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

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

correlation between melting deformation and fluid interaction in the continental crust of the high himalayas langtang valley nepal combined metamorphic stable isotope and structural studies from the high himalayan crystalline sequence in the langtang valley of north central nepal reveal a strong positive correlation between distance above the base of the section the main central thrust the amount of melt material and evidence of prolonged fluid and deformation histories thus suggesting that these processes are strongly interdependent kyanite grade rocks at the base of the section are unmelted and have undergone little syn or post metamorphic internal deformation and little prograde or retrograde fluid rock interaction by contrast sillimanite grade rocks higher in the section contain progressively larger volumes of melt have suffered increasingly complex syn and post metamorphic deformation and show increasing evidence for the presence of fluids although the factors that initiated these processes remain problematic it is suggested that fluid distributions within the langtang section have been passively controlled by the movement of melts these melts may have provided a primary control on deformation during both the magmatic stage and subsequently through the exsolution of exsolved aqueous fluids

oxygen isotope exchange and closure temperatures in cooling rocks retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model which predicts both the zonation profile of 18o within grains and the bulk 18o value of each mineral in the rock apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 18o values of each mineral pair within the rock in systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values slow cooling the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the dodson closure temperatures tc for the two phases and the mode of the rock adding a third phase into a system which undergoes slow cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two phase system in some systems oxygen isotope reversals can be developed if cooling is not slow then the mineral 18o values resulting from cooling will be partly dependent upon the initial temperature of the system concerned the model successfully simulates the mineral 18o values that are often observed in granitic rocks application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems

geomagnetic field intensity during the last 5 myr ltd dht shaw palaeointensities from volcanic rocks of the society islands french polynesia summary on the basis of the thellier palaeointensities reported so far it has been thought that the time averaged virtual dipole moment vdm and virtual axial dipole moment vadm for the last few million years are almost the same as the present geomagnetic dipole moment 8 1022 a m2 this estimate has been called into question however because recent studies have revealed that the thellier method occasionally overestimates palaeointensities by as much as twice the true values in contrast a recently developed palaeointensity technique namely the double heating technique of the shaw method combined with low temperature demagnetization the ltd dht shaw method can yield expected field intensities from samples which give unreliable palaeointensities using the thellier method therefore we have measured absolute palaeointensities from 0 5 4 6 ma volcanic rocks from the society islands french polynesia mainly using the ltd dht shaw method as a result 195 out of 361 samples passed the selection criteria some of which are compared with additional results obtained with coe s version of the thellier method in the thellier experiments 18 out of 40 samples passed the criteria giving palaeointensities both consistent and inconsistent with the ltd dht shaw results these samples are characterized mostly by two segmented arai diagrams if we take the ltd dht shaw palaeointensities 24 reliable site means are available and give a mean vadm of 3 64 2 10 1022 a m2 this is nearly half of the mean of the 0 5 ma thellier data selected from the latest palaeointensity database 7 46 3 10 1022 a m2 n 458 as well as the present dipole moment the ltd dht shaw palaeointensities newly determined in this study suggest that the present day field is so strong that it may not be typical of the past geomagnetic field

upstream sediment input effects on experimental dune trough scour in sediment mixtures 1 understanding causes of dune irregularity especially dune trough scour is important for the modeling of vertical sorting of sediment mixtures in morphological models of rivers with sediment mixtures sediment in dunes is generally sorted in a fining upward manner which affects the sediment transport composition depending on the scour depth distribution of the dunes why dunes become more irregular and develop deep scour holes in some conditions is only partially understood moreover there is a feedback between vertical sorting and dune irregularity in gravelly sands erosion resistant coarse layers may form that decrease or inhibit dune trough scour the causes of dune irregularity and the feedback by coarse sediment layers are explored in experiments and are demonstrated to be related partly to the upstream sediment boundary condition determined by the experimental setup sediment feeding or recirculating sediment recirculation flumes promote fining of the transported sediment and formation of a less or immobile gravel lag layer in the dune troughs sediment feed flumes may force the transported sediment at the flume entry to be equal to the bed sediment equal mobility condition and hence allow no lag layer formation experiments show that the dune trough scour in recirculation is less deep than in feed flumes and the vertical sorting and transport sediment composition are different the experiments also indicate that dune irregularity in feed flumes is related to dimensionless shear stress similarly as in uniform sediments interpretation of experiments of dune dynamics in sediment mixtures should therefore account for differences in the upstream sediment supply condition more in general it is hypothesized that the upstream sediment supply condition resembling either recirculation or feed flumes in rivers may affect dune and vertical sorting dynamics

anisotropic fe mg diffusion in biotite marked variations of biotite fe mg compositional profiles on approach to interfaces with garnet as a function of biotite crystallographic orientation are found in a natural high temperature metamorphic rock for biotite with 001 planes nearly parallel to the interface mg increases and fe decreases steeply toward the interface if oblique the profile is less steep and if nearly perpendicular the profile is flat these variations are evidences for anisotropic fe mg intradiffusion within biotite during cooling for precise application of geothermobarometry and geospeedometry the crystallographic orientation and compositional gradient in biotite must be considered

experimental geochemical mineralogical and o isotope constraints on the late magmatic history of the fichtelgebirge granites germany mineralogical experimental geochemical and stable isotope data give evidence for an aqueous late stage fluid which develops increasingly from the less to the more highly evolved granites of the fichtelgebirge g1 g4 in less evolved granites particularly g1 only minor chloritization and sericitization minor hydration and corrosion of zircons low leachable fractions of sr and zr the nearly chondritic ratios of zr hf and y ho and rare earth element ree patterns lacking the tetrad effect provide evidence of a less differentiated granitic melt system poor in complexing agents and water with insignificant late magmatic fluid rock interaction the subsequent low temperature alteration which is suggested by oxygen isotope composition of quartz and biotite did not affect the whole rock chemical composition in the highly evolved granites particularly g4 albitization sericitization and fluoritization ubiquitous hydration and corrosion of zircons high leachable fractions of sr and zr zr hf and y ho ratios very different from the chondritic ratios and ree patterns showing a marked tetrad effect point to the presence of a highly specialized fluid system rich in water and complexing agents such as fluorine leading to a marked magmatic hydrothermal alteration the oxygen isotopes also indicate alteration by high temperature fluids which were likely exsolved during the final stage of crystallization

high strain deformation and fluid infiltration diachronism of the middle crust new devonian permian alice springs ages 365 290 ma of shear zones in the strangways metamorphic complex central australia abstract the proterozoic granulite facies strangways metamorphic complex smc in the arunta region of central australia was partially hydrated during the devonian carboniferous 380 320 ma alice springs orogeny by amphibolite greenschist facies high strain deformation stable and radiogenic isotope analyses on coeval muscovite and biotite from shear zone samples of the smc were performed to analyse the composition and date the timing of fluid circulation and cooling stable isotope data 18 o and d indicate that in the south of the smc the source of fluids appear to be the thick successions of neoproterozoic heavitree quartzite while another source in the north and central part of the smc may be related to meteoric fluids the heterogeneous 18 o fluid and homogenous d fluid values for muscovite reflect changes in water rock ratios with initial high values in the south and lower in the north possibly suggesting a migration of fluids from south to north as indicated by the evolving 18 o fluid and homogenous d fluid values rb sr muscovite ages of 365 340 ma approximate the time of crystallization and fluid infiltration they also indicate that the shear zones were active at different times during the alice springs orogeny and that some were active prior to the peak carboniferous amphibolite facies metamorphism 40 ar 39 ar muscovite ages of 350 325 ma are mostly interpreted as cooling ages rb sr biotite ages of 335 290 ma are interpreted as cooling ages at 300 c sr closure temperature for biotite and extend the activity of the alice springs orogeny until the permian cooling histories of the wallaby knob and harry creek shear zones which serve as the tectonic boundaries of the smc to the north and south respectively indicate slow cooling 1 for c 30 ma 360 330 ma at both boundaries continuing until 290 ma in the western portion of the harry creek shear zone this slow cooling is associated with passive exhumation i e erosion rapid cooling 20 10 c ma 1 then occurred over c 10 ma 330 320 ma in the wallaby knob shear zone and in the east portion of the harry creek shear zone this change in cooling rate is associated with the combined initiation in the carboniferous of dextral and thrusting deformation by the pinnacles bore and erontonga shear zones

himalayan inverted metamorphism constrained by oxygen isotope thermometry inverted metamorphic field gradients are preserved in two amphibolite facies metapelitic sequences forming the crystalline core zone of the himalayan orogen in the sutlej valley nw india in the high himalayan crystalline sequence hhcs metamorphic conditions increase upwards from the staurolite zone at the base through the kyanite in and sillimanite in isograds finally to reach partial melting conditions at the top the structurally lower lesser himalayan crystalline sequence lhcs shows a gradual superposition of garnet in staurolite in and kyanite sillimanite in isograds although phase equilibria constraints imply inverted temperature field gradients in both units garnet biotite garb rim thermometry indicates final equilibration at a nearly uniform temperature around t 600 c across these sequences the p t path and garnet zoning data show that this apparent lack of thermal field gradient is mainly the consequence of a resetting of the garb equilibria during cooling in order to constrain peak temperature conditions 20 samples along the studied section have been analysed for oxygen isotope thermometry the isotopic fractionations recorded by quartz garnet and quartz aluminosilicate mineral pairs indicate temperatures consistent with phase equilibria and p t path constraints for metamorphic peak conditions together with barometry results based on net transfer continuous reactions the oxygen isotope thermometry indicates peak conditions characterized by 1 a temperature increase from t 570 to 750 c at a nearly constant pressure around p 800 mpa from the base to the top of the hhcs unit 2 a temperature increase from t 610 to 700 c and a pressure decrease from p 900 to 700 mpa from the base to the top of the lhcs metapelites oxygen isotope thermometry thus provides the first quantitative data demonstrating that the himalayan inverted metamorphism can be associated with a complete inversion of the thermal field gradient across the crystalline core zone of this orogen

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

patterns of oxygen isotope depletion multiple hydrothermal circulation systems and the cooling history of the stony mountain intrusive complex colorado abstract oxygen isotopic compositions of 27 whole rock and mineral samples from the three intrusive phases of the stony mountain stock san juan mountains colorado have been measured and are combined with published analyses to obtain a history of the meteoric water circulation that cooled the intrusions the order of intrusion was the outer diorite the gabbro and the inner diorite the new whole rock data give 18 o smow 1 9 to 3 7 for the gabbro and 0 4 to 4 9 for the inner diorite five mineral analyses are reported for the inner diorite three feldspars ranging from 0 2 to 4 3 one quartz 8 2 and one pyroxene 5 3 all three intrusions are significantly depleted in 18 o relative to normal igneous rocks this depletion occurred by subsolidus exchange of oxygen isotopes as shown by the correlation between the extent of depletion and the grain size and by the non equilibrium 18 o concentrations in coexisting minerals the patterns of 18 o depletion which are complex suggest that separate hydrothermal circulation systems were set up by the cooling of each intrusion in turn the minimum water to rock ratio required to effect the observed depletion in the inner diorite is approximately 0 18 although the ratio for the total flux was probably much greater the mineral data show marked 18 o depletion in the feldspars and little if any depletion in the quartz application of oxygen diffusion data for feldspar and quartz leads to the conclusion that the effective temperature for oxygen isotope exchange was below 600 c and above 400 c furthermore these data place constraints on convective cooling times for this intrusion minimum times for cooling of the 500 m diameter inner diorite through this temperature range are on the order of 14 to 900 years for comparison a maximum time limit of 4000 years would be needed if cooling occurred only by conduction

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

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

obtaining equilibrium oxygen isotope fractionations from rocks theory and examples a generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope closure temperatures examples using literature data and new analyses from 32 natural amphibolite facies schists demonstrate both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks by screening samples according to the theoretical models natural data are shown to have highly consistent mineral fractionations 2 reproducibilities of 0 16 to 0 54 that within uncertainty reproduce experimental determinations among the minerals quartz biotite muscovite and calcic amphibole this correspondence indicates that the proposed theoretically based selection criteria improve the likelihood of measuring equilibrium fractionations the new data further corroborate the expected progressive enrichment of 18o in the orthosilicates with increasing al si relative to fe mg ky grt 1 05 st grt 0 6 and st cld 0 3 at 525 575 c in contrast typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz biotite and amphibole that are strongly disequilibrium because of exchange during cooling theoretical screening of samples prior to isotope analysis allows robust independent assessment of theoretical and experimental determinations of equilibrium isotope fractionations

nitrogen diffusion in silicate minerals with implications for nitrogen transport and cycling in the lithosphere abstract diffusion laws for nitrogen in orthoclase quartz olivine and clinopyroxene were determined using a combination of experimental strategies that included in diffusion from c h o n vapor in diffusion from molten nh4cl and ion implantation followed by heating to mobilize n most experiments were conducted at pressures near 1 bar but seven experiments were also run in a piston cylinder device at ptotal 1 gpa the c h o n source experiments were wet h2o present those on ion implanted samples were dry all n sources were highly enriched in 15n to avoid spurious 14n signals resulting from surface contamination use of 15n also enabled depth profiling by nuclear reaction analysis nra using the 15n p 12c reaction which has a detection limit of 10 ppm nitrogen profiles in three samples were characterized by both nra and sims with similar results a total of 17 to 28 diffusion coefficient measurements were made on each of the four minerals 83 in all yielding results that conform for each mineral to an arrhenius relation of the form d d0exp ea rt the following values for the pre exponential constant d0 and activation energy ea were obtained mineral log d0 m2 s ea kj mol t range c orthoclase 13 9 0 4 95 4 6 6 500 900 quartz 12 4 0 3 147 3 5 7 550 1150 olivine 14 2 0 3 135 2 6 6 650 1400 clinopyroxene 14 1 0 3 135 6 7 4 750 1300 the various experimental strategies yield generally indistinguishable results at a given temperature and no significant effects of total pressure or h2o are discernible in the overall diffusion data set experiments that involved in diffusion from external n sources provide qualitative insight into the compatibility of n in the structures of the four minerals nitrogen concentrations attain the highest levels in orthoclase 100 8000 ppm atomic with quartz olivine and clinopyroxene falling in the tens to hundreds of ppm range the new diffusion laws enable us to model the diffusive release of n acquired at depth in the crust during fluid absent exhumation and cooling in general orthoclase is the least retentive of the minerals investigated and will lose most of its n during cooling from 600 c at tectonically typical rates olivine and clinopyroxene are the most retentive of n quartz is intermediate in retentivity between the mafic minerals and feldspar but in all cases the extent of n exchange with the surroundings depends critically on the specific t t path of the mineral of interest in subduction settings where n bearing fluid is released to the mantle wedge diffusion is sufficiently fast to homogenize individual mafic mineral grains with respect to n over geologically plausible time scales but equilibration on the outcrop or regional scale via volume diffusion is precluded by our data

mineral chemistry of hydrothermal biotite from the kahang porphyry copper deposit ne isfahan central province of iran abstract the kahang porphyry cu deposit located northeast of isfahan city in central of iran is associated with a composite miocene stock and ranges in composition from diorite through granodiorite to quartz monzonite field observations and petrographic studies show that the emplacement of the kahang stock occurred in several pulses each associated with its related hydrothermal activity early hydrothermal alteration started with a potassic style in the central part of the system and produced a secondary biotite k feldspar magnetite assemblage accompanied by chalcopyrite and pyrite mineralization propylitic alteration that took place at the same time as the potassic alteration occurred in the peripheral portions of the stock subsequent phyllic alteration overprinted earlier potassic and propylitic alterations biotite grains from the potassic and phyllic zones show distinct chemical compositions the feo tio2 mno k2o and na2o concentrations in biotite from the phyllic alteration zone are lower than those from the potassic alteration zone the f and cl contents of biotite from the potassic alteration zone display relatively high positive correlation with the xmg the fluorine intercept values iv f from the potassic and phyllic alteration zones are strongly correlated with the fluorine chlorine intercept values iv f cl biotite geothermometry for the potassic and phyllic alteration zones based on the biotite geothermometer of beane 1974 yields a temperature range of 422 to 437 c mean 430 c and 329 to 336 c mean 333 c respectively the position of data in log xf xoh ratio vs xmg and xfe diagram suggests that biotite formed under dissimilar composition and temperature conditions in the potassic and phyllic alteration zones calculated log fugacity ratios of fh2o fhf fh2o fhcl and fhf fhcl show that hydrothermal fluids associated with the potassic alteration were distinctively different from those fluids associated with the phyllic alteration zone at kahang porphyry cu deposit the results of this research indicate that the chemistry of biotite is related to the chemical composition of the magma and the prevailing physical conditions during crystallization

apparent oxygen isotope equilibrium during progressive foliation development and porphyroblast growth in metapelites implications for stable isotope and conventional thermometry the assumption of oxygen isotope and major element equilibrium during prograde metamorphism was tested using staurolite grade pelitic schists that have undergone sequential porphyroblast growth and multiple episodes of recrystallization of matrix minerals and foliation development textural relationships are used to infer a metamorphic history that involves garnet growth followed by staurolite growth with each porphyroblast growth event followed by at least one period of recrystallization of matrix minerals conventional geothermobarometry using qtz grt pl ms bt st equilibria yields peak p t conditions of c 625 c at 9 11 kbar consistent with kmnfmash petrogenetic grid predictions for stability of the assemblage grt st bt qtz grt oxygen isotope fractionations yield apparent temperatures of c 590 c and qtz st fractionations yield an apparent temperature of c 595 c diffusional modelling indicates that quartz isotopic compositions were reset by c 30 c via retrograde isotopic diffusional exchange with micas the isotopic temperatures appear to be in excellent agreement with one another and suggest oxygen isotope equilibrium was attained between garnet and staurolite at c 625 c however the agreement of qtz grt and qtz str isotopic temperatures is not consistent with petrographic observations garnet grew before staurolite and petrogenetic grid constraints that predict that garnet grows over a temperature interval of c 525 550 c given that i oxygen diffusion rates in staurolite and garnet are slow enough to render an individual porphyroblast effectively closed to exchange after it forms and ii matrix minerals are able to exchange isotopes via recrystallization during each period of deformation garnet and staurolite could not have simultaneously achieved oxygen isotope equilibrium with each other or with minerals in the recrystallized matrix thus the qtz grt fractionations which yield apparent temperatures that are in apparent agreement with peak metamorphic temperature and apparent temperatures for qtz st fractionations cannot be fractionations resulting from equilibrium isotopic exchange instead they are apparent fractionations between porphyroblasts formed at different temperature and times in the prograde p t d path and quartz that recrystallized and exchanged with micas and plagioclase during several phases of deformation

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

empirical determination of oxygen isotope fractionation factors for titanite with respect to zircon and quartz abstract measurements of oxygen isotope fractionations between coexisting igneous titanite ttn and zircon zrc have been used to formulate a self consistent empirical calibration of equilibrium oxygen isotope partitioning 1000 ln zrc ttn 1 02 10 6 t 2 this calibration is based on the average measured zrc igneous ttn 1 2 0 3 n 27 rocks and a closure temperature of titanite to oxygen diffusion of approximately 650 c the average measured fractionation between zircon and metamorphic titanite is 2 1 0 4 n 5 rocks these results show that zrc ttn can be used to distinguish igneous vs metamorphic or hydrothermal titanite the new zircon titanite fractionation has been combined with published experimental and empirical data to provide a new quartz titanite fractionation curve 1000 ln qtz ttn 3 57 10 6 t 2 this new calibration is consistent with qtz igneous ttn data from coexisting mineral pairs in granitic rocks modeling diffusional exchange of oxygen during cooling in a typical granitic rock using the new calibration results in the same qtz igneous ttn as measured 4 5 0 43 n 25 rocks both measured zrc igneous ttn and qtz igneous ttn differ from fractionations calculated using the semiempirical increment method the consistency of empirical zrc igneous ttn in a range of geologic environments allows the calculation of a more accurate fractionation factor for titanite

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

oxygen diffusion in magnetite the diffusion rate of oxygen in magnetite fe3o4 has been measured under hydrothermal conditions between 800 and 500 c at 1000 bars pressure and yields the arrhenius relation d 3 5 10 6 exp 45 000rt kcal g atom o the diffusion rate was found to be the same within the estimated uncertainty of a factor of 2 for measurements at 100 1000 and 2000 bars 20 200 mpa water pressure and for oxygen fugacities buffered by magnetite hematite and approximately ninio the activation energy and diffusion mechanism probably change to that given by castle and surman 10 at temperatures below 460 c the oxygen diffusion rate in magnetite is between those of quartz and hornblende closure temperatures fall in the range 380 c for 0 1 mm radius grains cooling at 1 c m y to 675 c for 1 mm radius and cooling at 1000 c m y if the present data are used but will be lower for the fine grain sizes and slow cooling rates if the castle and surman data are used

an experimental calibration of the temperature dependence of oxygen isotope fractionation between apatite and calcite at high temperatures 350 800 c the fractionation of oxygen isotopes between apatite and calcite has been measured experimentally using the direct exchange technique in a piston cylinder apparatus at high temperature and pressure 350 800 c 1 1 gpa the fractional approach to equilibrium ranged from 20 to 80 and varies systematically with temperature and run duration isotopic xrd and sem analyses of the run products indicate that the exchange occurs dominantly by recrystallization in the early stages of the experiments the equilibrium fractionation over the temperature range of this study ranges from 5 5 to 2 but the data at 350 and 425 c have relatively large errors for t 500 c an error weighted least squares fit yields 1000 ln ap cc 0 68 1 60 0 26 106t 2 the magnitude of the temperature dependence is intermediate to that for felsic and mafic minerals to calcite mineral pairs involving apatite are potentially useful indicators of temperature cooling rates and equilibrium disequilibrium phenomena in igneous and metamorphic rocks

episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the larderello travale geothermal field italy abstract microscale oxygen isotope analysis 18o 16o of minerals can identify distinct events of fluid rock interaction this method is however still limited to a few major and accessory minerals of which most are anhydrous minerals we present the systematic study of oxygen isotope distribution in white mica by secondary ion mass spectrometry sims texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the larderello travale geothermal field ltgf italy record stages of magmatic crystallization metamorphic hydrothermal replacement and fluid rock interaction the large range in intra and inter grain white mica 18o values between 1 and 14 reflects varying protoliths and degrees of fluid mineral interaction at variable temperatures 180 450 c present day temperatures p d t this variability reflects the large scale circulation of both 1 magmatic syn intrusive to early contact metamorphic hydrothermal fluids with high 18o values and 2 meteoric fluids with 18o values of 7 during a post intrusive late hydrothermal stage metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity m scale including detrital grains 18o 12 14 high na low mg partially altered white mica 18o 8 9 and late hydrothermal white mica 1 6 low na mid mg the late hydrothermal white mica has similar 18o values to other secondary minerals and is in equilibrium with meteoric dominated fluids with a 18o of 6 to 0 5 which circulated in the late hydrothermal stage downhole towards the lower reservoir white mica from two contact metamorphic micaschist samples shows either 1 homogeneous 18o values of ca 9 likely due to recrystallization in the contact metamorphic hydrothermal stage t ca 600 c or 2 a large spread in 18o from 2 to 12 within and across grains of variable texture and chemistry in the host rock and a cross cutting quartz white mica vein ca 300 c present day temperature hereafter p d t this contrasting 18o signature of white mica is also recorded in granite cored at up to 4 6 km depth the carboli granite contains white mica with a homogeneous magmatic 18o of 10 0 6 whereas older granite samples from radicondoli have magmatic to hydrothermal white micas that vary in 18o from 4 to 10 a pronounced intra grain 18o variability of up to 6 occurs in white mica domains with higher fe mg ti halos around inclusions of chloritized biotite as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4 6 km in the porto azzurro granite on elba italy altered white mica has 18o values of 2 6 down from 10 in unaltered grains the distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration at depth sample and grain scale secondly the actual preservation of these m scale variabilities indicates that volume diffusion is not detectable at microscale at p d t at or below 350 c where most of the heterogeneous white mica is found selective sample and grain scale fluid penetration occurs episodically and anisotropically on micro and megascale along faults fractures and cleavages producing lower 18o white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation

relationships between o isotope equilibrium mineral alteration and rb sr chronometric validity in granitoids implications for determination of cooling rate a combined study of mineral o and rb sr isotopes was carried out for a number of mesozoic granitoids in china in order to compare the degree of o isotope equilibrium between coexisting minerals with the validity of mineral rb sr isochrons for granitoids a scrutiny of both o isotope geothermometry and rb sr internal isochron dating for corresponding minerals indicates that equilibrium o isotope fractionation between rb sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in 87rb 86sr ratio is big enough to provide reasonably small uncertainties in age significant deviation of the rb sr isochron age from the actual age appears to depend on the difference in sr isotopic composition between an external fluid and the igneous minerals as a result o isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration post magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock the o isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature both rb and sr concentrations and the sr isotope ratios of isochron minerals also changed due to the mixing of different sr reservoirs nevertheless the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed this provides an insight into the effect of internal and external fluids on the validity of mineral rb sr chronometry in addition an alternative approach is proposed to construct the cooling curve by a combined use of o isotope temperature and mineral isotope age for the granitoids of interest comparing with the traditional method using the empirical closure temperature for rb sr chronometry the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

crystal surface integrity and diffusion measurements on earth and planetary materials abstract characterization of diffusion behavior in minerals is key to providing quantitative constraints on the ages and thermal histories of earth and planetary materials laboratory experiments are a vital source of the needed diffusion measurements but these can pose challenges because the length scales of diffusion achievable in a laboratory time are commonly less than 1 m an effective strategy for dealing with this challenge is to conduct experiments involving inward diffusion of the element of interest from a surface source followed by quantification of the resulting diffusive uptake profile using a high resolution depth profiling technique such as rutherford backscattering spectroscopy rbs nuclear reaction analysis nra or ion microprobe sims the value of data from such experiments is crucially dependent on the assumption that diffusion in the near surface of the sample is representative of diffusion in the bulk material historical arguments suggest that the very process of preparing a polished surface for diffusion studies introduces defects in the form of dislocations and cracks in the outermost micrometer of the sample that make this region fundamentally different from the bulk crystal in terms of its diffusion properties extensive indirect evidence suggests that in fact the near surface region of carefully prepared samples is no different from the bulk crystal in terms of its diffusion properties a direct confirmation of this conclusion is nevertheless clearly important here we use transmission electron microscopy to confirm that the near surface regions of olivine quartz and feldspar crystals prepared using careful polishing protocols contain no features that could plausibly affect diffusion this finding does not preclude damage to the mineral structure from other techniques used in diffusion studies e g ion implantation but even in this case the role of possible structural damage can be objectively assessed and controlled while all evidence points to the reliability of diffusivities obtained from in diffusion experiments we do not recommend experiments of this type using a powder source as a means of obtaining diffusant solubility or partitioning information for the mineral of interest

rates and mechanisms of isotopic exchange background the proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies this trend is likely to continue because of the development of micro analytical techniques e g laser ablation ion microprobe femtomole carrier gas methods etc for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record see mckibben et al 1998 equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems indeed determination of the distribution of the stable isotopes of c o h s in gases fluids and minerals has become a standard and extremely powerful approach in elucidating the temperatures material fluxes rock and fluid origins and time scales associated with ancient and active fluid rock interaction processes in the earth s crust see valley et al 1986 although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by urey 1947 the calibration of fractionation factors has proven to be difficult during the last four decades a great deal of progress has been made in determining the equilibrium fractionations of 18o 16o d h 13c 12c and 34s 32s among fluid and mineral components at high temperatures 400 c both from experimental exchange studies theoretical calculations using vibrational spectra semi empirical calculations using bond type considerations and empirical calibrations based on natural assemblages see references in o neil 1986 kyser 1987 criss 1991 1999 chacko et al this volume of these the experimental method is the most direct because it involves the least number of a priori assumptions despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water rock

patterns and processes of oxygen isotope exchange in a fossil meteoric hydrothermal system cuillins gabbro complex isle of skye scotland oxygen isotope compositions of whole rock specimens and mineral separates from the cuillins gabbro complex isle of skye scotland are employed to determine the patterns and processes of18o depletion in the outer unlayered gabbro oug and associated dikes whole rock 18o values range from 4 8 to 1 1 smow and dike 18o values range from 4 7 to 2 8 mineral separates from three oug samples yield 18o values from 5 3 to 4 8 for augite and 4 1 to 0 8 for plagioclase an early small scale hydrothermal circulation system was initiated by the oug prior to the large scale hydrothermal convection established by the later layered cuillins complex lcc dikes were emplaced in the oug after intrusion of the lcc and had only a minor effect on hydrothermal circulation in the oug there is evidence of enhanced fluid flow along dike gabbro contacts isotopic compositions of augite separates demonstrate a normal 18o value for the oug magma with all18o depletion in the oug due to subsolidus exchange processes including diffusion and surface reaction the mineral separates yield a pattern of18o depletion consistent with a diffusion mechanism the bulk of the exchange having occurred in the plagioclase secondary mineral formation played a subordinate role in the18o depletion of the oug the calculated water to rock mass ratio necessary to effect the observed18o depletion in the oug is on the order of 0 2 although a much greater amount of water circulation probably occurred the cooling duration required to explain the measured18o depletion in the oug by diffusion is very short 140 years at 750 c 2400 years at 550 c compared to the duration necessary for pure conductive cooling 105 to 106 years rapid local cooling rates in the oug due to meteoric water convection are consistent with the observed18o depletion in oug samples

influence of f oh 1 substitution on the relative mechanical strength of rock forming micas microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite wilson and bell 1979 kronenberg et al 1990 mares and kronenberg 1993 we propose a crystal chemical resolution to this conflict namely that 001 dislocation glide in biotite is rate limited by its fluorine content significant f oh 1 substitution and concomitant removal of hydroxyl h directed into the interlayer cavity potentially increases mechanical strength of biotite in two ways 1 it eliminates k h repulsion thereby strengthening the interlayer bonds and 2 it allows k to sink deeper into the interlayer cavity the resultant geometry being less favorable to basal slip in testing this hypothesis we analyzed the naturally deformed biotite studied by wilson and bell 1979 and documented its very low f content xf 0 02 compared to that of the biotite experimentally deformed by kronenberg et al 1990 our model and the comparative xf data explain why the biotite of wilson and bell 1979 deformed more easily in nature than its coexisting muscovite whereas the biotite of kronenberg et al 1990 was mechanically stronger than muscovite similarly deformed by mares and kronenberg 1993 our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their f oh 1 substitution our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families further testing of crystal chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size

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

oxygen isotope thermometry speedometry and hygrometry apparent equilibrium temperature versus closure temperature rather than indicating formation peak temperature oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures tae the isotopic fractionations and tae values deliver information about cooling history as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate despite that several models such as the fast grain boundary fgb model have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling extraction of cooling rate remains far from straightforward on the other hand there is a well defined quantitative relationship between the dodson closure temperature tc and the cooling rate but tc cannot be directly measured based on simulation results of existing models for a variety of rock systems including open systems with an infinite fluid reservoir closed systems with negligible fluid participation and semi open systems with moderate fluid participation this study demonstrates that tae of the mineral pair with the largest equilibrium isotope fractionation pleif is always bounded by their tc values regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange if the two tc values happen to be similar tae will serve as a good approximation of both tc provided that the equilibrium fractionation factor has been precisely determined as a function of temperature one such pair is quartz magnetite by contrast a mineral pair with similar tc but relatively small fractionation is susceptible to the disturbance from other minerals hence does not always have tae confined within their tc range the relationship of tae tc correspondence for pleif with similar tc can be used to constrain either cooling rate i e as a speedometry method or oxygen isotope diffusivity if one of them has been independently determined in the latter case the inferred oxygen diffusivity may be an index of water fugacity i e as a hygrometry method when compared with experimental diffusivity values measured under different fluid conditions

uranium mobilization from the variscan questembert syntectonic granite during fluid rock interaction at depth leaching of uranium fertile granites represents a major source of uranium as uraninite is easily dissolved in oxygenated aqueous solutions this phenomenon is well documented at surface conditions but remains poorly documented for granites at depth in this study we propose that surface derived oxidized hydrothermal fluids leached uranium from uraninite in the questembert peraluminous granite at temperatures greater than 70 to 160 c this variscan synkinematic granite is characterized by widespread and pervasive development of vertical and permeable c s structures these structures likely facilitated the infiltration of oxidized hydrothermal fluids from the surface their circulation at depth and the subsequent fluid rock interaction in the granite published oxygen isotope data shows that it has undergone subsolidus fluid rock interaction dated between 312 and 303 ma by 40ar 39ar analyses on muscovite these interactions were responsible for the concomitant decrease of the feldspar 18o values together with uranium leaching mass balance calculations suggest that this hydrothermal event could have liberated several hundred thousand tonnes of uranium from the questembert granite the liberated uranium may have been dispersed in early permian intramountainous basins and therefore disseminated over large areas this study emphasizes that the efficiency of uranium leaching may be directly related to the extent of subvertical structure development in granites emplaced along strike slip shear zones which allow for downward infiltration of oxidized surface derived fluids a specific and systematic sampling is required to better constrain the proposed model

coupled mineral fluid evolution of a basin and high kaolinization in the sw england granites in relation to the development of the plymouth basin

oxygen self diffusion in quartz under hydrothermal conditions oxygen self diffusion dox has been monitored in single crystal quartz in the temperature range 515 850 c under hydrothermal conditions in the beta field between 700 and 850 c the data are represented by two linear arrhenius relations for transport parallel and perpendicular to c values for do m2 s 1 are 2 09 10 11 parallel to c and 3 16 10 10 perpendicular to 101 0 values for h kj mol 1 are 138 54 parallel to c and 203 72 perpendicular to 101 0 at 700 c in the total pressure range 11 5 100 mpa dox is independent of water f h2o and oxygen f o2 fugacities between the ni nio and fe3o4 fe2o3 buffers the results are consistent with diffusion via a simple charged vacancy mechanism under an extrinsic point defect regime further experiments are required to confirm the nature of the mobile oxygen defect a key aspect of the results is the observation that at the low water fugacities of the present experiments a hydrogen containing defect appears to play no role in the oxygen transport mechanism this is in contrast to other published sets of data and leads directly to the requirement for detailed interlaboratory comparisons

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

diffusion of sr in fluorphlogopite determined by rutherford backscattering spectrometry abstract we have measured the diffusivity of sr in fluorphlogopite in a direction perpendicular to the basal planes in the c direction annealing experiments were performed on single crystals and most of them at ambient pressure the penetration profiles of the diffusant were determined by rutherford backscattering spectrometry sr diffusion parallel to c obeys an arrhenius law whose equation is d sr 2 7 10 14 exp 13 5900 3100 rt where d sr is given in m 2 s 1 and activation energy in j mol 1 the values measured in fluorphlogopite overlap the range previously determined in other silicates but the activation energy for fluorphlogopite is approximately half that of the other minerals for which diffusion data exist a summary of the available data on diffusion in micas of all compositions shows that activation energies for species such as sr o rb and k fall in the same range i e 100 175 kj per mol when plotted together these data define two groups of parallel arrhenian lines one group consists of the measurements performed in the c direction the other of those performed normal to the c direction the distinction between the two groups is clear and diffusion parallel to the c direction is slower by several orders of magnitude the corresponding closure temperature for fluorphlogopite when diffusion occurs in the c direction is of the order of 700 c relatively close to that of feldspars on the basis of some simple assumptions about diffusion anisotropy in micas we also estimated the closure temperature when diffusion takes place in a direction parallel to the basal planes in that case the value is noticeably lower of the order of 400 c however this value if applicable to other micas is higher than what is currently accepted which could have some implications for models of thermal histories or uplift rates of rocks