internally consistent thermodynamic data set for dense hydrous magnesium silicates up to 35 gpa 1600 c implications for water circulation in the earth s deep mantle abstract fluid undersaturated high pressure experiments were carried out in the system mgo sio 2 h 2 o in a multi anvil apparatus from 15 5 to 21 5 gpa and from 1000 to 1200 c in order to constrain fluid absent solid solid reactions from the experimentally determined fluid absent phase equilibria thermodynamic parameters for high pressure hydrous phases were retrieved by a linear programming method together with dry mantle minerals from the existing data set and high pressure hydrous phases from our previous study an internally consistent set of thermochemical and thermophysical parameters for dense hydrous magnesium silicates dhmss was evaluated up to 35 gpa and 1600 c high pressure hydrous phases involved in the data set are phase a phase e clinohumite phase d superhydrous phase b hydrous wadsleyite and hydrous ringwoodite in addition by calculating water bearing reactions we evaluated water activities in the fluid phase coexisting with mantle minerals the water activity decreases with increasing pressure or temperature suggesting that the amount of silicate component dissolved in the fluid phase increases with pressure or temperature the calculated petrogenetic grid consisting of 89 univariant reactions can be used for the discussion of water circulations in the mantle in a subducting slab peridotite water in phase a as the post serpentine phase is transferred to other dhmss via solid solid reactions down to the bottom of the transition zone at around 660 km depth in the stagnant slab heated from the surrounding mantle a free fluid will be released by the dehydration of superhydrous phase b or phase d the released fluid will be trapped by wadsleyite or ringwoodite in the surrounding mantle in a cold subducting slab without stagnation at 660 km depth dhmss will survive into the lower mantle in an upwelling plume through the water bearing transition zone a free fluid will be produced when the plume passes 410 km depth finally water will be released to the earth s surface by magmatism the fluid generated in the deep mantle dissolves significant amounts of a silicate component therefore at 410 and 660 km depths some chemical differentiation will be processed

deformation of mantle pyroxenites provides clues to geodynamic processes in subduction zones case study of the cabo ortegal complex spain abstract in the herbeira massif cabo ortegal complex spain a well exposed assemblage of deformed dunites and pyroxenites offers a unique opportunity to investigate key upper mantle tectonic processes four types of pyroxenites are recognized clinopyroxenites with enclosed dunitic lenses type 1 massive websterites type 2 foliated and commonly highly amphibolitized clinopyroxenites type 3 and orthopyroxenites type 4 field and petrological observations together with ebsd analysis provide new insights on the physical behavior of the pyroxenes and their conditions of deformation and reveal the unexpected journey of the cabo ortegal pyroxenites we show that during deformation type 1 pyroxenites due to their enclosed dunitic lenses are more likely to localize the deformation than types 2 and 4 pyroxenites and may latter act as preferred pathway for fluid melt percolation eventually resulting in type 3 pyroxenites all pyroxenite types display a similar response to deformation orthopyroxene deformed mostly by dislocation creep it shows kink bands and undulose extinction and its fabric is dominated by 001 100 clinopyroxene displays subgrain rotation dynamic recrystallization and fabric with 010 axes clustering next to the foliation pole and 001 axes clustering next to the lineation suggesting activation of 001 110 and 001 100 in some samples these observations are in good agreement with deformation at temperatures greater than 1000 c olivine in type 1 and type 4 pyroxenites shows 100 010 or 001 010 fabrics that are consistent with deformation at temperatures 1000 c and may indicate deformation in a hydrous environment the amphibole 001 100 fabric gives insights on a lower temperature deformation episode 800 to 500 c our results interpreted in the light of published experimental data together with the regional geological and geochemical studies are consistent with the following tectonic evolution of the cabo ortegal pyroxenites 1 delamination from an arc root in a mantle wedge setting at temperatures above 1000 c and 2 introduction into a relatively softer subduction channel where deformation was accommodated by localized shear zones thus preserving the high temperature fabrics of pyroxenites the cabo ortegal pyroxenites may therefore be seen as a rare exposure of deformed mantle wedge material

melting of enstatite from 13 to 18 gpa under hydrous conditions abstract experiments on mgsio 3 enstatite were conducted in the pressure range from 13 to 18 gpa under hydrous conditions in order to clarify the effect of water on the melting phase relations of enstatite at pressures corresponding to the earth s mantle transition zone in some previous experiments geol soc am bull 79 1968 1685 phys earth planet inter 85 1994 237 incongruent melting behavior to form mg 2 sio 4 forsterite and sio 2 enriched liquid up to 5 gpa was observed and congruent melting behavior at pressures up to 12 gpa was observed under hydrous conditions we found that the melting reaction changes from congruent to incongruent at around 13 5 gpa liquid formed above 13 5 gpa is enriched in mgo component relative to mgsio 3 because it coexists with stishovite sio 2 moreover the solidus temperature decreases drastically at around 13 5 gpa in unison with the change in the melting reaction the solidus temperature is about 1400 c at 13 gpa but approximately 900 c at 15 gpa our results show that the liquidus phase changes from clinoenstatite to stishovite with increasing pressure and water content above 13 5 gpa mgsio 3 enstatite is one of the major constituent minerals in the earth s mantle and it is expected that mgo enriched liquid will be generated in the transition zone if water is present

transposed high pressure granulite fabrics cabo ortegal nw spain implications on the scales of deformation localization abstract high pressure granulite facies rocks of the bacariza formation cabo ortegal nw spain were syn metamorphically deformed at the contacts with the bounding units peridotite and eclogite massifs this enabled the formation of meter thick spectacular shear zones with reworked and transposed foliations and lineations the texturally stable mineral assemblage of the new fabrics records an intense ductile deformation of the mineral aggregate at temperatures of 700 800 c associated with amalgamation of eclogite high pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel the lattice preferred orientation of the main constituent minerals garnet augite amphibole plagioclase quartz and biotite discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high grade conditions complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species

thermal hydrous and mechanical states of the mantle transition zone beneath southern africa abstract observations of p to s conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410 km discontinuity and the deepening of the 660 km discontinuity beneath the archaean cratons in southern africa consequently the mantle transition zone is 20 km thicker than beneath post archaean regions and the global average the discontinuity structure combined with tomographically imaged seismic velocities suggests a relatively cold t 100 k and water enriched oh 20 35 saturation transition zone beneath the cratons the thicker than normal transition zone correlates with surface geology and does not follow the track of african plate motion as numerical simulations of small scale convection imply the observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons although it may exist in a confined area near the boundary between the kheis thrust belt and the namaqua natal belt if thermal downwelling exists its temperature anomaly at the 660 km discontinuity must be below the levels detectable by the method used in this study t 60 k the spatial correlation between surface geology high velocities beneath the cratons and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle substantially deeper than most estimates our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots

decomposition of phase d in the lower mantle and the fate of dense hydrous silicates in subducting slabs abstracthigh pressure high temperature quench type experiments were carried out on serpentine to pressures of 53 gpa andtemperatures between 800 1800oc x ray analyses show that recovered phase assemblages varied considerably for thedifferent high pressure and high temperature studies the dense hydrous magnesium silicate phases decompose sequentiallyas the pressure increases and appear to serve as reservoirs for h 2 o in the mantle eventually releasing it at the highestpressure superhydrous phase b is stable up to pressure equivalent to the boundary between the transition zone and lowermantle phase d decomposes at pressures of 44 gpa equivalent to about 1250 km depth this may de ne the lower depthlimit for the presence of dense hydrous magnesium silicates beyond this pressure only nominally anhydrous phases havebeen recovered in high pressure experiments this may indicate a lack of stoichiometric hydrogen bearing silicate phasesat higher pressure 1998 elsevier science b v all rights reserved keywords x ray diffraction analysis lower mantle serpentine high pressure

seismic evidence for a chemical heterogeneity in the midmantle a strong and slightly dipping seismic reflector beneath the mariana subduction zone 1 a clear later phase 80 s after the direct p wave is observed in most of individual seismograms recorded by a short period seismometer network in japan j array from a cluster of deep earthquakes that occurred at the northern mariana subduction zone this phase 1 shows a p wave particle motion 2 arrives later from earthquakes with shallower focal depths 3 has a steeper incident angle than that of p wave and 4 shows a deviation of a few degrees in the arrival azimuth from that of p wave we interpret it as an s to p converted wave which takes off downward from the source and is reflected at a velocity discontinuity reflector below the earthquakes travel time inversion shows that the seismic reflector dips southwest by 20 at 24 25 n 144 75 e and at a depth of 1115 km with a lateral extension at least 100 100 km the location corresponds to the lower edge of a high velocity anomaly in global tomographic models amplitude and waveform analyses suggest a decrease of s wave velocity by 2 6 and an increase of density by 2 9 within the reflector there is almost no difference in p wave velocity 1 between the reflector and the surrounding mantle the estimated thickness of the reflector is 12 km these observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly the seismic reflector might be a piece of subducted oceanic crust as suggested by a previous study it also could be related to the break down of the d phase of dense hydrous magnesium silicates at midmantle pressure condition reported by recent mineral physics studies both scenarios imply that mechanical chemical segregation might occur within the subducted slab at midmantle condition

the effect of small amounts of h2o on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine abstract it has been suggested that in relatively cold subduction zones the interior of down going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high pressure polymorphs wadsleyite and ringwoodite resulting in the persistence of metastable olivine in the mantle transition zone the rapid transformation of this metastable olivine has been identified as a possible cause of deep focus earthquakes however it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates we conducted a series of high pressure experiments to hydrate and then partially transform olivine to ringwoodite we measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates the addition of h2o to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate decreasing the activation enthalpy for growth and weakening the ringwoodite and or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near constant growth rates throughout transformation the addition of approximately 300 ppmw 2h2o to olivine increases the growth rate of olivine to ringwoodite transformation at 1100 c and 18 gpa by almost a factor of 40 from 4 0 0 8 10 9 ms 1 to 1 5 0 3 10 7 ms 1 there is a corresponding decrease in the activation enthalpy of growth from 392 kj mol to 237 kj mol assuming activation volume of 0 cm3 mole analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface

hydrous phases and water transport in the subducting slab arc volcanoes are typically located 90 180 km above the surface of downgoing slabs as shown by wadati benioff deep seismic foci gill 1981 tatsumi 1989 the intimate relationship between the dip angles of the subducting slab and the locations of volcanic arcs indicates that subduction zone magmatism is triggered by material input from the subducting slab tatsumi and eggins 1995 the slab derived components are thought to be aqueous fluids or h2o rich partial melts of subducted oceanic crust therefore knowledge of the stability of hydrous phases and the chemical and physical properties of aqueous fluids in downgoing slabs is essential to understand the material transport in subduction zones in this section i will review the stability of hydrous phases in downgoing peridotite basalt and sediment systems and the chemical and the wetting properties of aqueous fluids recent experimental studies indicate that 3 4 gpa equivalent to 90 120 km depth is a key pressure where 1 the chemical compositions of silicate components dissolved in aqueous fluids equilibrated with mantle minerals approach the composition of mantle peridotite itself stalder et al 2001 mibe et al 2002 kawamoto et al 2004 2 the dihedral angle between olivine and aqueous fluids starts becoming smaller than 60 watson et al 1990 mibe et al 1998 1999 and 3 the immisciblity gap between peridotitic melts and aqueous fluids disappears and consequently hydrous minerals liberate supercritical aqueous fluids mibe et al 2004a 2006 the similarity between these pressures and the depths of downgoing slab underneath volcanic fronts where the maximum numbers of volcanoes are formed 124 38 km gill 1981 or 112 19 km tatsumi 1986 suggests that subduction zone magmatism can be triggered by the input of supercritical fluids from the downgoing peridotite and basalt many hydrous crystalline phases

post stishovite transition in hydrous aluminous sio2 abstract lakshtanov et al 2007 showed that incorporation of aluminum and some water into sio2 significantly reduces the post stishovite transition pressure in sio2 this discovery suggested that the ferroelastic post stishovite transition in subducted morb crust could be the source of reflectors scatterers with low shear velocities observed in the mid to upper lower mantle a few years later a similar effect was observed in anhydrous al bearing silica in this paper we show by first principles static calculations and by molecular dynamics using inter atomic potentials that hydrogen bonds and hydrogen mobility play a crucial role in lowering the post stishovite transition pressure a cooperative redistribution of hydrogen atoms is the main mechanism responsible for the transition pressure reduction in hydrous aluminous stishovite the effect is enhanced by increasing hydrogen concentration this perspective suggests a potential relationship between the depth of seismic scatterers and the water content in stishovite

double seismic discontinuities at the base of the mantle transition zone near the mariana slab 1 we use p to s converted phases from teleseimic data recorded at island and ocean bottom stations in mariana to investigate the upper mantle structure in the region we find evidence for double seismic discontinuities at the base of the transition zone near the mariana slab a shallower discontinuity is imaged at depths of 650 715 km and a deeper interface lies at 740 770 km depth the large lateral extent at near constant depths for both features is consistent with horizontal interfaces rather than small scale scatterers the amplitude ratios of the seismic signals suggest that the shear velocity contrast across the two interfaces is comparable these characteristics support the notion that the discontinuities are the results of phase transformations in olivine ringwoodite to post spinel and non olivine component ilminite to perovskite respectively for the pyrolite model of mantle composition

the metamorphic evolution of the high pressure mafic granulites of the bacariza formation cabo ortegal complex hercynian belt nw spain abstract the high pressure mafic granulites of the bacariza formation outcrop in the two uppermost structural units of the cabo ortegal complex la capelada unit and cedeira unit were separated by a variscan thrust in both cases they appear as heterogeneous metabasites in normal contact between ultramafic rocks and other more homogeneous and less differentiated metabasic rocks also affected by catazonal metamorphism the main difference between the mafic granulites in the two units is the degree of deformation which is higher in the underlying cedeira unit petrologie and mineralogical data indicate that the high pressure hp granulites gt cpx amp pl qtz scp rt ilm czo are already retrograde m2 stage post dating an earlier eclogite facies metamorphism ml stage characterised by the mineral associations gt cpx amp ky qtz rt and gt cpx amp qtz zo rt the main structure relatted to the exhumation processes is the development of a general mylonitic foliation that although initiated in granulite facies conditions was mainly equilibrated in amphibolite facies m3 stage this foliation was affected by isoclinal folds which led to the formation of the variscan thrusts responsible for the present stacking position thrust conditions were transitional between amphibolite and greenschist facies m4 stage thermobarometric data point to different p t exhumation paths in the two units estimated p t conditions were higher in la capelada unit during ml p 13 kbar 860 c and m2 15 kbar 800 c than in the cedeira unit ml p 11 kbar 770 c m2 12 kbar 750 c temperatures for the m3 stage were comparable 720 c in both units but rocks from the cedeira unit show a much bigger drop in pressure this resulted in an isothermal decompression type path for the cedeira unit while both p and t decreased more steadily in la capelada rocks these were always located at deeper level than the cedeira rocks before the variscan stacking the difference in the two paths is related to different exhumation rates higher in rocks from the cedeira unit than in those from la capelada exhumation processes coeval with underthrusting and a different location of the rocks with respect to the main shear zone responsible for the exhumation would account for the distinct paths

platiniferous chromitite and the tectonic setting of ultramafic rocks in cabo ortegal nw spain high cr number cr chromitites with exceptional concentrations of platinum up to 7000 ppb and palladium up to 4400 ppb occur in sulphide poor dunites exposed along the western side of the ultramafic herbeira massif at cabo ortegal nw spain the pt pd enriched chromitites are restricted to the exposed top of the c 800 m thick herbeira layered complex which comprises two dunite dominated units lower and upper dunites separated by a garnet bearing pyroxenite dominated unit the herbeira layered complex is interpreted as lower crustal and is underlain by high al spinel harzburgite interpreted as mantle tectonite dunite lenses within this underlying tectonized harzburgite also contain high cr chromitites but these are pt pd depleted pt and pd have been strongly fractionated into the overlying herbeira layered complex highest pt and pd concentrations are restricted to the top of the upper dunite where a negative ru anomaly also becomes apparent the cabo ortegal ultramafics form part of a high grade terrane with hp granulites garnet pyroxenites and eclogites and are similar to pge rich chromite concentrations in arc root complexes in pakistan jijal and alaska talkeetna and tonsina the cabo ortegal chromitites are interpreted as having crystallized from a sulphide poor mantle derived suprasubduction melt deep within an arc massif subsequently thrust over the gondwanan continent during variscan collision

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle abstract we evaluated the water storage capacity in the mantle based on the phase relations of silicate systems containing water we conducted high pressure and temperature synthesis experiments on some silicate systems such as the superhydrous phase b phase c and phase g phase d and f compositions in the mgo sio2 h2o system and the cmas pyrolite 2 wt h2o system up to the uppermost part of the lower mantle superhydrous phase b is stable at temperatures below 1200 c at 18 gpa and below 1300 c at 20 gpa the following reaction sequences were observed with increasing temperature from 1200 to 1600 c in the pressure range from 18 to 20 gpa superhydrous phase b phase b liquid wadsleyite periclase liquid anhydrous phase b periclase liquid phase g is stable at temperatures below 1000 c at 18 gpa below 1100 c at 20 22 gpa and below 1200 c at 25 gpa wadsleyite stishovite liquid are stable above 1000 c at 18 gpa ringwoodite or phase e stishovite liquid above 1100 c at 22 gpa ilmenite stishovite liquid above 1100 c at 23 5 gpa and perovskite stishovite liquid above 1200 c at 25 gpa superhydrous phase b is stable at 1200 c at 18 5 and 25 gpa in cmas pyrolite 2 wt h2o composition whereas it decomposes at around 1400 c at 25 gpa in the composition there may be a layered structure in the mantle in its water storage potential solubility of water per unit mass i e the upper and lower mantles have relatively small water storage potentials whereas the transition zone has a larger potential the water content in the transition zone might be large because the primordial water trapped in the lower mantle and the recycled water transported by the slab subduction might have been released upwards and stored in the transition zone during geological time

insights into the post emplacement history of the saunstein granitic dyke showing heterogeneous deformation and inconsistent shear sense indicators bavarian forest germany the recent mapping survey undertaken in the bavarian forest led to the discovery of several eew trending sinistral shear zones usually affecting granitic dykes or areas intruded by dykes we studied a granitic dyke affected by one of these shear zones the deformation in the dyke as well as in other dykes of the region is not homogeneous and concentrates mainly next to the contact to the country rock the differences in mineralogy between dyke and host probably controlled the distribution of strain favoring the deformation of the lithology richer in quartz i e the granite dyke next to a rheological boundary i e the contact to the host rock as inferred from microfabrics and quartz textures quartz deformation occurred by dislocation creep accommodated by grain boundary migration and subgrain rotation recrystallization intracrystalline glide on prism planes in the direction indicate a temperature of w500e550 c only in the most strained fine grained bands was deformation achieved by grain boundary sliding a late shear sense reversal after the principal sinistral deformation in the dyke led to the development of a dextral quartz shape fabric this reflects on the one hand how quartz is able to record late low strain overprints and on the other hand how oblique quartz foliations may lead to wrong interpretations when being used without corroboration of other shear sense indicators

