retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

trace element zoning in garnet as a monitor of crustal melting x ray composition maps of sc y p and cr in garnet have been measured using the electron microprobe zoning is discontinuous and is correlated with fluid absent crustal melting reactions the magnitude of the discontinuity is a function of the amount of muscovite that melts and the partition coefficients of solids versus melt trace element zoning is apparently not modified by diffusive processes so the zoning can be used to monitor the extent of melting and to examine kinetics of melting and the participation of accessory phases in crustal anatexis

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

major and trace element zoning in garnets from calcareous pelites in the nw shelburne falls quadrangle massachusetts garnet growth histories in retrograded rocks

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

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

the significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization the spatial disposition compositional zoning profiles and size distributions of garnet crystals in 11 specimens of pelitic schist from the picuris range of new mexico usa demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks an ordered disposition of garnet centers and a significant correlation between crystal radius and near neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre existing crystals compositional zoning profiles require diffusionally controlled growth the rate of which increases exponentially as temperature increases with time an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process but decline in the later stages because of suppression effects in the vicinity of earlier formed nuclei simulations of porphyroblast crystallization based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics quantitatively replicate textural relations in the rocks the simulations employ only two variable parameters which are evaluated by fitting of crystal size distributions both have physical significance the first is an acceleration factor for nucleation with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock the second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it crystal size distributions for the picuris range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals the same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet phlogopite sphene and pyroxene in rocks from both regional and contact metamorphic occurrences these commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

oscillatory and sector zoned garnets record cyclic rapid thrusting in central nepal 1 garnets with oscillatory zoning repeated compositional reversals and sector zoning crystallographically controlled compositional differences were discovered in rocks from near the main central thrust in the langtang region central nepal this zoning results from crystal growth kinetic effects related to extraordinarily rapid possibly cyclic thrusting if cyclic thrusting occurred then timescales of thrust rate inhomogeneities are 25 kyr implying that satellite determination of modern fault movement rates may not accurately reflect slip rates or strain partitioning in orogens on timescales exceeding thousands of years sector zoning in ca but not y mn or mg implies maximum intracrystalline diffusion rates for ca of 10 21 cm2 s at 550 c but faster rates for other elements

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

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

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

geochronology in migmatites a sm nd u pb and rb sr study from the proterozoic damara belt namibia implications for polyphase development of migmatites in high grade terranes sm nd garnet u pb monazite and rb sr biotite ages from a composite migmatite sample damara orogen namibia constrain the time of high grade regional metamorphism and the duration of regional metamorphic events sm nd garnet whole rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534i5 528i11 and 539i8 ma these results provide substantial evidence for pre 500 ma pan african regional metamorphism and melting for this segment of the orogen other parts of the migmatite yield younger sm nd ages of 488i9 ma for melanosome and 496i10 492i5 and 511i16 ma for the corresponding leucosomes garnet from one xenolith from the leucosomes yields an age of 497i2 ma major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components but the flat shape of the elemental patterns suggests late stage retrograde equilibration rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments monazite from the leucosomes is reversely discordant and records 207 pb 235 u ages between 536 and 529 ma indicating that this monazite represents incorporated residual material from the first melting event monazite from the mesosome mes 2 and the melanosome mel 3 gives 207 pb 235 u ages of 523 and 526 ma and 529 and 531 ma respectively which probably indicates another thermal event previously published 207 pb 235 u monazite data give ages between 525 and 521 ma for composite migmatites and 521 and 518 ma for monazite from neosomes monazite from granitic to granodioritic veins indicates another thermal event at 507 505 ma these ages are also recorded in 207 pb 235 u monazite data of 508 ma from the metasediment met 1 from the migmatite and also in the sm nd garnet ages obtained in this study taken together these ages indicate that high grade metamorphism started at c 535 ma or earlier and was followed by thermal events at c 520 ma and c 505 ma the latter event is probably connected with the intrusion of a large igneous body donkerhoek granite for which so far only imprecise rb sr whole rock data of 520i15 ma are available rb sr biotite ages from the different layers of the migmatite are 488 469 and 473 ma these different ages indicate late stage disturbance of the rb sr isotopic system on the sub sample scale nevertheless these ages are close to the youngest sm nd garnet ages indicating rapid cooling rates between 13 and 20u cm a x1 and fast uplift of this segment of the crust similar sm nd garnet and u pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730i30uc the preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water undersaturated peraluminous melts can be sluggish this study shows that geochronological data from migmatites can record polymetamorphic episodes in high grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events

