geobarometry geothermometry and late archean geotherms from the granulite facies terrain of south india geobarometry and geothermometry of garnet bearing chamockitic and metapelitic assemblages from the high grade terrain from south india indicate three fields of equilibration high pressure granulites p 8 3 pm 1 0 kb t 760 pm 40 circ c are restricted to the charnockites and mafic granulites of the north slopes of the nilgiris these place a minimum constraint of 30 km on the maximum thickness of late archean crust medium pressure granulites p 6 4 pm 1 0 kb t 735 pm 40 circ c characterize the charnockites of the central nilgiris and the madras granulites to the east low pressure granulites p 5 0 pm 1 0 kb t 700 pm 20 circ c characterize the charnockites and metapelites south of the bhavani shear zone although migmatites from the kodaikanal massif may reflect somewhat higher temperatures all three granulite fields lie on a metamorphic geotherm piezothermic array which is strongly convex to the temperature axis and characteristic of convective heat transfer

evolution of synmetamorphic veins and their wallrocks through a western alps transect no evidence for large scale fluid flow stable isotope major and trace element systematics quartz rich synfolial veins and wallrocks from different areas of a western alps cross section are examined in an attempt to constrain the scale and mechanisms of fluid flow through metamorphic terrains different portions of this cross section underwent distinct p t evolutions as reflected by the mineralogy of veins a in the greenschist external dauphinois domain veins are characterized by quartz and calcite together with an aluminosilicate pyrophyllite na k bearing phyllosilicates and a femg bearing silicate chlorite b in narrow domains of the brianconnais and piemontais zones with a non eclogitic hp lt evolution followed by rapid cooling veins are characterized by quartz calcite ca and fe mg bearing silicates lawsonite and fe mg carpholite respectively and c in the major part of the brianconnais andpiemontais zones where low grade blueschistfacies metamorphism is followed by a greenschist facies evolution veins are characterized by the replacement of earlier lawsonite and carpholite by na and k bearing micas in order to constrain the provenance of fluids involved in vein formation we determined stable isotope c 0 h and major and trace element compositions in minerals and whole rocks for a large set of veins and wallrocks these rocks are compared with some non metamorphic shales from the northern paris basin considered as protoliths of the alpine schists stable isotope compositions of calcites are regionally distinct 6r80 28 0 smow in the northern paris basin 26 0 in the brianconnais zone 22 0 in the piemontais zone and 18 0 in the dauphinois zone within each studied region calcite g o values are quite homogeneous regardless of rock type vein or wallrock this feature is explained as an isotopic homogenization with the most abundant lithology in each zone rock buffered system where the overall ratio of calcite of marine origin to detrital silicate minerals determines the final isotopic composition of the whole rock w r after equilibration during alpine metamorphism g o gd values of analysed hydrated minerals and whole rocks si80 15 to t 25 0 and 6d 60 0 smow lie within the field of alpine cover rocks and of fluids in equilibrium with these rocks interaction with an external o or d depleted fluid such as hydrothermal waters meteoric water or basement fluid can be ruled out major and trace element systematics indicate an enrichment in si ca p sr y ta and locally eu in the veins with respect to the wallrock related to massive crystallization of quartz and carbonates together with phosphate and oxide potential fluid sources chemical composition of the fluid phase and the mechanisms of fluid flow during vein formation are discussed fluid flow takes place within tectonic units at both hectometric and decimetric scale fluid may also have migrated along and across the major tectonic contacts but this process is poorly documented

metamorphism and deformation along the emplacement thrust of the samail ophiolite oman abstract the base of the samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement the classic exposure of this metamorphic sole at wadi tayin consists of a few meters of garnet clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite overlying 150 m of lower amphibolite facies mafic and sedimentary schist the sole is bounded by faults but is internally coherent peak metamorphic temperatures decrease exponentially from 825 c at the top of the sole to 500 c at its base as documented by changes in grain size hornblende composition the disappearance of chlorite and epidote garnet clinopyroxene element partitioning and garnet hornblende partitioning the inverted peak temperature gradient decreases from 4000 c km in the upper 50 m to 100 c km in the lower 150 m contrary to earlier reports greenschist facies tectonites are not present at wadi tayin although the entire sole underwent greenschist facies retrogression driven by the influx of fluids from the overlying peridotite or underlying carbonate calculations based on an amphibolite constitutive relation reveal that although the section represents a lithologic continuum the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column

