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

constitution of the mantle 3 density elastic properties and the mineralogy of the 400 km discontinuity abstract on the basis of phase diagrams of the feo mgo sio 2 system and equations of state for minerals we have calculated internally consistent thermodynamic profiles of adiabatic bulk modulus seismic parameter and density for a series of petrological models it is shown that the sequences of phase assemblage changes across the 400 km discontinuity and the thickness of divariant transitions depend strongly on the iron content the ol px ratio and the temperature at the transition depth calculations show that the iron rich model is inadequate to described the seismic properties in the uppermost mantle and through the 400 km discontinuity the distribution of physical properties in the iron rich model at the depths of 360 400 km is of a gradient type which does not agree with the seismic evidence an important constraint on the upper mantle chemistry is an fe fe mg ration less than 0 17 comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match seismic observations through the 400 km discontinuity hence the sio 2 content in the isochemical upper mantle should be in excess of 36 4 mol and less than 44 4 mol from all the petrological models considered only a pyrolite model sio 2 40 mol fe fe mg 0 12 ol px 1 may satisfy available seismological constraints the results obtained indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification the pyrolite models considered show that the phase transitions occur over a range of 8 15 km at a depth near 400 km it is proposed that in the different tectonic regions of the earth a 400 km discontinuity might be considered as a combination of first and second order discontinuities with a phase transition sequence px px px px within a thickness range of around 15 km low temperature pyrolite model or as pseudo first order discontinuity through a narrow divariant loop px about 8 km wide high temperature pyrolite model

determination of fe mg partitioning between perovskite and magnesiowustite the partition coefficient of fe and mg between perovskite and magnesiowostite k mg fe pv mw has been measured by adopting a b 2 o 3 catalyst technique the present experimental results demonstrate that k mg fe pv mw is a factor of two closer to unity than that obtained in previous studies k mg fe pv mw increases from 0 16 to 0 35 with increasing mg of magnesiowustite from 50 to 86 on the basis of these results the compositions of enstatite and magnesiowustite included in diamond imply that the temperature at the top of the lower mantle is 1870 2040 k the mg 100mg mg fe of perovskite and magnesiowustite at the top of the lower mantle are 93 and 82 83 respectively for a pyrolitic mantle model the upper bound of the lower mantle temperature estimated in this study do not allow a larger fraction of perovskite than the pyrolitic mantle

elastic wave velocities of forsterite and its β spinel form and chemical boundary hypothesis for the 410 km discontinuity a method for measuring elastic wave velocities at pressures up to 15 gpa has been developed velocities of forsterite and spinel were measured using a piston cylinder apparatus with a pulse echo overlap method for pressures up to 4 gpa and using a split sphere apparatus with a pulse transmission technique for higher pressures to validate the results volume compression incompressibility and rigidity were estimated from measured travel times and comparison was made between the present estimates and data from x ray brillouin and acoustic measurements the v p and k values for spinet are 3 and 8 higher than previous measurements at 10 gpa respectively velocity jumps in v p and v s due to olivine to spinel transition were about 1 34 and 0 80 km s 1 respectively very different from seismological observations of velocity jumps at the 410 km discontinuity which were concentrated around 0 45 km s 1 for the p wave and 0 22 km s 1 for the s wave velocity jumps in mantle peridotite were calculated using the experimental results and comparison was made between these estimates and seismological observations at the 410 km discontinuity the evidence does not support the phase change hypothesis and a chemical boundary was proposed as the origin of the 410 km discontinuity an eclogitic mineral assemblage might be an alternative for the composition of the transition layer

iron partitioning in a pyrolite mantle and the nature of the 410 km seismic discontinuity pyrolite1 is a hypothetical mixture of distinct minerals which iswidely believed to represent the composition of the earth s mantle the main pressure induced phase transformations of the olivine component of pyrolite occur at about 13 5 gpa to and 24 gpa to mgsio3 rich perovskite magnesiowustite 2 3 which are thought to be responsible for the seismic discontinuities at 410 and 660 km depths in the mantle recent seismological studies however have demonstrated that the 410 km seismic discontinuity is sharper in some areas than that expected from the to transformation in mantle olivine with a fixed composition4 5 6 7 moreover some mineral physics studies suggest that the seismic velocity jump at the 410 km discontinuity is inconsistent with that associated with the to transformation in olivine8 9 here we present a phase equilibria study of a material having pyrolite composition at pressures of 6 16 gpa we found that the iron content in olivine changes significantly with increasing pressure as a result of the formation of a relatively iron rich majorite phase at these pressures this variation in iron content can overcome or at least reduce both of the above difficulties encountered with the pyrolite model of mantle composition by showing that the component mineral systems cannot be treated as separate

slab dehydration and mantle upwelling in the vicinity of the sumatra subduction zone evidence from receiver function imaging of mantle transition zone discontinuities

evolution of the gypsum karst of sorbas se spain the gypsum karst of sorbas is developed within messinian gypsum characterized by an interbedded sequence of selenitic gypsum beds and marls this geological configuration has led to the development of an interstratal karst in which galleries have developed in the marl levels but not in the gypsum ones up to six cave passage levels have been detected in the gypsum caves following stratification planes between the impervious intercalated marls and the pervious gypsum strata the explanation for this lies in the hydrogeological history of the area initially the gypsum karst evolved as a multi layer semi confined aquifer under phreatic conditions enabling the formation of small proto conduits in the individual gypsum beds while the intervening marls and clays acted as impervious barriers during a second stage after lowering of the piezometric level vadose conditions were established in which mechanical erosion processes in the intercalated marls and clays became predominant this genetic duality means that the gypsum karst at sorbas can be taken as an example of interstratal karstification where the contemporary underground erosion processes and those of karstic evolution should be considered products of the hydrogeological development the total lowering of the piezometric level during the quaternary is at least of 120 m in the centre of the basin interstratal vadose caves more than 120 m below the surface possess phreatic proto channels in their upper levels which identify the past phreatic conditions in the genesis of the caves

ringwoodite growth rates from olivine with 75ppmw h2o metastable olivine must be nearly anhydrous to exist in the mantle transition zone it has been previously demonstrated that as little as 300 ppmw h 2o increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis to further test this hypothesis we present new ringwoodite growth rate measurements from olivine with 75 ppmw h 2o at 18 gpa and 700 900 and 1100 c these growth rates are nearly identical to those from olivine with 300 ppmw h 2o and significantly higher than those from nominally anhydrous olivine we infer that transformation of olivine with 75 300 ppmw h 2o is primarily enhanced by hydrolytic weakening of reaction rims which reduces the elastic strain energy barrier to growth we present a new method for fitting non linear nominally anhydrous data to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine based on previous thermokinetic modeling these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone metastable persistence of olivine into the mantle transition zone would therefore require 75 ppmw h 2o

precise determination of phase relations in pyrolite across the 660 km seismic discontinuity by in situ x ray diffraction and quench experiments abstract mineral assemblage changes in a pyrolite composition with increasing pressure were observed by in situ x ray diffraction and quench experiments at pressures near that of the 660 km seismic discontinuity and at a fixed temperature of 1600 c according to results obtained by in situ x ray diffraction experiments ringwoodite rw was observed with majorite garnet and casio 3 rich perovskite at pressures of about 20 22 gpa dissociation of ringwoodite to mgsio 3 rich perovskite and magnesiowustite mw was completed at 22 0 0 2 gpa according to matsui et al s periclase pressure scale and at 21 7 0 1 gpa according to shim et al s gold pressure scale majorite garnet persisted to about 24 gpa where pyrolite transformed to a lower mantle mineral assemblage i e mgsio 3 perovskite casio 3 rich perovskite and magnesiowustite thus majorite garnet coexists with the lower mantle assemblage at pressures of about 22 24 gpa in the quench experiments an assemblage of mgsio 3 perovskite magnesiowustite casio 3 rich perovskite and majorite garnet was synthesized at 22 5 gpa and 1600 c in which mg perovskite contained 2 8 wt al 2 o 3 and was significantly poorer in fe than coexisting magnesiowustite the fe mg partition coefficient between mg perovskite and magnesiowustite including ferric iron k app 0 27 0 06 is very close to that in the al free system which suggests that these p t conditions are in the vicinity of those of ringwoodite decomposition both the results of in situ x ray diffraction and quench experiments in the present study yield a convergent result that ringwoodite decomposes into mg perovskite and magnesiowustite before the garnet to perovskite transition at 1600 c in pyrolite the relation between the al content in mg perovskite and k app in pyrolite is non linear which is consistent with the fe mg partitioning between mg perovskite and magnesiowustite previously reported for a simpler mgo feo al 2 o 3 sio 2 system

