strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

magmatic fractionation by compositional convection in a sheet like magma body constraints from the nosappumisaki intrusion northern japan the mechanism of fractionation in a sheet like magma body was investigated based on observations from the nosappumisaki intrusion northern japan this is one of a number of late cretaceous shoshonite sills in the nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing 50 vol cumulus crystals and a overlying middle zone free of such cumulus crystals these zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones it is inferred that massive crystal settling or flotation from a magma initially laden with 20 vol of crystals primocrysts in a sheet like magma body formed a cumulus pile and an overlying crystal free melt layer after the formation of the compound structure with partial freezing of the upper and lower marginal zones the internal part of the intrusion slowly solidified during this period the melt in the central part of the intrusive sheet underwent fractional crystallization this is shown by patterns of vertical variation in the composition of the interstitial melt the overall patterns are inverted mirrorimage s shaped and s shaped for incompatible and compatible elements respectively the incompatible element abundances show maxima near the bottom of the middle zone which corresponds to the final solidification horizon they show minima in the cumulate zone which suggests discharge of an evolved melt from the crystal pile fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer the melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile this conclusion is based on the following observations 1 there is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone 2 leucocratic pipe like structures which represent relict pathways for the evolved melt are developed in the cumulate zone these structures increase in diameter decrease in number density and increase in abundance of incompatible elements with height 3 cumulus augite in the cumulate zone has ca rich and al poor dissolved rims the extent of dissolution increases with stratigraphic height suggesting downwelling of an h2o rich melt from the overlying melt layer 4 there is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone 5 the initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical a mass balance model for compositional convection was constructed to quantify the melt transportation processes model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions the optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt the initially formed compound structure is a highly preferential environment for compositional convection

fracturing of garnet crystals in anisotropic metamorphic rocks during uplift discussion ji rt al 1997 proposed a shear lag model for the development of the ordered fractures in garnet crystals from the morin shear zone in the grenville province quebec such fractures are also very commonly observed but poorly understood elsewhere such as in the east athabasca mylonite triangle of the snowbird tectonic zone hanmer et al 1994 snoeyenbos et ai 1995 jiang 1996 hanmer and williams unpublished data major points of their argument may be summarized as follows

crystal size distribution and composition of garnets in eclogites from the dabie orogen central china crystal size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation growth environment and the metamorphic evolution of their host rocks episodic nucleation and growth histories for garnets in eclogites from the dabie orogen are revealed by variations in crystal size geochemical zoning and mineralogy of inclusions in garnets the studied garnets show pseudo lognormal crystal size distributions csds prograde chemical zoning patterns indexed by mineral inclusions and are mainly distributed at random constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns the csd shapes were evaluated in terms of a size dependent proportionate growth model and a thermally accelerated diffusion controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet initial increasing nucleation rate followed by a subsequent medial stage of nearly constant growth and finally declining nucleation rate is inferred from the csd data this study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through csd shapes of minerals in metamorphic rocks

samples from the crystallising boundary layer of a zoned magma chamber the spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr b p tephra deposit of laacher see volcano the melt showed vertical chemical zonation immediately prior to eruption and throughout most of the magma volume only a small fraction of crystals were present the eruption also ejected crystal rich nodules ranging from mafic to felsic in composition which are samples of the materials crystallising at the boundaries of the chamber new data on nodule petrography and chemical compositions of whole rocks minerals and interstitial glasses are presented volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol i e the crystals interlock one exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt felsic nodules show much less pronounced differences with their interstitial melt felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor modal mineralogy glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber a broader range of mineral compositions are often found mineral assemblages were especially diverse in the upper part of the chamber it is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and or time

sigmoidal inclusion trails punctuated fabric development and interactions between metamorphism and deformation porphyroblast inclusion fabrics are consistent in style and geometry across three proterozoic metamorphic field gradients comprising two pluton related gradients in central arizona and one regional gradient in northern new mexico garnet crystals contain curved sigmoidal inclusion trails in low grade chlorite schists these trails can be correlated directly with matrix crenulations of an older schistosity s1 the garnet crystals preferentially grew in crenulation hinges but some late crenulations nucleated on existing garnet porphyroblasts at higher grade biotite staurolite and andalusite porphyroblasts occur in a homogeneous s2 foliation primarily defined by matrix biotite and ilmenite biotite porphyroblasts have straight to sigmoidal inclusion trails that also represent the weakly folded s1 schistosity staurolite and andalusite contain distinctive inclusion rich and inclusion poor domains that represent a relict s2 differentiated crenulation cleavage together the inclusion relationships document the progressive development of the s2 fabric through six stages garnet and biotite porphyroblasts contain stage 2 or 3 crenulations staurolite and andalusite generally contain stage 4 crenulations and the matrix typically contains a homogeneous stage 6 cleavage the similarity of inclusion relationships across spatially and temporally distinct metamorphic field gradients of widely differing scales suggests a fundamental link between metamorphism and deformation three end member relationships may be involved 1 tectonic linkages where similar p t time histories and similar bulk compositions combine to produce similar metamorphic and structural signatures 2 deformation controlled linkages where certain microstructures particularly crenulation hinges are favourable environments for the nucleation and or growth of porphyroblasts and 3 reaction controlled linkages where metamorphic reactions particularly dehydration reactions are associated with an increase in the rate of fabric development a general model is proposed in which 1 garnet and biotite porphyroblasts preferentially grow in stage 2 or 3 crenulation hinges and 2 chlorite consuming metamorphic reactions lead to pulses in the rate of fabric evolution the data suggest that fabric development and porphyroblast growth may have been quite rapid of the order of several hundreds of thousands of years in these rocks these microstructures and processes may be characteristic of low pressure first cycle metamorphic belts

shape size spatial distribution and composition of garnet crystals in highly deformed gneiss of the otter lake area quebec and a model for garnet crystallization garnet biotite sillimanite gneiss 700 c 7 kbar of the otter lake area in the western grenville province canadian shield occurs as granitic gneiss group 4 that forms a large part of the otter complex and as widely distributed more heterogenous metasedimentary gneiss group 2 in one sample of group 4 gneiss qtz25 pl34 kfs28 bt10 grt2 5 sil1 the true diameter determined by serial grinding of subhedral garnet crystals ranges from 0 2 to 3 0 mm with a mode at 1 0 mm nearest neighbour measurements in this sample and in surfaces of nine additional samples all 5 garnet confirm that garnet crystals are distributed mainly at random slight clustering was detected in two samples in one sample of group 4 gneiss microprobe analyses on sections through crystal centres obtained by serial slicing reveal that small crystals and margins to large crystals contain more fe and mn and less mg than the broad central regions of large crystals based on these and previous results together with theoretical considerations a crystallization model is proposed in which i garnet was produced by the continuous reaction ms bt qtz grt kfs h2o ii nucleation occurred by the random selection of randomly distributed ms bt qtz triple junctions iii the rate of linear growth remained constant and iv as temperature increased the rate of nucleation first increased slowly then remained nearly constant and finally declined within population compositional homogenization was followed on cooling by local fe mg mn exchange with biotite

crystal size distribution of periclase in contact metamorphic dolomite marbles from the southern adamello massif italy crystal size distributions csd of periclase in contact metamorphic dolomite marbles are presented for two profiles near the cima uzza summit in the southern adamello massif italy the database was combined with geochemical and petrological information to deduce the controls on the periclase forming reaction the contact metamorphic dolomite marbles are exposed at the contact of mafic intrusive rocks and are partially surrounded by them brucite is retrograde and pseudomorphs spherical periclase crystals prograde periclase growth is the consequence of limited infiltration of waterrich fluid at t near 6058c stable isotope data show depletion in 13 c and 18 o over a narrow region 40 cm near the magmatic contact whereas the periclase forming reaction front extends up to 4 m from the contact csd analyses along the two profiles show that the median grain size of the periclase crystals does not change but that there is a progressively greater distribution of grain sizes including a greater proportion of larger grains with increasing distance from the contact a qualitative model based on the textural and geochemical data attributes these variations in grain size to changing reaction affinities along a kinetically dispersed infiltration front this study highlights the need to invoke disequilibrium processes for metamorphic mineral growth and expands the use of csds to systems of mineral formation driven by fluid infiltration

boundary layer fractionation constrained by differential information from the kutsugata lava flow rishiri volcano japan detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber kutsugata lava a suitable target of such study is a quaternary alkali basalt 51 3 53 2 wt in sio2 from rishiri volcano northern japan despite the narrow range in the whole rock compositional variation chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition in the north lava 51 3 51 9 wt in sio2 the less differentiated portion of the kutsugata lava most crystals which include low ni olivine and plagioclase were derived from the mushy boundary layer the main part of the magma body was principally aphyric 30 vol by such mechanisms as compaction and compositional convection this fractionated melt was mixed with the overlying main magma causing differentiation of the kutsugata magma the average temperature of the extracted melt is 1010 c significantly lower than 1100 c estimated for the main magma a quantitative model of magmatic differentiation which includes thermal and compositional evolution of a mushy boundary layer can successfully reproduce the observed compositional trends of the north lava

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization crystal size distributions csds measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates growth times and the degree of overstepping t of reactions during metamorphism csds are described through use of a population density function n dn dl where n is the cumulative number of crystals per unit volume and l is a linear crystal size plots of ln n vs l for olivine pyroxene and magnetite in high temperature 1000 c basalt hornfelses from the isle of skye define linear arrays indicating continuous nucleation and growth of crystals during metamorphism using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at t 10 c and nucleation rates between 10 4 and 10 1 cm3 s garnet and magnetite in regionally metamorphosed pelitic schists from south central maine have csds which are bell shaped we interpret this form to be the result of two processes 1 initial continuous nucleation and growth of crystals and 2 later loss of small crystals due to annealing the large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth the extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known finally the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs contact metamorphosed rocks because contact metamorphism involves high temperatures for short durations resulting csds are linear and unaffected by annealing similar to those produced by crystallization from a melt because regional metamorphism involves prolonged cooling from high temperatures primary linear csds are later modified by annealing to bell shapes

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

applications of diffusion data to high temperature earth systems diffusion in general is such a widespread phenomenon in nature that a basic knowledge can serve very effectively to address well known applications not only to thermochronology and petrology but also to environmental geology e g paleoclimate sedimentology and applied geosciences e g assessing contamination of gas stations indeed the list of potential applications of diffusion phenomena is so long that an exhaustive treatment would quickly exceed the scope of this review the reader is therefore referred to reviews covering various aspects of diffusion in the earth sciences including diffusion in melts watson 1994 chakraborty 1995 diffusion processes during diagenesis berner 1980 krom and berner 1980 boudreau 2000 and diffusion phenomena in general e g lasaga 1998 ganguly 2002 watson and baxter 2007 zhang 2008 all of these treatments are based on the same general concepts of diffusion which do not need restating here in great detail applications of diffusion profiles in igneous systems have been thoroughly reviewed a recent reviews in mineralogy and geochemistry volume rimg 69 putirka and tepley 2008 and there is also an excellent volume on low temperature thermochronology rimg 58 reiners and ehlers 2005 describing the impact and application of diffusion theory for age and thermal history determination with these precedents in mind we focus the present review on the application of diffusion in high temperature regimes i e systems at t 500 c both igneous and metamorphic systems are addressed but because of the recent precedents noted above we emphasize recent developments and challenges in metamorphic studies in a broad sense diffusion describes the statistical non directional movement of molecules or heat through a medium of interest as independent theories of diffusion were developed in the early 19th century and compared with the principles of statistical variance it was

convective exchange between pore fluid and an overlying reservoir of denser fluid a post cumulus process in layered intrusions abstract we describe experiments on buoyancy driven exchange between fluid in a porous bed of glass balls and an overlying reservoir of denser fluid fingers of the dense fluid penetrated downwards into the porous medium when the two fluids had equal viscosities the rate of convective exchange was proportional to permeability and the density difference between the fluids and was inversely proportional to viscosity when the fluids had different viscosities the rate further depended on the viscosity ratio raised to the power 0 4 if a layer of dense new melt is emplaced above a porous cumulus pile forming the floor of a magma chamber an analogous process may occur as the new melt can percolate down into the pore space and expel the less dense intercumulus liquid convective exchange may proceed at rates of metres per year to tens of metres per year in basic to ultrabasic magmatic systems this conclusion strongly depends on the porosity of the cumulus pile and is less sensitive to the magmatic viscosities chemical and textural effects will result from this process such as re equilibration and resorption of cumulus crystals when intercumulus melt exchange is incomplete lateral geochemical gradients may be preserved in cumulate rocks

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

metamorphic fractional crystallization and internal metasomatism by diffusional homogenization of zoned garnets metamorphic fractional crystallization involves fractionation of material into the interior of a crystallizing mineral such as garnet with consequent change in the effective bulk composition in assemblages with thermodynamic variance of 3 or greater the mineral assemblage compositions of coexisting phases and the zoning profile preserved in the garnet are a function of the amount of material that is fractionated homogenization of zoned garnet by intracrystalline diffusion at elevated temperatures operates on a time scale of millions to tens of millions of years depending on the temperature time path and radius of the garnet one effect of homogenization is to drive chemical reactions by a metasomatic mechanism internal metasomatism even in the absence of changes in pressure or temperature this process may affect the appearance or disappearance of minerals and may in some rocks be responsible for the first appearance of kyanite or the disappearance of staurolite in barrovian terranes

pressure temperature trajectory of the sanbagawa metamorphism deduced from garnet zoning abstract most garnets in a small volume of the pelitic sanbagawa schists nucleated simultaneously grew and then ceased with their growth maintaining near equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism chemical trends of zoned garnets systematically shift from the pyrope poor to pyrope rich portions in a ternary mn fe mg system in order of ascending metamorphic grade a ternary mn fe mg model system suggests that the higher grade garnets started to grow at lower pressures and higher temperatures than the lower grade ones during prograde metamorphism it follows that high grade metamorphism took place at lower pressures in the garnet to oligoclase biotite zones of the sanbagawa metamorphic terrain in central shikoku

