on the relative influence of heat and water transport on planetary dynamics the dynamics of a planet and its evolution are controlled to a large extent by its viscosity in this study we demonstrate that the dependence of mantle viscosity on temperature and water concentration introduces strong dynamic feedbacks we derive a dimensionless parameter to quantitatively evaluate the relative strength of those feedbacks and show that water and heat transport are equally important in controlling present day dynamics for the earth a simple parameterized evolution model illustrates the strong feedbacks and behavior of the system and agrees well with our analytic results the analysis identifies characteristic times for changes of viscosity temperature and water concentration and demonstrates for time scales greater than a few hundred million years that the system should either be degassing while warming or regassing while cooling this yields a characteristic evolution in which after an initial period of rapid adjustment the mantle warms while degassing and subsequently cools rapidly while regassing as the planet continues to cool the entire surface ocean may eventually return to the mantle our results suggest that a simple relationship may exist between the rate of change of water concentration and the rate of change of temperature in the mantle this connection is extended by deriving an explicit equation for the urey ratio that depends on both heat and water transport

textural and compositional evolution of au hosting fe s as minerals at the axi epithermal gold deposit western tianshan nw china abstract the axi deposit is the largest low sulfidation epithermal gold deposit in chinese western tianshan central asian orogenic belt in this contribution combined petrological observations electron microprobe analysis empa and x ray backscattered electron images bse have been conducted on fe s as minerals to reveal their textural and compositional evolution pyrite and arsenopyrite are ubiquitous their texture and composition show gradual changes with time four generations of pyrite can be identified named py1 py2 py3 and py4 from early to late the coarse grained euhedral to subhedral py1 mostly 100 300 m from the quartz chalcedony stage is the earliest and predates gold mineralization sometimes they contain silicate and arsenopyrite apy1 inclusions compositionally they are enriched in pb up to 0 60 wt but depleted in as 0 06 7 61 wt mostly syn mineralization feature subsequent py3 occurs as very fine grained disseminations of euhedral crystals 10 50 m in quartz polymetallic sulfides stage and carbonate quartz stage they display clear oscillatory zoning and contain arsenopyrite apy3 sphalerite and sb te mineral inclusions their au content vary greatly from 0 02 wt to 1 02 wt with corresponding as content varying from 0 07 wt to 8 28 wt the post mineralization py4 crystals 30 5000 m mainly occur as foliated sheaf or radiaxial aggregates in carbonate quartz stage they have the lowest au with a mean value of 0 04 wt as with a mean value of 0 32 wt co with a mean value of 0 04 wt and ni with a mean value of 0 04 wt concentrations although dramatically high s contents with a mean value of 52 72 wt correspondingly there are three generations of arsenopyrite apy1 20 50 m and apy3 3 15 m mainly occur as inclusions in py1 and py3 respectively whereas the latest apy4

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

geology and geochemistry of the early permian axi low sulfidation epithermal gold deposit in north tianshan nw china abstract the axi gold deposit containing 12 mt of ore with an average au grade of 5 g t is the largest low sulfidation epithermal gold deposit in chinese tianshan it is hosted by late devonian to early carboniferous continental arc volcanic rocks in the tulasu volcanic sedimentary basin in north tianshan nw china the orebody is controlled by annular and radial faults associated with volcanic crater and later nw trending faults four stages of ore formation could be identified based on petrographic studies i chalcedony ii gold bearing pyrite quartz veins and breccias iii gold bearing marcasite quartz carbonate veins and iv quartz barite calcite veins electrum precipitated in stage ii 285 195 c log f s 2 6 7 to 13 and stage iii 190 95 c log f s 2 15 8 to 25 6 the chalcedony formed in stage i is strongly enriched in lree la yb 4 5 36 2 compared with later gold bearing pyrite quartz veins and quartz barite calcite veins la yb 1 2 2 0 o h isotopic data indicate that fluid mixing is a major mechanism causing electrum precipitation in stage ii hydrothermal breccias consisting of chalcedony altered wall rocks and gold bearing pyrite quartz veins stage ii are cemented by gold bearing marcasite quartz carbonate assemblages stage iii which implies that brecciation plays an important role in stage iii ore formation re os isotopic analyses of gold bearing pyrite samples yield an 187 re 188 os 187 os 188 os isochron age of 299 35 ma with an initial 187 os 188 os ratio of 0 57 0 37 k ar ages of four intensively sericitized andesitic wall rocks range between 286 and 293 ma these early permian ages are much younger than the late devonian early carboniferous host volcanic rocks but overlap the age range of postcollisional granitoid in the tulasu basin the re os isotopic compositions and the ages suggest that mantle derived components were involved in the axi ore forming system in a postcollisional setting

