomatic metamorphic fluids inferred from preiswerkite in chromitite pods of the khoy ophiolite in iran role of chromitites as capsules of trapped fluids abstract we report a new occurrence of preiswerkite a rare na and al rich trioctahedral mica na analog of eastonite from a high al cr of chromite 0 4 chromitite pod of the khoy meta ophiolite in northwestern iran the preiswerkite is euhedral and associated with ca cr al garnet clinochlore titanite and calcite preiswerkite and associated minerals fill cavities in the chromitite pod and are absent in the surrounding peridotites the preiswerkite rich aggregate is possibly representative of aqueous fluid supplied to the mantle section of the khoy ophiolite peraluminous conditions which are indispensable for preiswerkite formation were established only in the pool of fluid trapped by the high al chromitite the fluid and preiswerkite precipitates were isolated from the peridotite by the chromitite capsule the na rich fluid was a precursor of the serpentinization fluid the fluid is an analog of the fluid released from the subducting slab which can precipitate jadeite under high pressure conditions

geochronology and eruptive history of the katmai volcanic cluster alaska peninsula abstract in the katmai district of the alaska peninsula k ar and 40ar 39ar ages have been determined for a dozen andesite dacite stratocones on the arc front and for 11 rear arc volcanoes 10 of which are monogenetic tied to mapping and stratigraphic studies our dating emphasized proximal basal lavas that rest on basement rocks in order to estimate ages of inception of each polygenetic cone oldest among arc front cones is alagogshak volcano 690 43 ka succeeded in the holocene by the active mount martin cone mount mageik consists of four overlapping subedifices basal lavas of which give ages of 93 71 and 59 ka and holocene the three small prehistoric cones of trident volcano yield ages of 143 101 58 and 44 ka falling mountain and mount cerberus dacite domes near the 1912 novarupta vent are related compositionally to the trident group and give ages of 70 ka and 114 ka mount katmai which underwent caldera collapse in 1912 consists of two subedifices that overlapped in space and time and is the only arc front center here to include basalt and rhyolite one cone began by 90 ka the other by 47 ka snowy mountain also consists of two contiguous cones which started around 200 and 171 ka respectively the younger remaining active into the holocene devils desk the only mafic cone on the arc front was short lived at about 245 ka in the rear arc 1 mount griggs produced mafic to silicic andesite in several episodes between 292 ka and the holocene 2 the savonoski river cluster includes a pliocene dacite dome and five small mafic cones 390 88 ka 3 gertrude creek cone 49 8 sio2 yields an age of 500 ka and 4 the saddlehorn creek cluster includes five pliocene basalt to andesite remnants eruptive volumes were reconstructed permitting estimates of average eruption rates for edifice lifetimes since the mid pleistocene total volume erupted along the arc front here is 210 47 km3 and in the rear arc 39 6 km3 of which mount griggs alone accounts for 35 5 km3 most productive has been mount katmai at 70 18 km3 yielding a rate of 1 km3 kyr followed by mount mageik 0 33 km3 kyr and mount griggs 0 3 km3 kyr since 50 ka

triaxial tests on snow at low strain rate part ii constitutive behaviour fine grained dry snow with a density range of 190 435 kg m 3 was tested in triaxial compression at 12 c with confining pressures varying between 0 and 40 kpa the tests were strain rate controlled with strain rates ranging between 7 4 x 10 7 s 1 and 6 6 10 5 s 1 the analysis of the test results revealed that the relationship between yield stress and viscous strain rate is best given by a power law similar to polycrystalline ice however the power law exponent n is a function of density and varies between 1 8 low density snow p 320 kg in 3 the tests also showed that lower density snow displays a significant non linear stress strain response before yielding two further aspects of the constitutive behaviour of snow were identified 1 the strain rate independence of the post yield work hardening behaviour in compression and 2 the independence of the axial yield stress in relation to the confining pressure the experimental observations are discussed with respect to the mechanical properties of polycrystalline ice which is the constituent material of the load bearing ice skeleton