phosphate lu hf geochronology abstract igneous metamorphic and biogenic apatite contains lu and hf in proportions that make this mineral suitable for dating by the 176lu 176hf method we present a new method for separation of lu and hf from apatite that involves a single extraction column step for hf and a second exchange column step for lu this procedure allows rapid sample processing prior to analysis by mc icp ms results from the igneous gardiner skaergaard and khibina intrusions indicate that the closure temperature for the lu hf isotopic system in apatite is relatively high the lu hf isochron age of 60 ma for the skaergaard intrusion east greenland is older than the generally accepted emplacement age but does not appear to be a product of mixing a centimeter sized apatite from the otter lake area grenvillian province yields high and variable 176lu 177hf ratios showing that metamorphic apatite may in some cases allow for mono mineral dating the most important application of the method is undoubtedly its potential use for dating sedimentary formations although we find biogenic apatite in porous matrix highly susceptible to post depositional resetting of the lu hf isotopic system the preservation of a near depositional age for a shark tooth from the impermeable london clay and recent work on crystalline authigenic phosphorites earth planet sci lett 201 2002 203 show the lu hf system to be a promising tool for direct dating of sedimentary formations

growth history of garnet in pelitic schists of the sanbagawa metamorphic terrain in central shikoku abstract during the prograde metamorphism of the sanbagawa metamorphic terrain in central shikoku garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote etc in the quartz albite layer it has grown euhedrally in the micaceous layer its crystal morphology varies according to the different minerals in contact and their different orientations garnet enclosed in or sandwiched between micas is always ellipsoidal extended parallel to the basal planes of the micas however when the edge of a garnet has intruded at an acute angle into the micas euhedral garnet has grown it is concluded that the growth of garnet at the interface with micas ceased when the 110 surface of garnet came into contact with the basal plane of the micas in the tectonic c axis direction other models such as chloritization resorption pressure solution and mechanical erosion or grinding do not adequately explain the origin of the flattened garnet during retrograde metamorphism chloritization and resorption of garnets took place in the matrix resulting in the rough and irregular surface of garnet garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism

garnet polycrystals and the significance of clustered crystallization polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei if clustering is early or impingement of larger grains if clustering occurs later in the growth history the timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance if any of clustering electron backscattered diffraction ebsd analysis of garnet bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study the northern appalachians vermont maine new york usa north american cordillera north cascades range washington snake range nevada usa western rocky mountains british columbia canada southern menderes massif turkey santander massif colombia and the sanandaj sirjan zone hamadan iran in some samples polycrystals comprise 20 30 of garnets analyzed and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common some early coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal in thin section these garnets may be undetectable as polycrystals based on morphology or zoning in some polycrystals zoning is unrelated to the location of internal grain boundaries in others fe mn mg zoning has a different pattern than that of ca zoning patterns may vary on the scale of a single thin section in addition some polycrystals are characterized by high angle misorientation boundaries that may be in special non random orientations an observation that indicates that these polycrystals are not random clusters of grains the presence of internal grain boundaries may affect diffusion pathways and length scales and may facilitate communication of porphyroblast interiors with matrix phases thereby influencing reaction history of the rock and the composition zoning of garnet

relative sea level history of marguerite bay antarctic peninsula derived from optically stimulated luminescence dated beach cobbles abstract calmette bay within marguerite bay along the western side of the antarctic peninsula contains one of the most continuous flights of raised beaches described to date in antarctica raised beaches extend to 40 8 m above sea level masl and are thought to reflect glacial isostatic adjustment due to the retreat of the antarctic peninsula ice sheet using optically stimulated luminescence osl we dated quartz extracts from cobble surfaces buried in raised beaches at calmette bay the beaches are separated into upper and lower beaches based on osl ages geomorphology and sedimentary fabric the two sets of beaches are separated by a prominent scarp one of our osl ages from the upper beaches dates to 9 3 thousand years ago ka as of 1950 consistent with previous extrapolation of sea level data and the time of ice retreat from inner marguerite bay however four of the seven ages from the upper beaches date to the timing of glaciation we interpret these ages to represent reworking of beaches deposited prior to the last glacial maximum lgm by advancing and retreating lgm ice ages from the lower beaches record relative sea level fall due to holocene glacial isostatic adjustment we suggest a holocene marine limit of 21 7 masl with an age of 5 5 7 3 ka based on osl ages from calmette bay and other sea level constraints in the area a marine limit at 21 7 masl implies half as much relative sea level change in marguerite bay during the holocene as suggested by previous sea level reconstructions no evidence for a relative sea level signature of neoglacial events such as a decrease followed by an increase in rsl fall due to ice advance and retreat associated with the little ice age is found within marguerite bay indicating either 1 no significant neoglacial advances occurred within marguerite bay 2 rheological heterogeneity allows part of the antarctic peninsula i e the south shetland islands to respond to rapid ice mass changes while other regions are incapable of responding to short lived ice advances or 3 the magnitude of neoglacial events within marguerite bay is too small to resolve through relative sea level reconstructions although the application of reconstructing sea level histories using osl dated raised beach deposits provides a better understanding of the timing and nature of relative sea level change in marguerite bay we highlight possible problems associated with using raised beaches as sea level indices due to post depositional reworking by storm waves

combining compositional zoning and foliation intersection axes fias in garnet to quantitatively determine early p t t paths in multiply deformed and metamorphosed schists north central massachusetts usa quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite staurolite garnet grade rocks from the northeastern flank of the pelham dome north central massachusetts distinguishes the effects of acadian deformation and metamorphism from extensive overprinting alleghanian shearing the p t conditions and the metamorphic path during the acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts fias this acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an alleghanian overprint for each sample where garnet first nucleated during one of these stages in the tectonic history the pt of core growth was determined by plotting the intersection of the mn fe and ca isopleths calculated for the core composition on a p t pseudosection for that sample using thermocalc combining the pt data from all these samples indicates that the temperature and pressure increased throughout acadian orogenesis causing episodic garnet growth during the alleghanian locally intense shearing especially against the margin of the pelham dome formed the dominant schistosity which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of acadian deformation and metamorphism in the rock matrix shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700 c in those rocks lying directly adjacent to the pelham dome and resulted from the rocks of the northfield syncline being thrust a large distance southwards over the gneisses in the dome

a garnet population in yellowknife schist canada data are presented on a garnet population in a specimen of garnet biotite plagioclase quartz schist from the cordierite zone of an archaean thermal dome in the southern slave province of the canadian shield garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10 m scale are corrugated crystal distribution as revealed by dissection of a small cubic volume of rock is random the size distribution is normal with a mean diameter of 0 81 mm and a standard deviation of 0 32 mm in the largest crystal of the population mean radius 0 83 mm mn 100 mn fe mg mn ca decreases from 14 5 at the centre to 7 5 and then increases in the outer margin to 8 5 fe increases continuously from 67 at the centre to 77 at the surface mg increases from 12 5 to 13 5 and then falls sharply to 11 ca remains unchanged at 4 0 and then drops to 3 3 progressively smaller crystals have progressively lower mn and higher fe concentrations at their centres while all crystals have the same margin composition growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located a garnet forming model is presented in which reaction was initiated by a rise in temperature nucleation sites were randomly selected the nucleation rate increased with time and then declined crystal faces advanced at a constant linear rate which implies an increase in volume proportional to surface area initially the composition of garnet deposited on crystal surfaces was determined by van laar equations of equilibrium which demanded the withdrawal of mn and fe from within chlorite crystals this transfer reaction was then accompanied by an ion exchange reaction which moved mn and fe to garnet surfaces from biotite in exchange for mg the exchange reaction provides an explanation for the high overall concentration of mn and fe in garnet and for the observed mn and mg reversals in the margins of crystals the increase of garnet volume in the garnet population is found to be parabolic i e v 5

petrology and li be b geochemistry of muscovite biotite granite and associated pegmatite near yellowknife canada prosperous granite rb sr 2520 25 ma occurs as several plutons 1 380 km2 outcrop area in a thick succession of metamorphosed greywacke mudstone of the yellowknife supergroup the average mineral content of the sparrow pluton in vol is quartz 32 plagioclase 31 k feldspar 24 muscovite 9 biotite 3 and apatite 1 average trace element concentrations in ppm are li 140 be 4 b 28 zn 47 rb 250 sr 76 zr 75 and ba 360 the central portion of the pluton is slightly richer in k sr and ba than the margin li is concentrated in mica li in biotite li in muscovite 4 7 and be and b in muscovite and plagioclase countless pegmatite dikes occur in the sparrow pluton and in schist hornfels to the east the outer limit is marked by the cordierite isograd 9 km from the granite contact dikes vary greatly in size 1 km to a few cm in length in mineral content quartz albite k feldspar muscovite tourmaline beryl spodumene in major element composition especially the na k ratio and in trace element content li 18 5000 ppm be 5 260 ppm b 20 150 ppm compared with prosperous granite the pegmatite bodies are richer in p and rb and poorer in ti fe mg zr and ba dikes rich in tourmaline beryl and spodumene occur in overlapping zones situated progressively farther from the centre of the sparrow pluton the composition of tourmaline is related to host rock the highest concentrations of fe and zn occur in crystals from pegmetite and the highest concentrations of mg and v occur in crystals from tourmalinized schist while those from granite and quartz veins occupy on intermediate position complex compositional zoning is present in some tourmaline crystals in pegmatite estimates of temperature 500 600 c and pressure 2 4 kb of granite emplacement based on the distribution of andalusite and sillimanite in the contact rocks suggest that the final stage of granite emplacement occurred at sub solidus conditions a vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability some of the pegmatite constituents including li be and b were possibly derived from yellowknife graywacke and mudstone

a comment on diffusion and the displacement of atoms in metamorphic transformations experimental data on diffusion in olivine are used to define certain terms diffusion coefficient jump frequency characteristic distance random walk that are useful in a discussion of atom displacements under natural conditions examples of atom displacements in two metamorphic terranes of the canadian precambrian shield are then examined as follows i in a high grade metamorphic terrane in the mid proterozoic grenville province otter lake area mg concentration gradients about dolomite microcrystals in calcite and na gradients about albite microcrystals in k feldspar are viewed as stranded mg ca and na k interdiffusion gradients formed by exsolution during slow cooling from 700 to 400 c ii in the archean slave province yellowknife area the crystallization of sillimanite near andalusite but within crystals of quartz possibly occurred by coupled al si and oxygen vacancy interdiffusion in quartz at 550 c and the crystallization of garnet from chlorite occurred by the two way crystal boundary diffusion of several kinds of atoms across distances ranging to 3 mm iii in the otter lake area the crystallization of orthopyroxene hornblende spinel reaction zones at boundaries between crystals of olivine and plagioclase in metagabbro evidently occurred by the mechanism of interstitial diffusion that transported mg fe mn and o atoms across the reaction zone from olivine to the plagioclase hornblende spinel boundary and si al ca and na atoms from plagioclase to the olivine orthopyroxene boundary accompanied by nasi caal interdiffusion in plagioclase and the addition of hydrogen and minor ti zn f cl and k from beyond the reaction zone also centimetric reaction zones with abundant biotite and plagioclase at boundaries between k feldspar gneiss and deformed amphibolite dykes evidently formed by the reaction strained hornblende in amphibolite k feldspar in gneiss biotite in amphibolite plagioclase in gneiss with crystal boundary diffusion of na ca atoms and of k atoms across the reaction zone

metamorphic evolution of garnet epidote biotite gneiss from the moine supergroup scotland and geotectonic implications metapelitic gneisses from the glenfinnan group of the moine supergroup scotland contain sparse large and numerous small garnets associated with complex zoned epidote and plagioclase in a biotite matrix the large garnets show four zones ai aiv whereas the small garnets show three or fewer zones indicating successive garnet nucleation with increasing nucleation densities garnet zones ai and aiv grew under static conditions whereas the formation of aii and aiii was accompanied by deformation garnet zones ai and aii were formed in the assemblage all biotite epidote plagioclase quartz fluid apatite garnet chlorite muscovite ilmenite sphene magnetite zone aiii in the assemblage garnet muscovite sphene magnetite and zone aiv in the assemblage garnet sphene ilmenite the chemical zonation and microstructures of garnet a indicate two important discontinuities one at the transition between garnet zones ai and aii and a second between zones aii and aiii which correlate with complex zonation shown by epidote and plagioclase these discontinuities may result from polymetamorphic garnet growth during different orogenic cycles affecting the moine supergroup geothermobarometric calculations and gibbs method modelling provide evidence that garnet zone ai grew rapidly during heating from about 550 to 560 c at pressures of about 4 6 kbar in contrast the formation of zone aii was accompanied by nearly isothermal compression from 6 to 8 5 kbar 560 575 c indicating crustal stacking after a certain period of cooling garnet zone aiii grew during renewed heating at p t conditions of about 640 c and pressures between 5 and 9 kbar growth of garnet aiv was accompanied by further temperature rise reaching maximum conditions of about 670 c at 5 kbar

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

crystal growth mechanisms of quartz overgrowths in a cambrian quartz arenite abstract mechanisms of crystal growth during quartz cementation can be investigated by determining the relationship between growth increments and crystal radii crystal growth is theoretically described as 1 dr dt k for polynuclear growth and spiral growth where dr dt is the rate r is the radius of the crystal and k is some constant 2 dr dt kr for mononuclear growth and 3 dr dt k r for diffusion limited growth empirical results in this study where r detrital grain radius and dr dt quartz overgrowth thickness define another crystal growth mechanism size dependent growth described by the rate equation dr dt kr grain radii and quartz overgrowth thicknesses were measured in 16 thin sections of quartz arenite from the galesville sandstone cambrian wisconsin u s a and plotted to determine correlation coefficients between radius and rate correlation coefficients ranged from 0 145 to 0 855 ten of 16 samples show statistically significant correlations between detrital grain radii and overgrowth thicknesses representing dr dt kr standard deviations sorting of detrital grain size correlate with the correlation coefficient between radius and rate samples indicating no correlation may in fact grow by dr dt kr but for smaller grain size ranges the overgrowth thicknesses cannot be measured precisely enough to detect small variations size dependent growth dr dt kr is a new type of mineral water interaction that has also been observed in ancient dolomites as well as metamorphic garnets and other various minerals grown in the laboratory and now in quartz cemented sandstone this indicates that crystal growth theory may not include some significant growth mechanisms