five generations of monazite in langtang gneisses implications for chronology of the himalayan metamorphic core monazite grains from greater himalayan sequence gneisses langtang valley nepal were chemically mapped and then dated in situ via th pb ion microprobe analysis correlation of ages and chemistry reveals at least five different generations of monazite ranging from c 9 to 300 ma petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks yielding an age for the melting of greater himalayan rocks within the main central thrust sheet c 16 ma and for the timing of thrust sheet emplacement that are younger than commonly viewed chemical characterization of monazite is vital prior to chronological microanalysis and many ages previously reported for monazite from the greater himalayan sequence are interpretationally ambiguous

ultrahigh pressure metabasaltic garnets as probes into deep subduction zone chemical cycling 1 we demonstrate an approach to examining the metamorphic history of subducting oceanic crust that can complement records of subduction zone chemical cycling derived from studies of igneous rocks produced at volcanic arcs by merging methods utilizing garnet zoning to establish prograde reaction histories with in situ high resolution trace element geochemistry and application to coesite bearing mafic eclogites representing subduction to depths beneath arcs we are able to directly identify geochemical manifestations of reactions contributing to element mobility in the subducting slab that are only inferred in studies of volcanic arcs or theoretical metamorphic models specifically we identify a prograde metamorphic reaction based solely on the zoning of geochemistry and mineral inclusions within garnet and infer that these features are a record of the breakdown of coexisting clinozoisite titanite and probable liberation of trace element laden fluid from the rock during prograde metamorphism we are then able to assign a specific depth interval for the reaction through calculation of the p t dependence of the reaction for these eclogites and comparison with a published p t trajectory because of the robust preservation of records of petrologic and geochemical processes by garnet this methodology is particularly suited for study of ultrahigh pressure uhp eclogites in which severe retrograde alteration generally related to exhumation commonly obscures prograde history

improved dissolution and chemical separation methods for lu hf garnet chronometry 1 garnet based lu hf geochronology using conventional hf hno3 dissolution methods may be compromised by full or partial digestion of hf rich zircon inclusions this study integrates two complimentary methods to substantially reduce zircon digestion while assuring complete digestion of garnet and sample spike equilibration handpicked garnet fractions are heat treated to 1000 c in an evacuated silica glass ampoule to anneal zircon inclusions and then dissolved with 12 m hcl at 210 c and cold 28 m hf analyses of heat treated garnet and their high temperature breakdown products orthopyroxene spinel quartz from gore mountain new york demonstrate the method is capable of complete dissolution of garnet and routinely achieving sample spike equilibration independent analyses of annealed and unannealed zircons dissolved by hcl cold hf demonstrate the benefit of heat treatment prior to dissolution analyses of zircon rich garnets from peletic paragneisses of labrador canada show the potential of this method by returning lu hf ratios commensurate with the degree of heat treatment finally a complementary chemical separation method for lu and hf is presented that supports analysis of garnet by mc icp ms

trace element zoning in a metamorphic garnet trace element zoning has been measured in an amphibolite garnet from the tauern window austria by using an ion microprobe humps in the zoning profiles of na sc v y and the heavy rare earth elements mark a period of open system behavior these humps correspond to a part of the major element zoning profile that is interpreted as a p t reversal the source of the mass excesses of these elements remains ambiguous they were derived either on a thin section scale by the breakdown of trace element enriched refractory minerals or externally from unusual trace element enriched fluids p t paths determined from garnet zoning may require modification if open system behavior is important during garnet growth

trace element zoning as a record of chemical disequilibrium during garnet growth trace element concentrations in pelitic garnets from the picuris range of new mexico display precipitous changes coincident with abrupt variations in ca concentration these patterns probably arise from the transitory participation of different trace element enriched phases in the garnet forming reaction changes in the reactant and product assemblages occur at different times during the reaction history for crystals of different size so they cannot be the result of any event affecting the entire rock such as a change in pressure temperature or fluid composition instead they reflect kinetic factors that cause ca y yb p ti sc zr hf sr na and li to fail to achieve chemical equilibrium during garnet growth caution is needed to avoid misinterpreting excursions in the concentration of these elements as event markers recording simultaneous rockwide changes in intensive parameters when in fact they may record transient disequilibrium states that are local in scope and not contemporaneous

