fluid infiltration and transport of major minor and trace elements during regional metamorphism of carbonate rocks wepawaug schist connecticut usa geochemical alteration of metacarbonate layers due to fluid infiltration during greenschist and amphibolite facies metamorphism was investigated based on 149 whole rock analyses of the wepawaug schist usa ankerite albite ank ab rare ankerite oligoclase ank ol biotite bt amphibole amp and diopside di index mineral zones developed during progressive metamorphism acadian orogeny and devolatilization co2 h2o s loss ank ab ank ol and bt zone rocks were altered along some lithologic contacts and in reaction selvages around veins mass changes included gains of na y and in many cases p and fe k rb and ba were typically lost and sr was lost except where calcite precipitated in and around calcite bearing veins na gain and k loss reflect muscovite destruction and albite growth in response to infiltration of fluids from surrounding metapelitic and metapsammitic metaclastic rocks at higher metamorphic grades k rb and ba were lost when biotite broke down to form amphibole or diopside these elements were transported out of metacarbonate layers down chemical potential gradients at lithologic contacts and in selvages na was lost as plagioclase reacted to form clinozoisite zoisite czo zo calc silicates cm to dm scale thickness rich in amphibole and czo zo amp zones or diopside and zo di zones formed at layer margins and in selvages via extreme metasomatism including major 1 addition of al si and in many rocks fe 2 destruction of calcite and 3 loss of ca and volatiles volatile losses near 90 percent relative to low grade precursors demonstrate that calc silicate formation resulting from al si metasomatism is an important source of co2 to be considered when assessing regional devolatilization al mass gains for calc silicates indicate that large quartz veins and some lithologic contacts were loci for massive time integrated fluid fluxes of 105 m3 m 2 heavy rare earth elements hree were gained in the ank ab through the amp zones and a di zone example gained light and mid ree including sm and nd the ree gains coincide with gains of y and in most cases p suggesting that p and y complexes transport ree volume was lost progressively from the unveined interiors of metacarbonate beds attaining 20 to 30 percent loss in the di ii zone mass addition due to veining reduced volume losses and in some cases produced volume gains reaction transport modeling indicates that fluid flow was mostly layer parallel was focused along lithologic contacts fractures and permeable horizons and was accompanied by diffusion and or mechanical dispersion at high angles to the flow directions the metasomatic fluids infiltrated from surrounding metaclastic and or syn metamorphic igneous rocks thus interaction of fluids derived from different rock types must be considered when assessing mass transfer in lithologically diverse mountain belts

petrochronology and tims thermal ionization mass spectrometers or tims were developed by the pioneers of mass spectrometry in the mid 20th century and have since been workhorses for generating isotopic data for a wide range of elements later developed mass spectrometric techniques have many advantages over tims including higher spatial resolution with in situ techniques such as secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms and greater versatility in terms the elements that can be easily and well measured the reason tims persists as an important method for geochronology is that for some key parent daughter systems e g u pb sm nd it can produce isotopic data and resultant dates with 10 100 times higher precision and more quantifiable accuracy than in situ techniques even when sample sizes are very small such as those that might result from single crystals or even small portions of zoned crystals for many questions in the geosciences the highest achievable precision and accuracy are required to resolve the timescales of processes and or correlate events globally as an example modern tims u pb geochronology is capable of producing dates with precision and accuracy better than 0 1 of the age for single crystals with only a few picograms pg of pb therefore it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in mesozoic and younger rocks if these dates and rates can be connected with other igneous processes such as magma transfer emplacement and crystallization then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage as another example sm nd geochronology of garnet permits dates with precision better than 1 million years for garnets of any age including multiple concentric growth zones in single crystals such

initial pb sr nd isotopic heterogeneity in a single allanite epidote crystal implications of reaction history for the dating of minerals with low parent to daughter ratios we analyzed 17 fragments from a zoned allanite epidote crystal ca 2 2 mm 4 0 mm which had formed during different prograde and retrograde stages of ultra high pressure uhp and amphibolite facies metamorphism 240 230 ma sulu belt e china for the isotopic composition of pb nd and sr and contents of pb u and th sr and rb and nd and sm since most fragments had 238u 204pb and 232th 204pb values less than 1 corrections for in situ pb growth are small and uncertainties in the recalculation of the pb isotopic compositions to 240 ma are insignificant the recalculated pb falls on a linear trend in the 206pb 204pb vs 207pb 204pb diagram with the allanite defining the low 206pb 204pb end 17 07 of this trend and the epidote defining its high 206pb 204pb end 17 56 the recalculated data scatter in the 206pb 204pb vs 208pb 204pb diagram which implies that the initial pb isotopic variation reflects the involvement of at least three different pb sources the low 87rb 86sr values account for a change in 87sr 86sr by in situ 87sr growth of less than 0 0007 which implies that the isotopic heterogeneity of 87sr 86sr 0 70601 0 7200 is a primary feature the pb and sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product it is likely that such an initial isotopic heterogeneity also exists for nd but it could not be resolved in the present study initially heterogeneous pb and sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates especially for minerals with relatively low parent to daughter ratios

