evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks the morphologies of calcite grain boundaries were analyzed to provide insight into the evolution of pore networks in unfractured rock two synthetic calcite rocks were fabricated by hot isostatically pressing hip ing dried analytical grade powders of pure caco3 and caco3 plus 5 al2o3 at 600 c and 200 mpa confining pressure for 3 hours hip 1 some samples were hiped a second time at different temperatures and pressures to investigate the stability of the structures hip 2a c sem and tem were used to image both grain faces and grain boundary cross sections structures on grain faces vary from open shallow basins with peripheral rims to labyrinths of irregular ridges and channels to isolated circular depressions all of these structures are mirrored across the plane between grain faces the grain size in both the single and two phase samples increased markedly during hip 1 migrating boundaries either dragged pores along or broke away leaving grain interiors dotted with small voids the structures present after hip 1 were not stable but evolved considerably in a way dependent on the conditions of the hip 2 confining pressure had the most pronounced effect with low confining pressure the grain boundary porosity evolved into isolated circular depressions but the total pore volume did not noticeably decrease with high confining pressure the pore volume virtually disappeared the structures present after hip 1 are strikingly similar to those that develop in intragranular cracks during healing we infer that grain boundaries and intragranular cracks heal by similar processes decomposition localized melting impurities and anisotropic surface energies played no evident role in forming the grain boundary structures the timing of the formation of the porosity and of the subsequent healing processes is more difficult to ascertain some structures appear to have evolved gradually throughout the constant high temperature stage of hiping the most obvious structures however appear to have evolved on grain boundary cracks that opened during cooling

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

magnetofossil spike during the paleocene eocene thermal maximum ferromagnetic resonance rock magnetic and electron microscopy evidence from ancora new jersey united states previous workers identified a magnetically anomalous clay layer deposited on the northern united states atlantic coastal plain during the paleocene eocene thermal maximum petm the finding inspired the highly controversial hypothesis that a cometary impact triggered the petm here we present ferromagnetic resonance fmr isothermal and anhysteretic remanent magnetization first order reversal curve and transmission electron microscopy analyses of late paleocene and early eocene sediments in drillcore from ancora new jersey a novel paleogeographic analysis applying a recent paleomagnetic pole from the faeroe islands indicates that new jersey during the initial eocene had a 6 9 degrees lower paleolatitude 27 3 degrees for ancora and a more zonal shoreline trace than in conventional reconstructions our investigations of the petm clay from ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria this result the first identification of ancient biogenic magnetite using fmr argues that the anomalous magnetic properties of the petm sediments are not produced by an impact they instead reflect environmental changes along the eastern margin of north america during the petm that led to enhanced production and or preservation of magnetofossils

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

experimental deformation of synthetic wet jadeite aggregates 1 predictions on interplate coupling and shear heating in the deeper levels of subduction zones require an understanding of the rheology of eclogite the strength of eclogite is probably limited by the flow strength of its major constituent omphacite representing a solid solution between the clinopyroxene end members diopside and jadeite here we report the results of deformation experiments on the end member jadeite naalsi2o6 carried out on fine grained synthetic aggregates crystallized in the stability field of jadeite from a synthetic glass precursor a standard procedure was established to yield samples with a uniform microstructure and an average grain size of approximately 10 m the presence of micropores and the detection of small amounts of freezable water by fourier transform infrared spectroscopy ftir in both undeformed and deformed samples indicates the presence of a free aqueous fluid hence wet conditions deformation experiments were carried out in a modified griggs type apparatus at a confining pressure of 2 5 gpa temperatures between 800 c and 1100 c and at strain rates of 4 10 4 s 1 to 2 10 6 s 1 a molten eutectic cscl nacl mixture was used as pressure medium the microfabrics of the deformed samples and the mechanical data indicate deformation in the dislocation creep regime the mechanical data are fit by a power law using a global inversion method yielding an activation energy of q 326 27 kj mol 1 a stress exponent n 3 7 0 4 and a preexponential factor of ln a 3 3 2 0 extrapolation of this flow law for synthetic jadeitite to low geological strain rates and comparison with available flow laws for diopside indicates jadeite to be significantly weaker than diopside in the dislocation creep regime

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

eustatic variations during the paleocene eocene greenhouse world we reconstruct eustatic variations during the latest paleocene and earliest eocene 58 52 ma dinoflagellate cysts grain size fractions and organic biomarkers in marine sections at four sites from three continents indicate an increased distance to the coast during the paleocene eocene thermal maximum petm the same trend is recognized in published records from other sites around the world together the data indicate a eustatic rise during the petm beginning 20 to 200 ka before the globally recorded negative carbon isotope excursion cie at 55 5 ma although correlations are tentative we recognize other global transgressions during chrons c25n and c24n the latter may be associated with eocene thermal maximum 2 53 5 ma or the x event 52 ma these results suggest a link between global sea level and hyperthermal intervals potentially because of the melting of small alpine ice sheets on antarctica thermal expansion of seawater or both however the early onset of sea level rise relative to the cie of the petm suggests contributions from other mechanisms perhaps decreasing ocean basin volume on sea level rise copyright 2008 by the american geophysical union

refining ancient carbon dioxide estimates significance of coccolithophore cell size for alkenone based pco2 records 1 long term alkenone based pco2 records are widely applied in paleoclimate evaluations these pco2 estimates are based on records of the carbon isotope fractionation that occurs during marine haptophyte photosynthesis p37 2 in addition to the concentration of aqueous co2 co2 aq the magnitude of p37 2 is also influenced by algal growth rates and cell geometry to date the influence of haptophyte cell geometry on the expression of ancient p37 2 values has received little attention this study evaluates changes in cell geometry of ancient alkenone producing algae at deep see drilling project site 516 in the southwest atlantic ocean by analyzing individual coccolith dimensions which are proportional to algal cell volume and surface area we show that during part of the early miocene mean cell sizes of alkenone producing algae were smaller relative to modern emiliania huxleyi cell size variations coincide with significant changes in p37 2 with a distinct 6 decrease in p37 2 at 20 3 ma associated with a 27 increase in haptophyte cell sizes these changes in cell size impact p37 2 based interpretations of growth rate variation and co2 aq estimates for this southwest atlantic site after correcting for cell geometry co2 aq estimates at site 516 are consistent with those reported from other oligotrophic sites during this time resulting in overall low atmospheric pco2 estimates 350 ppmv for the early miocene

grain scale deformation in ultra high pressure metamorphic rocks an indicator of rapid phase transformation abstract conspicuous grain scale deformation is observed in some ultra high pressure uhp metamorphic rocks of the dora maira massif western alps although no significant strain is discernible on the mesoscopic scale in a jadeite kyanite quartz rock some of the jadeite crystals reveal 100 deformation twins indicating local differential stress levels above 0 3 gpa many kyanite crystals show marked kink or deformation bands with a slip system 100 001 in contrast the adjacent coarse grained quartz matrix grain size ca 0 2 mm which has formed from coesite during exhumation from 100 km depth reveals a foam structure the quartz grain boundary configuration is controlled by interfacial free energy the grains are optically strain free and there is no crystallographic preferred orientation preservation of this foam microstructure which indicates grain growth during low stress annealing precludes that deformation of the jadeite and kyanite crystals is a result of a late stage low temperature overprint the orientation distribution of jadeite and kyanite with and without twins or deformation bands respectively has been investigated with a combination of universal stage and ebsd techniques on the scale of a thin section there is no preferred orientation of twinned jadeite and bent kyanite crystals and undeformed crystals respectively thus the orientation of the inferred local shortening direction is random this precludes deformation driven by a homogeneous far field tectonic stress but suggests an internally controlled stress field which is highly inhomogeneous on the scale of a few grain diameters laboratory experiments show that the coesite to quartz transformation proceeds within hours after decompression from 3 0 to 2 7 gpa at 800 c the microstructures of incompletely transformed samples indicate that the quartz growing at the expense of coesite undergoes crystal plastic deformation and recrystallizes with a very fine grain size during transformation in this case the deformation of quartz is attributed to the volumetric strain v 10 inherent in the coesite quartz transformation which causes a highly inhomogeneous stress field inside the sample related to the progress of the transformation we propose that a similar process has taken place in the polyphase natural rock during exhumation with the transient stresses causing mechanical twinning of jadeite and bending or kinking of kyanite when the transformation had gone to completion grain growth obliterated the microstructures of the quartz matrix while the deformed jadeite and kyanite crystals preserved the record of inhomogeneous deformation at high temperatures the peak differential stresses locally exceeded 0 3 gpa which indicates very high strain rates and a correspondingly rapid transformation of coesite to quartz comparable to the laboratory results

the effect of dislocation density on the aqueous solubility of quartz and some geologic implications a theoretical approach the internal energy associated with dislocations is calculated in quartz as a function of dislocation density these calculations show that at dislocation densities of 109 1010 1011 and 1012 cm dislocation cm3 the energy stored per mole of quartz is 2 2 19 7 170 and 1426 j respectively the density of free dislocations in natural quartz rarely exceeds 109 line cm cm3 and the resulting increase in free energy is probably too small to affect any geochemical processes significantly this applies to the overwhelming volume of rocks in the earth s crust the density of dislocations in tangles may exceed 1011 line cm 2 which is significant but the volume of quartz containing tangles is a very small fraction of most quartz grains nevertheless where fluid rock ratios are low the aqueous activity of sio2 may be significantly increased by the dissolution of quartz containing dislocation tangles such dissolution could be geologically significant in active greenschist facies fault zones and mylonites where repeated cold working would produce a sustained source and large population of dislocation tangles in such a setting the locally high internal energies of quartz containing dislocation tangles would lead to strain solution the resulting high activity of aqueous silica would enhance the stability of feldspars and would establish chemical potential gradients of sio2 which would drive the diffusive mass transfer of sio2 out of the fault zone

on the duration of the paleocene eocene thermal maximum petm 1 the paleocene eocene thermal maximum petm is one of the best known examples of a transient climate perturbation associated with a brief but intense interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean atmosphere system one key to quantifying the mass of carbon released identifying the source s and understanding the ultimate fate of this carbon is to develop high resolution age models two independent strategies have been employed cycle stratigraphy and analysis of extraterrestrial helium heet both of which were first tested on ocean drilling program odp site 690 these two methods are in agreement for the onset of the petm and initial recovery or the clay layer main body but seem to differ in the final recovery phase of the event above the clay layer where the carbonate contents rise and carbon isotope values return toward background values here we present a state of the art age model for the petm derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during odp leg 208 walvis ridge southeastern atlantic integrated with published records from site 690 weddell sea southern ocean odp leg 113 during leg 208 five paleocene eocene p e boundary sections sites 1262 to 1267 were recovered in multiple holes over a depth transect of more than 2200 m at the walvis ridge yielding the first stratigraphically complete p e deep sea sequence with moderate to relatively high sedimentation rates 1 to 3 cm ka where a is years a detailed chronology was developed with nondestructive x ray fluorescence xrf core scanning records on the scale of precession cycles with a total duration of the petm now estimated to be 170 ka the revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and heet

north american continental margin records of the paleocene eocene thermal maximum implications for global carbon and hydrological cycling the impacts of the paleocene eocene thermal maximum petm 55 ma one of the most rapid and extreme warming events in earth history are well characterized in open marine and terrestrial environments but are less so on continental margins a major carbon sink here we present stable isotope carbonate content organic matter content and c n ratio records through the petm from new outcrop sections in california and from cores previously drilled on the new jersey margin foraminifer 18o data suggest that midlatitude shelves warmed by a similar magnitude as the open ocean 5 c 8 c while the carbon isotope excursion cie recorded both in carbonate and organic matter 13c records is slightly larger 3 3 4 5 than documented in open ocean records sediment accumulation rates increase dramatically during the cie in marked contrast to the open ocean sites in parallel mass accumulation rates of both organic and inorganic carbon also increased by an order of magnitude the estimated total mass of accumulated carbon in excess of pre cie rates suggests that continental margins at least along north america became carbon sinks during the cie mainly because of weathering feedbacks and rising sea level this result is significant because it implies that the negative feedback role of carbon burial on continental margins was greater than previously recognized

arctic late paleocene early eocene paleoenvironments with special emphasis on the paleocene eocene thermal maximum lomonosov ridge integrated ocean drilling program expedition 302 we reconstruct the latest paleocene and early eocene 57 50 ma environmental trends in the arctic ocean and focus on the paleocene eocene thermal maximum petm 55 ma using strata recovered from the lomonosov ridge by the integrated ocean drilling program expedition 302 the lomonosov ridge was still partially subaerial during the latest paleocene and earliest eocene and gradually subsided during the early eocene organic dinoflagellate cyst dinocyst assemblages point to brackish and productive surface waters throughout the latest paleocene and early eocene dinocyst assemblages are cosmopolitan during this time interval suggesting warm conditions which is corroborated by tex86 reconstructed temperatures of 15 18 c inorganic geochemistry generally reflects reducing conditions within the sediment and euxinic conditions during the upper lower eocene spectral analysis reveals that the cyclicity recorded in x ray fluorescence scanning fe data from close to eocene thermal maximum 2 53 ma presence confirmed by dinocyst stratigraphy is related to precession within the lower part of the petm proxy records indicate enhanced weathering runoff anoxia and productivity along with sea level rise on the basis of total organic carbon content and variations in sediment accumulation rates excess organic carbon burial in the arctic ocean appears to have contributed significantly to the sequestration of injected carbon during the petm

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

mgsio3 phase boundaries measured in the laser heated diamond cell abstract new data on phase equilibria in the mgsio3 system relevant to the stability of mg si perovskite perovskite are obtained in the laser heated diamond cell using improved techniques for accurate pressure measurement and novel heating techniques eliminating temperature gradients the data of the mgsio3 ilmenite akimotoite perovskite phase boundary measured in the range of 19 25 5 gpa and 1800 2350 k are consistent with the pressure and temperature conditions generally assumed at 660 km depth and agree within the experimental uncertainties with earlier multi anvil quench experiments the triple point wadsleyite stishovite akimotoite perovskite is at 21 gpa and 2300 k the slope of the akimotoite perovskite boundary is not well constrained by reversal experiments alone due to large hysteresis but results using glass starting materials yield a value of 0 004 0 002 gpa k the slopes of the phase transitions to mg si perovskite in both olivine and pyroxene systems are less negative than those required to cause layering in mantle convection at the pressure of the 660 km seismic discontinuity 23 8 gpa the phase boundary between akimotoite and perovskite yields a slightly lower temperature than that of the ringwoodite to perovskite periclase transition in the mg2sio4 system 1900 100 k garnet majorite is not observed above 21 gpa

discontinuous grain growth in recrystallised vein quartz implications for grain boundary structure grain boundary mobility crystallographic preferred orientation and stress history abstract grain growth is a process driven by the reduction of interfacial free energy leading to an increase in grain size and a reduction of the number of grains in a given volume of a single phase polycrystalline material discontinuous grain growth dcgg as opposed to normal grain growth ngg involves just a few grains blasts growing at the expense of a matrix that itself undergoes much slower modification in grain size hence producing a bimodal grain size distribution as a transient state prior to impingement of the blasts such a transient state has been found in a quartz layer in metamorphic rocks of the sesia zone western alps there large strain free quartz grains occur that reveal concave outward grain boundaries against a matrix of small grains with a pronounced crystallographic and a weak dimensional preferred orientation the misorientation of the blasts with respect to the matrix grains systematically exceeds 30 this finding suggests that the mobility of high angle grain boundaries in a natural quartzite depends on the degree of their misorientation among other factors as known for metals and alloys the width of the grain boundaries must be sufficiently small to allow for this effect dcgg is promoted by a pronounced crystallographic preferred orientation cpo that leaves only a few grain boundaries with sufficient misorientation to be mobile in contrast to ngg dcgg wipes out any pre existing cpo and information on activated glide systems or kinematics during the preceding stage of flow is lost after dcgg has gone to completion a complementary new cpo could even develop that could be difficult to interpret with respect to tectonic history interfacial free energy driven grain growth reflects a period of low stress annealing and thus constitutes a first order signal

grain growth in synthetic marbles with added mica and water evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder through primary recrystallization and normal grain growth to a size stabilized by second phases to form the marbles reagent grade caco3 was mixed with 0 1 and 5 volume mica and heat treated under pressure with added water densification with negligible recrystallization occurred within one hour at 500 c and 500 mpa confining pressure primary recrystallization occurred at 500 550 c causing increases of grain size of factors of 2 5 resulting samples had uniform grain size gently curved grain boundaries and near equilibrium triple junctions they were used subsequently for normal grain growth studies normal grain growth occurred above 550 c at 800 c grain size d increased from 7 m d0 to 65 m in 24 hours growth rates fit the equation d n d 0 n kt where k is a constant and n 2 6 minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size dmax which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0 3 and 1 once dmax is reached normal growth continues only if second phases are mobile or coarsen or if new driving forces are introduced that cause unpinning of boundaries normal grain growth in solnhofen limestone was significantly slower than in pure synthetic marble suggesting that migration is also inhibited by second phases in the limestone

microstructures of synthetic polycrystalline coesite aggregates the effect of pressure temperature and time the preservation of coesite bearing ultrahigh pressure metamorphic rocks in orogenic belts indicates that continental crust can be buried to a depth exceeding its present thickness the exhumation of these rocks requires large scale flow patterns not yet understood by analogy to the use of flow laws for quartz for the higher crustal levels rheological data and a flow law for coesite are required for the reconstruction and modelling of these important deep processes the first aim of the present study was the synthesis of coesitite samples appropriate for deformation experiments synthesis of coesite from silica glass follows the ostwald step rule with an interface controlled slow second step from metastable quartz tocoesite normal grain growth of coesite crystals is not possible at temperatures up to 1100 c because of the dependence of interfacial energy on crystallographic orientation this energy becomes independent of orientation at 1170 c and a foam microstructure tends to develop allowing for normal grain growth this temperature dependence should also hold true for natural rocks however normal grain growth was not measurably observed in an experiment with 1080 min duration at a temperature of 1170 c consequently the desired grain size of synthetic coesitite samples had to be achieved by controlling the nucleation rate which was successfully done by choosing appropriate p t t paths in the synthesis experiments

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

coesite and quartz characteristic of crystallization from shock produced silica melt in the xiuyan crater article i nfo abundant coesite and quartz coexist with the shock produced silica glasses that retain the morphology of primary quartz in the gneiss clasts of polymict breccia from the xiuyan crater quartz occurs as idiomorphic needle like and spherulitic crystals and polycrystalline aggregates coesite occurs as granular or idiomorphic dendritic and needle like crystals and polycrystalline aggregates of stringers the occurrence and morphology of coesite and quartz characterize crystallization from an undercooled dense silica melt during decompression coesite firstly crystallizes from shock produced dense silica melt at high pressure as pressure decreases to the stability field of quartz quartz crystallizes from the melt whereas coesite remains metastable state a growth rate of 10 3 m s is evaluated for the crystallization of both coesite and quartz based on an estimation of the pressure duration of 10 ms in the coesite stability field for the crater higher pressure in silica melt could play a key role for promoting the rapid growth of coesite and quartz by reducing the viscosity of silica melt and the glass transition temperatures substantially the occurrence of both coesite and quartz embedded in silica glass provides unambiguous evidence for an impact origin of the crater 2010 elsevier b v all rights reserved

the effect of porosity on the strength of quartz aggregates experimentally deformed in the dislocation creep regime abstract the presence of porosity results in significant although transient strengthening of quartz aggregates experimentally deformed in the dislocation creep regime experiments were conducted on four quartz aggregates with porosities of

polycrystalline calcite to aragonite transformation kinetics experiments in synthetic systems the kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates with and without additional minerals present the reaction progresses as a function of time were measured at four temperature pressure conditions 1 550 c 1 86 gpa 2 600 c 2 11 gpa 3 650 c 2 11 gpa and 4 700 c 2 29 gpa experiments reveal that mg calcite and fe calcite transforms to aragonite at considerably slower rates than pure calcite and that sr bearing calcite and calcite quartz aggregates transform at significantly higher rates than pure calcite the reaction progresses vs time data for pure calcite were fitted to cahn s grain boundary nucleation and interface controlled growth model evidence for interface controlled growth is provided by petrographic observations of grain boundaries the activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble the discrepancy in rates and activation energy may be attributed to the nature of grain boundaries to deformational strain or the presence of impurities in the studied samples and likely to uncertainties in experimental conditions the results of this study imply that the variation of local petrologic conditions in addition to temperature pressure and grain size may play an important role in determining the rates for the calcite to aragonite transformation in nature

predictability of the atlantic overturning circulation and associated surface patterns in two ccsm3 climate change ensemble experiments abstractpredictability of the atlantic meridional overturning circulation amoc and associated oceanic and atmospheric fields on decadal time scales in the community climate system model version 3 ccsm3 at t42 resolution is quantified with a 700 yr control run and two 40 member perfect model climate change experiments after taking into account both the mean and spread about the mean of the forecast distributions and allowing for the possibility of time evolving modes the natural variability of the amoc is found to be predictable for about a decade beyond that range the forced predictability resulting from greenhouse gas forcing becomes dominant the upper 500 m temperature in the north atlantic is even more predictable than the amoc by several years this predictability is associated with subsurface and sea surface temperature sst anomalies that propagate in an anticlockwise direction along the subpolar gyre and tend to be prominent during the 10 yr following peaks in the amplitude of amoc anom

a theoretical approach to the kinetics of calcite aragonite transition application to laboratory experiments a theoretical approach to the calcite aragonite transformation is presented for the case of a dislocation glide mechanism obeying first order kinetics the kinetic parameters are determined from recent experimental data from the literature the sensitivity of the model to these and other parameters like the grain size and the deviatoric stresses are discussed the model results in the definition of what could be the actual transformation domains of the caco3 polymorphs in the p t plane these domains are different from the classical ones i e different from the stability fields defined with respect to clapeyron s curve they can evolve in the p t plane when a deviatoric stress is applied to the system in this case we can define a domain in which the two senses of the calcite aragonite transition are activated simultaneously the application of the model to laboratory experiments needs the integration of the kinetic differential equation k varying withp andt using this method we demonstrate the role of p t t path on reaction rate and the role of experimental uncertainties on the reaction parametersk andn finally we discuss the problem of the synthesis of polymorphs out of their stability field and of their formation by grinding these problems difficult to tackle in terms of diffusion can be approached with the proposed dislocation transformation kinetic model

an experimental study into the rheology of synthetic polycrystalline coesite aggregates coesite has been found as a relic in ultrahigh pressure metamorphic uhpm crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at uhpm conditions we performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3 1 to 3 7 gpa temperatures of 700 to 1160 c and strain rates betweeen 6 10 7 and 1 10 3 s 1 the problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress the correlation between the mechanical data and the microstructural record of the deformed samples as a function of temperature and imposed strain rate is consistent with deformation by dislocation creep we deduced parameters of a power law as n 3 1 and q 275 50 kj mol 1 extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures due to the lack of preserved coesite other than as single crystal inclusions nevertheless the extrapolation indicates a low strength of order 10 mpa for natural strain rates at typical uhpm conditions absent deformation of the uhpm brossasco granite dora maira massif western alps thus implies low stresses deformation must have been localized in very weak shear zones during burial and exhumation

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas