coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

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

self induced fracture generation in zircon the mineral zircon has particular geological significance because of its use in the age determination of rocks and as a structural analogue phase for radioactive waste forms despite its widespread utility however radiation damage of the crystal structure metamictization causes a volume expansion of the crystal lattice and the generation of internal stresses which can induce fractures in the crystal a model has been developed which describes the state of stress in n concentric spherical shells each of which may have different material properties the case of a three shelled composite sphere has been examined explicitly in combination with fracture mechanics theory the model predicts that spatially distinct radial and or concentric fracture sets can be produced and such fracture patterns accurately describe the distribution and types of self induced fractures observed in natural zircons in general the formation of such microfractures in zircon is a function of the degree of metamictization shell thickness and the confining pressure the model predicts that the maximum depths in the earth s crust at which radial versus concentric fractures can form in a crystal are different suggesting that the types of fractures found in a suite of zircons from a particular rock might be potential indicators of the depth of burial because metamictization induced fractures may serve as potential pathways for the rapid leaching of various elements from the zircon crystal this may also have important implications in interpreting the u pb ages of fractured zircons or in evaluating the suitability of related crystalline phases as hosts for nuclear waste

raman spectroscopic study of garnet inclusions in diamonds from the mantle transition zone raman spectra of syngenetic majoritic garnets were obtained from the brazilian sno luiz diamond suite these garnets have a low majoritic content 3 have a characteristic signature in the frequency region of the sio4 sio6 stretching vibrations this vibration gives a broad peak between 800 and 900 cm 1 just before in intense band 900 930 cm 1 classically assigned to si o stretching vibrations of the sio4 tetrahedra in all known garnets this broad band is also present in non majoritic garnets si 3 that result front the transformation within the diamonds of majoritic garnets into low pressure garnet pyroxene intergrowths we show that the presence of significant tio2 content in these normal garnets leads to raman spectra mimicking those of majoritic garnets the occurrence of a broad shoulder between 900 and 900 cm 1 as well as broad bands near 960 and 1030 cm 1 in the raman spectra are no longer diagnostic features indicative of a majoritic garnet when the tio2 content or the garnets is high 1 wt as is the case for the sao luiz diamonds the full width at half maximum fwhm of the strong raman mode near 910 cm 1 is the only reliable signature that discriminates si 3 from si 3 garnets the frequency of the diamond raman mode used as an in situ piezometer shows that the actual pressure of the garnet inclusions is between 0 5 and 0 8 gpa

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

tectonic overpressure and underpressure in lithospheric tectonics and metamorphism the lithostatic pressure concept is most commonly applied on a geological scale for lithospheric processes and related evolution of metamorphic rock complexes here various aspects of non lithostatic overpressure and underpressure phenomena in lithospheric tectonics and metamorphism are reviewed on the basis of recently published literature the main conclusion from this short review is that these phenomena certainly exist in nature on all time and space scales including geological ones they are in particular responsible for some geological processes which are otherwise difficult to explain such as downward water suction into the interior of subducting slabs magnitudes of overpressure and underpressure are strongly variable and may potentially reach up to 100 of the lithostatic pressure and up to a gpa level these magnitudes depend mainly on the rheology of deforming rocks and on the nature of related tectonic process rheological heterogeneity of deforming rock units which is common in nature has a tendency to enhance overpressure and underpressures large overpressure can typically be expected in rheologically strong dry bending rock units in particular in the mantle lithosphere however rheological weakness of rocks and small local deviatoric stresses do not guarantee the absence of large overpressures in these rocks therefore the influence of significant tectonic overpressure and or underpressure cannot be excluded for any metamorphic complex a priori but should be instead tested by exploring realistic thermomechanical models for envisaged tectono metamorphic scenarios many lithospheric rocks subjected to large overpressures and underpressures cannot be studied as they do not exhume to the surface some controversy exists concerning overpressure magnitudes for the ultrahigh pressure uhp rocks and several conflicting hypotheses are proposed which need to be thoroughly tested in the future in this respect the alpine region may offer a unique opportunity for the testing of geological scale overpressures in u hp rocks by combining structural geological and petrological data with realistic lithospheric scale numerical modelling

evolution of host inclusion systems a visco elastic model a visco elastic model is suggested to recover the p t t paths of spherically symmetrical host inclusion systems from residual pressure patterns for a coesite garnet aragonite garnet and diamond garnet system with partial transition of high pressure polymorphs to low pressure ones the model predicts that the exhumation history of ultra high pressure metamorphic rocks from depths below 90 km began with pressure decrease to 1 gpa 40 km depth while the temperature remained high 800 c then the system cooled and decompressed to ambient conditions non spherical inclusion shapes in the 3 d model lead to heterogeneities in residual pressure and shear stress distributions as well as to significant gradients of shear stress which may affect the kinetics and thermodynamics of phase transformation in the inclusions

contrasted metamorphic evolutions in thrusted cover units of the brianconnais zone french alps a model for the conservation of hp lt metamorphic mineral assemblages the evolution of organic matter silicate and fluid phases in cover units of the three structural zones of the vanoise area allows one to distinguish differentp t cooling paths these paths depend on whether the cover has been transported western and northern vanoise or not southern vanoise all units first underwent a common high pressure low temperature hp lt metamorphic stage 300 c 6 kbar of blueschist type fe mg carpholite chloritoid facies the cover units transported on the extermal colder zone coal measure series metamorphosed in albite chlorite facies preserved their hp mineralogy fe mg carpholite lawsonite and organic matter content oils wet gases and kerogen while the unit which remained in contact with its more thermally conductive basement polymetamorphic now shows extensive greenschist facies overprinting breakdown of fe mg carpholite and lawsonite appearance of chlorite pyrophyllite chloritoid and clinozoisite absence of oils and wet gases these two tectonic evolutions might serve as a more general model for the preservation of hp lt mineral assemblages

diamond graphite relationships in ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan graphite inclusions in diamond k bearing clinopyroxene garnet and kyanite and graphite coatings around microdiamonds from the ultrahigh pressure kokchetav massif northern kazakhstan were investigated by means of scanning electron microscopy laser raman spectroscopy and x ray diffraction important observations include the increasing size of the graphite crystals from the diamond graphite interface to the external part of the graphite coatings the high volume fraction of graphite in the graphite coatings the presence of composite crystals of successively formed diamond core graphite mantle zone diamond rim external graphite coating the spherulitic texture of the graphite coatings the formation of both ordered and disordered graphite and the absence of any fluid associated with the graphite inclusions in diamond and other ultrahigh pressure metamorphic uhpm minerals these features combined with the presence of oriented graphite flakes in k bearing clinopyroxene and in diamond bearing garnet and kyanite and the presence of intergranular diamonds without graphite coatings in the quartzo feldspathic matrix require reconsideration of the generally accepted diamond graphitization model for interpreting the formation of graphite in uhpm rocks metastable growth of graphite within the diamond stability field is suggested and the following crystallization scheme for the c polymorph is proposed graphite was the first c polymorph formed in the diamond stability field followed by diamond the graphite coatings around diamond formed during the final stage of uhpm conditions

mineral and fluid inclusions in zircon of uhp metamorphic rocks from the ccsd main drill hole a record of metamorphism and fluid activity abstract the chinese continental scientific drilling ccsd main drill hole 0 3000 m in donghai southern sulu orogen consists of eclogite paragneiss orthogneiss schist and garnet peridotite detailed investigations of raman cathodoluminescence and microprobe analyses show that zircons from most eclogites gneisses and schists have oscillatory zoned magmatic cores with low pressure mineral inclusions of qtz pl kf and ap and a metamorphic rim with relatively uniform luminescence and eclogite facies mineral inclusions of grt omp phn coe and rt the chemical compositions of the uhp metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks similar uhp metamorphic p t conditions of about 770 c and 32 kbar were estimated from coexisting minerals in zircon and in the matrix these observations suggest that all investigated lithologies experienced a joint in situ uhp metamorphism during continental deep subduction in rare cases magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence implying that the cores have been largely altered possibly by fluid mineral interaction during uhp metamorphism abundant h 2 o co 2 h 2 o or co 2 dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons coexisting with low p mineral inclusions these fluid inclusions should have been trapped during magmatic crystallization and thus as primary only few h 2 o and or co 2 dominated fluid inclusions were found to occur together with uhp mineral inclusions in zircons of metamorphic origin indicating that uhp metamorphism occurred under relatively dry conditions the diversity in fluid inclusion populations in uhp rocks from different depths suggests a closed fluid system without large scale fluid migration during subduction and exhumation