effects of water transportation on subduction dynamics roles of viscosity and density reduction abstract the effects of water on subduction dynamics e g plate migration rate slab geometry stress field and back arc spreading are investigated by using a 2 d self consistent model for lithosphere subduction and whole mantle convection we solve water transportation coupled with hydrous mineral phase changes mantle flows and water transportation are interactive through constitutive and state equations for hydrous rocks our model has successfully reproduced the water distribution in a mantle wedge and along the slab with sufficient resolution comparable to that of previous models that focus on the mantle wedge structure as a result low density owing to hydration reduces subduction rates back arc spreading and slab stagnation on the phase boundary at 660 km depth whereas low viscosity owing to hydration enhances rapid subduction trench migration and slab stagnation we attribute these results to mechanisms that cause the hydrous buoyancy of subducting plates to reduce the slab pull force and the accompanying tensile stress on overlying lithosphere in addition hydrous weakening diminishes the mechanical coupling of the subducted slab with the wedge mantle and overriding lithosphere thus water is capable of generating two opposite situations in the stress field of the overlying lithosphere and the subduction rate water is therefore expected to be an important mechanism for generating broad styles of the subduction structure and kinematics as observed in actual subduction zones such as tonga and mariana such observed variation in the subduction mode can be caused by variation in buoyancy corresponding to the water content from relatively dry to several thousands of parts per million for the wedge mantle and slab surface whereas the extremely buoyant case does not appear to occur in nature water in the mantle is thus key to better understand the whole mantle scale slab dynamics as well as island arc volcanic processes

observation and inversion of surface wave group velocities across central india abstract surface wave group velocities across central india are measured from computer plots of frequency time analysis using multiple filter technique periods range from 5 to 56 sec for fundamental mode rayleigh wave 5 to 16 sec for first higher mode rayleigh wave and 5 to 49 sec for fundamental mode love wave the wave paths from koyna earthquakes to bokaro cross purely through the peninsular plateau the group velocities for these paths are found to be similar to those of surface waves recorded at poona from earthquake which lie to the northeast of bokaro in sub himalayan regions from this it is concluded that the shield structure of the peninsula extends to the sub himalayan regions to northeast of the peninsular plateu with a negligible part of sedimentary layer group velocities of fundamental mode rayleigh and love waves are also obtained for the paths from koyna earthquakes to shillong and are found to be similar to the above mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of bengal basin northern limit of this basin sedimentary layer has been identified mean and standard deviation of observed group velocity at each period are obtained for each of rayleigh fundamental rayleigh first higher and love fundamental modes the group velocities closely agree with those obtained earlier for the path between new delhi and kodaikanal and are also found to be similar to those observed in some shield regions monte carlo inversion method is applied and crust and upper mantle structure of the indian shield across central india is obtained crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study

influence of plate tectonic mode on the coupled thermochemical evolution of earth s mantle and core we investigate the influence of tectonic mode on the thermochemical evolution of simulated mantle convection coupled to a parameterized core cooling model the tectonic mode is controlled by varying the friction coefficient for brittle behavior producing the three tectonic modes mobile lid plate tectonics stagnant lid and episodic lid the resulting compositional structure of the deep mantle is strongly dependent on tectonic mode with episodic lid resulting in a thick layer of subducted basalt in the deep mantle whereas mobile lid produces only isolated piles and stagnant lid no basaltic layering the tectonic mode is established early on with subduction initiating at around 60 myr from the initial state in mobile and episodic cases triggered by the arrival of plumes at the base of the lithosphere crustal production assists subduction initiation increasing the critical friction coefficient the tectonic mode has a strong effect on core evolution via its influence on deep mantle structure episodic cases in which a thick layer of basalt builds up experience less core heat flow and cooling and a failed geodynamo thus a continuous mobile lid mode existing from early times matches earth s mantle structure and core evolution better than an episodic mode characterized by large scale flushing overturn events