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

thermal effects of gabbro accretion from a deeper second melt lens at the fast spreading east pacific rise motivated by recent observations a ridge model has been developed which includes both a shallow melt lens and a second deeper melt lens above the moho to assess the thermal effect of the gabbro accretion from the lower melt lens at the fast spreading east pacific rise modeling results argue for a limited role of the lower melt lens in constructing the layer 3 gabbros if significant 10 amount of the layer 3 gabbros is emplaced near the moho then the liberated latent heat during crystallization would generate a large molten section of the lower crust due to the inability of cold seawater to penetrate into this hot ductile region a large molten section of the lower crust is not consistent with recent seismic tomographic results of the east pacific rise thus it is likely that the amount of the gabbro being emplaced from the lower melt lens at the epr is significantly less than that suggested by oman ophiolite studies such a lower melt lens is short lived or a large portion of the sill intrusions observed at oman was emplaced at a few kilometers off axis 3 4 km

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

compaction control of melt distribution at fast spreading mid ocean ridges models of mid ocean ridge processes in which oceanic crust forms from a sill like intrusion in the upper crust provide a link between geophysical studies of mid ocean ridges and geological studies of ophiolites these models predict that the lower crust forms by subsidence of largely solidified material beneath the sill such a downwelling provides a means to counter the normal tendency of entrained melt to separate from the higher density solid phase in this case the melt distribution may be controlled by the melt fraction that is just unable to escape the downward motion as much as by thermal constraints reasonable physical parameters for such a system yield melt distributions with a good match to the observed seismic velocity structure of the east pacific rise detailed estimates of lower crustal melt distribution may help to distinguish these two controls

source composition and distribution of the fluid in the kurile mantle wedge constraints from across arc variations of b nb and b isotopes b and nb concentrations and b isotope composition were determined for lavas covering the entire length of the kurile arc b nb and 11b of these lavas show clear across arc variations in which both are highest at the trench side and continuously decrease as the slab depth increases b nb in the volcanic front lavas 8 25 is significantly higher than those of morb and oib indicating that the mantle wedge is extensively metasomatized by b rich fluid derived from the subducting slab the frontal 11b values are also out of the range of morb and oib and systematic difference exists between northern kurile nk 5 4 0 5 and central and southern kurile csk 4 3 0 3 however these two parameters approach or coincide with mantle values at the most back arc side the 11b and 87sr 86sr linearly correlate with nb b suggesting control by simple mixing of two isotopically homogeneous components that is slab derived fluid and mantle wedge the estimated 11b and 87sr 86sr of the fluid are 7 0 0 6 and 0 70326 0 00005 for nk and 6 3 0 3 and 0 7033 for csk respectively these fluid compositions are consistent with a source constituted by 95 altered oceanic crust and 5 oceanic sediment with the slight difference between nk and csk resulting from the different lithologies of subducting sediments the mantle wedge beneath the kurile arc has b sr and nd isotope compositions identical with morb but shows significantly higher b nb the above trends bear a close resemblance to those observed in the izu arc strongly suggesting their universality in the arc system the across arc decline of b nb observed both in kurile and izu implies that the fluid flux from the slab continuously decreases as the slab depth increases however this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore arc region in contrast to b k is uniformly enriched across the arc this requires control by a k bearing mineral phase either in the slab phengite or the mantle wedge phlogopite although such a mineral is responsible for only a small part of the whole fluid transport

the eclogite garnetite transformation in the morb h2o system abstract to decipher phase relations of oceanic crust in the coldest slab at the mantle transition zone multi anvil experiments were conducted in the morb h 2 o system at pressures of 10 19 gpa and temperatures of 700 1500 c garnet and stishovite were recognized in all run charges above 15 gpa garnet drastically increases nasi na 2 msi 5 o 12 component m ca mg fe 2 jadeite occurs instead of omphacite na k hollandite containing 7 mol naalsi 3 o 8 and ca perovskite with 60 mol catio 3 were observed at p 17 gpa with decomposition of omphacite and increase of modal ratio of garnet there is a sharp increase of density at 440 km the density increase due to appearance of ca perovskite at 570 km is estimated approximately 100 km shallower than that of previous estimation

