multichronometric evidence for an in situ origin of the ultrahigh pressure metamorphic terrane of dabieshan china abstract u pb zircon data on gneisses indicate a late proterozoic protolith at 700 ma at bixiling and a late archean to early proterozoic for the shuanghe sheet within the ultrahigh pressure metamorphic terrane of dabieshan china u pb zircon and sm nd dates on the gneisses of the shuanghe sheet constrain a metamorphic peak at 230 ma rb sr and 40ar 39ar analyses of phengite from five gneisses in the bixiling complex yield ages at 198 212 ma overlapping the 190 220 ma age on eclogites the host gneisses record a similar metamorphic evolution as the coesite bearing eclogites since both the peak and the retrograde evolution were contemporaneous this argues for an in situ tectonic relationship

oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

mineral inclusions in zircons of para and orthogneiss from pre pilot drillhole ccsd pp1 chinese continental scientific drilling project the pre pilot drillhole ccsd pp1 chinese continental scientific drilling project ccsd with depth of 432 m is located in the donghai area in the southwestern sulu terrane the core samples are mainly comprised of paragneiss orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite bearing kyanite quartzite all analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism coesite and coesite bearing ultrahigh pressure uhp mineral assemblages were identified by raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss eclogite and phengite bearing kyanite quartzite samples in the paragneiss samples uhp mineral inclusion assemblages mainly consist of coe omp grt phe coe jd phe ap preserved in the mantles m and rims r of zircons these uhp mineral inclusion assemblages yield temperatures of 814 852 c and pressures of 28 kbar presenting the p t condition of uhp peak metamorphism of these country rocks according to the mineral inclusions and cathodoluminescence images of zircons the orthogneisses can be divided into two types uhp og1 and non uhp og2 in og1 orthogneisses low pressure mineral inclusion assemblage mainly consisting of qtz phe ab ksp ap were identified in zircon cores c while coesite or coesite bearing uhp mineral inclusions were identified in the mantles m and rims r of the same zircons these features suggest that the og1 orthogneisses together with the paragneisses phengite bearing kyanite quartzite and eclogite experienced widespread uhp metamorphism in the sulu terrane however in the zircons of og2 orthogneiss samples no uhp mineral inclusions were found inclusions mainly comprised qtz phe ap and were identified in cores c mantles m and rims r of og2 zircons the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims these features indicate that the og2 orthogneisses in pre pilot drillhole ccsd pp1 did not experience uhp metamorphism therefore we should not rule out the possibility that some orthogneisses in sulu terrane might represent relatively low pressure granitic intrusives emplaced after the uhp event

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

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

oxygen isotope equilibrium between eclogite minerals and its constraints on mineral sm nd chronometer abstract sm nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the sulu terrane in eastern china the results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and sm nd isotope systems of eclogite minerals the omphacite garnet pairs of oxygen isotope equilibrium at eclogite facies conditions yield meaningful triassic sm nd isochron ages whereas those of oxygen isotope disequilibrium give non triassic ages of geological meaninglessness this can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than or close to those of nd diffusion and thus attainment of isotopic equilibrium in the omphacite garnet o system suggests achievement of nd isotope equilibrium in the same mineral pairs the presence or absence of fluid in the eclogite protoliths is a major rate controlling factor for isotopic equilibration during high grade metamorphism it appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the sm nd mineral chronometer in addition the exact timing of the ultrahigh pressure metamorphism in the dabie sulu terranes is constrained at early triassic rather than late triassic

nd and sr systematics in an eclogite xenolith from tanzania evidence for frozen mineral equilibria in the continental lithosphere combined isotopic nd sr and petrographic analyses are reported for an eclogitic xenolith from the vissury mine tanzania an exceedingly large difference in measured nd values in garnet and clinopyroxene 200 units suggest a major mineral equilibration at 1 75 0 014 ga nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in sr and nd isotope compositions is present within the clinopyroxene the variation in measured nd isotope compositions in different generations of garnet corresponds to 16 units exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to sr and nd petrographically different generations of garnet are present one garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron features like this can originate either if a system is not closed e g affected by a migrating fluid or if a remanent isotopic heterogeneity survived the homogenisation at 1 75 ga the subsolidus partitioning of sm and nd between garnet and clinopyroxene results in higher fractionation of the smnd ratio in garnet than in the liquid crystal partitioning since the closure temperature for the sm nd exchange between garnet and clinopyroxene may be around 850 c in a dry system many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria the isotopic data suggest that this eclogite was emplaced at 1 75 ga into the lithosphere mantle at temperatures of about 1040 c and then cooled as a closed system at a rate of 6 50 c my down to 840 c where all the exchange reactions were efficiently frozen in at 1 75 ga ago the coincidence of the emplacement of granites 1 7 1 8 ga into the central part of the tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere

an inverse method of modeling thermal histories from apatite fission track data abstract apatite fission track ft ages and track length distributions are important sources of information about the thermal histories of rocks recent advances in the understanding of track annealing in apatite provide a solution to the forward problem of predicting ft ages and track length distributions that result from a given thermal history in this paper a method is presented to address the inverse problem estimation of the thermal history from ft data the inverse method uses an iterative approach the downhill simplex to systematically modify a starting thermal history to achieve satisfactory statistical agreement between the predicted and observed ft age and track length distribution however because of analytical uncertainties a unique thermal history does not exist monte carlo simulations are used to take into account the uncertainties in the data and yield a spectrum of possible thermal histories the results are based on the isothermal annealing model of laslett et al 1 chem geol isot geosci sect vol 65 but because the method uses forward predictions and not an analytical formula specific to a single annealing model alternative annealing models could be used within the same framework this method is an improvement in interpreting fission track data because the range of thermal histories permitted by the data can be evaluated to demonstrate the method it is applied to several sets of test data created by forward modeling of idealized thermal histories the results show that the age at which the sample cooled through its closure temperature and the most general features of the thermal history are revealed however data from partially annealed samples have wide spectra that reflect large uncertainties in the form of the thermal history for such samples ft data alone are not likely to provide strong tests of geologic hypotheses conversely geologic information may play a crucial role in constraining the thermal history spectrum in application to uplift related cooling this inverse method can provide much more realistic assessments of the cooling rate and its uncertainty than are otherwise possible

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

structures and fabrics in glacial ice a review abstract glaciers ice sheets and ice caps represent tectonic systems driven by gravity their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements all glacial ice has primary stratification exhibited by variations in grain size bubble content and debris content as it deforms with deformation dominated by plastic flow and recrystallization accompanied locally by fracture under tension a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten foliation develops from inherited mostly stratification or introduced mostly ice veins or fracture traces fabric elements and from dynamic recrystallization it is largely dependent on the accumulated strain which is highest at the base and near the margins of glaciers ice sheets and ice streams folds develop largely passively due to initial amplification of irregularities in the primary stratification to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures buckle folds and boudinage mostly on a small scale occur where viscosity contrast is large mostly in basal ice thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

effective shear viscosity and effective bulk viscosity of firn of a temperate glacier kesselwandferner otztal alps 1967 1978

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

comparison of laser ablation icp ms and isotope dilution ree analyses implications for sm nd garnet geochronology abstract high precision garnet chronometry using the sm nd u pb rb sr or lu hf isotope systems is gaining widespread application for constraining the prograde and high temperature history of metamorphic belts however due to the low concentrations of the chronometrically important elements garnet analyses are often susceptible to incorporation of small quantities of accessory phases such as zircon monazite and allanite here we assess the extent to which such inclusions affect garnet analyses by comparing in situ analyses of sm and nd concentrations in the crystal lattice obtained by laser ablation inductively coupled mass spectrometry la icp ms with those obtained by isotope dilution id on separates of the same garnets the results show that the garnets studied here are largely free from inclusions in general however the low concentrations of sm and nd in the garnet lattice mean that the incorporation of even very small quantities of accessory phases will result in a significant change in the measured garnet sm and nd concentration simple modelling of the effect of such inclusions on age information shows that garnet inclusion disequilibrium and the extent to which inclusions decrease the apparent parent daughter ratio are critical such modelling demonstrates that the incorporation of old lree enriched minerals such as monazite will yield an apparently lower age than the true garnet age however this study indicates that erroneous age information can be identified if duplicate garnet analyses are performed in addition la icp ms is shown to be a useful technique in verifying the purity of samples for isotope analysis

high net accumulation rates at campo de hielo patago nico sur south america revealed by analysis of a 45 97 m long ice core a45 97mlongicecorewasrecoveredintheaccumulationareaof glaciar tyndall 50 59 05 s 73 31 12 w 1756ma s l campo de hielo patagonico sur southern patagoniaicefield during december1999 thefirncorewassubjectedtovisualstratigraphic observationandbulkdensitymeasurementsinthefield andlatertoanalysesofwaterisotopes d 18 o dd major dissolved ions and snow algalbiomass the drillhole remained dry down to about 43m depth where a water soaked layer appeared seasonal cycles were found for d 18 o dd and the d excess although the amplitudes of the cycles decreased with depth major dissolved ions na k mg 2 ca 2 cl so4 2 and algal biomass exhibit rapid decreases in the upper 3m probably due to meltwater elution annual increments defined by the d 18 o and d excess peaks suggest that the minimum net accumulation rates at this locationwere17 8ma 1 in1997 98 1998 99 and 411 0ma 1 in1998 99 1999 2000 theseare much higher valuesthanthose previously obtained frompast ice core studies in patagonia but are of the same order of magnitude as those predicted from various observations in ablation areas of patagonian glaciers

oxygen isotope geochemistry of ultrahigh pressure metamorphic rocks from 200 4000 m core samples of the chinese continental scientific drilling abstract oxygen isotope studies were carried out for ultrahigh pressure metamorphic rocks from the main hole of the chinese continental scientific drilling in the sulu orogen the samples of interest include various types of lithology mainly eclogite and gneiss in a depth of 200 to 4000 m containing five continuous core segments between contrasting lithologies the results show a large variation in 18o value from 10 41 to 9 63 for constituent minerals distinct 18o depletions are observed in the frequently alternated layers of eclogite and gneiss in which the variations of 18o values are gradual regardless of lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals special attention was paid to the relationship between distance petrography and 18o value of adjacent samples the results show o isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm corresponding to the maximum scales of fluid mobility during the continental collision amphibolite facies retrograde metamorphism during exhumation caused mineral reactions and o isotope disequilibria between some of the minerals considerable changes occur in 18o and petrography at the contact between eclogite and gneiss suggesting that the contact between different lithologies is the most favorable place for fluid activity despite the widespread retrogression retrograde fluid was internally buffered in the stable isotope compositions the retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl although locally external fluids became available along fault zones and lithological layers it still acted within the exhumed slabs with internal origins fluid flow also took place after the amphibolite facies retrogression but it only affects the o isotope compositions of feldspar and mica premetamorphic protoliths are deduced to have heterogeneous 18o values due to varying degrees of meteoric water rock interaction before continental collision minimum depth of the 18o depletion is up to 3300 m together with areal 18o depletion in surface outcropped rocks along the dabie sulu orogenic belt at least 66 000 km3 of supracrustal rocks were interacted with meteoric water along the northern margin of the south china block during the neoproterozoic

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

geochemistry and origin of eclogites from the type locality koralpe and saualpe eastern alps austria abstract in the koralpe and saualpe polycrystalline basement which is now a part of the austroalpine nappe system eclogites and metagabbros occur intercalated in gneisses and kyanite bearing schists we report here the chemical as well as sr and o isotopic composition of such eclogites and one metagabbro a low ti kyanite bearing and a high ti kyanite free group can be distinguished which may be related by igneous differentiation the low ti group which comprises eclogites and the metagabbro has chemical characteristics indicating formation of the igneous precursors as cumulates by crystallization of pyroxene and plagioclase from a mafic magma in contrast the protoliths of the high ti eclogites appear to be basaltic liquids with normal to enriched type morb affinities which are likely to have formed at an oceanic ridge their sr and o isotope relationships strongly suggest that they have suffered both a phase of low temperature and a phase of high temperature hydrothermal alteration by reaction with seawater

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

sm nd disequilibrium in high pressure low temperature himalayan and alpine rocks in order to decipher the causes of sm nd isotopic disequilibrium in high pressure low temperature rocks sm nd isotopic analyses were carried out on minerals from four himalayan tso morari unit and four alpine dora maira monte viso sesia lanzo eclogitic rocks of different lithologies and different intensities of post eclogitic metamorphism in most of these

oxygen diffusion rates in quartz exchanged with co2 oxygen self diffusion rates were determined in quartz samples exchanged with18o enriched co2 between 745 and 900 c and various pressures and the diffusion profiles were measured using an ion microprobe the activation energy q and preexponential factor d0 at p co2 p tot 100 bar for diffusion parallel to the c axis are 159 13 kj g atom and 2 10 0 75 0 55 10 8 cm2 s this rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1 bar quartz o2 exchange experiment the oxygen diffusion rate measured at 0 6 bar 888 c and at 900 c in vacuum is in agreement with the previous 1 bar exchange experiments with18o2 the effect of higher co2 pressures is small at 900 c the diffusion rate exchanged with co2 is 2 35 10 15 cm2 s at 100 bar 2 24 10 15 cm2 s at 3 45 kbar and 8 13 10 15 cm2 s at 7 2 kbar there is probably a diffusing species other than oxygen that enhances the oxygen diffusion rate in these quartz co2 systems relative to that occurring at very low pressures or in a vacuum the effect of this diffusing species however is not as strong as that associated with h2o preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled high grade metamorphic terrane will vary depending upon whether a water rich phase was present or not closure temperatures will be approximately 100 c higher in rocks where no water rich phase was present during cooling the measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression

timing of high pressure metamorphism and exhumation of the eclogite type locality kupplerbrunn prickler halt saualpe south eastern austria constraints from correlations of the sm nd lu hf u pb and rb sr isotopic systems sm nd lu hf rb sr and sims u pb data are presented for meta gabbroic eclogites from the eclogite type locality hauy 1822 kupplerbrunn prickler halt and other areas of the saualpe se austria and pohorje mountains slovenia mg rich eclogites derived from early gabbroic cumulates are kyanite and zoisite rich whereas eclogites with lower mg contents contain clinozoisite kyanite calculated p t conditions at the final stages of high pressure metamorphism are 2 2 0 2 gpa at 630 740 c kyanite rich eclogites did not yield geologically meaningful sm nd ages due to incomplete nd isotope equilibration whereas sm nd multifraction garnet omphacite regression for a low mg eclogite from kupplerbrunn yields an age of 91 1 1 3 ma the sm nd age of 94 1 0 8 ma obtained from the fe rich core fraction of this garnet dates the initial stages of garnet growth zircon that also crystallized at eclogite facies conditions gives a weighted mean u pb sims age of 88 4 8 1 ma lu hf isotope analysis of a kyanite eclogite from kupplerbrunn yields 88 4 4 7 ma for the garnet omphacite pair two low mg eclogites from the gertrusk locality of the saualpe yield a multimineral sm nd age of 90 6 1 0 ma a low mg eclogite from the pohorje mountains 70 km to the se gives a garnet whole rock lu hf age of 93 3 2 8 ma these new age data and published sm nd ages of metasedimentary host rocks constrain the final stages of the eo alpine high pressure event in the saualpe pohorje part of the south easternmost austroalpine nappe system suggesting that garnet growth in the high pressure assemblages started at c 95 94 ma and ceased at c 90 88 ma probably at the final pressure peak zircon and amphibole crystallization was still possible during incipient isothermal decompression rapid exhumation of the high pressure rocks was induced by collision of the northern apulian plate with parts of the austroalpine microplate following jurassic closure of the permo triassic meliata back arc basin

isotopic disequilibrium in ultrahigh pressure and retrograde metamorphism of eclogite and gneiss from the chinese continental scientific drilling in the sulu orogen china evidence from mineral nd sr o isotopic composition diffusion rates of sr and o in minerals are often comparable while nd has a lower diffusion rate during thermal overprint s thus the o isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of rb sr and sm nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism this study presents a combination of investigation on sm nd rb sr and o isotopic compositions of minerals separated from ultrahigh pressure eclogite and gneiss that were collected from the main hole of the chinese continental scientific drilling project located in the sulu orogen eastern china oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620 740 and 460 590 c representing diffusion cessation of isotopic exchange during the eclogite facies recrystallization and later amphibolite facies retrograde overprint rb sr mineral regressions of two eclogite samples give consistent triassic ages of 244 ma corresponding to eclogite facies metamorphism while the same minerals do not yield meaningful sm nd isochron ages this phenomenon likely suggests that rb sr isotopic equilibrium was achieved during eclogite facies metamorphism and preserved during late amphibolite facies retrogression in contrast sm nd isotopic equilibrium between the minerals of eclogite was not achieved under uhp metamorphic conditions regressions of epidote and biotite of one gneiss sample give a triassic sm nd age of 243 34 ma corresponding to the time of the eclogite facies metamorphism and a jurassic mineral rb sr age of 187 5 1 8 ma these results imply that fluids have played an important role to achievement of the sm nd isotopic equilibrium during eclogite facies metamorphism and re equilibration of the rb sr isotopic system during later retrograde overprint

airborne radar stratigraphy and electrical structure of temperate firn bagley ice field alaska u s a helicopter borne 135 mhz short pulse radar profiles of the bagley ice field southern alaska u s a have been analyzed to determine the nature of radio wave propagation through crevassed stratified wet and draining firn diffractions characterize the firn horizons along the trunk and reflections characterize those of overlying snow fields dynamic stacking was used to form and determine firn depths and unstacked diffraction analysis to determine firn layer properties refractive indices range from 4 1 to 4 5 for the near surface from 1 7 to 2 1 at about 13 17 m depth along the main trunk and to 2 6 to 58 m depth within a snowfield average trunk values correspond with volumetric water contents of about 0 09 which agree with values measured for other glaciers the analysis of the airborne recorded diffractions is confirmed by deriving the approximate refractive index of water from a supraglacial lake bottom diffraction the lack of snowfield diffractions suggests that crevassing and not firn structure caused the trunk diffractions the reasonable values of the indices imply that the diffractions originated from single points or edges orthogonal to the profile transect and they predict low interlayer transmission losses the snowfield penetration suggests that several hundred meters might be penetrated in uncrevassed accumulation zones with improved system design

temporal relationship between granite cooling and hydrothermal uranium mineralization at dalongshan in china a combined radiometric and oxygen isotopic study abstract a combined study using multi radiometric dating and oxygen isotopic geothermometry was carried out for mesozoic quartz syenite alkali feldspar granite and associated hydrothermal uranium mineralization at dalongshan in the middle lower yangtze valley of east central china radiometric dating of the quartz syenite yields a whole rock rb sr isochron age of 135 6 4 3 ma a zircon u pb isochron age of 132 9 2 2 ma and k ar ages of 126 2 118 3 and 94 4 ma for hornblende biotite and orthoclase respectively the alkali feldspar granite yields a whole rock rb sr isochron age of 117 3 3 3 ma a zircon u pb isochron age of 114 7 2 1 ma and k ar ages of 112 2 109 3 and 88 4 ma for hornblende biotite and orthoclase respectively oxygen isotope thermometry for both granites gives temperatures of 685 to 720 555 to 580 435 to 460 and 320 to 330 c for hornblende magnetite biotite and orthoclase respectively when paired with quartz the systematic differences among the ages by the different techniques on the different minerals are used to reconstruct the cooling history of the granite the results yield rapid cooling rates of 27 4 to 58 6 c ma from 800 to 300 c in the early stage but slow cooling rates of 6 3 to 7 2 c ma from 300 to 150 c in the late stage the regular sequence of oxygen isotope temperatures for the different quartz mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of both radiometric and o isotopic systems during granite cooling pitchblende u pb isochron dating yields an uranium mineralization age of 106 4 2 9 ma which is younger than the age of the granite emplacement and thus considerably postdates the time of magma crystallization but is close to the closure time of the k ar system in the biotite this points to a close relationship between granite cooling and ore forming process it appears that hydrothermal mineralization took place in the stage of slow cooling of the granite whereas the rapid cooling of the granite was concurrent with the migration of hydrothermal fluids along fault structures therefore the activity of the ore forming hydrothermal system is temporally dictated by the cooling rates of the granite and may lag about 25 to 30 ma behind the crystallization timing of associated granite

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

late proterozoic magmatism and metamorphism recorded in gneisses from the dabie high pressure metamorphic zone eastern china evidence from zircon u pb geochronology this study presents zircon ages and geochemical and nd isotopic data for metagranites and quartzites from the susong high pressure metamorphic zone in the dabie sulu ultrahigh pressure collisional belt this belt belongs to the eastern part of the qinling dabie orogenic belt that formed during early mesozoic collision of the north and south china blocks two metagranites give zircon u pb model ages of 785 7 and 205 12 ma likely representing a late proterozoic magmatic event and an early mesozoic overprint they have low initial end values 12 4 and 11 1 at 780 ma favouring a crustal origin flat heavy rare earth elements ree patterns gdn ybn 1 2 probably reflect that melting took place at a shallow crustal section where heavy ree bearing mineral phases are unstable all zircons of three quartzites yield young discordant u pb ages and define a discordia with u pb model ages of 784 6 and 213 3 ma identical to those of the metagranites we assume that all detrital zircons had lost radiogenic pb prior to the early mesozoic overprint probably facilitated by fluid participation during a metamorphic event contemporaneous with the intrusion of the metagranites this simultaneous metamorphic and magmatic event was probably related to a rift setting along the periphery of the yangtze south china block during late proterozoic

strontium diffusion kinetics in amphiboles and significance to thermal history determinations abstract strontium tracer diffusion coefficients have been determined in hornblende and tremolite single crystals using experiments conducted at a confining pressure of 200 mpa at temperatures ranging from 700 to 960 c a quasi hydrothermal experimental technique was devised to avoid solution precipitation effects depth profile analyses were made with an ion microprobe the arrhenius relation for diffusion parallel to the c crystallographic direction in the gore mountain hornblende over this temperature interval has an activation energy q 260 12 kj mol and a pre exponential factor d 0 4 9 18 1 3 10 8 m 2 s at 800 c transport rates parallel to the c axis measured in the two tremolite samples amnh 30563 and barrie township ontario are 3 2 10 21 m2 s and 2 0 10 21 m2 s respectively these values are factors of 3 and 5 slower than those observed for the hornblende sample at the same temperature transport rates parallel and perpendicular to the c axis are the same within the estimated uncertainty for all three compositions experiments that differed in o 2 by a factor of 6 107 mno mn3o4 mno and magnetite wustite mw for the hornblende gave the same strontium diffusion rates within a factor of two crystals with cleavage faces or polished with alumina or diamond gave the same d values provided the crystals were annealed prior to the diffusion experiment using a combination of closed system rb sr systematics and strontium diffusion kinetics it is now possible to determine the thermal histories of hornblende bearing igneous and metamorphic rocks since the diffusion rates for strontium in amphiboles are relatively slow hornblende will be more retentive of sr than of other species for which transport rates are known ar o h and will be a useful tool to record higher temperature portions of cooling paths