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

generation of plate like behavior and mantle heterogeneity from a spherical viscoplastic convection model 1 how plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical viscoplastic computations we present mainly internally heated convection results with temperature dependent viscosity and explore parts of the rayleigh number ra yield stress y phase space as well as the effects of depth dependent y bottom heating and a low viscosity asthenosphere convective planform and toroidal poloidal velocity field ratio tpr are affected by near surface viscosity variations and tpr values are close to observed values for our most plate like models at the relatively low convective vigor that is accessible at present most models favor spherical harmonic degree one convection though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra an asthenospheric viscosity reduction improves plate like nature as expected for our incompressible computations pure bottom heating produces strong plumes that tend to destroy plates at the surface this implies that significant internal heating may be required both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer

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

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

grain damage hysteresis and plate tectonic states abstract shear localization in the lithosphere is an essential ingredient for understanding how and why plate tectonics is generated from mantle convection on terrestrial planets the theoretical model for grain damage and pinning in two phase polycrystalline rocks provides a frame work for understanding lithospheric shear weakening and plate generation and is consistent with laboratory and field observations of mylonites grain size evolves through the competition between coarsening which drives grain growth and damage which drives grain reduction the interface between crystalline phases controls zener pinning which impedes grain growth damage to the interface enhances the zener pinning effect which then reduces grain size forcing the rheology into the grain size dependent diffusion creep regime this process thus allows damage and rheological weakening to co exist providing a necessary positive self weakening feedback moreover because pinning inhibits grain growth it promotes shear zone longevity and plate boundary inheritance however the suppression of interface damage at low interface curvature wherein inter grain mixing is inefficient and other energy sinks of deformational work are potentially more facile causes a hysteresis effect in which three possible equilibrium grain sizes for a given stress coexist 1 a stable large grain weakly deforming state 2 a stable small grain rapidly deforming state analogous to ultramylonites and 3 an unstable intermediate grain size state perhaps comparable to protomylonites a comparison of the model to field data suggests that shear localized zones of small grain mylonites and ultra mylonites exist at a lower stress than the co existing large grain porphyroclasts rather than as predicted by paleopiezometers or paleowattmeters at a much higher stress this interpretation of field data thus allows localization to relieve instead of accumulate stress the model also predicts that a lithosphere that deforms at a given stress can acquire two stable deformation regimes indicative of plate like flows i e it permits the coexistence of both slowly deforming plate interiors and rapidly deforming plate boundaries earth seems to exist squarely inside the hysteresis loop and thus can have coexisting deformation states while venus appears to straddle the end of the loop where only the weakly deforming branch exists

plate generation and two phase damage theory in a model of mantle convection summary the formation of narrow rapidly deforming plate boundaries separating strong plate interiors are integral components of the generation of plate tectonics from mantle convection the development of narrow plate boundaries requires the interaction of a non linear rheology and convection one such non linear rheology is two phase damage theory which employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby forming a theoretical model for void generation two phase damage theory was recently extended to allow for deformational work to increase the fineness reduce the grain size of the matrix phase we present results testing two phase damage theory in a 2 d convectively driven system where we allow for 1 pure void generating damage 2 pure fineness generating damage and 3 combined void and fineness generating damage pure void generating damage is found to be unsuccessful at producing plate like features finenessgenerating damage is successful at inducing plate like behaviour in certain circumstances including increasing viscosity sensitivity to fineness and certain regimes of damage input and healing rate cases with combined void and fineness generating damage produce significantly more localization than the end members due to the apparent increase of deformational work input into fineness generation the interaction of microcracks and grain size reduction in two phase damage theory suggests a rheological model for shear localization necessary for the formation of plate tectonic boundaries

global scale water circulation in the earth s mantle implications for the mantle water budget in the early earth abstract we investigate the influence of the mantle water content in the early earth on that in the present mantle using numerical convection simulations that include three processes for redistribution of water dehydration partitioning of water into partially molten mantle and regassing assuming an infinite water reservoir at the surface these models suggest that the water content of the present mantle is insensitive to that of the early earth the initial water stored during planetary formation is regulated up to 1 2 oms om ocean mass 1 4 10 21 kg which is reasonable for early earth however the mantle water content is sensitive to the rheological dependence on the water content and can range from 1 2 to 3 oms at the present day to explain the evolution of mantle water content we computed water fluxes due to subducting plates regassing degassing and dehydration for weakly water dependent viscosity the net water flux is almost balanced with those three fluxes but for strongly water dependent viscosity the regassing dominates the water cycle system because the surface plate activity is more vigorous the increased convection is due to enhanced lubrication of the plates caused by a weak hydrous crust for strongly water dependent viscosity the degassing history is insensitive to the initial water content of the early earth as well as rheological strength the degassing flux from earth s surface is calculated to be approximately o 10 13 kg yr consistent with a coupled model of climate evolution and mantle thermal evolution

