the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

numerical modelling of spontaneous slab breakoff dynamics during continental collision slab detachment or breakoff is directly associated with phenomena like morphological orogenesis occurrenceofearthquakesandmagmatism atdepththedetachmentprocessisslowand characterized by viscous rheolgy whereas closer to the surface the process is relatively fast and plastic using a 2d mantle model 1500 km deep and 4000 km wide we investigated with finite difference and marker in cell numerical techniques the impact of slab age convergence rate and phase transitions on the viscous mode of slab detachment in contrast to previous studies exploring simplifiedbreakoffmodelsinwhichtheblockageresponsibleforinducingbreakoffiskinematically prescribed we constructed a fully dynamic coupled petrological thermomechanical model of viscousslabbreakoff inthismodel forcedsubductionofa700 km longoceanicplatewasfollowed bycollisionoftwocontinentalplatesandspontaneousslabblockingresultingfromthebuoyancyof thecontinentalcrustonceithadbeensubductedtoadepthof100 124 km typically fivephasesof model development can be distinguished a oceanic slab subduction and bending b continental collision initiation followed by the spontaneous slab blocking thermal relaxation and unbending in experiments with old oceanic plates in this phase slab roll back occurs c slab stretching and necking d slab breakoff and accelerated sinking and e post breakoff relaxation our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models the results also demonstrate a non linear dependence of the duration of the breakoff event on slab age a positive correlation being characteristic of young 50 ma slabs while for older slabs the correlation is negative the increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness which increases both the slab thermal relaxation time and duration of the neckingprocess inolderslabsthistendencyiscounteractedbynegativeslabbuoyancy whichgen eratehigherstressesthatfacilitateslabneckingandbreakoff apredictionfromourbreakoffmodels isthattheolivine wadsleyitetransitionplaysanimportantroleinlocalizingviscousslabbreakoffat depths of 410 510 km due to the buoyancy effects of the transition

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

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

the characterization of viscous remanent magnetization in large and small magnetite particles previous work on the properties of viscous remanent magnetization vrm have shown that it can play an important role in some paleomagnetic investigations the results of this study on the properties of vrm of very fine single domain sd and coarse multidomain md magnetite particles ranging in size from 0 02 to 60 m indicate that the md samples are very sensitive to the previous magnetic history of the samples the af demagnetization causes a dramatic decrease in the ability of md samples to acquire vrm compared to the same sample with an undemagnetized thermal remanent magnetization trm thermal demagnetization of the same md samples causes an increase in the ability to acquire vrm components submicron sd samples show vrm acquisition and decay curves that are independent of previous magnetic treatment both md and sd samples are extremely sensitive to the length of time of zero and weak field storage that occurs prior to the application of the external vrm field a model using the total energy of the system is used to describe the above results

evolution of the interior of mercury influenced by coupled magmatism mantle convection system and heat flux from the core to discuss mantle evolution in mercury i present two dimensional numerical models of magmatism in a convecting mantle thermal compositional and magmatic buoyancy drives convection of temperature dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4 5 gyr calculated history the stratified structure persists for 4 5 gyr when the reference mantle viscosity at 1573 k is higher than around 1020 pa s the planet thermally contracts by an amount comparable to the one suggested for mercury over the past 4 gyr mantle upwelling however generates magma only for the first 0 1 0 3 gyr at lower mantle viscosity in contrast a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0 3 0 8 gyr convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size these models however predict larger contraction of the planet coupling between magmatism and mantle convection plays key roles in mantle evolution and the difficulty in numerically reproducing the history of magmatism of mercury without causing too large radial contraction of the planet warrants further exploration of this coupling

