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

submillimeter scale zonation of δ18o in quartz and feldspar isle of skye scotland abstract laser probe analysis of oxygen isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains two samples collected within 10 m of each other in the maol na gainmhich epigranite mge isle of skye scotland are similar petrographically yet display very different 18o zonation patterns sk1 has isotopically zoned quartz 18 o 1 8 8 2 with intragranular gradients of 18o as great as 5 mm and individually homogeneous grains of k feldspar 0 2 which also display a large range of values 18 o 1 7 to 7 3 oxygen isotope gradients of more than 11 mm were measured in this sample in contrast sk4 has homogeneous quartz 18o 5 7 0 3 n 14 and a zoned k feldspar phenocryst 18 o 3 3 to 6 9 isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains the isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the mge pluton although the average values of 18o obtained by laser probe are identical to the average bulk values values in k feldspar suggest exchange at 400 c with infiltrating meteoric water 18 o 10 the marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous these microanalytical data for quartz and feldspar provide the first oxygen isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures

ion microprobe evidence for the mechanisms of stable isotope retrogression in high grade metamorphic rocks retrograde interdiffusion is widely proposed as the dominant factor in producing the stable isotopic fractionation among minerals in slowly cooled igneous and metamorphic rocks mineral zonation consistent with interdiffusion of stable isotopes has never been directly observed however leaving doubt as to the mechanism responsible for the bulk mineral isotopic compositions commonly measured ion microprobe analyses of oxygen isotope ratios in magnetite were combined with conventional bulk mineral analyses and diffusion modeling to document the relationship between mineral zonation and the mechanism of retrogression inferred from bulk mineral data two samples of magnetitebearing quartzo feldspathic lyon mountain gneiss from the adirondack mountains n y were studied in detail conventional stable isotope analysis of both samples indicates that isotope thermometers are discordant and were reset by as much as 200 c from the estimated peak temperature of 750 c the relative order of apparent temperatures recorded by various thermometers differs between the two samples however with t qtz fsp t mt qtz and t mt fsp in one sample and t qtz fsp t mt qtz and t mt fsp in the other diffusion modeling using the fast grain boundary model shows that the former pattern of apparent temperatures is consistent with closed system interdiffusion during cooling whereas the latter is not the modeling predicts that 0 5 mm diameter magnetite grains common to this rock type will contain isotopic zonation of 1 rims lower in 18 o than cores and that the cores of smaller 0 1 mm grains will be similarly lower than to the cores of large 0 5 mm grains ion microprobe analysis reveals that the zoning patterns of magnetite grains from the first sample contain clear core to rim zonation in multiple grains core rim 1 1 0 4 and predicted grain size vs core composition variations consistent with diffusion controlled resetting of bulk mineral fractionations in contrast the second sample shows irregular inter and intra granular variations over an 8 range consistent with open system alteration these results provide direct documentation of the importance of interdiffusion in affecting stable isotope distributions in slowly cooled rocks the correlations of bulk mineral resetting with zonation show that bulk mineral data when interpreted with detailed modeling can be used to determinate what processes controlling retrogression

isotopic exchange in mineral fluid systems ii oxygen and hydrogen isotopic investigation of the experimental basalt seawater system abstract oxygen and hydrogen isotopic exchange reactions between basalt and seawater at t 300 to 500 c were investigated using oceanic tholeiitic basalt 18 o 5 7 d 70 natural seawater 18 o 1 2 d 5 and artificial seawater 18 o 5 3 as starting materials the starting basalts varied in the crystallinity from holocrystalline to glass but were ground to approximately the same grain size 100 mesh the water rock mass ratios ranged from 1 to 3 and the duration of the experiments ranged from 167 to 576 days in general depletion of 18o and enrichment of d in basalts occur at all temperatures with the magnitudes of change being greater as temperature time and to a lesser degree glass content increases the trends in isotopic shifts are directly related to changes in the style and intensity of mineralogic alterations in the basalt e g smectite at 300 c talc actinoiite at 400 500 c the changes in the 18 o values of basalts and seawater in the experimental systems were observed to follow closely with those expected from a first order rate law rate constants for the oxygen isotopic exchange between rock and water range from 10 9 5 to 10 8 0 moles oxygen m2 of solid surface sec for temperatures of 300 to 500 c the activation energy for the isotopic exchange reaction was calculated to be 11 5 kcal mol an application of our experimental rate data to natural systems suggests that the oxygen isotope equilibrium between basalt and seawater in the mid oceanic ridge may take place within approximately 1000 years at 350 c our experimental data also suggest the equilibrium oxygen isotopic fractionation factors between the altered basalt and seawater to be 3 5 0 5 at 300 c 2 0 0 4 at 400 c and 0 5 0 25 at 500 c the observed hydrogen isotopic fractionation factors between the altered basalts and seawater in our experimental systems were about 74 at 300 c about 62 at 400 c and about 48 at 500 c these fractionation factors are probably attributable to the equilibrium fractionation between fe rich secondary phases and seawater