solidus and liquidus profiles of chondritic mantle implication for melting of the earth across its history abstract we investigated the melting properties of a synthetic chondritic primitive mantle up to core mantle boundary cmb pressures using laser heated diamond anvil cell melting criteria are essentially based on the use of x rays provided by synchrotron radiation we report a solidus melting curve lower than previously determined using optical methods the liquidus curve is found between 300 and 600 k higher than the solidus over the entire lower mantle at cmb pressures 135 gpa the chondritic mantle solidus and liquidus reach 4150 150 k and 4725 150 k respectively we discuss that the lower mantle is unlikely to melt in the d layer except if the highest estimate of the temperature profile at the base of the mantle which is associated with a very hot core is confirmed therefore recent suggestions of partial melting in the lowermost mantle based on seismic observations of ultra low velocity zones indicate either 1 a outer core exceeding 4150 k at the cmb or 2 the presence of chemical heterogeneities with high concentration of fusible elements our observations of a high liquidus temperature as well as a large gap between solidus and liquidus temperatures have important implications for the properties of the magma ocean during accretion not only complete melting of the lower mantle would require excessively high temperatures but also below liquidus temperatures partial melting should take place over a much larger depth interval than previously thought in addition magma adiabats suggest very high surface temperatures in case of a magma ocean that would extend to more than 40 gpa as suggested by siderophile metal silicate partitioning data such high surface temperature regime where thermal blanketing is inefficient points out to a transient character of the magma ocean with a very fast cooling rate

high mantle temperature during cretaceous avalanche abstract a new sequence of axisymmetrical spherical mantle convection simulations has been computed to monitor the influence of mantle avalanches on the mantle thermal state during an avalanche that is characterized by a high rate of mixing between upper mantle and lower mantle a complex perturbation of the thermal field occurs due to the advection of cold upper mantle into the lower mantle and the hot return flows from the lower mantle the computation shows that the avalanches trigger global upper mantle warming of several tens of degrees and make it possible to explain both core mantle boundary and transition zone origins for mantle plumes recently the chemical compositions of dsdp odp drill holes on crust older than 80 myr have been interpreted to reflect a higher mean mantle temperature during the mesozoic humler et al earth planet sci lett 173 1999 7 23 here we have used and extended the data set for oceanic crust younger than 80 myr combining the mean chemistry for old oceanic crust and the mean depth of the ocean basin versus the square root of age we propose a continuous curve for the temperature evolution of the upper mantle for the last 130 myr it shows a mantle thermal high around 125 ma with amplitude 50 k and duration 30 myr comparable with those obtained from theoretical convection models it is proposed that such a high mantle temperature may be due to the thermal effects of an avalanche which may have started 180 myr ago

constraints on lower mantle composition from molecular dynamics simulations of mgsio3 perovskite abstract we have carried out molecular dynamics simulations of mgsio 3 perovskite pv in order to determine its equation of state and elastic parameters under pressure and temperature conditions of the earth s lower mantle combining this information with density and elastic constants for mg fe o magnesiowustite mw and including the effects of iron on the density of perovskite we investigated possible compositional models for the lower mantle by calculating seismic velocities and densities as functions of depth and comparing them to the globally averaged seismological models ak135 f and prem the calculations were done along several possible geothermal gradients both adiabatic and non adiabatic and for a range of mg fe partitioning coeffcients k fe mg pv mw 0 10 0 35 our results indicate that the most probable compositional models for the lower mantle are pyrolitic with x mg 0 87 and x pv 0 54 for increasing temperatures at the top of the lower mantle the best fitting models increase slightly in both iron and silica content

the orthorhombic to high p monoclinic phase transition in mg fe pyroxenes can it produce a seismic discontinuity the orthorhombic to high p monoclinic phase transition in mg fe sio 3 pyroxene with a mantle relevant composition x fs 0 1 is expected to occur at 300 km depth woodland and angel 1997 however the divariant nature of the phase transition in the mg fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity new experimental results with binary mg fe pyroxenes constrain the divariant loop to be 0 2 gpa wide at the composition of x fs 0 4 and on the order of 0 15 gpa for a mantle relevant composition this implies that the phase transition will be complete over a depth interval of about 5 6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high p monoclinic phase transition in mg fe sio 3 pyroxene is indeed narrow enough to produce a jump in seismic velocities the experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition the expected location of this phase transition coincides with a small magnitude seismic discontinuity the x discontinuity occasionally observed in seismic profiles at 300 km depth and thus provides a viable petrologic explanation for the origin of this discontinuity if it truly exists

long term erosion rate measurements in gypsum caves of sorbas se spain by the micro erosion meter method abstract the present work deals with the results of long term micro erosion measurements in the most important gypsum cave of spain the cueva del agua sorbas almeria se spain nineteen mem stations were positioned in 1992 in a wide range of morphological and environmental settings gypsum floors and walls carbonate speleothems dry conduits and vadose passages inside and outside the cave on gypsum and carbonate bedrocks and exposed to variable degree of humidity different air flow and hydrodynamic conditions four different sets of stations have been investigated 1 the main cave entrance las vinicas spring 2 the main river passage 3 the abandoned laboratory tunnel and 4 the external gypsum surface data over a period of about 18 years are available the average lowering rates vary from 0 014 to 0 016 mm yr 1 near the main entrance and in the laboratory tunnel to 0 022 mm 1 on gypsum floors and 0 028 mm yr 1 on carbonate flowstones the denudation data from the external gypsum stations are quite regular with a rate of 0 170 mm yr 1 the observations allowed the collecting of important information concerning the feeding of the karst aquifer not only by infiltrating rainwater but under present climate conditions also by water condensation of moist air flow this contribution to the overall karst processes in the cueva del agua basin represents over 20 of the total chemical dissolution of the karst area and more than 50 of the speleogenetically removed gypsum in the cave system thus representing all but a secondary role in speleogenesis condensation corrosion is most active along the medium walls being slower at the roof and almost absent close to the floor this creates typical corrosion morphologies such as cupola while gypsum flowers develop where evaporation dominates this approach also shows quantitatively the morphological implications of condensation corrosion processes in gypsum karst systems in arid zones responsible for an average surface lowering of 0 047 mm yr 1 while mechanical erosion produces a lowering of 0 123 mm yr 1

is low spin fe2 present in earth s mantle abstract recently x ray spectroscopy indicated that the low spin ls electronic configuration of fe 2 and fe 3 ions substituting in silicate perovskite of various compositions and also in magnesiowustite is stable at room temperature above pressures of 49 to 120 gpa j badro g fiquet f guyot j p rueff v v struzhkin g vanko g monaco iron partitioning in earth s mantle toward a deep lower mantle discontinuity science 300 2003 789 791 j badro j p rueff g vanko g monaco g figuet f guyot electronic transition in perovskite possible non convecting layers in the lower mantle science 305 2004 383 386 j li v v struzhkin h k mao j shu r j hemley y fei b mysen p dera v prakapenka g shen electronic spin state of iron in lower mantle perovskite proc natl acad sci 101 2004 14027 14030 j m jackson w sturhahn g shen j zhao m y hu d errandonea j d bass y fei a synchrotron mossbauer study of mg fe sio 3 perovskite up to 120 gpa am mineral 90 2005 199 205 j f lin v v struzhkin s d jacobsen m y hu p chow j kung h liu h k mao r j hemley spin transition of iron in magnesiowustite in the earth s lower mantle nature 436 2005 377 380 simple thermodynamic relationships combined with crystal field theory provide a minimum clausius clapeyron slope of 0 23 to 0 4 gpa k for the high spin hs to low spin transition of fe ions in perovskite consistent with lower mantle temperatures stabilizing the disordered hs state this computation of p t utilizes experimentally determined parameters only p and t at the transition and is supported by microstructural analysis of the change in volume and entropy considerations similarly p t min 0 18 to 0 31 gpa k for magnesiowustite high spin fe 2 should be stable in silicates throughout earth s mantle for compositions of fe mg fe 0 1 consistent with disorder prevailing at high temperature although partial conversion of octahedral fe 3 to ls may occur similarly partial conversion of octahedral fe 2 in magnesiowustite to the ls is possible for fe rich compositions which transform at relatively low pressures