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

u pb garnet chronometry in high grade rocks case studies from the central damara orogen namibia and implications for the interpretation of sm nd garnet ages and the role of high u th inclusions garnets from different migmatites and granites from the damara orogen namibia were dated with the u pb technique after bulk dissolution of the material measured 206pb 204pb ratios are highly variable and range from ca 21 to 613 variations in isotope 208pb 204pb 206pb 204pb and trace element th u u nd sm nd ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions due to their very low 206pb 204pb ratios garnets from pelitic migmatites from the khan area yield pb pb ages with large errors precluding a detailed evaluation however the 207pb 206pb ages ca 550 500 ma appear to be similar to or older than u pb monazite ages 530 1 517 1 ma and sm nd garnet ages 523 4 512 3 ma from the same sample it is reasonable to assume that the pb pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the u pb system in garnet relative to the u pb system in monazite and the sm nd system in garnet for igneous migmatites from oetmoed pb pb garnet ages 483 15 492 16 ma and one sm nd garnet whole rock age 487 8 ma are similar whereas the monazite from the same sample is ca 30 40 ma older 528 1 ma these monazite ages are however similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area although there is a notable difference in closure temperature for the u pb and sm nd system in garnet the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment restitic garnet from an unmigmatized granite from omaruru yields similar u pb 493 30 506 30 ma and sm nd 493 6 488 7 ma garnet ages whereas the monazite from this rock is ca 15 25 ma older 516 1 514 1 ma whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite the garnet ages may indicate the time of melt extraction for igneous garnets from granites at oetmoed the similarity between pb pb 483 34 474 17 ma and sm nd 492 5 484 13 ma garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust differences between garnet and monazite u pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re equilibration in the temperature range between 750 and 900 c e g braun et al 1998 for garnet analyses that have low 206pb 204pb ratios the influence of high inclusions is small however the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers for garnet with higher 206pb 204pb ratios the overall similarity between the pb pb and sm nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic pb signature upon the garnet the results presented here show that garnet with low 238u 204pb ratios together with sm nd garnet data and u pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high grade terranes

space time distribution of afterslip following the 2009 l aquila earthquake 1 the inversion of multitemporal dinsar and gps measurements unravels the coseismic and postseismic afterslip slip distributions associated with the 2009 mw 6 3 l aquila earthquake and provides insights into the rheological properties and long term behavior of the responsible structure the paganica fault well resolved patches of high postseismic slip 10 20 cm appear to surround the main coseismic patch maximum slip 1 m through the entire seismogenic layer above the hypocenter without any obvious depth dependent control time series of postseismic displacement are well reproduced by an exponential function with best fit decay constants in the range of 20 40 days a sudden discontinuity in the evolution of released postseismic moment at 130 days after the main shock does not correlate with independent seismological and geodetic data and is attributed to residual noise in the insar time series the data are unable to resolve migration of afterslip along the fault probably because of the time interval six days between the main shock and the first radar acquisition surface fractures observed along the paganica fault follow the steepest gradients of postseismic line of sight satellite displacements and are consistent with a sudden and delayed failure of the shallow layer in response to upward tapering of slip the occurrence of afterslip at various levels through the entire seismogenic layer argues against exclusive depth dependent variations of frictional properties on the fault supporting the hypothesis of significant horizontal frictional heterogeneities and or geometrical complexities we support the hypothesis that such heterogeneities and complexities may be at the origin of the long term variable behavior suggested by the paleoseismological studies rupture of fault patches with dimensions similar to that activated in 2009 appears to have a 500 year recurrence time interval documented by paleoseismic and historical studies in addition to that paleoseismological evidence of large 0 5 m coseismic offsets seems to require seismic events recurring every 1000 2000 years characterized by 1 multisegment linkage 2 surface ruptures larger than in 2009 and 3 complete failure of the 2009 coseismic and postseismic patches

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

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

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

miocene faulting at plate tectonic velocity in the himalaya of central nepal abstract combination of geochemical zoning in metamorphic garnet and monazite plus in situ th pb isotopic dating of monazite yields p t conditions ages and convergence rates for the main central thrust mct and affiliated faults in central nepal inferred rates were 1 5 0 9 cm yr langtang thrust 19 ma 2 2 0 7 cm yr main central thrust 15 ma and 7 3 cm yr ramgarh thrust 10 ma the lower values are similar to modern convergence rates across the himalaya but the ramgarh thrust may have briefly absorbed all indo asian convergence at 10 ma when foreland and marine sedimentation rates markedly increased and at least one major strike slip fault in tibet experienced a hiatus in movement variable rates of convergence across the himalaya on myr timescales imply myr variations in strain rates throughout all components of the indo asian orogen

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

exhumation of ultrahigh pressure rocks beneath the hornelen segment of the nordfjord sogn detachment zone western norway the nordfjord sogn detachment zone of western norway represents an archetype for crustal scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure uhp terranes in this paper we investigate the role of normal sense shear zones with respect to uhp exhumation using structural geology thermobarometry and geochronology of the hornelen segment of the nordfjord sogn detachment zone the hornelen segment of the zone is a 2 6 km thick top w shear zone primarily developed within amphibolite grade allochthonous rocks that juxtaposes the uhp rocks of the western gneiss complex in its footwall with lower grade allochthons and carboniferous devonian basins in its hanging wall new thermobarometry and sm nd garnet geochronology show that these top w fabrics were initiated at lower crustal depths of 30 40 km between 410 ma and 400 ma structural geology and quartz petrofabrics indicate that top w shear was initially relatively evenly distributed across the shear zone and then overprinted by discrete ductile brittle detachment faults at slower strain rates during progressive deformation and exhumation these results require a three stage model for uhp exhumation in which normal sense shear zones exhumed uhp rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile brittle structures

biological pump processes in the cryopelagic and hemipelagic arctic ocean canada basin and chukchi rise abstract the object of this study was to clarify the characteristics of the biological pump system operating in permanently or seasonally ice covered ocean cryopelagic conditions by examining the export of particulate organic carbon poc and other components of oceanic particles in the canada basin at 120 200 and 3067 m and in hemipelagic chukchi rise waters at 120 m the first time series sediment trap ts trap b96 200m was tethered to an ice ocean environmental buoy ioeb and deployed at 79 1 n 132 2 e in march 1996 it was recovered in july 1997 at 76 7 n 131 8 e having drifted exclusively in the cryopelagic environment the second ts trap s97 120m also tethered to an ioeb was launched at 75 2 n 142 5 e in october 1997 and recovered at 80 0 n 155 9 e having first traversed the cryopelagic canada basin to the west and then the hemipelagic chukchi rise the third ts trap cd04 3067m was deployed in august 2004 beneath the cryopelagic drift route of s97 120m at a water depth of 3067 m on a mooring in the interior of the 3824 m deep canada abyssal plain all three ts traps intercepted marine particles in 21 time series sets of 17 day intervals for a total of 357 days each the tmf annual dried mass flux intercepted by ts trap b96 200m was only 0 4 g m 2 yr 1 and the f c org mole annual export flux of poc was 7 0 mmol c m 2 yr 1 the export of f si bio mole si in diatom frustules and lithogenic particles in mole al were both extremely small ballast particle flux was three orders of magnitude smaller than in global epipelagic areas where the biological pump represents an important vehicle for carbon export to depth the f c org and ballast particles were rather small in the second drifting ts trap s97 120m we conclude that the biological pump is currently ineffective in the cryopelagic canada basin so that instead of removal to deep waters carbon from primary production 2 4 mmol c m 2 yr 1 is remineralized or converted to dissolved organic carbon doc within the surface layer at 4 3 gm 2 yr 1 the tmf sampled by moored ts trap cd04 3067m was an order of magnitude larger than the mass flux to the traps drifting at 120 and 200 m the lithogenic particle flux was about 18 times larger than that in s97 120m f c org sampled by the moored trap was 14 2 mmol c m 2 yr 1 1 4 2 times larger than the poc fluxes encountered by the 120 m and 200 m drifting ts traps the 14 c values in the poc collected at 120 m indicated that the poc was mostly autochthonous in contrast the poc exported to 3067 m had an apparent 14 c age of 1900 years indicating it was predominantly derived from aged allochthonous carbon the particle composition was largely invariant throughout the annual cycle suggesting that the upper ocean ecosystem plays a minimal role in transporting poc and other marine particles instead the majority of poc and other particulate matter is transported laterally to deep ocean layers from the shelf slope reservoir on the chukchi rise we found a mosaic of environmental regimes ranging from the intermittent development of a polynya in some locations where an efficient biological pump operates to an area characterized by re suspended allochthonous poc with an exceptionally high f al such spatial heterogeneity and associated sensitivity to the various physical and biogeochemical forcing factors are distinctive characteristics of hemipelagic arctic environments

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

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

dehydration melting and fluid recycling during metamorphism rangeley formation new hampshire usa reflect the dewatering of structurally lower levels after nappe emmuscovite and biotite dehydration melting reactions near the peak placement if so then nappe emplacement occurred at t 475 c of metamorphism played a significant role in the reaction and fluid history of the rangeley formation in southwestern new hampshire usa evidence for in situ melting includes 1 the consistency among theoretical phase equilibria observed reaction textures and the inferred p t conditions 2 disseminated centimeter scale

trace element distributions in silicates during prograde metamorphic reactions implications for monazite formation to assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism ree were measured across coexisting zoned silicates in garnet through kyanite grade pelitic schists from the great smoky mountains western blue ridge terrane southern appalachians to establish ree concentrations and distributions before and after the monazite in isograd and to identify the role major silicates play in the formation of monazite results indicate significant scavenging of light rare earth elements lree from silicates during the monazite in isograd reaction however the absolute concentration of lree hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated lree possibly adsorbed onto grain boundaries even though direct evidence for allanite is lacking in a majority of the samples laser ablation icp ms analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates allanite breakdown initially formed monazite and monazite production drew lree liberated from allanite major silicates and possibly from crystal boundaries in many rocks the reaction was further promoted by the staurolite in reaction allowing for rapid isogradic monazite growth

lu hf geochronology and trace element distribution in garnet implications for uplift and exhumation of ultra high pressure granulites in the sudetes sw poland combining lu hf garnet geochronology with in situ trace element analyses in garnet allowed its to gain new insight into the metamorphic evolution of uhp uht rocks in the stary gieraltow region in the polish sudetes prograde garnet growth recorded by rayleigh type heavy ree hree zoning in the felsic granulites indicates that the obtained 386 6 4 9 ma lu hf age represents the time of garnet crystallization on a prograde uhp metamorphic path the surrounding rocks were metamorphosed at the same time as indicated by 381 2 6 7 ma sm nd garnet age obtained for the mid crustal metapelites the second metamorphic episode which affected most of the lower crust in the orlica snieznik massif osm occurred at ca 340 ma as determined by u pb zircon and sm nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event which took place on a retrograde path trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and hree in garnets from mafic and felsic protoliths over the course of the metamorphic evolution this had strong impact on the isotopic dating results and led to decoupling of the sm nd and lu hf clocks which recorded timing of the two different metamorphic episodes separated by as much as 40 ma moreover the preservation of the hree growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 degrees c implies that the lu hf system under relatively dry conditions does not undergo significant diffusional re equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth under hydrous conditions at least some resetting will take place as documented by the partially relaxed hree zonation profile in the amphibolitised mafic granulite which yielded a 10 ma younger age the hree distribution study appeared to be a particularly valuable and essential tool which allowed us to distinguish garnet growth from post growth complexities and hence provide improved age interpretation medium ree on the other hand did not show any obvious correlation with the isotopic signature of garnet two distinct metamorphic episodes recorded in the stary gieraltow region show that buoyancy driven uplift of uhp rocks can be arrested at the base of a continental crust if not supported by any additional force in our case study the uhp rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime the multistage discontinuous uplift revealed by the uhp rocks of the osm provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast track exhumation histories recognized in uhp terranes elsewhere c 2006 elsevier b v all rights reserved