oxygen isotope zoning in garnet oxygen isotope zoning was examined within garnet with the use of the stable isotope laser probe four metamorphic garnets from the regional metamorphic terrane in vermont and the skarn deposit at carr fork utah were examined and were found to be concentrically zoned in 18o values the largest variations in 18o values were observed in the regional metamorphic garnets where 18o values change by 3 per mil from core to rim these oxygen isotope zoning profiles reflect the changes in the 18o values of the rocks during garnet growth which are caused by infiltration of fluids and by dehydration reactions during metamorphism

determination of oxygen diffusion rates in magnetite from natural isotopic variations the oxygen isotope compositions of magnetite grains hosted in a calcite marble from the bancroft terrane of the ontario grenville province vary systematically with grain size the 18 o values of magnetite and corresponding closure temperatures t c based on calcite magnetite range from 18 o 2 6 t c 505 c for a grain radius of 0 075 mm to 18 o 5 5 t c 660 c for a grain radius of 1 15 mm the 18 o of the calcite is constant within a scale of 100 m at a value of 12 3 the observed isotopic variations can be fit to the diffusion model of dodson by the method of least squares r 0 98 to yield an activation energy q 211 20 kj mole and a pre exponential factor d o 4 3 3 3 1 9 x 10 7 cm 2 s for a cooling rate of 4 c m y the activation energy estimate is independent of the assumed cooling rate but the calculated pre exponential factor varies as follows d o cm 2 s d t d t x 1 08 x 10 7 note d t d t is in c m y the activation energy is identical to an experimental determination by giletti and hess but the pre exponential factor is 100 times lower the difference is attributed to the water rich conditions in the experiments and the absence of fluid in the slowly cooled marbles investigated in this study the strong dependence of diffusion rate on water presence or f h 2 o may be used as a sensor for water rich fluids during cooling in natural systems

formation of low δ18o rhyolites after caldera collapse at yellowstone wyoming usa we present a new model for the genesis of low 18o rhyolites of the yellowstone caldera based on analyses of zircons and individual quartz phenocrysts low 18o rhyolites were erupted soon after the massive caldera forming lava creek tuff eruption 602 ka 1000 km3 and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites these zircons are isotopically zoned and out of equilibrium with their host low 18o melts and quartz diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron in which the alteration resistant quartz and zircon initially retained their 18o values these results link melting to caldera collapse rule out rapid or catastrophic magma meteoric water interaction and indicate wholesale melting rather than assimilation or partial melting

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites diffusive exchange as a guide to cooling history ion microprobe analysis of magnetites from the adirondack mountains ny yields oxygen isotope ratios with spatial resolution of 2 8 m and precision in the range of 1 1 sigma these analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of 18o at this scale high precision micro analyses of this sort will permit wide ranging new applications in stable isotope geochemistry the analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite magnetites are near to fe3o4 in composition and optically and chemically homogeneous a combination of ion probe plus conventional brf5 analysis shows that individual grains are homogeneous with 18o 8 9 1 smow from the core to near the rim of 0 1 1 2 mm diameter grains depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in 18o these low 18o values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite these are the sharpest intracrystalline gradients measured to date in geological materials this discovery is confirmed by bulk analysis of 150 350 m diameter magnetites which average 1 2 lower in 18o than coarse magnetites due to low 18o rims conventional analysis of coexisting calcite yields 18o 18 19 suggesting that bulk 18o cc mt 9 3 and yielding an apparent equilibration temperature of 525 c over 200 c below the temperature of regional metamorphism consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories the data for oxygen in calcite under hydrothermal conditions at high p h2o indicates that diffusion is faster in magnetite and modelling of the low 18o rims on magnetite would suggest that the adirondacks experienced slow cooling after grenville metamorphism followed by a brief period of rapid cooling possibly related to uplift conversely the data for calcite at low p h2o show slower oxygen diffusion than in magnetite modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals suggesting that the low 18o rims form by exchange with late low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets in either case the ion microprobe results indicate that two distinct events are recorded in the post metamorphic exchange history of these magnetites recognition of these events is only possible through microanalysis and has important implications for geothermometry

