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

oxygen isotope trends and anomalies in granitoids of the tibetan plateau abstract extreme transhimalayan oceanic mantle i type and high himalayan anatectic collision s type granitoids in the himalaya kashmir and tibet range in their original oxygen isotope composition from 6 to 7 5 and from 10 to 12 5 18osmow respectively in the sutured paleozoic and mesozoic orogen of the eastern tibetan plateau purely oceanic batholiths seem absent shalouli shan 8 10 but high himalayan values are matched in dongda shan and at luxi towards the east oceanic values are found again only on the margin of the yangtse platform the regional pattern of normal oxygen isotope data provides a background for anomalous feldspars to 1 in lhasa granite and in the active geothermal system at yangbachen recharged by 6000 m a s l meteoric water for lhasa granite oxygen isotope exchange paths and preliminary k ar dating of feldspars suggest exchange with meteoric water from over 3000 m altitude before 20 ma ago

assessment of grain scale homogeneity and equilibration of carbon and oxygen isotope compositions of minerals in carbonate bearing metamorphic rocks by ion microprobe abstract nineteen samples of metamorphosed carbonate bearing rocks were analyzed for carbon and oxygen isotope ratios by ion microprobe with a 5 15 m spot three from a regional terrain and 16 from five different contact aureoles contact metamorphic rocks further represent four groups calc silicate marble and hornfels 6 brucite marble 2 samples that contain a reaction front 4 and samples with a pervasive distribution of reactants and products of a decarbonation reaction 4 the average spot to spot reproducibility of standard calcite analyses is 0 37 2 standard deviations sd for 18 o and 0 71 for 13 c ten or more measurements of a mineral in a sample that has uniform isotope composition within error of measurement can routinely return a weighted mean with a 95 confidence interval of 0 09 0 16 for 18 o and 0 10 0 29 for 13 c using a difference of 6sd as the criterion only four of 19 analyzed samples exhibit significant intracrystalline and or intercrystalline inhomogeneity in 13 c at the 100 500 m scale with differences within individual grains up to 3 7 measurements are consistent with carbon isotope exchange equilibrium between calcite and dolomite in five of six analyzed samples at the same scale because of relatively slow carbon isotope diffusion in calcite and dolomite differences in 13 c can survive intracrystalline homogenization by diffusion during cooling after peak metamorphism and likely represent the effects of prograde decarbonation and infiltration all but 2 of 11 analyzed samples exhibit intracrystalline differences in 18 o up to 9 4 intercrystalline inhomogeneity in 18 o up to 12 5 and or disequilibrium oxygen isotope fractionations among calcite dolomite calcite quartz and calcite forsterite pairs at the 100 500 m scale inhomogeneities in 18 o and 13 c are poorly correlated with only a single mineral dolomite in a single sample exhibiting both because of relatively rapid oxygen isotope diffusion in calcite intracrystalline inhomogeneities in 18 o likely represent partial equilibration between calcite and fluid during retrograde metamorphism calcite is in oxygen isotope exchange equilibrium with forsterite in one of four analyzed samples in equilibrium with dolomite in none of six analyzed samples and in equilibrium with quartz in neither of two analyzed samples there are no samples of contact metamorphic rock with analyzed reactants and products of an arrested metamorphic reaction that are in oxygen isotope equilibrium with each other the degree of departure from equilibrium in analyzed samples is variable and is often related at least in part to alteration of 18 o of calcite during retrograde fluid rock reaction in situ sub grain scale carbon and oxygen isotope analyses of minerals are advisable in the common applications of stable isotope geochemistry to metamorphic petrology correlation of sub mm scale stable isotope data with imaging will lead to improved understanding of reaction kinetics reactive fluid flow and thermal histories during metamorphism

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

processes leading to eclogitization densification of subducted and tectonically buried crust 1 the transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling however a complex of partially eclogitized granulites on the island of holsnoy in the western part of the norwegian caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust the rocks on holsnoy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex metamorphism then occurred in a spatially heterogeneous manner with several distinct physical processes involving feedbacks between deformation fluid infiltration and chemical reactions operating simultaneously in different parts of the rock mass field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks a reservoir flux systems model is used to represent the various processes contributing to eclogitization of the granulites which operated at vastly different rates over disparate intervals of time the model suggests that the metamorphic process may have been brief 1 myr and ultimately self limiting arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite

mechanisms of oxygen isotopic exchange and isotopic evolution of 18 o 16 o depleted periclase zone marbles in the alta aureole utah insights from ion microprobe analysis of calcite oxygen isotope ratios have been measured by ion microprobe and millimeter scale dental drill along detailed sampling traverses across the boundary between periclase bearing 18o 11 8 and periclase free 18o 17 2 marble layers in the periclase per zone of the alta stock aureole utah these data define a steep coherent gradient in 18o that is displaced a short distance 4 cm into the periclase free cal fo layer sem and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale clear well polished calcite grains are isotopically homogeneous within analytical uncertainty 0 27 2sd more poorly polished pitted texturally retrograde turbid looking calcite has lower and more variable 18o values and replaces clear calcite along fractures cleavage traces or grain boundaries despite significant lowering of the 18o values in calcite throughout both layers during prograde metamorphism ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in 18o diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion 30 000 62 000 years at 575 600 c exceed significantly the timescale 1 250 years estimated for the prograde development of the 18o gradient at the boundary between these two marble layers the ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains thus the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization

a stable isotope study of retrograde alteration in sw connemara ireland dalradian metamorphic rocks lower ordovician meta igneous rocks mgs and caledonian granites of the connemara complex in sw connemara all show intense retrograde alteration alteration primarily involves sericitization and saussuritization of plagioclase the alteration of biotite and hornblende to chlorite and the formation of secondary epidote the alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs and was the result of a phase of fluid influx into these rocks in hand specimen k feldspar becomes progressively reddened with increasing alteration mineralogical alteration in the mgs and caledonian granites took place at temperatures 275 15 c and in the mgs pfluid is estimated to be 1 5 kbar during alteration the d values of alteration phases are 18 to 29 fluid inclusions 47 to 61 chlorites and 11 to 31 epidotes chlorite 18o values are 0 2 to 4 3 while 18o values for quartz k feldspar pairs show both positively sloped mgs and highly unusual negatively sloped caledonian granites arrays diverging from the normal magmatic field on a plot the stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200 140 c the retrograde fluid had d 20 to 30 in all lithologies but the fluid 18o varied both spatially and temporally within the range 4 to 7 the fo2 of the fluid that deposited the epidotes in the mgs varied with its 18o value with the most 18o depleted fluid being the most oxidizing the d values together with low 0 18o values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin this meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration the variation in retrograde fluid 18o values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid rock interaction infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the galway granite soon after intrusion at 400 ma however convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later

a proposed nomenclature for stages and series for the cambrian of laurentia an internally consistent succession of six named regional stages from youngest to oldest sunwaptan steptoean marjuman delamaran dyeran montezuman with their boundaries defined by regional extinction events within the trilobite record is proposed for the trilobite bearing rocks of laurentia sub trilobite stage s are recommended but unnamed the three youngest stages have been named previously but the lower boundary of the marjuman is shifted downward the three older stages within the trilobite record are new if deemed necessary four regional series are also suggested from youngest to oldest millardan lincolnian waucoban and begadean each of the three youngest series accommodates two stages

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

timescales and mechanisms of fluid infiltration in a marble an ion microprobe study using a recently developed ion microprobe technique a detailed oxygen isotope map of calcite grains in a coarse grained marble has been constructed supported by trace element mn sr fe analysis and cathodoluminescence cl imaging in order to constrain scales of oxygen isotope equilibrium timescales and mechanisms of metamorphic fluid infiltration and fluid sources and pathways results are compared with a previous study of this sample wada 1988 carried out using a cryo microtome technique and conventional oxygen isotope analysis the marble from the high temperature low pressure hida metamorphic belt in north central japan underwent granulite facies followed by amphibolite facies metamorphic events the latter associated with regional granite intrusion the cl imaging indicates two types of calcite a yellow luminescing ylc and a purple luminescing plc variety the ylc which occupies grain boundaries fractures replacement patches and most of the abundant deformation twin lamellae post dates the dominant plc calcite and maps out fluid pathways systematic relationships were established between oxygen isotope and trace element composition calcite type and texture based on 74 18o 16o and 17 trace element analyses with 20 30 m spatial resolution the ylc is enriched in mn and fe and depleted in 18o and sr compared to plc and is much more 18o depleted than is indicated from conventional analyses results are interpreted to indicate infiltration of 18o depleted metamorphic or magmatic fluid initial 18o 9 10 5 along grain boundaries fractures and deformation twin lamellae depleting calcite grains in sr and enriching them in mn and fe the sample is characterised by gross isotopic and elemental disequilibrium with important implications for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks

controls on the mechanisms of fluid infiltration and front advection during regional metamorphism a stable isotope and textural study of retrograde dalradian rocks of the sw scottish highlands vein controlled retrograde infiltration of h2o co2 fluids into dalradian epidote amphibolite facies rocks of the sw scottish highlands under greenschist facies conditions resulted in alteration of calcite rich marble bands to dolomite and spatially associated 18o enrichment of about 10 on a scale of metres fluid inclusion data indicate that the retrograde fluid was an h2o salt mixture with a low co2 content and that the temperature of the fluid was about 400d c detailed petrographic and textural backscattered electron imaging studies at one garnet grade locality show that advection of fluid into marbles proceeded by a calcite calcite grain edge flow mechanism while alteration of non carbonate wall rock is associated with veinlets and microcracks stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall rocks perpendicular to dolomite veins and fluid fluxes in the range 2 4 28 6 m3 m2 have been computed from measured advection distances coincidence of isotope and reaction fronts is considered to result from reaction enhanced kinetics of isotope exchange at the reaction front front advection distances are related to the proportion of calcite to quartz in each marble band with the largest advection distance occurring in nearly pure calcite matrix this relationship indicates that fluid flow in carbonates is only possible along fluid calcite calcite grain edges however experimental constraints on dihedral angles in calcite fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite calcite grain junctions rather than to an open calcite pore geometry the regional extent of the retrograde infiltration event has been documented from the high 18o of dolomite ankerite carbonates from veins and host rocks over an area of least 50 50 km in the sw scottish highlands isotopically exotic 18o rich retrograde fluids have moved rapidly upwards through the crust inducing isotopic exchange and mineral reaction in wall rocks only where lithology pore geometry or mineral solubilities pressure and temperature have been appropriate for pervasive infiltration to occur

the role of the grain boundary at chemical and isotopic fronts in marble during contact metamorphism carbon and oxygen isotopic profiles around a low pressure metasomatic wollastonite reaction front in a marble of the hida metamorphic terrain central japan display typical metamorphic fluid enhanced isotopic zonations isotopic profiles obtained from detailed microscale analyses perpendicular to the chemical reaction front in calcite marble show that diffusion enhanced isotopic exchange may control these profiles carbon and oxygen isotopic behaviour in grain boundaries is remarkably different oxygen isotopic troughs 18o depleted rims around the calcite grain boundaries are widely observed in this contact aureole demonstrating that diffusion of oxygen in calcite grain boundary dominates over lattice diffusion in calcite in contrast no difference is observed in carbon isotopic profiles obtained from grain cores and rims there is thus no specific role of the grain boundary for diffusion of carbonic species in the metamorphic fluid during transportation carbon chemical species such as co2 and co3 ions in metamorphic fluid migrate mainly through lattice diffusion the carbon and oxygen isotope profiles may be modelled by diffusion into a semi infinite medium empirically lattice diffusion of oxygen isotopes is almost six times faster than that of carbon isotopes and oxygen grain boundary diffusion is ten times faster than oxygen lattice diffusion oxygen isotopic results around the wollastonite vein indicate that migration of the metamorphic fluid into calcite marble was small and was parallel to the aquifer from the stability of wollastonite and the attainment of oxygen isotopic equilibrium we suggest that diffusion of oxygen occurred through an aqueous fluid phase the timescale of formation of the oxygen isotopic profile around the wollastonite vein is calculated to be about 0 76 106 years using the experimentally determined diffusion constant

late caradoc graptolitic faunal gradients across the iapetus ocean late caradoc graptolite assemblages across the iapetus ocean in wales and scotland became progressively more disparate despite the narrowing of the ocean we compare faunal distributions in continuous sections from opposite sides of iapetus at whitland in south wales and hartfell score in the southern uplands of scotland a comprehensive graptolite range chart is given for each the graptolite assemblages from the clingani biozone are subdivided into a lower ensigraptus caudatus subzone and upper dicellograptus morrisi subzone at both localities though the faunas differ in detail higher in the sequence the distinctive scottish linearis biozone fauna is not recognizable at whitland its presumed equivalent being a fauna dominated by variable morphotypes of the genus normalograptus this suggests that environmental gradients depth and or temperature were changing more rapidly than the geography significantly the subsequent inception of limestone deposition at whitland was approximately contemporaneous with widespread replacement of black mudstones by oxic bioturbated sediments elsewhere in the welsh basin and in scotland this was possibly a response to an early phase of cooling prior to the end ordovician glaciation in wales the claimed hiatus at the caradoc ashgill boundary may rather reflect biofacies variation

thin ice growth the thickening of ice covers has traditionally been calculated using a method in which the thickness is assumed proportional to the square root of the accumulated freezing degree days particularly for newly formed thin ice this method overpredicts the ice thickness consideration of the thermal resistance between the top of the ice and the atmosphere results in a method which predicts linear growth with time for thin ice and transitions to the t growth at large thicknesses the method proposed here is not new but seems to be rarely used even though it requires selection of only one coefficient data from several sources including initial river ice growth sea ice growth and sludge freezing are used to validate the method and bracket the coefficient

ion microprobe study of marble from naxos greece grain scale fluid pathways and stable isotope equilibration during metamorphism in metamorphic terranes modification of the stable isotope compositions of marbles by fluid infiltration has been extensively modeled to try to constrain both the magnitudes and time scales of metamorphic fluid fluxes we have made a study of key localities on the aegean island of naxos greece in which we combined textural observations with micro scale trace element and stable isotope analysis to test the assumptions upon which modeling of metamorphic fluid flow is based and to identify fluid transport mechanisms integration of ion microprobe analyses of 18 o 16 o 13 c 12 c mn and fe with detailed cathodoluminescence imaging provides evidence that water rich fluids were channeled along cracks and grain boundaries within marble with limited equilibration between the fluid and rock very low values of 18 o smow down to 2 along grain boundaries indicate a meteoric source for fluids rather than a metamorphic source in contrast to previous studies in this example the simplifying assumptions made previously in chromatographic theory to constrain metamorphic fluid fluxes time scales and porosities from bulk isotope data are incorrect by making micro scale measurements we can determine infiltration mechanisms and distinguish separate fluid flow events these observations and measurements provide a basis for constructing more realistic models of metamorphic fluid flow

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

the correlation of reaction and isotope fronts and the mechanism of metamorphic fluid flow infiltration of a metabasite sill from islay scotland by an h2o co2 fluid caused 1 modification of delta 18 o and 2 carbonation at the sill margins maps of delta 18 o and reaction progress were constructed from a 20 x 47 7 metre sample grid across

diffusion in carbonates fluorite sulfide minerals and diamond this chapter focuses on diffusion in non silicate minerals that do not fall under any of the other chapter categories this includes minerals that are not oxides fluorite sulfide minerals diamond and those like the carbonates that are not among the oxide minerals discussed in chapter 17 of this volume van orman and crispin 2010 although there are numerous minerals in these categories limited diffusion data exist for many of them so the primary phases that will be considered are carbonates fluorite diamond and sulfide minerals carbonate minerals are major constituents of sediments and many sedimentary rocks fractionation of o and c isotopes between carbonate rocks and fluids can provide insight into paleoclimates and sedimentary histories the behavior of the isotopes of major elements in calcite and aragonite has become of increasing interest due to the important role of co2 and carbonate dissolution precipitation in the oceans and the connections to global climate e g arita and wada 1990 chacko et al 1991 dickson 1991 beck et al 1992 cole 1992 graham et al 1998 cole and chakraborty 2001 ca mg diffusion rates affect the process of dolomitization hardie 1987 fisler and cygan 1999 and characterization of diffusion of these species is important for refinements in application of the calcite dolomite geothermometer e g essene 1982 farver and yund 1996 and better understanding of deformation mechanisms for example calcium isotopic fractionation between seawater and calcite and aragonite has been observed in marine samples zhu and macdougall 1998 de la rocha and depaolo 2000 schmitt et al 2003 with possible applications as a thermometer ca isotopic fractionation also occurs in biologic systems as in the growth of foraminifera skulan et al 1997 zhu and macdougall 1998 diffusion of major elemental species plays an important rate limiting

correlation for the lower palaeozoic the global correlation standards now agreed for the ordovician and silurian systems and partly so for the cambrian system are summarized correlation of the international series and stages with the traditional and revised british series and stages is reviewed as well as with those in north america laurentia

