contrasting provenance and timing of metamorphism from paragneisses of the aracuai ribeira orogenic system brazil hints for western gondwana assembly abstract the aracuai orogen and the ribeira belt make up a complex neoproterozoic cambrian orogenic system the aracuai ribeira orogenic system aros located from the eastern to southeastern brazil along the aros the ediacaran rio doce magmatic arc represents a geotectonic connection between the aracuai and the ribeira orogenic domains although the nature and evolution of the rio doce plutonic rocks is regionally well established it lacks detailed studies on the paragneisses found along the western and central regions of this magmatic arc besides information on the nature and provenance of their sedimentary protoliths the paragneisses provide data to unravel the palaeogeographic scenario from the precursor to arc related basins six samples of al rich gneisses covering a large aros region were selected for electron microprobe emp mineral analyses in order to obtain geothermobarometric data and monazite ages as well as for laser ablation inductively coupled plasma mass spectrometry la icp ms isotopic analyses on zircon u pb lu hf and monazite u pb the different age spectra from detrital zircon grains and contrasting hf isotopic signatures suggest a complex sedimentary history located in the western sector of the study region the samples rc 02 and rc 34 with an 80 age peak of detrital zircon grains from 2158 ma to 1830 ma e hf t from 2 2 to 22 7 and hf t dm model ages from 3530 ma to 2440 ma suggest sediment sources located in the sao francisco craton basement the samples rc 03 also from the western sector and rc 46 from the southern sector have a more complex assemblage of detrital zircon grains with an 87 age peak from 987 ma to 592 ma e hf t from 14 9 to 2 9 and hf t dm model ages from 2220 ma to 720 ma indicating provenance from mainly juvenile sources of distinct ages candidates to be juvenile sources for rc 03 and rc 46 sedimentary protoliths are the rhyacian juiz de fora and pocrane complexes in the basement of the rio doce arc the neoproterozoic rio negro arc system of the ribeira belt and aros ophiolite complexes samples rc 30 and rc 38 from the eastern sector of the study region with most detrital zircon ages between 650 ma and 552 ma and very negative e hf t 25 3 to 16 5 suggest main sediment sources in the rio doce arc by extending u pb analyses on metamorphic zircon and monazite we have identified a complex timing of metamorphism represented by metamorphic ages ranging from 621 ma to 480 ma with the main collisional activity between 580 ma and 540 ma geothermobarometric studies on garnet porphyroblasts syn kinematic to the d 2 regional foliation show a retrograde metamorphic path typical of continental collision belts starting with p t conditions of t max 733 c and p max 6 43 kbar our data also suggest i the studied paragneisses represent distinct neoproterozoic basin stages shifting from passive to active margin settings ii if the rio negro arc system really provided sediments for the basin stage represented by the rc 03 and rc 46 paragneisses it would have amalgamated with the aros before 614 ma iii the final amalgamation of western gondwana took place around 540 ma in the focused region iv an important re heating period 520 480 ma can be related to the aros gravitational collapse after western gondwana assembly

experimental study of subsolidus phase relations and mixing properties of pyroxene in the system cao al2o3 sio2 abstract subsolidus phase relations in the system cao al2o3 sio2 cas were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene anorthite where pyroxene is a binary solid solution of ca tschermak cats caal2sio6 and ca eskola caes ca0 5alsi2o6 endmembers reversals were obtained on the following reactions bar c 3an gr 2ky q p 22t 700 3an cor gr 3ky p 21 8t 950 3cats gr 2cor p 55t 53900 and 6cats 1 x caesx 2 1 2x gr 4 1 2x cor 9xan observed slopes indicate 9 8 j mol k of al si disorder in ca tschermak pyroxene and 5 3 j mol k of al si disorder in anorthite at 1300 c it is suggested that al si disorder in anorthite increases by 1 9 j mol k from 700 c to 1300 c compositions of cats caes pyroxene in equilibrium with anorthite and pbo rich liquid were experimentally determined at 1400 1430 c and 22 7 30 8 kbar microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3an 2cats 2caes j bar k 2rtln xcats xcaes 60200 86 4t 5 06 13 10 7p p 0 resulting in h0j 39 8 kj mol and s0 461 8 j mol k for the ca eskola endmember at 1300 c the obtained properties of the ca eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase quartz or kyanite

the mount hay block central australia another puzzle piece for paleo mesoproterozoic tectonic history abstract the 100 km 2 mount hay block arunta region central australia is a 15 km thick crustal cross section comprised of granulite facies tectonites containing a tectonothermal record from ca 1803 to ca 1552 ma as demonstrated by the combination of prior work and new monazite geochronology and thermobarometry igneous and sedimentary protoliths formed ca 1803 1798 ma during 1810 1790 ma stafford tanami time between ca 1790 and 1740 ma nearly continuous magmatism in the aileron province and possibly ages of recrystallized zircon and monazite in the mount hay block record subduction along the southern margin of the proterozoic australia mawson continent geologic mapping and zircon and monazite geochronology demonstrate that the mount hay block was penetratively deformed between ca 1720 ma and 1700 ma during 1735 1690 ma strangways time this includes the 8 km thick mount hay sheath fold which records ne sw subhorizontal shear once paleozoic tilting of the mount hay block is removed while still in the deeper crust 798 33 c 7 6 0 7 kbar fabrics along the northern edge of the mount hay block were transposed by the cross cutting 7 km thick capricorn ridge shear zone at ca 1551 7 5 5 ma during chewings time once restored the capricorn ridge shear zone records ne sw extension a jump in strain rate while at high temperatures 725 c initiated severe localization accommodated initially by pseudotachylite formation and cataclasis then mylonitization during fe rich fluid flux shearing may have continued to pressure temperature conditions as low as

effects of a perovskite post perovskite phase change near core mantle boundary in compressible mantle convection 1 numerical simulations of compressible isochemical mantle convection are used to investigate the effect of the perovskite to post perovskite phase transition at around 2700 km depth which has recently been discovered by high pressure experiments and ab initio calculations on the convective planform temperature and heat transport characteristics of the mantle the usual phase transitions at 410 km olivine spinel and 660 km spinel perovskite are also included the exothermic post perovskite phase change at 2700 km depth destabilizes the lower thermal boundary layer increasing the heat flow increasing interior mantle temperature and increasing the number and time dependence of upwelling plumes the resulting weak highly time dependent upwellings also have a smaller horizontal spacing than the plumes that occur in the absence of the phase transition while the influence of post perovskite phase change may be smaller than that of some other complexities such as compositional stratification it appears to have an important enough effect that it should not be ignored in dynamical studies of mantle convection

zircon hf o isotopes as melt indicator melt infiltration and abundant new zircon growth within melt rich layers of granulite facies lenses versus solid state recrystallization in hosting amphibolite facies gneisses central erzgebirge bohemian massif abstract in the central erzgebirge within the bohemian massif lenses of high pressure and ultrahigh pressure felsic granulites occur within meta sedimentary and meta igneous amphibolite facies felsic rocks in the felsic granulite melt rich parts and restite form alternating layers and were identified by petrology and bulk rock geochemistry mineral assemblages representing the peak p t conditions were best preserved in melanocratic restite layers in contrast in the melt rich leucocratic layers garnet and related hp minerals as kyanite are almost completely resorbed both layers display differences in accessory minerals melanosomes have frequent and large monazite and fe ti minerals but lack xenotime and apatite leucosomes have abundant apatite and xenotime while monazite is rare here we present a detailed petrographic study of zircon grains abundance size morphology inclusions in granulite facies and amphibolite facies felsic gneisses along with their oxygen and hafnium isotope compositions our data complement earlier u pb ages and trace element data ree y hf u on zircons from the same rocks tichomirowa et al 2005 our results show that the degree of melting determines the behaviour of zircon in different layers of the granulites and associated amphibolite facies rocks in restite layers of the granulite lenses small inherited and resorbed zircon grains are preserved and new zircon formation is very limited in contrast new zircons abundantly grew in the melt rich leucocratic layers in these layers the new zircons u pb age trace elements hf o isotopes best preserve the information on peak metamorphic conditions due to intense corrosion of other metamorphic minerals the new zircons often contain inherited cores compared to cores the new zircons and rims show similar or slightly lower hf isotope values slightly higher hf model ages and decreased oxygen isotope ratios the isotope compositions hf o of new zircons indicate partial hf isotope homogenization in the melt and melt infiltration from an external source new zircon was most likely formed by a peritectic reaction with melt above the wet solidus peritectic zircon conversely the amphibolite facies host gneisses lack indications of significant melt production pre metamorphic zircons experienced mainly solid state recrystallization and variable pb loss with only minor new zircon formation however subtle changes in cathodoluminescence pattern in the hf and o isotopes and in the lu hf yb hf ratios of zircons suggest that small volumes of melt were locally present in difference to granulites melt was internally produced the detection of low degree melts inferred from zircon geochemistry is extremely important for the rheology because these amphibolite facies rocks could act as large scale ductile shear zones the new zircon data support a different p t path for closely spaced amphibolite and granulite facies rocks

intersector element partitioning in tourmaline a potentially powerful single crystal thermometer hourglass sector zoning and related polar overgrowths are common features of metamorphic tourmaline developing as a result of variations in element preference on the different growth surfaces for sector zoned crystals three domains are present for each growth zone c c and a with compositional differences most distinct for ca and ti and among c and c sectors intersector differences vary commonly showing decreasing fractionation from core to rim attributed to increasing metamorphic grade here we show that intersector element partitioning is temperature dependent and derive empirical geothermometers based on c c and c a partitioning of ca and ti these thermometers are applicable over a range of temperatures and bulk rock compositions intersector partitioning is not affected by re equilibration and records and preserves complete t histories of individual tourmaline grains from prograde to peak and on to retrograde growth information on element mobility is preserved by tourmaline composition because intersector partitioning is independent of element concentration these factors make intersector partitioning an ideal tool to elucidate the thermal history of tourmaline grains and thus their host environment and tourmaline s refractory nature preserves these signatures even into the sedimentary record

metamorphic field gradients in the central alps metamorphic field gradients were determined across the entire amphibolite grade central alps c 50 100 km p t were calculated from 116 samples acquired from our own field work from samples provided to us by others and from rocks with mineral compositions described in the literature only fluid conserved equilibria were used to determine p t the use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors the results are presented in contour maps temperature increases from 500 to 550 c along the limit of amphibolite grade metamorphism in the north and west to c 675 c toward the south at the insubric line near the town of bellinzona maximum recorded pressures of c 7 kbar are in a central region c 20 km north of the insubric line and decrease both to the north 5 5 kbar and south 4 5 kbar the p t results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep 40 c of the kyanite sillimanite phase boundary comparison of p t conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source examination of the two dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aected the central alps during the tertiary in the second heat aected only the southern parts of the area and overprinted the previously established p t gradients

effect of post perovskite rheology on the thermal evolution of the earth abstract secular cooling of the earth mantle is a complex process affected by many factors here we present the results of a modelling study focused on efficiency of cooling in the presence of rheologically distinct post perovskite we evaluate combined effects of variable thermal expansivity and diffusivity initial thermal condition and heat source model and concentrate on the effects of rheologically weak post perovskite cooling of the core is included in the model core is assumed to be an isothermal heat reservoir with temperature controlled by heat flux through core mantle boundary our 2d axisymmetric convection model has pressure temperature and phase dependent viscosity and includes the effects of an endothermic phase transition at 660 km depth and an exothermic perovskite post perovskite phase transition in the lowermost mantle in agreement with previous studies we conclude that depth dependent material parameters tend to delay secular cooling presence of the weak post perovskite on the other hand significantly enhances core cooling and its effect on core temperature is opposite and comparable in magnitude

is the long wavelength geoid sensitive to the presence of postperovskite above the core mantle boundary 1 the analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle d 00 this work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in d 00 w e assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere our results show that the long wavelength component of the geoid is very sensitive to the presence of postperovskite areas in d 00 especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density

synthesis of staurolite in melting experiments of a natural metapelite consequences for the phase relations in low temperature pelitic migmatites we document experiments on a natural metapelite in the range 650 775 c 6 14 kbar 10 wt of added water and 700 850 c 4 10 kbar no added water staurolite systematically formed in the fluid present melting experiments above 675 c but formed only sporadically in the fluid absent melting experiments the analysis of textures phase assemblages and variation of phase composition and fe mg partitioning with p and t suggests that supersolidus staurolite formed at near equilibrium during fluid present melting reactions the experimental results are used to work out the phase relations in the system k2o na2o feo mgo al2o3 sio2 h2o appropriate for initial melting of metapelites at the upper amphibolite facies the p t grid developed predicts the existence of a stable p t field for supersolidus staurolite that should be encountered by aluminous fe rich metapelites during fluidpresent melting at relatively low temperature and intermediate pressures 675 700 c 6 10 kbar for xh2o 1 in the knfmash system but not during fluid absent melting the implications of these findings for the scarcity of staurolite in migmatites are discussed

the applicability of the grips geobarometry in metapelitic assemblages although the garnet rutile ilmenite plagioclase silica quartz grips geobarometer has been experimentally calibrated and widely applied its applicability to metapelitic rocks has not yet been discussed carefully in this paper this barometer was recalibrated by fitting the available reversed phase equilibrium data incorporating different combinations of activity models of garnet plagioclase and ilmenite the resultant grips barometer formalisms reproduce the experimental pressures well within 0 2 kbar the grips and garnet aluminium silicate plagioclase quartz gasp barometer values are concordant within 1 kbar for pressures above 6 kbar for natural metapelites but the difference of pressure determinations between these two barometers becomes larger when pressure and or the grossular content of garnet decrease however the pressure difference is independent of either temperature or almandine in garnet or anorthite in plagioclase or iron content in ilmenite after testing and application of the grips barometer to aluminosilicate bearing metapelites and metapelitic assemblages within limited geographical areas as well as within contact thermal aureoles it is concluded that this barometer may be applied to low to high grade medium to high pressure metapelites the application of the grips barometer to metapelites is not advocated in situations where calcium is deficient in garnet 0 05 or plagioclase 0 17 or for pressures below 6 kbar

origin of seismic anisotropy in the d layer inferred from shear deformation experiments on post perovskite phase abstract seismic anisotropy is one of the significant features in the d layer of the earth and is thought to be derived from the lattice preferred orientation lpo of constituent materials or shape preferred orientation spo of heterogeneous materials such as melt and inclusions recent experimental and theoretical studies strongly suggest that the d layer consists mainly of a mgsio3 post perovskite phase together with ferro periclase to understand the anisotropy in the d layer we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline cairo3 as an analogue of mgsio3 and measured the lpo of the post perovskite phase crystallographic orientation analysis of the deformed post perovskite phase showed strong lpos with the dominant slip system being 100 010 calculation of the elastic wave velocities considering the effect of lpos of post perovskite and ferro periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized s wave is considerably faster than that of vertically polarized s wave thus the seismic anisotropy observed in the d layer can be reasonably explained by the lpo of the mixture of post perovskite and ferro periclase where the lpo may result from the horizontal shear flow generated by the mantle convection

internal heating and thermal constraints on the mantle we have carried out numerical simulations to study the average temperature and thermal structure of an internally heated mantle it is found that in an incompressible mantle the hottest parts are localized in regions of slow flow in the middle and upper mantle whereas in a compressible mantle they occur over laterally extensive regions near the core mantle boundary cmb for chondritic concentrations of heat producing elements hpe s the average thermal conductivity in the mantle must be high to avoid heating the core and the temperature of the cmb tcmb is low to avoid large scale melting the mantle may have been extensively molten in the archaean

limits on the precision of geobarometry at low grossular and anorthite content in many cases the grossular content of garnet and or the anorthite content of the plagioclase used in thermobarometry are very low leading to a large pressure uncertainty the dependence of this uncertainty on mole fractions of grossular and anorthite is evaluated by propagating uncertainties in composition and activity terms into the gasp geobarometer for a series of hypothetical rocks equilibrated at 550 8c and 6 kbar results are 6 0 65 kbar 1 standard deviation at high mole fractions xgrs 5 0 15 xan 5 0 98 increasing to 6 1 55 kbar at low mole fractions xgrs 5 0 03 xan 5 0 133 specific results vary depending on errors chosen p t conditions thermodynamic database and activity models used but an overall trend of increasing uncertainty with decreasing mole fraction is robust these theoretical conclusions are supported by a data set of 42 amphibolite facies metapelitic samples for which pressure and temperature were determined with and without grossular anorthite bearing equilibria if grossular and anorthite mole fractions are large the difference in p determination is low 0 5 kbar but if these mole fractions are 0 10 and 0 30 respectively the difference in p determination is variable and can be high 3 kbar a general guideline is that if the product of grossular and anorthite mole fraction is 0 05 then grossular anorthite bearing equilibria should be used only with great caution

the potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle abstract recent seismic tomographic models have revealed broad low velocity anomalies in the lower mantle beneath africa and the central pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics we have considered the possible impact from radiative heat transfer which can be described by a nonlinear temperature dependent coefficient in the thermal conductivity in 2d numerical simulations results for rayleigh numbers up to 106 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior this nonlinearity is responsible for producing a strong attractor in the mantle convective system which greatly simplifies its time dependent dynamics the possible link of the simplified lower mantle time dependent dynamics with polar wander is discussed the main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long term rotational dynamics

inferring the viscosity and the 3 d density structure of the mantle from geoid topography and plate velocities summary in a dynamic earth mantle mass heterogeneities induce gravity anomalies surface velocities and surface topography these lateral density heterogeneities can be estimated on the basis of seismic tomographic models recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates the relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle here we use this model combined with a generalized least squares method in order to infer the viscosity profile of the earth from the surface observations and to get some new insight into the 3 d density structure of the mantle the computed radial viscosity profile presents a continuous increase of more than two orders of magnitude the asthenosphere has a viscosity close to 2 x lo2 pa s no sharp discontinuity is requested at the upper lower mantle interface the largest viscosity 7 x 10 pas is reached in the middle of the lower mantle at greater depth approaching the core mantle boundary the viscosity decreases by one order of magnitude the model suggests that the well known degree 2 and order 2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs it also slightly increases the degree 2 and order 0 in the lower mantle and decreases it in the upper mantle in other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes

spin state of ferric iron in mgsio3 perovskite and its effect on elastic properties abstract recent studies have indicated that a significant amount of iron in mgsio 3 perovskite pv is fe 3 fe 3 fe 10 60 due to crystal chemistry effects at high pressure p and that fe 3 is more likely than fe 2 to undergo a high spin hs to low spin ls transition in pv in the mantle we have measured synchrotron mossbauer spectroscopy sms x ray emission spectroscopy xes and x ray diffraction xrd of pv with all iron in fe 3 in the laser heated diamond anvil cell to over 100 gpa fe 3 increases the anisotropy of the pv unit cell whereas fe 2 decreases it in pv synthesized above 50 gpa fe 3 enters into both the dodecahedral a and octahedral b sites approximately equally suggesting charge coupled substitution combining sms and xes we found that the ls population in the b site gradually increases with pressure up to 50 60 gpa where all fe 3 in the b site becomes ls while fe 3 in the a site remains hs to at least 136 gpa fe 3 makes pv more compressible than mg endmember below 50 gpa because of the gradual spin transition in the b site together with lattice compression the completion of the spin transition at 50 60 gpa increases bulk modulus with no associated change in density this elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with fe 3

garnet biotite geothermometry revised new margules parameters and a natural specimen data set from maine the garnet biotite geothermometer has been recalibrated using recently obtained margules parameters for iron magnesium calcium garnet mn interactions in garnet and al interactions in biotite as well as the fe oxidation state of both minerals fe mg and dwal margules parameters for biotite have been retrieved by combining experimental results on 6 al free and 6 al bearing biotite using statistical methods margules parameters per mole of biotite are bt w 5 40 719 2 30t j mole mgfe

chemical and textural equilibration of garnet during amphibolite facies metamorphism the influence of coupled dissolution reprecipitation metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low temperature amphibolite facies metamorphic reactions garnet is widely assumed to be a reactant in staurolite isograd reactions and the evidence for this has been assessed in the late proterozoic dalradian pelitic schists of the scottish highlands the 3d imaging of garnet porphyroblasts in staurolite bearing schists reveals a good crystal shape and little evidence of marginal dissolution however there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction staurolite forms directly adjacent to the garnet and its nucleation is strongly associated with deformation of the muscovite rich fabrics around the porphyroblasts cloudy fluid inclusion rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet such cloudy garnet typically has a mg rich mn poor composition and is interpreted to have formed during a coupled dissolution reprecipitation process triggered by a local influx of fluid all garnet in the muscovite bearing schists present in this area is potentially reactive irrespective of the garnet composition but very few of the schists contain staurolite the staurolite producing reaction appears to be substantially overstepped during the relatively high pressure barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history the remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning such zoning in metamorphic garnet created during coupled dissolution reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re equilibration fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists in addition the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history however the partial re equilibration of the porphyroblasts during coupled dissolution reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions

empirical garnet biotite plagioclase quartz gbpq geobarometry in medium to high grade metapelites on the basis of the net transfer reactions among garnet biotite plagioclase and quartz for both mg and fe end member models the garnet biotite plagioclase quartz gbpq geobarometer was empirically calibrated under physical conditions of p 1 0 11 4 kbar and t 515 878 c based on the input garnet biotite temperatures and garnet aluminosilicate plagioclase quartz gasp pressures of 224 natural aluminosilicate bearing metapelitic samples collated from the literature the calibrations are internally consistent with the asymmetric quaternary solid solution model of garnet the symmetric quaternary solid solution model of biotite and the al avoidance ternary solid solution model of plagioclase in calibrating the garnet biotite geothermometer and the gasp geobarometer the resulting two gbpq barometer formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent metapelites the two gbpq barometry formulae yielded identical pressures whether the sample was included or not included in calibrating the gbpq barometry the random error of the gbpq barometry may be expected as 1 2 kbar the dp dt slopes of these two gbpq formulae are close to that of the gasp barometer in p t space applications of the gbpq barometry of aluminosilicate absent metapelites to the rocks within a thermal contact aureole or rocks within a limited geographical area without post metamorphic structural discontinuity show no obvious pressure change it may be concluded that the two gbpq barometry formulae derived in this study may be used as practical tools for metamorphic pelites under the conditions of 515 878 c and 1 0 11 4 kbar in the composition range of xgros 3 in garnet xan 17 in plagioclase and x bio 3 in biotite application of the gbpq barometer beyond the calibration range should be undertaken with caution

the partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism glen roy scotland garnet porphyroblasts in sillimanite bearing pelitic schists contain complex textural and compositional zoning with considerable variation both within and between adjacent samples the sillimanite bearing schists locally occur in regional barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism garnet in these dalradian rocks from the scottish highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution reprecipitation followed by partial dissolution individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co exists with those containing largely unmodified characteristic bell shaped mn profiles this highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite producing reactions the modification of garnet via a dissolution reprecipitation process releases ca into the effective whole rock composition displacing the pressure temperature positions of subsequent isograd reactions this represents the first report of internal metasomatism controlling reaction pathways the behaviour of garnet highlights the importance of kinetic factors especially deformation and fluids in controlling reaction progress and how the resulting variability influences subsequent prograde history the lack of a consistent metamorphic response within and between adjacent schists suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions

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

simultaneous inversion for the earth s mantle viscosity and ice mass imbalance in antarctica and greenland redistribution of mass in the earth due to pleistocene deglaciation and to present day glacial melting induces secular changes in the earth s gravitational field the earth is affected today by the former mechanism because of the viscous memory of the mantle and by the latter because of ongoing surface mass redistribution and related elastic response a self consistent procedure allows us to invert simultaneously for the lower and upper mantle viscosity and for the present day mass imbalance in antarctica and greenland using the observed time variations of the long wavelength gravity field from satellite laser ranging slr analyses the procedure is based on our normal mode relaxation theory for the forward modeling and a newly developed inversion scheme based on the levenberg marquardt method we obtain a large viscosity increase across the 670 km depth transition zone separating the upper and the lower mantle with the lower mantle viscosity varying over the range 5 1021 to 1022 pa s and the less resolved upper mantle viscosity of the order of 1020 pa s when antarctica is the only present day source its rate of melting is 240 gt yr 1 corresponding to a sea level rise of 0 7 mm yr 1 when greenland is added as a source of ice loss the rates of melting are 280 gt yr 1 for antarctica and 60 gt yr 1 for greenland corresponding to sea level rises of 0 8 and 0 2 mm yr 1 slr data indicate that ice melting in the polar regions of the earth is ongoing

the importance of radiative heat transfer on superplumes in the lower mantle with the new post perovskite phase change abstract a new post perovskite phase has been identified recently from both x ray observations and first principles calculations this novel phase change occurs at a depth about 200 km above the core mantle boundary for temperatures of around 2800 k using a two dimensional extended boussinesq model we have studied the dynamical consequences of this deep mantle phase transition on mantle convection with particular emphasis on the effects on lower mantle plume structures we have employed a depth dependent viscosity with a viscosity maximum in the mid lower mantle and two phase transitions one at 670 km depth and the other at a depth of 2650 km the phase transition at 670 km is endothermic while we have examined both exothermic and endothermic possibilities for the new phase transition our results show the following situations favorable for the development of superplumes 1 endothermic phase transition for both constant and radiative thermal conductivities and 2 exothermic phase transition and radiative thermal conductivity smaller unstable plumes are found for exothermic phase transition and constant thermal conductivity extremely high lateral temperature increases exceeding 1500 degrees are found inside the lower mantle plume for constant thermal conductivity this lateral temperature contrast is mitigated by the presence of radiative thermal conductivity in order for superplumes to prevail in the lower mantle with a deep exothermic phase change we must invoke some form of radiative thermal conductivity for stabilizing and promoting large upwellings with a width exceeding at least 500 km strong lateral heterogeneities in both the seismic velocity and density fields can be produced in the plumes and the surrounding mantle for this new phase transition with a large clapeyron slope close to 10 mpa k

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

the impact of variability in the rheological activation parameters on lower mantle viscosity stratification and its dynamics the recently discovered spin state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties such as the activation creep parameters in the deep mantle wentzcovitch et al 2009 have computed the elastic parameter of ferropericlase that is affected by this crossover the bulk modulus at high realistic conditions using first principles density functional theory plus hubbard u as a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law using a parameterized model capturing the basic physics we have studied the dynamical consequences within the framework of a 2 d cartesian convection model our models reveal a series of asthenospheres and are characterized first by a low viscosity channel below the 670 km boundary second asthenosphere caused by an increase of temperature and decrease in grain size reduction superplascity with the post spinel transition which results in partially layered convection further the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km third asthenosphere caused by spin crossovers followed by a viscosity hill in the bottom half of the lower mantle the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle our numerical simulations reveal a tendency to the formation of small scale convection below the 670 km boundary and bigger stabilized plumes and or plume clustering in the deep lower mantle other material properties at the bottom of the mantle decrease of thermal expansivity radiative thermal conductivity also exert significant influence on the multi scale lower mantle plume dynamics

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

permo triassic and paleoproterozoic metamorphism related to continental collision in yangpyeong south korea abstract gneisses and migmatites exposed in the yangpyeong area in the northern gyeonggi massif provide insight into the paleoproterozoic and triassic metamorphic events in south korea garnet biotite gneiss and sillimanite garnet biotite gneiss in the western part of the area reveal paleoproterozoic metamorphism 1888 1871 ma at p t conditions of 760 820 c and 8 10 kbar and 710 750 c and 5 7 kbar respectively these rocks were overprinted by low p t type metamorphism 590 650 c 3 4 kbar during the triassic ca 237 ma in contrast a cordierite rich migmatite near the post collisional triassic igneous complex in the eastern part of the area was strongly metamorphosed during the triassic ca 235 ma at 750 790 c and 7 8 kbar the similar triassic ages in the western and eastern areas suggest that low p t type metamorphism occurred as a second stage of regional metamorphism which is characterized by the production of cordierite with an irregularly shaped garnet the metamorphic grade of the triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions and the paleoproterozoic metamorphism is well preserved in the western part with low grade triassic metamorphism the new discovery of the triassic metamorphic event in the yangpyeong area in addition to the previously reported triassic post collision igneous event supports the idea that the continental collision belt between the north and south china blocks extends from the hongseong area into the odesan area through the yangpyeong area in south korea

the viscosity of earth s lower mantle inferred from sinking speed of subducted lithosphere abstract the viscosity of the mantle is indispensable for predicting earth s mechanical behavior at scales ranging from deep mantle material flow to local stress accumulation in earthquakes zones but mantle viscosity is not well determined for the lower mantle particularly only few constraints result from elaborate high pressure experiments karato 2008 and a variety of viscosity depth profiles result from joint inversion of the geoid and postglacial rebound data forte and mitrovica 1996 kaufmann and lambeck 2000 mitrovica and forte 2004 here we use inferred lower mantle sinking speed of lithosphere subduction remnants as a unique internal constraint on modeling the viscosity profile this entails a series of elaborate dynamic subduction calculations spanning a range of viscosity profiles from which we select profiles that predict the inferred sinking speed of 12 3 mm yr van der meer et al 2010 our modeling shows that sinking speed is very sensitive to lower mantle viscosity good predictions of sinking speed are obtained for nearly constant lower mantle viscosity of about 3 4 1022 pa s viscosity profiles incorporating a viscosity maximum in the deep lower mantle as proposed in numerous studies only lead to a good prediction in combination with a weak postperovskite layer at the bottom of the lower mantle and only for a depth average viscosity of 5 1022 pa s

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

metamorphic evolution and 40ar 39ar geochronology of the wuguan complex eastern qinling area china implications for the late paleozoic tectonic evolution of the qinling orogen abstract the late paleozoic tectonic evolution of the qinling orogen is poorly constrained the wuguan complex which lies between the north qinling and south qinling tectonic belts represents a late paleozoic metamorphic belt of the qinling orogen in this contribution we report new metamorphic pressure temperature p t constraints and 40ar 39ar ages of garnet amphibolites and garnet bearing metapelites from the wuguan complex textural observations and mineral compositions especially garnets with two stages of growth indicate the wuguan complex experienced two distinct metamorphic events conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak p t conditions of 660 725 c and 8 0 12 0 kbar with a clockwise p t path whereas the later metamorphic overprint records p t conditions of 560 610 c and 7 5 9 0 kbar hornblendes from amphibolites yield 40ar 39ar ages of 306 299 ma whereas muscovites from garnet bearing metapelites yield 40ar 39ar ages of 252 251 ma the two groups of 40ar 39ar ages correspond to the cooling time of the two distinct metamorphic events respectively combining with previously published zircon u pb ages from the wuguan complex and two groups of metamorphic ages from the guishan complex and the xiongdian eclogite belt we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during carboniferous subduction of oceanic crust the later metamorphic overprint might have been related to the initial collision between the sino korean and yangtze cratons during the late permain

recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer the garnet al silicate plagioclase gasp geobarometer has been recalibrated using four recent garnet activity models four analogous garnet biotite temperature models and two recent plagioclase activity models a typical sillimanite bearing sample that formed at about 5 25 kbar 575 c shows a possible p range of 0 7 kbar due to t error 1 kbar due to range of garnet activity model 0 9 kbar due to range of plagioclase activity model and 5 4 kbar due to range of experimental end member reversals extended by one sigma calibrations were further constrained with the kyanite sillimanite k s phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer samples with low grossular or anorthite component were rejected the end member calibrations are constrained to pass through the the best determined portion of the gasp experimental reversals at 1230 c 26 6 kbar these individual end member calibrations provide self consistent models that tend to compensate for error in the garnet and plagioclase activity expressions the models were also tested on a set of 59 samples from the alps the recommended calibration is the average garnet activity model and average garnet biotite t model of holdaway 2000 the fuhrman and lindsley 1988 plagioclase activity model and h grs 6628521 s grs 258 76 to combine with the remaining phases in the berman database to produce the optimum end member gasp curve these thermodynamic data are for the gasp geobarometer only error is about 0 8 kbar absolute and about 0 6 kbar relative geological error is the largest component of error in many of these samples care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet that the peak t portions of garnet and plagioclase are selected that the peak t al silicate is determined and that the t calculated is the most accurate possible these calibrations represent an improvement over previous published gasp calibrations these eight models are available for distribution as three programs t p p t intersection for the dos based personal computer

crustal material recycling induced by subduction erosion and subduction channel exhumation a case study of central tibet western china based on p t t paths of the eclogite bearing baqing metamorphic complex

recalibration of the garnet muscovite gm geothermometer and the garnet muscovite plagioclase quartz gmpq geobarometer for metapelitic assemblages the garnet muscovite gm geothermometer and the garnet muscovite plagioclase quartz gmpq geobarometer have been simultaneously calibrated under conditions of t 1 4 450 760 c and p 1 4 0 8 11 1 kbar using a large number of metapelitic samples in the compositional ranges x grt fe 1 4 0 53 0 81 x grt mg 1 4 0 05 0 24 x grt ca 1 4 0 03 0 23 in garnet x pl ca 1 4 0 17 0 74 in plagioclase and fe 1 4 0 04 0 16 mg 1 4 0 04 0 13 al 1 4 1 74 1 96 in muscovite on the basis of 11 oxygens the resulting gm thermometer yielded similar temperature estimates mostly within 50 c to that of the garnet biotite thermometer and successfully discerned the expected systematic temperature change of prograde sequences thermal contact zones and an inverted metamorphic zone the resulting gmpq barometer yielded similar pressure estimates mostly within 1 0 kbar to the garnet aluminum silicate plagioclase quartz gasp barometer and placed the aluminosilicate bearing samples in the appropriate aluminosilicate stability fields application of the gmpq barometer to thermal contact aureoles or rocks within limited geographical areas confirmed the expected constant pressures that should exist in these settings the random errors of the gm thermometer and the gmpq barometer are estimated to be 16 c and 1 5 kbar respectively when biotite or aluminosilicate is absent in metapelites metamorphic p t conditions may be determined by simultaneously applying the gm thermometer and the gmpq barometer

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

empirical garnet muscovite geothermometry in metapelites abstract two empirical garnet muscovite geothermometers assuming no ferric iron model a and 50 ferric iron model b in muscovite respectively were calibrated under the physical conditions of p 3 0 14 0 kbar and t 530 700 c the input temperatures and pressures were determined by simultaneously applying the garnet biotite thermometer am mineral 85 2000 881 and the gasp geobarometer am mineral 86 2001 1117 to natural metapelites to confirm internal thermodynamic consistency holdaway s am mineral 85 2000 881 garnet mixing properties were adopted muscovite was treated as a symmetric fe mg al vi ternary solid solution and its margules parameters were derived in this work the resulting two formulae reproduced the input garnet biotite temperatures well within 50 c and gave identical results for a great body of natural samples moreover they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence inverted metamorphic zone and thermal contact aureole pressure estimation has almost no effect on the two formalisms of the garnet muscovite geothermometer assuming analytical error of 5 for the relevant components of both garnet and muscovite the total random uncertainty of the two formulations will generally be within 5 c the two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 c low to high pressure in the composition ranges xalm 0 51 0 82 xpyr 0 04 0 22 and xgros 0 03 0 24 in garnet and fe tot 0 03 0 17 and mg 0 04 0 14 atoms p f u in muscovite

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

the bent hawaiian emperor hotspot track inheriting the mantle wind bends in volcanic hotspot lineaments best represented by the large elbow in the hawaiian emperor chain were thought to directly record changes in plate motion several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks inverse modeling suggests that although deep flow near the core mantle boundary may have played a role in the hawaiian emperor bend capture of a plume by a ridge followed by changes in sub pacific mantle flow can better explain the observations thus hotspot tracks can reveal patterns of past mantle circulation

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

lower mantle dynamics with the post perovskite phase change radiative thermal conductivity temperature and depth dependent viscosity the new post perovskite phase near the core mantle boundary has important ramifications on lower mantle dynamics we have investigated the dynamical impact arising from the interaction of temperature and depth dependent viscosity with radiative thermal conductivity up to a lateral viscosity contrast of 10 4 on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post perovskite transition at 2650 km depth the phase boundaries are approximated as localized zones we have employed a two dimensional cartesian model using a box with an aspect ratio of 10 within the framework of the extended boussinesq approximation our results for temperature and depth dependent viscosity corroborate the previous results for depth dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle once the post perovskite phase change is brought into play this aspect is especially emphasized when the radiative thermal conductivity is restricted only to the post perovskite phase these results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole mantle convection model streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved 2005 elsevier b v all rights reserved

ab initio study of the composition dependence of the pressure induced spin crossover in perovskite mg1 x fex sio3 abstract we present ab initio calculations of the zero temperature iron high to low spin crossover in mg 1 x fe x sio 3 perovskite at pressures relevant to earth s lower mantle equations of state are fit for a range of compositions and used to predict the fe spin transition pressure and associated changes in volume and bulk modulus we predict a dramatic decrease in transition pressure as fe concentration increases this trend is contrary to that seen in ferropericlase and suggests the energetics for spin crossover is highly dependent on the structural environment of fe both local density approximation lda and generalized gradient approximation gga exchange correlation methods are used and both methods reproduce the same compositional trends however gga gives a significantly higher transition pressure than lda the spin transition is made easier by the decreasing spin flip energy with pressure but is also driven by the change in volume from high to low spin volume trends show that high spin fe 2 is larger than mg 2 even under pressure but low spin fe 2 is smaller at ambient conditions and approximately the same size as mg 2 under high pressure indicating that low spin fe 2 is less compressible than high spin fe 2 we find large changes between high and low spin in the slope of volume with fe concentration although these changes are small in absolute magnitude for small fe content they are still important when measured per fe and could be relevant for calculating partitioning coefficients in the lower mantle

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

ferrous iron diffusion in ferro periclase across the spin transition we present first principle calculations on absolute diffusion rates of high and low spin ferrous iron in ferro periclase the diffusivity of high and low spin iron depends on the physical conditions pressure and temperature iron concentration and the value of the chosen hubbard u we also find that low spin iron swaps back to high spin during migration our results show that throughout the earth s lower mantle iron diffuses at a faster rate than magnesium up to an order of magnitude thus the effect of the spin transition only has a small impact on the rheology of periclase in the earth s mantle using reasonable vacancy concentrations we find that ferro periclase is much weaker than mgsio3 perovskite throughout the mantle we conclude therefore that ferro periclase controls the viscosity of the mantle in regions where it becomes interconnected such as regions of high strain around slabs or near plumes at very high pressures such as in super earths low spin iron diffusion is expected to become much slower than magnesium eventually making ferro periclase more viscous than pure periclase c 2010 elsevier b v all rights reserved

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

compressible convection in the earth s mantle a comparison of different approaches numerical models of mantle convection using the boussinesq extended bousinessq and anelastic liquid approximation are compared for steady state solutions there is good quantitative agreement between the results if they are scaled in a proper way time dependent extended boussinesq and anelastic liquid flows show only qualitative agreement the main difference being a distortion of timescale compressibility induces an asymmetry in the structure of upper and lower boundary layers that cannot be observed in boussinesq fluids

syn metamorphic b bearing fluid infiltrations deduced from tourmaline in the main central thrust zone eastern nepal himalayas abstract mode of occurrence and chemical composition of tourmaline in pelitic schists from the main central thrust mct zone of the lesser himalayan sequences lhs are described in detail with the aim of deducing the chemical characteristics of tourmaline formed through b bearing fluid infiltration and of estimating the composition of the syn metamorphic fluids metasomatic tourmalines from the tourmalinized wall rocks show significant increases in xca ca ca na at almost constant xmg mg mg fe2 from the cores or mantles to the rims tourmaline in tourmaline rich 1 0 vol pelitic schists from the biotite zone to the kyanite zone also show marked increase in xca at almost constant xmg and are interpreted as a product of b bearing fluid infiltration abundant margarite and anorthite formed in the pelitic schists intercalated with the metadolostone layers suggesting that the b rich fluid became ca enriched as it interacted with metadolostone layers and metasomatically introduced ca into the pelitic schists infiltration of such b and ca rich fluids into pelitic schists likely resulted in production of abundant tourmaline with the compositional trend of increasing xca at almost constant xmg most of the tourmaline in tourmaline rich pelitic schists are in equilibrium with plagioclase suggesting that the fluid composition was buffered by the pelitic schists composition of tourmaline in the pelitic schists with

electronic spin transitions in iron bearing mgsio3 perovskite the electronic spin state of iron in perovskites with the chemical composition mg0 9375fe0 0625sio3 mg0 8750fe0 1250sio3 and mg0 9375fe0 0625si0 9375fe0 0625o3 have been determined at 0 k and various pressures between 40 160 gpa using ab initio methods the results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60 160 gpa while ferrous iron has a much narrower span between about 130 145 gpa in general the lowest spin transition pressures are associated with the most energetically favorable substitution configurations where iron atoms are closest together the effect of spin state on calculated elastic properties is found to be small c 2006 elsevier b v all rights reserved

petrology of the regional sillimanite zone west central new hampshire u s a with implications for the development of inverted isograds abstracr sillimanite from the regional sillimanite zone in west central new hampshire is fibrolitic and overprints f2 folds nappe stage of earlier mica foliation regional sillimanite zone samples show no evidence for earlier staurolite parageneses despite the fact that staurolite is abundant at lower grade because sillimanite was produced directly from garnet chlorite by the prograde heating reaction garnet chlorite muscovite quartz sillimanite biotite h20 the pressure at which this reaction occurs is sensitive to the mno and cao contents of garnet and phase equilibrium arguments reveal that at the regional pressures of west central new hampshire 2 4 kbar staurolite parageneses are only possible in rocks with low mno cao the inferred p t path is counterclockwise with nearly isobaric initial heating at 2 kbar followed by loading t heating to a peak metamorphic temperature of 600 t 25 c at 4 kbar followed by nearly isobaric cooling garnets were compositionally homogenized near the metamorphic peak but subsequent cooling was rapid 100 c m y these data argue against the folded isotherm model of chamberlain 1986 instead the present distribution of metamorphic grades is interpreted to be the result of regional stacking of high grade thrust sheets on lower grade rocks followed by depression of high grade rocks to lower structural levels

long wavelength character of subducted slabs in the lower mantle abstract the high resolution seismic tomography provides the detailed information about the fate of the subducted plates in the mantle while the slab deformation in the transition zone is quite complex and differs among the individual subduction zones it could be generally concluded that most slabs seem to penetrate into the lower mantle the character of the fast seismic velocity anomalies however changes when the slabs cross the 670 km discontinuity while in the upper mantle tomography resolves relatively narrow plate like structures in the lower mantle it rather indicates thick blobs of seismically fast material such thickening of the downgoing slabs is traditionally attributed to the effect of the viscosity increase in the lower mantle here we study the deformation of the slabs in the lower mantle in a viscous model with a non linear composite rheology including the diffusion creep the dislocation creep and a power law stress limiter we perform numerical simulations of the subduction process in a 2d cartesian domain and concentrate especially on the effect of the change of the rheological parameters across the 670 km boundary on the slab deformation in the upper part of the lower mantle the thickening or the buckling of the plate is indeed observed in the models with relatively low yield stress 0 1 gpa of the stress limiting mechanism and with a viscosity increase at the 670 km discontinuity further we show that both the major mantle phase transitions and the strength of the crust layer have significant consequences for the creation of the buckling instabilities in the lower mantle

inferences of viscosity from the oceanic geoid indication of a low viscosity zone below the 660 km discontinuity abstract we have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid and tomographic structure beneath the oceans in order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents we have considered only the intermediate degrees of the oceanic geoid l 12 25 a genetic algorithm has been used to determine the family of viscosity models which give the best correlation with the observed geoid our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms the viscosity increase in the lower mantle predicted by previous inferences but suggests that the main viscosity jump occurs at a depth of about 1000 km and not at the usually stated 660 km boundary somewhere in the depth range of 400 1000 km a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle unfortunately the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel

calibration of the garnet biotite al2 sio5 quartz geobarometer for metapelites a garnet biotite al2sio5 quartz gbaq geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under p t conditions of 450 950 c and 1 17 kbar in the calibration activity models of garnet and biotite identical to those in the garnet biotite geothermometer of holdaway 2000 were used therefore the gbaq geobarometer and the garnet biotite geothermometer can be simultaneously applied to iteratively estimate metamorphic p t conditions successful application of the gbaq geobarometer to natural metapelites certify its validity most importantly when plagioclase is absent or cao in garnet and or plagioclase are deficient this geobarometer may prove useful for estimating metamorphic pressures the random error of the present gbaq geobarometer is inferred to be around 1 8 kbar an electronic spreadsheet is available as table s4 to apply the gbaq geobarometer in combination with the garnet biotite geothermometer this article is protected by copyright all rights reserved

intergranular diffusion rates from the analysis of garnet surfaces implications for metamorphic equilibration novel approaches to garnet analysis have been used to assess rates of intergranular diffusion between different matrix phases and garnet porphyroblasts in a regionally metamorphosed staurolite mica schist from the barrovian type area in scotland x ray maps and chemical traverses of planar porphyroblast surfaces reveal chemical heterogeneity of the garnet grain boundary linked to the nature of the adjacent matrix phase the garnet preserves evidence of low temperature retrograde exchange with matrix minerals and diffusion profiles documenting cation movement along the garnet boundaries garnet quartz and garnet plagioclase boundaries preserve evidence of sluggish mg mn and fe diffusion at comparable rates to volume diffusion in garnet whereas diffusion along garnet biotite interfaces is much more effective evidence of particularly slow al transport probably coupled to fe3 exchange is locally preserved on garnet surfaces adjacent to fe oxide phases the ca distribution on the garnet surface shows the most complex behaviour with long wavelength heterogeneities apparently unrelated to the matrix grain boundaries this implies that the ca content of garnet is controlled by local availability and is thought likely to reflect disequilibrium established during garnet growth geochemical anomalies on the garnet surfaces are also linked to the location of triple junctions between the porphyroblasts and the matrix phases and imply enhanced transport along these channels the slow rates of intergranular diffusion and the characteristics of different boundary types may explain many features associated with the prograde growth of garnet porphyroblasts thus minerals such as quartz fe oxides and plagioclase whose boundaries with garnet are characterized by slow intergranular diffusion rates appear to be preferentially trapped as inclusions within porphyroblasts as such grain boundary diffusion rates may be a significant kinetic impediment to metamorphic equilibrium and garnet may struggle to maintain chemical and textural equilibrium during growth in pelites

conflicting structural and geochronological data from the ibituruna quartz syenite se brazil effect of protracted hot orogeny and slow cooling rate the ibituruna quartz syenite was emplaced as a sill in the ribeira aracuai neoproterozoic belt southeastern brazil during the last stages of the gondwana supercontinent amalgamation we have measured the anisotropy of magnetic susceptibility ams in samples from the ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric a large magnetic anisotropy dominantly due to magnetite and a consistent magnetic fabric have been determined over the entire ibituruna massif the magmatic foliation and lineation are strikingly parallel to the solid state mylonitic foliation and lineation measured in the country rock altogether these observations suggest that the ibituruna sill was emplaced during the high temperature similar to 750 degrees c regional deformation and was deformed before full solidification coherently with its country rock unexpectedly geochronological data suggest a rather different conclusion la icp ms and shrimp ages of zircons from the ibituruna quartz syenite are in the range 530 535 ma and la icp ms ages of zircons and monazites from synkinematic leucocratic veins in the country rocks suggest a crystallization at similar to 570 580 ma i e an ht deformation 35my older than the emplacement of the ibituruna quartz syenite conclusions from the structural and the geochronological studies are therefore conflicting a possible explanation arises from ar 40 ar 39 thermochronology we have dated amphiboles from the quartz syenite and amphiboles and biotites from the country rock together with the ages of monazites and zircons in the country rock ar 40 ar 39 mineral ages suggest a very low cooling rate 3 degrees c my between 570 and similar to 500 ma and similar to 5 degrees c my between 500 and 460 ma assuming a protracted regional deformation consistent over tens of my under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time this may be a characteristic of slow cooling hot orogens that rocks deformed at significantly different periods during the orogeny but under roughly unchanged temperature conditions may display almost indiscernible microstructure and fabric

empirical garnet muscovite plagioclase quartz geobarometry in medium to high grade metapelites abstract based on the net transfer reactions among garnet muscovite plagioclase and quartz for both mg and fe end member models the garnet muscovite plagioclase quartz gmpq geobarometry was empirically calibrated under the physical conditions of p 1 0 11 4 kbar and t 505 745 c for 128 natural metapelitic rock samples collected from the literature the input temperatures and pressures were simultaneously determined by the garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gmpq calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same al avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet biotite geothermometer and the gasp geobarometer a symmetric fe mg al vi ternary solid solution model of muscovite was adopted and the margules parameters of muscovite were obtained through regression the mg and fe model reactions along with the assumption of whether the ferric iron content in muscovite is 0 or 50 resulted in four gmpq barometry formulae the gmpq barometry formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent samples the gmpq barometry formulae yield identical pressures for every sample whether the sample was included or not in calibrating the barometers for each of the mg or fe model reaction the formulae gave identical pressures within 40 bars the random error of the gmpq barometry may be expected as 1 4 kbar the d p d t slopes of these gmpq formulae are close to that of the gasp barometer in the p t space applications of the gmpq barometry to aluminosilicate absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference it may be concluded that the gmpq barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505 745 c and 1 11 4 kbar in the composition range of x gros 3 in garnet and x an 17 in plagioclase

polymetamorphism in medium to high grade pelitic metamorphic rocks west central maine a regional petrologic study of pelitic metamorphic rocks has been undertaken with emphasis on the area bounded by the cities of augusta lewiston norway rangeley kingfield and madison this report involves a regional isograd map interpretation of the nature of the various reactions and discussion of petrologic problems of polymetamorphism isograd distribution and widespread occurrence of muscovite in the k feldspar sillimanite zone a future contribution will deal more specifically with mineral chemistry geotherrnometry and geobarometry of much of the area after a low grade metamorphism m 1 the region suffered a higher grade early metamorphism m 2 which in many places produced andalusite and cordierite as well as staurolite this was followed by a somewhat higher pressure metamorphism m 3 involving staurolite chlorite and sillimanite in some areas m 3 has produced extensive retrograde effects with increasing grade in ms the major mineral reactions are 1 formation of staurolite from reaction of garnet chlorite and muscovite 2 formation of sillimanite from staurolite chlorite and muscovite 3 reaction of staurolite to form sillimanite biotite and in many rocks garnet and 4 formation of k feldspar from the partial reaction of muscovite quartz and plagioclase a possible fourth metamorphism m 4 produced narrow aureoles with andalusite and staurolite around the three latest plutons analysis of the isograd pattern demonstrates that the pressure was rather uniform throughout the area at a given time but increased about 0 5 kbar or more between m 2 and m 3 metamorphism the more regional distribution of isograds to the south is consistent with higher initial temperatures and more closely spaced magmatic heat sources but not necessarily with increasing pressure to the south the pressure increase between m 2 and m 3 might have been produced by intrusion of magma above the present level and or extrusion of rhyolitic volcanics

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

calibration of a ti in muscovite geothermometer for ilmenite and al2sio5 bearing metapelites abstract the ti in muscovite geothermometer was empirically calibrated as ln t oc 7 258 0 289 ln ti 0 158 mg fe mg 0 031 ln p kbar using ilmenite and al2sio5 bearing assemblages in metapelites under p t conditions of 450 800 c and 0 1 1 4 gpa the calibration was conducted for muscovites containing ti 0 01 0 07 fe 0 03 0 16 mg 0 01 0 32 and mg fe mg 0 05 0 73 respectively on the basis of 11 oxygen per formula unit such compositional range covers more than 90 natural muscovites and the random error of this thermometer is estimated to be of 65 c the geothermometer was validated against a set of independently determined temperature conditions between different degrees in samples from different prograde inverted and contact metamorphic terranes application of this thermometer beyond the calibration conditions is not encouraged

dynamical consequences of depth dependent thermal expansivity and viscosity on mantle circulations and thermal structure the effects of both depth dependent thermal expansivity and depth dependent viscosity on mantle convection have been examined with two dimensional finite element simulations in aspect ratio ten boxes surface rayleigh numbers between 107 and 6 107 have been considered the effects of depth dependent properties acting singly or in concert are to produce large scale circulations with a few major upwellings the interior of the mantle is cooled by the many cold instabilities which are slowed down and eventually swept about by the large scale circulation the interior temperature of the mantle can be influenced by the trade off between depth dependent properties and internal heating for chondritic abundance of internal heating and depth dependent thermal expansivity the viscosity increase across the mantle can be no greater than a factor of around ten in order to keep the lower mantle adiabatic the thermal contrasts between the cold blobs and the surrounding mantle are strongly reduced by depth dependent properties whereas the lateral differences between the hot upwelling and the ambient lower mantle can be significant over several hundred degrees depth dependent properties also encourage the formation of a stronger mean flow in the upper mantle which may be important for promoting long term polar motions

effect of phase transitions on compressional wave velocities in the earth s mantle the velocities of seismic waves in the earth are governed by the response of the constituent mineral assemblage to perturbations in pressure and stress the effective bulk modulus is significantly lowered if the pressure of the seismic wave drives a volume reducing phase transformation a comparison between the amount of time required by phase transitions to reach equilibrium and the sampling period thus becomes crucial in defining the softening and attenuation of compressional waves within such a two phase zone these phenomena are difficult to assess experimentally however because data at conditions appropriate to the earth s deep interior are required here we present synchrotron based experimental data that demonstrate softening of the bulk modulus within the two phase loop of olivine ringwoodite on a timescale of 100 s if the amplitude of the pressure perturbation and the grain size are scaled to those expected in the earth the compressional wave velocities within the discontinuities at 410 520 and possibly 660 km are likely to be significantly lower than otherwise expected the generalization of these observations to aluminium controlled phase transitions raises the possibility of large velocity perturbations throughout the upper 1 000 km of the mantle

dynamical consequences in the lower mantle with the post perovskite phase change and strongly depth dependent thermodynamic and transport properties article i nfo we have carried out numerical simulations of large aspect ratio 2 d mantle convection with the deep phase change from perovskite pv to post perovskite ppv using the extended boussinesq approximation for a fluid with temperature and pressure dependent viscosity we have investigated the effects of various ppv phase parameters on the convective planform heat transport and mean temperature and viscosity profiles since ppv is expected to have a relatively weak rheology with respect to pv and a large thermal conductivity we have assumed that the transition from pv to ppv is accompanied by both a reduction in viscosity by 1 to 2 orders of magnitude and by an increase in thermal conductivity by a factor of 2 furthermore we have analyzed the combined effects of a strongly decreasing thermal expansivity in pv and steeply increasing thermal conductivity according to recent evidence from high pressure experiments and first principle calculations as long as the thermal expansivity and conductivity are constant ppv exerts a small but noticeable effect on mantle convection it destabilizes the d layer causes focusing of the heat flux peaks and an increase of the average mantle temperature and of the temporal and spatial frequency of upwellings when the latest depth dependent thermal expansivity and conductivity models are introduced the effects of ppv are dramatic on the one hand without ppv we obtain a very sluggish convective regime characterized by a relatively cool mantle dominated by large downwellings that tend to stagnate beneath the transition zone with ppv on the other hand we observe an extremely significant increase of the average mantle temperature due to the formation of large sized and vigorous upwellings that in some cases tend to cluster thus forming superplumes if a very large thermal conductivity at the core mantle boundary is assumed k 20 wk 1 m 1 we obtain a quasi steady state regime characterized by large and stable plumes with long lifetimes the combination of strongly depth dependent expansivity and conductivity is a viable mechanism for the formation of long wavelength long lived thermal anomalies in the deep mantle even if a low viscosity ppv atop the core mantle boundary is included

are mantle plumes adiabatic abstract the issue concerning the state of adiabaticity of mantle plumes has been examined in a cartesian two dimensional box with an aspect ratio of six we have investigated in the quasi steady state regime high rayleigh number convection with both depth dependent viscosity and thermal expansivity for both the boussinesq and the extended boussinesq approximations we have also assessed the influence of various forms of thermal conductivity and internal heating we have generalized the classical bullen s parameter equation from one dimension to multidimensions for assessing the local state of adiabaticity inside plumes and in their surroundings we have extracted from the local geotherms and the local thermodynamic properties the corresponding bullen s parameter profiles and the two dimensional maps portraying the state of adiabaticity in the mantle histograms characterizing the frequency of adiabaticity are also employed for quantification purposes in general superadiabatic thermal gradients are found inside the thick plume limbs and sometimes along the central part of the plume the centers of plume heads are subadiabatic or nearly adiabatic but the edges of the plume heads are strongly subadiabatic alternating strips of subadiabaticity and adiabaticity are found in the downwellings the ambient mantle outside the plumes is generally adiabatic and is sometimes perforated with islands of marked deviations from adiabaticity

stable intermediate spin ferrous iron in lower mantle perovskite iron has the ability to adopt different electronic configurations and transitions in its spin state in the lower mantle can significantly influence mantle properties and dynamics experimental results for two lower mantle perovskite compositions show that the intermediate spin state of iron is stable throughout the bulk of the lower mantle

weakening of calcium iridate during its transformation from perovskite to post perovskite magnesium silicate perovskite the dominant mineral in the lower mantle is thought to transform into a post perovskite phase in the mantle s lowermost region laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post perovskite which could explain the anomalous physical properties of the lowermost mantle

intermediate spin ferrous iron in lowermost mantle post perovskite and perovskite the earth s lowermost mantle displays an important mineralogical transition from perovskite to post perovskite but the spin state of iron in these phases remains poorly known experimental results suggest that iron occurs in the intermediate spin state in both of these phases which implies that changes in physical properties of the lower mantle must be governed by factors other than spin transitions in iron

inferring radial models of mantle viscosity from gravity grace data and an evolutionary algorithm convective flow in the mantle can be thought of and modeled as exclusively driven by density heterogeneities in the mantle itself and the resulting lateral variations in the earth s gravity field with this assumption and a model of mantle rheology a theoretical relationship can be found between 3d mantle structure and flow related quantities that can be measured on the earth s surface like free air gravity anomalies this relationship can be used to set up an inverse problem with 1d mantle viscosity as a solution in the assumption that seismic velocity anomalies be of purely thermal origin and related to density anomalies by a simple scaling factor we invert the large scale length component of the above mentioned measurements jointly with seismic observations waveforms and or travel times to derive an accurate 5 layer spherically symmetric model of upper and lower mantle viscosity we attempt to account for non uniqueness in the inverse problem by exploring the solution space formed of all possible radial profiles of earth viscosity by means of a non deterministic global optimization method the evolutionary algorithm ea for each sampled point of the solution space a forward calculation is conducted to determine a map of gravity anomalies whose similarity to grace gravity recovery and climate experiment is then measured the procedure is iterated to convergence according to ea criteria the robustness of the inversion is tested by means of synthetic tests indicating that our gravity data set is able to constrain less than 6 radial layers each with uniform viscosity independently of the tomographic model or the scaling factor adopted to convert seismic velocity into density structure the ea optimization method finds viscosity profiles characterized by low viscosity in a depth range corresponding to the transition zone and relatively uniform elsewhere

interconnection of ferro periclase controls subducted slab morphology at the top of the lower mantle abstract the electrical conductivity of mantle rocks during phase transformation from ringwoodite to silicate perovskite and ferro periclase was measured at 25 gpa and various temperatures ranging from 1300 to 1900 k the electrical conductivity was high at the initial stage of annealing suggesting that ferro periclase forms interconnected layers in aggregates of silicate perovskite and ferro periclase that are representative of lower mantle rock at 1900 k the electrical conductivity quickly decreased and reached that of silicate perovskite suggesting the cut off of the interconnected ferro periclase because of rounding of crystals below 1700 k the high conductivity values were maintained for experimental duration the interconnection of ferro periclase which has a lower viscosity than silicate perovskite can be maintained in a cold descending slab over geological time scales 1 my indicating that a colder slab is less viscous than the warmer mantle surrounding it the low viscosity slab can be prevented from penetrating into the deeper part of the lower mantle by the high viscosities encountered at a depth of 1000 km referred to as the viscosity hill that cause stagnation at this depth as observed by seismic tomography

petrogenesis of the puerto eden igneous and metamorphic complex magallanes chile late jurassic syn deformational anatexis of metapelites and granitoid magma genesis a suite of schists gneisses migmatites and biotite granitoids from the puerto eden igneous and metamorphic complex peimc and biotite hornblende granitoids of the south patagonian batholith southern chile has been studied for that purpose the chemistry of minerals and the bulk rock composition of major and trace elements including rb sr and sm nd isotopes were determined mineralogical observations and geothermobarometric calculations indicate high temperature and low pressure conditions ca 600 700 c and 3 to 4 5 kbar for an event of metamorphism and partial melting of metapelites in late jurassic times previously determined by shrimp u pb zircon ages structures in schists gneisses migmatites and mylonites indicate non coaxial deformation flow during and after peak metamorphic and anatectic conditions andalusite schists and sillimanite gneisses yield initial 87sr 86sr ratios of up to 0 7134 and end150 values as low as 7 6 contemporaneous biotite granitoids and a coarse grained orthogneiss have initial 87sr 86sr ratios between 0 7073 and 0 7089 and end150 values in the range 7 6 to 4 4 this indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source thus a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids the peimc is considered as a segment of an evolving kilometre sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in late jurassic times a biotite hornblende granodiorite and a muscovite garnet leucogranite show initial 87sr 86sr ratios of 0 7048 and 0 7061 and end100 values of 2 6 and 1 8 respectively and are thus probably related to early cretaceous magmas not involved in the anatexis of the metasedimentary rocks

neoproterozoic a type magmatism in the western sierras pampeanas argentina evidence for rodinia break up along a proto iapetus rift a type orthogneisses of mid neoproterozoic age 774 6 ma u pb shrimp zircon age are reported for the first time from the grenvillian basement of the western sierras pampeanas in argentina these anorogenic meta igneous rocks represent the latest event of rodinia break up so far recognized in grenvillian basement exposures across andean south america moreover they compare well with a type granitoids and volcanic rocks along the appalachian margin of laurentia blue ridge thus adding to former evidence that the western sierras pampeanas grenvillian basement was left on the conjugate rifted margin of eastern laurentia during rodinia break up and the consequent opening of the iapetus ocean

radiosonde observations of equatorial atmosphere dynamics over indonesia 1 equatorial waves and diurnal tides this paper describes the characteristics of the mean winds equatorial waves with periods ranging from 4 to 20 days and diurnal tides determined by analyzing the profiles of wind velocity temperature and humidity obtained every 5 7 hours in the height range up to about 35 km with a height resolution of 150 m during an observation campaign conducted february 27 to march 22 1990 in east java indonesia the structures of the mean winds in the troposphere and lower stratosphere seemed to be affected by the australian monsoon and the quasi biennial oscillation respectively frequency spectra indicated that the equatorial waves as well as the diurnal tides were dominant below about 25 km while gravity waves with periods shorter than 4 days became more significant above 25 km a 7 day oscillation showing an antiphase relation between the eastward and northward components and exhibiting large amplitudes was observed in the lower troposphere the time height variations of the activity of this 7 day oscillation were clearly correlated with a region of high relative humidity perturbations in the zonal wind and temperature with wave periods varying from 15 to 17 days were also enhanced in the troposphere while kelvin waves with periods of about 7 and 20 days were detected in the lower stratosphere and activity near the tropopause was conspicuously enhanced we found that the 20 day kelvin wave greatly modified the structure of the tropopause such as the minimum temperature the tropopause height and the values of the brunt vaisala frequency squared n2 which further suggests the effects of kelvin waves on the transportation of tropospheric water vapor into the stratosphere and on the downward mixing of stratospheric minor constituents into the troposphere the observed profiles of the diurnal oscillation were compared with those of a numerical model assuming only migrating tides which reasonably agreed above about 25 km below 25 km however the observed amplitudes were 1 1 5 m s exceeding those of the model about 10 times moreover the phase profiles involved fluctuations with small vertical scales suggesting interference by many nonmigrating tides with short vertical wavelengths

polar wandering and the forced responses of a rotating multilayered viscoelastic planet changes in the earth s rotation produced by the waxing and waning of large mobile ice sheets can be used to infer the viscosity of the mantle from a comparison with the recent secular polar drift as revealed in the international latitude service data and also in the estimates of the nontidal acceleration of the length of the day we have developed by using an analytical approach a physical model of a rotating earth consisting of an elastic lithosphere a viscoelastic mantle and an inviscid core forcings from the two major ice sheets laurentide and fennoscandia have been considered sensitivity analysis of the parameters governing the external forcings such as the length and the time of termination of the deglaciation phase show in general that there exist two families of mean mantle viscosities 0 1022 p and 0 1023 p which can match the data satisfactorily however we find it an improvement to incorporate some amount of shrinkage from the antarctic ice sheet obtaining simultaneously a better fit with the two rotational data sets to the lower viscosity solutions hence forcings from the southern hemisphere may be able to remove the ambiguity associated with the extraction of mantle viscosity from rotational data we have also solved the initial value problem of the components of instantaneous angular velocity by means of the laplace transform for applied loads characterizing the continental ice sheets which since the mid pliocene have periodically expanded and contracted over the northern hemisphere in consequence of this continual forcing polar wander of around 5 10 degrees can take place for a mean mantle viscosity 0 1022 p recent reanalysis of paleomagnetic data in conjunction with hot spot tracks have revealed that several degrees of true polar wander could have occurred in the last 5 m y these calculations thus provide a physical explanation for the observed movement of the rotation axis membrane stresses of 0 10 bars can develop from this secular polar motion

metamorphism and deformation at different structural levels in a strike slip fault zone ross lake fault north cascades usa continental crust is displaced in strike slip fault zones through lateral and vertical movement that together drive burial and exhumation pressure temperature deformation p t d histories of orogenic crust exhumed in transcurrent zones record the mechanisms and conditions of these processes the skagit gneiss complex a migmatitic unit of the north cascades washington usa was metamorphosed at depths of 25 30 km in a continental arc under contraction and is bounded on its eastern side by the long lived transcurrent ross lake fault zone rlfz the p t d conditions recorded by rocks on either side of the rlfz vary along the length of the fault zone but most typically the fault separates high grade gneiss and plutons from lower grade rocks the ruby mt elijah ridge area at the eastern margin of the skagit gneiss exposes tectonic contacts between gneiss and overlying rocks the latter rocks including slivers of methow basin deposits are metamorphosed and record higher grade metamorphism than in correlative rocks along strike along the rlfz in this area the skagit gneiss and overlying units all yield maximum p t conditions of 8 10 kbar at 650 c indicating that slices of basin rocks were buried to similar mid crustal depths as the gneiss after exhumation of fault zone rocks to 15 km depth intrusion of granitoid plutons drove contact metamorphism resulting in texturally late andalusite cordierite in garnet schist in the elijah ridge area of the rlfz an overlapping step over or series of step overs that evolved through time may have facilitated burial and exhumation of a deep slice of the methow basin indicating that strike slip faults can have major vertical displacement tens of kilometres that is significant during the crustal thickening and exhumation stages of orogeny

implications of post perovskite transport properties for core mantle dynamics abstract recent evidence on perovskite to post perovskite phase change in the lowermost mantle suggests that post perovskite piles or lens should be present in the relatively cold downwelling areas while the roots of the hot upwelling plumes consist of perovskite post perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions that can even result in a viscosity paradox in the lowermost mantle viscosity in cold downwellings transformed to post perovskite might be lower than the viscosity of warm perovskite plumes such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the d we investigate the dynamics of the cold slabs transformed to post perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2d cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep different creep parameters are used for the perovskite lower mantle and for the post perovskite lens or layer respectively while perovskite is considered to be deformed via linear diffusion creep post perovskite is modeled either as a constant viscosity material with viscosity ranging from 1020 pa s to 1023 pa s or it is deformed purely by the dislocation creep the presence of the rheologically distinct post perovskite not only strongly influences the slab deformation above the core mantle boundary cmb but also results in different dynamic regimes of the cmb region characterised by e g the different length scales of the upwelling plumes depending on the creep parameters of post perovskite further the presence of very low viscosity lens or a layer above the cmb strongly enhances the cmb heat flux in the cold slab areas according to the amount of post perovskite present in the d and the rheology of post perovskite the heat flux across the cmb can vary considerably with time which may have important implications for the core mantle coupling and the character of the earth s geomagnetic field