formation of garnet clusters during polyphase metamorphism pre alpine garnets of variscan age from metapelitic basement units in northern italy were strongly retrogressed at near surface conditions prior to alpine contact metamorphism the replacement by sheet silicates caused a significant volume increase during retrogression producing pervasive fracturing up to several hundreds of angular fragments formed from each crystal electron backscatter diffraction analysis documents a maximum misorientation of 22 of some fragments as a result of local rotation during fracturing new garnet growth is observed on the garnet fragments during contact metamorphic overprinting resulting in garnet clusters fragments can be identified due to calcium rich domains fragment orientations were inherited and only minor new nucleation occurred these garnets develop features typically associated with multiple nucleation models but here they reflect multiple metamorphic events we propose that clusters can be indicative of multiple metamorphic events which were separated by a period of intense retrograde alteration

the effect of reaction overstep on garnet microtextures in metapelitic rocks of the ilesha schist belt sw nigeria garnet bearing assemblages of k rich and k poor metapelites from the ilesha schist belt sw nigeria are investigated k rich samples contain the assemblages a garnet staurolite muscovite chlorite magnetite b andalusite garnet staurolite muscovite chlorite magnetite and c sillimanite andalusite garnet muscovite chlorite magnetite k poor samples contain the assemblages d garnet staurolite cordierite chlorite and e garnet cordierite chlorite staurolite all assemblages contain quartz plagioclase biotite and ilmenite p t pseudosections calculated in the system cao na2o k2o tio2 mno feo mgo al2o3 sio2 h2o o2 suggest peak metamorphism at 590 20 c at 5 0 5 kbar followed by retrogression to 550 c at 3 0 kbar in agreement with field evidence domain assemblages mineral compositions modes and geothermobarometry the absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant p t x in equilibrium with associated minerals on the thin section scale however the garnet in reaction did not begin until the establishment of a significant temperature overstep of 80 c with consequent rapid dehydration of chloriteand muscovite bearing assemblages resulting in interface controlled garnet growth under locally hydrostatic pressure conditions garnet in rock types a and b formed with a characteristic pattern of biotite and quartz inclusions a house of cards texture additionally clusters and aggregates of unzoned garnet grains formed in rock types a and d garnet resorption in rock types a c resulted from prograde stauroliteand retrograde andalusite forming reactions

diffusion control of garnet growth harpswell neck maine usa a detailed analysis of chemical zoning in two garnet crystals from harpswell neck maine forms the basis of an interpretation of garnet nucleation and growth mechanisms garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation giving rise to complex two dimensional zoning patterns depending on the orientation of the thin section cut contours of ca zoning cross those of mn fe and mg indicating a lack of equilibrium among these major garnet constituents zoning of fe mg and mn is interpreted to reflect equilibrium with the rock matrix whereas ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal image analysis reveals that the growth of garnet is more rapid along triple grain intersections than along double grain boundaries moreover different minerals are replaced by garnet at different rates the relative rate of replacement by garnet along double grain boundaries is ordered as muscovite chlorite plagioclase quartz flux calculations reveal that replacement is limited by diffusion of si along double grain boundaries to or from the local reaction site it is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet with si and al transport being rate limiting in these samples

microstructural chemical and textural records during growth of snowball garnet the growth history of two populations of snowball garnet from the lukmanier pass area central swiss alps was examined through a detailed analysis of three dimensional geometry chemical zoning and crystallographic orientation the first population collected in the hinge of a chevron type fold shows an apparent rotation of 360 the first 270 are characterized by spiral shaped inclusion trails gradual and concentric mn zoning and a single crystallographic orientation whereas in the last 90 crenulated inclusion trails and secondary mn maxima centred on distinct crystallographic garnet domains are observed microstructural geochemical and textural data indicate a radical change in growth regime between the two growth sequences in the first 270 growth occurred under rotational non coaxial flow whereas in the last 90 garnet grew under a non rotational shortening regime the second population collected in the limb of the same chevron type fold structure is characterized by a spiral geometry that does not exceed 270 of apparent rotation these garnet microstructures do not record any evidence for a modification of the stress field during garnet growth concentric mn zoning as well as a single crystallographic orientation are observed for the entire spiral electron backscatter diffraction data indicate that nearly all central domains in the snowball garnet are characterized by one 001 axis oriented sub parallel to the symmetry axis and by another 001 axis oriented sub parallel to the orientation of the internal foliation these features suggest that the crystallographic orientation across the garnet spiral is not random and that a relation exists among the symmetry axis the internal foliation and the crystallographic orientation

unraveling the schneeberg garnet puzzle a numerical model of multiple nucleation and coalescence a quantitative kinetic model for the growth of the different garnet porphyroblast microstructures type 1 and type 2 of the western schneeberg complex wsc is presented these porphyroblasts formed by a multiple nucleation and coalescence mechanism our numerical simulation shows that at constant diffusion rates 1 low interface reaction rates result in a fully amalgamated porphyroblast type 2 2 intermediate reaction rates result in a porphyroblast where coalescence of grains closer to the margin prevented amalgamation of those in the centre similar to type 1 porphyroblasts and 3 high interface reaction rates result in a porphyroblast microstructure with an atoll form all three microstructures are characterised by distinctive cluster size distributions a 2 d cluster size distribution analysis of type 1 porphyroblasts of wsc shows that these did not form because of intermediate interface reaction rates but because the diffusion rate of nutrients was too low to keep pace with the interface reaction rate

crystal growth and the formation of chemical zoning in garnets a crystal growth model is developed which generalizes the rayleigh fractionation process the new growth model allows some insight into the interpretation of nonequilibrium behavior of minerals primarily the chemical zoning profiles exhibited by metamorphic minerals a nonlinear equilibrium term for exchange of constituents between a growing mineral and a reservoir is initially incorporated into the usual isothermal fractionation model criteria are established to decide when a simple distribution term is sufficient to describe the growth and exchange process the model is then extended to allow for temperature changes during a cooling or heating event finally an exact solution is obtained for the temperature dependent case incorporating a time dependent growth rate the growth models are successfully used to obtain growth rates of 0 01 to 0 09 cm million year and describe the magnesium and iron zoning profiles of garnets from phillipston massachusetts the generalized model confirms the development of zoning during the retrograde growth of garnet in the late stages of the acadian orogeny

development of garnet porphyroblasts by multiple nucleation coalescence and boundary misorientation driven rotations two types of garnet porphyroblast occur in the schneeberg complex of the italian alps type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz decussate mica and small garnet grains and a margin containing large garnet grains orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains individual garnet grains without subdomains are small 50 m faceted and idioblastic and have simple zoning profiles with ca rich cores and ca poor rims subdomains of larger garnet grains are similar in size to the individual small garnet grains type 2 porphyroblasts comprise only ellipsoidal garnet with small subdomains in the centre and larger subdomains at the margin each subdomain has its own ca high ca dropping towards subdomain boundaries garnet grains with or without subdomains all have the same ca poor composition at rims in contact with other minerals the compositional zonation patterns are best explained by simultaneous multiple nucleation followed by growth and amalgamation of individual garnet grains the range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre the difference between type 1 and type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast electron backscatter diffraction shows that small individual garnet grains are randomly oriented large marginal garnet grains and subdomain bearing garnet grains have a strong preferred orientation clustering around a single garnet orientation misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations the only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other this process was driven by the reduction of subdomain boundary energy associated with misorientation loss rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals quartz and mica around the garnet an analytical model in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz suggests that at the conditions of metamorphism the rotation required to give a strong preferred orientation can occur on a similar time scale to that of porphyroblast growth

formation of garnet polycrystals during metamorphic crystallization garnet polycrystals may form throughout the metamorphic history of a rock starting at the earliest stages of garnet growth when closely spaced nuclei coalesce in mica schist from townshend dam vt electron back scattered diffraction ebsd analysis shows that garnet polycrystals possess two or more distinct lattice orientations separated by high angle boundaries 28 61 the minimum rotational displacements required to bring these lattice orientations into concordance with each other are commonly normal to the same low energy planes that occur as crystal faces of euhedral garnet there is no evidence for intracrystalline deformation and the polycrystals therefore probably represent individual garnet crystals that coalesced during growth the boundaries cross cut growth zoning and inclusion trails of the polycrystals indicating that early formed polycrystals once coalesced behave chemically and physically as single crystals statistical analysis of a 3d high resolution x ray computed tomographic data set of a large sample 912 cm3 of a townshend dam schist combined with microprobe and ebsd analyses of garnet are consistent with a high degree of clustering at all stages of garnet growth the formation and prevalence of polycrystals implies that garnet nuclei impinged on each other and coalesced and that coalescence was a common feature throughout garnet growth in the rock