stress dependent power law flow in the upper mantle following the 2002 denali alaska earthquake abstract far field continuous global positioning system gps time series data following the 2002 m7 9 denali alaska earthquake imply that mantle viscoelastic rheology is stress dependent a linear viscous mantle cannot explain fast early displacement rates at the surface that rapidly decay with time whereas a power law rheology where strain rate is proportional to stress raised to the power of 3 5 0 5 provides decay rates and spatial patterns in agreement with observations this is consistent with laboratory measurements for hot wet olivine implying a hydrated mantle and a relatively thin 60 km thick lithosphere beneath south central alaska these results suggest that the viscous strength of the lithosphere varies both spatially and temporally and that effective viscosities inferred from different loading events or observational time periods can differ by up to several orders of magnitude thus the very conditions that enable the inference of rheologic strength transient loading and unloading events significantly alter the effective viscosity

the micromorphology of gypsum and halite in reg soils the negev desert israel gypsum and halite are the most common salts in reg soils developed on alluvial parent material under extremely arid conditions in the negev desert israel the aim of this paper is to document the changes in the micromorphology of these salts at different stages of reg soil development on two alluvial fan chronosequences the micromorphological analyses included thin section observations and scanning electron microscope and electron microprobe analyses in this arid soil environment gypsum and halite possess a variety of crystal forms which may change with depth in a single profile and or between profiles of different ages the variety of crystal forms results from changes in the microenvironmental conditions that occur in desert reg soils over time poikilitic lenticular gypsum is found in all reg soils and is distributed throughout the profiles the conditions needed for such crystals to form are high ionic impurities and deposition in a void system where space is not limiting microcrystalline alabastrine gypsum is only found in mature reg soils and is crystallized when the profile has high amounts of fine material and a well developed desert pavement in a well developed reg soil profile indicators such as a well developed desert pavement and high amounts of fine earth limit the leaching depth and cause gypsum deposition from supersaturated soil solutions under high evaporation rates close to the surface prismatic and fibrous gypsum are less common low amounts of prismatic gypsum are found in young and mature soils while fibrous gypsum is found only in mature soils in re cemented shattered gravel the halite crystal form is mainly cubic with low amounts of host material incorporated into the crystal it occurs predominantly in mature reg soil profiles through the crystallization from supersaturated soil solutions at the depth of maximum water penetration although the alabastrine prismatic and fibrous gypsum and cubic halite are deposited in a displacive manner no correlation was found between their occurrence and the distribution of shattered gravel in the soil profile

mantle transition zone beneath a normal seafloor in the northwestern pacific electrical conductivity seismic thickness and water content abstract we conducted a joint electromagnetic em and seismic experiment to reveal the mantle structure beneath a normal seafloor at 130 145 ma in the northwestern pacific where the seafloor is relatively flat and the underlying mantle is expected to be normal free from tectonic perturbations in the experiment we deployed state of the art instruments in two arrays from 2010 2015 here we report the result of analyses of the em and seismic data for investigating the mantle transition zone mtz structure the em data analysis revealed that an electrical conductivity structure below both arrays was approximated by an average 1 d model of the north pacific and showed a possible downward increase in conductivity at the top of the mtz from the p wave receiver function analysis perturbations in the mtz thickness from a global average were estimated to be 20 km and 2 km below the northern and southern arrays respectively from which temperature profiles in the mtz below these two arrays were then estimated we jointly interpreted the profiles of electrical conductivity and thus estimated temperature with reference to the experimental values of the effects of water on the electrical conductivities of mtz minerals wadsleyite and ringwoodite from mineral physics the upper bound of the water content below the northern array was determined to be 0 4 wt or 0 04 wt depending on different results of mineral physics and that below the southern array was determined to be slightly smaller the lower bound of the water content was not constrained by our data our results indicate that the mtz beneath the normal seafloor in the northwestern pacific is drier than subduction zones and may be a water poor region in a plum pudding mantle model

in situ observations of phase transition between perovskite and cairo 3 type phase in mgsio 3 and pyrolitic mantle composition abstract in situ observations of the perovskite cairo3 phase transition in mgsio3 and in pyrolitic compositions were carried out using a laser heated diamond anvil cell interfaced with a synchrotron radiation source for pure mgsio3 the phase boundary between the orthorhombic mg perovskite and cairo3 type phases in the temperature range of 1300 3100 k was determined to be p gpa 130 3 0 0070 0 0030 t 2500 k using platinum as a pressure calibrant we confirmed that the cairo3 type phase remained stable up to pressures of at least 156 gpa and temperatures of 2600 k the consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the cairo3 type phase the phase change from an orthorhombic mg perovskite to a cairo3 type bearing assemblage in a pyrolitic mantle composition was also observed at p 125 gpa which corresponds to the same mantle depth as the seismic discontinuity the phase boundary between the orthorhombic mg perovskite and cairo3 type bearing assemblage was determined to be p gpa 124 4 0 008 0 005 t 2500 k using gold as a pressure calibrant this transition boundary indicates that the temperature at a depth of 2700 km is about 2600 k and the adiabatic temperature gradient in the lower mantle is estimated to be 0 31 k km the partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic mgsio3 perovskite and cairo3 type mgsio3 were estimated from ab initio calculations our experimental and theoretical results indicate that the d layer consists of a cairo3 type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the earth s mantle

stability of a hydrous δ phase alooh mgsio2 oh 2 and a mechanism for water transport into the base of lower mantle abstract the global water cycle in the earth is one of the most important issues in geodynamics because water can affect the physical and rheological properties of the mantle however it is still a matter of debate whether water can be transported into the lower mantle and core here we report a new reaction between aluminous perovskite and water to form alumina depleted perovskite and hydrous phase alooh mgsio2 oh 2 along the mantle geotherm in the lower mantle chemical analysis of the coexisting phases showed that the perovskite and post perovskite phases were depleted in al2o3 whereas hydrous phase contains at least 44 mol of mgsio2 oh 2 component at 68 gpa and 2010 k and 23 mol of this component at 128 gpa and 2190 k the present experiments revealed that hydrous phase alooh mgsio2 oh 2 can coexist with alumina depleted mgsio3 perovskite or post perovskite under the lower mantle conditions along the slab geotherm thus this hydrous phase in the slabs can transport water into the base of the lower mantle

a mineralogical model for density and elasticity of the earth s mantle 1 we present a thermodynamic model of high pressure mineralogy that allows the evaluation of phase stability and physical properties for the earth s mantle the thermodynamic model is built from previous assessments and experiments in the five component cfmas system cao feo mgo al2o3 sio2 including mineral phases that occur close to typical chemical models of the mantle and reasonable mantle temperatures in this system we have performed a system gibbs free energy minimization including pure end member phases and a nonideal formulation for solid solutions solid solutions were subdivided into discrete pseudocompounds and treated as stoichiometric phases during computation of chemical equilibrium by the simplex method we have complemented the thermodynamic model with a model of shear wave properties to obtain a full description of aggregate elastic properties density bulk and shear moduli that provide a useful basis for the consideration of seismic and geodynamic models of the earth s mantle the thermodynamic model described here is made available for research and training purposes through a web interface http www earthmodel org we examine its validity in light of experiments from mineral physics and briefly discuss inferences for mantle structure

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

temperature fluctuation and thermodynamic properties in earth s lower mantle an application of the complete travel time equation of state abstract the complete travel time equation of state ct eos enables us to obtain all the thermodynamic parameters from travel time or velocity measurements of elastic waves at simultaneously high pressure and temperature here we formulate the adiabatic ct eos and apply it to the lateral heterogeneity of seismic wave velocity in the lower mantle we obtained internally consistent results for temperature thermal expansivity and the gruneisen constant in the lower mantle at the depth 2741 km just above the d layer the most likely results are 2500 k for temperature 1 0 10 5 k 1 for thermal expansivity and 1 0 for the gruneisen constant we also discuss the geophysical implication of the small thermal expansivity suggested in the deep lower mantle

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

radiative heat transfer in a hydrous mantle transition zone abstract the structure and dynamics of earth s interior depend crucially upon heat flow and thus upon the thermal conductivity of its constituents we measured optical absorbance spectra of hydrous wadsleyite and hydrous ringwoodite at simultaneous high pressure and high temperature conditions up to 26 gpa and 823 k in order to determine their radiative conductivities and to study the potential influence of hydration in the transition zone on thermal conductivity of the mantle we report radiative thermal conductivities of 1 5 0 2 wm 1 k 1 for hydrous wadsleyite and 1 2 0 1 wm 1 k 1 for hydrous ringwoodite at transition zone conditions the analytically derived radiative thermal conductivities of anhydrous wadsleyite and ringwoodite are 40 and 33 higher respectively the total thermal conductivities calculated from temperature and pressure dependent optical absorption measurements maintain an energy transmission window in the infrared and visible spectral range at high pressures and temperatures the results indicate that the mantle transition zone may contribute significantly to heat transfer in the mantle and demonstrate the importance of radiative heat transfer in controlling geodynamic processes in earth s mantle

mantle transition zone structure beneath the south pacific superswell and evidence for a mantle plume underlying the society hotspot abstract underside reflections of shear waves from the discontinuities at 410 and 660 km depth are used to map lateral variations in the thickness of the mantle transition zone beneath the south pacific superswell and surrounding regions differential travel times of reflected waves indicate that the transition zone is about 25 km thinner than normal and thus hotter than normal over an area 500 km or less in diameter beneath the society hotspot there is no general difference however in transition zone thickness between the superswell area and its surroundings our observations support the inference that the thermal flux from lower to upper mantle beneath the south pacific superswell occur on the lateral scale of a mantle plume several hundred kilometers rather than that of a superplume several thousand kilometers

phase transitions and short timescale sinusoidal motions abstract while the effects of phase transitions of mantle materials are considered in convection studies models of geophysical processes that occur on shorter timescales such as seismic normal modes and earth tides often ignore these effects a common justification is that the latent heat released from the material changing phase could not conduct away from the boundary on timescales shorter than those of convection and thus the phase transition would not proceed in this study we first examine the behavior of a phase boundary to a periodic pressure perturbation by solving the heat equation if all the latent heat is released at an infinitely thin boundary we find that the phase transitions do not proceed however if the latent heat is released over a region of 1 5 km thickness which might occur due to the divariant nature of the phase boundaries then some of the material changes phase regardless of the period of the forcing we apply these results to predictions of seismic normal mode center frequencies and elastic love numbers the perturbations to the normal mode frequencies can be two orders of magnitude greater than the differences between the observed frequencies and those predicted using the preliminary reference earth model however we have not considered kinetics the energetics of the mechanisms of the phase transitions in this formulation this work suggests a greater knowledge of the kinetics near equilibrium phase boundaries is required because the kinetics may be the limiting factor in these short period small amplitude motions

the structure and sharpness of mg fe 2sio4 phase transformations in the transition zone abstract to calculate accurately the pressure interval and mineral proportions i e yields across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non ideality of fe mg mixing in these mg fe 2 sio 4 solid solutions in order to constrain the activity composition relations that describe non ideal mixing fe mg partitioning experiments have been conducted between magnesiowustite and mg fe 2 sio 4 olivine wadsleyite and ringwoodite as a function of pressure at 1400 c using known activity composition relations for magnesiowustite the corresponding relations for the three polymorphs were determined from the partitioning data in all experiments the presence of metallic iron ensured redox conditions compatible with the earth s transition zone the non ideality of the mg fe 2 sio 4 solid solutions was found to decrease in the order w wadsleyite femg w ringwoodite femg w olivine femg these partitioning data were used along with published phase equilibria measurements for the mg 2 sio 4 and fe 2 sio 4 end member transformations to produce an internally consistent thermodynamic model for the mg 2 sio 4 fe 2 sio 4 system at 1400 c using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations by combining these results with fe mg partitioning data for garnet the widths of transition zone phase transformations in a peridotite composition were calculated the olivine to wadsleyite transformation at 1400 c in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation this transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow the entire range of estimated widths for the 410 km discontinuity 4 35 km could therefore be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and h 2 o contents the wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400 c this transformation yield was also found to be near linear

adiabatic temperature profile in the mantle abstract the temperature at the 410 km discontinuity is re evaluated by comparing the depth of the discontinuity with the olivine wadsleyite transition pressure obtained using in situ x ray diffraction experiments by katsura et al 2004a and equation of state eos of mgo by tange et al 2009 tange scale and matsui et al 2000 the newly estimated temperature is 1830 48 k 70 k higher than that by our previous estimation the eoses of the major mantle minerals olivine wadsleyite ringwoodite and perovskite are also recalculated using the tange scale the adiabatic temperature gradient is calculated using the thermal expansion coefficient obtained from these eoses the adiabatic temperature gradient gradually decreases with increasing depth without a phase transition and abruptly increases in association with phase transitions the adiabatic temperature gradients are found to be 04 0 5 and 0 3 k km in the upper and lower parts of the mantle respectively the temperatures at a depth of 200 km the bottom of the mantle transition zone the top of the lower mantle and a depth of 2700 km are found to be 1720 40 2010 40 1980 40 and 2730 50 k the mantle potential temperature is found to be 1610 35 k

genesis and evolution of gypsum tumuli tumuli are hollow subcircular domes of the most superficial stratum of gypsum principally found in outcrops of macrocrystalline gypsum they vary from a few centimetres to several metres in diameter and reach maximum heights of a little more than 1 m the relationships between the morphostatistical parameters that define these formations are h r 3 and e r 9 where h is the elevation of the raised layer e its thickness and r the mean radius their genesis has caused some controversy over the involvement of phenomena such as hydration of anhydrite or tectonic processes capable of explaining this folding this paper shows their genesis linked to the dissolution of macrocrystalline gypsum and reprecipitation of microcrystalline gypsum within the same gypsiferous layer it has been calculated that to reach the theoretical saturation within the few centimetres thickness of the cap of the tumulus water infiltration velocities are required of between 0 002 cm s 1 for an uplifted stratum of 2 cm thickness and 0 03 cm s 1 for 30 cm thickness these velocities imply the existence of very slow rates of infiltration and or capillary movement of water within the gypsiferous layer the secondary microcrystalline gypsum is precipitated in the intercrystalline and intracrystalline voids of the gypsum crystals producing an increase in porosity and associated volume that causes the doming of the gypsiferous layer the development of tumuli is a cyclic process which is favoured by a sequence of short wet and dry intervals which in turn facilitate the almost simultaneous processes of dissolution and precipitation these conditions predominate in arid and semiarid climates where intense evaporation can occur suddenly following sporadic infiltration copyright 1999 john wiley sons ltd

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

the composition and geotherm of the lower mantle constraints from the elasticity of silicate perovskite abstract a newly developed parameterization of the third order isentropic finite strain equation of states eos is used in conjunction with experimental data and theoretical results on mgsio3 perovskite new geotherms for the lower mantle are derived by comparison with preliminary reference earth model prem the geotherms are adiabatic up to 1500 km depth and super adiabatic thereafter a description of the critical steps in obtaining the new parameterization is also given

subduction of continental crust and terrigenous and pelagic sediments an experimental study abstract an important role for the recycling of terrigenous sediments in the mantle was proposed by armstrong 1 2 aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust over the pressure interval 6 24 gpa this composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments in addition to terrigenous sediments during subduction the subsolidus phase relationships displayed by lithologies derived from these materials are complex with major roles being played by k hollandite garnet na clinopyroxene and stishovite continentally derived lithologies achieve densities similar to or greater than surrounding mantle at depths greater than 200 km the geodynamical implications of these buoyancy relationships are discussed a reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies during partial melting at relatively low pressures 5 10 gpa orthoclase wadeite and k hollandite are eliminated near the solidus whereas the stability field of na clinopyroxene extends to temperatures well above the solidus liquids formed by small to moderate degrees of partial melting of subducted continentally derived lithologies in this pressure interval have high k na ratios and high sio2 contents at higher pressures 16 24 gpa the stability fields of k hollandite and stishovite extend towards the liquidus whereas sodium bearing phases are eliminated much closer to the solidus resultant partial melts consequently possess relatively low k na ratios and sio2 contents lead possesses a high crystal liquid partition coefficient in k hollandite whereas uranium is excluded the broad stability field of k hollandite during partial melting of these lithologies at pressures above 15 gpa therefore has the capacity to cause fractionation of lead from uranium the petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle

phase relation and mineral chemistry in pyrolite to 2200 c under the lower mantle pressures and implications for dynamics of mantle plumes 1 phase relations in pyrolite were studied at pressures of 24 and 30 gpa and temperatures up to 2200 c at 24 gpa and 1600 c an assemblage of mg perovskite magnesiowustite ca perovskite and garnet was observed with the proportion of garnet increasing continuously with temperature at 30 gpa only the two perovskites and magnesiowustite were observed and no significant changes in mineral proportions with temperature were observed iron was almost equally partitioned between mg perovskite and magnesiowustite at temperatures below about 1800 c at 24 and 30 gpa with increasing temperature however iron was concentrated more in magnesiowustite at both of these pressures the decrease of al content in mg perovskite with temperature which is caused by an increase in the proportion of garnet is likely to be responsible for the iron partitioning behavior at 24 gpa while the minor elements na al and cr in magnesiowustite may play an important role in fe partitioning at 30 gpa the density difference between ascending hot plumes t 200 and 400 c and the surrounding mantle was calculated using the present experimental data the density difference t 400 c at 30 gpa 806 km depth is 0 06 g cm3 but increases to 0 22 g cm3 at 24 gpa 670 km depth the large increase in the density difference that occurs in a hot plume at the uppermost part of the lower mantle originates from an increase of the proportion of garnet and may be a trigger for the plume s passage through the boundary between the lower mantle and the mantle transition zone

topography of the mantle transition zone discontinuities beneath alaska and its geodynamic implications constraints from receiver function stacking the 410 and 660 km discontinuities d410 and d660 respectively beneath alaska and adjacent areas are imaged by stacking 75 296 radial receiver functions recorded by 438 broadband seismic stations with up to 30 years of recording period when the 1 d iasp91 earth model is used for moveout correction and time depth conversion significant and spatially systematic variations in the apparent depths of the d410 and d660 are observed the mean apparent depth of the d410 and d660 for the entire study area is 417 12 km and 665 12 km respectively and the mean mantle transition zone mtz thickness is 248 8 km which is statistically identical to the global average for most of the areas the undulations of the apparent depths of the d410 and d660 are highly correlated indicating that lateral velocity variations in the upper mantle above the d410 contribute to the bulk of the observed apparent depth variations by affecting the traveltimes of the p to s converted phases from both discontinuities beneath central alaska a broad zone with greater than normal mtz thicknesses and shallower than normal d410 is imaged implying that the subducting pacific slab has reached the mtz and is fragmented or significantly thickened within the proposed northern cordilleran slab window an overall thinner than normal mtz is observed and is most likely the result of a depressed d410 this observation when combined with results from seismic tomography investigations may indicate advective thermal upwelling from the upper mtz through the slab window

stability of clinopyroxene at pressure temperature conditions of the transition region abstract the stability of diopside at conditions of 16 19 gpa and 1250 1900 c was determined using a 6 8 type multi anvil apparatus at pressures of 17 18 gpa diopside undergoes a decomposition reaction to an assemblage of casio3 perovskite ca mg sio3 garnet or ilmenite depending on temperature the clapeyron slope for the breakdown reaction download download full size image is 2 mpa k 1 when coupled with a re examination of clinopyroxene stability in other more complex peridotite systems the experimental data are consistent with a recent proposal that the 520 km discontinuity in the transition region is caused by the exsolution of casio3 rich perovskite from garnet rich in majorite component

impact of quarrying gypsum in a semidesert karstic area sorbas se spain the messinian gypsum of sorbas is intensively karstified with hundreds of dolines and numerous highly unusual exo and endokarstic forms amongst these the tumuli are especially spectacular as are the large caverns adorned with numerous speleothems some of which have not been described anywhere else in the world the extraction of the gypsum has impacted practically all the elements of the environment landscape water soil subsoil flora fauna etc and in particular all of the karstic forms in addition to endangering the caves and the natural treasures that they contain it affects the surface waters markedly modifying the hydrographic network groundwaters which now occur in the bottom of a quarry are also affected

triangulated finite difference methods for global scale electromagnetic induction simulations of whole mantle electrical heterogeneity long period global scale electromagnetic induction data have long been recognized as containing potentially useful information for constraining our understanding of the dynamics and physiochemical state of earth s deep interior a key component to realizing the potential of this data set is the ability to compute the induction response of a fully 3 d earth mantle in global spherical geometries in this contribution a novel finite difference method is described which preserves many algorithmic advantages of staggered grid discretizations in cartesian computational electromagnetics but is generalized for the sphere by introducing a hybrid tensor product mesh consisting of concentrically nested triangulated spherical shells topologically connected node for node in the radial direction such a discretization allows for a flexible distribution of mesh nodes in the lateral sense and avoids the problems associated with excessive node density near the poles which results from conformally mapping the standard cartesian mesh modified finite difference templates for spherical geometries are derived and presented as is a comparison with integral equation solutions for a simple double hemisphere test model as a first step toward applying the algorithm for mantle induction studies results of a conservative thermally activated mantle heterogeneity model confirm that mantle structure remains observable in the presence of the strongly conductive ocean and may manifest important diagnostic signatures in the orientation of the horizontal component of induced magnetic field

reconciling pacific 410 and 660 km discontinuity topography transition zone shear velocity patterns and mantle phase transitions a r t i c l e i n f o article history we observe that the depths of the 410 and 660 km seismic discontinuities are on average slightly positively correlated globally this is due in large part to a modestly depressed 660 km discontinuity and a large depression of the 410 km discontinuity across the paci c the clapeyron slope dp dt the change in pressure depth of a phase transition with a change in temperature can be used to predict the depth of a phase transformation assuming lateral temperature variations the phase change of olivine to spinel is well understood experimentally almost certainly produces the 410 km discontinuity and has a positive clapeyron slope at the base of the transition zone both the olivine component of the mantle spinel and the pyroxene component garnet transform to perovskite and periclase observations of 660 km discontinuity depths are often consistent with the negative clapeyron slope of the spinel to perovskite and periclase transition with an apparent anti correlation with the depth of the 410 km discontinuity however under the paci c the depression of the 410 km discontinuity and slow seismic velocities indicate that the mantle is warmer than average using a negative clapeyron slope for the perovskite forming reaction the 660 km discontinuity is predicted to be shallow in this region however we observe that it is either depressed or has little de ection from its average depth we nd that if the clapeyron slope associated with the 660 km discontinuity changes sign from negative to positive between 1920 and 2020 k we can explain the correlation of discontinuity structure with seismic velocities in the transition zone this shift in sign is in accord with the dominant 660 km transition forming reaction shifting from spinel to garnet near this temperature

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

kinetics of the post garnet transformation implications for density and rheology of subducting slabs abstract mechanisms and kinetics of the post garnet transformation in natural pyrope were examined at 28 1 31 3 gpa and 1320 2150 k by time resolved x ray diffraction measurements using synchrotron radiation combined with microstructural observations of the recovered samples the water content of samples recovered was estimated to be approximately 400 wt ppm h 2 o from infrared spectroscopic measurements the transformation occurs by the grain boundary reaction and reaction rims are formed along the grain boundaries of parental garnet grains decomposed post garnet assemblages consisting of mg and ca perovskites aluminous phase and stishovite show very fine grained symplectic texture analyses of kinetic data obtained have revealed that growth rates in the post garnet transformation are strongly time dependent extrapolations of the present kinetic data suggest that the post garnet transformation cannot complete at less than ca 1730 k even under geological timescales because of the slow growth kinetics due to the presence of the metastable garnet subducting slabs become buoyant in the lower mantle at less than ca 1730 k for the basaltic crust and at ca 1400 1600 k for the underlying peridotite layer rheological weakening of the basaltic crust is expected in the lower mantle through grain size reduction by the metastable post garnet transformation occurring under large overpressure conditions the kinetics of the post garnet transformation significantly affect both the density and rheology of subducting slabs in the lower mantle and possibly has important roles in the mixing and survival of chemically differentiated slab materials

geological and environmental implications of the evaporite karst in spain in spain evaporite outcrops cover approximately 7 of the total area of the country most of the evaporitic formations are made up of ca sulfates gypsum anhydrite or ca sulfates and halite certain paleogene marine evaporites also contain k mg chlorides and some tertiary continental formations bear substantial amounts of na sulfates in the subsurface glauberite and thenardite mesozoic evaporitic formations commonly wedge out towards the ground surface passing into condensed sequences and dissolution collapse breccias some of these highly porous breccias constitute major regional aquifers in several areas interstratal karstification of the evaporites has given rise to gravitational deformations such as basin structures monoclines and collapse structures covering several square kilometers that record a cumulative subsidence in excess of 200 m teruel and calatayud grabens a widespread consequence of evaporite dissolution processes in spain is the hydrochemical degradation of surface waters some of the largest and most outstanding lake systems from an environmental perspective occur in karstic depressions developed in evaporitic formations fuente de piedra gallocanta bujaraloz and banolas lakes sinkhole activity is a major geohazard in several evaporite karst areas the sinkhole risk has a particularly high impact in sectors where tertiary evaporites are overlain by quaternary alluvial aquifers calatayud zaragoza and madrid areas some of the detrimental effects of subsidence include severe damage to historical monuments calatayud the demolition of a whole village puilatos or the derailment of a freight train zaragoza area the deepest gypsum caves are found in triassic diapiric structures el sumidor cave 210 m deep and the longest ones are developed in horizontally lying neogene sequences sorbas caves and estremera maze cave the cardona diapir hosts salt caves up to 4 300 m long whose genesis is related to flooding of mine galleries caused by the interception of a phreatic conduit the main anthropogenic impacts on the endokarstic systems are related to the disposal of wastewaters and the destruction of caves by quarrying the fluvial valleys that cross tertiary evaporitic outcrops commonly show peculiar geological characteristics related to dissolution induced synsedimentary subsidence phenomena 1 thickened alluvium filling dissolution basins up to several tens of kilometers long and more than 100 m deep the largest thickenings are found in areas where the bedrock contains halite and glauberite 2 superimposed alluvial units locally bounded by angular unconformities 3 abundant deformational structures and paleosinkholes related to the rockhead and or interstratal karstification of the substratum these fluvial valleys typically are flanked by a prominent gypsum escarpment rock falls favored by the dissolutional enlargement of joints derived from these scarps are the type of mass movement which has caused the highest number of casualties in spain

a temperature profile of the mantle transition zone the temperature distribution across the transition zone has been determined on the high pressure phase equilibria in the system mg2sio4 fe2sio4 recently determined by katsura and ito 1988 and ito and takahashi 1988 the temperature at the base of the transition zone 655 km depth is estimated to be about 1600 c assuming that the spinel dissociation is responsible for the sharp seismic discontinuity implied by study of reflected waves this estimation would be little modified for the chemically stratified mantle models since the lower mantle is characterized by the stability of perovskite the temperatures in the region from 350 km to 655 km depth are calculated for a peridotitic mantle model assuming an adiabatic temperature gradient the and transformations in mg fe 2sio4 make the temperature profile steep in the depth regions of 390 415 km and 500 530 km respectively the temperature would increase from about 1400 c to 1600 c over the depths interval 350 between 655 km

in situ infrared spectra of hydroxyl in wadsleyite and ringwoodite at high pressure and high temperature the infrared spectra of hydroxyl in synthetic hydrous wadsleyite mg 2 sio 4 and ringwoodite mg 2 sio 4 were measured at room temperature up to 18 8 gpa for wadsleyite and up to 21 5 gpa for ringwoodite high temperature spectra were measured in an externally heated diamond anvil cell up to 650 c at 14 2 gpa for wadsleyite and up to 900 c at 18 4 gpa for ringwoodite the synthetic samples reproduce nearly all the important oh bands previously observed at ambient conditions only subtle changes were observed in the infrared spectra of both minerals both upon compression at room temperature and upon heating at high pressure for wadsleyite upon compression to 18 8 gpa the frequencies of the bands at 3600 cm 1 remain almost unchanged while the main bands at 3200 3400 cm 1 shift to lower frequencies during heating at 14 2 gpa to 650 c the bands at 3200 3400 cm 1 broaden and shift to slightly lower frequencies for ringwoodite upon compression to 21 5 gpa the main bands at 3115 cm 1 progressively shift to lower frequencies during heating at 18 4 gpa to 900 c no frequency shift was observed for the band at 3700 cm 1 but the band initially at 3115 cm 1 shifts very slightly to higher frequencies which should yield almost the same band positions at 1300 1400 c as those measured at ambient conditions our data suggest that water speciation in hydrous wadsleyite and ringwoodite at ambient conditions may be comparable to that under mantle conditions except perhaps for subtle changes in hydrogen bonding the low oh stretching frequencies in wadsleyite and ringwoodite under transition zone conditions imply a large h d fractionation during degassing of the deep mantle this may explain the apparent disequilibrium between the hydrogen isotopic composition of the upper mantle and the ocean

elasticity composition and temperature of the earth s lower mantle a reappraisal the interpretation of seismological models for the earth s lower mantle in terms of chemical composition and temperature is sensitive both to the details of the methodology employed and also to uncertainties in key thermoelastic parameters especially for the dominant mg fe sio3 perovskite phase here the alternative approaches adiabatic decompression of the lower mantle for comparison at zero pressure with laboratory data and the projection of laboratory data to lower mantle p t conditions for direct comparison with seismological observations are assessed along with the equations of state on which they are based it is argued that adiabatic decompression of the lower mantle is best effected by a strategy in which consistent third order finite strain expressions are required simultaneously to fit the strain dependence of both the seismic parameter and the density this procedure accords due weight to the most robust seismological observations namely the wave speeds and reduces to an acceptable level the otherwise very strong covariance among the fitted coefficients it is demonstrated that this approach can readily be adapted to include the effects of relaxation from the hill average to the reuss lower bound of the aggregate bulk modulus which governs the radial variation of density for the projection of laboratory thermoelastic data to lower mantle p t conditions the preferred equation of state is of the mie gruneisen type involving the addition at constant volume of the pressure along a finite strain principal 300 k isotherm and the thermal pressure calculated from the debye approximation to the lattice vibrational energy a high degree of consistency is demonstrated between these alternative equations of state possible departures of the lower mantle from conditions of adiabaticity and large scale homogeneity are assessed these are negligible for the prem model bullen parameter b 0 99 0 01 but significant for ak135 b 0 94 0 02 reviving the possibility of a substantially superadiabatic temperature gradient experimentally determined thermoelastic properties of the major mg fe sio3 perovskite and mg fe o magnesiowustite phases are used to constrain the equation of state parameters employed in the analysis of seismological information although the scarcity of information concerning the pressure and temperature dependence of the elastic moduli for the perovskite phase remains a serious impediment nevertheless it is demonstrated that the combination of a pyrolite composition simplified to the three component system sio2 mgo feo with molar xpv 0 67 and xmg 0 89 and a lower mantle adiabat with a potential temperature of 1600 k consistent with the preferred geotherm for the upper mantle and transition zone is compatible with the prem seismological model within the residual uncertainties of the thermoelastic parameters for the perovskite phase in particular such consistency requires values for the perovskite phase of ks ks p t and q ln ln v t of approximately 3 8 and 2 and g t near 0 022 gpa k 1 for the lower mantle assemblage more silicic models which have sometimes been advocated can also be reconciled with the seismological data but require lower mantle temperatures which are much higher by about 700 k for pyroxene stoichiometry however the absence of seismological and rheological evidence for the pair of thermal boundary layers separating convection above and below which is implied by such models remains a formidable difficulty under these circumstances the simplest possible model that of grossly uniform chemical composition throughout the mantle is preferred

a joint interpretation of electromagnetic and seismic tomography models suggests the mantle transition zone below europe is dry abstract the three dimensional 3d electrical conductivity distribution in the mantle transition zone mtz below europe is resolved by directly inverting geomagnetic response functions from 12 selected european observatories comparison of the conductivity image with the results of global seismic tomography reveals a good correlation between low conductivity and high velocity anomalies at depths of 400 700 km beneath the mediterranean subduction zone in central europe interpretation of the 3d conductivity model combined with seismic tomography indicates that the european mtz is drier than the mtz beneath the western pacific subduction zone even though the subducted slab material is stagnating similarly in both regions

using stable isotopes δ 17 o δ 18 o and δd of gypsum hydration water to ascertain the role of water condensation in the formation of subaerial gypsum speleothems abstract we analyzed the stable isotopes 17 o 18 o and d of gypsum hydration water ghw in a variety of speleothems as well as condensation and infiltration waters in five caves of the semiarid gypsum karst of sorbas basin almeria se spain microclimate parameters air temperature relative humidity and effective condensation rate were also monitored over an annual cycle we found that the mother solution from which the majority of gypsum speleothems grow is composed of a mixture of condensation 60 and infiltration water 40 that undergoes evaporation although evaporation of infiltration water alone was thought to be responsible for secondary gypsum precipitation in vadose caves our results suggest that condensation can be a major source of water for the formation of gypsum speleothems the modelled d excess and 17 o trajectories of water during the evaporative process confirm that the majority of speleothems precipitate from a mixture of condensation and infiltration water under relative humidity of 75 85 similar to that measured in the cave atmosphere during winter these findings have important implications for future studies of gypsum speleothems as paleoenvironmental archives

gypsum karst evolution in a diapir a case study pinoso alicante spain gypsum deposits are highly soluble and can undergo rapid weathering especially where water intervenes though certain human activities can accelerate the karstification processes an example of karst formed in gypsum at pinoso alicante spain is analysed to show how human activity has impacted on its evolution the karst here is associated with a huge diapir in the triassic terrain that boasts of a large number of highly unusual exokarstic landforms here rock salt has been extracted commercially for over 25 years using solution mining via deep boreholes the brine extracted contains in excess of 300 g l over the whole period more than 12 million tonnes of salt have been extracted using this method after preliminary treatment to remove impurities it travels along a brine pipeline to the coast the enormous volume of the voids generated have probably favoured the acceleration of certain exokarstic and endokarstic processes

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part ii stochastic barotropic modeling abstract a stochastic barotropic model linearized about the 850 mb flow is used to investigate the relationship between wind variations associated with the madden julian oscillation mjo and eddy kinetic energy variations in the tropics such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the mjo and the suppression of eddy activity during the easterly phase of the mjo the concentration of eddy energy during the westerly phase results from the strong east west and north south gradients of the large scale wind fields the model shows that barotropic wave propagation and wave mean flow interaction tend to concentrate small scale rossby wave energy in regions of convergence which may be an important mechanism for organizing convection into tropical cyclones the structure and barotropic energetics of the wave activity are similar to those observed but the modeled eddies are smaller in scale and do not move westward as do the observed e

two types of deep seismicity in subducting slabs the relation between variation of seismic activity with depth and temperature structure in the subducting slabs in five subduction zones kuril ne japan izu bonin mariana and tonga is investigated when the temperature of the core of the slab at depths of 300 350 km is higher than the kinetic cut off temperature of the olivine spinel phase transformation there is no seismicity maximum in the slab below that depth range when the temperature of the core of the slab at 300 350 km depths is lower than the kinetic cut off temperature there is a depth range of relatively high seismic activity in the slabs moreover estimated temperature in the depth range of relatively high seismicity is near the kinetic cut off temperature it is inferred that the phase transformation of meta stable olivine triggers deep earthquakes and causes high seismic activity below the depths of 300 350 km in relatively low temperature slabs when the phase transformation is the cause of deep earthquakes in relatively high temperature slabs however it is considered that the phase transformation occurs in nearly equilibrium conditions and meta stable olivine does not exist other mechanisms in addition to the phase transformation are required to explain observed deep seismicity especially in relatively high temperature slabs

pressure effect on electrical conductivity of mantle olivine abstract we report new measurements of electrical conductivity of olivine that were made in a multi anvil press using solid buffers mo moo2 to stabilize oxygen partial pressure the temperature range was 1000 c 1400 c and the pressure range was 4 10 gpa when pressure effects are interpreted as activation volumes in the arrhenius equation they yield values of order 0 6 0 6 cm3 mole values of the pre exponential factor 0 are of the order of hundreds of s m and the zero pressure activation energy is about 1 5 ev in agreement with laboratory measurements at lower pressure conditions in addition we analyzed older literature data which produced a similar result but with larger variation the common practice of assuming that the pressure effect on electrical conductivity of olivine in the earth s upper mantle depends weakly on pressure has rested on uncertain grounds these new observations suggest that for upper mantle conditions depths of 80 200 km neglecting pressure effects on olivine conductivity is justified a weak pressure effect also supports the small polaron fe3 model as the dominant conduction mechanism although the mainly positive activation volumes argue for a component of lattice deformation when applied to the depth range 200 400 km in the earth the laboratory data yield conductivities of order 10 2 s m slightly lower than the range of geophysical conductivity profiles but well within this range after allowing for probable effects of oxygen activity and temperature uncertainty

reflections of p p from upper mantle discontinuities beneath the mid atlantic ridge summary precursors to p p observed in japan from a large deep earthquake mb 6 4 in the fiji islands are analysed to investigate reflection properties of upper mantle discontinuities beneath an area close to the mid atlantic ridge mar in the equatorial atlantic ocean traveltime analysis indicates that the precursors are interpreted as underside reflections of p p from discontinuities at depths of 655 and 415 km in the mantle the latter reflection is much weaker and more intermittent than the former slant stacking and beam forming are attempted to measure the slowness and incidence azimuth of p p and its precursors the obtained slownesses are 3 4 and 2 8 s deg 1 for p p and p p respectively little deviations of the ray incidence off the diametral azimuth are found for both phases amplitude ratio p dp p p here d 655 or 415 is studied as a function of the wave period and the distance of reflection point of p p to the axis of mar the overall trend of the spectral amplitude ratio for p 655p is consistent with the theoretical value for the 670 km discontinuity of the prem model dziewonski anderson 1981 p 415p signals were very weak and it was difficult to obtain an unbiased estimate of p 415p p p the amplitude ratios p 655p p p of filtered seismograms with a passband of 0 5 4 0 s have a mean value of 0 13 and a standard deviation of 0 03 this amplitude ratio is in good agreement with those of previous studies on the reflections from the 650 km discontinuity beneath antarctica and surrounding seas when the reflection points of p p are calculated under the assumption of a spherically symmetric earth the observed p 655p p p seems to decrease as the points become close to the axis of mar thus it could be inferred that some changes in the reflectivity of the discontinuity might occur at or near the spreading center another interpretation may also be possible the low velocity beneath the mar region would cause focussing of p p rays

upper mantle anisotropy and transition zone thickness beneath southeastern north america and implications for mantle dynamics a variety of models for mantle flow beneath southeastern north america have been proposed including those that invoke westward driven return flow from the sinking farallon slab small scale convective downwelling at the edge of the continental root or the upward advective transport of volatiles from the deep slab through the upper mantle we use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern united states to test several of these proposed mantle flow geometries near the coast stations exhibit well resolved null no splitting behavior for sks phases over a range of back azimuths consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry farther inland we identify splitting with mainly ne sw fast directions consistent with asthenospheric shear due to absolute plate motion apm lithospheric anisotropy aligned with appalachian tectonic structure or a combination of these phase weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations at most stations we find transition zone thicknesses that are consistent with the global average 240 km with two stations showing evidence for a slightly thickened transition zone 250 km our results are relevant for testing different models for mantle dynamics beneath the southeastern united states but due to the sparse station coverage we are unable to uniquely constrain the pattern of mantle flow beneath the region our sks splitting observations support a model in which mantle flow is primarily vertical either upwelling or downwelling beneath the southeastern edge of the north american continent in contrast to the likely horizontal apm driven flow beneath the continental interior however our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material we expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks including the transportable array and flexible array components of usarray

high pressure phase transformation in camgsi2o6 and implications for origin of ultra deep diamond inclusions the decomposition of diopside to casio3 cubic and mgsio3 orthorhombic perovskites was demonstrated by in situ x ray diffraction measurements under simultaneous high pressure and high temperature conditions using a combination of synchrotron radiation and a multianvil apparatus at spring 8 quench experiments also confirmed the decomposition of diopside in a wide temperature range of 1000 1900 c and at pressures 23 27gpa these results strongly support the idea that some mineral inclusions with pyroxene stoichiometry discovered in natural diamonds may have been originated from the lower mantle moreover observed temperature dependence of mgsio3 content dissolved in casio3 perovskite suggests that some of the inclusions were formed at relatively low temperatures 1200 c in the deep mantle presumably related to slab subductions

numerical model of heat flow in back arc regions abstract we numerically modeled mantle wedge flow driven by subducting plates and its consequences to surface heat flow in back arc regions where an increase in surface heat flow has been observed in almost all subduction zones in order to calculate the steady state temperature and velocity field in a generic subduction zone model we utilized a new technique which does not require a priori assumptions about the position of the slab wedge boundary we demonstrated that calculated surface heat flow is very sensitive to the thermal boundary condition prescribed at the base of the model for a simple temperature dependent viscosity since an increase of surface heat flow is a common feature of many back arc regions it should be rather independent on various thermal conditions applied at the bottom we showed that such a robust behavior can be reproduced if moreover a strong pressure dependence of viscosity is taken into account the pattern of increased surface heat flow is then stable for a wide range of settings including various dip angles and age of the subducting slab the temperature and pressure dependent viscosity resulted in a nearly isothermal mantle wedge where temperatures exceeded 1200 c even in relatively shallow depths and consequently elevated surface heat flow is consistent with recent hot back arcs evidence circulation in the wedge was obtained since the employed viscosity law resulted in creation of a zone of substantially reduced viscosity in shallow depths below the continental crust

trans pacific temperature field in the mantle transition region derived from seismic and electromagnetic tomography abstract the trans pacific temperature field for the depth range 350 850 km was inferred from global seismic tomography and semi global electromagnetic tomography the seismic tomography incorporated millions of reported first arrival times and 7000 pp p differential travel times measured on broadband seismograms the electromagnetic tomography used voltage data from trans pacific submarine cables and magnetic field data from circum pacific geomagnetic observatories the resultant p wave velocity anomalies and electrical conductivity anomalies were converted to temperature anomalies using a proposed conversion formula and experimental results for mantle minerals respectively these conversions show consistently high temperature anomalies of 200 300 k in the mantle transition region beneath the hawaiian hotspot at subduction zones where slab related cold anomalies and wedge mantle related hot anomalies are likely to coexist in close proximity the seismic and electromagnetic tomography did not always give consistent features in part because of the preferred sensitivity of electromagnetic tomography to hot anomalies low temperature anomalies of 200 300 k associated with subducted slabs are clearly resolved in the seismic tomography but are less apparent in the electromagnetic tomography the high temperature anomaly in the intervening zone between the mariana and philippine slabs is very pronounced in the electromagnetic tomography but is marginal in the seismic tomography

morphology and micro fabrics of weathering features on gyprock exposures in a semiarid environment ebro tertiary basin ne spain abstract gyprock is a common rock in evaporitic formations which have a wide distribution across all continents however outcrops of gyprock are rather limited occurring mainly in low rainfall areas i e southwest of usa italy spain gyprock is one of the most common lithology in the ebro tertiary basin here semiarid conditions propitiate the development of specific weathering processes and landforms with significant implications for understanding the genesis of soils and the geomorphological evolution of these areas gypsum crystallization within the pores and fissures of gyprock close to the surface leads to volume increase and pressures with the consequent weathering of this rock as a result considerable porosity develops producing peculiar morphologies on gyprock outcrops domes and decimetre scale blisters specific microscopic features voids with plain walls horse shoe pores and serrated pores as well as microscopic fabrics grating fabric skeletal fabric fan fabric and botryoidal fabric this paper presents a morphological characterization of the material forming the various types of weathering features from microscopic to mesoscopic scale the macromorphology and micromorphology of those features were used to interpret their genesis

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

silicon self diffusion of mgsio3 perovskite by molecular dynamics and its implication for lower mantle rheology 1 the pressure effect of silicon self diffusion of mgsio3 perovskite was investigated by molecular dynamics md the viscosity variation of mgsio3 perovskite in the lower mantle was derived by the nabarro herring herring nabarro model for the md calculation the spontaneous jumping of atoms by self diffusion was reproduced without using artificial forces and the consistency of migration enthalpy hm with experimental data was improved the results showed that migration enthalpy increases monotonically with increasing pressure the viscosity of mgsio3 perovskite in the lower mantle increases monotonically with increasing depth the obtained depth profile is distinguishable from that of mgo periclase and can be utilized to determine which mineral dominates the lower mantle rheology depending upon the assumed shape of the depth profile for lower mantle viscosity we considered the dominant mineral as 1 mgsio3 perovskite for the monotonic shape case or 2 mgo periclase for the hill shape case that has the highest viscosity zone in the middle of the lower mantle

solution and recrystallisation processes and associated landforms in gypsum outcrops of sicily abstract four small areas of messinian upper miocene age gypsum outcropping in western sicily are described messinian age evaporites are found in sicily over a 1000 km 2 area here gypsum outcrops extensively as a consequence of soil erosion induced by human impact geomorphological maps show how the rocky surfaces are characterized by a wide range of forms there are large medium small and microsized forms which can be identified as belonging to different morphotypes the morphotypes can be classified into two main categories those that originated by solution and those that originated through recrystallisation four areas illustrated by geomorphological maps were specifically chosen to describe a type of medium sized form dome like hills these medium sized forms are covered by a mosaic of smaller forms related to both the previous categories different types of karren and of expansion forms the types of karren can be explained as the results of the solution process under different hydro dynamical behaviour the dome like hills and other related expansion forms are more difficult to understand these expansion forms can be explained by the same process that leads to the development of gypsum tumuli the outcrops of gypsum lacking soil cover and influenced by alternating seasonal water conditions of surplus and deficit are affected by both solution and recrystallisation processes during the wet season the water soaks into the rocky mass filling all the fissures and pores of the outer rocky layer from a few centimetres to some metres below the surface during the dry season there is a capillary upward motion of the water solution near the surface gypsum precipitates from the oversaturated solution increasing the crystal size or forming new crystals in this way during the dry season there is a pressure increase in the outer gypsum layers which is responsible for the development of a gypsum weathering crust and characterised by many different forms such as gypsum tumuli pressure ridges pressure humps and other related small forms the crust may also lead to the development of mega tumuli and dome like hills from the morphostructural point of view the dome like hills do not seem to be controlled by the strike dip or fissuring of the gypsum beds their evolution seems to be linked to the fact that on most of the dome surfaces the weathering crust is evolving through a nearly isotropic field of stresses resulting in volume increase in the outer gypsum layer

locally layered convection inferred from dynamic models of the earth s mantle the structure of convection in the mantle is still the subject of considerable debate the now standard modelling of the convective flow as driven in a viscous mantle by density anomalies derived from seismic tomography has successfully explained the longest wavelength degree 2 to 8 geoid anomalies and provided important information concerning the viscosity structure of the mantle1 8 with this approach however the predicted response of surface topography to convective stresses the dynamic topography has a typical magnitude of several kilometres which does not conform with observations9 11 a possible source of this discrepancy lies in the severe underestimation by tomography of density anomalies due to deflections of the boundary between the upper and lower mantle at 660 km depth here we model the mantle flow implied by seismically derived density heterogeneities using an empirical method to account for the 660 km boundary topography the predicted dynamic surface topography thus obtained is significantly reduced to values that conform with the observations in addition the 660 km boundary topography appears to have a strong influence on the computed mantle circulation inducing local layering of the convective flow