high precision microsampling and preparation of zoned garnet porphyroblasts for sm nd geochronology abstract zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes with growth histories spanning many millions of years well known challenges exist in the physical preparation purification and geochronologic analysis of zoned garnets which have limited the progress and application of this potentially powerful geochronologic tool here we present details of an integrated methodology specifically for the acquisition of high resolution sm nd geochronologic records from zoned garnets that overcome these challenges we develop and demonstrate our methods on large test garnets from a shear zone in the tauern window in western austria chemically contoured microdrilling permits physical sampling of many discrete age annuli each as narrow as 100 m within a single garnet refined hno 3 hf hclo 4 partial dissolution methods cleanse coarse crushed 75 150 m size garnet annuli of detrimental inclusions yielding high 147 sm 144 nd ratios 0 9 to 4 6 and consequently accurate and precise ages experiments on powdered garnet 147 sm 144 nd 143 nd 144 nd with 10 20 ppm 2 rsd precision good enough for 147 sm 144 nd 1 0 in total this integrated sampling and analytical protocol permits the acquisition of 3 11 discrete high precision

synchronous peak barrovian metamorphism driven by syn orogenic magmatism and fluid flow in southern connecticut usa recent work in barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times this result is inconsistent with most geodynamic models of crustal over thickening and conductive heating wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times instead the presence of additional sources of heat and or focusing mechanisms for heat transport such as magmatic intrusions and or advection by metamorphic fluids may have contributed to the contemporaneous development of several different metamorphic zones here we test the hypothesis of temporally focussed heating for the wepawaug schist a barrovian terrane in connecticut usa using sm nd ages of prograde garnet growth and u pb zircon crystallization ages of associated igneous rocks peak temperature in the biotite garnet zone was dated via sm nd on garnet at 378 9 1 6 ma 2r whereas peak temperature in the highest grade staurolite kyanite zone was dated via sm nd on garnet rims at 379 9 6 8 ma 2r these garnet ages suggest that peak metamorphism was pene contemporaneous within error across these metamorphic grades ion microprobe u pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn metamorphic peak igneous activity at 380 6 4 7 ma 2r indistinguishable from the peak ages recorded by garnet a 388 6 2 1 ma 2r garnet core age from the staurolite kyanite zone indicates an earlier episode of growth coincident with ages from texturally early zircon and a previously published monazite age along the prograde regional metamorphic t t path the timing of peak metamorphism and igneous activity as well as the occurrence of extensive syn metamorphic quartz vein systems and pegmatites best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes the first at c 389 ma and the second corresponding to the regionally synchronous peak metamorphism at c 380 ma

authigenic albite in carbonate rocks a tracer for deep burial brine migration euhedral post depositional albite from the eastern and western alps the western carpathians and some greek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in carbonate rocks this study focuses on coarsely crystalline homogeneously nucleated albite in order to avoid problems related to the presence of inseparable detrital material in fine grained albite varieties all albite samples show a very restricted compositional variability and are typically 99 mol ab component unit cell parameters determined by rietveld analysis are slightly more variable than previously accepted but confirm high al si ordering characteristic of low albite the oxygen isotopic composition of albite ranges from 19 4 to 28 3 vsmow there is no direct relationship between the 18o value and the inferred temperature of albite formation nor is there one with stoichiometry the coarse crystal size up to several millimetres in diameter petrographic evidence showing albite cross cutting stylolites greater abundance of albite in carbonate rocks subject to high grade diagenetic or weak metamorphic overprinting and available fluid inclusion data suggest that albite precipitation is favoured at higher temperatures in carbonates than in sandstones pore fluids were invariably brines as suggested by the inferred high positive 18ofluid values the common association of albite bearing carbonates and evaporites and reports of saline fluid inclusions in albite the presence of authigenic albite may thus be a useful tracer of palaeobrine carbonate reactions particularly in deep burial and incipient metamorphic settings

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

young ages from old garnet in the polyphase metamorphic terrane of the limpopo belt south africa abstract the p t t path of high grade metamorphic rocks is significant for understanding vertical motions and heating of the crust in general garnet is used to constrain p t paths and metamorphic zircon is used to obtain t for different metamorphic stages however this approach may be complicated by polyphase tectonothermal events the central zone of the limpopo belt in south africa is a complex precambrian metamorphic terrane that has experienced three distinct high grade metamorphic events at 3 22 ga m1 2 62 ga m2 and 2 02 ga m3 based on zircon dating a previous investigation of a metapelite enclave in the bulai pluton in the central zone showed that metamorphic zircon in the enclave recorded mainly m2 granulite facies metamorphism and was protected by the bulai pluton however titanite and rutile u pb ages of the same sample record the m3 event here we investigated garnet sm nd isotopic systematics of four metapelite samples from the evelyn enclave in the bulai pluton we constructed single mineral sm nd isochrons for garnet using micro samples in order to avoid the risk of isotopic disequilibrium between different minerals or whole rocks metamorphic zircon with sillimanite inclusions has yielded a granulite facies metamorphic age of ca 2 6 ga however the sm nd isotopic data for the garnet record much younger ages of ca 2 03 ga these ages are consistent with the m3 event in this area but were rarely identified in previous zircon dating studies this suggests that the m3 event reached sufficiently high temperatures to totally reset the rutile and titanite u pb and garnet sm nd systems moreover the decoupling of the age of garnet formation and the date it records needs to be carefully considered when reconstructing p t t paths and again highlights the usefulness of garnet in the study of polyphase metamorphic terranes

garnet growth as a proxy for progressive subduction zone dehydration the release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism seismicity the growth and maturation of continents and the global geological water co 2 cycle while models exist to predict slab volatile release from hydrous phases it is challenging to reconstruct and test these fluid fluxes in nature here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones using thermodynamic analysis including the effects of garnet fractionation and fluid removal we show the proportional relationship between garnet and water production in two end member crustal lithologies pelitic sediment and hydrated mid oceanic ridge basalt morb in three representative subduction geotherms dehydrating minerals such as lawsonite chlorite amphibole and epidote contribute to garnet growth especially between 1 4 and 3 0 gpa where geophysical models and observations predict dehydration the average production ratio for altered morb compositions is 0 52 wt water as fluid per vol garnet in cooler geotherms honshu japan and nicaragua and 0 27 in hotter geotherms cascadia north america whereas for pelite the production ratios are about half 0 24 and 0 13 respectively garnet growth correlates with production of 3 3 5 9 wt water in hydrated morb and 1 8 3 1 wt water in pelite representing 42 100 of the water lost between 0 5 and 6 5 gpa from a fully saturated starting material garnet abundance its pressure temperature growth span and its growth chronology may be used to recognize reconstruct and test models for progressive subduction zone dehydration

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

evaluating chemical equilibrium in metamorphic rocks using major element and sm nd isotopic age zoning in garnet townshend dam vermont usa abstract a quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet sm nd ages suggests that major element matrix equilibrium was maintained to a first order throughout a ca 40 cm wide rock sample during garnet growth however cm scale sm nd isotopic heterogeneity limits the sm nd age precision required to evaluate more subtle age differences within individual garnet crystals central wafers from 1 3 cm diameter garnet grains within a 1 21 104 cm3 block of pelitic schist were used to document concentric growth zoning of major elements with decreasing mn and ca and increasing fe and mg from cores to rims garnets also preserve growth zoning patterns for hree and mree and show evidence for resorption and partial recrystallization of the outermost rims similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium p t x conditions for major elements over the sample volume comparison of 28 rock sm nd isotope values from the sample indicates substantial cm scale heterogeneity which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation therefore sm nd isotopic compositions of thirty eight concentric core to rim garnet segments from ten large 1 3 cm garnets and two small 1 4 mm bulk garnets with narrow ranges of mn content are paired with sixteen matrix whole rock sm nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383 1 6 8 ma to 324 5 3 3 ma four of the garnets have anomalously young rims that likely result from post growth alteration chlorite quartz and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post growth resorption recrystallization effects excluding these young rims yields a range of ages from 383 1 6 8 oldest core to 374 9 1 8 ma youngest rim sm nd age precisions 1 5 m y and high mswd result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix however garnet cores with high mn n 7 mantles with intermediate mn n 14 and rims with low mn n 8 including the 2 smaller bulk garnet analyses define three distinct multi grain isochrons of 380 3 2 0 ma n 23 mswd 14 377 3 1 4 ma n 30 mswd 18 and 376 5 1 0 ma n 24 mswd 18 respectively yielding an average garnet growth duration of 3 8 2 2 m y these three composite mn age zones define a mn vs age relationship that reflects depletion of mn in the rock matrix as it is sequestered by growing garnet correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained to a first order throughout the ca 4 m y duration of garnet growth

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

strontium isotope zoning in garnet implications for metamorphic matrix equilibration geochronology and phase equilibrium modelling in principle garnet growth rates may be calculated from 87 rb 86 sr and 87 sr 86 sr measurements in garnet subsamples and the surrounding rock matrix because of low rb sr garnet should passively record the matrix decay of 87 rb to 87 sr as a progressive increase in 87 sr 86 sr from core to rim this concept was tested by collecting rb sr data for five garnet grains from four major orogenic belts eastern vermont c 380 ma western new hampshire c 320 ma southern chile c 75 ma and northwestern italy c 35 ma both normal sr isotope zoning increasing 87 sr 86 sr from core to rim and inverse sr zoning decreasing 87 sr 86 sr from core to rim were observed garnet and matrix iso tope data commonly yielded grossly inaccurate model ages incomplete rb and sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v measured zoning patterns and the age disparities initially the reactive matrix is dominated by rapidly equilibrating rb rich mica which imparts high 87 sr 86 sr values in garnet cores progressive participation of slower equilibrating sr rich plagioclase buffers or even reduces 87 sr 86 sr possibly leading to flat or decreas ing 87 sr 86 sr from garnet cores to rims unusually high 87 sr 86 sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages so metamorphic ages growth rates and associated heating and loading rates are likely suspect although rb sr may be the most suscep tible because of the profound disparities between mica and feldspar zircon reactivity might influence the lu hf system by up to a few per cent the sm nd system seems generally immune to these effects pseudosection analysis and conventional garnet geochronology which presume complete matrix equilibration during metamorphism may require modification to account for differences between whole rock v reactive matrix compositions

late cretaceous uhp metamorphism recorded in kyanite garnet schists from the central rhodope mountains bulgaria in this study we report the first discovery of microdiamond inclusions in kyanite garnet schists from the central rhodope mountains in bulgaria these inclusions occur in garnets from metapelites that are part of a meta igneous and meta sedimentary melange hosted by variscan hercynian orthogneiss ultra high pressure uhp conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750 800 c and pressures in excess of 4 gpa the discovery of uhp conditions in the central rhodopes of bulgaria compliments the well documented evidence for such conditions in the southernmost greek part of the rhodope massif dating of garnets from these uhp metapelites chepelare shear zone using sm nd geochronology indicates a late cretaceous age 70 5 92 7 ma for the uhp metamorphic event this is significantly younger than previously reported ages and suggests that the uhp conditions are associated with the late mesozoic subduction of the vardar ocean northward beneath the moesian platform europe the present day structure of the rm is the result of a series of subduction exhumation events that span the cenozoic alongside subsequent post orogenic extension and metamorphic core complex formation

field measurement of high temperature bulk reaction rates i theory and technique knowledge of metamorphic reaction rates is crucial to accurately interpret rock and mineral chemistry the local equilibrium assumption used in geochronology geothermobarometry and material flux estimates requires that local reaction rates among system phases are fast relative to local rates of p t x change natural metamorphic reaction rates are essentially unknown and difficulties and disagreements exist regarding extrapolations of existing laboratory data to natural conditions growing recognition of natural effects that could be attributed to slower reaction rates justifies the need for a field based quantification of reaction rates to assess the accuracy of lab based predictions we describe in detail the theory and methodology of a technique for extracting bulk reaction rates directly from isotopic data derived from natural samples baxter and depaolo 2000 reaction rates measured using this technique may be judiciously applied to isotopic exchange or net reaction kinetics in other natural systems the technique requires collection of whole rock and garnet 87sr 86sr data along a sampling traverse normal to a lithologic contact where there was prior to metamorphism a sharp isotopic discontinuity garnet data provide information on syn metamorphic conditions and the time interval for the exchange process forward modeling of the reactive transport process using numerical methods and the equations for diffusive reactive transport allow determination of the reaction rate and bulk sr diffusivity that provides the best fit to the data measurement of reaction rates is best constrained if an isotopic step is preserved at the contact the size of which is directly proportional to the bulk reaction rate this contribution is intended to serve as a template for future use and development of this technique to acquire natural reaction rate data from metamorphic systems

trace element mapping by la icp ms assessing geochemical mobility in garnet a persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements particularly the rare earth elements rees this is despite the fact that trace elements are generally less vulnerable to diffusive resetting and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage including the growth and or dissolution of accessory minerals we outline a technique for the routine acquisition of high resolution 2d trace element maps by la icp ms and introduce an extension of the software package xmaptools for rapid processing of la icp ms data to visualise and interpret compositional zoning patterns these methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet which are argued to be largely dependent on the interplay between element fractionation mineral reactions and partitioning and the length scales of intergranular transport samples from the peaked hill shear zone reynolds range central australia exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing u pb monazite and sm nd garnet geochronology ultimately the remarkable preservation of original growth zoning and its subtle modification by subsequent re equilibration is used to see through multiple superimposed events thereby revealing a previously obscure petrological and temporal record of metamorphism metasomatism and deformation

nd isotope re equilibration during high temperature metamorphism across an orogenic belt evidence from monazite and garnet abstract monazite is a powerful u th pb geochronometer commonly used to determine the tempo of orogenic processes as a light rare earth element lree enriched mineral monazite also records key information for understanding the behavior of sm nd isotope systematics during tectonothermal events however sm nd isotope studies of monazite have received far less attention than its use as a u pb geochronometer here we investigate coupled u pb ages and sm nd isotopic compositions of monazite from partially melted metasedimentary rocks in the jiao liao ji orogenic belt jljb north china craton to provide insight into sm nd isotope behavior during high temperature metamorphism and crustal melting we utilize several complementary dating systems in the same sample including garnet lu hf and sm nd zircon u pb and monazite u pb geochronology to gain precise time constraints on the p t evolution of the jljb garnet lu hf isochron dates of 1 97 1 94 ga are interpreted to represent prograde garnet growth consistent with the prograde metamorphic zircon dates of 1 96 1 90 ga garnet sm nd dates agree with retrograde metamorphic zircon dates and all monazite u pb dates define a relatively narrow age range of 1 86 1 83 ga interpreted to represent retrograde cooling the lack of older monazite grains matching zircon or garnet dates suggests the dissolution of pre existing monazites into partial melt monazite in all samples irrespective of occurring as garnet inclusions or in the matrix yields homogenous nd isotope compositions end 5 at 1 85 ga which are consistent with the garnet and bulk rock values thus nd isotope re equilibration between bulk rock and minerals occurs synchronously across the orogenic belt during high t metamorphism 750 c where melting was involved with no evidence of a pre metamorphic isotope signature this suggests mobility in the sm nd isotope system during high grade metamorphism and melting where the nd isotope signature of monazite likely represents the most recent tectonothermal event

field measurement of high temperature bulk reaction rates ii interpretation of results from a field site near simplon pass switzerland bulk metamorphic reaction rates have been measured using sr isotopes as tracers of reactive transport processes at a field site near simplon pass switzerland employing the technique described in part i of this study baxter and depaolo 2002a the reaction rate r inferred similar to the value first reported in baxter and depaolo 2000 is 1 4 1 1 0 4 x 10 7 yr 1 or r 7 0 5 5 2 0 x 10 9 g cm 2 yr normalized to the geometric surface area of plagioclase which is several orders of magnitude slower than extrapolations of laboratory data this rate suggests that attainment of local equilibrium may require 10 s of millions of years and consequently equilibrium may not be closely approached in dynamic metamorphic systems if significant changes in p t x occur over shorter timescales these data suggest that there is a problem in current extrapolations or applicability of laboratory kinetic data to natural high temperature systems possible reasons for this discrepancy include differences in reactive surface area reactant supply limited by slow diffusion of major elements or an armoring effect by which access to grain interiors is limited on the basis of microchemical patterns in plagioclase we identify grain boundary migration gbm as the rate limiting process for accessing grain interiors and thus for bulk reaction calculation of reaction quotients for six balanced mineral reactions suggests local disequilibrium for garnet with respect to its ca content the characteristic equilibrium lengthscales le for sr and ca are both about 1 meter equilibrium based geochemical tools such as isochron geochronology or geothermobarometry are compromised within a distance le of the lithologic contact bulk sr diffusivity measured at the field site changes from 10 9 to 10 7 m yr suggesting that the characteristics of the intergranular transporting medium itm changed significantly during the early retrograde history of the field site

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

timing of prograde metamorphism in the zanskar himalaya the timing of the prograde metamorphism of continental crust buried during the collision of india and eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism our knowledge of the himalaya indicates that continental collision at 55 50 ma culminated in crustal melting and leucogranitic magmatism at 24 18 ma however the timing of events in the intervening 30 m y is poorly understood here we present sm nd isotope analyses of garnets from the zanskar himalaya that also preserve information on crustal and orogenic evolution during that period the prograde p t t paths indicate that heating and burial from 6 to 10 kbar continued until at least 25 30 ma and imply peak temperatures 700 c close to those necessary to cause the crustal melting in the himalaya at ca 20 ma without a requirement for additional heat sources such as shear heating on major crustal shear zones the persistence of high temperatures as late as 20 ma in the himalaya may be related to postcollisional thrusting south of the suture zone and within the indian plate at 40 35 ma

garnets as open systems during regional metamorphism if a garnet fractures during metamorphism its interior will be in chemical communication with matrix phases solids and fluids and may participate in late prograde or retrograde metamorphic reactions the core of a growth zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally in the vicinity of fractures or extensively resulting in dissolution and reprecipitation of the garnet interior and or production of new phases the results of petrologic techniques such as thermobarometry pressure temperature path calculations and geochronometry may be misinterpreted if sites of garnet and mineral inclusion compositional modification are not identified brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions the conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure retrograde pressure temperature path and thermoelastic properties of the inclusion and garnet inclusion induced fracturing is favored by nearly isothermal decompression a common metamorphic pressure temperature path other fractures may form as a result of propagation of an inclusion host boundary or by mechanisms external to the garnet

interactions between serpentinite devolatilization metasomatism and strike slip strain localization during deep crustal shearing in the eastern alps the greiner shear zone in the tauern window eastern alps changes from a zone of distributed dominantly sinistral shear in supracrustal rocks to a series of narrow gully forming dextral splays where it enters basement gneisses within these splays granodiorite is transformed into quartz poor biotite and or chlorite schists reflecting hydration removal of si ca and na and concentration of fe mg and al stable isotope analyses show a prominent increase in d and a decrease in 18o from granodiorite into the shear zones these changes indicate significant channelized flow of an externally derived low 18o high d fluid through the shear zones the shear zone schists are chemically similar to blackwall zones developed around serpentinite bodies elsewhere in the greiner zone and the stable isotope data support alteration via serpentinite derived fluid monazite in schist from one shear zone yields spot dates of 29 20 ma indicating that the fluid influx and switch from sinistral to dextral shear occurred at or shortly after the thermal peak of the alpine orogeny c 30 ma we suggest that alpine metamorphism of serpentinites released large amounts of high d low 18o si undersaturated fe mg saturated fluids that became channelized along prior zones of weakness in the granodiorite infiltration of this fluid facilitated growth of chlorite and biotite which in turn localized later dextral strain in the narrow splays via cleavage parallel slip this dextral strain event can be linked to other structures that accommodated tectonic escape of major crustal blocks during dextral transpression in the eastern alps this study shows that serpentinite devolatilization can play an important role in modifying both the chemistry and rheology of surrounding rocks during orogenesis

disequilibrium for ca during growth of pelitic garnet compositional zoning in hundreds of almandine rich garnets in amphibolite by facies micaceous quartzites from the picuris range north central new mexico usa indicates that although mn mg and fe achieve chemical equilibrium at hand sample scale during garnet growth ca does not instead ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment yielding disequilibrium for ca at scales larger than the region immediately surrounding an individual porphyroblast detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite and core analyses were made of 97 additional crystals each analysis was made on a section that passed precisely through the morphological centre of the crystal located by means of 3 d imagery from computed x ray tomography the data reveal strong correlations between crystal size and concentrations of mn mg and fe but not ca in garnet cores a relationship between crystal size and isolation rigorous cross correlations among concentrations of mn mg and fe but not ca and systematic variations in ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations we interpret these observations as the result of thermally accelerated diffusion controlled garnet growth in circumstances that promoted rapid intergranular diffusion and thus rock wide equilibration of mn mg and fe but that prevented equilibration at similar scale for ca because of its more sluggish intergranular diffusion the anomalous behaviour of ca is made evident in these garnets by the presence of sharp spikes in ca concentration which are demonstrably not a consequence of any simultaneous rock wide event such as a change in pressure temperature or some other intensive parameter instead ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast to the degree that such kinetic factors introduce departures from chemical equilibrium for ca thermobarometric estimates that involve grossular contents of pelitic garnet will be in error

timing duration and inversion of prograde barrovian metamorphism constrained by high resolution lu hf garnet dating a case study from the sikkim himalaya ne india abstract we investigated lu hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c 500 c to biotite dehydration melting at c 800 c in the sikkim himalaya india exceptionally precise lu hf ages obtained for the barrovian metasedimentary sequence in the lesser himalaya lh correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth the youngest age is reported for the garnet zone 10 6 0 2 ma and then the ages become progressively older in the staurolite 12 8 0 3 ma kyanite 13 7 0 2 ma and sillimanite 14 6 0 1 ma zones the oldest age of 16 8 0 1 ma was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting directly below the main central thrust mct these ages provide a tight constraint on the timing and duration of the barrovian sequence formation which lasted about 6 ma the age pattern is clearly inverted with respect to structural depth but shows normal correlation with the metamorphic grade i e earlier garnet growth in higher grade rocks the timing of the peak metamorphic conditions estimated for the garnet and kyanite zones suggests that thermal climax occurred in all zones within a relatively short time span about 10 13 ma ago the metamorphic inversion in the sikkim himalaya must therefore have occurred after the metamorphic peak was attained in all grades lu hf garnet ages in the overthrust higher himalaya hh continue to be older but do not show a clear progression there is about 6 ma jump to 22 6 0 1 ma within the mct zone and even older ages of 26 28 ma were obtained for migmatites from the lower part of the hh the lu hf system in garnets from the hh records the time of melting at or near the thermal peak rather than dates initial garnet growth as in the lh the age pattern obtained in this study provides precise time constraints on the continental collision models which must take into account early and prolonged anatexis in the hh progressive delay in initial garnet growth in different barrow zones in the lh nearly contemporaneous metamorphic peak in all grades and post peak inversion of the coherent lh block during exhumation

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

dating the longevity of ductile shear zones insight from 40ar 39ar in situ analyses we attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally controlled in situ 40ar 39ar analyses of pre kinematic muscovite biotite and k feldspars of syn kinematic phengite and k feldspar and of post kinematic phengite within the same samples of sinistral shear zones from the western tauern window eastern alps additionally two samples were dated by the rb sr method microsampling relative sequences of mineral formation based on microstructural cross cutting relationships were confirmed by in situ 40ar 39ar analyses showing that syn kinematic minerals are in general younger than pre kinematic minerals and older or of equal age than the post kinematic minerals of the same sample from the rim to the core of the western tauern window syn kinematic phengite and k feldspar reveal a set of formation ages varying between 33 and 15 ma for the northernmost and peripheral shear zone ahorn shear zone between 24 and 12 ma for the intermediate shear zone network tuxer shear zones and between 20 and 7 ma for the southernmost and central shear zone greiner shear zone the age variation of syn kinematic phengite and k feldspar analyses is larger than the analytical error of each age obtained in addition isochron calculations of the syn kinematic minerals reveal atmospheric like 40ar 36ar intercepts therefore the obtained age values of the syn kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period the time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn kinematic phengite and k feldspar post kinematic phengite laths show the youngest formation ages and overlap with the youngest syn kinematic formation ages this relationship indicates that post kinematic growth occurred immediately after syn kinematic mineral formation at the end of ductile sinistral shear hence the termination of deformation is dated by the ages of these post kinematic phengite blasts pre kinematic minerals are characterized by break down and exsolution reactions and their age values are heterogeneous and often affected by the presence of extraneous ar these age values are usually older than but sometimes overlapping with ages of the syn kinematic minerals using the temporal constraints obtained by the ages of pre syn and post kinematic minerals we could assess partly overlapping time intervals of syn kinematic mineral formation of 19 myr 33 15 ma in the ahorn shear zone 13 myr 24 12 ma in the tuxer shear zones and 14 myr 20 7 ma in the greiner shear zone this indicates successive localization and propagation of ductile shear zones in the western tauern window from lower metamorphic sites at the rim towards higher metamorphic sites in the center

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

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

u pb geochronology of grossular andradite garnet abstract this study presents a new laser ablation inductively coupled plasma mass spectrometry la icpms based u pb geochronometric method for dating grossular andradite grandite garnet grandite is a primary skarn mineral therefore dating its growth directly dates hydrothermal activity as zircon u pb geochronometry provides a high resolution record of magmatic processes grandite u pb dating has the potential to provide a complementary record for hydrothermal systems this study characterizes four garnets of variable grossular andradite content and age as potential reference materials for la icpms u pb geochronology willsboro andradite 1020 ma adirondacks usa red and yellow mali grandite 200 ma southern mali and lake jaco grossular 35 ma coahuila mexico isotope dilution thermal ionization mass spectrometry id tims u pb analyses of willsboro andradite yield a mean 206pb 238u date of 1022 16 ma we use willsboro andradite as a primary reference material for la icpms u pb characterization of the two other garnets the non radiogenic pb pbc concentration in grandite is variable between specimens willsboro andradite and mali grandites contain almost purely radiogenic pb pb however lake jaco grossular has a higher pbc concentration comparatively and yields discordant u pb ratios for specimens with highly variable pb pbc linear regression is employed to derive a lower intercept age la icpms u pb dates for yellow mali grandite 202 2 ma 206pb 238u weighted mean and lake jaco grossular 35 2 ma lower intercept agree with independent id tims u pb 202 0 1 2 ma and u th he dates 35 5 ma derived for each specimen the precision of la icpms u pb dates varies between 1 10 2 for the analyzed garnets considering the short estimated lifespans of pluton related hydrothermal systems tens of thousands to a few million years neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels as a performance test for the method we present new u pb andradite data from a late miocene skarn system on serifos island greece this garnet yields a lower intercept age of 9 15 0 36 ma in agreement with biotite rb sr and zircon u pb dates from the causative pluton