lithospheric deformation induced by loading of the hawaiian islands and its implications for mantle rheology the long term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on earth in this study we have developed 3 d finite element models for computing the load induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding low temperature plasticity and high temperature creep we have determined the lithospheric deformation and stress due to volcano loading in the hawaiian islands region for the last few million years by comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the hawaiian islands region we have sought to constrain lithospheric rheology our calculations show that the load induced surface deformation is controlled by low temperature plasticity and frictional sliding but is insensitive to high temperature creep lithospheric strength predicted from laboratory derived low temperature plasticity needs to be reduced significantly and a frictional coefficient f ranging from 0 1 to 0 7 is required in order to account for the observations however f 0 1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity our results therefore suggest a value for f between 0 25 and 0 7 finally the maximum stress that accumulates in the deformed lithosphere beneath the hawaiian islands is about 100 200 mpa for models that match the observations and this stress may be viewed as the largest lithospheric stress on earth

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

the role of fluid pressure on frictional behavior at the base of the seismogenic zone to characterize stress and deformation style at the base of the seismogenic zone we investigate how the mechanical properties of fluid rock systems respond to variations in temperature and strain rate the role of fluids on the processes responsible for the brittle ductile transition in quartz rich rocks has not been explored at experimental conditions where the kinetic competition between microcracking and viscous flow is similar to that expected in the earth our initial analysis of this competition suggests that the effective stress law for sliding friction should not work as efficiently near the brittle ductile transition as it does at shallow conditions

hysteresis in mantle convection plate tectonics systems 1 we use simulations of mantle convection with surface yielding to show that multiple tectonic regimes are possible for equivalent system parameter values models with the same lithospheric strength parameters and the same vigor of convection can display different modes of tectonics within the region of multiple solutions the evolutionary pathway of the system is the dominant factor that determines the tectonic mode e g whether mantle convection operates in a plate tectonic like mode the extent of the multiple regimes window is found to increase with the temperature dependent viscosity contrast across the system the implication for models that seek to predict the tectonic regimes of planets is that the temporal evolution of the planet needs to be taken into account a further implication is that modeling studies can lead to different conclusions regarding the tectonic state of a planet extra solar planets in particular despite the final model parameter values remaining equivalent

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

phase relations of peridotites under h2o saturated conditions and ability of subducting plates for transportation of h2o phase relations of peridotites under h2o saturated conditions up to 28 gpa and 2600 c have been clarified based on the high pressure experimental results in the mgo feo sio2 h2o mgo sio2 al2o3 h2o cao mgo al2o3 sio2 h2o and natural systems based on the phase assemblages deduced and the chemical compositions of the phases the maximum h2o contents for two bulk rock compositions lherzolite and harzburgite have been calculated using mass balance then the potential ability of subducting plates for transportation of h2o is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a 15 130 ma the subduction velocity u0 2 25 18 cm year and angle 30 and 60 and the mantle potential temperature tp 1300 and 1350 c the results confirm the importance of choke point a cusp around 6 2 gpa and 550 c at which the stability field of major hydrous phases is minimized in terms of temperature although the choke point is exceeded by the mg sursassite bearing assemblage that has the maximum h2o content of 0 4 0 7 wt around 5 7 gpa and 550 700 c if the geotherm of the coldest part across the subducting plate passes below the choke point maximum 4 6 wt h2o can subduct whereas h2o less than 0 4 0 7 wt can subduct above the choke point the critical conditions are clarified as follows if 30 and tp 1300 c a 15 ma with u0 9 cm year or a 30 ma with u0 2 25 cm year is the critical condition if 60 and tp 1300 c or 30 and tp 1350 c the critical conditions slightly shift to an older age or a greater velocity e g a 30 ma with u0 3 5 cm year the slab thermal parameter au0sin is useful to predict roughly the critical condition however within the younger plate the spatial extent of a cold region with major hydrous phases is thinner hence less h2o is subducted even with the same slab thermal parameter this indicates that the age of the plate and the subduction velocity act differently on the h2o transportation ability of subducting plates

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

water circulation and global mantle dynamics insight from numerical modeling we investigate water circulation and its dynamical effects on global scale mantle dynamics in numerical thermochemical mantle convection simulations both dehydration hydration processes and dehydration melting are included we also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed due to effective slab dehydration at shallow depths water still affects significantly the global dynamics by weakening the near surface oceanic crust and lithosphere enhancing the activity of surface plate motion compared to dry mantle case as a result including hydrous minerals the more viscous mantle is expected with several orders of magnitude compared to the dry mantle the average water content in the whole mantle is regulated by the dehydration hydration process the large scale thermochemical anomalies as is observed in the deep mantle is found when a large density contrast between basaltic material and ambient mantle is assumed 4 5 comparable to mineral physics measurements through this study the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection

generation of plate tectonics from lithosphere mantle flow and void volatile self lubrication abstract the formation of plate tectonics from mantle convection necessarily requires nonlinear rheological behavior recent studies suggest that self lubricating rheological mechanisms are most capable of generating plate like motion out of fluid flows the basic paradigm of self lubrication is nominally derived from the feedback between viscous heating and temperature dependent viscosity here we propose a new idealized self lubrication mechanism based on void e g pore and or microcrack generation and volatile e g water ingestion we test this void volatile self lubrication mechanism in a source sink flow model this leads to a basic nonlinear system which permits the excitation of strike slip toroidal motion a necessary ingredient of plate like motion out of purely divergent i e poloidal or characteristically convective flow with relatively inviscid void filling volatiles the void volatile mechanism yields a state of highly plate like motion i e with uniformly strong plate interiors weak margins and extremely focussed strike slip shear zones moreover the void volatile model obeys a chemical diffusion time scale that is typically much longer than the thermal convection time scale the model thus complies with the observation that plate boundaries are long lived and survive even while inactive the void volatile model of self lubrication therefore predicts self focussing shear zones plate generation and plate boundary longevity through what has long been suspected to be a key ingredient for the existence of plate tectonics i e water

spontaneous development of arcuate single sided subduction in global 3 d mantle convection models with a free surface we present temporally evolving 3 d global mantle convection models with single sided subduction and a free surface in both 3 d cartesian and fully spherical geometry special focus is given to the spontaneous development of three dimensional structures at the surface and in the upper mantle we find that an arcuate shape is the natural form for trenches and slabs cartesian models are used first to study the dynamic evolution of subduction zones spreading ridges and interconnected transform features these experiments highlight the strong variation of spontaneously developing arcuate slab curvature and subduction polarity along the trench strike the spontaneous development of spreading ridges leads to lateral offsets between separated segments that are characterized by normal transform motion spherical models then allow insights into the evolution of plate tectonics on a sphere investigated are the spontaneous evolution of slab geometry trench motion and subduction induced mantle flow two new dynamical features are discovered back slab spiral flow and slab tunneling

geology and geochemistry of axi gold deposit xinjiang the axi gold deposit is a large epithermal gold deposit in west tianshan mountain of xinjiang its ore forming process can be divided into four stages namely chalcedony quartz stage pyrite quartz stage marcasite carbonate quartz stage and barite carbonate stage electrum occurs as the most important gold bearing mineral stage and are the major periods for gold deposition from stage 195 285 logf s2 6 7 13 to stage 95 190 logf s2 15 8 25 6 the temperature and sulfur fugacity of the ore forming fluid decrease obviously the reactions between the ore forming fluid and the wall rocks caused the increase of lile contents in the ore forming fluid the variations of eu behaviors in veins formed at different stage suggest that stage i vein crystallized in a rather oxidized fluid relative to stage vein stage i vein is strongly enriched in lree la yb 4 5 36 2 in comparison with veins formed at stages and la yb 1 2 2 0 stage vein ree 67 5 10 6 is different from stage vein ree 1 0 10 6 4 2 10 6 co2 3 and so2 4 are important in transporting and enriching ree in the ore forming fluid geochemistry of volcanic rocks and veins formed at different stage suggests that the magmatic fluid was the primary ore forming fluid and meteoric water participated in the mineralization from stage to stage which caused obvious decrease of temperature and sulfur fugacity and hence induced the electrum precipitation at stages and