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

ion microprobe analysis of oxygen isotope ratios in quartz from skye granite healed micro cracks fluid flow and hydrothermal exchange quartz grains in hydrothermally altered granites from the isle of skye are highly heterogeneous and not equilibrated in oxygen isotope ratio at the 20 m scale ion microprobe analysis of one grain shows a gradient of 13 over 400 m and a greater range in 18o than all quartz previously analyzed on the isle of skye other crystals from the same outcrop are homogeneous digitized cathodoluminescence images reveal patterns of magmatic zoning and brittle fracturing not otherwise detectable the ion probe analysis correlates low 18o values on a micro scale to one set of healed cracks thus quartz exchanges oxygen isotopes primarily by solution and reprecipitation along fractures in contrast to more reactive feldspar that appears to exchange from the grain boundary inward macroscopic models of isotope exchange are not realistic for these rocks the flow of hydrothermal fluids was heterogeneous anisotropic and crack controlled

contrasting oxygen diffusion in nepheline diopside and other silicates and their relevance to isotopic systematics in meteorites oxygen self diffusion coefficients have been determined in natural nepheline na3 na k al4si4o16 and synthetic diopside camg sio3 2 at one atmosphere by monitoring the rate of 18o exchange between co2 gas and minerals of pre determined grain size 10 12 5 and 15 m radius oxygen self diffusion in nepheline between 1000 and 1300 c fits a linear arrhenius relation of the form d 5 9 10 9 exp 25 0 2 5 103calrt and for diopside between 1150 and 1350 c d 6 3 exp 96 7 5 8 103calrt the activation energy for oxygen diffusion in nepheline is similar to that of melilite 31 9 0 1 kcal mol but the oxygen diffusivity is lower by two orders of magnitude the diffusion of oxygen in diopside is similar to that of nepheline at 1375 c however the activation energy is almost four times higher this contrast suggests a difference in the crystal chemical and structural controls of the minerals both nepheline and melilite are characterized by strongly distorted alkali sites within their frameworks which could account for their relatively low activation energies and high diffusivities more importantly nepheline and melilite have sheet like structures and high anionic porosities which may provide ready migration paths for oxygen the concentration of open space between the silicate sheets in melilite may explain the greater diffusivity of oxygen in melilite than in nepheline diopside forsterite anorthite spinel are characterized by high activation energies 80 to 100 kcal and are refractory with very stable bonds and low anion porosities furthermore these minerals are much denser and closer packed than nepheline or melilite making it difficult for oxygen to migrate within the crystalline structure and have access to the cation oxygen bonds oxygen diffusion studies constrain interpretations of post solidus oxygen exchange among minerals and may be used to explain the stable isotope systematics observed in ca al rich inclusions of carbonaceous chondrites the slow exchange and diffusion of oxygen in diopside suggests that the isotopic composition of clinopyroxenes in ca al rich inclusions of carbonaceous chondrites has not changed appreciably since their incorporation into the meteorite however minerals such as nepheline and melilite which readily exchange oxygen record the isotopic composition of the last nebular gas to which they were exposed

sulfur diffusion in rhyolite melts diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800 1100 c water contents of 0 7 3 wt and oxygen fugacities from the quartz fayalite magnetite buffer to air experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses regression of the diffusion data in dry rhyolite melt gives dsulfur 0 05 exp 221 80rt which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as h2o co2 and cl diffusion of sulfur in melt with 7 wt dissolved water is 1 5 to 2 orders of magnitude faster than diffusion in the anhydrous melt depending on temperature sulfur is known to dissolve in silicate melts as at least two different species s2 and s6 the proportions of which vary with oxygen fugacity despite this oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions this result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity the most likely candidate for the diffusing species is the sulfide ion s2 re equilibration between s2 and s6 in oxidized melts must generally be slow compared to s2 diffusion in order to explain the observed results in a silicic melt undergoing degassing sulfur will tend to be fractionated from other volatile species which diffuse more rapidly this is consistent with analyses of tephra from the 1991 eruption of mount pinatubo philippines and from other high silica volcanic eruptions

oxygen diffusion in an fe rich basalt melt abstract the self diffusion of oxygen has been measured in an feti basalt melt at one atmosphere from 1320 1500 c under different oxygen fugacities diffusivities were determined by measuring isotopic exchange between spheres of melt and flowing oxygen in a vertical tube furnace the oxidation state of the melt was varied by performing experiments in either o2 or co2 gas the diffusion data for experiments in both gases can be fit to the equation d 1 4 10 exp 60 7 10 3 rt cm 2 s the observed self diffusivities for oxygen in the basalt melt are one to two orders of magnitude lower than published chemical diffusivities at one atmosphere diffusivities in basalt melt at each temperature in o2 and co2 gas are similar within experimental error 0 2 log d units at the 1 level and indicate that oxygen self diffusion is insensitive to the oxidation state of fe in natural basalt melts the redox independence of oxygen self diffusion observed in this study suggests that oxygen transport in natural magmas will not be significantly affected by changes in their redox state caused by degassing and or crystallization

oxygen bulk diffusion measurements and tem characterization of a natural ultramylonite implications for fluid transport in mica bearing rocks oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample c 5 m grain size 15 20 biotite 20 quartz 60 65 feldspars and minor fe oxides from the gerrish island shear zone se maine usa the diffusion experiments were performed at 250 550 c and 100 mpa water pressure oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample the arrhenius parameters for transport parallel to the foliation are dbulk0 2 0 10 11 m2 s 1 and q 30 6 kj mol 1 the bulk diffusivity perpendicular to the foliation is about a factor of 3 5 less than that parallel to the foliation with the same activation energy the values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions the microstructure of the ultramylonite was characterized using transmission electron microscopy tem the nature and distribution of the interconnected fluid cannot be completely resolved from the tem analysis however the low percentage of three grain channels and open grain interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces the results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates strong lithological control and pervasive nature of fluid transport in mica bearing rocks

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

microstructural characterization of metamorphic magnetite crystals with implications for oxygen isotope distribution the microstructures of magnetite crystals in three samples from a single outcrop of granulite facies marble were characterized by transmission electron microscopy tem to determine how exsolution history can affect physical properties and mineral reactivity during retrograde metamorphism the microstructure of sample 90lp9 consists of dislocation dislocation arrays with dislocation spacings of 100 to 500 nm and linear channels filled with layer silicates acid etching and ion milling of polished 90lp9 magnetite grains show dislocation arrays clustered near grain boundaries with rheologically hard magnetite diopside and monticellite but rarely near grain boundaries with softer calcite samples lp204 1 magnetite grains contain coherent l underscore brace 100 r underscore brace al mn fe spinel precipitates approximately 40 nm diameter approximately 1 3 nm thick approximately 10 sup 4 platelets micro m sup 3 and very few dislocations larger more widely spaced spinel precipitates are present in a third sample 94ak3 extremely low dislocation densities in powders of lp204 1 and intermediate dislocation densities in powders of 94ak3 compared with extremely high dislocation densities in powders of 90lp9 are related to strong dislocation pinning effects by precipitates the different exsolution behavior of the three magnetite samples is attributed to small but important differences in al content because dislocations can provide fast pathways for exchange that enhance diffusion especially in very slowly cooledmore rocks these microstructural results may explain previously reported subgrain scale oxygen isotopic heterogeneity in 90lp9 magnetite compared with relative isotopic homogeneity in lp204 1 magnetite less

microscale isotopic zoning in calcite and graphite crystals in marble chemical and isotopic zoning in minerals is a clear indication of disequilibrium and provides an important key for elucidating metamorphic processes here i present carbon and oxygen isotope data from calcite and graphite crystals in a marble from the hida metamorphic belt central japan showing isotopic zoning within individual crystals within 100 m of the calcite graphite grain boundary the 13c contents gradually decrease from the constant values of the crystal core to values 1 lower 18o contents in calcite decrease by up to 7 within 200 300 jim of the boundary at a calcite calcite grain boundary similar patterns of 13c and 18o depletions were found whereas the 13c contents of graphites remained constant except in a narrow zone at the calcite contact these isotopic patterns are interpreted as reflecting isotopic exchange with metamorphic fluids that were relatively depleted in 13c and 18o these results strongly suggest that the most accurate geothermometry will be obtained if only the cores of crystals are used and that isotopic zoning may provide another way to elucidate polymetamorphic processes

oxygen grain boundary diffusion in natural and hot pressed calcite aggregates abstract oxygen grain boundary diffusion rates were experimentally determined in natural solnhofen limestone and hot pressed calcite aggregates at 300 to 500 c and 100 mpa water pressure the solnhofen limestone was pre annealed for 24 h at 700 c and 100 mpa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments the synthetic calcite aggregates were formed by hot isostatic pressing of an intimate mixture of 1 2 m calcite grains plus 5 wt 0 3 m alumina grains at 700 c and 200 mpa for 3 10 days values for the product of the grain boundary diffusion coefficient d and the effective grain boundary diffusion width were determined from 18 o concentration profiles measured with an ion microprobe there is no measurable difference between d values obtained for pre annealed solnhofen samples and hot pressed calcite aggregates the temperature dependence for oxygen grain boundary diffusion at 100 mpa is described by the arrhenius parameters d 0 3 8 10 14 m 3 s and q 127 17 kj mol comparison of the results of this study with previously published data show that the rate of oxygen grain boundary diffusion in calcite aggregates is four to five orders of magnitude greater than that of volume diffusion in calcite single crystals and the activation energy is less grain boundary diffusion of oxygen in calcite aggregates is five to six orders of magnitude greater than that of calcium when the latter is extrapolated to the temperature range of this study in addition oxygen grain boundary diffusion rates in calcite aggregates are about a factor of 4 10 and 100 times greater than that in feldspar and quartz aggregates respectively under similar experimental conditions

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

oxygen self diffusion in calcite dependence on temperature and water fugacity oxygen self diffusion in natural calcite single crystals was studied hydrothermally at 400 800 c and 10 350 mpa confining pressure diffusion coefficients d were determined from18o concentration profiles measured with an ion microprobe at 100 mpa confining pressure the arrhenius parameters yield an activation energy q 173 6 kj mole and pre exponential factor d0 7 10 9 m2 s over the temperature range 400 800 c and there is no measurable anisotropy constant temperature experiments at 700 c indicate that there is a strong linear correlation slope 0 9 of d with water fugacity over the range 4 240 mpa consistent with the interpretation that in the presence of water the dominant oxygen bearing transport species in calcite is molecular water a sample containing 1180 ppm mn shows a marked increase in observed d values at temperatures below 600 c however when pre annealed at 700 c the d values obtained at 550 c are within a factor of four of the value obtained for the low 100 ppm mn sample the results of this study confirm that in contrast to carbon oxygen diffusion rates in calcite are greatly enhanced when water is present the difference in the effect of water on doxygen and dcarbon in calcite may provide valuable information for evaluating the role of fluids in the thermal histories of calcite bearing rocks

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

oxygen isotope and trace element zoning in hydrothermal garnets windows into large scale fluid flow behavior single garnet crystals from a large multicyclic hydrothermal system in far east russia are strongly growth zoned microscale ion microprobe analysis reveals consistent intracrystalline patterns in oxygen isotope composition and boron and iron contents reflecting evolution of the hydrothermal system from magmatic to meteoric dominated conditions and repetitive influx of magmatic fluids large intracrystalline variations in oxygen isotope composition 12 18 o delimit temporal evolution of magmatic and meteoric fluid input within different portions of the hydrothermal system and suggest that fluid flow regimes differed between central and distal portions of the system these results provide compelling evidence that microanalytical data can be extrapolated from intracrystalline scales to study macroscale processes in complex large scale fluid rock systems

a new look at stable isotope thermometry interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks the fast grain boundary fgb model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion eiler et al 1991 1992 this model assumes that grain boundary diffusion is much faster than volume diffusion and it accounts for exchange among all minerals in a rock previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks modeling results are described for amphibolites and hornblende granites and gneisses biotite granites schists and gneisses pelitic and semi pelitic rocks garnet peridotites anorthosites gabbros pyroxenites and related rocks and calc silicate rocks examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase pyroxene in pyroxene bearing anorthosites and garnet quartz in garnetiferous quartzites in contrast the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures closed system processes are capable of producing a variety of patterns of stable isotope resetting discordance mineral zonation and fractionation reversals examples include large reversals of quartz feldspar fractionations in micaceous rocks and oscillatory zonation in feldspar from some quartz rich rocks these results permit reinterpretation of many studies of stable isotope thermometry speedometry and retrograde alteration history fgb modeling of mineral zonation provides an important new guide to applying recently developed micro analytical tools to slowly cooled rocks application of the fgb model to quartzo feldspathic gneisses from the adirondack mountains new york demonstrates the usefulness of diffusion modeling in discriminating closed system diffusion controlled retrogression from open system retrogression and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity

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

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

equilibrium dihedral angles in the system h2o co2 nacl calcite and implications for fluid flow during metamorphism fluid calcite calcite dihedral angles have been measured for fluids in the system h2o co2 nacl between 1 and 2 kbar and 550 750 c it is found that the calcite calcite h2o dihedral angle decreases steadily with addition of nacl from a value of about 80 pure water to 44 60 wt nacl the co2 h2o system displays a well defined minimum at x co 2 0 5 with a dihedral angle of 50 in contrast to those of pure co2 and h2o which are 90 and 80 respectively experiments containing fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the co2 h2o nacl system which can be approximately correlated with the compositions of the two fluid phases such bimodality was only observed for immiscible fluids in the h2o nacl system if the quench rate exceeded about 200 c per min this is probably due to the extremely rapid establishment of the single phase dihedral angle on quenching the fluid phase topology in devolatilising marbles will only be a connected network for very saline brines and fluids with x co 2 close to 0 5 fluids trapped in fluid inclusions in calcite grains in marbles may be predominantly h2o rich or co2 rich and of low salinity all other fluid compositions in the h2o co2 nacl calcite system will occupy isolated pores the largest of which will grow at the expense of the smallest escape of fluid produced during devolatilisation reactions under such conditions will occur by fluid overpressuring and hydrofracture in contrast previous experimental studies of quartz fluid dihedral angles between 950 and 1100 c watson and brenan 1987 predict that quartz dominated lithologies will permit pervasive flow of h2o nacl fluids but not of h2o co2 fluids documented geological examples of differences in permeability and fluid flow mechanism between metamorphic argillites psammites and limestones which support the results of the experimental studies are discussed

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

multiple sulfur and iron isotope composition of magmatic ni cu pge sulfide mineralization from eastern botswana we report here high precision multiple sulfur and iron isotope compositions for a series of mineralized samples from ni cu pge sulfide deposits in the archean tati greenstone belt and the phikwe complex of eastern botswana mineralized samples from the phoenix and selkirk ni cu pge deposits in the tati greenstone belt display slightly positive 34s isotope values ranging from 0 2 to 0 8 v cdt 33s values of sulfides at phoenix and selkirk are 0 01 to 0 08 v cdt suggesting either a dominantly mantle sulfur source or effective eradication of a crustal 33s anomaly through equilibration with large amounts of silicate melt in the selebi phikwe belt a granite gneiss terrane with abundant amphibolite lenses of either volcanic and or intrusive nature mineralized lower grade samples from the phikwe phokoje and dikoloti ni cu pge deposits have more variable 34s values ranging from 3 1 to 0 3 and display significant mass independent anomalies 33s values ranging from 0 89 to 0 27 suggesting that barren sulfides associated with distal or low temperature sea floor hydrothermal activity contributed sulfur to these deposits iron isotopes of sulfides from these deposits show a relatively small range of negative 56fe values 0 29 to 0 04 consistent with high temperature fractionations in magmatic systems with the exception of one sample from the dikoloti ni cu pge deposit of the selebi phikwe greenstone belt which shows a more negative 56fe value of 0 61 consistent with assimilation of sedimentary or hydrothermal sulfides rather than fractionations in high temperature magmatic systems data from this study highlight the complexity and variability that characterize ore forming processes in magmatic systems we suggest that the presence of sulfur bearing lithologic units in host rocks of mafic and ultramafic intrusions may not be essential toward the assessment of the prospectivity of a province to host orthomagmatic nickel sulfides geologic settings without any or little sulfur in the stratigraphy which have been traditionally neglected in terms of their prospectivity should thus be revisited and possibly reassessed considering the potential importance of external source of sulfur to generate ore deposits

oxygen isotope zoning in garnet a record of volatile transport abstract this study examines oxygen isotope zoning in garnets from a barrovian metamorphic terrane in eastern vermont using a co 2 laser extraction system previous strontium isotopic and structural studies of these garnets have shown that they grew over an approximately 10 ma interval during thrusting and nappe emplacement rosenfeld 1970 christensen et al 1989 our studies show that the garnets are strongly zoned in 18 o this zoning is the result of equilibration of garnet with water derived from dehydration of subjacent pelites during nappe stage deformation the magnitude and nature of 18 o zoning depends upon the garnet s location in the outcrop studied the garnets examined in this study come from an isotopically low 18 o paragonitic schist 18 o whole rock 9 that is adjacent to a relatively high 18 o schist 18 o whole rock 12 5 garnets from the paragonitic schists within 10 m of the contact with the isotopically heavier schists have relatively homogenous 18 o values varying from 9 5 in the core to 10 5 at the rim garnet in the paragonitic schists 85 m from the contact are more strongly zoned with 18 o ranging from a low of 6 0 in the cores to a high of 9 0 at the rims of the garnet these zoning patterns were produced by continuous infiltration of relatively high 18 o waters derived from the subjacent schists into the paragonitic schists during garnet grade metamorphism it is possible to determine the time integrated fluid fluxes by comparison of observed 18 o zoning profiles in garnet with those calculated from the equation describing combined advection diffusion of a tracer using this method we calculate time integrated fluid fluxes of 1 5 10 4 cm 3 cm 2 fluxes of this magnitude could have been produced by dewatering of 1 5 km of schist during garnet grade metamorphism

evidence for oxygen isotope disequilibrium in selected geothermal and hydrothermal ore deposit systems abstract there is ample evidence from geothermal systems that temperatures estimated from the oxygen isotope fractionation between alteration phases and coexisting aquifer fluids agree closely 25 c with measured bore hole temperatures conversely many hydrothermal systems exhibit varying degrees of apparent fluid rock oxygen isotope disequilibrium measured rock water equilibrium rock water a comparison of data from a variety of hydrothermal systems indicates that an increase in the degree of isotopic equilibration can be related in a general way to an increase in the temperature salinity and intensity of fracturing this relationship is entirely consistent with calculations carried out for a simple closed system model that estimates the fraction of isotope exchange between rock and fluid as a function of temperature rock type salinity of the fluid grain size water rock ratio and the rate of isotope exchange the fact that many silicate fluid reactions and their isotopic counterparts are fast leads us to conclude that observed variations in the degree of exchange may have resulted from local self sealing of the fracture network prior to equilibration the ability for fractures to remain open or to propagate allowing continued fluid flow may be the deciding factor in the attainment of isotopic equilibration

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

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

oxygen isotope microanalyses of diagenetic quartz possible low temperature occlusion of pores abstract secondary ion mass spectrometry sims was used to obtain 18 o measurements on 20 m spots in authigenic and detrital quartz from mississippian sandstones of the western canadian sedimentary basin detrital quartz shows a large range in 18 o from 5 to 25 which overlaps the analyses of authigenic cements 20 34 combining these data with the burial history and precipitation sequence of minerals indicates that meteoric water may have precipitated quartz at temperatures between 15 and 35 c over a period of 180 ma 300 120ma this nearly isothermal precipitation may have occluded pores in samples from the central portion of the basin but quartz precipitation continued up 75 c at the basin margin conventional bulk analyses of these samples gave 18 o values of 13 4 for detrital quartz and 19 8 for authigenic quartz models based on bulk isotopic analyses can bias the interpretation of the timing of diagenetic quartz precipitation toward much higher temperatures the sims measurements in diagenetic silicates can identify discrete episodes of precipitation allowing a more accurate interpretation for the timing of diagenetic events