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

the duration of prograde garnet crystallization in the uhp eclogites at lago di cignana italy abstract the distinct core to rim zonation of different rees in garnet in metamorphic rocks specifically sm relative to lu suggests that sm nd and lu hf isochron ages will record different times along a prograde garnet growth history therefore ree zonations in garnet must be measured in order to correctly interpret the isochron ages in terms of the garnet growth interval which could span several m y new ree profiles garnet crystal size distributions and garnet growth modeling combined with previously published sm nd and lu hf geochronology on a uhp eclogite of the zermatt saas fee zsf ophiolite lago di cignana italy demonstrate that prograde garnet growth of this sample occurred over a 30 to 40 m y interval relative to peak metamorphism at 38 to 40 ma garnet growth is estimated to have begun at 11 to 14 kbar pressure at 70 to 80 ma although such a protracted garnet growth interval is surprising this is supported by plate tectonic reconstructions which suggest that subduction of the liguro piemont ocean occurred through slow and oblique convergence these results demonstrate that ree zonations in garnet coupled to crystal size distributions provide a powerful means for understanding prograde metamorphic paths when combined with sm nd and lu hf geochronology

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

sm nd dating of spatially controlled domains of garnet single crystals a new method of high temperature thermochronology ganguly and tirone meteorit planet sci 36 2001 167 175 recently presented a method of determining the cooling rates of rocks from the difference between the core and bulk ages of a crystal as determined by a single decay system here we present the first application of the method using the core and bulk ages of garnet single crystals according to the sm nd decay system in two rock samples with contrasting cooling rates which can be constrained independently the samples belong to the metamorphic core complex valhalla british columbia and the mid crustal magmatic arc exposure of the salinian terrane california we have micro sampled the garnet crystals over specific radial dimensions and measured the nd isotopes of these small sample masses as ndo via solid source mass spectrometry to determine the sm nd age difference between the core and bulk crystals using a peak metamorphic p t condition of 8 1 kbar 820 30 c spear and parrish j petrol 37 1996 733 765 the core 67 3 2 3 ma and bulk 60 9 2 1 ma ages of the british columbian garnet sample yield a cooling rate of 2 13 c myr which is in very good agreement with the cooling rates that we have derived by modeling the retrograde fe mg zoning in the same garnet and assuming the same peak metamorphic p t condition considering earlier cooling rate data derived from closure temperature vs age relation of multiple geochronological systems spear and parrish j petrol 37 1996 733 765 a cooling rate of v15 20 c myr seems most reasonable for the valhalla complex diffusion kinetic analysis shows that the sm nd core age of the selected garnet crystal could not have been disturbed during cooling consequently the core age of the garnet crystal 67 3 2 3 ma corresponds to the peak metamorphic age of the valhalla complex the salinian sample on the other hand yields indistinguishable core 78 2 2 7 ma and bulk 77 9 2 9 ma ages as expected from its fast cooling history which can be constrained by the results of earlier studies the sm nd decay system in garnet has relatively high closure temperature usually s 650 c therefore the technique developed in this paper fills an important gap in thermochronology since the commonly used thermochronometers are applicable only at lower temperatures simultaneous modeling of the retrograde fe mg zoning in garnet spatially resolved sm nd ages of garnet single crystals and resetting of the bulk garnet sm nd age

prograde temperature time evolution in the barrovian type locality constrained by sm nd garnet ages from glen clova scotland abstract the timing of garnet growth during metamorphism associated with the grampian orogeny in the sillimanite zone of the barrovian type locality in glen clova scotland was determined by sm nd geochronology two high precision garnet whole rock ages were achieved by employing hf partial dissolution of garnet separates to optimize purity multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data an early stage at 472 9 2 9 ma during d 2 deformation under garnet zone conditions c 500 550 c and a later stage at 464 8 2 7 ma during or slightly after d 3 deformation mostly under sillimanite zone conditions peak temperature of c 660 c but possibly including some growth during kyanite zone conditions when combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of barrow9s zones were achieved roughly contemporaneously the difference between garnet zone and sillimanite zone peak temperature attainment is 2 8 3 7 ma the near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over thickened crust we suggest that local igneous intrusions with synmetamorphic ages provided that additional heat

