grain growth kinetics of cairo3 perovskite and post perovskite with implications for rheology of d layer abstract grain growth kinetics of cairo 3 perovskite and post perovskite aggregates was investigated by time series experiments at pressures of 2 and 3 gpa and temperatures ranging from 1373 to 1773 k in a piston cylinder apparatus the experiments were conducted in the stability field of both perovskite and post perovskite the increase of grain size g with time t follows a growth law g n g 0 n k t k k 0 exp h rt the growth exponents n for perovskite and post perovskite are 2 6 0 5 and 4 5 0 9 respectively the activation enthalpy h of post perovskite grain growth is 251 35 kj mol grain growth of post perovskite is distinctly slower than that of perovskite sluggish grain growth of the post perovskite is caused by its strong anisotropic shape the phase transition from the perovskite to the post perovskite at the d discontinuity would lead to maintenance of small grain size in the d layer if diffusion creep is the dominant deformation mechanism of post perovskite the phase transformation would induce the significant softening of the d layer due to slow growth rate of post perovskite compared with a situation without the phase transition

integrating the uranium series and elemental diffusion geochronometers in mixed magmas from volcan quizapu central chile magma mixing is a ubiquitous process in silicic magmatic systems we present pre and post mixing time scales for a single magma mixing event associated with the 1846 1847 eruption of volcan quizapu we compare and contrast 226 ra 230 th crystal residence ages between the mixed lavas of the 1846 1847 effusive eruption and the largely homogeneous plinian dacite eruption of 1932 both eruptions are dominated by the same dacite magma whereas the ear lier eruption contains evidence for a single episode of mafic recharge and mixing the use of uranium series mineral separate dating on a mixed magma requires deconvolution of different crystal populations we present a four component mixing model for the crystal separates to obtain dacite derived plagioclase model ages as well as andesite derived recharge plagioclase ages plagioclase model ages average 3 6 ka for dacite derived plagioclase from the mixed 1846 1847 eruption 1 4 kyr older than average plagioclase model ages of the 1932 eruption suggesting that magma mixing and reheating led to some resorption of young crystal rims in the 1846 1847 magmatic episode and or that some growth of plagioclase rims occurred during the repose time of 86 years the andesite derived plagioclase residence ages are younger than the corresponding dacite derived plagioclase residence ages but they are still of the order of thousands of years in addition we use mg diffu sion in plagioclase to constrain time scales of magma mixing we show that the 1846 1847 eruption was triggered by large volume mafic recharge magma that mixed and mingled with the resident dacite within probably days to weeks of the ensuing eruption

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

diffusion closure temperature and age of a mineral with arbitrary extent of diffusion theoretical formulation and applications the commonly used expression of the closure temperature tc of a diffusing species in a mineral as derived by m h dodson contrib mineral petrol 40 1973 259 264 is applicable only to systems which have undergone sufficient diffusion so that even the composition at the center of individual grains is significantly removed from that established at the onset of cooling we have extended dodson s formulation to include cases with arbitrarily small amount of diffusion and applied it to calculate tc and age profiles which would develop in single crystals of different geometries these results permit evaluation of the extent of resetting of mineral age and of ion exchange geothermometers during cooling the measured age profile of a mineral can also be used to constrain its cooling rate these applications have been illustrated for the cases of garnet biotite fe mg exchange geothermometer and the cooling age and closure temperatures of the sm nd and lu hf decay systems in garnet

grain growth kinetics of ferropericlase at high pressure abstract grain growth kinetics of mg 0 85 fe 0 15 o ferropericlase was investigated at pressures of 5 and 10 gpa and temperatures of 1673 1873 k using a kawai type multi anvil apparatus presintered ferropericlase aggregate with an average grain size of 4 0 m was used as starting material of grain growth annealing experiments the grain growth kinetics of ferropericlase is described by g n g 0 n k 0 t exp e p v r t where g is the average grain size at annealing time t g 0 the initial average grain size p the pressure r the gas constant and t is the absolute temperature with n 2 6 0 4 log 10 k 0 7 5 1 7 m 2 6 s e 262 30 kj mol v 4 7 0 7 cm 3 mol compared at same pressure and temperature grain growth rate of ferropericlase is similar to olivine and faster than those of wadsleyite and ringwoodite the present results show that at the top of the lower mantle p 25 gpa and t 1773 k grain size of ferropericlase in single phase system evolves to 2 10 2 m after significant geological time 10 my while a previous study predicts that grain size of mgo in two phase system is as small as 8 10 6 m at same condition

slab weakening during the olivine to ringwoodite transition in the mantle the strength of subducted slabs in the mantle transition zone influences the style of mantle convection intense deformation is observed particularly in relatively old subducted slabs in the deep mantle transition zone understanding the cause of this regional and depth variation in slab deformation requires constraint of the rheological properties of deep mantle materials here we report results of in situ deformation experiments during the olivine to ringwoodite phase transformation from which we infer the deformation process under the conditions of cold slabs deep in the mantle transition zone we find that newly transformed fine grained ringwoodite deforms by diffusion creep and that its strength is substantially smaller than that of coarser grained minerals but increases with time scaling analysis based on a model of transformation kinetics and grain size evolution during a phase transformation suggests that a cold slab will be made of a mixture of weak fine grained and strong coarse grained materials in the deep transition zone whereas a warm slab remains strong because of its large grain size we propose that this temperature dependence of grain size may explain extensive deformation of cold slabs in the deep transition zone but limited deformation of relatively warm slabs in relatively cold subducted slabs olivine may transform to finer grained weaker ringwoodite than in warm slabs according to deformation experiments under conditions analogous to those in the mantle transition zone and scaling analysis

crystal origins and magmatic system beneath ngauruhoe volcano new zealand revealed by plagioclase textures and compositions abstract the textural variation and compositional zoning of plagioclase in pre historic and historic basaltic andesite lava flows from ngauruhoe volcano reveals extensive crystal recycling from a multi level magma system most phenocrysts have a calcic an80 90 resorbed core with diffuse or no zonation that is depleted in fe and mg some cores display patchy zonation from replacement by high an crystallization prior to resorption the cores are mantled by oscillatory zoned rims of lower an content the resorbed cores crystallized from magmas more mafic than those erupted at ngauruhoe and slow cooling and prolonged storage resulted in loss of an zoning patterns and depletion of fe and mg by diffusion these crystals are likely to have originated from deep cumulates or intrusions and were subsequently entrained in ascending magmas patchy textured cores were produced during decompression in a water under saturated magma and staged ascent the diversity in crystal cores reflect different conduits and ascent histories the crystal rims grew in a more differentiated magma reservoir and are in equilibrium with the erupted melt most of the zoning patterns in the rim zone require water pressure and or temperature changes these changes could have been caused by convective self mixing in a closed system and or the intrusion of hydrous melts of similar bulk composition other crystals display rimward elemental enrichments consistent with mafic recharge previously reported rimward enrichment in 87sr 86sr compositions can be explained by the re cycled origin of the crystal cores and progressive crustal assimilation at shallower depths in the magma system where subsequent crystal growth occurred a steady state balance of intrusion and crystallization in the deep crust and remobilization and entrainment of co magmatic crystals in newly formed and ascending magmas has produced relatively monotonous batches of basaltic andesite throughout the volcano s life trace element zonation fe and mg in the plagioclase mostly reflects variable diffusive loss through the crystal history and the patterns in different crystals are highly variable this along with uncertainties in estimating trace element partitioning coefficients highlights the difficulty of reconstructing melt compositions and subterranean residence time in such magmas

the olivine spinel transformation and the rheology of subducting lithosphere during subduction relatively hot lithospheric slabs are expected to deform in a superplastic flow regime below a depth of 300 km if olivine reacts to fine grained spinel this process reduces the flow strength of such slabs below the phase boundary so that they become aseismic at about this depth in colder slabs temperatures are too low for superplastic deformation and a high flow strength and seismic activity are maintained to a depth of 600 700 km

si and o diffusion in mg fe 2sio4 wadsleyite and ringwoodite and its implications for the rheology of the mantle transition zone si and o diffusion rates on polycrystalline mg fe 2 sio 4 wadsleyite and ringwoodite have been determined at pressures between 16 and 22 gpa and temperatures between 1400 and 1600 c using a kawai type multi anvil high pressure apparatus pre synthesized polycrystalline wadsleyite or ringwoodite was used as starting materials diffusing sources of 29 si and 18 o enriched mg fe 2 sio 4 thin film were deposited on the surface of wadsleyite and ringwoodite by pulsed laser deposition the diffusion profiles were obtained using secondary ion mass spectrometry in the depth profiling mode all measured diffusion profiles were composed of volume and grain boundary diffusion regimes arrhenius relations for volume and grain boundary diffusion rates of si and o in wadsleyite and ringwoodite have been determined the results show that si is the slowest diffusing element among the major elements in both wadsleyite and ringwoodite under mantle transition zone as well as slab conditions therefore si should be the rate controlling species for high temperature creep in wadsleyite and ringwoodite comparisons with si and o diffusion rates obtained in this study and those in olivine and silicate perovskite suggest that si and o diffusion rates are enhanced at both 410 and 660 km seismic discontinuities the deformation mechanism maps constructed from si diffusion data in wadsleyite and ringwoodite suggest that the dominant deformation mechanism operating in the mantle transition zone is dislocation creep which may be the origin of the observed global seismic anisotropy under cold subduction zone conditions grain size sensitive diffusion creep becomes dominant when the grain size is reduced to less than 10 100 m below the metastable olivine wedge

magnesium diffusion in plagioclase dependence on composition and implications for thermal resetting of the 26al 26mg early solar system chronometer article history experimental data are reported on mg diffusion in plagioclase crystals with a range of anorthite content xan at temperatures between 800 and 1150 c oriented and polished single crystals of anorthite xan 0 93 labradorite xan 0 67 andesine xan 0 43 and oligoclase xan 0 23 were each embedded in powered source material enriched in natural mgo or 25mgo and suspended in a furnace at constant temperature diffusion profiles in quenched samples were measured from the polished surface using sims depth profiling the diffusion coefficient does not depend significantly on the mg concentration gradient and little anisotropy is observed between the b and c directions in labradorite diffusion coefficients increase systematically with decreasing xan and the entire data set is described by ln d 6 06 1 10 7 96 0 42 xan

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

a new mg in plagioclase geospeedometer for the determination of cooling rates of mafic rocks abstract we present a new mg in plagioclase geospeedometer based on the diffusive exchange of mg between plagioclase and clinopyroxene that allows cooling rates of a wide range of mafic rocks to be determined the major element composition of plagioclase is shown to play an important role in driving the flux of mg as well as the partitioning of mg between plagioclase and clinopyroxene therefore commonly used analytical solutions to the diffusion equation are not applicable in this system and an alternate approach is developed practical aspects of the method such as the role of anorthite zoning patterns and influence of shapes of cooling paths and grain geometries in controlling the mg concentration profile shapes are discussed propagation of uncertainties in the knowledge of diffusion and partition coefficients shows that the method is capable of resolving small differences in cooling rates e g log d t d t c yr 1 3 0 3 in one example the approach is illustrated by application to two samples from the lower oceanic crust

grain growth kinetics of majorite and stishovite in morb abstract grain growth rates of majorite and stishovite in their aggregates with morb composition were studied at 18 gpa and temperatures ranging from 1873 to 2223 k the rates of grain growth were expressed by g n g 0 n k 0 t exp h r t where g is the grain size at time t s and g 0 is the initial grain size we determined the grain growth kinetic parameters for majorite where n 9 1 1 3 h 950 249 kj mol and log k 0 32 5 2 3 m 9 1 s for the graphite capsule and log k 0 31 3 3 9 m 9 1 s for the re capsule kinetic parameters for stishovite were determined to be n 6 6 0 9 h 599 226 kj mol and log k 0 30 6 0 9 m 6 6 s for the graphite capsule and log k 0 29 0 1 1 m 6 6 s for the re capsule

oxygen isotope exchange and closure temperatures in cooling rocks retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model which predicts both the zonation profile of 18o within grains and the bulk 18o value of each mineral in the rock apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 18o values of each mineral pair within the rock in systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values slow cooling the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the dodson closure temperatures tc for the two phases and the mode of the rock adding a third phase into a system which undergoes slow cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two phase system in some systems oxygen isotope reversals can be developed if cooling is not slow then the mineral 18o values resulting from cooling will be partly dependent upon the initial temperature of the system concerned the model successfully simulates the mineral 18o values that are often observed in granitic rocks application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems

deformation across the mantle transition zone a theoretical mineral physics view abstract the dynamics of the earth s mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor the transition zone tz might play a crucial role as the interface connecting the upper to the lower mantle here we examine the rheology of the main tz minerals wadsleyite ringwoodite and majorite garnet based on a mineral physics approach using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self diffusion in the tz minerals we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models we show that pure climb creep is expected to contribute to the plasticity of the tz without the need of significant diffusion related hydrolytic weakening matching well the geophysical observations our model results predict that crystallographic preferred orientations cpo might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding tz despite their lower temperatures

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

texture of mg fe sio3 perovskite and ferro periclase aggregate implications for rheology of the lower mantle abstract rheology of the lower mantle characterizes the dynamics of the earth s interior and it is often controlled by the textures of the constituting material which are mg fe sio3 perovskite and ferro periclase aggregate we conducted high pressure experiments to synthesize the mg fe sio3 perovskite and ferro periclase aggregates and measured two important textures of grain size and dihedral angle the grain growth rates of perovskite and ferro periclase in two phase aggregates were influenced by the iron content and increased with factors of 1 5 in iron rich system this difference in grain growth rates indicates that the viscosity of aggregates of iron rich system is only a few times greater than that of iron poor system for likely diffusion creep in the lower mantle in contrast the change of the dihedral angle of perovskite periclase perovskite at triple grain junction with variation of iron content was not observed systematically but the dihedral angle decreases from 110 to 105 with an increase of temperature from 1673 to 2273 k the dihedral angle of 105 110 would imply the interconnected network spatially of ferro periclase in the aggregates and the connectivity increases with temperature as a result at higher temperature ferro periclase plays more important role for understanding the rheology of the lower mantle because ferro periclase is a few order of magnitude softer than mg fe sio3 perovskite

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

seismological and experimental constraints on metastable phase transformations and rheology of the mariana slab abstract a wedge with a low seismic wave velocity implying the presence of metastable olivine and the depression of the 660 km seismic discontinuity have been observed inside the mariana slab based on these seismic observations and numerical calculation of thermal structures of the slab we suggest that the phase transformations from olivine to ringwoodite and from ringwoodite to perovskite ferropericlase the post spinel transformation occur at the depth of 630 km and 550 c and at 690 km and 700 c respectively in the central coldest part of the mariana slab combining this information with recent experimental kinetic data we constrain the details of non equilibrium phase transformations and rheology of the mariana slab the observed depth of the metastable olivine inside the mariana slab can be explained by growth controlled olivine ringwoodite transformation under relatively dry condition such as the water content of about 150 wt ppm h2o on the other hand nucleation process controls the depth of the post spinel transformation in the mariana slab and therefore the observed depressions of the 660 km discontinuity should depend on both the overpressure needed for the nucleation and the negative clapeyron slope for the transformation when the overpressure for the nucleation is 0 5 gpa the observed depressions can be explained by the clapeyron slope of 0 7 mpa k grain size evolution and viscosity structures in the mariana slab are estimated based on these metastable phase transformations the slab weakening due to the grain size reduction is limited in about 40 km in width below the metastable olivine wedge at the depths from 630 to 690 km whereas the width of the weakened portion of the slab increases to more than 120 km at deeper than the 690 km depth after the post spinel transformation these viscosity structures are consistent with the behaviors of the mariana slab estimated from seismic tomography the slab vertically descends to the bottom of the transition zone and substantially deforms and thickens at the top of the lower mantle

evaluation of a relaxation geospeedometer for volcanic glasses abstract the cooling of silicate melts influences the character of volcanic processes primarily through the effect of temperature on melt viscosity but also through the derivative thermal properties expansivity and heat capacity if the cooling rates of volcanic facies can be established then the undoubtedly complex processes of erupting cooling and degassing magmas can be more readily understood the frozen structure of every glass volcanic or synthetic is influenced by the previous cooling history moreover the relaxation of the glass structure back to equilibrium during reheating across the glass transition is also dependent on the quenched in structure and thus can be used to determine the cooling rate history in this study differential scanning calorimetry dsc measurements have been used to determine the enthalpic relaxation of four volcanic glasses each glass has been heated at known heating rates in the calorimeter to temperatures above the glass transition interval the first heating of the glass involves the relaxation of the structure frozen in during the volcanic cooling subsequent controlled heating and cooling in the calorimeter enables the determination of a set of kinetic parameters which can be used to model the heat capacity curves by modeling the transient values of heat capacity across the glass transition interval the relaxation of enthalpy and the volcanic cooling rate across the glass transition interval can be obtained from our study we can demonstrate four different cooling rates for volcanic glasses over a range of three orders of magnitude from 4 c min 1 to 2 c day 1 we propose that the enthalpic relaxation of silicate melts can be applied as a geospeedometer to the study of volcanic rocks from a variety of occurrences

the role of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

fe mg interdiffusion rates in clinopyroxene experimental data and implications for fe mg exchange geothermometers chemical interdiffusion of fe mg along the c axis 001 in natural diopside crystals x di 0 93 was experimentally studied at ambient pressure at temperatures ranging from 800 to 1 200 c and oxygen fugacities from 10 11 to 10 17 bar diffusion couples were prepared by ablating an olivine x fo 0 3 target to deposit a thin film 20 100 nm onto a polished surface of a natural oriented diopside crystal using the pulsed laser deposition technique after diffusion anneals compositional depth profiles at the near surface region 400 nm were measured using rutherford backscattering spectroscopy in the experimental temperature and compositional range no strong dependence of d fe mg on composition of clinopyroxene fe mg ratio between di93 di65 or oxygen fugacity could be detected within the resolution of the study the lack of fo2 dependence may be related to the relatively high al content of the crystals used in this study diffusion coefficients d fe mg can be described by a single arrhenius relation with d text fe text mg 2 7 7 pm 4 2 7 times 10 7 text exp 3 20 7 pm 1 6 0 text kj text mol text rt m 2 text s d fe mg in clinopyroxene appears to be faster than diffusion involving ca species e g d ca mg while it is slower than d fe mg in other common mafic minerals spinel olivine garnet and orthopyroxene as a consequence diffusion in clinopyroxene may be the rate limiting process for the freezing of many geothermometers and compositional zoning in clinopyroxene may preserve records of a higher compared to that preserved in other coexisting mafic minerals temperature segment of the thermal history of a rock in the absence of pervasive recrystallization clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed 1 100 c e g resetting may be expected in slowly cooled mantle rocks many plutonic mafic rocks or ultra high temperature metamorphic rocks

grain growth kinetics in wadsleyite effects of chemical environment abstract grain growth kinetics in wadsleyite was investigated using a multianvil high pressure apparatus fine grained wadsleyite aggregates were synthesized by isostatic hot pressing and were subsequently annealed under high pressure and temperature in a controlled chemical environment wadsleyite samples show normal grain growth characterized by a log normal grain size distribution following the relation l n l 0 n k t where n is a constant l the grain size at time t l0 the grain size at time t 0 and k is a rate constant that depends on temperature t and chemical environments f o 2 oxygen fugacity in pa coh water content in h 106si as k a d f o 2 r d exp h d r t a w f o 2 r w c oh q exp h w r t with a d 10 4 9 6 1 8 0 7 4 m n s 1 p a r d rd 0 12 0 11 0 20 0 14 h d 410 230 500 270 kj mol a w 10 18 2 1 4 24 0 1 7 m n s 1 p a r w rw 0 14 0 05 0 22 0 06 q 1 7 0 3 2 2 0 3 and h w 120 60 160 70 kj mol with assumed value of n 2 3 values in parentheses denote parameters for n 3 both water and oxygen fugacities significantly enhance grain growth kinetics the large value of the parameter describing the water fugacity dependence q 1 5 2 5 cannot be explained solely by a simple model in which grain growth is controlled by diffusion of atoms defects across the grain boundaries the interaction of grain boundaries with charged defects or the density of hydrated ledges may be important factors that control the grain growth kinetics of wadsleyite when compared at similar thermo chemical conditions grain growth of wadsleyite is found to be more sluggish than grain growth of olivine the present results show that a small wadsleyite grain size 100 000 h 106si is present a small grain size

slab dynamics in the transition zone abstract seismic images of horizontal slab segments in across or below the transition zone invoke scenarios in which slabs are laid down in the mantle through progressive trench retreat however observations of subduction characteristics do not exhibit a clear correlation between trench retreat and slab dip in the transition zone instead analysis of a range of subduction characteristics demonstrates that while transition zone slabs include slabs of many ages and subduction velocities with both retreating and advancing trench motion they also have several enigmatic characteristics such as faster sinking rates and a larger range in slab dip than slabs that extend to either shallower or deeper depths comparison of subduction characteristics with several analytical and numerical models of subduction dynamics suggests that many of the possible mechanisms for trapping slabs in the transition zone e g positive buoyancy sources viscous resistance slab weakening are only viable if the slab is already shallow dipping two scenarios for the formation of stagnant slabs are proposed 1 trench retreat prior to slabs entering the transition zone or caused by the negative buoyancy forces associated with the wadsleyite and ringwoodite phase transitions and 2 slow lateral migration of slabs in stable subduction zones

miocene to pliocene development of surface and subsurface temperatures in the benguela current system 1 the initiation of the benguela upwelling has been dated to the late miocene but estimates of its sea surface temperature evolution are not available this study presents data from ocean drilling program odp site 1085 recovered from the southern cape basin samples of the middle miocene to pliocene were analyzed for alkenone based u37k sstuk and glycerol dialkyl glycerol tetraether gdgt based tex86 temptex water temperature proxies in concordance with global cooling during the miocene sstuk and temptex exhibit a decline of about 8 c and 16 c respectively the temperature trends suggest an inflow of cold antarctic waters triggered by antarctic ice sheet expansion and intensification of southern hemisphere southeasterly winds a temperature offset between both proxies developed with the onset of upwelling which can be explained by differences in habitat alkenone producing phytoplankton live in the euphotic zone and record sea surface temperatures while gdgt producing thaumarchaeota are displaced to colder subsurface waters in upwelling influenced areas and record subsurface water temperatures we suggest that variations in subsurface water temperatures were driven by advection of cold antarctic waters and thermocline adjustments that were due to changes in north atlantic deep water formation a decline in surface temperatures an increased offset between temperature proxies and an increase in primary productivity suggest the establishment of the benguela upwelling at 10 ma during the messinian salinity crisis between 7 and 5 ma surface and subsurface temperature estimates became similar likely because of a strong reduction in atlantic overturning circulation while high total organic carbon contents suggest a biogenic bloom in the pliocene the offset between the temperature estimates and the cooling trend was reestablished

intraplate volcanism due to convective instability of stagnant slabs in the mantle transition zone the study of volcanism can further our understanding of earth s mantle processes and composition continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the mantle transition zone mtz such as in europe eastern china and western north america here we use two dimensional numerical models to explore the evolution of stagnant slabs in the mtz and their potential to sustain mantle upwellings that can support volcanism we find that weak slabs may go convectively unstable within tens of million years upwellings rise out of the relatively warm underbelly of the slab are entrained by ambient mantle flow and reach the base of the lithosphere the first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab ultimately convective instability also acts to separate the compositional components of the slab harzburgite and eclogite from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle such a physical filtering process may sustain a long term compositional gradient across the mtz

zirconium in rutile speedometry new constraints on lower crustal cooling rates and residence temperatures abstract the incorporation of zirconium into the mineral rutile tio2 has been both empirically and experimentally calibrated as a measure of rutile crystallization temperatures watson et al 2006 this temperature sensitive system has been employed as a geothermometer with applications to a number of different geologic settings and rock types experimentally measured kinetics for zr diffusion in rutile cherniak et al 2007 indicate that zr can be lost to temperature dependent diffusion warranting further investigation of the geologic significance of calculated temperatures coupling diffusion kinetics with numerical solutions to the diffusion equation provides a means to forward model the time and temperature dependency of the system modeled results indicate a strong dependency of zr concentration in rutile on both 1 initial cooling rate following high temperature metamorphism crystallization and 2 temperature and duration of long term geologic residence zr concentrations measured in rutile from lower crustal xenoliths that resided at 25 45 km depths for 2000 my reveal zr concentrations in the approximate grain center that are consistent with temperatures measured by independent thermometers forward models for zr diffusion show that preserving a zr record of these initial temperatures in the center of a rutile crystal with a 50 m radius requires rapid cooling 300 c ma from magmatic metamorphic temperatures followed by a long term residence 2000 my at temperatures

near conductive cooling rates in the upper plutonic section of crust formed at the east pacific rise abstract a new geospeedometer based on diffusion modeling of mg in plagioclase is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast spreading mid ocean ridges the investigated natural sample suites include gabbroic rocks formed at three different locations along the fast spreading east pacific rise these samples cover a depth interval of 0 840 m below the sheeted dike gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined we demonstrate that the cooling rates we obtained are robust reproducible and consistent across different vertical sections at fast spreading ridges and decrease significantly with increasing sample depth covering almost 4 orders of magnitude ranging from 1 c y 1 for the shallowest samples to 0 0003 c y 1 for the deepest samples both the absolute cooling rates and the rate of decrease of cooling rate with depth are consistent with conductive thermal models in contrast the absolute cooling rates determined from the deeper samples 300 m below dgb and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off axis hydrothermal circulation within the upper plutonic section of the crust

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

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

paleotemperatures and ice volume of the past 27 myr revisited with paired mg ca and 18o 16o measurements on benthic foraminifera 1 we explore the applicability of paired mg ca and 18o 16o measurements on benthic foraminifera from southern ocean site 747 to paleoceanographic reconstructions on pre pleistocene timescales we focus on the late oligocene through pleistocene 27 0 ma history of paleotemperatures and the evolution of the 18o values of seawater 18osw at a temporal resolution of 100 200 kyr absolute paleotemperature estimates depend on assumptions of how mg ca ratios of seawater have changed over the past 27 myr but relative changes that occur on geologically brief timescales are robust results indicate that at the oligocene to miocene boundary 23 8 ma temperatures lag the increase in global ice volume deduced from benthic foraminiferal 18o values but the smaller scale miocene glaciations are accompanied by ocean cooling of 1 c during the mid miocene phase of antarctic ice sheet growth 15 13 ma water temperatures cool by 3 c unlike the benthic foraminiferal 18o values which remain relatively constant thereafter temperatures vary by 3 c and reach maxima at 12 and 8 5 ma the onset of significant northern hemisphere glaciation during the late pliocene is synchronous with an 4 c cooling at site 747 a comparison of our 18osw curve to the haq et al 1987 sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18o 16o ratios at 12 11 ma in particular when benthic foraminiferal 18o values do not support a further increase in ice volume the 18osw curve comes to a maximum that corresponds to a major mid miocene sea level regression the agreement between the character of our mg ca based 18osw curve and sequence stratigraphy demonstrates that benthic foramaniferal mg ca ratios can be used to trace the 18osw on pre pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater mg ca ratios the mg ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes which can lead to misinterpretations of the 18o record

mechanism and kinetics of the α β transition in san carlos olivine mg1 8fe0 2sio4 the mechanism and kinetics of the a b transformation in san carlos olivine a mg1 8fe0 2sio4 containing ca 526 wt ppm h2o were studied in situ by time resolved synchrotron x ray diffraction xrd in the range 12 5 14 5 gpa and 870 1150 k time series of xrd spectra reveal the early formation of an intermediate phase that further transforms to wadsleyite b phase this intermediate phase may be either a metastable ringwoodite g phase or the disordered spinelloid phase proposed in the pseudomartensitic reaction model both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries growth rates in the range 1 7 10 12 2 1 10 9 m s 1 are estimated by fitting the transformation time data to the cahn rate equation and define an activation enthalpy for growth ha 204 55 kj mol compared to the previous studies performed on pure forsterite mg2sio4 these new kinetic results point out an increase of transformation rates with the iron content hence the persistence of a metastable olivine wedge to depths 660 km might be restricted to even colder and or dryer subduction zones than previously estimated

dynamical mechanisms controlling formation and avalanche of a stagnant slab abstract we performed a numerical study to understand the dynamical mechanism controlling the formation and avalanche of a stagnant slab using two dimensional dynamical models of the integrated plate mantle system with freely movable subducting and overriding plates we examined slab rheology as a mechanism for producing various styles of stagnating or penetrating slabs that interact with the 410 km and 660 km phase transitions the simulated results with the systematically changed rheological parameters are interpreted using a simple stability analysis that includes the forces acting on the stagnant slab slab plasticity that memorizes the shape produced by past deformation generates slab stagnation at various depths around the 660 km phase transition the slab stagnates even beneath the 660 km phase boundary with a gentle clapeyron slope feedbacks between trench backward migration and slab deformation promote each other during the slab stagnation stage slab viscosity also determines the final state of the subducted slab that is it continues stagnation or initiates penetration a low viscosity slab can finally penetrate into the lower mantle because the growth time of the rayleigh taylor instability is shorter after the avalanche the direction of the trench migration changes depending on the lower mantle slab viscosity

mg in plagioclase experimental calibration of a new geothermometer and diffusion coefficients abstract the temperature sensitive exchange of mg between plagioclase pl and clinopyroxene cpx has been studied experimentally accounting for different anorthite contents in plagioclase x an and various silica activities a sio 2 in the system the partitioning of mg between plagioclase and clinopyroxene was determined over a temperature range of 1100 1200 c using plagioclase single crystals of different compositions x an 0 5 0 8 surrounded by different clinopyroxene bearing matrix powders to account for different silica activities from 0 55 to 1 0 the experimental design also allows the diffusivity d mg pl of mg in plagioclase under these conditions to be determined both k mg pl cpx defined as k mg pl cpx c mg pl c mg cpx and d mg pl decrease with temperature and increase with a sio 2 isothermal data for different x an in plagioclase show a linear increase of ln k mg pl cpx with increasing x an but d mg pl appears to be insensitive to x an the partitioning data allow a new geothermometer to be calibrated which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist t k 9219 2034 x an ln k mg pl cpx 1 6 ln a sio 2 application of this geothermometer to experimental data from this study reproduces the experimental temperatures within 20 c diffusion of mg in plagioclase is described by d mg pl m 2 s 1 1 25 10 4 m 2 s 1 exp 320 924 j mol 1 rt a sio 2 2 6

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

modeling the dynamics of subducting slabs cold dense subducting lithosphere provides the primary force driving tectonic plates at earth s surface the force available to drive the plates depends on a balance between the buoyancy forces driving subduction and the mechanical and buoyancy forces resisting subduction because both the buoyancy and rheology of the slab and mantle depend on temperature composition grain size water content and melt fraction unraveling which of these processes exert a first order control on slab dynamics and under what circumstances other processes become first order effects can be challenging laboratory and numerical models of slab dynamics provide a powerful method for testing the combined effects of buoyancy and strength changes that accompany the slab evolution in the upper mantle transition zone and lower mantle recent studies have focused on understanding how rheologic variations newtonian versus non newtonian viscosity or water content geometry 2d versus 3d and plate motions trench roll back or advance influence the evolution of slabs in the upper mantle and how they sink into the lower mantle these models suggest that spatial and temporal variations in slab strength and the history of subduction determine whether slabs sink directly into the lower mantle or are trapped in the transition zone

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

testing emplacement models for the rustenburg layered suite of the bushveld complex with numerical heat flow models and plagioclase geospeedometry abstract conventional models of the genesis of the rustenburg layered suite rls of the bushveld complex postulate that it is the product of a single multiply replenished magma chamber that cooled as a single massive unit i e a monolithic magma chamber this concept has been challenged by alternative hypotheses involving out of sequence emplacement of some of its major layers as sills both scenarios have partial support from high precision u pb geochronological data to test these contradictory hypotheses new zircon geothermometric data and compositional data for plagioclase from the upper critical zone are presented and the available geochronological data are tested against the predictions of the thermal evolution of the entire rls for both emplacement and cooling scenarios crystallization temperatures of zircon within units of the upper critical zone were estimated using ti in zircon thermometry and range between 799 45 c and 865 64 c the data are used in conjunction with a detailed numerical model of heat flow in the bushveld complex that is used to approximate the duration and extent of thermal perturbations felt throughout the system and delimit the time intervals during which zircon crystals could have grown at the measured crystallization temperatures the results show that prolonged cooling histories 1 myr that would be experienced near the centre of a large slowly cooled plutonic magma chamber would erase signs of plagioclase zoning at 4 kbar and moderate f h 2 o however plagioclase crystals with zoning profiles spanning at least 10 mole an are common in the upper critical zone in contrast emplacement of the major units of the rls at widely separated intervals as recorded by the existing high precision u pb dataset would impose transient thermal effects on the system against a backdrop of generally much lower ambient temperatures throughout its evolution in this scenario apparent ages of zircon crystals would record the time of cooling through temperatures near the water saturated biotite granite solidus corresponding to the final intercumulus mineral assemblage in both mafic and ultramafic macrolayers subsequent reheating of these rocks would be unable to reset primary zircon crystallization dates but might in some examples result in some amount of remelting or regrowth of zircon comparison of the evolving temperature fields of the two types of emplacement and cooling histories shows that the existing geochronological data and preservation of plagioclase zoning are consistent with out of sequence emplacement and rapid cooling of individual ultramafic sills into previously solidified lithologies of the rls but not with the conventional view of a single monolithic magma body

grain growth kinetics of ringwoodite and its implication for rheology of the subducting slab abstract grain growth rates of mg2sio4 and mg0 9fe0 1 2sio4 ringwoodites have been studied under the conditions of the mantle transition zone at a pressure of 21 gpa and temperatures ranging from 1473 to 2023 k using a kawai type high pressure apparatus the grain growth rates were expressed by gn g0n k0texp h rt where g m is grain size at time t s and g0 is the initial grain size at t 0 with n 4 5 0 8 and h 414 kj mol and log k0 20 4 1 6 m4 5 s for mg0 9fe0 1 2sio4 ringwoodite and h 456 kj mol and logk0 20 2 1 4 m4 5 s for mg2sio4 ringwoodite using these kinetic parameters for grain growth we estimated the grain size of ringwoodite in the cold subducting slabs to be less than 100 m this suggests that the dominant deformation mechanism in the subducting slab is diffusion creep it is also likely that some cold slabs are softer than the warmer surrounding mantle

decadal time gaps between mafic intrusion and silicic eruption obtained from chemical zoning patterns in olivine abstract intrusion of mafic magma in a silicic crustal reservoir has been reported in numerous studies of subduction related volcanoes and is considered a main process for triggering explosive eruptions and for transferring geochemical signatures from the mantle to the crust here we determine the timescales between mafic silicic encounters and eruption by exploiting the fact that during such interactions magmas exchange crystals and their mineral compositions will strive to equilibrate to the new environmental conditions specifically we model the fe mg zoning patterns of olivine found in the dacites andesite and quenched mafic inclusions of the holocene zoned eruption of volcan san pedro 36 s chilean andes the san pedro eruption is an example where mixing between silicic and mafic magmas produced intermediate compositions e g andesite at depth in the magma reservoir for modeling the compositional profiles we took into account the anisotropy of fe mg diffusion in olivine and also the effects of diffusion in multiple dimensions aspects that have been largely neglected in previous works the retrieved timescales for olivine melt equilibration by diffusion display an array ranging from 1 year for the most silica rich dacite to 50 years for the andesite this array of timescales rather than a single value provides new insight into the details of mafic silicic mixing at depth they reveal processes that occur on a longer timescale than the days to weeks time gap for cases where the mafic intrusion triggered the eruption and they might be associated with the instances where seismic activity or significant inflation of the volcanic edifice was not immediately followed by eruption on the other hand the year to decade timescales that we report for producing magmas of intermediate composition e g andesites are much shorter than the 10 3 10 5 years inferred from u series disequilibria data for igneous differentiation by closed system fractionation to produce similar magma compositions at subduction zones

solute partitioning under continuous cooling conditions as a cooling rate indicator a mathematical model is developed to describe the concentration profiles as a function of time of a solute dissolved in two coexisting finite phases under continuous cooling conditions a temperature dependent partitioning of the solute between the two phases is included the model relates to the use of experimentally determined solute profiles to infer the rates at which various rocks cooled the model employs finite difference equations and the thomas tridiagonal method to solve the appropriate differential equations describing the solute partitioning to illustrate the application of the model the partitioning of zirconia between coexisting ilmenite and ulvospinel is considered it is shown that cooling rates can be calculated not only from solute concentration profiles within the grains but also from the ratio of the solute concentrations at a given distance on both sides of the interface or from the ratio of the average solute concentrations in the two phases the cooling rates of a suite of apollo 15 elbow crater gabbros are calculated from measured ratios of concentrations at 10 m from the interfaces between the phases a comparison is made of calculated concentration ratios 10 m from the interface and cooling rates for equilibrium versus uniform initial concentrations it is shown that the elbow crater gabbros could not have had a uniform concentration profiles at the previously suggested initial temperature of 1350 k also considered are the effects of the initial temperature the grain sizes and the diffusion coefficients of zr in the phases on the calculated cooling rates finally suggestions are made for other systems where the present model can be applied to estimate the thermal histories of rocks

a geophysical perspective on the bulk composition of mars we invert the martian tidal response and mean mass and moment of inertia for chemical composition thermal state and interior structure the inversion combines phase equilibrium computations with a laboratory based viscoelastic dissipation model the rheological model which is based on measurements of anhydrous and melt free olivine is both temperature and grain size sensitive and imposes strong constraints on interior structure the bottom of the lithosphere defined as the location where the conductive geotherm meets the mantle adiabat occurs deep within the upper mantle 250 500 km depth resulting in apparent upper mantle low velocity zones assuming an fe fes core our results indicate 1 a mantle with a mg molar mg mg fe of 0 75 in agreement with earlier geochemical estimates based on analysis of martian meteorites 2 absence of bridgmanite and ferropericlase dominated basal layer 3 core compositions 13 5 16 wt s core radii 1640 1740 km and core mantle boundary temperatures 1560 1660 c that together with the eutectic like core compositions suggest the core is liquid and 4 bulk martian compositions that are overall chondritic with a fe si wt ratio of 1 63 1 68 we show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters specifically we find that the inversion results are reproduced by stagnant lid convection models for a range of initial viscosities 1019 1020 pa s and radioactive element partitioning between crust and mantle around 0 001 the geodynamic models predict a mean surface heat flow between 15 25 mw m2

grain growth in the forsterite diopside system abstract grain growth experiments in dunite clinopyroxenite and wehrlites with various forsterite diopside ratios were performed to investigate the effect of modal composition on grain growth kinetics in the earth s uppermost mantle the experiments were conducted using a piston cylinder apparatus at 1200 c and 1 2 gpa for 2 763 h under dry conditions normal grain growth ngg occurred in dunite clinopyroxenite and relatively forsterite poor wehrlites 70 vol of forsterite grain growth rates of forsterite and diopside in relatively forsterite poor wehrlites were much slower than those in dunite and clinopyroxenite in the forsterite rich wehrlites 80 vol of forsterite ngg of diopside and abnormal grain growth agg of forsterite occurred the growth rate of diopside was significantly slower than that in clinopyroxenite while the growth rate of forsterite by agg was found to be similar to that by ngg in dunite the presence of 20 vol diopside had the effect of inhibiting the forsterite grain growth during the initial stage resulting in agg of forsterite thus overtaking the growth rate in dunite our experimental results suggest that there would be a significant difference in grain growth rate and consequently in effective viscosity between olivine rich peridotites depleted mantle and relatively olivine poor peridotites fertile mantle in the case of grain size sensitive creep variation of mean grain sizes in the upper mantle would result not only from differences in temperature and phase assemblage but also from the variation of modal compositions

seismological evidence for compositional variations at the base of the mantle transition zone under japan islands abstract temperature and water content anomalies just above the 660 km discontinuity under the japan islands are estimated from seismological observations two sets of seismological observations of p wave velocity perturbations and depth variations of the 660 km discontinuity are used which are 1 long wavelength 500 km variations from seismic tomography based on a grid parameterization and waveform analysis of scs reverberations and 2 moderate wavelength 150 km variations from seismic tomography with a block parameterization and receiver function analysis to estimate temperature and water content anomalies partial derivatives of velocity and depth variations with respect to temperature and water content determined by mineral physics studies are used under southwest japan low temperature and high water content anomalies are obtained from both sets of seismological observations which have already been found by a previous study under northeast japan however there are discrepancies between the results estimated from the two data sets possibly due to the different resolution scales this discrepancy is dismissed when examining distributions of the temperature and water content anomalies these anomalies can be grouped into two geographical locations one group under southwest japan the other under northeast japan the two groups are clearly separated and exhibit common features in both sets of the seismological observations the grouping is interpreted by a compositional difference between the subducting pacific slab under southwest japan and a normal mantle under northeast japan

olivine spinel geospeedometry analysis of diffusion controlled mg fe2 exchange subsolidus mg fe2 exchange between olivine and spinel is governed by mg fe2 interdiffusion incomplete exchange results in mg fe2 heterogeneity in both olivine and spinel which provides information on the thermal histories of the host rocks a composite sphere model has been developed to obtain quantitative cooling rates or heating duration from the mg fe2 heterogeneity the model assumes that a spherical core of spinel and a surrounding semi infinite spherical shell of olivine interact by diffusion controlled exchange of mg and fe2 the differential equations describing the model are solved numerically by finite difference approximations the numerical solution reveals that cooling rates or heating duration can be estimated from the relationship between the grain size of spinel and temperature calculated from the chemical compositions of the core of a spinel grain and of olivine far away from it the calculated temperature is employed in place of mg mg fe2 at the center of spinel to obtain the absolute temperature of thermal events this olivine spinel geospeedometer has been applied to peridotites gabbro and picrites from some ophiolite complexes in japan to estimate their cooling rates the estimated cooling rates for the peridotites range from 10 4 to 10 1 c yr and those for the picrites from 103 to 104 c yr the geospeedometer has been extended to estimate the heating duration of lherzolite xenoliths in basalt from the ichinomegata crater northeast japan the estimated heating duration of the xenoliths is less than one day

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

sensitivity of the short to intermediate wavelength geoid to rheologic structure in subduction zones 1 the rheologic structure of subduction zones determines how the negative buoyancy of sinking lithosphere slabs couples to the surface to cause surface deformation and plate motion however the rheologic structures determined using different methods are inconsistent analysis of the geoid and some dynamical models conclude that slabs must be weak whereas other dynamical models and experimentally determined rheology predict that slabs are strong using idealized three dimensional models of a subduction zone we show that lateral variations in viscosity lvvs within a slab caused by yielding render the geoid insensitive to the viscosity structure outside of the bending region of the slab spectral analysis of the geoid up to harmonic degree 360 features at wavelengths down to 110 km shows that lvvs cause significant mode coupling with changes to the long wavelength geoid 1000 km caused by short wavelength 500 km changes to dynamic topography models with either a uniformly weak slab or a strong slab with yielding provide a good match to the observed geoid spectrum and characteristic features of the geoid over subduction zones therefore observations other than the geoid are needed to constrain the strength profile of subducting slabs

the kinetics of the reaction of majorite plus ferropericlase to ringwoodite implications for mantle upwellings crossing the 660 km discontinuity we have measured the kinetics of reaction between mgo and majoritic garnet at 20 gpa and 1773 2123 k as a proxy for the reaction between perovskite and ferropericlase during mantle upwelling across the 660 km seismic discontinuity ringwoodite forms a layer between mgo and garnet and in the case of aluminous garnets the interface between ringwoodite and garnet develops a fingering instability resulting in a complex intergrowth at this interface by contrast the mgo ringwoodite interface is always planar for an initial planar mgo garnet interface two thicknesses are therefore defined 1 a layer thickness x1x1 which is the maximum thickness of ringwoodite which forms a plane parallel bounded layer next to the mgo and 2 an interface thickness x2x2 which is the maximum extent of the intergrowth region away from the ringwoodite layer the growth of both of these regions can be described by apparent rate constants kiki which are arrhenius with view the mathml sourceln k10 6 36 0 25 m2 s and e1 456 40 kj mole1 456 40 kj mol for the ringwoodite layer and view the mathml sourceln k20 9 2 3 3 m2 s and e2 371 53 kj mole2 371 53 kj mol for the intergrowth region the fingering instability is caused by the incompatibility of aluminium in ringwoodite and its low chemical diffusivity in garnet which results in an increase of surface area at the ringwoodite garnet interface to minimise the aluminium concentration at the interface the intergrowth region contains a fine grained mixture of ringwoodite and garnet which coarsens very slowly with time this might result in a transient weakening of upwelling regions of mantle just above the 660 km seismic discontinuity allowing some viscous decoupling between the upper and lower mantle

slow cooling of the lowermost oceanic crust at the fast spreading east pacific rise the cooling history of the lowermost ocean crust formed at the east pacific rise and exposed at the hess deep rift is constrained using fresh primitive olivine gabbro and troctolite cores recently recovered by integrated ocean drilling program expedition 345 the mg in plagioclase and ca in olivine geospeedometers constrain the initial subsolidus cooling rates down to closure temperatures in the range of 900 750 c these independent geospeedometers yield similar slow cooling rates 0 005 0 0001 c yr 1 with a mean value of 0 0011 c yr 1 making these results robust despite the uncertainties inherent in the methods the slow cooling rates derived using natural samples provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling the cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex this implies that lower crust formed at fast spreading rates remains hot in the near axis environment indicating limited hydrothermal fluid ingress and reaction

geospeedometry of metamorphic rocks examples in the rantasalmi sulkava and kiuruvesi areas eastern finland biotite garnet diffusion couples compositional zoning in biotite garnet pairs in metamorphic rocks from eastern finland has been studied the mg profiles in the garnet side of biotite garnet crystal pairs have been interpreted by means of lasaga s theory geospeedometry however the binary interdiffusion equations are first reformulated by starting from a ternary system and using the lattice fixed frame of reference this frame of reference gives the fluxes directly by means of the numbers of diffusing ions which helps to check the 1 dimensionality of the analysis assumed in lasaga s theory it is also shown that the recently argued effect of the third cation ca is negligible in our samples we were able to investigate satisfactory profiles in three samples from different areas the values for the cooling rate are a few degrees per million years if the diffusion data obtained by freer are adopted the cooling rates are in agreement with recent estimates based on the k ar ages on biotite in the same areas

hydrothermal anorthitization of plagioclase within the magmatic hydrothermal transition at mid ocean ridges examples from deep sheeted dikes hole 504b costa rica rift and a sheeted dike root zone oman ophiolite abstract the unique petrological record from the deep sheeted dikes of odp hole 504b gives insight into the magmatic hydrothermal transition and the hydrothermal reaction zone at mid ocean ridges a high temperature peak hydrothermal event at hole 504b defined as stage 1 corresponds to an early stage in the evolution of an axial reaction zone and perhaps to an initial high temperature phase of black smoker activity the temperatures and or the a ca a na 2 ratios of hydrothermal fluids overlapped and even exceeded those inferred for the reaction zones of many modern black smoker systems the volume of hydrothermally altered rock at this stage of evolution of the reaction zone was much less than 10 stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches there is no epidote associated with stage 1 wallrock reactions involved the replacement of magmatic clinopyroxene by magnesiohornblende and of magmatic plagioclase by hydrothermal plagioclase these secondary minerals were not in equilibrium hydrothermal plagioclase varies in composition from an 54 to an 95 and has low mgo and feo tot it has distinctive bright cathodoluminescence and a porous appearance stage 1 at hole 504b occurred at pressures between 38 and 85 mpa 0 38 and 0 85 kbar and temperatures in excess of about 425 c these p t conditions closely approach the two phase field for seawater stage 1 is overprinted by retrograde alteration that caused further replacement within halos and patches as well as 10 40 alteration mostly to chlorite and albite of all the fresh sheeted dikes that were unaffected during stage 1 stage 1 alteration at hole 504b is unique among oceanic and ophiolitic samples however we report one area from the oman ophiolite that exhibits features similar to stage 1 at hole 504b early halos around hornblende veins contain secondary hornblende and porous hydrothermal calcic plagioclase an 55 an 78 with low mgo and feo tot fluid inclusions in quartz show that some early hydrothermal fluids had 5 6 2 3 wt nacl equivalent at temperatures constrained to be above 320 and below 510 c as well as a saline brine with 31 to 40 wt equivalent nacl and similar temperature constraints although evidence for stage 1 alteration of sheeted dikes is far from ubiquitous it is now identified in an oceanic setting hole 504b and in an ophiolitic example from oman

aging of marine organic matter during cross shelf lateral transport in the benguela upwelling system revealed by compound specific radiocarbon dating organic matter accumulation and burial on the namibian shelf and upper slope are spatially heterogeneous and strongly controlled by lateral transport in subsurface nepheloid layers much of the material deposited in depo centers on the slope ultimately derives from the shelf supply of organic matter from the shelf involves selective transport of organic matter we studied these selective transport processes by analyzing the radiocarbon content of co occurring sediment fractions here we present radiocarbon data for total organic carbon as well as three tracers of surface ocean productivity phytoplankton derived alkenones membrane lipids of pelagic crenarchaeota crenarchaeol and calcareous microfossils of planktic foraminifera in core top and near surface sediment samples the samples were collected on the namibian margin along a shelf slope transect 85 to 1040 m at 24 s and from the upper slope depo center at 25 5 s in core top sediments alkenone ages gradually increased from modern to 3490 radiocarbon years with distance from shore and with water depth crenarchaeol while younger than alkenones also increased in age with distance offshore it was concluded that the observed ages were a consequence of cross shelf transport and associated aging of organic matter radiocarbon ages of preserved lipid biomarkers in sediments thus at least partially depend on the relative amount of laterally supplied pre aged material present in a sample highlighting the importance of nepheloid transport for the sedimentation of organic matter over the namibian margin

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

time scales of magmatic processes from modeling the zoning patterns of crystals the advent of polarized light microscopy in the middle of the 19th century allowed mineralogists and petrologists interested in igneous rocks to recognize the widespread occurrence of fine scale heterogeneities in the optical properties of minerals e g see young 2003 for details the interpretation of mineral zoning patterns as archives of magmatic processes has been with us for some time e g larsen et al 1938 tomkeieff 1939 the development of the electron microprobe in the 1960 s allowed mineral zoning profiles to be quantitatively analyzed and modeled e g bottinga et al 1966 moore and evans 1967 enhanced textural observations permitted recognition of many kinds of detailed structures during the growth and dissolution of magmatic minerals fig 1 e g anderson 1983 pearce and kolisnik 1990 and these processes were also explored using experimental and numerical models e g albarede and bottinga 1972 lofgren 1972 kirkpatrick et al 1976 loomis 1982 major element zoning patterns are routinely measured and with the arrival of the ion microprobe the identification of heterogeneous distribution of trace elements opened the window for more realistic and sophisticated scenarios and models fig 1 e g kohn et al 1989 blundy and shimizu 1991 singer et al 1995 we now measure abundances of naturally occurring isotopes at the scale of tens of micrometers e g davidson et al 2007a ramos and tepley 2008 and this allows in situ dating of crystals e g cooper and reid 2008 figure 1 a nomarski dic image of a plagioclase crystal from a gabbroic xenolith of volcan san pedro costa et al 2002 the crystal shows a well defined core and a rim separated by an oscillatory zoned interval ion microprobe analysis pits and the trace of the electron microprobe traverse between the pits are evident b major

creep mechanism in mg2geo4 effects of a phase transition we deformed polycrystalline mg2geo4 olivine and spinel in a griggs type solid medium apparatus flow of mg2geo4 olivine can be represented by 6 5 l07 3 5 exp 105 kcal mol rt where kbar is the differential stress and s 1 is the natural strain rate for mg2geo4 spinel the flow law is 2 6 104 2exp 73 kcal mol rt the low stress exponent and activation enthalpy coupled with fine grain size 3 m suggest that mg2geo4 spinel deformed by a superplastic mechanism flow parameters for the olivine phase suggest a dislocation creep mechanism comparison of theoretical superplastic flow laws for mg2geo4 olivine with the spinel phase data suggests that strain rates in mg2geo4 spinel are only about a factor of 3 lower than for mg2geo4 olivine of the same grain size a similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion transformation from mg2geo4 olivine to spinel reduced grain size to approximately 3 m thus we might expect a similar reduction in grain size in the earth s transition zone which could result in superplastic deformation of the transformed phase and cause a weak decoupling zone at the transition boundary in the mantle superplasticity brought about by transformation induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle

water partitioning between nominally anhydrous minerals in the mgo sio2 h2o system up to 24 gpa implications for the distribution of water in the earth s mantle multi anvil experiments have been conducted in the mgo sio2 h2o system at pressures of 15 24 gpa and temperatures of 1200 1600 c to investigate the partitioning of water between mantle phases the water contents were obtained from fourier transform infrared spectroscopy on doubly polished and clear crystals most of the mineral phases coexisted with a hydrous melt the results show that among all magnesium silicate phases 1 mgsio3 perovskite is the only phase which does not dissolve a detectable amount of water and 2 the high pressure polymorphs of mg2sio4 wadsleyite and ringwoodite dissolve about one order of magnitude more water than the polymorphs of mgsio3 clinoenstatite majorite and akimotoite the following partition coefficients of water were measured dwadsleyite clinoenstatite 3 8 at 15 gpa and 1300 c dringwoodite akimotoite 21 at 19 gpa and 1300 c dakimotoite perovskite 1 at 24 gpa and 1600 c and dpericlase perovskite 1 at 24 gpa and 1500 c these results suggest that most of the water in the earth s mantle is partitioned in the transition zone

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

p and sh velocity structure in the upper mantle beneath northeast china evidence for a stagnant slab in hydrous mantle transition zone abstract using high dense regional body waves for three deep earthquakes that occurred around russia china border we investigate both s and p wave velocity structures in the mantle transition zone beneath northeast china and northern part of north china craton where the northwestern pacific plate is imaged to subhorizontally lie above the 660 km discontinuity we observe an increasing trend of s p travel time residuals along the epicentral distance within a distance range of 11 16 5 indicating a velocity anomaly in mtz we seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector both sh and p data suggest a 140 20 km high velocity layer lying above a slightly depressed and broad 660 km discontinuity shear velocity reduction of 2 5 in the deeper part of the transition zone is required to compensate for the significantly large relative time between ab and cd triplicate branches and the increased trending of s p travel time residuals as well the mtz as a whole is featured by low shear velocity and high v p v s ratio a water rich mantle transition zone with 0 2 0 4 wt of h 2 o may account for the discrepancy between the observed v p and v s velocity structures our result supports the scenario of a viscosity dominated stagnant slab with an increased thickness of 140 km which was caused by the large viscosity contrast between the lower and upper mantles the addition of water and eastward trench retreat might facilitate stagnation of the subducting pacific slab beneath northeast china

mg diffusion in anorthite implications for the formation of early solar system planetesimals we have measured the self diffusion coefficients of mg ca and sr in anorthitic plagioclase in order to assess the potential of mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism diffusion couples were constructed from polished single crystals of natural anorthite and synthetic isotopically enriched anorthite glass couples were annealed at atmospheric pressure and 1200 1400 c and isotopic concentration profiles were measured with an ion microprobe the results show that mg diffusion in anorthite is surprisingly fast with d mg being over 2 orders of magnitude greater than d sr this indicates that the diffusion coefficient of mg in anorthite cannot be approximated with that for sr mg diffusion in the c direction is also slightly faster than in the b direction while ca and sr diffusion appear to be isotropic the results provide important constraints on the thermochronological history of anorthite bearing mineral assemblages that preserve radiogenic 26 mg excesses in a planetesimal heated by the decay of 26 al the temperature at any point depends on the planetesimal size time of formation thermal conductivity and depth within the planetesimal given sufficient heating 26 mg heterogeneities produced by the in situ decay of 26 al in ca al rich inclusions cais and chondrules will be erased by diffusive equilibration using the self diffusion coefficient for mg in anorthite measured in this study we show that the common occurrence of 26 mg excesses in these inclusions requires that they must be stored in small 15 km bodies or the outermost rims of larger bodies for the first 1 2 million years of the solar system s history for early formed bodies larger than 15 km most of the mass will have been heated sufficiently for any radiogenic 26 mg to have been diffusively homogenized in the mg rich planetary environment

topography of the 660 km discontinuity beneath northeast china implications for a retrograde motion of the subducting pacific slab 1 clear s to p converted waves at the 660 km discontinuity s660p are observed at stacked seismograms of deep earthquakes occurring in northeast china and japan sea recorded by broadband seismic arrays in north america and europe differential travel times between the s660p and p waves are used to constrain depth variations of the 660 beneath northeast china a rapid change in the depth of the 660 is observed within a narrow longitudinal range of 130 8 e 131 4 e where the lower boundary of the subducting slab encounters the 660 towards the west the 660 deepens steadily as it approaches the coldest core of the slab the maximum depression occurs at the west end of the studied region with an amplitude of 20 km to the east the 660 appears to be flat showing no obvious effect of the subducting slab these observations are consistent with a scenario for a retrograde moving pacific slab lying over the upper and lower mantle boundary progressively from west to east

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods