causes of the uniformly high crystallinity of graphite in large epigenetic deposits in low pressure environments precipitation of graphite is hindered at low to moderate temperatures by the high solubility of carbon in c o h fluids and by kinetic barriers to nucleation those low temperature fluids that do attain saturation tend to precipitate graphite continuously during flow and cooling thereby producing widely dispersed films of low crystallinity graphite in contrast at high temperatures particularly when combined with high pressures the precipitation of graphite is enhanced by decreased solubility of carbon in c o h fluids and by improved nucleation under those conditions the longevity of fluid systems in high temperature high pressure terranes permits efficient long term scavenging of dispersed carbon from the crust the latter may be redistributed in a much more concentrated form as fluids rise cool and decompress and as the carbon is finally precipitated as highly crystalline graphite in fracture systems the combined effects of the thermochemical controls on carbon solubility and the geological controls on fluid generation movement and p t pathways are the reason that large epigenetic graphite deposits form dominantly at high temperatures and pressures those high temperature high pressure conditions in turn account for the uniformly high crystallinity of the fluid deposited graphite

graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms japan this study is an attempt to correlate the graphitization process of carbonaceous matter during metamorphism with metamorphic grade graphitization can be parameterized using crystal structure and chemical and isotopic compositions the extent of graphitization could be characterized mainly by temperature duration of metamorphism and rock composition we compared the graphitization trends for two metamorphic terrains a contact aureole of the kasuga area and a regional metamorphic terrain of high temperature low pressure type of the ryoke metamorphic terrain in northern kiso area central japan and for two different lithologies carbonate and pelite using x ray diffractogram dta tg analysis and chemical and stable isotope analyses during contact metamorphism graphitization and carbon isotopic exchange reactions proceeded simultaneously in pelitic and carbonate rocks the decreases in basal spacing d 002 of the carbonaceous matter in carbonate rocks is greatly accelerated at temperatures higher than about 400 c furthermore carbon isotopic ratios of graphite in carbonate rocks also change to 13c enriched values implying exchange with carbonates the beginning of this enrichment of 13c in the carbonaceous matter coincides with an abrupt increase of the graphitization processes carbon isotopic shifting up to 5 in pelites could be observed as metamorphic temperature increased probably by about 400 c carbonaceous matter in pelitic rocks is sometimes a mixture of poorly crystallized organic matter and well crystallized graphite detritus dta tg analysis is an effective tool for the distinction of detrital graphitic material two sources for the original carbon isotopic composition of carbonaceous matter in pelites in the kasuga contact aureole can be distinguished about 28 and 24 regardless of the presence of detrital graphite and were mainly controlled by depositional environment of the sediments graphitization in limestones and pelitic rocks in regional metamorphism proceeds further than in a contact aureole in the low temperature range the differences in extent of graphitization between the two metamorphic regions is large however at temperatures higher than 600 c the extent of graphitization in both regions is indistinguishable the degree of graphitization is different in limestones and pelitic rocks from the ryoke metamorphic terrain we demonstrate that the graphitization involves a progressive re construction process of the crystal structure the sequence of the first appearance of crystal inter planar spacing correlates with the metamorphic grade and indicates the crystal growth of three dimensional structured graphite

shear stress in the graphitization of carbonaceous matter during the low grade metamorphism from the northern parang mountains south carpathians implications to graphite geothermometry abstract low grade metamorphic rocks from the northern parang mountains south carpathians were examined to infer the temperature of metamorphism and to obtain new data to study the applicability of the geothermometers relying on the graphite crystallinity to rocks that underwent different shear stress conditions there were analyzed samples of graphitic carbonaceous matter bearing rocks that exposed dissimilar degrees of strain for which the temperature of metamorphism was calculated using the graphite geothermometers based on the lattice parameter c0 determined through x ray diffractometry and based on the peak area ratio of the first order raman spectrum the temperature was also inferred from the chemical composition of the chlorite and from the fe mg partitioning between chlorite and chloritoid the mineral chemistry being determined through electron microprobe analyses the results obtained have shown i differences of up to more than 150 c between vicinal strained and less strained zones when applying the geothermometers based on the graphite crystallinity and ii considerable discrepancies between the temperatures calculated based on the graphite crystallinity and those based on the chlorite and chloritoid chemical compositions these inconsistencies are most likely caused by the fact that the calibration of the graphite geothermometers considers temperature as being exclusively responsible for enhancing the degree of graphite crystallinity and therefore these graphite geothermometers should be adjusted accordingly by introducing a correction factor that quantifies the degree of strain of the samples analyzed in order to obtain more accurate results

infrared reflectance spectra 4 12 µm of typical lunar samples a laboratory study of the mid infrared spectral reflectance properties of typical apollo lunar rock and soil samples was conducted using a modern interferometric spectrometer the results show that characteristic spectral features exist in the 4 12 micrometer range that are diagnostic of rock composition and mineralogy especially for the plagioclase rich rocks and soils this is the first phase of a comprehensive infrared reflectance study underway to better understand the use of infrared emission spectroscopy for mapping the surface composition of the moon and other silicate bodies

40ar 39ar mineral age constraints for the tectonothermal evolution of a variscan suture in southwest iberia abstract dallmeyer r d fonseca p e quesada c and ribeiro a 1993 40ar 39ar mineral age constraints for the tectonothermal evolution of a variscan suture in southwest iberia tectonophysics 222 177 194 mafic units interpreted to represent an internally disrupted ophiolite succession are exposed along a variscan suture situated between the south portuguese and ossa morena zones in southern iberia structural characteristics of various units adjacent to the suture zone suggest maintainence of an oblique transpressional sinistral tectonic regime during late paleozoic plate convergence this involved initial limited north directed subduction beneath the ossa morena zone and was followed by oblique collision of the south portuguese terrane metamorphic evolution of rocks adjacent to the suture occurred during maintainence of a moderately high geothermal regime five samples of amphibolite were collected within the hanging wall of the suture these included samples from the ophiolitic terrane and from the ossa morena zone two samples were also collected within the post kinematic beja gabbro which intruded the other two tectonic units following their structural juxtaposition and penetrative ductile deformation hornblende concentrates from these seven samples record well defined 40ar 39ar plateau ages which range between c 342 and 335 ma these results imply that the regional tectonostratigraphic units were maintained at moderate crustal levels until the visean and that their juxtaposition and internal ductile deformation occurred in the late devonian subsequent cooling was probably effected by transpressional uplift during oblique collision following closure of intervening oceanic elements

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

a carbon isotope study of graphites from the kerala khondalite belt southern india evidence for co_ 2 infiltration in granulites field relations of arrested charnockite patches felsic pegmatite dikes and coarse cordierite graphite veins in the granulite facies supracmstal belt of southern kerala suggest paleo fluid channels we have investigated the stable isotopic composition of graphites associated with these structures as well as those in association with charnockites adjacent to calcsilicate horizons the remarkably high delta 13 c values displayed by graphites from the structurally controlled zones 8 6 to 13 4 o together with the narrow spread in individual localities pm 1 2 0 correlate with precipitation from isotopically homogeneous co 2 rich fluids that either infiltrated along structural pathways faults shears or were transferred through magmatic conduits graphites from charnockite calcsilicate associations show extreme 13 c enrichment delta 13 c up to 4 8 changing fluid regimes are detected from up to 5 0 variation in delta 13 c among graphites sampled adjace

constraints on graphite crystallinity in some spanish fluid deposited occurrences from different geologic settings epigenetic vein type graphite mineralization originates by deposition from c o h fluids in high temperature environments consequently fluid deposited graphite is uniformly highly crystalline in volumetrically large occurrences this work examines the factors controlling graphite crystallinity in fluid deposited occurrences with reference to some case studies from southern spain where vein type graphite is associated with a variety of host rocks possible causes influencing high crystallinity of graphite in these occurrences include 1 large graphite occurrences are generated from large volumes of fluids that maintain their temperatures for long periods of time which is easier at higher temperatures 2 high temperature conditions are required for a fluid to precipitate a major part of its dissolved carbon during a small temperature decrease 3 carbon is incorporated into c o h fluids mainly through devolatilization reactions which also require high temperatures 4 highly crystalline graphite generated at high t high p conditions is less susceptible to resorption as p decreases or by subsequent fluid flow 5 graphite precipitation involves high activation energy that can be overcome only if the temperature is high enough these causes can be extrapolated to most vein type graphite deposits worldwide

on boundary value problems of the ideal fluid thermocline abstract recent developments of ideal fluid thermocline models are briefly reviewed using density coordinates boundary value problems are formulated for the ideal fluid thermocline with continuous stratification ekman pumping and surface density are specified as the upper boundary conditions no flow is permitted through the ocean s eastern boundary nor its bottom each water column is divided into three parts i e the stagnant abyssal water with specified stratification the unventilated thermocline with its potential vorticity specified and the ventilated thermocline with its potential vorticity determined by a global dynamic balance the unventilated thermocline is further divided into the shallow and deep parts potential vorticity is specified a priori for the latter however for the former potential vorticity has to be chosen in the process of calculating the solution so as to make the solution self consistent numerical integration of the ideal fluid thermocline equations is reduced to repea

hydrothermal graphite in new hampshire evidence of carbon mobility during regional metamorphism graphite precipitated from hydrothermal fluids pervades the sillimanite grade metasedimentary and plutonic rocks of new hampshire hydrothermal graphite occurs as microscopic veinlets halos of spherulites around shear zones and metre thick veins carbon isotope analyses of the graphite range from 25 to 9 13c pdb intermediate between the two biogenic crustal reservoirs of reduced organic matter and carbonate it is proposed that carbon was mobilized from sediments as co2 and ch4 during metamorphic devolatilization reactions the carbon bearing species were transported in aqueous fluids through hydraulic fractures graphite precipitated when aqueous fluids with different co2 ch4 ratios were mixed in fractures

geological setting and controls on the development of graphite sillimanite and phosphate mineralization within the jiamusi massif an exotic fragment of gondwanaland located in north eastern china abstract china is the world s biggest producer and exporter of graphite and also possesses the world s largest reserves of crystalline graphite one of the main production areas is in northeastern china around jixi in heilongjiang province where graphite sillimanite and phosphate form in original sedimentary horizons within the mashan group the mashan group is composed of khondalitic metasediments and tectonically interleaved orthogneisses all metamorphosed to granulite facies the peak conditions have been calculated at 800 850 c at 5 3 6 2 kb and the p t t path defines a tight clockwise loop involving simultaneous decrease in temperature and pressure falling to 410 490 c at less than 3 kb this metamorphic event is interpreted as resulting from continent continent collision with emplacement of mantle derived mafic magma and possible delamination at the base of the crust metamorphism has been precisely dated at 500 ma using u pb zircon shrimp techniques it appears likely that this terrane formed part of an originally more extensive orogenic belt of late pan african age rocks of similar age are recorded from east antarctica western australia india and sri lanka suggesting that the mashan group was most likely juxtaposed with these areas within gondwanaland the mechanism by which the mashan group arrived at its present position between the siberian and north china cratons is poorly constrained but it appears likely that along with the north china south china and tarim blocks it underwent a northward drift from an initial location off northern australia to finally dock with the siberian craton in the late permian to late jurassic

ion microprobe analysis of graphite from ca 3 8 ga metasediments isua supracrustal belt west greenland relationship between metamorphism and carbon isotopic composition in situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca 3 8 ga isua supracrustal belt west greenland the 13c values of micron scale graphite globules in the metasediments and the carbonate rocks vary from 18 to 2 and from 7 to 3 respectively the maximum 13c value of graphite globules in the metasediment rises from 14 to 5 as the metamorphic grade increases from epidote amphibolite to upper amphibolite facies in a single hand specimen the 13c values of graphite inclusions in garnet are 7 lower on average than those outside garnet similarly graphite armored by quartz apparently shows a few permil lower 13c values than those on grain boundaries between noncarbonate minerals the fact that early crystallized minerals include relatively 13c depleted graphite indicates that the regional metamorphism increased the 13c values of the isua graphite this is consistent with the regional trend of 13c enrichment accompanied by the increase of metamorphic grade the minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 c these observations indicate that isotopic exchange with isotopically heavy carbonate caused 13c enrichment of isua graphite the 13c values of graphite reported here 13c 18 were produced either as a metamorphic modification of organic carbon with initially much lower 13c values or as an abiological reaction such as decomposition of carbonate if the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13c enrichment of isua graphite previously reported large 13c depletion of graphite especially armored by apatite mojzsis et al 1996 was probably premetamorphic in origin this supports the existence of life at isua time ca 3 8 ga

carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan abstract carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan was separated from the host rocks and its x ray diffraction and chemical composition were studied its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well ordered graphite near the biotite isograd as graphitization is a rate process the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism in an individual metamorphic terraan however the degree of graphitization is a useful indicator of relative metamorphic temperature in lower grade rocks

the fate of graphite in prograde metamorphism of pelites an example from the ballachulish aureole scotland abstract graphite bearing slates and phyllites 0 4 1 2 vol graphite are progressively metamorphosed in the 3 kbar aureole of the 425 ma ballachulish intrusion scotland two major dehydration reactions are crossed the chlorite out reaction at ca 550 c forming cordierite biotite and the muscovite out reaction at 625 c forming al 2 sio 5 k feldspar graphite persists to the highest grades and shows no significant variation in abundance with grade except for a possible decrease in the highest grade rocks variable graphite abundance in rocks at the same grade reflects primary sedimentological heterogeneity texturally graphite grains and aggregates in the rock matrix become coarser grained and more widely separated as grade increases these thermally induced textural modifications of graphite are superimposed on mechanically induced features such as graphite segregations along cleavages and crenulations that formed prior to contact metamorphism mass balance modelling assuming internal fluid generation shows that the amount of graphite consumed during contact metamorphism in the aureole ranges between 0 1 and 0 3 vol depending on the amount of chlorite and muscovite in the protolith because the amount of c dissolved in a c o h fluid decreases with increasing pressure and the ballachulish aureole is at relatively low pressure these results are a maximum for regional metamorphism suggesting that graphite will persist through a regional metamorphic cycle if it is initially present in volumes ca 0 2 vol

significance of graphite occurrences in the aracena metamorphic belt iberian massif the significance of syngenetic and epigenetic graphite occurrences from the variscan high temperature low pressure aracena metamorphic belt is discussed in the framework of the tectono thermal evolution of this southern zone of the iberian massif syngenetic graphite is associated with both low to medium grade metamorphic rocks la umbria series precambrian in age and high grade granulite facies rocks the precambrian fuente del oro series and a cambrian calc silicate series epigenetic fluid deposited occurrences correspond to overgrowths on existing metamorphic graphite grains and vein type mineralization two types of graphitized particles with remarkable differences in reflectance anisotropy and size can be distinguished in the precambrian metapelites of the la umbria series large 150 m length platy crystals with high reflectance and anisotropy are interpreted as detrital and are considered indirect evidence of an old orogenic cycle prior to the cadomian orogeny during which metamorphism exceeded greenschist facies the coexistence of two types of particles explains the scattering of values of the c parameter of graphite determined by xrd c 6 72 6 74 a and the anomalously high temperatures of the dta exothermic peak close to 600 c of graphite with respect to that inferred from mineral assemblages in these rocks the presence of graphite rich quartzites and gneisses within the fuente del oro series and the calc silicate series is evidence of sedimentation under reducing conditions in a continental shelf the characteristics of graphite reflect the high grade metamorphic conditions attained in the southern area of the aracena metamorphic belt pervasive flow of fluids related to a major variscan extensional event resulted in overgrowths on the pre existing graphite in the gneisses and quartzites of the calc silicate series as evidenced by the heterogeneous isotopic composition of graphite single crystals in these rocks a later stage of graphite precipitation is represented by scarce vein type occurrences in mafic granulites that document channelled flow of fluids

constraints on the application of carbon isotope thermometry in high to ultrahigh temperature metamorphic terranes nine marble horizons from the granulite facies terrane of southern india were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite the marbles in trivandrum block show coupled lowering of 13c and 18o values in calcite and heterogeneous single crystal 13c values 1 to 10 for graphite indicating varying carbon isotope fractionation between calcite and graphite despite the granulite facies regional metamorphic conditions the stable isotope patterns suggest alteration of 13c and 18o values in marbles by infiltration of low 13c 18o bearing fluids the extent of alteration being a direct function of the fluid rock ratio the carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated recrystallized in the presence of an externally derived co2 rich fluid and that the infiltration had occurred under high temperature and low fo2 conditions during metamorphism the onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid calcite carbon reservoir following a rayleigh type distillation process within fluid rich pockets pathways in marbles resulting in the observed zonation the results suggest that calcite graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration however marble horizons in the madurai block where the effect of fluid infiltration is not detected record clear imprints of ultrahigh temperature metamorphism 800 1000 c with fractionations reaching 1000 c and would be consistent with uht metamorphism

fluid deposited graphitic inclusions in quartz comparison between ktb german continental deep drilling core samples and artificially reequilibrated natural inclusions we used raman microsampling spectroscopy rms to determine the degree of crystallinity of minute 2 15 m graphite inclusions in quartz in two sets of samples experimentally reequilibrated fluid inclusions in a natural quartz grain and biotite bearing paragneisses from the ktb deep drillhole in se germany our sequential reequilibration experiments at 725 c on initially pure co2 inclusions in a quartz wafer and the j krautheim 1993experiments at 900 1100 c on organic compounds heated in gold or platinum capsules suggest that at a given temperature 1 fluid deposited graphite will have a lower crystallinity than metamorphosed organic matter and 2 that the crystallinity of fluid deposited graphite is affected by the composition of the fluid from which it was deposited we determined that the precipitation of more crystalline graphite is favored by lower f h2 higher f o2 and that the crystallinity of graphite is established by the conditions including gas fugacities that pertain as the fluid first reaches graphite saturation graphite inclusions within quartz grains in the ktb rocks show a wide range in crystallinity index reflecting three episodes of carbon entrapment under different metamorphic conditions isolated graphite inclusions have the spectral properties of totally ordered completely crystalline graphite such crystallinity suggests that the graphite was incorporated from the surrounding metasedimentary rocks which underwent metamorphism at upper amphibolite facies conditions much of the fluid deposited graphite in fluid inclusions however shows some spectral disorder the properties of that graphite resemble those of experimental precipitates at temperatures in excess of 700 c and at elevated pressures suggesting that the inclusions represent precipitates from c o h fluids trapped under conditions near those of peak metamorphism at the ktb site in contrast graphite that is intimately associated with chlorite and other presumably low temperature silicates in inclusions is highly disordered and spectrally resembles kerogens this graphite probably was deposited during later greenschist facies retrograde metamorphism at about 400 500 c the degree of crystallinity of fluid deposited graphite is shown to be a much more complex function of temperature than is the crystallinity of metamorphic graphite to some extent experiments can provide temperature calibration of the crystallinity index however the difference in time scales between experimental runs and geologic processes makes it difficult to infer specific temperatures for naturally precipitated graphite

triple junction migration during paleozoic plate convergence the aracena metamorphic belt hercynian massif spain new data on the petrology and structure of the aracena metamorphic belt shows that this is a subduction related low pressure high temperature complex developed by plate convergence at the north margin of gondwana during the paleozoic the low pressure inverted metamorphic gradient in morb derived amphibolites resulted from heating from the continental hanging wall during subduction this implies that the previous heating of the continental rocks was related to subduction of an oceanic ridge and the creation of a slab window beneath the continental margin this slab window brought the asthenosphere in contact with the continental margin inducing a shallow thermal anomaly and partial melting of the lithospheric mantle resulting in boninite magmatism

new insights into carbon fluid chemistry and graphite precipitation sims analysis of granulite facies graphite from ponmudi south india slow graphite carbon self diffusion rates and a strong dependence of 13 c 12 c fractionations on fluid chemistry can be combined with secondary ion mass spectrometry sims analyses of 13 c 12 c of graphite to fingerprint graphite precipitation mechanisms and carbon chemistry of crustal fluids the d 13 c variations in single graphite grains in granulite facies rocks from ponmudi south india approach 10 and are interpreted to reflect three distinct generations of graphite formation and fluid infiltration illustrating the potential of sims microdomain graphite analyses for deciphering crustal fluid influx histories our analyses provide insight into a longstanding debate in crustal metamorphism of south india indicating that the fluid rock interaction history included early middle and late interactions with c o h fluids 1999 elsevier science b v all rights reserved

age constraints to the relationships between magmatism metamorphism and tectonism in the aracena metamorphic belt southern spain the aracena metamorphic belt amb southwest iberian peninsula is characterized by the following geological elements a a high temperature low pressure ht lp metamorphic belt a few kilometres wide and more than 200 km long b a linear belt of oceanic amphibolites with a low pressure inverted metamorphic gradient c crustal scale ductile shear zones and d mafic noritic intrusions of high mg andesite boninite composition the relationships between these elements led to the proposal of a model of ridge subduction for this sector of the hercynian belt of europe this interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously the results of a geochronological study ar ar rb sr and sm nd clearly support a late paleozoic tectonic evolution at an active continental margin the time evolution of the metamorphism in the oceanic domain ranging from 342 6 0 6 ma in the west to 328 4 1 2 ma in the east over a distance of 70 km along the metamorphic belt support a tectonic model of triple junction migration responsible for the creation at depth of a slab free window with decisive consequences for the thermal evolution of the region the origin of the linear metamorphic belt of ht lp regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple junction at the continental margin

graphitization in a high pressure low temperature metamorphic gradient a raman microspectroscopy and hrtem study the graphitization of carbonaceous material cm in a high pressure metamorphic gradient is characterized along a cross section in the schistes lustres formation western alps along this 25 km cross section both the cm precursor and the host rock lithology are homogeneous and the prograde evolution of the pressure temperature metamorphic conditions from the lower blueschist facies 13 kbar 330 c to the eclogite facies 20 kbar 500 c is tightly constrained by literature data raman microspectroscopy shows that at the micrometre scale this process is progressive and continuous with increasing metamorphic grade and that the structure of cm is very sensitive to temperature variations at the nanometre scale hrtem the cm is composed of a mixture of a microporous phase and an onion ring like phase both known as non graphitizing under the effect of temperature at ambient pressure the hp lt graphitization produces structurally and microtexturally heterogeneous cm with increasing metamorphic grade the graphitization of the two types of cm proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of cm the microporous material is progressively transformed into graphite through a macroporous transitional stage in this case graphitization mainly occurs on the pore walls as a result of pore growth in the case of concentric onion ring like material graphitization occurs in the regions with the largest radius of curvature i e on the outer part of the ring in comparison with 1 bar experiments pressure seems to induce microtextural changes which allows the subsequent structural modifications of the starting material

the role of finite mixed layer thickness in the structure of the ventilated thermocline abstract a model of the ventilated thermocline consisting of three adiabatic layers surmounted by a mixed layer of finite thickness is presented the mixed layer depth density increase continuously northward and these attributes of the mixed layer are specified the effect of the mixed layer on the thermocline circulation is explicitly calculated the mixed layer thickness and its variation play a significant role in shifting the trajectories of the streamlines westward the shadow zones enlarge more rapidly south of the outcrop lines the finite mixed layer depth and its increase northward produce shadow zones in each of the adiabatic layers of the thermocline the augmentation of the ventilation rate i e the rate at which fluid enters the thermocline from the mixed layer in excess of the ekman pumping is directly proportional to the northward gradient of the mixed layer potential vorticity the enhancement is greatest in regions where the circulation is surface intensified i e above the eastern sh

the role of the mixed layer in setting the potential vorticity of the main thermocline abstract a steady ventilation model is used to assess the effect of the mixed layer on the structure of the main thermocline the potential vorticity is found in a subtropical gyre after imposing the thickness and density of the mixed layer the ekman pumping and the hydrography on the eastern boundary the modeled potential vorticity becomes comparable in value to observations in the north atlantic if the mixed layer deepens poleward as is observed in winter the isopycnal gradients in potential vorticity are reduced on the denser ventilated surfaces if the mixed layer outcrops deviate from latitude circles and more realistically sweep southward along the eastern boundary the age of the subducted fluid is also in reasonable agreement with observations of the tritium helium age by jenkins this study suggests that ventilation may form much of the extensive region of nearly uniform potential vorticity observed on the 26 75 surface in the north atlantic with lateral mixing by eddies being required

metamorphism of carbonaceous material in the tono contact aureole kitakami mountains japan abstract optical and x ray studies of carbonaceous material in the tono contact metamorphic aureole kitakami mountains northeast japan have revealed that metamorphic graphitization proceeded through two discontinuous changes first optically anisotropic domains develop within the coaly phytoclast forming transitional material and then ordered graphite crystallizes by the decomposition of pre existing carbonaceous materials coaly material disappears in the uppermost chlorite zone transitional material appears in the middle of the lower chlorite zone graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso grad to the cordierite isograd where all the carbonaceous materials have converted to graphite the apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials the temperature of graphitization in the present area is at least 100 c higher than the temperature in the sanbagawa and new caledonia high pressure metamorphic terrains probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism

mineralogical and isotopic characterization of graphite deposits from the anatectic complex of toledo central spain graphite is found dispersed in high grade metapelitic rocks of the anatectic complex of toledo act and was mined during the mid twentieth century in places where it has been concentrated guadamur and la puebla de montalban mines some samples from these mines show variable but significant alteration intensity reaching very low t hydrothermal supergene conditions for some samples from the waste heap of the guadamur site 100 c and 1 kbar micro raman and xrd data indicate that all the studied act graphite is of high crystallinity irrespective of the degree of hydrothermal alteration chemical differences were obtained for graphite 13c composition act granulitic graphite shows 13cpdb values in the range of 20 5 to 27 8 indicating a biogenic origin interaction of graphite with hydrothermal fluids does not modify isotopic compositions even in the most transformed samples from mining sites the different isotopic signatures of graphite from the mining sites reflect its contrasted primary carbon source the high crystallinity of studied graphite makes this area of central spain suitable for graphitic exploration and its potential exploitation due to the low carbon content required for its viability and its strategic applications in advanced technologies such as graphene synthesis

tectonometamorphic evolution of the aracena metamorphic belt sw spain resulting from ridge trench interaction during variscan plate convergence 1 the aracena metamorphic belt is a high temperature low pressure ht lp band located at the southern end of the european variscan chain it marks a suture between armorica and avalonia this belt is characterized by 1 the presence of mid ocean ridge basalt derived metabasites that were affected by an inverted ht lp metamorphism related to sw verging thrusting whose thermal peak diachronously migrated eastward 2 the occurrence of a ht lp metamorphism related to an extensional event affecting continental rocks belonging to the ossa morena zone with peak temperatures that are 150 c higher than that recorded in the oceanic metabasites and 3 the presence of near trench magmatism with high mg andesite composition these and other characteristics can be interpreted in terms of a ridge trench trench triple junction that migrated along the armorican margin during variscan oblique convergence

correlation between seismic anisotropy and major geological structures in sw iberia a case study on continental lithosphere deformation in this paper we postulate a correlation between the lithospheric structure and seismic anisotropy inferred from geophysical data and the major structures and tectonic history past tectonothermal features of the outcropping basement rocks in sw iberia oblique plate convergence orogenesis and terrane accretion have been documented for pre mesozoic times in this area however the deviations and areal variations observed between the azimuthal anisotropy of seismic waves and the structural trends of the accessible crust indicate that the tectonic model of orogen parallel lithospheric deformation does not appear to succeed in relating hercynian surface geology and the structural geology of the subcontinental lithospheric mantle the model of polyorogenic lithosphere deformation proposed accounts for this divergence and is supported from additional perspectives including the manifestation of thermal effects induced in the crust by lithospheric mantle shear zones the incidence of crust mantle coupling and or decoupling and the scales of subcrustal heterogeneity

effects of horizontal advection on upper ocean mixing a case of frontogenesis abstract the wind stress on the ocean surface induces horizontal advection and vertical mixing two mechanisms which are capable of changing the surface water density advection and mixing can enhance or partially destroy the effects of each other frontogenesis due to convergent ekman transports forced by a wind stress field is one example a bulk model for the study of advection and mixing in such circumstances is constructed from a one dimensional model continuity of mass requires that water masses either downwell convergence or escape laterally confluence this distinction leads to a study of two extreme cases of frontogenesis each herein treated separately the model reduces to two coupled highly nonlinear prognostic equations for the mixed layer buoyancy and mixed layer depth scaling of the equations leads to the definition of a mixing parameter a nondimensional number which measures the relative importance of advection and mixing for large scale ocean frontogenesis this parameter is of th

structural and metamorphic evolution of the almaden de la plata core seville spain in relation to syn metamorphic shear between the ossa morena and south portuguese zones of the iberian variscan fold belt abstract the almaden de la plata core is a tectonic unit situated in the southern branch of the iberian variscan fold belt it has traditionally been regarded as an eastern extension of the aracena metamorphic belt which is located at the south southeast boundary of the ossa morena with the south portguese zone low pressure high temperature parageneses gt crd sill are found in the highest grade areas while a metamorphic gradient of medium pressure type gt sta bearing schists and gneisses is observed for the middle stages of the prograding metamorphic evolution later processes indicate an evolution to the very low grade locally retrograding previous parageneses five main tectonic events are observed d 1 oriented inclusions within cordierite and garnet and d 2 are not present in the surrounding area d 2 and d 3 have a blastomylonitic high grade metamorphism and mylonitic low grade character respectively d 4 is responsible for the present boundaries of the almaden de la plata core while d 5 corresponds to late hercynian fractures events 1 4 book place under high to low and very low metamorphic grades the metamorphic climax was reached pre to syn d 2 the metabasic rocks acebuches amphibolites show medium to high grade associations hbl plag cpx with a well developed n120 e mineral lineation these features change towards the contact with the south portuguese materials of the pulo do lobo group where metabasites are transformed into very fine grained amphibolitic schists greenschist associations with actinolite and relicts of hornblende and clinopyroxene the later are mylonitic rocks with n150 e stretching lineations and s c structures as well as sigmoidal pressure shadows indicating a southward vergence the low grade metabasites are interpreted as the bottom of a d 3 main thrust which put the ossa morena and south portuguese zones into contact these results support the hypothesis that the acebuches amphibolites of the aracena belt do not pass gradually into the pulo do lobo group but are thrust upon that unit

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

a numerical model of the ventilated thermocline abstract a steady state numerical solution is found for an idealized rectangular ocean basin driven by wind and surface buoyancy flux a three dimensional primitive equation model is used in agreement with recent analytical modeling the thermocline in the numerical solution consists of three regions quite distinct in their ventilation characteristics forming the greater part of the subtropical thermocline is an unventilated pool zone located in the core of the subtropical gyre and a ventilated zone to the east the unventilated shadow zone lies farther east and toward the equator analysis of potential vorticity on constant density surfaces is used to study the structure of the thermocline a small but intense zone of convection located in the western boundary outflow caused by rapid heat loss to the atmosphere produces source water for the ventilated zone this water of extremely low potential vorticity mode water is distributed widely into the subtropical thermocline the pool forms equ

a study of inherited zircons in granitoid rocks from the south portuguese and ossa morena zones iberian massif support for the exotic origin of the south portuguese zone abstract trace element and u pb isotopic analyses of inherited zircon cores from a sample of gil marquez granodiorite south portuguese zone spz and almonaster nebulite ossa morena zone omz in the aracena metamorphic belt have been obtained using laser ablation inductively coupled plasma mass spectrometry these data reveal differences in the age of deep continental crust in these two zones inherited zircon cores from the ossa morena zone range at 600 100 ma 1 7 2 ga and 2 65 2 95 ga while those from the south portuguese zone range at 400 500 and 700 800 ma these data support the exotic origin of the south portuguese zone basement relative to the rest of iberian massif the young ages of inherited zircon cores and nd model ages of magmatic rocks of the south portuguese zone are comparable to results from granulite facies xenoliths and granitic rocks from the meguma terrane and avalonia and support a correlation between the basement of the southernmost part of the iberian massif and the northern appalachians