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

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

phase transformations of ca3al2si3o12 grossular garnet to the depths of the earth s mantle transition zone abstract high pressure and high temperature phase transformations of ca 3 al 2 si 3 o 12 grossular garnet were examined at 19 26 gpa and 700 2000 k using kawai type multi anvil apparatus coupled with in situ x ray diffraction xrd recovered samples were analyzed by a combination of micro focused x ray diffraction xrd and transmission electron microscopy tem the results show that grossular garnet gradually transforms to an al rich casio 3 perovskite at 22 26 gpa and 1000 1400 k the transition boundary can be expressed as p gpa 0 0082 t k 33 05 when the garnet completely disappears we observed orthorhombic casio 3 perovskite with a grossular composition at 20 24 gpa and temperatures above 1500 k the cas phase with the composition caal 4 si 2 o 11 appears to accommodate excess al from the perovskite along with two distinct populations of al bearing casio 3 perovskites with al content of 3 7 and 10 0 wt al 2 o 3 respectively the pressure and temperature of these transitions correspond to the lowermost part of the transition zone and therefore it suggests that ca rich aluminosilicates could provide alternative candidates to explain multiple seismic reflections near the 660 km depth discontinuity

global variations of earth s 520 and 560 km discontinuities abstract we investigate seismic discontinuities in the mantle transition zone mtz by analyzing ss precursors recorded at global seismic stations our observations confirm the global existence of the 520 km discontinuity although substantial regional depth variations in the 520 km discontinuity are generally correlated with temperature in the mid mtz they cannot be fully explained by the clapeyron slope of the wadsleyite ringwoodite phase transition suggesting both thermal and compositional heterogeneities in the mtz a second discontinuity at 560 km depth previously interpreted as splitting of the 520 km discontinuity is most commonly detected in cold subduction zones and hot mantle regions the depth separation between the 520 and 560 km discontinuities varies from 80 km in cold regions to 40 km in hot areas the exsolution of calcium perovskite ca pv from majorite garnet has been proposed to explain the velocity and density changes across the 560 km discontinuity however the gradual exsolution of perovskite and partitioning of ca and al between perovskite and garnet appear inconsistent with the relatively sharp discontinuity in seismic observations and thus need to be revisited in the future nevertheless because the only known transition in major minerals at this depth in the mtz is the formation of ca pv the existence of the 560 km discontinuity may imply localized high calcium concentrations in the mid mtz possibly related to the recycling of oceanic crust

frequency dependent analysis of sks skks waveforms observed in australia evidence for null birefringence abstract we analyzed sks skks waveforms recorded by permanent broadband stations in australia and found a lack of birefringence signals in general are absent on the transverse t component of high quality data over a wide range of back azimuths at periods longer than 6 s including the dominant periods of 8 10 s for sks skks there is no exception out of 89 high quality observations at periods between 1 3 and 3 s eight observations show appreciable signal on the t component the anomalous signals on the t component however are unlikely to be a result of azimuthal anisotropy in the upper mantle 1 for a particular earthquake station pair anomalous signal is present only in either skks or sks but not in both since the fresnel zones of these two phases overlap in the upper mantle this observation alone rules out simple homogeneous layers of azimuthally anisotropic upper mantle as the main cause of anomalous signals 2 the small number of anomalous observations are restricted to high frequencies over limited back azimuths and distances 3 signals on the t component do not resemble the time derivative of waveforms on the radial component these observations and an analysis of fresnel zones suggest that the anomalous waveforms are probably caused by scattering associated with small scale heterogeneities in a corridor trending roughly east west in the lower mantle if the structure lies near a depth of approximately 1000 km the source of anomalous propagation correlates with the estimated position of paleo pacific lithosphere that was subducted more than 30 60 ma ago suggesting remnant slab in the lower mantle as a plausible cause of anomalous sks skks arrivals the lack of azimuthal anisotropy beneath australia may be related to its high speed of current plate motion in the hotspot frame of reference a feature unique to the indo australian plate sparse sks skks data from the indian shield also support this view

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

subaqueous eruption and emplacement of the shinjima pumice shinjima moeshima island kagoshima bay sw japan abstract the shinjima pumice is a fines depleted pumice lapilli tuff emplaced several thousands years ago at about 100 140 m below sea level this 40 m thick deposit comprises many poorly defined flow units which are 1 10 m thick diffusely stratified and showing upward coarsening of pumice clasts with a sharp to transitional base parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts especially in the lower half of the flow units pumice clasts of block to coarse lapilli size commonly have thermal contraction cracks best developed on the surfaces demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement these features are suggestive of the direct origin of the shinjima pumice from subaqueous eruptions a theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water logged mass flows

stability of majorite mg fe sio3 at high pressures and 1800 c abstract phase relations in the system mgsio 3 fesio 3 were studied experimentally up to a pressure of 24 gpa at 1800 c alumina free tetragonal garnet exists in the pressure range from 16 to 23 5 gpa the compositional range of the tetragonal garnet phase extends from pure mgsio 3 to mg 0 76 fe 0 24 sio 3 at 18 2 gpa and 1800 c the sequence of the phase transitions in mg rich part of this system at 1800 c is clinopyroxene 17gpatetragonal garnet 22 5gpailmenite 23gpamg perovskite the lattice constants of tetragonal garnet increase with increasing iron content from a 11 517 0 002aandc 11 435 0 003afor mgsio 3 toa 11 543 0 002a and c 11 476 0 003a for mg 0 76 fe 0 24 sio 3 cubic garnet was not observed in the pressure and temperature range studied in this work the alumina free tetragonal garnet observed in this study may exist in the harzbergite layer in the descending slab

coesite and clinopyroxene exsolution lamellae in chromites in situ ultrahigh pressure evidence from podiform chromitites in the luobusa ophiolite southern tibet abstract we report in situ petrological evidence of deep mantle origin from podiform chromitites in the luobusa ophiolite southern tibet analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite which indicate an ultrahigh pressure origin of over 3 gpa 100 km deep the presence of these lamellae coupled with abundant micro inclusions of clinopyroxene requires high solubility of sio2 and cao in the host chromite and suggests a precursor of chromite a cafe2o4 structured high pressure polymorph stable at pressures over 12 5 gpa 380 km deep these nano scale observations and geological occurrence indicate that the mantle peridotite under the tibetan mid ocean ridge was transported from the deep mantle at least 100 km probably more than 380 km deep by the mantle convection it implies that the root of mantle upwelling has a much deeper origin than previously believed

elasticity of majorite and a majorite pyrope solid solution to high pressure implications for the transition zone 1 high seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals such as majorite garnet here we present brillouin scattering measurements of the elasticity of polycrystalline mg majorite mj100 mg4si4o12 and a mj50py50 majorite pyrope solid solution to pressures similar to those at the top of the transition zone 15 1 1 gpa and 12 8 1 gpa respectively the pressure derivatives of the adiabatic bulk ks and shear moduli of both mj50py50 and mj100 are 4 2 3 and 1 4 2 respectively and are equal to those of pyrope within the experimental uncertainties we conclude that neither the adiabatic compression of majorite garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically high velocity gradients are consistent with gradual transformations between minerals with very different elastic properties the transformation of 25 or more of clinopyroxene into majorite above 520 km and the formation of ca silicate perovskite below 520 km could produce the observed seismic gradients

high pressure elasticity of sodium majorite garnet na2mgsi5o12 garnet is the second most abundant mineral phase in the upper mantle and transition zone settings the crystal structure of garnet is quite flexible and hence it is able to accommodate various cations including large incompatible cation such as sodium we used first principles simulation based on density functional theory and two widely used approximations local density approximation lda and generalized gradient approximation gga to explore the crystal structure equation of state and elasticity of sodium bearing majorite garnet with na 2 mgsi 5 o 12 stoichiometry at pressures relevant to the upper mantle and transition zone we find that the pressure volume results based on lda can be explained by a birch murnaghan finite strain equation of state with v 0 1447 6 0 1 a 3 k 0 177 4 0 4 gpa and k 0 3 93 0 02 the results based on gga can be explained by a birch murnaghan finite strain equation of state with v 0 1525 8 0 2 a 3 k 0 160 2 0 4 gpa and k 0 3 96 0 02 the full elastic moduli tensor for na majorite with tetragonal symmetry exhibits slight deviation from the cubic symmetry with c 11 c 33 c 12 c 13 and c 44 c 66 the magnitude of the tetragonal strain also captures the slight deviation from the cubic symmetry at pressures corresponding to the upper mantle and mantle transition zone the compressional wave velocity v p and shear wave velocity v s for the na majorite garnet are fast compared to a wide variety of garnets such as pyrope grossular almandine and majorite garnet although single crystal anisotropy of na majorite is greater than pyrope it is still low compared to the major mantle phases

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

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

partitioning of rare earth elements between casio3 perovskite and coexisting phases constraints on the formation of casio3 inclusions in diamonds minerals with casio3 composition were found as inclusions in diamonds and are considered to be originally of perovskite structure to constrain their genesis and consequently the extent of circulation of mantle material rare earth element ree partitioning between casio3 perovskite and coexisting majoritic garnet 20 gpa 1520 c or mgsio3 perovskite 25 gpa 1600 c was determined by combining the technologies of high pressure synthesis and trace element analysis using ion probe it is consistent with previous experiments and confirms that casio3 perovskite is the main ree depository especially of the light ree kce capv gt is 1900 and decreases gradually to 18 5 of kyb capv gt for capv mgpv it decreases gradually from 57 7 of kce to about 10 of kyb estimated ree concentrations in the source lithology of the casio3 inclusions according to these partitioning coefficients either peridotitic or eclogitic paragenesis show strong enrichment in light ree e g cen 18 163 very different from normal mantle peridotite and subducted oceanic crust it is proposed that interaction with carbonatic melt in the deep mantle may have played an important role in the formation of these casio3 inclusions in diamonds as well as in their ascending transportation

elasticity of pyrope and majorite pyrope solid solutions to high temperatures abstract majorite garnet solid solutions are major mineral phases in the earth s upper mantle and transition zone here we present the first brillouin scattering measurements of the elasticity of majorite mj mg 4 si 4 o 12 pyrope py mg 3 al 2 si 3 o 12 solid solutions mj 50 py 50 and mj 80 py 20 and single crystal elasticity of pure synthetic pyrope at temperatures up to 800 c the temperature derivatives of the adiabatic bulk k s and shear moduli for all compositions along the mj py join are the same within the experimental uncertainties k s t 14 0 14 5 20 mpa k t 8 3 9 2 10 mpa k the temperature dependence of the acoustic velocities for mj py solid solutions is about half that of other major transition zone minerals this implies that temperature variations in the transition zone inferred from lateral velocity heterogeneity can be significantly underestimated if the properties of majoritic garnet are not taken into account

phase transition and equation of state of dense hydrous silica up to 63 gpa although it has previously been considered to be essentially anhydrous al free stishovite can contain up to 1 3 wt of h2o perhaps through the direct substitution si4 4h according to recent studies yet the stability of such substitution and its impact on the properties of silica and rutile structured hydrous phases such as alooh and phase h are unknown at the conditions of the deeper mantle we have synthesized hydrous and anhydrous al free stishovite samples at 723k and 9gpa and 1473k and 10gpa respectively synchrotron x ray diffraction patterns show that the unit cell volume of hydrous stishovite is 1 3 greater than that of anhydrous stishovite at 1bar suggesting significant incorporation of oh in the crystal structure 3 2 0 5 wt h2o at 300k we found a lower and broader transition pressure from rutile type to cacl2 type 28 42gpa in hydrous dense silica we also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts after the phase transition the unit cell volume of hydrous silica becomes the same as that of anhydrous silica showing that the proton incorporation through a direct substitution can be further stabilized at high pressure the lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica rich heterogeneities such as subducted oceanic crust

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

continental crust formation on early earth controlled by intrusive magmatism the global geodynamic regime of early earth which operated before the onset of plate tectonics remains contentious as geological and geochemical data suggest hotter archean mantle temperature and more intense juvenile magmatism than in the present day earth two crust mantle interaction modes differing in melt eruption efficiency have been proposed the io like heat pipe tectonics regime dominated by volcanism and the plutonic squishy lid tectonics regime governed by intrusive magmatism which is thought to apply to the dynamics of venus both tectonics regimes are capable of producing primordial tonalite trondhjemite granodiorite ttg continental crust but lithospheric geotherms and crust production rates as well as proportions of various ttg compositions differ greatly which implies that the heat pipe and plutonic squishy lid hypotheses can be tested using natural data here we investigate the creation of primordial ttg like continental crust using self consistent numerical models of global thermochemical convection associated with magmatic processes we show that the volcanism dominated heat pipe tectonics model results in cold crustal geotherms and is not able to produce earth like primordial continental crust in contrast the plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various ttg rocks using a systematic parameter study we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data low medium and high pressure ttg our study thus suggests that the pre plate tectonics archean earth operated globally in the plutonic squishy lid regime rather than in an io like heat pipe regime

superplumes plates and mantle magmatism in two dimensional numerical models 1 two dimensional numerical models are systematically presented for magmatism in a convecting mantle internally heated by incompatible radioactive elements plate tectonics is self consistently modeled as a part of convection of newtonian fluid with temperature and stress history dependent viscosity driven by thermal and compositional buoyancy while magmatism is modeled as an upward migration of basaltic magma generated by decompression melting ridge magmatism actively takes place to chemically differentiate the mantle and a superplume is well developed in the lowermost mantle as a broad hot region enriched in basaltic component provided that the lithosphere is mechanically strong enough to inhibit spontaneous initiation of subduction by its own weight and that the internal heating rate is sufficiently high hot narrow plumes frequently rise up from the superplume to induce deep magma generation and hot spot magmatism which in turn frequently induces new plate margins the frequent plate margin formation keeps the activity of plate tectonics rather steady the thermal and chemical state of the mantle remains rather steady too as the internal heating rate becomes lower however the narrow plumes from superplume become sporadic and significantly colder deep magma generation becomes rare and new plate margin formation becomes less frequent as a consequence the activity of plate tectonics and the thermal and chemical state of the mantle become fluctuating much with time the mantle structure and plume behavior modeled here match with many observations for those of the earth from the archean to the present

thermal equation of state of majorite with morb composition in situ synchrotron x ray diffraction experiments were conducted using the speed 1500 multi anvil press at spring 8 on majoritic garnet synthesized from natural mid ocean ridge basalt morb whose chemical composition is close to the average of oceanic crust at 19 gpa and 2200 k pressure volume temperature data were collected using a newly developed high pressure cell assembly to 21 gpa and 1273 k data were fit to the high temperature birch murnaghan equation of state with fixed values for the ambient cell volume v0 1574 14 4 a3 and the pressure derivative of the isothermal bulk modulus k tk t 4 this yielded an isothermal bulk modulus of kt0 173 1 gpa a temperature derivative of the bulk modulus kt t p 0 022 5 gpa k 1 and a volumetric coefficient of thermal expansivity a bt with values of a 2 0 3 10 5 k 1 and b 1 0 5 10 8 k 2 the derived thermoelastic parameters are very similar to those of pyrope the density of subducted oceanic crust compared to pyrolitic mantle at the conditions in earth s transition zone 410 660 km depth was calculated using these results and previously reported thermoelastic parameters for morb and pyrolite mineral assembledges these calculations show that oceanic crust is denser than pyrolitic mantle throughout the mantle transition zone along a normal geotherm and the density difference is insensitive to temperature at the pressures in lower part of the transition zone

thermal equation of state of cafe2o4 type mgal2o4 abstract in situ x ray diffraction measurements of cafe 2 o 4 type mgal 2 o 4 have been conducted at pressures up to 42 gpa and temperatures to 2400 k using kawai type multianvil apparatus with sintered diamond anvils additional measurements have also been conducted at pressures to 12 gpa using diamond anvil cell with helium as a pressure medium at room temperature and at temperatures to 836 k at the ambient pressure using a high temperature x ray diffractometer the analysis of room temperature data yielded v 0 240 1 2 a 3 k 0 205 6 gpa and k 0 4 1 3 a fit of the present data to high temperature birch murnaghan equation of state eos yielded k 0 t p 0 030 2 gpa k and 0 a 0 b 0 t with values of a 0 1 96 13 10 5 k 1 and b 0 1 64 24 10 8 k 2 the present data set was also fitted to mie gruneisen debye mgd eos and we obtained 0 1 73 7 q 2 03 37 and 0 1546 104 k density changes of morb have been estimated using the newly obtained thermoelastic parameters assuming that the al rich phase in this composition possesses the cafe 2 o 4 type structure under the lower mantle p t conditions the calculated densities along geotherms for the normal mantle and subducting cold slabs are both significantly higher than those of typical seismological models confirming the conclusion of some recent results on morb by laser heated diamond anvil cell experiments

the effect of pressure on the elastic properties and seismic anisotropy of diopside and jadeite from atomic scale simulation the full elastic constants tensors of diopside camgsi2o6 and jadeite naalsi2o6 have been calculated using a planewave and pseudopotentials based implementation of density functional theory within the generalised gradient approximation at pressures between 0 and 20 gpa both minerals stien over this pressure range with the isotropic average bulk moduli increasing by 50 and the shear moduli by 20 however in detail the behaviour of the individual elastic constants varies and this drives changes in the anisotropy overall and in contrast to predictions based on the extrapolation of calculations based on inter atomic potential models the elastic anisotropy of diopside decreases with increasing pressure the elastic anisotropy of jadeite increases slightly at low pressure exhibits a maximum at around 10 gpa and then begins to slowly decrease despite the small changes in the total and maximum anisotropy the shear wave anisotropy for certain propagation directions vary dramatically with pressure for example the anisotropy experienced by a shear wave propagating in the 010 direction in diopside doubles between 5 and 15 gpa

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

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

seismic evidence for a steeply dipping reflector stagnant slab in the mantle transition zone studies of seismic tomography have been highly successful at imaging the deep structure of subduction zones in a study complementary to these tomographic studies we use array seismology and reflected waves to image a stagnant slab in the mantle transition zone using p and s sh waves we find a steeply dipping reflector centred at ca 400 km depth and ca 550 km west of the present mariana subduction zone at 20n 140e the discovery of this anomaly in tomography and independently in array seismology this paper helps in understanding the evolution of the mariana margin the reflector stagnant slab may be the remains of the hypothetical north new guinea plate which was theorized to have subducted ca 50 ma

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

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

multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes the hypothesis that a single mushroom like mantle plume head can generate a large igneous province within a few million years has been widely accepted the siberian traps at the permian triassic boundary and the deccan traps at the cretaceous tertiary boundary were probably erupted within one million years these large eruptions have been linked to mass extinctions but recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years this observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal single stage plume model suggests here we present numerical experiments to demonstrate that the entrainment of a dense eclogite derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces the characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations such a process may contribute to multiple episodes of large igneous provinces

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

radial 1 d seismic structures in the deep mantle in mantle convection simulations with self consistently calculated mineralogy 1 numerical thermo chemical mantle convection simulations in a spherical annulus geometry with self consistently calculated mineralogy and mineral physics are used to predict detailed deep mantle seismic structures particularly local radial profiles of shear wave velocity vs and bulk sound velocity vb the predicted structures are compared to seismological observations and used to guide the interpretation of seismic observations and to test the model the mantle composition is described as a mixture of morb mid oceanic ridge basalt and harzburgitic end members in the na2o cao feo mgo al2o3 sio2system to assess the influence of chemical variability four different sets of end member compositions are evaluated results confirm that the post perovskite ppv phase causes anti correlated s wave and bulk sound velocities in the deep mantle due to ppv being fast in vs but slow in bulk sound velocity local 1 d seismic profiles display great lateral variability and often have multiple discontinuities in the deep mantle due to morb layers in folded slabs with a positive vs anomaly and negative bulk sound anomaly or the perovskite ppv phase transition the ppv transition is not visible inside piles of segregated morb because of the high temperature and small velocity contrast of ppv in morb piles of segregated morb are seismically slow in both vs and bulk sound despite being expected to be fast in vs because they are hotter than the surrounding material anelasticity has a significant influence on profiles of vs only in the lower thermal boundary layer which corresponds to below 2600 to 2800 km depth depending on region where temperatures are higher than the extrapolated adiabat these results indicate the importance of using a joint geodynamical mineralogical approach to predict and aid in the interpretation of deep mantle seismic structure because interpretations based on seismology and mineral physics alone may be misleading and do not capture the strong lateral variability in 1 d structure obtained here for example multiple reflections arising from folded slabs and the precise balance between thermal and compositional influences on seismic structure

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

absence of density crossover between basalt and peridotite in the cold slabs passing through 660 km discontinuity 1 interaction between subducting slabs and surrounding mantle near 660 km discontinuity is one of the key issues in the dynamic of deep earth distinguishing phase transformations occur in the both major parts of the slab basaltic crust and underlying peridotite post spinel transformation in peridotite takes place at 660 km whereas post garnet transformation in basalt is considered to occur at 720 800 km creating density crossover in this paper we determined the post garnet phase boundary in mid ocean ridge basalt representative for oceanic crust by in situ x ray diffraction studies the phase boundary can be expressed as p gpa 0 0041 t k 17 92 the present results imply that the basaltic component of the cold slab transformed to perovskite assembly at lower pressure than the surrounding mantle and provide direct evidence for penetration of the oceanic crust into the lower mantle without gravitational separation from the peridotite body of the slab

analytical transmission electron microscopy study of a natural morb sample assemblage transformed at high pressure and high temperature natural mid ocean ridge basalt morb samples recovered from diamond anvil cell dac experiments performed between 33 to 89 gpa and 1700 to 2600 k were studied with a transmission electron microscope tem we used the focused ion beam fib lift out technique to prepare the recovered high pressure laser heated samples for tem study observations of tem sections show the presence of five phases for samples transformed at pressures ranging from 33 to 45 gpa al bearing mg perovskite ca silicate perovskite stishovite and two al rich phases the al rich phases were identified by selected area electron diffraction saed patterns and chemical composition analysis and include the new aluminous nal phase with hexagonal structure and the calcium ferrite cf type phase chemical analyses obtained by analytical transmission electron microscopy atem show that mg silicate perovskite is the major host for al with significant amounts also distributed between the cf type and nal phases and less than 1 wt in stishovite beyond pressures of 40 gpa 1100 km depth the al content of mg perovskite and cf type phase increases between 45 and 50 gpa the nal phase disappears this mineralogical change may explain reported seismic anomalies in subduction zones at mid mantle depths

melting of subducted basalt at the core mantle boundary the geological materials in earth9s lowermost mantle control the characteristics and interpretation of seismic ultra low velocity zones at the base of the core mantle boundary partial melting of the bulk lower mantle is often advocated as the cause but this does not explain the nonubiquitous character of these regional seismic features we explored the melting properties of mid oceanic ridge basalt morb which can reach the lowermost mantle after subduction of oceanic crust at a pressure representative of the core mantle boundary 135 gigapascals the onset of melting occurs at 3800 kelvin which is 350 kelvin below the mantle solidus the sio2 rich liquid generated either remains trapped in the morb material or solidifies after reacting with the surrounding mgo rich mantle remixing subducted morb with the lowermost mantle