rain induced planar collapse of eolian dune slipfaces abstract decameter scale eolian dunes were observed following a burst of intense rainfall in the sand sea of eastern saudi arabia planar translational collapse structures developed over large areas of the dune slipfaces and were observed in pristine condition hours after their formation individual slipped blocks were approximately 10 cm deep over areas up to one hundred square meters observed geometries indicate a sequence of deformation beginning with downslope creep progressing to propagation of updip tensile fractures finally thrust ramps developed at the downdip limits movement halted in all cases after about 10 displacement a mechanical model is developed here based in part on snow avalanches and landslides to account for the observed deformation sequence rainwater loading increases differential stress in the dry internal parts of the dune rainwater infiltration also forms an impermeable wetting front below which air increases in pressure bringing the effective stress state towards the frictional sliding criterion triggering downslope creep of the cohesive wet slab above the wet slab has cohesion and tensile strength via proliferation of intergranular rainwater fluid bridges downslope creep led to updip tensile cracks releasing the slab promoting slip and leading to downdip compressional failure with thrust structures exposure of the overpressured substrate in between the opening tensile cracks enabled pressure to bleed off ensuring the mechanism is self limiting and therefore less hazardous than snow avalanches the preservation potential of these structures is reasonably high due to their scale and likelihood of rapid burial by dune migration but the distribution is patchy due to pre existing moisture content of the dunes inhibiting the process and the footprint of intense rainstorms usually being relatively small compared with the sand seas

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

the stability and composition of phengitic muscovite and associated phases from 5 5 to 11 gpa implications for deeply subducted sediments abstract the stability and composition of phengitic muscovite was investigated from 5 5 11 gpa 700 1150 c in synthesis experiments performed in a multianvil apparatus starting materials consisted of natural minerals with a bulk composition similar to that of a k deficient intermediate dioctahedral trioctahedral mica phengitic muscovite was found to be stable from 5 5 11 gpa at 900 c phengite melting occurs between 1075 1150 c at 7 8 gpa and 1000 1050 c at 10 gpa at 10 11 gpa 800 c octahedral cation deficient ocd muscovite and k hollandite are observed rather than phengite the average phengite content of muscovite is positively correlated with pressure ranging from 3 65 si pfu at 5 5 gpa to 3 81 si pfu at 11 gpa the maximum phengite content of stable muscovite appears to be 3 80 3 85 si pfu most phengite examined exhibits at least minor solid solution towards phlogopite averaging 2 04 0 06 2 octahedral cations pfu the hydrous phases phengite lawsonite topaz oh and mg pumpellyite occur between 6 8 gpa 700 900 c with increasing pressure lawsonite and mg pumpellyite dehydrate to form garnet between 8 9 gpa with increasing temperature mg pumpellyite lawsonite and topaz oh devolatilize to form garnet and kyanite between 900 1000 c at 7 8 gpa phengitic muscovite and topaz oh would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and mg pumpellyite would be stable to 240 300 km allowing the transport of h2o contained in these phases deep into the upper mantle in warmer subduction zones the dehydration of mg pumpellyite lawsonite and topaz oh as well as the melting of phengite would result in fluid release at depths of 180 240 km the presence of phengite at far greater depths than the zone of melt generation beneath arcs 100 150 km requires a mechanism such as the partitioning of k be b ba and rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

petrogenesis of metamorphic rocks within a subduction accretion terrane signy island south orkney islands whole rock and mineral analyses of polydeformed mica schist quartzite marble and amphibolite are presented from signy island south orkney islands part of the scotia metamorphic complex whole rock chemistry suggests that the amphibolites are the metamorphosed equivalent of enriched tholeiitic and alkali basalts of an oceanic intraplate basalt series these together with limestones and mn rich cherts of an oceanic island assemblage were tectonically mixed with trench or trench inner slope basin sediments in a subduction zone environment variation in mineral chemistry indicates an increase in temperature and decrease in pressure during metamorphism pressures of 8 kbar and temperatures of approximately 545 c were reached during amphibolite facies metamorphism in the latter stages of deformation these new data provide good evidence to support the previous interpretation of the scotia metamorphic complex as a subduction complex

phase petrology and p t conditions of mafic blueschists from the meliata unit west carpathians slovakia blueschists occurring as layers in calcite marbles of the meliata unit occur along the so called roznava tectonic line situated in the southern part of the gemericum slovakia mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated the most common minerals in the blueschists are blue amphibole epidote and albite some fe2 and al rich rocks also contain garnet and chloritoid respectively na pyroxene with a maximum 50 jadeite component was also found the blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples almandine and spessartine rich garnet has very low mgo content 3 wt the si content in phengite ranges between 3 3 and 3 5 pfu calculated on the basis of 11 oxygens the zoning patterns of blue amphibole garnet and chloritoid suggest their formation during a prograde stage of metamorphism the p t conditions of metamorphism are estimated to be about 380 460 c and 10 13 kbar pressures of 7 5 8 5 kbar and temperatures of 350 370 c were obtained for some actinolite and aegirine rich rocks apart from chlorite other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

implications of magma convection for the size and temperature of magma chambers at fast spreading ridges we present a new thermal model that resolves the discrepancy between observations of narrow 4 km magma chambers on the northern east pacific rise epr and predictions based on previous thermal models of wide 20 km chambers by including cooling of the lower crust by convection within the magma chamber for this cooling to produce a solid crust within a few kilometers of the axis the temperature of the magma chamber must be significantly below the solidus temperature of the lower crust this model is internally consistent for an average temperature of 1240 c for primary magmaleaving the mantle and an average magma chamber temperature of 1150 c these temperatures imply 10 mgo for primary magma which is at the low mg end of the range of proposed compositions and 6 mgo for magma residing in the chamber which is comparable to the most evolved basalts commonly erupted along the epr there is a trade off in the inputs to the model such that raising the primary magma temperature requires a lower magma chamber temperature to maintain the same maximum temperature of the lower crust it is reasonable for the magma chamber to be more evolved than most erupted lavas if magma mixing exerts substantial control over the composition of the erupted lavas aphyric 10 mgo basalt should be buoyant in 6 mgo basalt therefore the mixing need not involve the entire chamber prior to eruption

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

metamorphism and melting of picritic crust in the early earth abstract partial melting experiments with models of archean oceanic crust maoc with 11 13 and 15 wt mgo are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of ttg like melts and continental crust on the early earth the approach investigates the possibility that the average early crust composition was comparatively mgo rich and evolved to lower magnesium content during the secular cooling of the earth high pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in maoc 15 to predominantly tonalitic melts in maoc 11 and higher melting temperatures with increasing magnesium in the bulk composition tonalitic melts were generated in maoc 11 and 13 at pressures 12 5 kbar along with the residual phases garnet clinopyroxene plagioclase quartz orthopyroxene in the presence of quartz and at lower pressures in the absence of amphibole basaltic melts were generated at pressures 15 kbar predominantly in the presence of granulite facies residues such as amphibole garnet plagioclase orthopyroxene that lack quartz in all maoc compositions the tonalitic melts generated in maoc 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern morb is a viable source for the generation of early archean high si low mgo melts and therefore ttg like continental crust in the archean the favoured settings for the generation of these melts at pressures up to 15 kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones both without interaction of the melts with the mantle during passage to the surface tonalitic melts may have formed in deeper subduction zones at 20 kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction this process may have become more important during the progressive cooling of the earth

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

continuous monitoring of snowpack displacement at high spatial and temporal resolution with terrestrial radar interferometry terrestrial radar interferometry is used in geotechnical applications for monitoring hazardous earth or rock movements in this study we use it to continuously monitor snowpack displacements as test site the dorfberg slope at davos switzerland was measured continuously during march 2014 the line of sight displacement was retrieved at a spatial resolution of millimeter to centimeter and a temporal resolution of up to 1 min independent of visibility the results reveal several temperature driven diurnal acceleration and deceleration cycles the initiation of a small full depth glide avalanche was observed after 50 cm total differential displacement the maximum measured displacement of another differential glide area reached 43 cm h without resulting in a full depth avalanche even after a total measured differential displacement of 4 5 m in regard of the difficulty to predict full depth glide avalanches on the regional scale the presented method has big potential for operational snow glide monitoring on critical slopes