high resolution early paleozoic ordovician silurian time scales for much of the geologic time scale resolving power appears to be limited by the duration of biostratigraphic zones and subzones yet these zones exploit the appearance and disappearance events of only a small fraction of the species they span computer algorithms can sequence an order of magnitude more events and make explicit the uncertainties that arise when calibrating the resulting time line an illustrative case history builds an ordovician and silurian time scale from the geologic record of the entire graptolite clade over 1900 species from more than 400 localities and sections worldwide the same approach can be applied to any stratigraphic interval with detailed biostratigraphic observations it provides the foundation for time scales and paleobiologic time lines using logic similar to graphic correlation optimizing algorithms search for a composite sequence of species first and last appearance events that minimizes the implied shortcomings of all the field observations the algorithms minimize the number of species coexistences that are implied but never observed and the net adjustments needed to bring local range charts into agreement with a single composite sequence of events after total section thicknesses have been rescaled to help normalize for variation in depositional rate mean stratigraphic thicknesses are used to scale the intervals between adjacent events in the composite sequence the resulting scaled composite sequence is converted to a relative geologic time scale by identifying stage and zone boundaries within the sequence of graptolite events this relative scale is in turn calibrated by dated volcanic ash beds that were incorporated in the search for the optimal sequence these dated events are also used to test for linearity of the scaled composite the final time scale has a potential resolving power of 0 02 0 1 m y more than ten times better than can be achieved by traditional zones graptolite zones vary widely in duration from as short as 0 1 m y to nearly 5 0 m y the mean duration of zones or zonal groupings calibrated here is 1 44 m y in the ordovician and 0 91 m y in the silurian the average uncertainty in locating zone boundaries in a single composite sequence is about one fourth of the mean zone duration the variance resulting from differences in time scales developed from differing numbers of field observations and in response to changes in the optimization criteria gives a very conservative measure of the overall robustness of the method these differences indicate an average uncertainty in the age of graptolite zone boundaries that is more nearly equal to the mean zone duration for zone duration the mean uncertainty amounts to about one half of the length of an average zone

natural constraints on metamorphic reaction rates abstract quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources most common are predictions made on the basis of laboratory kinetic data however the applicability of such laboratory based predictions has long been questioned and many observations in the field now suggest much slower rates here published quantitative field based constraints on high temperature 400 c reaction rates are assembled from a variety of sources reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory based predictions a general rate law for regional metamorphism has been derived which best describes these field based data log 10 r net 0 0029 t 9 6 1 where r net is the net reaction rate g cm 2 a and t is temperature c at the same time natural reaction rates attending contact metamorphism differ from laboratory based predictions by less than two orders of magnitude and are in close agreement at higher temperatures thus while existing laboratory based kinetic data may be judiciously applied to some contact metamorphic systems laboratory based kinetic predictions clearly misrepresent regional metamorphism to explain this kinetic discrepancy regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited or transient availability of aqueous fluid in the intergranular medium the general field based rate law may be applied to regional metamorphic and other environments i e ultrahigh pressure or ultrahigh temperature metamorphism if similar system characteristics mainly low aqueous fluid content can be inferred

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

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

migration of reaction and isotopic fronts in infiltration zones assessments of fluid flux in metamorphic terrains abstract criteria are presented for recognising fluid infiltration driven reactions in metamorphic rocks and quantifying the associated fractionations of oxygen and carbon isotopic compositions in medium or high grade metamorphic rocks with local equilibrium between fluid and solid phases reactions or isotopic changes driven by infiltrating fluid are expected to migrate as relatively sharp fronts with limited diffusive broadening fluid leaving such reaction fronts is compositionally buffered by the reacting phases in suitably reactive rocks a series of reaction fronts will develop each driven by infiltration of fluid buffered by the upstream reaction knowledge of the spatial arrangement of such fronts allows calculation of the time integrated fluid fluxes total fluid volume per unit area distinctive stable isotope fractionations are predicted adjacent to infiltration driven reactions with the distribution and magnitude of the fractionation effects dependent upon the relative velocities of the reaction and isotopic fronts the various possibilities are illustrated by reference to a hypothetical infiltration driven decarbonation reaction the calculated isotopic fractionations do not in general resemble those predicted by the open or closed system water rock ratio calculations

fluid rock history of granulite facies humite marbles from ambasamudram southern india an extensive humite bearing marble horizon within a supracrustal sequence at ambasamudram southern india was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics at peak metamorphic temperatures of 775 73 c based on calcite graphite carbon isotope thermometry the mineral assemblages suggest layer by layer control of fluid compositions clinohumite calcite bearing assemblages suggest xco2 0 9 at 790 c and local wollastonite scapolite grossular bearing zones formed at xco2 of c 0 3 retrograde reaction textures such as scapolite quartz symplectites after feldspar and calcite and replacement of dolomite diopside or tremolite dolomite after calcite forsterite or calcite clinohumite are indicative of retrogression under high xco2 conditions calcite preserves late proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting isotopic zonation of calcite and graphite grains was likely produced by localized co2 fluid infiltration during retrogression contrary to the widespread occurrence of humite marbles related to retrograde aqueous fluid infiltration the ambasamudram humite marbles record a prograde to peak metamorphic humite formation and retrogression under conditions of low xh2o

the proterozoic cambrian transition within the charnian supergroup of central england and the antiquity of the ediacara fauna a revised interpretation for the charnian supergroup is suggested from ichnological and lithological comparisons with the cambrian of nuneaton and other parts of the avalonian superterrane late neoproterozoic volcaniclastic sediments contain an ediacara fauna in the maplewell group these are succeeded by one or more inferred erosional breaks and overlain by passive margin sediments quartzarenites glauconitic greywackes pelites of the brand group which yield cambrian trace fossils including abundant large teichichnus this transformation is compared with that between the neoproterozoic caldecote volcanic formation and the cambrian hartshill formation at nuneaton the hanging rocks formation of the brand group lies above a major erosional break but shares a volcanogenic island arc petrology with the underlying maplewell group conglomeratic sandstones at the base of the brand hills formation contain purple pelite clasts comparable with underlying lithologies plus clasts of weathered diorite showing granophyric textures identical to those seen in the enigmatic south charnwood diorites nd sm isotope studies confirm the geochemical similarity between these diorites and those that intruded the caldecote volcanics at nuneaton previously u pb zircon dated at 603 2 ma these strong similarities with the nuneaton succession suggest that deposition of the brand hills formation in the cambrian was preceded by the local unroofing of diorite and erosion of underlying mudrocks much as seen in other parts of avalonia a date of 603 2 ma for the south charnwood diorites provides a minimum age for the local ediacara fauna but this is in need of substantiation

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

stable isotope evidence of heterogeneous fluid infiltration at the ubehebe peak contact aureole death valley national park california stable isotope ratios of carbon and oxygen are used to define quantitatively the effects of magmatic fluid infiltration in marbles contact meta morphosed by the 173 ma ubehebe peak quartz monzonite death valley national park california in previous studies of fluid infiltration quantitative interpretation of aureole wide isotopic data has been difficult due to small data sets for this study sampling strategies were developed to obtain a data set that was large and unbiased enough to be statistically representative of the carbonates north of the ubehebe peak intrusion a total of 357 samples of marble were analyzed for bulk carbonate isotopic ratios separate analyses of coexisting calcite and dolomite were also obtained for an additional 31 samples unmetamorphosed samples 1900 3000 m from the intrusion have 18 o permil smow values of 25 5 0 8 1 and 13 c permil pdb values of 0 4 0 6 1 samples in the tremolite zone 750 1900 m have 18 o values ranging from 19 4 to 27 7 permil with a median value of 25 2 permil and 13 c ranges from 5 1 to 0 5 permil with a median value of 0 9 permil forsterite zone samples 0 750 m have isotopic ratios shifted to values as low as 11 1 permil 18 o and 9 1 permil 13 c despite this shift most forsterite zone samples retain sedimentary isotopic compositions with median 18 o values of 25 0 permil and 13c values of 1 2 permil 18o values for igneous minerals show no evidence for interaction with heated meteoric or metamorphic fluids the shifts in isotopic compositions within the marbles are interpreted to be the result of magmatic infiltration the effects of this infiltration were quantified by identifying samples with isotopic alteration that can only be attributed to infiltration the results show that magmatic fluid infiltration was limited in extent and very heterogeneous there is no evidence for infiltration of isotopically reactive fluids beyond 850 m from the intrusive contact and within this 850 m zone only 28 percent of the samples have been infiltratively altered with respect to 18 o and 20 percent are depleted in 13 c compositions the isotopic data when evaluated in conjunction with geostatistical and petrologic data indicate that the geometry of the hydrothermal flow system was mainly vertical and away from the pluton infiltration was restricted to large nearly vertical tube like zones of increased permeability these higher permeability zones likely reflect an initial heterogeneity of the host rocks and show no significant evidence for reaction enhanced permeability given the heterogeneity of the system and a lack of knowledge about many basic parameters controlling fluid infiltration it is shown that the best method of calculating the amount and composition of the infiltrating fluid may be a mass balance approach fluid rock ratio the application of mass balance models is discussed and shown to be valid only under limited conditions since infiltration at ubehebe peak was largely vertical the observed isotope alteration patterns represent an infiltration side and not a front because this precludes the use of traditional mass balance calculations a new infiltration side inside model is proposed that allows the isotopic data to be evaluated the inside model uses the ratio of the areal amounts of infiltrative alteration to calculate a fluid composi tion fluid amounts cannot be obtained from this model results for the ubehebe

the accuracy of δ11b measurements of foraminifers values of 11b reported in the literature for holocene samples of the same foraminiferal species show large variability 6 relative to cited precision 1 this is indicative of significant inter laboratory biases and raises concerns about the accuracy of foraminiferal proxy ph records to investigate this problem we have measured the 11b of modern ocean carbonates using several different analytical procedures we have used total evaporation negative thermal ionisation mass spectrometry te ntims with various sample preparation and loading procedures and multi collector inductively coupled plasma mass spectrometry mc icpms the 11b of pure boric acid solutions measured by te ntims and mc icpms agree well demonstrating no fundamental biases between the techniques yet the 11b values measured by te ntims for non foraminiferal carbonates are about 2 lighter than those by mc icpms whereas foraminifers measured by the same standard te ntims procedure are 2 to 6 heavier than those by mc icpms significantly we found that foraminiferal samples kept in acidic solution over several months yielded lower 11b values up to 5 lower and better reproducibility when re measured by te ntims we infer that organic material released on foraminiferal dissolution causes biases in ntims measurements no residual organics should be present in the mc icpms measurements as matrix is separated from sample before analysis moreover we demonstrate by standard addition the absence of any matrix influence in the mc icpms procedure beyond the inherent uncertainties in the standard addition approach 0 35 2sigma degradation of the inferred organic residue either by long term storage in acidic solution or by loading samples in 30 h2o2 reduces but does not totally remove the variable bias between te ntims and mc icpms 11b measurements despite these problems careful analysis of similar samples by ntims may permit data to be obtained with consistent relative differences that still yield valuable proxy records but there is clearly considerable potential for inaccuracies to result in such an approach from hard to detect changes in matrix

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

growth rate and salinity profile of first year sea ice in the high arctic this paper describes the growth of sea ice and the salinity profiles observed in eclipse sound near pond inlet baffin island canada during the winter of 1977 78 a numerical method of calculation has been developed to incorporate the variations in snow conditions and physical properties of ice and snow during the growth season it is shown that the growth rate can be predicted reasonably well it is also shown that the vertical salinity profile in the ice towards the end of the season provides a record of previous climatological conditions a dependence has been shown between the predicted growth rate and the measured salinity

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

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

the oxygen isotope anatomy of a slowly cooled metamorphic rock the distribution of o isotope ratios in a slowly cooled metamorphic rock is investigated by a combination of ion microprobe and laser probe techniques the sample has unusually simple geometry centimeter size domains of feldspar and garnet separated by a one millimeter to one centimeter thick layer of millimeter size magnetite grains isotopic zonation at two different scales is documented a sharp decrease in 18 o in the outer 10 100 m of individual magnetite grains is in agreement with modeling of isotopic reequilibration during cooling by interdiffusion between feldspar and magnetite the presence of this zonation in the outer portions of magnetite grains that are adjacent to garnet indicates extensive transport along grain boundaries during diffusive exchange millimeter scale zonation in the polygranular layer of magnetite is caused by the interaction of volume diffusion and grain boundary transport the effects of which depend upon local textures grain boundaries enhance bulk diffusion in polygranular aggregates and allow exchange between nontouching grains the results document a complex textural control to millimeter scale o isotope zonation and geothermometry in slowly cooled rocks and document the importance of fast grain boundary transport in controlling diffusive exchange