assessment of the garnet clinopyroxene fe mg exchange thermometer using new experimental data a detailed high pressure experimental study of two mafic xenoliths in which coexisting garnet and clinopyroxene plagioclase spinel and olivine were crystallized over a p t range of 10 30 kbar and 950 1200 c has revealed significant differences in temperatures from those estimated for coexisting garnets and clinopyroxenes using the ellis green fe mg exchange thermometer the results show perfect matching at 30 kbar 1150 1200 c but increasing deviation at lower pressure and lower temperature with the ellis green calibration reaching a t overestimate of c 145 c at 10 12 kbar and 950 c the grossular content of the garnet increases from c 21 mol at 10 kbar to 26 31 mol at 30 kbar these results confirm other recent experimental studies that show that the pressure correction and possibly to a lesser extent the correction for grossular content applied by ellis green are not appropriate for lower pressure conditions and give estimated temperatures that are significantly high when applied to granulitic terranes formed at c 10 kbar the new reconnaissance results allow a graphical interpolation of a garnet clinopyroxene geothermometer based on the fe mg exchange reaction which should be applicable to assemblages formed under lower crustal conditions

experimental transport of si al and mg in hydrothermal solutions an application to vein mineralization during high pressure low temperature metamorphism in the french alps a study of hydrothermal vein mineralization in meta argillites subjected to high pressure low temperature metamorphism reveals that ferromagnesian e g chlorite and pure aluminosilicate e g pyrophyllite mineralization can be correlated with regimes of increasing and decreasing temperature respectively an experimental study of the transport of silica aluminum and magnesium in hydrothermal solutions has been undertaken to simulate variations in the physical conditions during metamorphism and the accompanying mass transport in a closed system thermodynamic and kinetic analysis of the experimental results indicates that local equilibrium among aqueous and mineral phases controls the distribution and composition of hydrothermal vein mineralization and that vein mineralogy can be used to infer the sense of variation of pressure and temperature during metamorphism

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

preservation of detrital shocked minerals derived from the 1 85 ga sudbury impact structure in modern alluvium and holocene glacial deposits detrital shocked minerals can provide valuable residual records of eroded impact structures recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2 02 ga vredefort dome impact basin in south africa to evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a holocene glacial erosional history we investigated 4000 detrital zircons and 20 000 quartz grains at the less eroded 1 85 ga sudbury basin in ontario canada for the presence of shocked sand grains modern alluvium from rivers within and outside the basin and holocene glaciofluvial sands eskers and outwash deltas across the basin were investigated for shocked minerals shocked zircon and or quartz were found in all modern rivers and most holocene glacial deposits within but not outside the basin petrography and scanning electron microscopy sem backscattered electron bse cathodoluminescence cl imaging and analysis energy dispersive x ray spectroscopy eds electron backscatter diffraction ebsd were used to document shock microstructures of the total detrital zircons surveyed 3 118 3978 were identified as shocked holocene samples contained higher average percentages of shocked zircon 63 1361 or 4 6 with a high of 29 compared to modern alluvium 55 2617 or 2 1 with a high of 6 ebsd analysis revealed a range of shock microstructures including planar fractures deformation microtwins and crystal plastic deformation at sudbury detrital shocked quartz is rare compared to zircon only 15 grains 0 08 were identified all with decorated planar deformation features pdfs these results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a youthful stage of erosion despite its age in addition to modern alluvium our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems despite physical differences the observation that the two largest precambrian impact basins continue to contribute detrital shocked minerals 2 b y after impact suggests that a shocked mineral record of impacts on early earth should reside in precambrian siliciclastic rocks

on the significance of aragonite occurrences in the western alps aragonite occurrences from two areas of the western alps are described it is shown that aragonite has been formed under blueschist metamorphic conditions in the western vanoise while it has been precipitated under sub surface conditions in the queyras region an uplift p t path of the aragonite bearing rocks of the western vanoise is constructed using two independent methods 1 temperature estimates using the sr content of aragonite in the successive veins and 2 the kinetics of the aragonitecalcite solid state transition the uplift p t path has an unusual shape with an important temperature decrease 100 to 150 c for a pressure decrease of 0 2 gpa following blueschists metamorphism p 0 7 gpa t 300 c thermal models show that this unusual p t path of the western vanoise can be explained if one maintains a low temperature between 125 and 175 c at the base of the tectonic unit containing the aragonite bearing rocks during part of its burial history followed by the whole of its exhumation a tectonic scenario is proposed to account for the observed and modelled p t path

garnet forming reactions in mafic granulites from enderby land antarctica implications for geothermometry and geobarometry

shear structures and microstructures in micaschists the variscan cevennes duplex french massif central abstract structural microstructural and quartz c axis orientations have been investigated in shear zones within the cevennes micaschist series southeastern french massif central these shear zones define a hinterland dipping duplex formed during crustal thickening they are characterised by superposed cleavages a well defined stretching lineation isoclinal folds shear planes and abundant quartz lenses these shear zones were formed under p t metamorphic conditions of ca 500 c and 450 mpa the dominant deformation mechanism was dissolution crystallisation fluid circulation responsible for element transfer was limited in space and dissolved silicate minerals are now found in the quartz lenses study of the syntectonic quartz lenses suggests fluid overpressure that has favoured brittle behaviour of the rock and formation of quartz lenses at different stages of the shearing event quartz lenses have accentuated the initial anisotropy of the micaschists and have strongly enhanced folding boudinage and the development of shear zones these deformation criteria for shear zone recognition can be applied in understanding the structure and significance of monotonous micaschist series in orogenic belts

retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism staurolite kyanite schists from the dalradian of scotland and ireland show two types of retrograde alteration in the irish examples staurolite and kyanite are replaced by muscovite and paragonite often with margarite and sometimes chloritoid plagioclase in these samples may be albitized in contrast one scottish sample shows replacement of both staurolite and plagioclase by muscovite retrogression resulted from infiltration of fluid but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration it is demonstrated by the use of activity diagrams showing the relationships between al silicate white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down temperature from a granitic or arkosic source in contrast if growth of retrograde albite is actually accompanied by na metasomatism then the infiltrating fluids are likely to have been moving up temperature through pelitic lithologies large fluid rock ratios of around 10 1 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids

fe mg interdiffusion rates in clinopyroxene experimental data and implications for fe mg exchange geothermometers chemical interdiffusion of fe mg along the c axis 001 in natural diopside crystals x di 0 93 was experimentally studied at ambient pressure at temperatures ranging from 800 to 1 200 c and oxygen fugacities from 10 11 to 10 17 bar diffusion couples were prepared by ablating an olivine x fo 0 3 target to deposit a thin film 20 100 nm onto a polished surface of a natural oriented diopside crystal using the pulsed laser deposition technique after diffusion anneals compositional depth profiles at the near surface region 400 nm were measured using rutherford backscattering spectroscopy in the experimental temperature and compositional range no strong dependence of d fe mg on composition of clinopyroxene fe mg ratio between di93 di65 or oxygen fugacity could be detected within the resolution of the study the lack of fo2 dependence may be related to the relatively high al content of the crystals used in this study diffusion coefficients d fe mg can be described by a single arrhenius relation with d text fe text mg 2 7 7 pm 4 2 7 times 10 7 text exp 3 20 7 pm 1 6 0 text kj text mol text rt m 2 text s d fe mg in clinopyroxene appears to be faster than diffusion involving ca species e g d ca mg while it is slower than d fe mg in other common mafic minerals spinel olivine garnet and orthopyroxene as a consequence diffusion in clinopyroxene may be the rate limiting process for the freezing of many geothermometers and compositional zoning in clinopyroxene may preserve records of a higher compared to that preserved in other coexisting mafic minerals temperature segment of the thermal history of a rock in the absence of pervasive recrystallization clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed 1 100 c e g resetting may be expected in slowly cooled mantle rocks many plutonic mafic rocks or ultra high temperature metamorphic rocks

regional metamorphism of the vassalboro formation south central maine usa a case study of the role of fluid in metamorphic petrogenesis progressively metamorphosed carbonate rocks of the vassalboro formation serve a natural laboratory for the investigation of fluid rock interactions during metamorphism an integrated study of their whole rock chemistry mineralogy mineral chemistry and mineral abundances has led to the following results a prograde mineral reactions involved hydrolysis as well as dehydration and decarbonation the hydrolysis reactions have resulted in an almost quantitative extraction of k and na from high grade metacarbonates b prograde mineral reactions buffered the x co 2 of coexisting metamorphic fluid during almost all of the rocks metamorphic history c the carbonate rocks were infiltrated by large volumes of h 2 o rich fluid while the buffering reactions progressed during the metamorphic event some high grade metacarbonates were infiltrated by at least 3 rock volumes of fluid combined buffering and infiltration appears to be a general metamorphic phenomenon in carbonate bearing rocks to depths of 30 km in the crust d there is an excellent positive correlation between the heat budget of individual metacarbonate rock samples and calculated fluid rock ratios convective heat transfer by metamorphic fluids is therefore likely to be an important item in the heat budget of metamorphic terrains consideration of these results leads to a model of regional metamorphism which involves infiltration of rock by large volumes of acid h 2 o rich fluid large scale acid infiltration metasomatism in this model the infiltrating fluid exerts enormous control over the mineralogical evolution of the carbonate rocks during the metamorphic event it drives mineral reactions it changes whole rock chemistry and it supplies heat

implications to garnet clinopyroxene geothermometry of non ideal solid solution in jadeitic pyroxenes garnet clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the sesia zone in the western alps discrepancies in apparent temperatures of up to 600 c have been found suggesting substantial deviation from ideal behaviour of the garnet clinopyroxene equilibria in certain assemblages assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low kd 2 0 values and correspondingly high apparent temperatures 1 000 c the garnets coexisting with high na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer numerous examples of coexisting garnet clinopyroxene pairs indicate that there is no obvious relationship of kd with jadeite content in pyroxenes with jadeite content less than 70 but at higher values kd varies inversely with the jadeite content the dependence of kd upon the jadeite content in the high na pyroxenes is believed to be due to the preference of m2 sites for fe2 in the enstatite jadeite substitution fe2 mgna 1al 1 this substitution is usually very limited and has no demonstrable effect upon x fe cpx when the m1 fe2 mg content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes however when the m1 fe2 mg content is low as in jadeitic pyroxenes the non ideality of the enstatite type exchange has a marked effect on x fe cpx and consequently on garnet clinopyroxene calculations

high pressure low temperature metamorphism in northern hubei province central china the qinling dabie accretionary fold belt in east central china represents the e w trending suture zone between the sino korean and yangtze cratons a portion of the accretionary complex exposed in northern hubei province contains a high pressure low temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote amphibolite eclogite facies the hongan metamorphic belt can be divided into three metamorphic zones based on progressive changes in mineral assemblages zone in the south is characterized by transitional blueschist greenschist facies zone is characterized by greenschist facies zone in the northern most portion of the belt is characterized by eclogite and epidote amphibolite facies sequences changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic p t conditions during formation of this belt preliminary p t estimates for zone i metamorphism are 5 7 kbar 350 450 estimates for zone iii eclogites are 10 22 kbar 500 50 the petrographic chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the sino korean and yangtze cratons

fluid inclusion evidence for fluid mixing mascot jefferson city zinc district tennessee inverse correlation between homogenization temperature and salinity temperature of formation changes at near constant salinity and low salinity low to moderate temperature inclusion fluids are found in sphalerite formed in limestone an evolved basinal brine component was recognized by its characteristic temperature 100 degrees to 120 degrees c freezing point depression 1 fundamental chemical requirements for formation of pb zn ba f deposits from metal bearing basinal brines modified journal abstract

reassessment of the garnet clinopyroxene fe mg exchange thermometer i evaluation of the pattison and newton 1989 experiments this is part 1 of a two part study on the garnet grt clinopyroxene cpx fe mg exchange equilibrium widely used in geothermometry of amphibolites granulites and eclogites the experimental data set previously published by pattison and newton pn on this equilibrium comprises potentially the largest source of data used for extraction of thermodynamic properties for grt and cpx and for formulation of a new thermometer expression part 2 of this study however the experiments suggested unusual grt and cpx solution properties and resulted in a geothermometer expression that generally returns low temperatures for granulites to verify the reliability of the experiments and to test the experimental technique of grt buffering used in the pn study 69 of the 125 pn runs were reanalysed using wavelength dispersive spectroscopic analysis guided by backscattered electron images special attention was given to analysis of compositional changes in grt in addition to cpx with minor exceptions the reanalysis supports the analytical results of the original pn study the mechanism of equilibration in the runs was a multicomponent solution precipitation process involving dissolution of metastable grt and cpx starting materials and precipitation of newly formed cpx and grt the latter assumed to represent equilibrim compositions the experiments cannot be regarded either as reversed or as strictly bracketing nevertheless cpx compositional parameters including mg na ca and al show the same smooth variations with temperature pressure and composition as found by pn suggestive of a close approach to equilibrium grt in run products retained the moderate heterogeneity of the starting grt although there are subtle compositional trends indicating small changes in grt mg consistent with mass balance constraints these results uphold the essential validity of the technique of grt buffering used in the experiments nevertheless it is possible that the final grt compositional may not coincide with the nominal starting grt compositions requiring assignment of compositional uncertainties to grt for purposes of thermodynamic modelling of the data cpx analyses also require assignment of non zero compositional uncertainties

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

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

how well known are the thermodynamics of fe mg ca garnet evidence from experimentally determined exchange equilibria many thermodynamic calculations relating to metamorphic rocks hinge on the thermodynamic parameters of garnet though some models are widely used it is not clear whether their underlying premise is correct that a single set of equations can be written for the activities of the end members of garnet covering the whole compositional range a voluminous body of data can be used to constrain the thermodynamics of garnet namely fe mg exchange experimental data involving garnet and another mineral particularly clinopyroxene orthopyroxene and olivine however examination of these data reveals inconsistencies apparently stemming from differences between the thermodynamics of low ca and high ca garnets with a boundary of about xgca 0 15 in the two regions for the high p t of the experimental data the thermodynamics follow the regular model with values for the interaction parameters in the low ca region of about wgfemg 50r and wafe wgmgca 1300r in which r is the gas constant and in the high ca region of about wgfemg 1100r and wgcafe wgmgca 2200r using the subregular rather than the regular model does not remove the discrepancy the cause of the discrepancy needs to be identified if reliable calculations on rocks are to be made

an evaluation of garnet clinopyroxene geothermometry in granulites four new formulations of the garnet clinopyroxene geothermometer ellis and green 1979 ganguly 1979 saxena 1979 dahl 1980 have been evaluated in the adirondacks and five other granulite terranes using results from 94 mineral pairs the saxena and ganguly formulations give temperatures that are generally 100 150 c above those constrained by phase equilibria and other independent thermometry while the empirical calibration of dahl gives widely scattered erratic results despite some scatter in the data the ellis and green calibration appears to be more accurate and precise than the others and is the most useful garnet clinopyroxene thermometer currently available for quantitative thermometry in granulites all four formulations are sensitive to large variations 70 80 c in temperature suggesting that problems with accuracy and precision can be improved with further refinement of model based aspects of the thermometers

experimental evaluation of garnet clinopyroxene geothermometry as applied to eclogites this study performed equilibrium experiments in order to evaluate previously proposed formulations of the garnet grt clinopyroxene cpx thermometer as applied to eclogites the starting material is fine grained powder of natural eclogite 10 m whose main constituents are grt fe mg ca 44 28 28 cpx na pfu 0 55 0 60 phengite quartz and rutile experimental conditions are 1 100 1 250 c at 2 5 gpa and the run duration is 193 334 h the experimental run products mainly consist of grt cpx and glass in a preliminary experiment at 1 000 c for 144 h cpx grains are clearly zoned and most grt grains maintain primary compositions in the higher t 1 100 c and longer run 193 h experiments cpx in the run products becomes poorer in na and higher in fe mg compared with the starting material and each grain does not show clear chemical zoning garnet compositions become poorer in ca ca fe mn mg ca 0 2 0 25 and lower in fe mg compared with the starting material the average composition of cpx and the average of ca poor grt compositions in each run product were used to evaluate previously proposed formulations of the grt cpx thermometer temperatures calculated with formulations by pattison and newton 1989 and berman et al 1995 are much lower than the experimental temperatures even though these formulations are based on the compositional bracketing type experiment one of the reasons for this discrepancy might be uncertainty of solid solution properties of al in cpx because the value of the excess interaction parameter for al in the generally low al cpx modeled by berman et al 1995 is much higher than those proposed by independent experiments resulting in the estimated temperatures being significantly lower than the experimental temperatures

ultrahigh pressure metamorphism in the śnieznik mountains sudetes poland p t constraints and geological implications the petrology and p t conditions of eclogites and associated gneisses from the nieznik mountains sudetes sw poland were investigated to establish an exhumation path for these ultrahigh pressure rocks peak pressure conditions are represented by the assemblage garnet omphacite phengite kyanite coesite rutile post dating this assemblage but still in the eclogite facies garnet with lower grossular content and a first generation of amphibole are formed under lower pressure conditions giving rise to the assemblage garnet omphacite phengite kyanite quartz rutile calcic amphibole i continuing depressurization caused the successive destabilization of omphacite garnet kyanite and phengite in some places the primary eclogite assemblage was completely transformed into amphibolite facies mineral assemblages estimated peak pressures in the nieznik and miedzygorze units lie in the coesite stability field p 27 kbar at temperatures of about 700 800 c and are not significantly different to the zlote un

zeolitization of basalts in subaqueous freshwater settings field observations and experimental study abstract zeolitization of volcanic flows was studied using both field observations and experimental investigations field observations show that basalt flows located in two areas massif central france and cascade range usa exhibit similar features of zeolitization associated with the interaction between flow rocks and fresh water as well as the thermal history of the volcanic sheet results are reported of an experimental study which determined the conditions of hydrothermal crystallization under constant pressure 200 bars in a thermal gradient at relatively low temperatures 140 200 c and 210 280 c two sets of experiments with two kinds of reactants mineral and bulk rock were conducted in sealed gold tubes containing deionized water ph 6 3 with a fluid rock mass ratio of 10 the starting materials correspond to different types of alteration conditions 1 a phillipsite plus kaolinite assemblage which represents the chemical limited system sio 2 al 2 o 3 cao k 2 o h 2 o 2 an alkaline basalt and 3 an obsidian two phenomena were found to depend upon the thermal gradient a parallel distribution of minerals implying local equilibria and a nonparallel phenomenon implying an irreversible process dissolutioncrystallization controlling the succession of crystallization field and experimental data demonstrated that zeolitization resulted from the combined effects of an initial thermal gradient between the base and the interior of the flow and its cooling history