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

rheological implications of heterogeneous deformation at multiple scales in the late cretaceous sierra nevada california late cretaceous deformation in the east central sierra nevada arc of california was heterogeneous at multiple scales we quantify this heterogeneous deformation at centimeter meter and kilometer scales in the vicinity of the gem lake shear zone and infer variations in effective viscosity from our data at the centimeter scale variations in strain of different clast types in conglomerate suggest that effective viscosities varied by less than an order of magnitude lithology controlled the magnitude and nature of deformation recorded by clasts at the meter scale cleavage refraction between stratigraphic layers records variations in finite strain comparison of our observations with cleavage refraction models suggests a maximum effective viscosity contrast of 10 between layers bulk composition controls variations in deformation of the different layers at the kilometer scale variation in finite strain magnitude and orientation in similar rock types both within and outside the shear zone demonstrates that deformation inside the zone was relatively intense comparing these results to numerical models of heterogeneous regional deformation we estimate that the angle of oblique convergence inside the zone was 15 10 while outside it was greater than 60 these kilometer scale results imply regional deformation was moderately strike slip partitioned during the late cretaceous and suggest regional effective viscosity varied by a factor between 6 and 17 at each scale of observation the apparent range of effective viscosity varies by an order of magnitude or less consequently we infer that relatively modest strength variations produced the structures observed at hand sample to tectonic scales

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

microstructural evidences for thermal runaway and deep palaeoseismic activity cabo ortegal nw spain the occurrence of high pressure granulite facies tectonites with features at the outcrop and thin section scale characteristic of both pseudotachylytes and ultramylonites is reported in the cabo ortegal complex nw spain we interpret that these rocks formed due to a seismic event at great depth 1 4 gpa under high temperature conditions 740 c shear heating and thermal runaway mechanism would have been responsible for the failure of the still hot and mechanically rigid granulites and provoked melt formation and the accumulation at dilatational jogs ambient high p and t conditions would have led to subsequent recrystallization of the previously formed melt accounting for the present weak planar fabric

graphite and quartz petrofabrics examples from the ediacaran black quartzites of the ossa morena zone sw iberia abstract we study with the electron back scattered diffraction ebsd technique the fabric of metamorphosed and ductilely deformed phtanites or graphitic cherts a common lithotype in ediacaran supracrustals of the west european cadomian orogen currently they are black quartzites with a planolinear tectonite structure microscopically they present a sub mm scale alternation of coarse and fine grained dynamically recrystallized quartz bands we attribute intracrystalline plasticity partitioning to variations in graphite inclusion concentration constraining quartz grain boundary mobility during dynamic recrystallization under non coaxial strain regimes and moderate to elevated temperatures 400 650 c lattice preferred orientations of quartz c and axes are geometrically related to the external reference frame provided by foliations and lineations we also identify the involvement of 0001 r m and m c quartz intracrystalline slip systems deformation modes operated simultaneously and under identical temperatures in interleaved parallel domains mm to cm thick in adjacent coarse and fine grained bands medium to high t plasticity is congruent with the syntectonic temperature gradients associated with the amphibolite facies metamorphism of the country rocks we also present the first specific study published so far on natural deformation graphite lattice preferred orientation graphite inclusions as well as those of mica exhibit mineral shape fabrics that suggest operation of 0001 slip however ebsd measurements also record fabrics suggestive of m slip in spite of a rather small volumetric proportion graphite spatial organization at increased shear strains facilitated ductile deformation if a graphite network is established in the rock it can potentially increase rock electrical conductivity thus accounting for mid and lower crust anomalous electric conductivity

effects of h2o on the phase behaviour of the forsterite enstatite system at high pressures and temperatures and implications for the earth abstract in addition to many of the naturally occurring hydrous minerals known in the system mgo sio2 h2o various dense hydrous magnesium silicates 10 a phase phases a b c and d have been synthesized by high pressure and high temperature experiments in the past two decades in conjunction with these high pressure hydrous phases the effects of h2o on the observed phase transformations in the forsterite mg2sio4 enstatite mgsio3 system at high pressures and high temperatures are investigated on the basis of both experimental data and theory for forsterite the modified spinel phase may exist up to 7 1 wt h2o along with enstatite phase a up to 9 9 wt h2o along with stishovite phase a and up to only 3 2 wt h2o along with stishovite phase b the spinel phase of mg2sio4 is no longer stable when more than 3 2 wt h2o is present although both stishovite and the ilmenite type mgsio3 do not occur in dry mg2sio4 at high pressures a small amount of h2o would yield assemblages containing stishovite and the ilmenite phase in mg2sio4 at high pressures the post spinel phase assemblage perovskite periclase may be stable along with phase d up to 11 wt h2o in the case of enstatite except for the assemblage modified spinel stishovite which exists along with phase a up to 7 1 wt h2o and for the perovskite type mgsio3 along with periclase and phase d which exists up to 15 wt h 2 o all the high pressure phase transformations observed in dry mgsio3 may not occur at high pressures if 2 3 wt h2o is present based on experimental observations the various high pressure hydrous phases are stable from as low as 40 kbar to more than 280 kbar at various temperatures thus there is no lack of storage phases for h2o in the deeper interior of the earth s mantle according to the impact induced dehydration model advanced recently by shock wave experiments it is estimated that the earth s interior contains probably at least more than five times its present water contents in the near surface geochemical reservoirs hydrosphere crust so long as both forsterite olivine and h2o coexist in the upper mantle even in trace amounts there must exist a water line which is defined by the reaction 5mg 2 sio 4 3h 2 o 3mgsio 3 mg 7 si 2 o 14 h 6 forsterite water enstatite phase a in the upper mantle the equilibrium boundary of the water line has been in fact determined in two different phase assemblages by experiments but was not noticed if the earth s surface was covered by large scale melts during and shortly after its accretion h2o would be stored in phases a b below the water line and would liberate as water above the water line during freezing today s oceans were most likely derived from water released in the mantle when the palaeogeotherm dropped below the solidus of mantle materials stripping the free water above the water line from depths near 350 km thus the oceans were formed rapidly and early in the earth s history and slow degassing is still going on today the low velocity zone observed in today s mantle is interpreted as a region containing free water between the water line and the stability field of amphiboles in the suboceanic mantle and between the water line and the stability field of phlogopite in the subcontinental mantle in other words the low velocity zone is the free water zone and the water line is the base of the low velocity zone in the earth s mantle

stability of hydrous phases in the transition zone and the upper most part of the lower mantle phase relation for mg2sio4 20 4 wt h2o composition was studied in the pressure range from 11 gpa to 22 5 gpa the sequence of the phase transitions in the temperature range from 800 c to 1000 c with increasing pressure was as follows forsterite water phase a clinoenstatite water phase e water superhydrous phase b phase f water brucite phase f water coexistence of brucite and phase f was observed at pressures above 20 gpa these phases are stable at least up to 22 5 gpa and around 1000 c and are the candidates for the hydrous phase in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle

implications of seismic waveforms complex physical properties associated with stagnant slab 1 anomalously broadened p waveforms of deep focus events were observed at some regional stations after sampling flattened high velocity zones the source processes of these events are short and the p waveforms at other stations as well as the corresponding sh waves do not show such anomaly the rays of the anomalous waves propagated in the vicinity of those whose waveforms can be synthesized with a relatively simple model m3 11 or m2 0 for a flattened cold slab in the transition zone we suggest that the p waveform broadening is caused by sv converted waves at the heterogeneities within a narrow zone near the 660 km discontinuity or slab boundary the creation of such heterogeneities may involve in the complex structure of phase transformation with water and cold temperature anomaly associated with stagnant slab

water transport into the deep mantle and formation of a hydrous transition zone abstract the phase relations in the peridotite water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone results of preliminary experiments to clarify the effect of water on kinetics of the olivine wadsleyite transformation are given and show that water enhances the transformation kinetics significantly there are potential dehydration sites in the deep mantle four sites along the descending slabs depending on their temperature profiles i e the mantle wedge of the subduction zone mantle transition zone top of the lower mantle where hydrous ringwoodite and superhydrous phase b decompose and deep in the lower mantle around 1200 1500 km depth where decomposition of phase d g occurs we also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated the transition zone can serve as a water absorber for the mantle scale water circulation system some of the recent seismic works including the p wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone elevation of the 410 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions

the origin of the variscan upper allochthons in the ortegal complex northwestern iberia sm nd isotopic constraints on the closure of the rheic ocean northwestern iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the variscan orogeny by the late paleozoic collision between laurussia and gondwana are exposed lower allochthons contain ordovician ophiolite known as the lower ophiolite and are structurally overlain by devonian ophiolite upper ophiolite which are in turn structurally overlain by allochthons containing high grade metamorphic rocks with continental affinities and late cambrian early ordovician protolith ages known as the upper units geochemical and sm nd isotopic data from the upper ophiolite and the structurally overlying upper units exposed in the ortegal complex of galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages the general isotopic characteristics of each assemblage are similar to allochthons in other variscan complexes in nw iberia suggesting that the allocht

allochthonous terranes involved in the variscan suture of nw iberia a review of their origin and tectonothermal evolution abstract nw iberia includes a rather complete section of a variscan suture where different terranes with continental or oceanic affinities appear with clear structural relationships three groups of terranes namely upper ophiolitic and basal units and a frontal tectonic melange appear in galicia in cabo ortegal ordenes and malpica tui complexes they constitute a huge allochthonous pile thrust over the iberian parautochthonous and autochthonous domains which represent the section of the gondwanan margin that escaped continental subduction during the variscan cycle considering the allochthonous character of the nappe pile and the strong deformation associated to the variscan collision there are problems to identify the original tectonic setting of the terranes and thence it is difficult to reconstruct the paleogeographic context during the variscan and pre variscan times in detail key features to perform any model for the variscan convergence should consider the existence of two different high p metamorphic events dated at c 400 and 370 ma respectively separated in time for the generation of mafic ultramafic sequences at c 395 ma which constitute the most common ophiolites described in the variscan suture some dynamic models developed in nw iberia have important problems to explain the observed tectonothermal evolution especially the older high p metamorphic event and the exhumation of deeply subducted transitional type sections on the other hand the recently discovered participation of an older continental crust in the generation of different protoliths of the middle devonian ophiolites makes difficult the interpretation of these ophiolites in relation to open wide oceanic domains this paper describes the distribution structure lithologies geochronology and chemical composition of the terranes involved in the variscan suture of nw spain the scope of this description ranges from detailed regional aspects to the discussion of the development of the variscan orogen in the context of the assembly of pangea a two stage collisional model affecting a wide gondwanan platform may explain most of the evidences in nw iberia the generation of a long pull apart basin probably occurred after the first collision where the devonian ophiolites were formed these ophiolites have been in general described in the context of the rheic ocean but according to the new existing data they do not seem related to this ocean but rather they were formed after its closure

analysis of meteorological data and the surface energy balance of mccall glacier alaska usa we report on analysis of meteorological data for the period 27 may 20 august 2004 from two automatic weather stations on mccall glacier alaska usa aimed at studying the relationship between climate and ablation one station is located on a mountain ridge and the other in the ablation area where we also analyzed the energy balance the weather station on the glacier measured an average temperature of 5 38c at 2m height above surface and wind speed of 3 1ms 1 at 3m height a sonic height ranger and ablation stakes indicate a specific mass balance of 1 94 0 09mw e between 15 june and 20 august the specific mass balance calculated from the surface energy balance 2 06 0 18mw e is in close correspondence to this the latter is the sum of 0 12mw e of snowfall 0 003mw e of deposition and 2 18mw e of melt net radiation contributes 74 of the melt energy compared to ablation measurements in the early 1970s summer ablation was large this increase is explained by a combination of a relatively higher net radiation a lower albedo and larger turbulent heat fluxes that led to more energy being available for melting no single meteorological variable can be isolated as being the principal reason for the high ablation however the lower ice albedo 0 19 is possibly due to ash deposits from forest fires

hydration adjacent to a deeply subducting slab the roles of nominally anhydrous minerals and migrating fluids subduction related transport of water into the mantle has significant dynamical and geochemical implications dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle potentially hydrating nominally anhydrous minerals nam and impacting the viscosity and density structure of the mantle over a wide area we present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle a sharp decrease in water storage capacity across the lower boundary of the transition zone may produce secondary dehydration of hydrated nam leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle water stabilized in nam and as a free fluid can initiate upwelling within and above the transition zone a less abrupt change in water storage capacity across the base of the transition zone leads to high nam water contents in a channel adjacent to the slab where viscosity is reduced however seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity

structural analysis of high pressure shear zones bacariza formation cabo ortegal nw spain abstract high pressure granulites of the bacariza formation cabo ortegal complex nw spain exhibit spectacular examples of ductile shear zones developed at different scales in rocks containing pre existing foliations a detailed structural analysis was carried out on these shear zones in order to unravel and compare the role of various parameters controlling the deformation process i e heterogeneous simple shear components of homogeneous deformation heterogeneous volume change and degree of non coaxiality although heterogeneous simple shear largely dominated negligible deviations from the ideal simple shear model were detected involving shortening along the structural directions perpendicular to the stretching axis within the foliation plane of the finite strain ellipsoid the relationship between displacement parallel to a half shear zone and the normal distance from its boundary provided the basis for the estimation of the stress exponent in the power law constitutive flow equation associated with each shear zone which is interpreted as a rheological indicator these geometric and rheological results and the thermobaric conditions of high pressure shear zone deformation indicate that these shear zones accommodated dominant plastic rock flow coeval with high pressure and high temperature deformations under moderate stress levels concomitant with elevated strain rates

stability of hydrous magnesium silicates in the mantle transition zone phase relations in the mg oh 2 sio2 system have been studied up to 18 gpa a new hydrous phase phase e mg2 3si1 3h2 4o6 was encountered at 13 17 gpa and 1000 c phase f mg1 2si1 8h2 4o6 another new hydrous phase was observed at 15 5 gpa and 800 c the stability field of phase a was also briefly studied these new hydrous phases might be stabilized in the mantle transition zone especially in subducting slabs at those depths

petrogenetic grid in the system mgo sio2 h2o up to 30 gpa 1600 c applications to hydrous peridotite subducting into the earth s deep interior 1 a semiquantitative petrogenetic grid in the system mgo sio2 h2o up to 30 gpa and 1600 c was constructed using schreinemakers analysis on previous experimental data the grid includes stability relations of hydrous wadsleyite hydrous ringwoodite and dense hydrous magnesium silicates dhmss phase a phase d phase e and superhydrous phase b a sequence of chemical reactions among these hydrous phases was clarified in the mantle transition zone 410 660 km depth hydrous wadsleyite and hydrous ringwoodite are stable even at a standard mantle temperature of 1600 c whereas in the other depths nominal hydrous phases including dhmss are stable below 1400 c newly found phase transitions in hydrous wadsleyite and hydrous ringwoodite are multiphase and multireaction because their compositions are on neither mgo sio2 nor mg2sio4 h2o tie lines different from single transition in the dry mg2sio4 system the grid indicates that water in the subducting slab peridotite would be transported to the bottom of the upper mantle by several dhmss and hydrous polymorphs of olivine via solid solid reactions after antigorite decomposition finally dhmss would dehydrate to liberate free water at the upper mantle lower mantle boundary layer or deeper level we also examined a relation between the predicted depth distributions of dehydration reactions in slab peridotite along several possible pressure temperature paths and the mode of seismic frequency in each subduction zone the result suggests a possible origin of intermediate to deep seismicity by the dehydration of hydrous phases in the subducting slab

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

cation ordering in omphacite and effect on deformation mechanism and lattice preferred orientation lpo we present microstructural data and lattice preferred orientations lpos of omphacites from a suite of eclogites from the adula cima lunga nappe central alps our work shows a surprisingly strong correlation between the measured lpo and the ordering state of cations in omphacite estimates of deformation temperature from metamorphic petrology together with measured omphacite compositions and lpos determine the field ordering state on the omphacite phase diagram into which each sample falls lpos dominated by l type and s type signatures are developed in samples that fall in the p2 n field ordered structure and c2 c field disordered structure respectively dislocations with b 1 2 110 or b 001 are observed in the transmission electron microscope tem in all samples the former change from a perfect dislocation in the c2 c structure to a partial in p2 n any movement of a partial dislocation requires the formation or growth of a stacking fault furthermore in order to pass an obstacle a partial dislocation has to constrict to a unit dislocation the energy to form a constriction is high in omphacite due to the large separation width thus the activity of the b 1 2 110 dislocation is hindered in the p2 n structure relative to the c2 c structure which change the balance between the two and might give rise to the different lpos

metamorphic evolution and thermobaric structure of the subduction related bacariza high pressure granulite formation cabo ortegal complex nw spain abstract the high pressure bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses ultramafic mg rich mafic intermediate and common mafic granulites as well as of more exotic intercalations mineral assemblages in equilibrium in ultramafic to intermediate rocks contain garnet clinopyroxene and plagioclase with different amounts of zoisite clinozoisite kyanite quartz scapolite rutile and ilmenite depending on the granulite lithotype whereas granulite orthogneisses contain garnet phengite biotite k feldspar antiperthitic plagioclase quartz and rutile as primary phases thermobarometry of these rocks supports the existence of a high pressure metamorphism for which near peak p t conditions have been estimated at ca 790 c and 1 6 gpa the preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes dynamic retrogression began under still high pressure granulite facies conditions 1 4 gpa and ca 740 c and is postdated by symplectitization 1 3 gpa and ca 715 c subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift loading heating and subsequent decompression cooling are related to a single cycle in a subduction conduit setting this study suggests that high pressure granulite metamorphism might not be as uncommon in the high pressure metamorphic series as previously thought moreover it might constitute a diagnostic feature of convergent lithospheric settings whether or not associated with eclogite facies metamorphism in adjacent units

a single devonian subduction event for the hp ht metamorphism of the cabo ortegal complex within the iberian massif abstract ion microprobe data are reported for zircons from ultramafic mafic and metasedimentary high pressure hp rocks of the cabo ortegal allochthonous complex galicia nw spain the new data suggest that the protoliths of eclogites morb type and mg and al rich and mafic granulites have formed in early ordovician times 495 11 469 9 and 478 12 ma respectively involving oceanic and or continental mantle sources we have found no evidence for a hp metamorphic event at 480 ma and for the previously proposed second hp event in the silurian instead a single subduction induced pt t loop with a climax around 390 ma early devonian can be deduced from the relevant domains of zircons from the following rocks two different types of eclogites two hp paragneisses one garnet pyroxenite and one spinel harzburgite rapid cooling and exhumation minimum rates of ca 90 c ma and ca 5 8 km ma are derived for the time immediately after the peak conditions after cooling to ca 500 c slower cooling and exhumation rates ca 15 18 c ma and ca 0 18 km ma are probably artefacts due to an episode of reheating at ca 350 ma this interpretation is supported by the scatter of various mineral ages and the presence of synkinematic granitoids a subduction related acadian early devonian hp metamorphism followed by an early carboniferous collision can thus be demonstrated for the first time also for the iberian part of the european variscides

phase relations in the transition zone phase relations in the systems mgo al2o3 sio2 and cao mgo sio2 have been investigated experimentally at 166 226 kbar pressures and 1350 2500 c temperatures with a split sphere anvil apparatus ussa 2000 mgsio3 ilmenite is stable to lower pressures 175 kbar than found previously casio3 perovskite forms at p bar 15t c 151 000 three new high pressure phases are reported a phase with the diopsidic composition the cm phase a high pressure polymorph of al2o3 and superphase b with the tentative composition mg10si3o14 oh f 4 the invariant point for coexisting majorite mg4si4o12 perovskite mgsio3 beta phase mg2sio4 and liquid has been located at 224 kbar 2430 c with the eutectic melt composition on the enstatite forsterite join at 40 44 wt of forsterite the melting temperature of mgsio3 perovskite at 225 kbar is predicted to be 2600 c the new data made possible the calculation of a temperature pressure phase diagram for the system cao mgo al2o3 sio2 cmas in the pressure range 0 280 kbar the mineralogy of the earth s mantle and the possible mechanisms for producing large seismic velocity gradients in the transition zone are discussed on the basis of the derived phase relations

on the relationship between kilometer scale sheath folds ductile thrusts and minor structures in the basal high pressure units of the cabo ortegal complex nw spain abstract large sheath folds in the basal high pressure nappes of the cabo ortegal complex are described and a kinematic interpretation provided the principal penetrative and map structures relate to regional d2 deformation which produced foliations s2 bearing mineral and stretching lineations l2 and several types of folds a type sheath like and folded folds the latter structures are subparallel to the trend of the orogen their attitude suggests that the units involved shared a common tectonic evolution during progressive ductile deformation of an anisotropic medium reconstruction of major geological structures was accomplished through projection of map scale features onto the ductile flow plane and the plane perpendicular to the ductile flow direction the structures reconstructed illustrate their development in the deeper structural levels of an orogenic channel subjected to high pressure metamorphism during the early phases of the hercynian orogeny in nw iberia we argue that orogen normal tectonic displacements of up to a few hundreds of kilometers represent the minor components of the transpression with possibly thousands of kilometers along strike dextral displacement between the intervening plates during subduction collision

high hydrogen solubility in al rich stishovite and water transport in the lower mantle abstract stishovite is an important phase in subducting oceanic crust the post garnet assemblage from a precursor eclogite lithology contains up to 25 stishovite at pressures above 25 gpa this stishovite may contain up to 5 wt al 2 o 3 we measured the hydrogen contents of stishovite samples synthesized at 20 25 gpa and 1200 1800 c from several starting materials containing up to 10 wt al 2 o 3 ftir spectra of stishovite show major bands at 3111 3134 cm 1 with the frequencies increasing as h 2 o and al 2 o 3 contents increase and several minor bands at 2659 2667 3240 3261 3312 3334 and 3351 cm 1 the h 2 o content of al free stishovite is in the range of 16 30 wt ppm the maximum h 2 o content of al bearing stishovite 4 4 wt al 2 o 3 synthesized at 20 gpa and 1400 c is 3010 300 wt ppm most hydrogen in stishovite is associated with al 3 substitutional defects on the octahedral si 4 site the hydrogen can occupy 40 of vacancies created by incorporation of al 3 at 20 gpa this observation along with some anomalies in the ftir spectra may indicate an alternative mechanism of al 3 incorporation into stishovite via the formation of oxygen vacancies or interstitial al i defects we report the highest h 2 o concentrations in al stishovite to date and argue that it is the most important carrier of water into the lower mantle post garnet eclogitic assemblages

neoproterozoic high pressure metamorphism and tectonic constraint from the nappe system south of the sao francisco craton southeast brazil abstract the neoproterozoic giant nappe system exposed south of the sao francisco craton underwent a minimum of 150 km of near horizontal northeastward displacement this nappe system comprises an uppermost unit derived from a plutonic magmatic arc terrane with high pressure high temperature intermediate to mafic granulites at its base socorro guaxupe nappe underlain in sequence by a high pressure kyanite granulite nappe an amphibolite facies metapelite nappe and finally by a quartzite nappe which forms the lowest part of the pile and rests on a parautochthonous unit related to the passive margin of the craton the kyanite granulites crystallized ca 13 kbar 750 c and have only at their top sillimanite related to t increase overprinting t 890 c a consequence of downward heat advection from the overlying socorro guaxupe granulites equilibrated at 900 c pressures of 13 14 kbar are documented in the underlying metapelite nappe with temperatures of 640 670 c in which lenses of eclogite indicate p max of ca 17 5 kbar phyllites of the parautochthonous unit are directly overlain by hp granulites in the north and record p 7 kbar and t 500 c the distribution of metamorphic facies is reminiscent of inverted metamorphism with eclogites from subduction related metamorphism and kyanite granulites from early extrusion of subducted continental crust the metamorphic conditions of the parautochthon may represent a stage achieved during late metamorphic thinning of the nappe pile the overall picture suggests that high pressure units recrystallized in a subduction zone located to the wsw the frontal thrusts of the nappe system show a late north northeastward transported thin skinned pattern

tectonic evolution of the upper allochthon of the ordenes complex northwestern iberian massif structural constraints to a polyorogenic peri gondwanan terrane the upper allochthon of northwest iberia represents the most exotic terrane of this part of the european variscan belt recent advances in the metamorphic petrology structural geology and geochronology of the upper allochthon in the ordenes complex are integrated into a synthesis of its tectonic evolution constraining the main tectonothermal events important aspects of this synthesis are 1 the interpretation of cambro ordovician magmatism and earliest metamorphic event as the result of drifting of a peri gondwanan terrane 2 the subsequent shortening and crustal thickening of the terrane related to its subduction and accretion to laurussia 3 a younger cycle of shortening and extension resulting from convergence between lau russia and gondwana and 4 the emplacement of this exotic terrane as the upper allochthon together with underlying ophiolitic and basal allochthons during the laurussia gondwana collision implications derived from the well established tectonothermal sequence are discussed in the context of paleozoic paleogeography and geodynamics the evolution of this part of the belt is related first to the closure of the tornquist ocean and later to that of the eastern branch of the rheic ocean furthermore the relative paleopo sitions of the upper allochthon and the iberian autochthon in northern gondwana are discussed 2007 geological society of america all rights reserved

water partitioning between nominally anhydrous minerals in the mgo sio2 h2o system up to 24 gpa implications for the distribution of water in the earth s mantle multi anvil experiments have been conducted in the mgo sio2 h2o system at pressures of 15 24 gpa and temperatures of 1200 1600 c to investigate the partitioning of water between mantle phases the water contents were obtained from fourier transform infrared spectroscopy on doubly polished and clear crystals most of the mineral phases coexisted with a hydrous melt the results show that among all magnesium silicate phases 1 mgsio3 perovskite is the only phase which does not dissolve a detectable amount of water and 2 the high pressure polymorphs of mg2sio4 wadsleyite and ringwoodite dissolve about one order of magnitude more water than the polymorphs of mgsio3 clinoenstatite majorite and akimotoite the following partition coefficients of water were measured dwadsleyite clinoenstatite 3 8 at 15 gpa and 1300 c dringwoodite akimotoite 21 at 19 gpa and 1300 c dakimotoite perovskite 1 at 24 gpa and 1600 c and dpericlase perovskite 1 at 24 gpa and 1500 c these results suggest that most of the water in the earth s mantle is partitioned in the transition zone

pervasive seismic low velocity zones within stagnant plates in the mantle transition zone thermal or compositional origin abstract we exploit conversions between p and s waves for large scale high resolution imaging of the mantle transition zone beneath northwest pacific and the margin of eastern asia we find pervasive reflectivity concentrated in two bands with apparent wave speed reduction of 2 to 4 about 50 km thick at the top of the transition zone and 100 km thick at the bottom this negative reflectivity associated with the scattered waves at depth is interpreted jointly with larger scale mantle tomographic images and is shown to delineate the stagnant portions of the subducted pacific plate in the transition zone with largely positive shear wave velocity contrasts the upper reflectivity zone connects to broad low velocity regions below major intra plate volcanoes whereas the lower zone coincides locally with the occurrence of deep focus earthquakes along the east asia margin similar reflectivity is found in pacific northwest of the usa we demonstrate that the thermal signature of plates alone is not sufficient to explain such features alternative explanations for these reflective zones include kinetic effects on olivine phase transitions meta stability compositional heterogeneities within and above stagnant plates complex wave propagation effects in the heterogeneous slab structure or a combination of such factors we speculate that part of the negative reflectivity is the signature of compositional heterogeneities as revealed by numerous other studies of seismic scattering throughout the mantle and that such features could be widespread across the globe

melting and crystal liquid partitioning in the system mg2sio4 fe2sio4 to 25 gpa the melting relation of the mg2sio4 fe2sio4 system was studied at 8 5 gpa olivine and spinel melts incongruently into pyroxene and the liquid in the fe2sio4 rich portion of the system olivine melts congruently at least in the compositional range from mg0 5fe0 5 2sio4 to mg0 9fe0 1 2sio4 although the end member of mg2sio4 melts incongruently into periclase and the liquid at pressures above 8 gpa the melting relation of mg0 9fe0 1 2sio4 was also studied up to 25 gpa olivine and modified spinel of this composition melt incongruently into anhydrous phase b and liquid and into magnesiowustite and liquid at higher temperatures in the pressure range from 18 gpa to 23 gpa magnesiowustite is the liquidus phase at higher pressures at least up to 25 gpa in this composition the partition coefficients between crystal and liquid are dmg 1 19 0 06 dsi 0 88 0 04 dv 0 63 0 16 dcr 1 04 0 21 dmn 0 42 0 08 dfe 0 76 0 07 dco 1 24 0 44 dni 1 46 0 81 for modified spinel dmg 1 41 0 06 dsi 0 72 0 03 dv 0 44 0 11 dcr 0 94 0 12 dmn 0 30 0 07 dfe 0 56 0 06 dco 1 19 0 30 dni 1 65 0 87 for anhydrous phase b dmg 2 09 0 11 dv 0 64 0 06 dcr 1 01 0 05 dmn 0 7 0 04 dfe 1 45 0 08 dco 2 73 0 15 dni 4 72 0 25 for magnesiowustite the present results of melting relations and partitioning are applied for the fractionation in the mantles of the terrestrial planets such as mars and mercury

density of basaltic melt at high pressure and stability of the melt at the base of the lower mantle abstract we made density measurements of model morb and picritic melts up to about 15 gpa using the floating diamond method the compression behavior of the melts is fitted by the vinet equation of state and extrapolated to higher pressure a density crossover between the morb melt and the mantle peridotite is suggested at the base of the lower mantle thus the ultra low velocity zone in the d layer may be explained by the existence of gravitationally stabilized basaltic melt formed by melting of the accumulated slab materials this region might become unstable due to progressive melting of slab materials and or supply of volatiles

thermal expansion of wadsleyite ringwoodite hydrous wadsleyite and hydrous ringwoodite abstract the thermal expansions of wadsleyite ringwoodite hydrous wadsleyite and hydrous ringwoodite have been determined by high temperature powder x ray diffraction at ambient pressure wadsleyite and ringwoodite persist up to 700 c and their thermal expansion coefficients are measured as 34 0 5 10 6 and 30 7 6 10 6 k 1 respectively values that are slightly larger than those measured by j phys earth 27 1979 53 j phys earth 28 1980 273 hydrous wadsleyite and hydrous ringwoodite were dehydrated at 450 and 400 c respectively below their dehydration temperatures their thermal expansion coefficients are 30 1 14 10 6 and 27 3 9 10 6 k 1 respectively which are slightly lower than those of their anhydrous forms using the present results combined with the previous thermoelastic data the seismic velocity contrast at the 410 km discontinuity has been calculated for both dry and wet mantle if a pyrolite composition is assumed a hydrous mantle transition zone better explains seismic constraints