trace element zoning in garnet as a monitor of crustal melting x ray composition maps of sc y p and cr in garnet have been measured using the electron microprobe zoning is discontinuous and is correlated with fluid absent crustal melting reactions the magnitude of the discontinuity is a function of the amount of muscovite that melts and the partition coefficients of solids versus melt trace element zoning is apparently not modified by diffusive processes so the zoning can be used to monitor the extent of melting and to examine kinetics of melting and the participation of accessory phases in crustal anatexis

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

abandoned penguin rookeries as holocene paleoclimatic indicators in antarctica penguins are sensitive indicators of the antarctic climate and of the environmental parameters that limit their presence and distribution paleoenvironmental data obtained from the study of abandoned penguin rookeries pygoscelis adeliae along the victoria land coast in antarctica indicate 14c date of 11 13 ka for the oldest abandoned rookery and supply new information about the timing of glacier retreat in southern victoria land after the last glacial maximum the continuous presence of the adelie penguins is documented from 7 ka according to our data the limiting factors that control the presence of penguins along the coast of victoria land changed during the holocene whereas several colonies were occupied for very long periods other sites were used for more or less extended periods and then abandoned the greatest diffusion of rookeries occurred between 3 and 4 ka a period of particularly favorable environmental conditions that has never been repeated it was followed by a sudden decrease in the number of penguin rookeries shortly after 3 ka this event has been attributed to an increase of the sea ice extension and may have been correlated to a worldwide phase of climate change near the subboreal subatlantic boundary a minor phase of penguin reoccupation occurred locally in the eighth to fourteenth centuries a d because the presence and number of penguins reflect the state of health of the antarctic marine ecosystem it is important to evaluate the variations in their distribution in the past in the absence of human induced changes

analysis of experimental data on divalent cation diffusion kinetics in aluminosilicate garnets with application to timescales of peak barrovian metamorphism scotland we present a new statistical framework to analyze the diffusion data for divalent cations fe mg mn and ca in aluminosilicate garnet using published experimental data and an arrhenius relationship that accounts for dependence on temperature pressure garnet unit cell dimension and oxygen fugacity the regression is based on bayesian statistics and is implemented by the markov chain monte carlo approach all reported experimental uncertainties are incorporated and the data are weighted by the precision of the experimental conditions we also include a new term the inter experiment bias to compensate for possible inconsistencies among experiments and to represent any experimental variability not explicitly presented in the arrhenius relationship e g water content defect density at high temperatures where most experiments were conducted the diffusion coefficients calculated with the new parameters agree well with previous diffusion models e g chakraborty and ganguly in contrib mineral petrol 111 74 86 1992 carlson in am mineral 91 1 11 2006 however the down temperature extrapolation leads to notable differences at lower temperatures for common petrological applications for example at 600 c the diffusion coefficients of fe and mn are one half to one order of magnitude faster and the diffusion coefficient of ca is about one order of magnitude slower than calculated with parameters in carlson 2006 our statistical analysis also provides well defined uncertainty bounds for timescale estimates based on garnet diffusion profiles application of the newly derived coefficients indicates that the timescale of the thermal peak of barrovian metamorphism dalradian belt of scotland is about four to seven times longer than that estimated using previously published diffusion coefficients the peak is still geologically brief however of the order of 106 years 0 75 myr 0 70 0 36 myr 1 this brevity requires pulsed advective heat input as provided by syn orogenic mafic magmatism in these rocks

interface coupled dissolution reprecipitation in garnet from subducted granulites and ultrahigh pressure rocks revealed by phosphorous sodium and titanium zonation garnet zonation provides an unparalleled record of the pressure temperature time fluid evolution of metamorphic rocks at extreme temperature conditions 900 c however most elements preserve little zonation due to intracrystalline diffusional relaxation under these conditions slowly diffusing trace elements including p na and ti have the best chance of recording metamorphic histories here we map dramatic zoning patterns of these elements in subducted high pressure felsic granulite saxon granulite massif and ultrahigh pressure diamondiferous saidenbachite saxonian erzgebirge bohemian massif the results show that garnet replacement via interface coupled dissolution reprecipitation can strongly affect garnet compositions in subduction zones and that p na and ti record burial and exhumation histories that are otherwise lost to diffusion in these samples p diffuses the slowest and ti the fastest