late carboniferous high pressure metamorphism of the kassan metamorphic complex kyrgyz tianshan and assembly of the sw central asian orogenic belt abstract high pressure hp metamorphism of the kassan metamorphic complex kmc in the western kyrgyz tianshan has been related to either late ordovician or late carboniferous permian subduction processes we report sm nd ages for retrogressed eclogite samples and 40ar 39ar cooling ages for enclosing garnet muscovite samples from the kmc as new age constraints on hp metamorphism and rock exhumation these data will be used for an upgraded paleogeographic model for late paleozoic crustal consolidation in the southwestern central asian orogenic belt the retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace element patterns consistent with protoliths derived from garnet bearing mantle sources at rifting plate margins geothermobarometric data for a retrogressed eclogite sample indicate peak metamorphic conditions of 540 30 c at 1 6 0 1 gpa samples from different lithotectonic units of the kmc provide coherent sm nd garnet whole rock ages of 317 4 ma and 316 3 ma 2 the prograde major element zoning in the mm sized garnets in combination with the moderate peak metamorphic temperature support our interpretation of the sm nd garnet ages as unambiguous evidence for late carboniferous hp metamorphism the sm nd garnet growth ages overlap within error with the 40ar 39ar mica cooling ages of 314 2 ma and 313 2 ma 2 indicating rapid uplift of the subduction complex after peak metamorphism the ca 317 313 ma hp exhumation event of the kmc is contemporaneous with those of the atbashi and akeyazi ca 500 km east in nw china hp complexes and implies similar collision histories at the south tianshan suture to the east and west of the talas fergana fault tff the exhumation of the kmc and atbashi hp complexes overlaps with the initiation of the tff rolland et al 2013 suggesting incipient separation of the chatkal and atbashi complexes during rock exhumation and early plate collision

geochronology of shear zones a review abstract shear zones play a major role in the deformation of the crust at a variety of scales as expressions of strain localization during orogeny and rifting and also as reactivated structures they influence the geometry and evolution of orogenic belts and rifts crustal rheology magma ascent and emplacement and fluid flow consequently assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere the interpretation of thermochronologic data from shear zones is however not straightforward in the first place closure temperatures depend on a number of factors grain size cooling rate mineral composition and pressure among others on the other hand deformation related processes such as dynamic recrystallization neocrystallization and fluid circulation seem to be crucial for isotopic systems and thus the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks for this reason geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo or recrystallization fluid assisted deformation and inheritance of the protolith age s in order to robustly reconstruct p t e t paths of long term crustal scale shear zones structural microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems in addition geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications one of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion particularly the coupling with deformation processes based on natural theoretical and experimental data future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

rare earth diffusion kinetics in garnet experimental studies and applications abstract we determined the diffusion coefficient of sm in almandine garnet as function of temperature at 1 bar and fo 2 corresponding to that of wustite iron buffer and to a limited extent that of a few other selected rare earth elements in almandine and pyrope garnets both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields the experimental diffusion profiles were analyzed by secondary ion mass spectrometry and in addition by rutherford back scattering spectroscopy for two samples transmission electron microscopic study of an almandine crystal that was diffusion annealed did not reveal any near surface fast diffusion path using reasonable approximations and theoretical analysis of vacancy diffusion the experimental data were used to develop an expression of rare earth element ree diffusion coefficient in garnet as a function of temperature pressure fo 2 ionic radius and matrix composition calculation of the closure temperature for the sm nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently modeling of the ree evolution in melt and residual garnet suggests that for dry melting condition the ree pattern in the melt should commonly conform closely to that expected for equilibrium melting however for much lower solidus temperatures that would prevail in the presence of a h 2 o co 2 fluid the concentration of light ree in the melt could be significantly lower than that under equilibrium melting condition a reported core and rim differences in the ree content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the ree zoning was a consequence of entrapment in a magma derived from an external source for 32 000 yr before the eruption

short lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids collision tectonics and the associated transformation of continental crust to high pressure rocks eclogites are generally well understood processes but important contradictions remain between tectonothermal models and petrological isotopic data obtained from such rocks here we use 40ar 39ar data coupled with a thermal model to constrain the time integrated duration of an orogenic cycle the burial and exhumation of a particular segment of the crust to be less than 13 myr we also determine the total duration of associated metamorphic events to be 20 kyr and of individual heat pulses experienced by the rocks to be as short as 10 years such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events earthquakes such events triggered transient heat advection by hot fluid along deformation shear zones which cut relatively cool and dry subducted crust in contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust our non steady state cold crust model satisfactorily explains several otherwise contradictory geological observations the duration of the geological processes that shape the earth generates vigorous debate mountain building where crust is consumed continuously by burial and recycled as buried remnants rise to the surface is one process where new dating methods can resolve the issue high precision dating of rock remnants left by continental collision in the caledonides of southern norway shows that the entire cycle can happen quickly in about 13 million years in addition the rapid transport of hot fluid zones through the cold crust may explain many puzzling geological observations

single shot laser ablation split stream ss lass petrochronology deciphers multiple short duration metamorphic events single shot laser ablation split stream ss lass petrochronology offers spatial resolution of 1 m per surface analysis this enables the technique to interrogate metamorphic zircon overgrowths that are very thin or that preserve a composite age structure to demonstrate the advantages of ss lass the technique was applied to metamorphic zircon overgrowths in five rocks from the cordillera de la costa venezuela in these rocks ss lass was able to decipher discrete short duration 106 yr zircon growth events at c 33 0 c 28 3 c 23 0 and c 18 2 ma comparison with existing geo thermochronology suggests that the ss lass dates represent hydro thermal events that mark distinct episodes of tectonism affecting the northern margin of south america with an external error of 5 incorporated into isotope ratios as required to achieve satisfactory analytical precision ss lass dates for reference materials were accurate to within 1 5 of published values the ss lass technique is thus in favorable cases capable of providing 106 yr resolution for cenozoic rocks methods in geospeedometry which employ forward models to reproduce observed diffusion length scales have obtained anomalously short time scales of 104 106 yr for metamorphism the high spatial and temporal resolution of ss lass offers tremendous promise for investigating the veracity of these claims