widespread occurrences of variably crystalline 13c depleted graphitic carbon in banded iron formations abstract almost all evidence for the oldest traces of life on earth rely on particles of graphitic carbon preserved in rocks of sedimentary protolith yet the source of carbon in such ancient graphite is debated as it could possibly be non biological and or non indigenous in origin here we describe the co occurrence of poorly crystalline and crystalline varieties of graphitic carbon with apatite in ten different and variably metamorphosed banded iron formations bif ranging in age from 1 800 to 3 800 myr in neoarchean to palaeoproterozoic bif subjected to low grade metamorphism 13c depleted graphitic carbon occurs as inclusions in apatite and carbonate and arguably represents the remineralisation of syngenetic biomass in bif subjected to high grade metamorphism 13c depleted graphite co occurs with poorly crystalline graphite pcg as well as apatite carbonate pyrite amphibole and greenalite retrograde minerals such as greenalite and veins cross cutting magnetite layers contain pcg crystalline graphite can occur with apatite and orthopyroxene and sometimes it has pcg coatings crystalline graphite is interpreted to represent the metamorphosed product of syngenetic organic carbon deposited in bif while poorly crystalline graphite was precipitated from c o h fluids partially sourced from the syngenetic carbon along with fluid deposited apatite and carbonate the isotopic signature of the graphitic carbon and the distribution of fluid deposited graphite in highly metamorphosed bif is consistent with carbon in the fluids being derived from the thermal cracking of syngenetic biomass deposited in bif but extraneous sources of carbon cannot be ruled out as a source for pcg the results here show that apatite graphite is a common mineral assemblage in metamorphosed bif the mode of formation of this assemblage is however variable which has important implications for the timing of life s emergence on earth

detrital graphite particles in the cryogenian nantuo formation of south china implications for sedimentary provenance and tectonic history abstract the tectonic history of the yangtze block of south china prior to the ediacaran period has been a matter of debate in particular the tectonic history of the shennongjia terrane in the central northern margin of the yangtze block is poorly understood and the tectonic relationship between the shennongjia terrane and the huangling massif exposed in the huangling anticline is unclear previous researchers have reported u pb dates of detrital zircons and nd isotopic compositions of clastic rocks from cryogenian successions in the shennongjia terrane to identify ancient crustal remnants and the provenance of clastic sediments and to constrain the age and crustal evolution of the yangtze block here we describe abundant graphite discs from silty mudstone of the cryogenian nantuo formation in the shennongjia area of the yangtze block these graphite discs are opaque 23 171 m in size and highly reflective most are abraded or rounded although some show hexagonal crystalline morphology their raman spectra are distinct from those of indigenous carbonaceous material cm in the sedimentary matrix from the nantuo formation at the same locality raman cm geothermometric analysis indicates that graphite discs experienced a maximum metamorphic temperature of nearly 600 c which is inconsistent with the relatively low metamorphic grade of the host rock these observations indicate that the graphite discs are of detrital origin most likely derived from high grade metamorphic rocks in the basement of the yangtze block e g the archean paleoproterozoic kongling terrain in the core of the huangling anticline consistent with the hypothesis that the shennongjia region was a part of yangtze block by the end of the cryogenian period

vein graphite deposits geological settings origin and economic significance graphite deposits result from the metamorphism of sedimentary rocks rich in carbonaceous matter or from precipitation from carbon bearing fluids or melts the latter process forms vein deposits which are structurally controlled and usually occur in granulites or igneous rocks the origin of carbon the mechanisms of transport and the factors controlling graphite deposition are discussed in relation to their geological settings carbon in granulite hosted graphite veins derives from sublithospheric sources or from decarbonation reactions of carbonate bearing lithologies and it is transported mainly in co2 rich fluids from which it can precipitate graphite precipitation can occur by cooling water removal by retrograde hydration reactions or reduction when the co2 rich fluid passes through relatively low fo2 rocks in igneous settings carbon is derived from assimilation of crustal materials rich in organic matter which causes immiscibility and the formation of carbon rich fluids or melts carbon in these igneous hosted deposits is transported as co2 and or ch4 and eventually precipitates as graphite by cooling and or by hydration reactions affecting the host rock independently of the geological setting vein graphite is characterized by its high purity and crystallinity which are required for applications in advanced technologies in addition recent discovery of highly crystalline graphite precipitation from carbon bearing fluids at moderate temperatures in vein deposits might provide an alternative method for the manufacture of synthetic graphite suitable for these new applications

subtropical countercurrent in an idealized ocean gcm abstract in the north pacific there is a shallow eastward current called the subtropical countercurrent which flows across the central subtropical gyre the present article studies the generation mechanism of the subtropical countercurrent reproduced in an ocean general circulation model gcm with a simple geometry driven by surface wind stress and surface buoyancy forcing in the ocean gcm the deep mixed layer occurs in the northern part of subtropical gyre and shoals abruptly in the central subtropical gyre the mixed layer front the narrow transition zone of the mixed layer depth slants from the western central subtropical gyre to the northeast and the low potential vorticity fluid is formed at the intersection of the mixed layer front and the outcrop line since the surface density is almost zonally uniform and the mixed layer front slants northeastward the minimum potential vorticity fluids on denser isopycnals are formed in the northeastern region while those on lighter isopycnals are form