palaeotemperatures still exist in the greenland ice sheet the temperature distribution through the greenland ice sheet at the dye 3 borehole is a record of the past climatic changes in the arctic the numerical model of the temperature distribution now presented reproduces the observed temperature distribution within 0 03 k and shows that the basal ice is still cooled 5 k by the cold ice age climate the results suggest a mean ice age temperature of 32 2 c which is 12 k colder than the present temperature and a precipitation rate 50 25 of the present rate calculations of a more detailed temperature history through the present inter glacial period reveal evidence of the ad 1920 50 maximum the little ice age and the atlantic period

locking depth slip rate and seismicity distribution along the daofu kangding segment of the xianshuihe fault system eastern tibetan plateau abstract in this study 1338 relocated earthquakes ms 1 6 from 1981 to 2018 and 53 gps velocities from 2004 to 2017 were processed to obtain the geometry and kinematic characteristics of the daofu kangding segment of the xianshuihe fault system in southwestern china by analyzing the seismicity depth distribution of the selaha kangding fault we inferred its three branches the yalahe fault selaha fault and zheduotang fault converged into one fault at a depth of approximately 15 km and that a flower shaped structure was observed above this depth furthermore based on a 2d elastic dislocation model the creep rates creep depths slip rates and locking depths of the daofu kangding segment were analyzed in case of the daofu qianning fault the obtained left lateral slip rate was 4 5 1 1 mm a and the locking depth was 5 3 1 9 km because the fault creep rate from a depth of 2 6 km to the surface was 1 3 1 0 mm a the daofu qianning fault cannot generate earthquakes having magnitudes of greater than 7 in the near future however the slip rates were 7 5 1 6 2 3 1 5 and 1 9 1 5 mm a by assuming a locking depth of 15 km for the selaha fault yalahe fault and zheduotang fault respectively furthermore because the creep depth of the selaha fault was 3 4 1 6 km and the creep rate was 7 5 2 8 mm a the accumulated seismic moment since 1725 corresponded to a mw7 1 earthquake

site information and initial results from deep ice drilling on law dome antarctica the aim of deep ice drilling on law dome antarctica has been to exploit the special characteristics of law dome summit i e low temperature and high accumulation near an ice divide to obtain a high resolution ice core for climatic environmental studies of the holocene and the last glacial maximum lgm drilling was completed in february 1993 when basal ice containing small fragments of rock was reached at a depth of 1196 m accurate ice dating obtained by counting annual layers revealed by fine detail 18 o peroxide and electrical conductivity measurements is continuous down to 399 m corresponding to a date of ad 1304 sulphate concentration measurements made around depths where conductivity tracing indicates volcanic fallout allow confirmation of the dating for agung in 1963 and tambora in 1815 or estimates of the eruption date from the ice dating for the kuwae vanuatu eruption 1457 the lower part of the core is dated by extrapolating the layer counting using a simple model of the ice flow at the lgm ice fabric measurements show a large decrease 250 to 14 mm 2 in crystal size and a narrow maximum in c axis verticality the main zone of strong single pole fabrics however is located higher up in a broad zone around 900 m oxygen isotope 18 o measurements show holocene ice down to 1113 m the lgm at 1133 m and warm 18 o about the same as holocene ice near the base of the ice sheet the lgm holocene 18 o shift of 7 0 only 1 larger than for vostok indicates that law dome remained an independent ice cap and was not overridden by the inland ice sheet in the glacial

timescales of exhumation and cooling inferred by kinetic modeling an example using a lamellar garnet pyroxenite from the variscan granulitgebirge germany we present a numerical modeling approach to infer timescales of the exhumation and cooling history recorded in the chemical composition of minerals in a garnet pyroxenite from the granulitgebirge saxony germany the studied sample contains remarkable exsolution textures from former megacrysts that produced up to millimeter wide alternating lamellae of garnet grt and clinopyroxene cpx compositional profiles of major and minor elements measured with the electron microprobe perpendicular to the grt cpx interfaces reveal systematic zoning patterns for fe mg al si cr ti in clinopyroxene and ca fe mg mn in garnet respectively in addition to simple thermal modeling that is used to constrain the conditions of emplacement of the granulitgebirge massif at shallow crustal levels we combine thermodynamic data with a numerical finite difference scheme that simulates growth and simultaneous diffusive exchange between garnet and clinopyroxene along a virtual cooling path the latter model assumes local equilibrium at the interface diffusive fluxes are constrained by mass balance it is shown that zoning patterns such as fe mg exchange between garnet and clinopyroxene can be used to extract cooling rates and thus timescales of exhumation while the profiles for the minor elements are provisionally related to the growth history of the lamellae furthermore zoning profiles in the lamellae can only be reproduced with ultrahigh cooling rates similar to contact metamorphic conditions this in turn suggests that the massif was emplaced at temperatures above 900 c in agreement with the observed spatial extent of a contact aureole within low grade metasedimentary rocks surrounding the granulite massif as predicted by thermal modeling exhumation of the massif without cooling below 900 c requires an exhumation rate of several centimeters per year thus we propose an almost isothermal exhumation period of 1 ma followed by isobaric cooling from 900 to 600 c within less than 10 ka