fluid melt rock interaction in mafic eclogites and coesite bearing metasediments constraints on volatile recycling during subduction the question as to whether the metasomatizing slab agent needed to form island arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid melt rock interaction during high pressure low temperature metamorphism in the western alps typical anhydrous mafic eclogites monviso massif and coesite bearing metasediments dora maira massif have experienced a prograde p t path characteristic of a mature cold subduction zone microstructural petrologic geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high grade blueschist facies metamorphism most of the fluids were driven off the rocks at pressures lower than 1 and 1 6 gpa respectively with increasing pressure and temperature the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins partial melts or dense hydrous silicates tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments kmash system in subduction zones in contrast to the basalt and peridotite systems the kmash system is characterized by a wide range of pressure sensitive reactions which allow the reconstruction of dehydration hydration depths in the subducted slab independently of the thermal structure of subduction zones terrigenous sediments will begin to melt at pressures 2 gpa the melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids dense hydrous silicates will form in any subducted sediments at all depths these silicates can store significant amount of water in their structure in the subducted oceanic crust and upper mantle heterogeneous distribution of fluids inherited from hydrothermal alteration at mid ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large scale fluid infiltration in the overlying mantle wedge the situation where a cold competent and fluid rich subducted slab is bounded above by a hot relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards at sub arc depths 50 10 to 80150 km and for any geothermal gradient continuous internal buffering of volatile activities will retain the fluid phase in the slab this is in contrast with shallow sub forearc depths 20 to 50 10 km where tectonic vein arrays localized thrust faults serpentine diapirs mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge

paragonite eclogites from dabie shan china re equilibration during exhumation paragonite in textural equilibrium with garnet omphacite and kyanite is found in two eclogites in the ultrahigh pressure metamorphic terrane in dabie shan china equilibrium reactions between paragonite omphacite and kyanite indicate a pressure of about 19 kbar at c 700 c however one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet suggesting minimum pressures of 27 kbar at the same temperature the disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh pressure assemblages this hypothesis is tested by calibrating a garnet zoisite clinozoisite kyanite quartz coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh and high pressure terranes these four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite clinozoisite grossular kyanite quartz coesite h o the results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts for the paragonite eclogites in dabie shan the geobarometer indicates pressures in the quartz stability field confirming that the former coesite bearing paragonite eclogite has re equilibrated at lower pressures on the other hand garnets from other coesite bearing but paragonite free kyanite zoisite eclogites show a very wide variation in grossular content corresponding to a pressure variation from coesite into the quartz field this wide variation partly due to a rimward decrease in grossular component in garnet is caused by partial equilibration of the mineral assemblage during the exhumation

the effect of strain energy on growth rates during the olivine spinel transformation and implications for olivine metastability in subducting slabs we have studied growth kinetics during the transformation of mg1 8fe0 2sio4 san carlos olivine to its high pressure polymorphs wadsleyite phase and ringwoodite phase at 800 1200 c and nominal pressures of 16 20 gpa in experiments in which a large 500 600 m olivine single crystal contained in a matrix of either fine grained olivine or nacl was transformed reaction rims of wadsleyite ringwoodite form on the margins of the single crystal by incoherent grain boundary nucleation and interface controlled growth contrary to theoretical expectations the growth rate of these reaction rims decreases sharply as a function of time for instance at 1100 c and 18 gpa growth ceases on an experimental timescale after the rim width reaches 20 25 m in order to explain this observation we develop an elastic model based on the theory of a misfitting inclusion comparing the results of this model with the experimental data suggests that elastic strain energy which develops because of the large volume decrease associated with the transformation is responsible for the decreasing growth rates on the other hand experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form this is the case when the nucleation rate is low the growth rate is fast and the reactant olivine is fine grained on a geological timescale in subducting lithosphere where the grain size of the olivine is large the growth rates of grain boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation therefore current kinetic models of olivine metastability in subducting slabs which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure need to be reevaluated

modeling of radial microcracks at corners of inclusions in garnet using fracture mechanics radial microcracks around inclusions in garnet are important indicators of ultrahigh pressure metamorphism and have been used to interpret changes in pressure as a function of temperature during decompression of metamorphic terrains we investigate the growth of microcracks from inclusion corners using a two dimensional fracture mechanics based boundary element method code and propose that microcracks emanating from inclusions are primarily controlled by inclusion shape and size and do not require high confining lithostatic pressure or significant decompression to initiate results suggest that inclusions 100 m diameter develop microcracks during modest decompression the magnitude of decompression required to initiate microcracks decreases with increased degree of faceting and increased inclusion size the potential for crack growth at each corner depends on the corner sharpness and the overall shape of the inclusion we integrate model results with petrographic observations to understand mechanisms and conditions of garnet microcracking during regional metamorphism

thickening history of the western alps a model of evolution for the western segment of the alpine chain is presented available data concerning the structural metamorphic and geochronological evolution in the majorunits involved in the thickening processes are takeninto account io constrain the model the successive stages of the collisional history are correlated with the evolution of thermal structure the african paleomargin was deformed at the onset of collision just after the abduction of the oceanic crust after these initial stages progressive stacking affected more and more external domains of the european plate lastly the deformation of the entire pile is linked io the thermal reequilibration of the considered thickened crust the alpine chain is a good example of a collision zone where the interaction of thermal and mechanical effects can be treated quantitatively

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

detrital heavy mineral constraints on the triassic tectonic evolution of the west qinling terrane nw china implications for understanding subduction of the paleotethyan ocean we have examined the triassic sediments in the west qinling terrane northeastern tibet these sediments consist mainly of flysch and shallow sea and fluvial deposits with abundant lithic and heavy mineral detritus sandwiched between and overlying late paleozoic and early middle triassic ophiolitic melanges volcanic and metamorphic detritus dominates the lithic component of lower triassic sandstones accompanied by high cr spinel pyroxene and magnetite contents indicating a mixed ophiolite and metamorphic source detrital mineral geochemistry further suggests that ophiolitic high grade metamorphic basic and intermediate acidic igneous rocks must have been exposed and deeply eroded in their source area abundances of zircon rutile garnet tourmaline and epidote are greater in the middle triassic samples and granitic and volcanic sources are the major contributors of detrital clasts considering these new observations on sedimentary petrography and detrital heavy mineral geochemistry along with published data on paleocurrents detrital zircon u pb ages sedimentary facies and regional magmatism we suggest that these triassic sediments represent the sedimentary fill of a forearc basin that overlies a late paleozoic ophiolitic complex a south facing andean type convergent continental margin system developed along the southern margin of the north china block during the triassic in response to northward subduction of the paleotethyan ocean

the variscan french massif central a new addition to the ultra high pressure metamorphic club exhumation processes and geodynamic consequences abstract this paper presents the first documentation of coesite bearing eclogites in the eastern french massif central monts du lyonnais unit and discusses the exhumation processes for these very high pressure rocks a combination of mineralogical and geochronological datasets allows us to quantify a depth time path and related exhumation rates high pressure metamorphism is constrained to 400 420 ma minimum 28 kbar or ca 90 km by 360 380 ma the rocks were exhumed to 30 km depth these kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern french massif central these multidisciplinary approaches provide new information on paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks we suggest that a significant amount of exhumation of these rocks occurred during subduction prior to continental collision continental collision itself was responsible only for the final stages of exhumation under a transpressive regime

comment on first evidence for ultrahigh pressure metamorphism of eclogites in pohorje slovenia tracing deep continental subduction in the eastern alps by marian janak et al

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

how long to oceanic tracer and proxy equilibrium abstract the various time scales for distribution of tracers and proxies in the global ocean are critical to the interpretation of data from deep sea cores to obtain some basic physical insight into their behavior a global ocean circulation model forced to least square consistency with modern data is used to find lower bounds for the time taken by surface injected passive tracers to reach equilibrium depending upon the geographical scope of the injection major gradients exist laterally between the abyssal north atlantic and north pacific and vertically over much of the ocean persisting for periods longer than 2000 years and with magnitudes bearing little or no relation to radiocarbon ages the relative vigor of the north atlantic convective process means that tracer events originating far from that location at the sea surface will tend to display abyssal signatures there first possibly leading to misinterpretation of the event location ice volume glacio eustatic corrections to deep sea 18 o values involving fresh water addition or subtraction regionally at the sea surface cannot be assumed to be close to instantaneous in the global ocean and must be determined quantitatively by modelling the flow and by including numerous more complex dynamical interactions

continental collision crustal thinning and nappe forming during the pre miocene evolution of the alpujarride complex alboran domain betics the tectono metamorphic evolution of the alpujarride complex within the internal zones of the gibraltar arc indicates that the alboran crustal domain underwent several contractional and extensional events the superposition of structures and the metamorphic evolution observed in the alpujarride complex and in particular in the adra unit suggest that these events are as follows a the first thickening event d1 results from continental subduction and is evidenced by the presence of pre s2 high pressure mineral relicts b the thinning and associated vertical shortening event d2 is indicated by an almost isothermal decompression during which the s2 main foliation developed c kilometric scale n vergent recumbent folds of the condensed metamorphic sequences and thrust and nappe forming in the alpujarride complex suggest that a second thickening event d3 took place d the second thinning event d4 which affects the whole alboran domain is evidenced by extensional fault systems resulting in the opening of the alboran sea the oldest synrift deposits are of the late oligocene and early miocene suggesting that d1 to d3 events are pre miocene and e from the late tortonian to the pliocene the extensional systems were folded due to a continuous n s to nw se compression

the stacking of thrust slices in collision zones and its thermal consequences the thermal consequences of stacking up thrust slices in collision zones are investigated using simple one dimensional thermal models the metamorphic evolution geotherms and p t time paths of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects cooling and pressure decrease associated with erosion cooling by the lowest units screen effect heating by the upper units cover effect it is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone the metamorphic evolutions are very sensitive to the following tectonic parameters the number of thrusted units involved in thickening the time delay between each thrust and finally the mode of stacking of the different units over and understacking it appears that for a given depth of burial the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units the screen effect during understacking is more efficient for a short time delay 10 ma between each thrust overstacking leads to higher temperatures before uplift when compared to understacking it is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small 10 km it is shown that for an erosion controlled uplift the shape of the p t t path depends on the position of the rocks within a given unit and on the unit position within the pile finally the general metamorphic evolutions during crustal thickening hp lt metamorphism and subsequent overprint are discussed in terms of the previously mentioned parameters in the case of the western alps it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

coesite in the straumen kyanite eclogite pod norway the occurrence of coesite in the straumen kyaniteeclogite pod in the western gneiss region wgr norway is described although the microtextures of coesite and of its partial or complete transformation into quartz are similar to those described at the first coesite occurrence in norway the grytting dolomite eclogite pod there are some significant differences at this second locality these include the host mineral of the sio2 inclusions being garnet instead of clinopyroxene and also occasionally kyanite an initial coesite eclogite sub facies paragenesis was followed by a quartz eclogite sub facies paragenesis which in addition to rutile and dolomite or calcite contains the highest number 7 of silicate phases known in any norwegian eclogite garnet clinopyroxene kyanite quartz phengite zoisite and clinoamphibole this second occurrence of definite coesite i consolidates the very high pressury estimates about 30 kbar minimum previously deduced for certain eclogite pods occurring in the wgr and ii justifies the recognition of the norwegian coesite eclogite province constituted of a nappe terrane of about 30 kbar parageneses now residing in the wgr

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes