differential contraction of subducted lithosphere layers generates deep earthquakes abstract the origin of forces for generating deep earthquakes remains elusive we propose a new mechanical model that involves constant release of shear strain accumulated between the crust and mantle lithosphere caused by differential volume changes during phase transformations as a slab sinks this generates increasing down dip compression inside the slab with depth consistent with the global distribution of deep earthquake focal mechanisms using experimentally calibrated slab rheology we show that the estimated distribution of slab internal stress agrees well with the depth frequency relationship of deep earthquakes in both cold and warm slabs with the peak in seismicity at 600 km depth corresponding to an induced stress maximum this mechanism of residual stress within a cold slab also provides a solution to deep earthquakes occurring beneath former subduction zones such as western tonga and spain we further suggest that this model may reconcile existing geodynamic paradoxes concerning the absolute strength of slabs discrepancies on mantle rock strength between laboratory and geophysical inversions as well as the competition between earthquake generating stresses and slab pull forces

texture development and deformation mechanisms in ringwoodite ringwoodite mg2sio4 with spinel structure is an important phase in the earth s mantle transition zone controlled deformation experiments showed that ringwoodite underwent ductile deformation when compressed axially at 6 10 gpa and at room temperature in a multianvil d dia deformation apparatus texture evolution during cyclic compression has been recorded in situ using x ray transparent anvils with monochromatic synchrotron x ray diffraction and a two dimensional detector quantitative analysis of the images with the rietveld method revealed a 1 1 0 fiber texture by comparing this texture pattern with polycrystal plasticity simulations it is inferred that 111 slip is the dominant deformation mechanism in ringwoodite consistent with high temperature mechanisms observed in other spinel structured materials although strong ringwoodite textures may develop in the transition zone the contribution to bulk anisotropy is minimal due to the weak single crystal anisotropy 2005 elsevier b v all rights reserved

the eclogite garnetite transformation at high pressure and some geophysical implications mineral assemblages displayed by morb and alkali poor olivine tholeiites have been investigated over the pressure interval 4 6 18 gpa at 1200 c both compositions crystallize to form normal eclogites between 4 6 and 10 gpa and there is little change in the relative proportions of garnet and pyroxene over this range however the proportion of garnet increases rapidly above 10 gpa as pyroxene dissolves in the garnet structure and pyroxene free garnetites stishovite are produced by 14 15 gpa dependent upon composition the garnetite facies for both compositions possess zero pressure densities of 3 75 g cm3 implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km it is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with anderson s hypothesis that this region is of eclogitic composition

viscous entrainment by sinking plumes abstract geochemical models invoking several distinct reservoirs in the mantle with different time histories raise important questions about the exchange of mass between them if two of these reservoirs are the upper and lower mantle above and below about 700 km then sinking of cold slabs through this level is one of a number of possible ways in which mixing can occur in addition if slabs do penetrate the transition zone surrounding upper layer material will be dragged downwards we have examined the interaction of very viscous plumes or slabs with density and viscosity interfaces in a series of laboratory experiments using fluids of different viscosities and densities and have documented several mechanisms which can lead to significant entrainment and mixing if a slab remains planar as it passes through a density interface a boundary layer of lighter fluid is pulled into the lower layer and we predict the consequent mass flux when a near vertical slab becomes unstable to folding as it does if it has a sufficient viscosity contrast with its surroundings and its length is greater than about five times its thickness there is another more efficient entrainment mechanism upper layer fluid is trapped between the folds in the slab the effective entrainment increases as the density difference between the upper and lower layers decreases an increase in viscosity with depth also leads to buckling instability and folding of the surrounding material into the slab material on the other hand when there is substantial density difference between the layers a dense slab can cease to sink through the interface but spread out along the interface because it is unstable and incorporates enough upper layer fluid between its folds to become neutrally buoyant the range of slab behaviour occurring in the mantle is not known but we draw attention to the various possibilities and to the implications for mass flux between layers