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

precipitation of rutile and ilmenite needles in garnet implications for extreme metamorphic conditions in the acadian orogen u s a we report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the merrimack synclinorium connecticut and present a precipitation model for their origin the rocks were strongly metamorphosed and deformed during the devonian acadian orogeny the needles are primarily elongated parallel to in garnet rutile has anomalous extinction angles as great as 35 cf griffin et al 1971 rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long other oxide inclusions that may be present include submicrometer to micrometer scale twinned rutile bicrystals as well as srilankite and a crichtonite group mineral some garnet cores have unusual box shaped quartz inclusions which coexist with ti fe oxide needles and commonly contain micrometer scale rods of f oh cl apatite negative garnet crystal pores are also widespread ti fe oxide needles are restricted to garnet core regions rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite spinel cordierite and k feldspar consequently the garnet core regions represent an earlier distinct period of growth relative to the rims garnet cores contain 25 35 pyrope and a host of minor and trace constituents including tio2 0 07 0 6 wt cr2o3 0 01 0 10 wt na2o 0 01 0 03 wt p2o5 0 01 0 09 wt and zro2 up to 150 ppm na2o and zro2 correlate positively with tio2 titanium zoning is preserved in some garnets zoning profiles and two dimensional chemical mapping show that ti and to a lesser degree cr are depleted around ti fe oxide inclusions therefore we conclude that the needles are precipitates that formed from ti bearing garnet during exhumation and cooling garnet contained sufficient ti to form precipitates no ti source external to garnet was necessary titanium bearing garnets that contain oriented ti fe oxide needles are known primarily from ultrahigh pressure metamorphic rocks mantle peridotites and pyroxenites and high pressure granulites thus the presence of needle bearing garnets in connecticut strongly suggests that a previously unrecognized domain of extreme pressure and or temperature metamorphism exists in the acadian orogen

nanoscale processes of trace element mobility in metamorphosed zircon several examples of zircon grains from high to ultrahigh pressure uhp and ultrahigh temperature uht metapelites exhibit a characteristic yet atypical core rim interface domain 5 m wide observed in cathodoluminescence cl imaging the interface domain is located immediately against the magmatic core and is comprised of an irregular 0 2 m wide cl dark domain that is rimmed by a complex 0 5 m wide cl bright domain with cuspate margins the outer margin of the interface domain is rimmed by intermediate cl zircon with low contrast zoning to characterize the nature of the interface domain and to identify mechanisms of trace element mobility in metamorphosed zircon we analyzed several specimens prepared from zircon from the rhodope metamorphic complex eastern greece and the goshen dome western massachusetts usa via atom probe tomography apt the data reveal three types of geochemical anomalies each with a unique morphology 1 toroidal clusters with high concentrations of pb y al are found exclusively within the core of the rhodope grain these clusters are interpreted as decorated dislocation loops that formed during metamorphism and annealing of radiation damage to the lattice geochronological and geochemical data support this interpretation 2 complex cross cutting planar and linear features with anomalous concentrations of y p yb or u are spatially restricted to the core rim interface domain these features do not correlate with inherited geochemical variation oscillatory zoning or deformation induced microstructures instead the planar features likely formed in response to an interface coupled dissolution reprecipitation reaction that propagated into the crystal during metamorphism the observed cross cutting relationships are the product of either multiple events or complexity of the process that originally formed the domains 3 ellipsoidal features with high concentrations of y p yb al are found exclusively within the high y p yb planar features these features are interpreted as the product of spinodal decomposition that occurred during exhumation as the zircon passed the solvus where local equilibria favored nm scale exsolution to minimize the gibbs free energy the presence of multiple types of geochemical features in these examples indicates that trace element mobility in zircon is driven by multiple processes over the course of orogenesis given that these atypical domains are apparently restricted to zircon metamorphosed at uht and u hp conditions their presence may represent a marker of metamorphism at very high grade conditions

decrepitated uhp fluid inclusions about diverse phase assemblages and extreme decompression rates erzgebirge germany minute polyphase inclusions in garnet of quartzofeldspathic rocks saidenbachite from the saxonian erzgebirge germany contain microdiamond or graphite phlogopite quartz paragonite phengite and other minerals in minor amounts these inclusions are interpreted to represent an original dense coh silicate fluid trapped in crystallizing garnet at depths of 150 km inspection of the inclusion population in a single garnet by sem reveals two characteristic features i the shape of most inclusions indicates healed radial cracks in the host garnet and thus the buildup of a significant differential pressure p i e a contrast in pressure between the inclusion pi and the host mineral pe the mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of 750 50 c and pressures below 2 5 gpa ii the diverse types of inclusions appear to be randomly distributed in the garnet host thus the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet combining observations i and ii we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times and thus of distinct histories of pi as p at decrepitation is primarily controlled by inclusion size and shape applying a flow law for dislocation creep of garnet a low strength of garnet at 750 50 c for low geological strain rates is predicted thus differential pressure should have been kept low i e pi pe by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks to attain the differential pressure pi pe required for catastrophic brittle failure of the garnet host the decompression rate must have been extremely high as a robust lower bound a minimum exhumation rate on the order of 100 m year 1 is suggested which corresponds to ascent rates of magma

a garnet zircon oxygen isotope record of subduction and exhumation fluids from the franciscan complex california tectonic blocks of high grade metamorphic rock hosted by the franciscan me lange commonly preserve textural and geochemical evidence for metasomatism and blueschist facies overprinting within a subduction zone these exhumation events can obscure information on the nature of subduction fluids at depth that might otherwise be preserved by these rocks in this tectonic setting cation and oxygen isotope zoning in garnets from eclogite and garnet hornblende rocks with minimal textural evidence of retrogression reveals separate prograde and retrograde fluid histories garnet cores preserve a homogeneous prograde oxygen isotope history with both high do 11o and low 4o values in different samples probably recording altered ocean crust protolith compositions multiple episodes of garnet resorption and regrowth record a dramatically changing bulk oxygen isotope ratio resulting in 6 7o neoformed garnet rims with sharp core rim boundaries in both samples matrix omphacite and hornblende are in profound oxygen isotope disequilibrium with garnet cores but are in equilibrium with garnet rims despite the appearance of overall textural equilibrium in thin section this cryptic metasomatism found in samples from different locations within the franciscan complex brings into question existing thermobarometry on non inclusion phases for this complex as well as interpretations based on whole rock geochemical data zircon ages and oxygen isotope equilibrium with late forming garnet are consistent with postmetasomatic formation of zircon the ability of garnet and zircon to record multiple fluid events and tie them to p t t fluid history is a powerful tool in gaining an improved understanding of the complex fluid environments within subduction zones

trace element mapping by la icp ms assessing geochemical mobility in garnet a persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements particularly the rare earth elements rees this is despite the fact that trace elements are generally less vulnerable to diffusive resetting and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage including the growth and or dissolution of accessory minerals we outline a technique for the routine acquisition of high resolution 2d trace element maps by la icp ms and introduce an extension of the software package xmaptools for rapid processing of la icp ms data to visualise and interpret compositional zoning patterns these methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet which are argued to be largely dependent on the interplay between element fractionation mineral reactions and partitioning and the length scales of intergranular transport samples from the peaked hill shear zone reynolds range central australia exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing u pb monazite and sm nd garnet geochronology ultimately the remarkable preservation of original growth zoning and its subtle modification by subsequent re equilibration is used to see through multiple superimposed events thereby revealing a previously obscure petrological and temporal record of metamorphism metasomatism and deformation

holocene deglaciation sea level change and the emergence of the windmill islands budd coast antarctica a holocene deglaciation sequence for the windmill islands was determined from the 14c age of raised marine shorelines lakebottom sediments and adelie penguin remains found in abandoned rookeries a north south gradient in the elevation of the upper marine limit was observed with the highest marine limit 31 32 m observed on browning peninsula and hull island at the southern edge of the islands correspondingly the southern islands were found to have been deglaciated by 8000 corr yr b p while the northern islands were deglaciated by 5500 corr yr b p isostatic uplift rates were calculated as 0 5 to 0 6 m 100 yr with an estimated total uplift of around 53 m which indicates late pleistocene ice sheet thicknesses of 200 and 400 m over the islands and adjacent petersen bank respectively the margin of the late pleistocene grounded ice sheet extended an estimated 8 15 km offshore which coincides with the location of the 200 m isobath