long term stability of plate like behavior caused by hydrous mantle convection and water absorption in the deep mantle we investigate the cycling of water regassing dehydration and degassing in mantle convection simulations as a function of the strength of the oceanic lithosphere and its influence on the evolution of mantle water content we also consider pseudo plastic yielding with a friction coefficient for simulating brittle behavior of the plates and the water weakening effect of mantle materials this model can generate long term plate like behavior as a consequence of the water weakening effect of mantle minerals this finding indicates that water cycling plays an essential role in generating tectonic plates in vigorous plate motion the mantle water content rapidly increases by up to approximately 4 5 ocean masses which we define as the burst effect a burst is related to the mantle temperature and water solubility in the mantle transition zone when the mantle is efficiently cooled down the mantle transition zone can store water transported by the subducted slabs that can pass through the choke point of water solubility the onset of the burst effect is strongly dependent on the friction coefficient the burst effect of the mantle water content could have significantly influenced the evolution of the surface water if the burst started early in which case the earth s surface cannot preserve the surface water over the age of the earth

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

why is terrestrial subduction one sided subduction of the lithosphere at convergent plate boundaries takes place asymmetrically the subducted slab sinks downward while the overriding plate moves horizontally one sided subduction in contrast global mantle convection models generally predict downwelling of both plates at convergent margins two sided subduction we carried out two dimensional 2 d numerical experiments with a mineralogical thermomechanical viscoelastic plastic model to elucidate the cause of one sided subduction our experiments show that the stability intensity and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab two sided subduction occurs at low slab strength sin 0 15 the weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism the resulting weak interplate zone localizes deformation at the interface and decouples the strong plates facilitating asymmetric plate movement our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self sustaining one sided subduction processes

parameters controlling dynamically self consistent plate tectonics and single sided subduction in global models of mantle convection recent advances in numerical modeling allow global models of mantle convection to more realistically reproduce the behavior at convergent plate boundaries in particular the inclusion of a free surface at the outer boundary has been shown to facilitate self consistent development of single sided subduction this allows for a more extensive study of subduction in the context of global mantle convection as opposed to commonly used regional models our first study already indicated important differences between mantle convection with single sided subduction and mantle convection with double sided subduction here we further investigate the effect of various physical parameters and complexities on inducing earth like plate tectonics and its evolution in time results reinforce the previous finding that using a free surface instead of a free slip outer boundary dramatically changes subduction style with free surface cases displaying many episodes of single sided subduction which leads to more realistic slab dip stress state trench retreat rate and slab induced mantle flow longevity of single sided subduction is promoted by a layer of hydrated crust with a low yield strength to lubricate the subduction channel a low viscosity asthenosphere and a high strength of the slab determined by a combination of high diffusion creep viscosity and intermediate friction coefficient although its effective viscosity is in the observationally constrained range in the bending region the time evolution displays interesting events including subduction polarity reversals subduction shut off and slab break off

how mantle slabs drive plate tectonics the gravitational pull of subducted slabs is thought to drive the motions of earth s tectonic plates but the coupling between slabs and plates is not well established if a slab is mechanically attached to a subducting plate it can exert a direct pull on the plate alternatively a detached slab may drive a plate by exciting flow in the mantle that exerts a shear traction on the base of the plate from the geologic history of subduction we estimated the relative importance of pull versus suction for the present day plates observed plate motions are best predicted if slabs in the upper mantle are attached to plates and generate slab pull forces that account for about half of the total driving force on plates slabs in the lower mantle are supported by viscous mantle forces and drive plates through slab suction

origin and models of oceanic transform faults abstract mid ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics a fundamental problem of plate tectonics is how this pattern has formed and why it is maintained gross scale geometry of mid ocean ridges is often inherited from respective rifted margins indeed transform faults seem to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge both analog and numerical models of transform faults were investigated since the 1970s two main groups of analog models were developed thermomechanical freezing wax models with accreting and cooling plates and mechanical models with non accreting lithosphere freezing wax models reproduced ridge ridge transform faults inactive fracture zones rotating microplates overlapping spreading centers and other features of oceanic ridges however these models often produced open spreading centers that are dissimilar to nature mechanical models on the other hand do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading three main types of numerical models were investigated models of stress and displacement distribution around transforms models of their thermal structure and crustal growth and models of nucleation and evolution of ridge transform fault patterns it was shown that a limited number of spreading modes can form transform faults microplates overlapping spreading centers zigzag ridges and oblique connecting spreading centers however the controversy exists whether these patterns always result from pre existing ridge offsets or can also form spontaneously at a single straight ridge during millions of year of accretion therefore two types of transform fault interpretation exist plate fragmentation structures vs plate accretion structures models of transform faults are yet relatively scarce and partly controversial consequently a number of first order questions remain standing and significant cross disciplinary efforts are needed in the future by combining natural observations analog experiments and numerical modeling

two dimensional convection with a self lubricating simple damage rheology summary we present 2 d simulations of convection in the earth s mantle with temperature and damagedependent viscosity in a basally heated system the equation governing the temporal evolution of damage includes a source term for damage a healing term and an advection term a systematic study of the influence of the different damage terms on the convection pattern shows that 1 at least four different convective regimes can be distinguished depending on the size of the damage source term and 2 self lubricating behaviour is possible only if the advection term in the damage equation is small enough compared with the other terms we also demonstrate that good plate like behaviour in terms of a focused low viscosity bands lvb b homogeneous surface velocities within the plates and abrupt velocity jumps across the plate boundaries and c asymmetric subduction can be obtained with this kind of rheology although an increase in subduction asymmetry tends to make convection highly time dependent

a free plate surface and weak oceanic crust produce single sided subduction on earth 1 earth s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection subduction zones on present day earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle while global self consistent numerical models of mantle convection have reproduced some aspects of plate tectonics the assumptions behind these models do not allow for realistic single sided subduction here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates 1 the presence of a free deformable upper surface and 2 the presence of weak hydrated crust atop subducting slabs we show that assuming a free surface rather than the conventional free slip surface allows the dynamical behavior at convergent plate boundaries to change from double sided to single sided a weak crustal layer further improves the behavior towards steady single sided subduction by acting as lubricating layer between the sinking and the overriding plate this is a first order finding of the causes of single sided subduction which by its own produces important features like the arcuate curvature of subduction trenches citation crameri f p j tackley i meilick t v gerya and b j p kaus 2012 a free plate surface and weak oceanic crust produce single sided subduction on earth geophys res lett 39 l03306 doi 10 1029 2011gl050046

planforms of self consistently generated plates in 3d spherical geometry 1 in the past decade several studies have documented the effectiveness of plastic yielding in causing a basic approximation of plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity strong enough to form a rigid lid in the absence of yielding the vast majority of such research to date has been in either two dimensional or three dimensional cartesian geometry in the present study mantle convection calculations are performed to investigate the planform of self consistent tectonic plates in three dimensional spherical geometry the results are compared to those of similar calculations where a three dimensional cartesian geometry is used we found that when yield stress of the lithosphere is low 20 mpa a great circle subduction zone forms at low intermediate yield stresses 100 mpa plates spreading centers and subduction zones formed and were destroyed over time at high intermediate yield stresses 200 mpa two plates form separated by a great circle boundary that is a spreading centre on one side and a subduction zone on the other side at high yield stresses 400 mpa a rigid lid was observed the great circle subduction zone and the rigid lid are stable over time while at intermediate yield stresses some episodic behavior is observed the spherical cases showed a higher more earth like toroidal poloidal ratio of the surface velocity field than the cartesian cases

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

models of convection driven tectonic plates a comparison of methods and results summary recent numerical studies of convection in the earth s mantle have included various features of plate tectonics a number of different methods for modelling plate like behaviour have been used the differences in the methods of modelling plates may assume or predict significantly different plate deformation we describe three methods of modelling plates through material properties force balance and a thin power law sheet approximation we compare the results obtained using each method on a series of simple calculations from these results we are able to develop scaling relations between the different parametrizations while each method produces different degrees of deformation within the surface plate the surface heat flux and average plate velocity agree to within a few per cent the main results are not dependent upon the plate modelling method and therefore are representative of the physical system we set out to model

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 2 strain weakening and asthenosphere 1 in a previous paper tackley 2000b it was shown that a combination of temperature dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic like behavior in three dimensional models of mantle convection here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries strain weakening and the presence of a low viscosity asthenosphere introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus 1 dramatically improves plate quality even if the asthenosphere is restricted to regions around spreading centers 2 gives good smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude and 3 gives bimodal stable solutions over a range of yield stresses either immobile lid or plate behavior may be obtained depending on initial condition by contrast introducing an asthenosphere by using depth dependent viscosity has a much smaller effect on system behavior increasing strain weakening 1 improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates 2 weakens convergent zones and can make downwellings highly episodic and 3 does not lead to pure transform boundaries in these calculations time dependent damage evolution and instantaneous strain rate weakening give very similar results all cases that display platelike behavior have very long wavelength mantle heterogeneity consistent with earth s