small scale heterogeneity in the mid lower mantle beneath the circum pacific area abstract data from a western united states short period seismic network are investigated in order to detect anomalous later p wave arrivals within a time window from 50 s to 170 s after direct p waves for deep and intermediate depth earthquakes at circum pacific subduction zones we interpret the anomalous arrivals as s to p scattered waves from heterogeneities in the mid lower mantle the searched regions depths from 4800 km to 2200 km cover approximately 2 of the entire lower mantle the heterogeneous scattering objects are located by array processing of the seismic waves using semblance with careful investigations of arrival time slowness and azimuth of the later phases relative to direct p waves we usually observe two or three later phases which are separated in arrival time by 5 20 s and have similar arrival azimuth and slowness this observation suggests that a few scatterers often form a cluster with a typical spacing of 100 200 km vertically more than ten clusters or groups of s to p scattering objects are detected in the mid lower mantle beneath the circum pacific regions northern kuril izu bonin mariana northern fiji and west coast of south america the s to p scattering objects are found in a depth range from 1100 to 1800 km and the most prominent objects of each region tend to be located around the middle of the depth range the amplitudes of the scattered waves range from 1 to 10 of the direct p waves with exceptionally large values as large as 40 the associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1 and may occasionally exceed 4 of shear velocity considering their high efficiency at converting short period waves wavelengths of 5 10 km the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath south america which appear to be located in the former farallon plate and those at north of fiji which appear to be located around the pacific superplume the mid lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks the most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the pacific plate the extreme estimates of low shear velocity 4 may indicate the prominent drops in the shear modulus due to ferro elastic phase transition in stishovite sio 2 our observations imply that heterogeneity of kilometer scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle it is qualitatively consistent with some recent convection models

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

mid mantle heterogeneities associated with izanagi plate implications for regional mantle viscosity abstract we employed s to p converted waveforms to detect mid mantle scattering beneath northeastern china and the adjacent japan sea broadband and short period waveforms recorded by seismic arrays for eight moderate sized deep earthquakes were analyzed using a non linear array stacking technique and mid mantle scatterers within the depth range 930 1120 km were clearly revealed the heterogeneities have an overall lateral extent of 800 km and mostly occur within a region with a high velocity anomaly the accumulation of morb like slab materials at mid mantle depths might cause a different chemical composition than that of the surrounding peridotitic mantle the spatial isolation of the heterogeneities from the stagnant pacific slab suggests an origin related to the subduction of ancient izanagi plate in combination with the reconstruction history of plate motions we estimate the viscosity of the topmost lower mantle to vary from 1 0 10 22 to 1 6 10 23 pa s which can be used as an independent constraint on the rheology of the lower mantle on a regional scale

mineral assemblages of basalt in the lower mantle high pressure x ray diffraction and analytical transmission electron microscopy of laser heated diamond cell samples show temperature dependent phase assemblages in mid ocean ridge basalt morb at lower mantle pressures the assemblage observed below 2250 k consists of only stishovite plus an orthorhombic perovskite phase space group pbnm or cmcm having a composition close to that of morb but slightly depleted in sio2 with increasing temperature the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at 3250 k to orthorhombic and cubic perovskites plus stishovite and the cafe2o4 phase all of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle

mineral chemistry and density of subducted basaltic crust at lower mantle pressures subducted slabs are less dense than the surrounding mantle near the base of the transition zone 660 km depth because of the survival of garnet in former basaltic crust by this depth mantle peridotite has transformed to denser perovskitite1 2 the buoyancy of the former basaltic crust may contribute to the observed accu mulation or horizontal displacement of many slabs at the base of the transition zone3 here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology stable in the lower mantle synthetic mid ocean ridge basalt more glass subjected to pressures of 45 80 and 100 gpa in a laser heated diamond anvil cell transforms to an assemblage of aluminous mg fe silicate perovskite non quenchable casio3 perovskite stishovite and a sodic aluminous phase with the ca ferrite structure fig 1 per ovskititic more is about 0 06 g cm 3 more dense than a model lower mantle prem derived from seismological data thus even thermally equilibrated perovskititic slabs should encounter no sig nificant hindrance to subduction and convection in 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

source analysis of the crandall canyon utah mine collapse analysis of seismograms from a magnitude 3 9 seismic event on 6 august 2007 in central utah reveals an anomalous radiation pattern that is contrary to that expected for a tectonic earthquake and which is dominated by an implosive component the results show that the seismic event is best modeled as a shallow underground collapse interestingly large transverse surface waves require a smaller additional noncollapse source component that might represent either faulting in the rocks above the mine workings or deformation of the medium surrounding the mine seismic moment tensor results for nuclear explosions explosion and other mining cavity collapses and tectonic earthquakes are compared and the separation of the different populations indicates that the seismic moment tensor may be used for source type discrimination

lower mantle seismic scatterers below the subducting tonga slab evidence for slab entrainment of transition zone materials abstract we show evidence that materials with significantly different elastic properties are juxtaposed in the lower mantle immediately below the subducting tonga slab depths 1000 km like the anomalies preferentially located beneath the lower mantle slabs at other pacific subduction zones kaneshima 2009 array analyses of wave form data of short period seismic networks at western united states and japan for deep earthquakes at the tonga slab reveal s to p scatterers with a size less than the wavelengths 10 km the scatterers are located mostly outside of the slab by several tens of kilometers assuming a locally planar interface for the geometry of the scatterers the amplitudes and polarities of the s to p waves are modeled to constrain the properties of the scatterers we find that the scatterers are steeply dipping the vs increases oceanward across the interface and the vs contrasts are at least comparable to that associated with the post spinel transformation 6 it is unclear at this stage what these subslab scatterers represent so we discuss about three mechanisms which seem plausible from mantle dynamics viewpoints 1 they may represent basaltic rocks which were emplaced by partial melting immediately beneath the former oceanic lithosphere asthenosphere boundary lab before the slab started subducting 2 alternatively the elastic anomalies of the scatterers may be caused by localized presence of dehydrated water or 3 the scatterers may correspond to a sharp boundary between fine grained isotropic rocks in the immediate vicinity of the slab and coarse grained anisotropic rocks more distant from the slab the presence of pronounced and localized elastic anomalies preferentially beneath the slabs in the shallow lower mantle whatever its mechanism is implies that a geophysically observable amount of transition zone material is entrained by the subducting slabs into the lower mantle

distinct compositional thin layers at mid mantle depths beneath northeast china revealed by the usarray abstract we observe a clear seismic arrival at 35 45 s after the direct p wave in usarray recordings of two deep earthquakes occurring beneath northeast china velocity spectrum and beam forming analyses reveal that this arrival has a lower slowness value than the direct p wave and a back azimuth slightly different from the great circle direction the measured slowness and arrival time indicate that it is a transmitted s to p conversion from structures below the sources we employ the common conversion point ccp stacking and diffraction migration methods to determine the location and geometric features of the seismic structures the ccp stacking image indicates that the structure is a localized discontinuity at 1000 km with a dimension at 200 km by 50 km along the e w and n s directions respectively it is located at 150 km northeast to the two events the 2d migrated images on the other hand indicate that the sources structure are reflectors dipping northeastwards by 17 at a slightly shallower depths the reflectors have a length scale of 100 km ant their centers are 50 km away from the epicenters of the two earthquakes forward waveform modeling suggests that the dipping reflectors may be thin layers with a thickness of few kilometers the layers have a lower shear velocity and a higher density than that of the surrounding mantle which matches well with those predicted for mid ocean ridge basalt morb at mid mantle depths according to a recent ab initio study combined with the results from previous studies our observations here suggest that the former oceanic crust may be ubiquitously present in the lower mantle beneath subduction zones

high pressure transitions in the system mgal2o4 caal2o4 a new hexagonal aluminous phase with implication for the lower mantle abstract phase transitions in mgal 2 o 4 and caal 2 o 4 and in the binary system mgal 2 o 4 caal 2 o 4 were established at high pressures and high temperatures mgal 2 o 4 spinel dissociates to periclase and corundum at 15 16 gpa and 1200 c 1600 c with an equilibrium boundary of p gpa 12 3 0 0023 t c the mixture recombines into a phase with calcium ferrite structure at about 26 5 gpa at 1600 c at 780 c 1400 c caal 2 o 4 crystallizes with the calcium ferrite structure at pressure above 8 9 gpa at lower pressures phases iii and iv are stable on the join mgal 2 o 4 caal 2 o 4 solubility of mgal 2 o 4 component into caal 2 o 4 rich calcium ferrite is limited to about 10 mol at 20 21 gpa and 1200 c and a new phase with composition of mg x ca 1 x al 2 o 4 x 0 66 0 8 was found to be stable at pressure above 15 gpa powder x ray diffraction data show that the new phase possesses hexagonal symmetry moreover it is interpreted that an al rich phase observed in high pressure assemblage of mid ocean ridge basalt morb above 25 gpa by irifune and ringwood irifune t ringwood a e 1993 phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle earth planet sci lett 117 101 110 has the same structure as the hexagonal phase found in this study rather than calcium ferrite structure and another al rich phase synthesized at 24 27 gpa in high pressure experiments from natural pyrope rich garnet starting material is also similar to the hexagonal phase these results suggest that aluminous phase of the hexagonal structure would be one of major constituent minerals of subducted oceanic crust in the upper part of the lower mantle

spin transition of ferric iron in the calcium ferrite type aluminous phase we investigated fe free and fe bearing cf phases using nuclear forward scattering and x ray diffraction coupled with diamond anvil cells up to 80 gpa at room temperature octahedral fe3 ions in the fe bearing cf phase undergo a high spin to low spin transition at 25 35 gpa accompanied by a volume reduction of 2 0 and a softening of bulk sound velocity up to 17 6 based on the results of this study and our previous studies both the nal and cf phases which account for 10 30 vol of subducted morb in the lower mantle are predicted to undergo a spin transition of octahedral fe3 at lower mantle pressures spin transitions in these two aluminous phases result in an increase of density of 0 24 and a pronounced softening of bulk sound velocity up to 2 3 for subducted morb at 25 60 gpa and 300 k the anomalous elasticity region expands and moves to 30 75 gpa at 1200 k and the maximum of the v reduction decreases to 1 8 this anomalous elastic behavior of fe bearing aluminous phases across spin transition zones may be relevant in understanding the observed seismic signatures in the lower mantle

the effect of water on mantle phase transitions water has played an important role in the earth s evolution the incorporation of water as hydroxyl into solid mineral phases or as coexisting hydrous fluids and melts affects the chemical and physical properties of crust and mantle constituents i e it weakens rocks and minerals reduces viscosity and strength of the materials and depresses dramatically the melting temperature of silicate minerals e g karato 1990 inoue 1994 hirth and kohlstedt 1996 chen et al 1998 kubo et al 1998 mei and kohlstedt 2000 many recent studies have suggested the possible existence of water in the earth s mantle especially in the transition zone e g smyth and frost 2002 ohtani et al 2004 litasov et al 2005a hae et al 2006 where wadsleyite and ringwoodite can accommodate up to 3 wt of h2o in their structures e g kohlstedt et al 1996 low velocity zones observed seismologically at the top of the 410 km discontinuity may indicate the existence of trapped high density melt e g revenaugh and sipkin 1994 song et al 2004 matsukage et al 2005 sakamaki et al 2006 which is likely to be hydrous as it is not possible to melt the base of the upper mantle without water at these conditions electrical conductivity anomalies in the upper mantle and transition zone that are related to subduction zones have also been interpreted as an effect of water in the mantle e g fukao et al 2004 tarits at el 2004 hae et al 2006 koyama et al 2006 studies of the kinetics of the hydrous olivine wadsleyite transformation ohtani et al 2004 hosoya et al 2005 seem to be consistent with seismological observations e g koper et al 1998 that indicate the absence of a metastable olivine wedge in subducting

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

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

equations of state of na k al host phases and implications for morb density in the lower mantle abstract we report equations of state eos of two important silicate phases known to store na k and al at pressures and temperatures prevailing in the earth s lower mantle using laser heated diamond anvil cell we produced the ca ferrite type cf and the new aluminum rich nal phases using different synthetic and natural starting materials at pressures ranging from 23 to 65 gpa unit cell volumes were investigated by means of angle dispersive x ray diffraction at the id30 beamline of the esrf synchrotron ring grenoble france cf phases were synthesized in naalsio4 mgal2o4 and naalsio4 mg fe 2sio4 systems and for a natural mid ocean ridge basalt morb the insertion of mgal2o4 in the naalsio4 based lattice has reduced effects on the cf phase eos whereas that of mg fe 2sio4 is mainly to increase the room pressure volume cf phases most relevant to the earth s mantle show compression behavior comparable to the pure naalsio4 cf phase with bulk moduli values close to k0 190 gpa and k0 4 we also synthesized nal phases in naalsio4 mgal2o4 system and using cana and k rich silicate glasses as starting materials all nal compounds investigated show similar compression behavior except for one anomalously naalsio4 rich composition for nal phase with the most natural conditions relevant composition we refine bulk moduli of k0 182 gpa and k0 6 we then model the density profiles expected for the cf and nal phases using our newly determined eos and the most accurate chemical compositions reported in the literature these two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 k however the density profiles calculated for mineral assemblages corresponding to typical morb compositions point out to denser morbs compared to the pyrolite this is due to the high fe content in morb yielding a densest fe rich perovskite that counterbalances the lower densities of the cf and nal phases the density gap is calculated between 0 4 and 4 0 depending on the starting morb composition and pressure

subparallel dipping heterogeneities in the mid lower mantle 1 data from western u s short period seismic networks show anomalous later phases which we interpret as s to p converted waves from heterogeneities in the mid lower mantle depths of 1000 1850 km northeast of the mariana subduction zone the geometry of the objects is mapped by a two step approach 1 computing the s to p scattering likelihood with the arrival time slowness and azimuth of the later arrivals measured by array techniques relative to direct p waves and 2 by direct use of the waveforms using semblance at least three subparallel objects are detected in the lower mantle study region with the dips of 30 to 40 southward and with a lateral extent larger than 500 km by 300 km the elastic properties associated with the heterogeneous objects appear to change within several kilometers considering their high efficiency at converting short period waves wavelengths of 5 10 km previous seismic tomography studies have not predicted significant velocity anomalies around the heterogeneities this is probably because they are below the resolution limit of present methods the three major heterogeneities are separated from each other nearly 300 km vertically and significant anomalies seem absent between each object nevertheless semblance cross sections with a finer grid size 0 1 0 1 10 km of the clearest intermediate object at 1400 1600 km show a hint that the object is not a single interface but is actually composed of double interfaces nearly 50 km apart purely thermal anomalies of kilometers scale must be transient features in the mantle and thus are highly unlikely to be the cause of the heterogeneities they should rather represent either sharp chemical variations in major element composition or sharp changes in preferred orientations of anisotropic lattice of the lower mantle minerals thermal anomalies however may be superimposed on such variations because of heat generation either by concentrated radioactive nuclei or by viscous shear heating due to deformations a plausible tectonic interpretation of these dipping objects is that they represent remnants of subducted lithospheric plates which preceded the current subduction of the pacific plate although we cannot make conclusive arguments on what the mechanism of creating such heterogeneities from these observations alone we suggest that they may represent stretched thinned and duplexed rocks within chaotic mantle flows

elastic properties of stishovite and the cacl2 type silica at the mantle temperature and pressure an ab initio investigation a b s t r a c t the elastic constant tensors of stishovite and the cacl2 type silica at the earth s mantle temperature and pressure were determined using first principles calculations with local density approximation the elastic properties of stishovite show not only strong pressure dependence but also temperature dependence by increasing temperature the shear instability of stishovite is shifted to an elevated pressure with a slope of 5 4 1 4m pa k the softening of the shear modulus and the positive clapeyron slope result in crossing of the sound velocities at different temperatures which leads to the unusual positive temperature dependence of the sound velocities around the phase boundary the transition from stishovite to the cacl2 type silica at the lower mantle s temperature occurs at a depth far deeper than 1200 km and is accompanied by a velocity jump of 0 98 0 08 km si n s wave velocity v s and 0 45 0 15 km s in p wave velocity v p this transition is likely related to the seismic discontinuity at the depth of 1670 km in the vicinity of mariana island the unusual positive temperature dependence of v s of stishovite and strong anisotropy of stishovite and the cacl2 type silica around the phase boundary

structural aspects of diapiric melange emplacement the duck creek diapir abstract the duck creek melange is one of a series of diapiric mud intrusions into the slope cover sequence of the cascadia accretionary wedge the diapir originated deep within the accreted hoh sequences and has intruded approximately 1 km up into the 2 3 km thick pliocene quinault slope cover sequences at its current level of exposure in coastal seacliffs analyses of the geometry and kinematic indicators at the current exposure level strongly suggest that this part of the diapir intruded at a low angle to form an inclined sill or dyke like body at least 0 5 km in diameter microstructural analysis indicates a zonation of deformational features between the core and margins of the diapir resulting from an evolving series of fabrics and the increasing concentration of strain towards the margins specifically we interpret early independent particulate flow ipf fabrics as indicating deformation in a low effective stress environment with the possibility of turbulence during initial mobilization and intrusion the subsequent transition to cataclastic flow indicates deformation occurring in the presence of increasing effective stress and the creation of broad marginal shear zones suggests strain hardening the final transition to ipf fabrics limited to narrow strain softened marginal shear zone suggests decreasing effective stresses during the latest intrusion phase in addition the abundance of veining phases associated with the latter part of the intrusion history indicates that the diapir became a major fluid conduit for mineralizing fluids toward the end of its emplacement history

constraining the geometries of small scale heterogeneities a case study from the mariana region a new array based method for seismic waveform data is developed to detect small scale heterogeneities in the mantle and constrain their possible locations and sizes detecting small scale heterogeneities using scattered waves has been challenging because scattered phases usually suffer from low signal to noise ratios and determining the shapes of such heterogeneities is even more difficult because of isochronal artifacts in the proposed method the problem of the low signal to noise ratio is addressed by adopting dual bootstrap stacking which can detect weak signals even with a limited number of seismic traces as well as by ensuring reproducibility among multiple events the locations of detected scatterers and their sizes and shapes are then estimated using a new kind of hierarchical cluster analysis that could minimize the effect of isochronal artifacts the new method is applied to a previously analyzed data set of mariana earthquakes to facilitate a comparison with conventional methods in the mid to lower mantle beneath the mariana trench region a total of 39 scatterers are identified via s to p scattering with their potential volumes ranging from 8 104 to 1 106 km3