garnets as open systems during regional metamorphism if a garnet fractures during metamorphism its interior will be in chemical communication with matrix phases solids and fluids and may participate in late prograde or retrograde metamorphic reactions the core of a growth zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally in the vicinity of fractures or extensively resulting in dissolution and reprecipitation of the garnet interior and or production of new phases the results of petrologic techniques such as thermobarometry pressure temperature path calculations and geochronometry may be misinterpreted if sites of garnet and mineral inclusion compositional modification are not identified brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions the conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure retrograde pressure temperature path and thermoelastic properties of the inclusion and garnet inclusion induced fracturing is favored by nearly isothermal decompression a common metamorphic pressure temperature path other fractures may form as a result of propagation of an inclusion host boundary or by mechanisms external to the garnet

textures and high field strength elements in hydrothermal magnetite from a skarn system implications for coupled dissolution reprecipitation reactions there is increasing evidence to suggest that high field strength elements hfse could be mobile to some extent in hydrothermal fluid due to the influence of halogens e g fluorine and chlorine however in natural hydrothermal fluid systems coupled dissolution reprecipitation cdr reactions at fluid mineral interfaces that have been emphasized in the past decade may play a key role in controlling the final textures and mineral assemblages the influences of the cdr reactions in hydrothermal systems on hfse enrichment or depletion at the mineral scale are enigmatic in this study we show that enrichment of nb and zr can occur in magnetite on the mineral scale formed by hydrothermal fluids at medium to lower temperature in a skarn system four stages of mineralization and alteration of magnetite have been identified in the baishiya iron skarn deposit of the east kunlun mountains of china magnetite formed in stage 1 s1 developed obvious oscillatory zonation whereas that formed in stages 2 s2 and 3 s3 shows hydrothermal alteration and metasomatic textures and that in stage 4 s4 developed euhedral crystals with simple zoning systematic variations in the trace element compositions of different magnetite grains analyzed by empa and la icp ms suggest that the magnetite from s1 to s3 may have formed in a metasomatic process at relatively constant temperature whereas the magnetite from s4 formed by re equilibrium processes at lower temperature the magnetite from each stage can be divided into light and dark domains based on backscattered electron images the dark domains in the magnetite from s1 and s2 have higher nb ta 8 52 27 00 and zr hf 18 22 52 64 ratios and silicon contents than the light domains 0 55 5 66 and 2 54 16 43 respectively compared with other magnetite ores the ores from s1 and s2 are depleted of v and ni this depletion may be induced by increased oxygen and co crystallized sulfide however these variations are unlikely to be responsible for the enrichment of nb and zr in magnetite at equilibrium conditions conversely the dark domains of the magnetite from s1 and s2 are porous irregular and or oscillatory with quartz inclusions indicating nonequilibrium conditions these textural features could be attributed to the cdr reactions that are ubiquitous in skarn systems the increased silicon concentrations in magnetite due to the cdr reactions could affect the lattice parameters of the magnetite structure leading to an overall change in the volume of magnetite ores the reduplicative processes of volume change dissolution and porosity formation within magnetite are further imporved due to an incresed oxygen fugacity and co crystallized sulfide e g decreased temperature or increased sulfur fugacity at far from equilibrium or local equilibrium conditions resulting in oscillatory magnetite dark domains of s1 ripening of the transient porosity can trap nanoscale precipitates of columbite and zircon within pores of si magnetite and this precipitation could be attributed to the co crystallized phlogopite that would incorporate fluorine from the hydrothermal fluid and subsequently decrease the solubility of nb and zr in the skarn system this scenario highlights that nb and zr could be scavenged and enriched into in the reaction fronts porosity by controlling the reaction pathway at a local scale that does not reflect the overall fluid rock interaction history of the mineral assemblage

formation of elliptical garnet in a metapelitic enclave by melt assisted dissolution and reprecipitation metapelitic residual enclaves in the neogene volcanic province of se spain are residues left after melt extraction glass quenched melt of granitic composition occurs as inclusions in most minerals and as intergranular pockets the most common enclave types show one stage of garnet growth that is interpreted to have occurred at the same time as glass production some of these show a well developed foliation outlined by fibrolite biotite graphite and glass which wraps around elongate garnet crystals that have aspect ratios up to 10 1 based on microstructures and chemistry the garnet within these rocks shows clear core and mantle structure the core has an average composition of alm76 prp08 sps14 grs03 and contains primary inclusions of biotite and melt trapped during garnet growth a thin c 100 lm irregular mantle overgrows the garnet core enclosing oriented fibrolite inclusions in strain caps and biotite in strain shadows in places the overgrowths form skeletal elongated arms which extend parallel to the foliation the garnet mantle contains less mn and higher xmg but both core and mantle display flat mn profiles the contact being a sharp break ternary feldspar and grt bt thermometry yield temperatures in the range 800 900 c with no systematic differences among the different microstructural domains of elliptical garnet based on the observed intracrystalline microstructures the high amount of melt extraction in the rock by flattening component strain and the chemical zoning of garnet the formation of elliptical garnet is modelled by a multistage sequence this involves pressure solution and reprecipitation of the core followed by post kinematic partly mimetic growth of the garnet mantle

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

oriented multiphase needles in garnet from ultrahigh temperature granulites connecticut u s a this study investigates distinctive oriented needle shaped multiphase inclusions found within cores of garnets from felsic granulites of the upper member of the bigelow brook formation and the brimfield schist northeastern connecticut u s a the rocks crop out in the southern end of the central maine terrane and thus are a part of the acadian neoacadian orogen the typical mineral assemblage is garnet sillimanite k feldspar plagioclase quartz cordierite biotite spinel the sillimanite is commonly pseudomorphous after kyanite the multiphase needle inclusions and associated exsolved needles of rutile and ilmenite are mostly oriented parallel to of garnet the multiphase needles contain various combinations of quartz micas chlorite rutile graphite a siliceous compositionally variable phase phase cv zn spinel apatite zircon and rare ilmenite we hypothesize that they represent inclusions of fluid that adhered to exsolving ti fe oxide needles mostly rutile or extended along zones of weakness in garnet this requires that multiphase needle formation occurred in response to cooling and or decompression the needles ultimately decrepitated during retrogression we note that micaceous needle shaped multiphase inclusions are rarely described the closest analogs of which we are aware are found in uhp garnets of the erzgebirge perchuk 2008 the brimfield schist in this area underwent ultrahigh temperature metamorphism uhtm of 1000 c at a minimum pressure of 1 gpa ague et al 2013 here we provide new temperature estimates for the adjacent upper member of the bigelow brook formation ternary feldspar reintegration using the activity model of benisek et al 2004 and zr in rutile thermometry tomkins et al 2007 give average temperatures of 990 and 1010 c respectively at 1 gpa for this unit therefore the recently discovered uht zone in the brimfield schist of northeastern connecticut extends to the east to include the upper member of the bigelow brook formation consequently the first confirmed regional uht locality in the united states is larger than initially recognized and is at least 25 km long and 5 10 km wide the oriented elongate multiphase inclusions and petrographically obvious oriented ti fe oxide needles may be useful indicators of extreme temperature and or pressure rocks in other field areas

oxygen isotope heterogeneities and diffusion profile in composite metamorphic magmatic garnets from the pyrenees unusually large 1 8 o heterogeneities 4 within single crystals are reported in garnets from dioritic migmatites in the pyrenees these heterogeneities together with contrasting ca and p zoning allow the identification of different growth zones garnet cores with high 1 8 o values 12 14 are relatively poor in ca 7 9 mol grs and rich in p 400 900 ppm p 2 o 5 in contrast garnet rims with lower 1 8 o values 7 12 o are richer in ca 12 14 grs and poorer in p 100 200 ppm these growth zones can be ascribed to a metamorphic event followed by crustal partial melting and contamination by magmas from the mantle high 1 8 o intra crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct 1 8 o signatures at the garnet core rim interface compositional profiles in major divalent cations are consistent with the relaxation of an initial sharp step in ca fe and mg by ca fe mg interdiffusion at the same interface an o isotope profile is documented the analogy of ca and o isotope profiles suggests that the 1 8 o distribution may also result from a diffusion process in this particular case temperature garnet composition oxygen fugacity o diffusion appears to be of the same order of magnitude as ca fe mg interdiffusion considering a duration of 10 ma for the plutono metamorphic event in the pyrenees ca and o diffusivities in the range 10 2 2 m 2 s at 850 c are retrieved from the measured profiles like ca o diffusion in garnet at magmatic temperatures 850 900 c is both slow enough to preserve large 1 8 o heterogeneities and fast enough to generate relaxation profiles

anatomy of garnets in a jurassic granite from the south eastern margin of the north china craton magma sources and tectonic implications abstract the late jurassic jingshan granite located at the south eastern margin of the north china craton contains abundant garnets which can be subdivided into three types based on texture and composition i euhedral garnet in mafic biotite and garnet rich enclave grt i ii coarse grained garnet grt ii in the host granite and iii small euhedral garnet in aplite grt iii in general grt i has higher feo cao and lower mno contents than grt ii grt iii has higher mn but lower ca contents than others grt i has lower mree and hree contents than grt ii grt iii has prominent and distinctly negative eu anomaly as well as higher mree composition compared to the others systematic variations in oxygen isotope compositions are observed among the three garnet types with 18 o values of 4 7 in grt iii some of the grt ii and grt iii display two distinct zonings with cores having similar major and trace element compositions to grt i cathodoluminescence cl images revealed that the zircons from different garnet bearing samples possess fine scale oscillatory zoned magmatic rims with inherited cores in situ zircon u pb dating and trace element analyses show that the dark luminescent magmatic rims all have jurassic concordia ages 160 ma and similar trace element patterns most of the inherited cores also display similar triassic ages of 210 236 ma which is similar to the ages of ultrahigh pressure uhp metamorphic rocks of the dabie sulu orogen 230 ma in addition jurassic concordia ages were also found in a zircon inclusion in grt i implying that the grt i was formed shortly before the main magmatic event the age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events based on the zircon u pb ages from different garnet bearing samples the major element trace element oxygen isotope and zoning textures of the three kinds of garnet we suggest that grt i may be peritectic garnet whereas grt ii and iii are probably the results of magmatic dissolution precipitation processes and re equilibration of garnets with changing magmatic conditions during melting differentiation crystallization and cooling within the granite we conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the jingshan granites are from dabie sulu orogen representing the south china craton

dissolution reprecipitation of zircon at low temperature high pressure conditions lanzo massif italy abst r a c t an eclogite facies meta plagiogranite from the lanzo massif western alps italy contains crystals of zircon intimately associated with allanite zircon displays different microtextures ranging from pristine euhedral and magmatic to fractured porous varieties with mosaic zoning and pervasive recrystallization into euhedral microcrystals fractures and voids in the recrystallized zircon microcrystals are mainly filled by high pressure na rich pyroxene electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals with less than 2 misorientation among neighboring microdomains the textural change is coupled with chemical and isotopic modifications recrystallized zircon domains contain significantly less th and light to mid ree but are richer in sr than magmatic zircon crystals magmatic zircon preserves the protolith u pb age of 163 5 1 7 ma whereas zircon microcrystals have a mean age of 55 1 ma the coexisting allanite also contains inclusions of na rich pyroxene and has chemical features elevated sr and ni contents and lack of eu anomaly indicating formation at high pressure despite being associated texturally with zircon allanite yields a younger th pb age of 46 5 3 0 ma suggesting that the lanzo unit remained at relatively high pressure conditions for 8 m y zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution reprecipitation process recrystallization occurred with minimum transport in a low strain environment and was not significantly enhanced by metamictization the source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite

identification of growth mechanisms in metamorphic garnet by high resolution trace element mapping with la icp tofms garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism in addition to major elements zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification however mapping trace elements at low concentrations over large areas and with high resolution has remained a major challenge we present a comprehensive investigation of the te distribution in garnet from three alpine samples that underwent a complex evolution at different metamorphic conditions the te distribution in garnet grains is mapped in 2d in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry la icp tofms to achieve a lateral resolution of 5 m and limits of detection for the heavy rare earth elements ree down to 0 2 g g comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional la icpms spot analysis testifies to the accuracy of the measurements garnet in an amphibolite facies metapelite from campolungo central alps that recorded metamorphism to 600 c preserves y ree trace element zoning that closely matches that of ca in this sample there is no notable diffusive modification for trace elements y ree zoning is dominated by rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet a granulite facies garnet from malenco eastern central alps formed during subsolidus heating followed by peritectic melting reactions up to temperatures of 800 850 c major and trace element zoning are decoupled indicating diffusional resetting of major elements whereas trace elements still largely document the growth history enrichment of trace elements in the garnet mantle may be related to the consumption of biotite v cr and the dissolution of zircon zr and monazite y ree in the melt diffusion of y hree at the core mantle boundary occurred over a length scale of 200 m garnet in an eclogite from the sesia zone western alps p 2 gpa t 600 c displays pronounced fluid related veinlets visible in feo mgo and mno which cross cut the primary growth zoning surprisingly complex y ree and cr zoning is not affected by the veinlets indicating that they did not form by a crack seal mechanism but are rather related to a selective replacement process the trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes and assist in determination of p t conditions garnet dating diffusion modelling as well as documenting fluid induced modifications

