amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

petrology of the motaghairat mafic ultramafic complex eastern desert egypt a high mg post collisional extension related layered intrusion abstract the geodynamic settings of the precambrian mafic ultramafic complexes in the eastern desert of egypt have important bearing on understanding the geotectonic evolution of the arabian nubian shield we present a detailed petrological study on a layered mafic ultramafic intrusion that is located at the contact between the precambrian continental crust and the miocene red sea oceanic crust the motaghairat layered intrusion consists of basal lherzolite orthopyroxenite troctolite olivine gabbro and anorthosite on the top variations in modal mineralogy and mineral chemistry along with the chemical composition of these units suggest their derivation from a common high mg tholeiitic parent melt through fractional crystallization processes the parental magma was derived from a metasomatised mantle source the primitive mantle normalized patterns of the calculated melts exhibit enrichment in u relative to th and ba relative lree which indicate that the enriched lithospheric mantle source was metasomatised by fluids derived from a subducted oceanic crust rather than by a sediment melt geological and petrological evidences suggest that the layered motaghairat intrusion was emplaced during post orogenic extension following subduction break off and lithospheric delamination after the collision between the amalgamated island arc terranes and the saharan metacraton the heat source required to melt the metasomatised lithospheric mantle was derived from the upwelling of hot asthenosphere after the subduction break off

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

tholeiitic aleutian arc plutonism the finger bay pluton adak alaska the earliest representatives in the sequence of tertiary to recent magmatic rocks on adak island in the central aleutians are the finger bay volcanics and associated small shallow level gabbroic intrusives the tholeiitic finger bay pluton gabbro to quartz monzodiorite is among the least altered representatives of this earliest period the field relations mineralogy and geochemistry of the finger bay pluton contrast with the more recent calcalkaline plutons in the arc e g on adak hidden bay pluton 33 m y and on kagalaska kagalaska pluton 15 m y compared with the hidden bay pluton the finger bay pluton is smaller 8 km2 versus 100 km2 has a greater proportion of gabbro 84 versus 5 has a somewhat different mineralogy and has higher whole rock and mafic mineral feo mgo ratios higher k2o and higher concentrations of incompatable elements in rocks of equivalent sio2 particularly for the more siliceous units magmatic amphibole occurs only in the most siliceous units in the finger bay pluton whereas it is common in the calc alkaline plutons except for the size these differences parallel those between tholeiitic and calc alkaline volcanic rocks from the active aleutian volcanic centers textural miner alogical and trace element analyses indicate that mixing of magmas and gabbros generated some of the rocks of intermediate monzodiorite composition in the finger bay pluton most of the differences in magmatic trends between the calc alkaline and tholeiitic plutons are attributed to the physical conditions of magmatic evolution in the crust rather than to differences in initial magmatic types this conclusion rests on the similarity of fractionation independent isotope ratios and trace element ratios in aleutian magmas of all ages in particular compared to magmas of the north pacific ocean basin morb oceanic islands aleutian magmas show excess enrichment of ba k rb and u relative to ree and high 207pb 204pb ratios for a given 206pb 204pb ratio recognition of tholeiitic series plutons in the arc provides direct evidence for magmatic conditions accompanying fractionation and serves to emphasize the diversity of magmatic trends that are found over time and space at convergent plate margins

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

role of cryptic amphibole crystallization in magma differentiation at hudson volcano southern volcanic zone chile hudson volcano chile is the southern most stratovolcano of the andean southern volcanic zone and has produced some of the largest holocene eruptions in south america there have been at least 12 recorded holocene explosive events at hudson with the 6700 years bp 3600 years bp and 1991 eruptions the largest of these hudson volcano has consistently discharged magmas of similar trachyandesitic and trachydacitic composition with comparable anhydrous phenocryst assemblages and pre eruptive temperatures and oxygen fugacities pre eruptive storage conditions for the three largest holocene events have been estimated using mineral geothermometry melt inclusion volatile contents and comparisons to analogous high pressure experiments throughout the holocene storage of the trachyandesitic magmas occurred at depths between 0 2 and 2 7 km at approximately 972 c 25 and log fo2 10 33 10 24 0 2 one log unit above the nno buffer with between 1 and 3 wt h2o in the melt pre eruptive storage of the trachydacitic magma occurred between 1 1 and 2 0 km at 942 c 26 and log fo2 10 68 0 2 with 2 5 wt h2o in the melt the evolved trachyandesitic and trachydacitic magmas can be derived from a basaltic parent primarily via fractional crystallization entrapment pressures estimated from plagioclase hosted melt inclusions suggest relatively shallow levels of crystallization however trace element data e g dy yb ratio trends suggests amphibole played an important role in the differentiation of the hudson magmas and this fractionation is likely to have occurred at depths 6 km the absence of a garnet signal in the hudson trace element data the potential staging point for differentiation of parental mafic magmas i e 20 km e g annen et al in j petrol 47 3 505 539 2006 and the inferred amphibolite facies 24 km e g rudnick and fountain in rev geophys 33 267 309 1995 combine to place some constraint on the lower limit of depth of differentiation i e 20 24 km these constraints suggest that differentiation of mantle derived magmas occurred at upper mid to lower crustal levels and involved a hydrous mineral assemblage that included amphibole and generated a basaltic to basaltic andesitic composition similar to the magma discharged during the first phase of the 1991 eruption continued fractionation at this depth resulted in the formation of the trachyandesitic and trachydacitic compositions these more evolved magmas ascended and stalled in the shallow crust as suggested by the pressures of entrapment obtained from the melt inclusions the decrease in pressure that accompanied ascent combined with the potential heating of the magma body through decompression induced crystallization would cause the magma to cross out of the amphibole stability field further shallow crystallization involved an anhydrous mineral assemblage and may explain the lack of phenocrystic amphibole in the hudson suite

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

geochemistry of gabbros and granitoids m and i types from the nubian shield of egypt roots of neoproterozoic intra oceanic island arc abstract the neoproterozoic intrusive rocks of the wadi ranga area nubian shield of egypt comprise gabbros and granitoids emplaced during oceanic island arc and post collision stages the plutonic rocks of the island arc stage include hornblende gabbros dabbah pluton trondhjemite abu ghalaga pluton and tonalites with subordinate quartz gabbro and quartz diorite reidi and abu ghusun plutons whereas the post collision intrusives include granodiorite and monzogranite helifi hamata pluton the gabbros and granitoids of the island arc stage are largely calcic low k rocks which have either tholeiitic gabbro and trondhjemite or transitional tholeiitic to calcalkaline nature tonalites on the other hand the granitoids of the post collision stage are medium to high k calcalkaline rocks all the investigated granitoids are metaluminous the spider diagrams with enrichment in lile and strong nb depletion and the almost flat to slightly lree depleted ree patterns of the gabbro and trondhjemite are similar to those of the wadi ranga low k tholeiitic basalts and silicic volcanics respectively suggesting that the gabbro and trondhjemite are the plutonic equivalents of the wadi ranga immature island arc extrusives and they were derived from mantle source at the early immature island arc stage similar to the trondhjemite the tonalites show lile enrichment and strong nb depletions on the morb normalized spider diagrams however the tonalites have ree patterns which are enriched in lree la yb 1 71 5 54 the derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely therefore high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage the post collision granitoids show considerable enrichment in lile and to a lesser extent in hfse slight negative nb anomaly and strong negative p and ti anomalies relative to n morb their ree patterns are lree enriched la yb 5 19 with negative eu anomaly these characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization the geochemical characteristics of the trondhjemite and tonalites and the granodiorite monzogranite classify them as m type and i type granitoids respectively the partly tholeiitic intrusives of the wadi ranga area south eastern desert have lower k2o rb and lree compared to the m type calcalkaline intrusives of the north eastern desert implying northwardly dipping subduction zone the geochemical similarities between the intrusives of neoproterozoic and phanerozoic oceanic island arcs imply that they share similar style of subduction which differs from that of the archaean the generation of high sio2 up to 74 5 wt low k2o 0 56 1 78 wt and slightly lree depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post collision stage drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust the present study indicates that the intra oceanic island arcs continued to play a role in the generation of the continental crust after the archaean

water in snc meteorites evidence for a martian hydrosphere the shergotty nakhla chassigny snc meteorites purportedly of martian origin contain 0 04 to 0 4 percent water by weight oxygen isotopic analysis can be used to determine whether this water is extraterrestrial or terrestrial such analysis reveals that a portion of the water is extraterrestrial and furthermore was not in oxygen isotopic equilibrium with the host rock lack of equilibrium between water and host rock implies that the lithosphere and hydrosphere of the snc parent body formed two distinct oxygen isotopic reservoirs if mars was the parent body the maintenance of two distinct reservoirs may result from the absence of plate tectonics on the planet

geochemical changes in baslts and andesites across the archean proterozoic boundary identification and significance abstract to identify accurately changes in rock composition across the archean proterozoic boundary it is necessary to compare rocks from similar lithologic associations to constrain the effect of tectonic setting most basalts and andesites from the greenstone association volcanic dominated submarine supracrustal rocks possess a geochemical subduction zone component similar to their counterparts from modern arc systems basalts with island arc geochemical affinities dominate in archean greenstones while those with calc alkaline affinities are most abundant in proterozoic greenstones basalts with morb or oceanic within plate geochemical characteristics are rare in precambrian greenstones of all ages preserved early archean greenstone basalts 3500 ma reflect mantle sources less depleted than late archean greenstone basalts 2500 3500 ma proterozoic greenstone basalts are derived from relatively enriched mantle sources compared to all archean sources a feature which may be due to recycling of continental sediments into the mantle following rapid late archean continental growth precambrian andesites are geochemically similar to andesites from modern arcs but archean andesites are unique in that they are depleted in heavy ree and y results are consistent with archean andesite production by partial melting of descending mafic crust with amphibole garnet remaining in the residue while proterozoic and younger andesites are produced by fractional crystallization of basalts

trace element evidence for growth of early continental crust by melting of rutile bearing hydrous eclogite the early continental crust is composed dominantly of archean tonalite trondhjemite granodiorite ttg gneisses and is generally explained as the product of melting of metabasalts in the subducted crust however whether the melting occurs in shallow level amphibolite facies or in relatively deep eclogite facies is debated here i present experimental partition coefficients ds for 27 trace elements between garnet amphibole and tonalitic melts they are used together with published mineral melt trace element ds to model the melting of metabasalt in order to delimit the conditions for ttg production the results clearly show that model melts with trace element characteristics that completely mimic the ttg are in equilibrium with rutile bearing anhydrous and hydrous amphibole bearing eclogitic residues but not rutile free amphibole dominated residues rutile appears to be a necessary residual phase to account for the characteristic negative nb ta anomaly in the ttg these results thus suggest that the early continental material was produced under eclogite facies conditions based on the modeling results obtained using appropriate partition coefficients and the archean geotherm the preferred process for the ttg production is the melting of rutile bearing hydrous eclogite triggered by the release of h2o from the progressive breakdown of amphibole the pressure temperature p t conditions for ttg production via this process are constrained to 1 5 2 5 gpa 50 80 km and 850 1050 c by the p t stability boundaries of amphibole and rutile in the basalt system

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

trench parallel anisotropy and large delay times elasticity and anisotropy of antigorite at high pressures 1 using theoretical methods we calculate the full elastic constant tensor and equation of state of antigorite up to 10 gpa a pressure range that encompasses its experimentally observed stability field at ambient conditions the elastic constant tensor reveals significantly large acoustic anisotropy 38 in vp 35 in vs compared to the dominant mantle phase olivine the shear anisotropy is enhanced upon compression upon compression the full elastic constant tensor reveals anomaly at 2 69 gm cm 3 at 2 69 gm cm 3 we find that the pressure volume results for antigorite are well represented by a third order birch murnaghan formulation with k0 52 gpa k0 8 5 and v0 3037 a3 0 2 48 gm cm 3 our results on elasticity and anisotropy at conditions relevant to the mantle wedge indicates that a 10 20 km layer of antigorite might account for the observed shear polarization anisotropy with fast polarization parallel to the trench and associated large delay times of 1 2 sec observed in some subduction zone settings

mid mantle anisotropy elasticity of aluminous phases in subducted morb 1 aluminous phase with the calcium ferrite and calcium titanate structure constitutes around 20 by volume of the subducted mid ocean ridge basalt morb at lower mantle depths using first principle simulations we calculate the equation of state and elasticity of naalsio4 nacf and mgal2o4 mgcf and mgct up to 150 gpa encompassing the full range over which nacf mgcf and mgct has been observed experimentally we calculate the isotropically averaged elastic wave velocities and the anisotropy from our single crystal elastic constants the elasticity of these phases is sensitive to the chemistry the bulk modulus decreases with mgalna 1si 1 substitution with a k0 x 15 gpa whereas the shear modulus stiffens with a g0 x 10 gpa at lower mantle conditions the temperature derivative of bulk k t and shear g t modulus are 0 006 and 0 013 gpa k 1 respectively the chemistry is likely to have significant influence on the elasticity of these phases slab penetrating the lower mantle often develops significant anisotropy the full elastic constant tensor of these aluminous phases reveal significant anisotropy and are likely candidates to account for the large delay times observed in the tonga kermadec subduction zones

the low velocity layer in subduction zone structure and elasticity of glaucophane at high pressures abstract we investigated the structure equation of state and elasticity of glaucophane na2mg3al2si8o22 oh 2 up to 9 gpa which encompasses its experimentally observed stability field we find that the pressure volume results for glaucophane are well represented by a third order birch murnaghan formulation with k 0 81 gpa k 0 4 5 and v 0 899 4 a3 the full elastic constant tensor reveals significantly larger stiffness along the 1 0 0 plane the 1 0 0 direction is the relatively softer this could be rationalized in terms of the stacking of the stiffer tetrahedral units along 0 1 0 and 0 0 1 directions within the crystal structure glaucophane is a dominant mineral constituent of blueschist facies rock and has significantly lower velocities compared to garnet bearing eclogites in addition glaucophane is anisotropic and could account for the observed low velocity layer in the subducting slabs at depth range within the thermodynamic stability of glaucophane at high pressures beyond stability of glaucophane hydrous phase such as lawsonite could account for the observed low velocity layers in certain subduction zones

the building blocks of continental crust evidence for a major change in the tectonic setting of continental growth at the end of the archean abstract oceanic arcs are commonly cited as primary building blocks of continents yet modern oceanic arcs are mostly subducted also lithosphere buoyancy considerations show that oceanic arcs even those with a felsic component should readily subduct with the exception of the arabian nubian orogen terranes in post archean accretionary orogens comprise 50 produced in continental arcs felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs they have lower th yb nb yb sr y and la yb ratios reflecting shallow mantle sources in which garnet did not exist in the restite during melting the bottom line of these geochemical differences is that post archean continental crust does not begin life in oceanic arcs on the other hand the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs during the archean however oceanic arcs may have been thicker due to higher degrees of melting in the mantle and oceanic lithosphere would be more buoyant these arcs may have accreted to each other and to oceanic plateaus a process that eventually led to the production of archean continental crust after the archean oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges hence oceanic lithosphere is more subductable widespread propagation of plate tectonics in the late archean may have led not only to rapid production of continental crust but to a change in the primary site of production of continental crust from accreted oceanic arcs and oceanic plateaus in the archean to primarily continental arcs thereafter

silicic magmas of protector shoal south sandwich arc indicators of generation of primitive continental crust in an island arc protector shoal the northernmost and most silicic volcano of the south sandwich arc erupted dacite rhyolite pumice in 1962 we report geochemical data for a new suite of samples dredged from the volcano geochemically the dredge and 1962 samples form four distinct magma groups that cannot have been related to each other and are unlikely to have been related to a single basaltic parent by fractional crystallization instead the silicic rocks are more likely to have been generated by partial melting of basaltic lower crust within the arc trace element and sr nd isotope data indicate that the silicic volcanics have compositions that are more similar to the volcanic arc than the oceanic basement formed at a back arc spreading centre and volcanic arc basalts are considered to be the likely source for the silicic magmas the south sandwich islands are one of several intra oceanic arcs tonga kermadec izu bonin that have 1 significant amounts of compositionally bimodal mafic silicic volcanic products and 2 6 0 6 5 km s 1 p wave velocity layers in their mid crusts that have been imaged by wide angle seismic surveys and interpreted as intermediate silicic plutons geochemical and volume considerations indicate that both the silicic volcanics and plutonic layers were generated by partial melting of basaltic arc crust representing an early stage in the fractionation of oceanic basalt to form continental crust

mid neoproterozoic arc magmatism in the northeastern margin of the indochina block sw china geochronological and petrogenetic constraints and implications for gondwana assembly abstract the mid neoproterozoic high k calc alkaline granitoids in the northeastern margin of the indochina block sw china provide important insights into the relationship of the indochina block with the gondwana supercontinent here we report zircon la icp ms u pb data from the granitoids which show weighted mean 206pb 238u ages ranging from 754 to 767 ma representing their timing of emplacement the zircons from the granitoids have hf t values ranging from 0 2 to 7 9 with a mean of 1 9 to 4 6 and t dm c varying from 1713 to 2156 ma the granitoids are characteristized by high mg weakly negative eu anomalies enrichment of lile rb ba th and k and lree la ce nd strong depletion of hfse nb ta sr p and ti with most of the samples falling within the field of volcanic arc granitoids the hf isotopic data and geochemical signatures compare these granitoids with typical calc alkaline magmas generated in active continental margins within a subduction related island arc or continental arc tectonic setting with the magma mainly sourced from the partial melting of the ancient crust together with a minor mantle derived component the rocks reported in our study and their ages are similar to those occurring along the northwestern margin of greater india and suggest the juxtaposition of india and indochina within the gondwana reconstruction

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

ab initio investigation of high pressure phase relation and elasticity in the naalsi2o6 system 1 subducted crusts contain aluminous phases with incompatible elements such as sodium in order to understand the fate of subducted crusts and how they recycle within the earth it is important to investigate high pressure stability and elastic properties of host phases here we have studied naalsi2o6 jadeite a naalsio4 calcium ferrite cf type phase and sio2 stishovite by means of a first principles computation method jadeite was found to dissociate into an assemblage of the cf type phase and stishovite at about 18 gpa associated with remarkable increases of compressional wave 18 7 shear wave 26 4 bulk sound 13 3 velocities and density 15 1 proximity of this transition condition to the 660 km discontinuity in the mantle transition zone suggests that seismically observed splitting of the reflection could be related to this phase transition implying subduction of crustal materials down to the mantle transition zone

geomorphic classification of rivers a new methodology applied in an atlantic region galicia nw iberian peninsula the classification of rivers based on geomorphological criteria played in the past a secondary role for management decision making although in the last years they have experienced a shift from the scientific field to that of the technical currently managers require the most simplified form of classifications in order to use them in plans and planning projects management and restoration on the one hand this means that classifications should be directed toward simplifying the diversity of fluvial environments in a number of manageable types and on the other hand to apply to each geomorphic type a management model in this study we have developed a method of geomorphic classification of rivers according to specific stream power variables and median grain size the new method is dynamic ability to readjust and gain robustness with the incorporation of new data and predictive we obtained six types directly from the method although we added a seventh type bedrock rivers for its special singularity each geomorphic type presents concrete values of specific stream power and median grain size which additionally involves a hydraulic geometry adjusted to terms of balance the change from a geomorphic type to another would reflect a geomorphologic imbalance in the form of greater power and sediment size our results have been compared with six commonly used classifications e g rosgen classification system or river styles framework geomorphic classification of rivers stands as a flexible tool that allows the development of a personalized geomorphic classification for rivers of the same geomorphological province through the temporary revision of various sites that act as control points we can learn should the case arise the intensity and geomorphic change of the site geomorphic classification of rivers acts as an alert system for any geomorphologic disturbance its simple application and interpretation facilitate the implementation in the administrative environment or its attachment to other commonly used classifications