coupled effects of phase transitions and rheology in 2 d dynamical models of subduction author s arredondo km billen mi abstract 2017 american geophysical union all rights reserved observations of seismicity and seismic tomography provide a present day constraint on the geometry of slabs within the mantle however it is challenging to directly relate the observed shape of slabs to their time evolution phase transitions modify the buoyancy forces within slabs affecting how slabs deform as they sink into the lower mantle and how forces are transmitted to the surface plates here we show that if the crustal shear zone is sufficiently weak 1020 pa s then the coupled effects of a compositionally dependent phase transition model and a stress dependent rheology lead to oscillatory plate speeds and trench motion in response to buckling and folding of the slab on average subducting plate velocity is 4 cm yr and trench motion is 0 7 cm yr however during folding subducting plate speed and trench motion increases by a factor of 3 and trench motion switches from advance to retreat in models with a higher viscosity shear zone 1021 pa s average plate speed is lower 2 cm yr and there is only trench advance stress dependent weakening due to increasing driving stresses can also cause unexpected changes in plate velocity i e decrease or no change in addition due to the added negative buoyancy from the phase transitions slab breakoff occurs in models with a yield stress 500 mpa because of the oscillatory motion of the trench long flat slabs do not form in the transition zone this suggests that other factors such as interaction of the slab with deep mantle flow may be required to create long flat slabs

folding of viscous plumes impinging on a density or viscosity interface summary the possibility that slabs of subducted oceanic lithosphere may buckle and fold under longitudinal compressive stresses when they encounter the major seismic discontinuity near a depth of 670 km in the mantle has led us to explore experimentally the behaviour of very viscous plumes falling onto a density interface or a viscosity step we begin with the case of axisymmetric and planar streams falling through air onto the free surface of a viscous fluid which has the same or smaller density and observe the conditions under which they coil or fold in order to remove surface tension and to introduce effects of a viscous environment we study plumes falling onto a density interface between two generally very viscous liquid layers stability to buckling is found to depend largely on a geometrical criterion based on the length to thickness ratio of plumes but also the tendency to buckle is greater for large density differences across the interface and larger viscosity contrasts between plume and upper layer when plumes are unstable coils or folds are laid down with a horizontal scale in a fixed proportion to the thickness of the plume and the frequency of folding is determined by the velocity and thickness of the slab as it enters the region of compressive stress entrainment of upper layer fluid can have a marked effect on the fate of the folded plume material which may spread at the interface or continue to descend into the lower layer similar behaviour is observed at a viscosity interface application of results to the mantle depends on the nature of the 670 km discontinuity and is therefore inconclusive however cool slabs are likely to be unstable if mantle circulation involves two layers separated by a density interface or if whole mantle circulation involves a large viscosity increase with depth near 700 km

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

dynamic models of subduction geophysical and geological evidence in the tyrrhenian sea summary predictions based on a 2 d finite element model for subduction underneath the calabrian arc in southern italy are compared with a variety of geophysical and geological data such as the present day stress pattern within the slab uplift from the elevation of marine terraces in calabria and subsidence in the tyrrhenian marsili basin from odp leg 107 we model the behaviour of the slab driven by slab pull in agreement with the present tectonic style in this part of the mediterranean as suggested by several investigators the model accounts for the crustal lithospheric and mantle structures in a vertical cross section perpendicular to the calabrian subduction zone the shape of the slab is constrained on the basis of new tomographic images in the southern tyrrhenian sea which were obtained from the regional seismic stations of the istituto nazionale di geofisica while the rheological properties of the mantle are taken from global dynamic models density contrasts between the subducted slab and the surrounding mantle based on petrological models drive the flow in our viscoelastic model stress values displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached our estimates are appropriate for a time window of 100 kyr the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day two families of models with unlocked and locked subduction faults are considered the unlocked models allow for roll back of the trench of about 20 mm yr in agreement with some geological estimates the same family of models predicts uplift of the calabrian arc of about 1 mm yr l and subsidence in the marsili basin of 1 2 mm yr in agreement with geological surveys the deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern tyrrhenian sea with minor concentration within the lithospheric wedge locked models fail to reproduce these geophysical and geological observations predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern tyrrhenian sea deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate dynamic topography and horizontal motions are reduced when compared with the continuous plate with deviatoric stresses concentrated within the relict slab our results indicate that subduction is a major tectonic process in the southern tyrrhenian sea

dynamic modelling of the subduction zone of central mexico summary in central mexico some significant normal faulting events have occurred within the subducted oceanic cocos plate just below or near the down dip edge of the strongly coupled interface these normal faulting shocks followed large shallow thrust earthquakes in other subduction zones such events generally precede the up dip thrust events a vertical 2 d finite element modelling has been used to simulate the subduction of the cocos plate beneath the north american plate when the slab is driven by an active convergence velocity or slab pull we find that the latter mechanism plays only a minor role due to shallow subduction the modelling results show that the stress pattern is very sensitive to the geometry of the plates in particular normal faulting earthquakes that follow large thrust events can be explained on the basis of the flexural response of the overriding and subducting plates to the peculiar geometry of this subduction zone where the subducting slab becomes horizontal at about 100 km from the trench this horizontal part of the subducting plate down dip with respect to the main thrust zone is under an extensional stress field this provides an alternative explanation to the slab pull for the occurrence of normal faulting intraplate earthquakes in order for normal faulting earthquakes to occur in the early part of the seismic cycle it is necessary that the large up dip thrust events have a partial stress drop we find that for small fractional stress drop a wide region of extension remains below the down dip edge of the main fault plane following a large thrust earthquake thus the main thrust earthquakes do not invert the polarity of the active stress field which is compressional and extensional up dip and down dip respectively with respect to the main thrust fault larger fractional stress drops result in larger delays in the occurrence of normal faulting events after the main thrust events

estimate of the rigidity of eclogite in the lower mantle from waveform modeling of broadband s to p wave conversions broadband usarray recordings of the 21 july 2007 western brazil earthquake mw 6 0 depth 633 km include high amplitude signals about 40 s 75 s and 100 s after the p wave arrival they are consistent with s wave to p wave conversions in the mantle beneath northwestern south america the signal at 100 s denoted as s1750p has the highest amplitude and is formed at 1 750 km depth based on slant stacking and semblance analysis waveform modeling using axisymmetric finite difference synthetics indicates that s1750p is generated by a 10 km thick heterogeneity presumably a fragment of subducted mid ocean ridge basalt in the lower mantle the negative polarity of s1750p is a robust observation and constrains the shear velocity anomaly vs of the heterogeneity to be negative the amplitude of s1750p indicates that vs is in the range from 1 6 to 12 4 the large uncertainty in vs is due to the large variability in the recorded s1750p amplitude and simplifications in the modeling of s1750p waveforms the lower end of our estimate for vs is consistent with ab initio calculations by tsuchiya 2011 who estimated that vs of eclogite at lower mantle pressure is between 0 and 2 due to shear softening from the poststishovite phase transition

a speculation on the structure of the d layer the growth of anti crust at the core mantle boundary through the subduction history of the earth abstract the growth curve of the continental crust shows that large amounts of continental crust formed in the early part of the earth history are missing in order to test a hypothesis that the former crust was subducted to the deep mantle we performed phase assemblage analysis in the systems of mid oceanic ridge basalt morb anorthosite and tonalite trondhjemite granite ttg down to the core mantle boundary cmb conditions results show that all these materials can be subducted to the cmb leading to the development of a compositional layering in the d layer we speculate that there could be five layers of feo enriched melt from partial melting of morb morb crust anorthosite ttg and slab or mantle peridotite in ascending order although the polymorphic transformation of perovskite to post perovskite in mg fe sio3 may explain the seismic discontinuity at the top of the d layer d discontinuity the effects of solid solution on the sharpness of the transformation suggest that the compositional layering is more plausible for the origin of the d discontinuity the d layer can be an anti crust made up mostly of ttg anorthosite derived from the former continental crust tectonic style of the anti crust at the cmb is similar to that at the surface at both places chemically distinct layers are density stratified and are also characterized by the processes of accretion magmatism and metasomatism

phase transformations in a harzburgite composition to 26 gpa implications for dynamical behaviour of the subducting slab abstract the mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5 26 gpa at temperatures of 1300 1400 c the pyroxene garnet component of the harzburgite composition harzburgite minus 82 wt olivine transforms to majorite garnet by 18 19 gpa and further disproportionates to the assemblage of garnet stishovite mg 2 sio 4 spinel above 20 gpa at still higher pressures first ilmenite 22 24 gpa and then perovskite mgsio 3 24 26 gpa are found to coexist with garnet garnet disappears at 26 gpa and almost complete transition to perovskite is achieved at this pressure the mineral proportions and density profiles in the subducting oceanic lithosphere modelled by a combination of 80 harzburgite 20 primitive morb compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km under isothermal conditions the slab has a density similar to surrounding mantle to a depth of 600 km however between 600 and 700 km the slab is up to 0 08 g cm 3 denser than surrounding mantle this is caused primarily by the higher alumina content in pyrolite as compared to harzburgite which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite the presence of alumina also smears out the garnet perovskite transition in pyrolite over a depth interval of 50 km whereas this transformation is much sharper in the harzburgite composition calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser by 0 1 0 3 g cm 3 than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth these results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity

famennian charcoal of belgium abstract charcoal results from an incomplete combustion of biological material in anoxic conditions and has a high potential for three dimensional cellular preservation most of the belgian upper devonian flora is known through compression fossils mainly from the siliciclastic evieux formation no charcoal bearing layers had been reported to date we have recently identified organic rich beds from three late famennian localities val dieu quarry trooz quarry and arbre quarry the three localities are described from the sedimentological point of view the arbre and trooz quarries contain allochthonous specimens of charcoalified wood they are identified as callixylon in arbre in trooz the specimen belongs either to an undescribed species of callixylon or to another taxon of woody pro gymnosperm the val dieu quarry by contrast yielded a more diverse charcoalified assemblage including leafy remains branches and fertile axes related to ferns sphenopsids and pro gymnosperms this material experienced moderate transportation before burial the val dieu assemblage is exceptional for late devonian deposits as it represents a moderately disturbed litter interpreted as representing a swamp environment and its proximal margins this assemblage is an illustration that the fossil charcoal record can provide a unique opportunity to study in fine detail the evolutionary history and past diversity of life

the interplay between global tectonic processes and the seismic cycle in the umbria marche seismogenic region summary for the central apennines peninsular italy a series of tectonic mechanisms are reproduced by means of finite element models in order to study the effects of active tectonics on the seismic cycle in the umbria marche seismogenic zone continental extension and rift push effects induced by small scale convection are modelled within 2 d viscoelastic models of the crust lithosphere system in vertical cross sections perpendicular to the strike of the major tectonic structures under study namely the apennines and the colfiorito fault zone where the 1997 seismic sequence took place with the aim of constraining the active tectonic mechanisms at the regional scale and the behaviour of the fault in the seismogenic zone at the local scale modelled baseline rate of change are compared with newly acquired gps data retrieved from the two permanent gps receivers of camerino came and elba elba deliberately installed along the modelled transect these receivers are located at both edges of the continental extension in the front of the apennines close to the adriatic plate in the east and in the rear of the chain in the tyrrhenian domain the deformation pattern inferred from seismicity and from the geodetic data is consistent with small scale convection in the tyrrhenian domain which reproduces extension in the rear of the apennines and compression in the front of the chain a convective mechanism associated with backarc opening and doming of the asthenosphere provides an extensional rate along a baseline connecting two sites in the front of the chain camerino and in its rear elba comparable to the observed baseline rate of change the viscosity of the lower crust plays a fundamental role in determining the style of stress in the crust lithosphere system once constrained by means of the extensional baseline rate inferred from gps the modelled slip across the colfiorito fault and the modelled earthquake recurrence time are consistent with the 1997 normal fault event and with palaeoseismicity respectively

an experimental investigation of the pyroxene garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle abstract the pyroxene garnet transformation in a model pyrolite composition has been investigated over the pressure interval 4 0 to 17 5 gpa at temperatures of 1200 1400 c the transformation is largely completed in the pressure interval 14 16 gpa and a single phase aluminum deficient garnet majorite becomes stable above 16 gpa the mineral assemblages and the density and seismic velocity distributions for pyrolite are calculated as a function of depth on the basis of this and other recent experimental data density and seismic velocity profiles calculated for pyrolite are consistent with models based on seismic observations to a depth of 550 km if the existence of a high density velocity gradient between 550 km and 650 km is confirmed the presence of further phase transformations in majorite and or some degree of chemical heterogeneity in this depth interval would be indicated

is subducted lithosphere trapped at the 670 km discontinuity recent work1 2 on phase equilibria has shown that subducted oceanic lithosphere which is differentiated into a basaltic crust and a harzburgitic residue layer could be significantly denser than undifferentiated peridotite at depths between 650 and 700 km but would be less dense in the range 700 740 km it might thus be trapped in a gravitational stable layer between upper and lower mantle3 here using numerical experiments of isoviscous convec tion i explore the stability of such a layer assuming that it had previously been formed the results suggest that the survival of the intermediate layer is at best marginally feasible provided that new subducted lithosphere is efficiently trapped

phase transformations in diopside camgsi2o6 at pressures up to 25 gpa phase transformations in diopside have been studied at pressures up to 25 gpa and at temperatures of 1100 and 1500 c by a multiple anvil apparatus the following phase assemblages are found to be stable at given pressures diopside 15 and 18 gpa mg2sio4 spinel stishovite casio3 perovskite 19 and 20 gpa mgsio3 ilmenite casio3 perovskite 22 gpa and mgsio3 perovskite casio3 perovskite 24 5 gpa electron microprobe analysis reveals that the solubility of the mgsio3 molecule in casio3 cubic perovskite is limited to between 2 5 the solubility of the casio3 molecule in mgsio3 orthorhombic perovskite is more restricted being of the order of 2 or less these results are contradictory to earlier work in which diopside is reported to transform into a cubic perovskite structure of the same composition above 22 23 gpa

an experimental study of the garnet perovskite transformation in the system mgsio3mg3al2si3o12 phase transformations in the system mgsio3 mg3al2si3o12 have been studied at pressures between 22 and 28 gpa at 1500 c using a multi anvil apparatus majorite garnet with less than about 10 wt al2o3 was found to completely transform into an orthorhombic perovskite structure over a pressure interval between 24 and 27 gpa whereas pyrope garnet disproportionated into sub aluminous perovskite plus corundum solid solutions at pressures greater than 26 5 gpa alumina contents in the perovskite structure are dependent on temperature and increase from 11 wt at 1500 c to 17 wt at 1800 c at a pressure of 27 gpa the alumina content of perovskite also in creases with increasing pressure such that aluminous perovskite with the pyrope composition 25 mol al2o3 should be formed by about 30 40 gpa these observations are consistent with our recent experimental results on a pyrolite composition which demonstrated that no separate aluminous phase exists in the upper part of the lower mantle the entire inventory of al2o3 in pyrolite can be accommodated in solid solution in magnesium silicate perovskite

sound velocity measurements of casio 3 perovskite to 133 gpa and implications for lowermost mantle seismic anomalies abstract we report the measurements of aggregate shear velocity v s of casio 3 perovskite capv at high pressure p between 32 and 133 gpa and room temperature t on the basis of brillouin spectroscopy the sample had a tetragonal perovskite structure throughout the experiments the measured p v s data show the shear modulus and its pressure derivative at ambient condition to be g 0 115 8 gpa and g 1 20 respectively the zero pressure shear velocity is determined to be v s0 5 23 km s in good agreement with the previous estimate inferred from the ultrasonic measurements on ca si ti o 3 perovskite at 1 bar our experimental results are broadly consistent with the earlier calculations on tetragonal capv but exhibit lower velocity at equivalent pressure such tetragonal capv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle while primitive mantle includes certain amount of capv a depleted peridotite former harzburgite layer in subducted oceanic lithosphere is deficient in capv and enriched in ferropericlase in the lower mantle such harzburgite exhibits 0 9 faster v s and 0 7 slower bulk sound velocity v at the lowermost mantle p t conditions if capv is present in the tetragonal form in the surrounding mantle the observed fast v s and slow v anomalies in the d layer underneath the circum pacific region might be attributed in large part in the presence of subducted harzburgitic materials

interplay between subduction and continental convergence a three dimensional dynamic model for the central mediterranean summary the mediterranean region is attracting considerable attention due to the complexities of its tectonic setting which is considered worldwide a unique natural laboratory for studying the occurrence of extensional tectonics in a general context of continental convergence the tyrrhenian apennine system is controlled by the west dipping subduction of the adria ionian lithosphere and by the near north south convergence between the african and eurasian plates we provide the first 3 d dynamic model of the central mediterranean that quantifies the effects of subduction and convergence on surface deformation in simplified geometry the axis of the model extends from sicily to the alps along the subduction hinge line a convergence rate of 1 cm yr 1parallel to the subduction hinge has been applied to the tyrrhenian block in agreement with global plate motion models density contrasts within the slab cause the gravitational sinking and roll back of the slab in the southern tyrrhenian domain modelling results show a gradual decrease of hinge retreat from south to north with values ranging between 8 and 2 mm yr 1 indicating that the arculate geometry of the hinge line along the italian peninsula is ultimately controlled by the interplay between subduction and convergence the pattern of vertical velocity along directions perpendicular to the hinge with subsidence in the foredeeps and uplift at the eastern border of the tyrrhenian domain is maintained along the whole italian peninsula with higher values in the southern areas

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

melting and phase relations in the system mg2sio4 mgsio3 at 20 gpa under hydrous conditions an experimental study was performed on the melting and subsolidus phase relations in a synthetic magnesium silicate system at a pressure of about 20 gpa and over the temperature range of 1700 2500 c under hydrous conditions the modified spinel mg2sio4 is stable up to 2250 c above which it melts incongruently into phase b mg23si8o42h6 and liquid at higher temperatures phase b melts again incongruently into periclase and liquid the liquid coexisting with periclase has an atomic ratio mg si 1 5 at 2360 c and 1 7 at 2500 c for mgsio3 composition the ilmenite is stable at lower temperatures and it transforms to a garnet with increasing temperatures to 2000 c at above 2200 c clinopyroxene replaces the garnet and then melts congruently at 2250 c the triple point of clinopyroxene garnet and liquid is closely located at 20 gpa and 2250 c the binary system of mg2sio4 mgsio3 shows a eutectic melting between modified spinel and garnet with a eutectic point located at 2150 c and mg si 1 3

a comparison of garnet ilmenite perovskite phase equilibria in germanate and silicate systems at high pressures abstract phase equilibria have been investigated in the system cageo 3 mggeo 2 al 2 o 3 and on germanium analogues of basalt and a pyrolite related system at pressures between 2 and 11 gpa and at temperatures between 850 and 1200 c solid solubility relationships between garnet ilmenite and perovskite type phases were determined and compared with those in the corresponding silicate systems there are some interesting similarities where the germanate equilibria are able to provide insight into the phase chemistry of corresponding silicate systems however there are also some significant differences that limit the ability of germanate models to predict the probable behaviour of corresponding silicate systems at higher pressures