exsolution of rutile or apatite precipitates surrounding ruptured inclusions in garnet from uht and uhp rocks this study describes halos of rutile apatite needles and or plates centred on quartz inferred former coesite inclusions or multiphase inclusions in garnet both types of central inclusions are surrounded by cracks the multiphase inclusions contain mica or carbonate minerals and are interpreted to represent decrepitated fluid inclusions examples from two localities are examined i ultrahigh temperature uht metapelitic gneisses from the southern end of the central maine terrane in northeastern connecticut usa rutile only and ii ultrahigh pressure uhp diamondiferous saidenbachite from the saxonian erzgebirge rutile and apatite the needles of apatite or rutile are typically oriented in three directions within garnet chemical zonation in garnet shows clear ti or p depletion halos corresponding spatially with the rutile or apatite inclusion halos the radii of the inclusion halos when measured from the centre of the central inclusions are about two to several times the radii of the central inclusions we propose that the inclusion halos of rutile apatite formed by exsolution out of ti bearing and or p bearing garnet during retrogression because of its strength garnet can internally maintain higher pressures than the surrounding matrix nonetheless if the inclusion pressure is significantly greater than the confining lithostatic pressure then deformation of the host garnet can occur during exhumation high internal pressures relative to the matrix can result from retention of fluid entrapment pressure or a phase change with a large positive volume increase e g coesite quartz the differential stress imposed on the garnet adjacent to the central inclusion preceding and during mechanical failure would create dislocations and other crystalline defects which are ideal sites for exsolved precipitates to nucleate and grow the strain induced exsolution hypothesis is consistent with observations i the radii of the halos are roughly consistent with the pressure vessel model or the multi anvil model for an inclusion which predict that differential stress will drop off sharply away from the boundary of the central inclusion into the host garnet ii the rutile or apatite inclusions formed in ti or p bearing garnet adjacent to ruptured central inclusions rutile or apatite are rare or absent elsewhere in garnet even in areas of high ti or high p concentration in addition rutile is absent in areas surrounding the central inclusion that were depleted in ti prior to rupturing e g garnet rims that lost ti during retrogression thus both elevated ti p concentrations and proximity to inclusions which deformed the host garnet were necessary for halo formation iii reintegrated garnet ti or p contents in and adjacent to the halos obtained using wide beam electron probe microanalysis are consistent with local derivation of the ti or p necessary to form rutile and apatite from garnet elevated ti p concentrations in aluminous garnet are mostly found in uht hp uhp granulite and mantle environments consequently the phenomena described will most likely occur in such settings

peroxide concentrations in the dome summit south ice core law dome antarctica measurements of h2o2 concentrations have been made at intervals along the full length of the dome summit south dss ice core from law dome antarctica these results show mean peroxide concentrations of approximately 37 parts per billion by mass over the past 4 kyr and a tendency for concentrations to decrease with age the rate of h2o2 decay in antarctic ice appears slower than in greenland ice and the pattern of measurements suggests an abrupt increase in concentration during the emergence from the last glacial into the holocene there is some evidence for a recent increase in h2o2 but the data so far are not conclusive detailed examination of the timing of the seasonality in h2o2 and 18o shows that peroxide extrema occur very near the solstices these seasonal curves have been used to derive atmospheric seasonal h2o2 curves based on possible air snow partitioning models these show a broad low in atmospheric concentration with a sharp peak which is again very near the summer solstice

ice core age dating and paleothermometer calibration based on isotope and temperature profiles from deep boreholes at vostok station east antarctica an interpretation of the deuterium profile measured along the vostok east antarctica ice core down to 2755 m has been attempted on the basis of the borehole temperature analysis an inverse problem is solved to infer a local geophysical metronome the orbital signal in the surface temperature oscillations expressed as a sum of harmonics of milankovich periods by correlating the smoothed isotopic temperature record to the metronome a chronostratigraphy of the vostok ice core is derived with an accuracy of 3 0 4 5 kyr the developed timescale predicts an age of 241 kyr at a depth of 2760 m the ratio 13d 13t i between deuterium content and cloud temperature fluctuations at the top of the inversion layer is examined by fitting simulated and measured borehole temperature profiles the conventional estimate of the deuterium temperature slope corresponding to the present day spatial ratio 9 per mil oc is confirmed in general however the mismatch between modeled and measured borehole temperatures decreases noticeably if we allow surface temperature responsible for the thermal state of the ice sheet to undergo more intensive precession oscillations than those of the inversion temperature traced by isotope record with this assumption we obtain the long term temporal deuterium temperature slope to be 5 8 6 5 per mil oc which implies that the glacial interglacial temperature increase over central antarctica was about 15oc in the surface temperature and 10oc in the inversion temperature past variations of the accumulation rate and the corresponding changes in the ice sheet surface elevation are simultaneously simulated

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation