coherence of western boundary pressure at the rapid wave array boundary wave adjustments or deep western boundary current advection this study investigates the coherence between ocean bottom pressure signals at the rapid climate change programme rapid west atlantic variability experiment wave array on the western north atlantic continentalslope includingthewoodsholeoceanographicinstitutionlinew highlycoherentpressuresignals propagate southwestward along the slope at speeds in excess of 128 m s 21 consistent with expectations of barotropic kelvin like waves coherent signals are also evidenced in the smaller pressure differences relative to 1000 m depth which are expected to be associated with depth dependent basinwide meridional transport variations or an overturning circulation these signals are coherent and almost in phase for all time scales from 3 6 years down to 3 months coherence is still seen at shorter time scales for which group delay estimates are consistent with a propagationspeedofabout1 m s 21 over990 kmofcontinentalslopebutwithlargeerrorboundsonthespeed this is roughly consistent with expectations for propagation of coastally trapped waves though somewhat slower than expected a comparison with both eulerian currents and lagrangianfloat measurements shows that the coherence is inconsistent with a propagation of signals by advection except possibly on time scales longer than 6 months

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

population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

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

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

rare earth element mobility during prograde granulite facies metamorphism significance of fluorine mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the quetico subprovince of the superior province ontario canada and exhibit concentric zoning with a biotite rich margin orthopyroxene rich outer zone clinopyroxene rich central zone and occasionally patches of relict amphibolites within the clinopyroxene rich zone the granulites biotite orthopyroxene and clinopyroxene rich zones in the mafic gneisses are characterized by significant amounts of rare earth element ree bearing fluorapatite 1 10 vol and other ree rich minerals allanite monazite and zircon fluorapatite shows an increase in modal abundance from the biotite and orthopyroxene rich zones to the clinopyroxene rich zone but is rare in the relict amphibolites textural evidence and element partitioning indicate that fluorapatite and other ree rich minerals was part of the peak metamorphic assemblages whole rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of ree and high field strength elements hfse whereas the relict amphibolites are geochemically typical of tholeiitic basalts mass balance calculations reveal that ree and hfse were introduced into the mafic gneisses during the prograde granulite facies metamorphism pointing to ree mobility under granulite facies metamorphic conditions the presence of high f contents in the ree rich minerals and their associated minerals e g biotite and hornblende suggests that ree and hfse may have been transported as fluoride complexes during the granulite facies metamorphism this conclusion is supported by previously published results of hydrothermal experiments on the partitioning of ree between fluorapatite and f rich fluids at 700 c and 2 kbar

mineralogy geochemistry and zircon u pb hf isotopes of the paleoproterozoic granulite facies metamorphic rocks from the aketashitage area southeastern tarim craton abstract the key to understanding the generation of mantle derived magma and the formation of new continental crust is the recognition of the granulite facies metamorphism associated partial melting and coeval magmatism that are documented in active continental margins we firstly present a combined analysis of petrography mineral chemistry zircon u pb ages and hafnium hf isotopes in addition to whole rock geochemistry on the garnet biotite gneisses grt bi gneisses and mafic granulites from the aketashitage area southeastern tarim craton the implications of this analysis are discussed with respect to the thermal and tectonic evolution of the tarim craton the grt bi gneisses and mafic granulites are strongly peraluminous in composition and have flat ree patterns with negative eu anomalies that are identical to those from the middle crust but higher than lower continental crust both the grt bi gneisses and the mafic granulites have negative nb ta anomalies slightly positive zr hf anomalies and pronounced positive spikes in u and pb the characteristics in whole rock geochemistry and mineral compositions indicate that the grt bi gneisses and mafic granulites have the same source locality which is most likely the garnet bearing middle crust the retrograde metamorphic p t estimates for the grt bi gneisses and mafic granulites are t 574 c p 7 0 kbar and t 715 c p 7 0 kbar respectively sims zircon u pb dating results reveal a large spread in metamorphic ages 2036 1962 ma for these granulite facies rocks interpreted to be the consequence of a large amount of magmas being added to the crust in an uninterrupted manner zircon hf isotopes are characterized by variable but mostly negative ehf t values 18 2 to 0 6 reasonably uniform hf isotopic compositions and highly variable tdmc model ages between 2628 and 3599 ma representing a long lived reworking of the preexisting crust our data along with available geological evidences lead us to propose a model for the evolution of the dunhuang block in the paleoproterozoic that is the southeastern margin of the tarim craton the dunhuang block was possibly an active continental margin during the paleoproterozoic the steep subduction of oceanic slab resulted in the thickening of the continental crust which in turn caused the crust anatexis the increase in asthenosphere high heat flow and the intrusion of mantle derived magma induced extensive heating which preceded the granulite facies metamorphism and coeval magmatism in the middle crust of the southern tarim craton

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

monazite chemical composition some implications for monazite geochronology an investigation of the chemical composition of monazite from a number of localities has been carried out samples used include monazites from metamorphic rocks granitic rocks and a hydrothermal ore deposit the ree distribution pattern of monazite varies greatly in accordance with its geological environment a remarkable feature of the monazites studied is that their chondrite normalised ree distribution patterns are mostly uniform between grains within the same sample but differ significantly from sample to sample this characteristic apparently indicates that there is an important effect on the ree distribution of monazite exerted by the host rock or source material from which monazite crystallised another important feature shown by the monazites studied is that monazites in rocks containing garnet as a major mineral show extreme depletion of hree whereas monazites in rocks without garnet or monazite that formed after the garnet breakdown contain significantly higher amounts of y and hree this suggests that the phase assemblage especially garnet plays an important role in the ree distribution of monazites in these rocks the value of ree distribution in monazite is exemplified with regard to the origin of monazite in the lewisian metamorphic rocks which is a fundamental issue in monazite geochronology

coupled wind forced controls of the bering chukchi shelf circulation and the bering strait throughflow ekman transport continental shelf waves and variations of the pacific arctic sea surface height gradient abstract we develop a conceptual model of the closely co dependent bering shelf bering strait and chukchi shelf circulation fields by evaluating the effects of wind stress over the north pacific and western arctic using atmospheric reanalyses current meter observations satellite based sea surface height ssh measurements hydrographic profiles and numerical model integrations this conceptual model suggests bering strait transport anomalies are primarily set by the longitudinal location of the aleutian low which drives oppositely signed anomalies at synoptic and annual time scales synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along strait meridional pressure gradient in particular we show that storms centered over the bering sea excite continental shelf waves on the eastern bering shelf that carry northward velocity anomalies northward through bering strait and along the chukchi coast the integrated effect of these storms tends to decrease the northward bering strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the aleutian basin and the western subarctic gyre ekman suction then increases the water column density through isopycnal uplift thereby decreasing the dynamic height sea surface height and along strait pressure gradient storms displaced eastward over the gulf of alaska generate an opposite set of bering shelf and aleutian basin responses while ekman pumping controls canada basin dynamic heights proshutinsky et al 2002 we do not find evidence for a strong relation between beaufort gyre sea surface height variations and the annually averaged bering strait throughflow over the western chukchi and east siberian seas easterly winds promote coastal divergence which also increases the along strait pressure head as well as generates shelf waves that impinge upon bering strait from the northwest

fluid present melting of meta igneous rocks and the generation of leucogranites constraints from garnet major and trace element data lu hf whole rock garnet ages and whole rock nd sr hf o isotope data pan african high grade metamorphism in the proterozoic damara orogen namibia led to formation of garnet bearing leucosomes in potassic meta igneous gneisses producing a meta igneous migmatite in addition the migmatite gneiss mesosome plus leucosome was intruded by small scale leucogranitic melts with a high amount of accumulated biotite and garnet u pb zircon ages obtained on the mesosome and the leucosome indicate late proterozoic ca 850 ma concordia upper intercept ages which are interpreted as minimum ages of the precursor rock of the migmatite u pb monazite ages obtained on the leucogranite give a concordant age of 512 1 ma and two reversely discordant ages with pb 207 u 235 ages of 544 1 and 534 1 ma indicating the growth of monazite before or close to the age of high grade metamorphism in the damara orogen high precision lu hf garnet whole rock dating gave ages of 492 6 1 7 ma for the mesosome 497 6 1 7 ma for the leucosome and 494 0 1 7 ma for the garnet and biotite bearing leucogranite indicating that the growth of garnet postdates the growth of monazite during high grade metamorphism in addition it is suggested that melting and intrusion was coeval and occurred probably shortly after the main peak of metamorphism which occurred at c 512 1 ma p t estimates obtained by conventional thermobarometry c 690 720 degrees c and accessory mineral dissolution thermometry on the leucogranite c 730 degrees c suggest that partial melting occurred through limited fluid present melting of biotite via the reaction bt kfs pig qtz h2o double left right arrow grt melt outcrop evidence diffuse relationship between the gneiss domain and the leucosomes similar size of the leucosomes homogeneous distribution of leucosomes on the sample scale suggests that minor melt segregation had occurred whole rock sr nd hf and o isotope data of the mesosome indicate that it belongs to basement rocks from this area geochemical and isotope data obtained on the leucosomes argue for derivation by in situ melting of the mesosome both leucosome and leucogranite originated from the same source rock but the leucogranite represents an accumulated melt that was able to segregate and to intrude the gneiss domain the similar isotope features of the mesosome leucosome and leucogranite indicate a direct relationship for the gneiss and the melts chemical and mineral data favour a derivation of both types of melt through fluid present melting of isotopically and chemically comparable biotite plagioclase k feldspar quartz bearing gneisses c 2008 elsevier b v all rights reserved

in situ major and trace elements of garnet and scheelite in the nuri cu w mo deposit south gangdese tibet implications for mineral genesis and ore forming fluid records abstract garnet and scheelite are common hydrothermal silicate mineral and metal mineral in skarn or porphyry skarn deposits and can aid in constraining the composition of ore forming fluid based on in situ major and trace element analyses of garnet and scheelite from the nuri cu w mo deposit we discuss the factors and mechanisms controlling the incorporation of rare earth elements rees into the garnet and scheelite and investigate the associated hydrothermal fluid characteristics the garnets from the nuri deposit belong to the grossular andradite grandite solid solution series and range in composition from adr64grs34 to adr91grs8 the ree patterns of garnet are enriched in light rare earth elements lrees and depleted in heavy rare earth elements hrees the rees characteristics indicates that the incorporation of rees into the nuri garnet was controlled by the surface adsorption and element substitution in the mineral structure based on the cathodoluminescence cl images of scheelite two growth stages early and late stages are identified these two stages of scheelite belong to the scheelite powellite series and are enriched in lrees and depleted in hrees however the early scheelite exhibits negative or no eu anomalies 0 34 0 95 whereas the late scheelite presents positive eu anomalies 1 25 2 89 the mo content in the early scheelite is positively correlated with the eu value while the mo content in the late scheelite is negatively correlated with eu these characteristics indicate that the redox state of the ore forming fluid gradually changed from the oxidized state to the reduced state the preferred ree3 substitution for ca2 in the nuri scheelite is 3ca2 2ree3 ca where ca is a ca site vacancy the ree features of the scheelite are mainly controlled by the ree nature of the ore forming fluid according to the characteristics of trace elements in garnet and scheelite we suggest that the ore forming fluid of the nuri deposit is a magmatic fluid mixed with a slightly acidic external fluid chloride complexes may be the main transport ligand of the rees and the oxygen fugacity of the ore forming fluid gradually decreased

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

chromium and manganese zoning in pelitic garnet and kyanite spiral overprint and oscillatory zoning patterns and the role of growth rate x ray composition maps and quantitative analyses for mn ca and cr have been made for six pelitic and calc pelitic garnet crystals and al fe and cr analyses maps have been made for two kyanite crystals from lower and mid upper amphibolite facies rocks from the grenville province of western labrador using an electron microprobe analyser and a laser ablation icp ms garnet with spiral snowball internal fabrics si has spiral zoning in major elements implying that growth was concentrated in discrete regions of the crystal at any one time spiral zoning cr zoning is parallel to si in low amphibolite facies garnet with both straight and spiral internal fabrics indicating that the garnet overprinted a fabric defined by cr rich mica chlorite epidote and cr poor quartz plagioclase layers during growth overprint zoning and that cr was effectively immobile in contrast in mid upper amphibolite facies garnet porphyroblasts lacking si cr zoning is concentric implying that cr diffusion occurred cr zoning in kyanite porphyroblasts appears superficially similar to oscillatory zoning with up to three or four annuli of cr enrichment and or depletion present in a single grain however the variable width continuity cr concentration and local bifurcation of individual annuli suggest that an origin by overprint zoning may be more likely the results of this study explain previously observed nonsystematic cr zoning in garnet and irregular partitioning of cr between coexisting metamorphic mineral pairs in addition this study points to the important role of crystal growth rate in determining the presence or absence of inclusions and the type of zoning exhibited by both major and trace elements during fast growth inclusions are preferentially incorporated into the growing porphyroblast and slow diffusing elements such as cr are effectively immobile whereas during slow growth inclusions are not generally included in the porphyroblast and cr zoning is concentric

circulation and thermohaline variability of the hanna shoal region on the northeastern chukchi sea shelf we analyzed velocity and hydrographic data from 23 moorings in the northeast chukchi sea from 2011 to 2014 in most years the eastern side of hanna shoal was strongly stratified year round while weakly stratified regions prevailed on the shelf south and west of the shoal stratification differences cause differential vertical mixing rates which in conjunction with advection of different bottom water properties resulted in seasonally varying along isobath density gradients in agreement with numerical models we find that bottom waters flow anticyclonically around the shoal whereas most of the shelf responded barotropically to wind forcing there was a strong baroclinic component to the flow field northeast of hanna shoal resulting in no net vertically integrated transport on average in contrast there is a net eastward transport from west of the shoal which implies convergence north of the shoal convergence and along isobath density gradients may foster cross shelf exchange north of hanna shoal modal analyses indicate that the shelf south of the shoal and barrow canyon responded coherently to local and remote winds whereas the wind current response around hanna shoal was less coherent barotropic topographic waves of 3 day period were generated episodically northeast of the shoal and propagate clockwise around hanna shoal but are blocked from entering barrow canyon and are possibly scattered by the horizontally sheared flow and converging isobaths on the western side of the shoal analysis of water properties on the western side of hanna shoal suggests that these include contributions from the western and southern portions of the chukchi sea

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

origins of yttrium and rare earth element distributions in metamorphic garnet highly variable distributions of yttrium and rare earth elements y rees are documented in pelitic garnet from the picuris mountains new mexico and from passo del sole switzerland and in mafic garnet from the franciscan complex california the wide variety of these y ree zoning patterns and those described previously in other occurrences imply diverse origins linked to differing degrees of mobility of these elements through the intergranular medium during garnet growth in the picuris mountains large early nucleating crystals have radial profiles of y ree dominated by central peaks and annular maxima in patterns that vary systematically with atomic number superimposed on these features are narrow spikes in hrees and mrees located progressively rimward with decreasing atomic number in contrast profiles in small late nucleating crystals contain only broad central maxima for all y rees in garnet from passo del sole y ree zoning varies radically from sample to sample in some rocks crystals of all sizes display only central peaks for all y rees in others profiles exhibit irregular fluctuations in y ree contents that match up with small scale patchy zoning in y and ca x ray maps in the franciscan complex y ree in garnet cores fluctuate unsystematically but mantles and rims display concentric oscillatory zoning for both major elements and y rees our interpretation of the complexity of y ree distributions in metamorphic garnet centres on the concept that these distributions vary primarily in response to the length scales over which these elements can equilibrate during garnet growth very short length scales of equilibration due to low intergranular mobility produce overprint zoning characterized by small scale irregularities higher but still restricted mobility yields diffusion controlled uptake characterized by patterns of central peaks and annular maxima that vary with atomic number and are strongly influenced by t t paths during garnet growth still greater mobility permits progressively greater potentially rock wide equilibration with major and accessory phase assemblages leading to mineralogical controls an unchanging mineral assemblage during garnet growth produces bell shaped profiles resembling those produced by rayleigh fractionation whereas changes in major and or accessory phase assemblages produce profiles with distinct annuli and sharp discontinuities in concentration the very high mobility associated with influxes of y ree bearing fluids can cause these element distributions to be dominated by factors external to the rock yielding profiles characterized by abrupt shifts or oscillations that are not correlated to changes in mineral assemblages

trace element zoning in pelitic garnet of the black hills south dakota trace element ree cr ti y y and zr analysis of garnet from the garnet staurolite and lower sillimanite zones of an aluminous schist of the black hills south dakota indicates that ree zoning varies as a function of grade garnet zone garnet has high concentrations of rees cr ti y y and zr in the cores and low concentrations in the rims profiles of heavy rees contain inflections between the cores and rims which are approximately symmetric about the cores staurolite zone garnet contains cores enriched with y and heavy rees which decrease toward the rim and increase again at the rim edges but to lower concentrations than in the cores cr y ti zr and light ree zoning is less pronounced than heavy ree zoning and is less symmetric about the garnet cores almandine rich garnet of the lower sillimanite zone displays no major element zonation trace element ti cr y and zr concentrations are minimal and the zoning is irregular and not symmetric about the garnet cores garnet from all three zones has core to rim fe fe mg profiles that suggestgarnet growth was uninterrupted with respect to major element components and that mn zoning formed by a fractionation process analysis of trace element zoning in this garnet reveals that the major element zoning was relatively unaffected by volume diffusion reequilibration trace element zonation of all samples of garnet is best explained by a fractionation mechanism in conjunction with limited intergranular diffusion and changing partition coefficients during garnet growth heavy ree partitioning is especially dependent on the major element composition of garnet this research complements previous research by others on the use of trace elements as metamorphic petrogenetic indicators which demonstrated the importance of bulk rock composition and phase assemblage on trace element partitioning

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

yttrium zoning in metamorphic garnets abstract yttrium zoning has been measured in garnets from the straits schist connecticut usa using the synchrotron x ray fluorescence sxrf microprobe sxrf compositional maps for mn and fe show that the major elements are concentrically zoned about the gannet core in the classic bell shaped profiles considered indicative of growth zoning yttrium however displays oscillatory zoning which varies three orders of magnitude in abundance from about 3 to 4000 ppm and is correlated with textural zoning in garnet the major element abundances by contrast display no visible inflection related to textural zoning at this time it is not possible to unequivocally identify the mechanism which has produced this oscillatory zoning most likely this reflects either the breakdown of y enriched phases changes in the rate of garnet growth with time or changes in the effective y concentration of matrix phases due to breakdown and reprecipitation of garnet as the equilibrium reactions involving gannet change this study demonstrates that the sxrf microprobe provides excellent spatially resolved compositional maps for trace elements in porphyroblasts something other analytical techniques have been unable to do it also suggests that the diffusion rates for y in garnet are probably significantly slower than those for major cations such as mn fe mg and ca as a result trace element mapping potentially provides a clearer record of the reaction history of the rock provides details of how chemical zoning in garnet is produced and allows evaluation of assumptions about chemical equilibrium between minerals

secondary porosity in sandstones secondary porosity in sandstones is caused by dissolution and fracturing and is common in the sedimentary record secondary porosity commonly develops in the deep subsurface and thus provides an opportunity to extend exploration to depths traditionally considered unsuitable for exploration two contrasting routes of diagenesis exist in nature porosity reduction and porosity enhancement porosity reduction is commonly caused by compaction and cementation whereas porosity enhancement is primarily caused by dissolution of carbonate minerals two basic types of primary pores intergranular and intragranular and four basic types of secondary pores grain fractures rock fractures intergranular and intragranular can be differentiated on the basis of 1 position of pores 2 timing of origin and 3 processes of origin the proposed classification system is useful in inferring reservoir quality various types of secondary porosity are recognized using a comprehensive set of 20 criteria the various criteria are based on manner of breakage pore geometry grain geometry products of dissolution and sediment packing new evidence suggests that silicate minerals including quartz dissolve more commonly than have been reported the abundant occurrence of secondary porosity in reservoir sandstones emphasizes the importance of secondary porosity in evaluating deep reservoirs

u pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites abstract u pb isotope analyses on major and accessory minerals from pelitic schists in of southwestern connecticut are used to date metamorphism and deformation most of the major minerals have 206 pb 204 pb ratios less than 100 however u pb mineral isochron and mineral whole rock isochrons constructed using aliquots of the same mineral yield concordant ages with uncertainties as low as 5 ma staurolite kyanite and garnet are older than metamorphic monazites and zircons these data are summarized below 5 mineral 206 pb 204 pb system the straits schist trap falls fm monazite zircon 207 pb 206 pb 379 384 ma 380 2 ma staurolite 48 623 238 u 206 pb 394 400 ma 398 5 ma garnet 22 46 238 u 206 pb 383 ma 414 10 ma 235 u 207 pb 418 ma 207 pb 206 pb 623 ma kyanite 21 27 238 u 206 pb 399 ma chlorite anatase ilmenite 20 27 238 u 206 pb 350 370 ma apatite quartz kspar 18 19 transposed pegmatitic intrusions in the straits schist yield monazites with u pb ages analytically indistinguishable from those of metamorphic monazites and zircons in the schists growth of monazite and zircon in the schists may be related to high t fluid influx associated with pegmatite intrusion age discordance in the garnets from the straits schist is most consistent with continuous loss of intermediate daughter products of the 238 u decay chain from about 420 ma to 380 ma garnets from the adjacent trap falls formation do not display this behavior

lateral mixing across ice meltwater fronts of the chukchi sea shelf summer and fall hydrographic sections in the northeastern chukchi sea frequently capture 5 20 m thick intrapycnocline lenses or horizontal plumes of warm moderately salty summer bering sea water flowing northward from bering strait these features occur within the shallow 20 m depth pycnocline separating cold dilute surface meltwater from near freezing salty winter formed waters beneath the pycnocline an idealized numerical model suggests that the features arise from eddies and meanders generated by instability of the surface front separating meltwater from bering sea water warm bering sea water is transported across the front and into the pycnocline by the cross frontal velocities associated with the instabilities the accompanying lateral eddy heat fluxes may be important both in summer for promoting ice melt and in fall by delaying the onset of ice formation over portions of this shelf lateral heat flux magnitudes depend upon the stratification of the bering sea water

trace element partitioning between coexisting biotite and muscovite from metamorphic rocks western labrador structural compositional and thermal controls coexisting biotite and muscovite in ten metapelitic and quartzofeldspathic rocks from western labrador have been analyzed by electron microprobe for major and minor elements and by a laser ablation microprobe coupled to icp ms lam icp ms for selected trace elements li sc v cr mn co ni cu zn rb sr y zr nb cs ba ree hf and ta the samples have experienced a single prograde grenvillian metamorphism ranging from 490 to 680 c and from 7 to 12 kbar the trace element compositions of coexisting micas in the metamorphic rocks are used to assess the effects of crystal structure major element composition and temperature on the partitioning of each element between biotite and muscovite overall trace element distributions are systematic across the range of metamorphic grade and bulk composition suggesting that chemical equilibrium was approached most distribution coefficients biotite muscovite show good agreement with published data however distribution coefficients for co and sr are significantly different from previous determinations probably because of contamination associated with older data obtained by bulk analysis techniques the sequence of distribution coefficients is governed mainly by the ionic radii and charges of substituting cations compared to the optimum ionic radius of each crystallographic site in the micas in particular distribution coefficients exhibit the sequence cr3 0 615 a v3 0 64 a sc3 0 745 a in vi sites and ba2 1 61 a sr2 1 44 a and cs 1 88 a k 1 64 a rb 1 72 a na 1 39 a in xii sites the distributions of li sc sr and ba appear to be thermally sensitive but are also controlled by major element compositions of micas v and zr partitioning is dependent on t and may be used to cross check thermometry calculations where the latter suffer from retrograde re equilibration and or high concentrations of fe3 the ranges and dependence of distribution coefficients on major element compositions provide important constraints on the values that can be used in geochemical modeling

electron microprobe petrochronology the term petrochronology has increasingly appeared in publications and presentations over the past decade the term has been defined in a somewhat narrow sense as the interpretation of isotopic dates in the light of complementary elemental or isotopic information from the same mineral s kylander clark et al 2013 although complementary isotopic and elementary information are certainly a central and critical part of most if not all petrochronology studies the range of recent studies that might use the term covers a much broader scope the term petrochronology might alternatively be defined as the detailed incorporation of chronometer phases into the petrologic and tectonic evolution of their host rocks in order to place direct age constraints on petrologic and structural processes as noted by kylander clark et al 2013 the linkage between geochronology and petrology can involve a variety of data including mineral textures and fabrics the distribution of mineral modes or volume proportions compositional zoning mineral inclusion relationships and certainly major element trace element and isotopic composition of the chronometer and all other phases electron probe micro analysis epma has a central and critical role to play in establishing the linkage between chronometer phases and their host assemblage the basic instrument is an electron microscope which can be used in either scanning or fixed beam modes with integrated wavelength dispersive spectrometers wds energy dispersive spectrometers eds electron detectors to image secondary and backscattered signals a light optical system and optionally cathodoluminescence cl detection the electron microprobe is used to investigate the distribution composition and compositional zonation of all mineral phases the data that underpin thermobarometric analysis and modeling of p t histories the microprobe with m scale spatial resolution can also characterize compositional zonation in very small accessory phases including monazite xenotime zircon allanite titanite apatite and others this as discussed below can be a

observed increases in bering strait oceanic fluxes from the pacific to the arctic from 2001 to 2011 and their impacts on the arctic ocean water column 1 mooring data indicate the bering strait throughflow increases 50 from 2001 0 7 sv to 2011 1 1 sv driving heat and freshwater flux increases increase in the pacific arctic pressure head explains two thirds of the change the rest being attributable to weaker local winds the 2011 heat flux 5 1020j approaches the previous record high 2007 due to transport increases and warmer lower layer ll temperatures despite surface temperature sst cooling in the last decade warmer ll waters arrive earlier 1 6 1 1 days yr though winds and sst are typical for recent decades maximum summer salinities likely set in the bering sea remain remarkably constant 33 1 psu over the decade elucidating the stable salinity of the western arctic cold halocline despite this freshwater flux variability strongly driven by transport exceeds variability in other arctic freshwater sources remote data winds sst prove insufficient for quantifying variability indicating interannual change can still only be assessed by in situ year round measurements

sources of stream sediments in the granulite terrain of the walawe ganga basin sri lanka indicated by rare earth element geochemistry abstract thirty eight samples of stream sediments draining high grade metamorphic rocks in the walawe ganga river basin sri lanka were analysed for their ree contents together with samples of metamorphic suites from the source region the metamorphic rocks are enriched in light ree lree compared to heavy ree hree and are characterised by high la lu ratios and negative eu anomalies the chondrite normalised patterns for these granulite grade rocks are similar to that of the average post archaean upper crust but they are slightly enriched with la and ce the ree contents of the n sm n and lower la n lu n ratios relative to other fractions the lower la n lu n ratio is related to the depletion of heavy minerals in the

deformation induced garnet zoning compositional zoning patterns in garnet porphyroblasts from kyanite bearing samples of the devonian littleton formation north central massachusetts reveal complex patterns of growth that are related to multiple deformation and metamorphic events garnet porphyroblasts exhibit asymmetrical and irregular zoning patterns in xmn xca and fe fe mg zoning reversals in mn and fe fe mg and patch distribution in ca appear to occur around the boundaries of the textural zones also the compositions of the garnet at the textural boundaries are variable for all traverses these observations suggest that the garnet zoning was not only modified from diffusion processes but was also influenced by pre existing microfabrics through the effects of preferential dissolution and resorption in partial disequilibrium relationships between chemical and textural truncations indicate that the zoning patterns of garnet were strongly modified from preferential dissolution and precipitation during the development of successive foliations that occurred in zones of high strain stress cleavage seams and zones of low strain stress respectively

secondary porosity in sandstones abstract secondary porosity in sandstones is caused by dissolution and fracturing and is common in the sedimentary record secondary porosity commonly develops in the deep subsurface and thus provides an opportunity to extend exploration to depths traditionally considered unsuitable for exploration two contrasting routes of diagenesis exist in nature porosity reduction and porosity enhancement porosity reduction is commonly caused by compaction and cementation whereas porosity enhancement is primarily caused by dissolution of carbonate minerals the dissolution of carbonate minerals in the deep subsurface can be attributed to release of carbon dioxide during thermal maturation of kerogen clay carbonate reaction and flushing of undersaturated meteoric water from erosional unconformity are equally important mechanisms in generating secondary porosity at various depths two basic types of primary pores intergranular and intragranular and four basic types of secondary pores grain fractures rock fractures intergranular and intragranular can be differentiated on the basis of 1 position of pores 2 timing of origin and 3 processes of origin the proposed classification system is useful in inferring reservoir quality various types of secondary porosity are recognized using a comprehensive set of 20 criteria the various criteria are based on manner of breakage pore geometry grain geometry products of dissolution and sediment packing examples of secondary porosity gathered in part from investigation of 2 000 thin sections will serve as a guide for recognizing secondary porosity in outcrop in hand specimen under the scanning electron microscope and most importantly under the petrographic microscope new evidence suggests that silicate minerals including quartz dissolve more commonly than have been reported the abundant occurrence of secondary porosity in reservoir sandstones emphasizes the importance of secondary porosity in evaluating deep reservoirs since the first recognition of secondary porosity by nutting in 1934 chepikov in 1959 and 1961 not only developed the first set of criteria for recognition of secondary porosity but also introduced the concept in which development of secondary porosity is related to arrival of oil end of article last page 547

formation of metamorphic and metamorphosed garnets in the low t uhp metagranite during continental collision in the dabie orogen abstract a combined study of major and trace elements in garnet was carried out for low t uhp metagranite in the dabie orogen the results show different fashions of element zoning in the garnet suggesting occurrence of both metamorphic and metamorphosed garnets three generations of garnet growth are distinguished on the basis of the assumption that ca contents mn contents and fe mg ratios of garnet in metagranites are a function of pressure and temperature the first generation of garnet grt i occurs in the core of a skeletal garnet showing homogeneously low x grs values and fe mg ratios but high x sps values this implies that the core garnet grew at the highest temperature but the lowest pressure representing a metamorphosed residue of magmatic garnet in protolith granite the second generation of garnet grt ii occurs in the mantles and cores of many garnet grains exhibiting increased x grs values but decreased fe mg ratios this is ascribed to a continuous increase in temperature and pressure till the peak pressure corresponding to metamorphic growth or overgrowth during the prograde subduction the third generation of garnet grt iii occurs in the rims of all garnet grains displaying decreased x grs values and fe mg ratios in response to a pressure decrease but a temperature increase till the peak temperature these rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation grt i shows steep mree hree patterns and profoundly negative eu anoalies consistent with growth from granitic melt this kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements such as ree rb ba sr pb th u nb and ta despite the low t uhp metamorphism grt ii and grt iii in one sample exhibit steep mree hree patterns with a continuous decrease in ree contents this suggests their growth from the almost same matrix of mineral assemblages plagioclase k feldspar muscovite during metamorphism however grt ii in the other sample displays flat to steep mree hree patterns with an increase of ree contents from core to mantle this implies that the matrix of mineral assemblages for grt ii changes from common rock forming minerals e g feldspar muscovite and biotite to ree rich minerals e g epidote allanite zircon amphibole apatite and titanite grt iii in the two samples all displays significantly lowered ree contents compared to those of grt i and grt ii this may be due to a decrease in pressure and an increase in temperature during the initial exhumation therefore the two stage growth of metamorphic garnet is evident in the low t uhp metagranite during the continental subduction zone metamorphism in addition the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims this suggests that the peak pressure p max did not occur contemporarily at the peak temperature t max corresponding to the hot exhumation

topographic waves over the continental slope abstract current meter data taken during a one year period over the continental slope and upper rise in three cross isobath sections have been examined for energy distribution coherence and phase propagation of topographic waves a peak at 15 days is present in the energy preserving spectrum of the near bottom currents on the rise and slope phase propagation is offshore and little energy is found in reflected waves these results are consistent with earlier findings on the lower rise at site d onshore energy flux associated with topographic waves is deflected by the continental slope and wave energy propagates along isobaths on the lower slope and upper rise the along isobath coherence scale is about 200 km the waves are probably generated by meanders in the gulf stream

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

multistage growth of garnet in ultrahigh pressure eclogite during continental collision in the dabie orogen constrained by trace elements and u pb ages abstract major and trace elements for garnet and other minerals and u pb ages and trace elements for zircon as inclusions in garnet or as matrix were determined on thin sections of uhp eclogite from the western part of the dabie orogen the results provide temporal constraints on growth of garnet during continental subduction zone metamorphism major elements show homogeneous distribution across garnet grains with weak or no zonation of ca and almost flat distribution patterns of mg and fe however their ree compositions display two groups of patterns relative to chondrite group i garnet is characterized by steep patterns from lree to hree whereas group ii garnet shows enrichment of mree relative to lree and hree these two groups of garnet also have different parageneses of mineral inclusions group i garnet always contains abundant inclusions of amphibole epidote allanite rutile plagioclase quartz zircon whereas group ii garnet occasionally contains inclusions of omphacite rutile apatite zircon la icpms u pb dating and trace element analysis of the zircon provide direct constraints on the growth time of the two groups of garnet four types of zircon were identified based on their cl images ree distributions and u pb ages types i and ii zircons are metamorphosed after protolith magmatic zircon with type i being modified by solid state recrystallization and type ii by dissolution recrystallization types iii and iv zircons are metamorphic and grew in the stability field of garnet they gave consistent 206pb 238u ages of 239 6 ma and 222 4 ma respectively in association with groups i and ii garnets this provides a temporal constraint on the two stage growth of garnet respectively predating and postdating the peak uhp metamorphic event therefore we conclude that there are two stages of zircon and garnet growth during continental collision which occurred during the final subduction from hp to uhp phase and the early exhumation from uhp to hp eclogite facies respectively

significance of secondary porosity in interpreting sandstone composition secondary porosity resulting from dissolution of unstable framework grains such as feldspar and rock fragments including chert is common in sandstones extensive dissolution of framework grains may result in misinterpreting the original composition of a sandstone and hence in misinterpreting its provenance to avoid these problems secondary porosity caused by grain dissolution must be recognized sandstone composition may be properly evaluated by including the dissolved portion of a framework grain as a grain rather than as porosity while point counting this should be useful in interpreting original composition of sandstones and in paleogeographic reconstruction of prospect areas

trace element fractionation during high grade metamorphism and crustal melting constraints from ion microprobe data of metapelitic migmatitic and igneous garnets and implications for sm nd garnet chronology abstract rare earth element ree and other trace element y sr ti cr v na abundances in garnet from a garnet bearing metapelite a pelitic migmatite a syn tectonic granite and a post tectonic leucogranite were measured by secondary ion mass spectrometry sims in order to identify the effective variables on the trace element distribution between garnet and the host rock garnet from the garnet bearing metapelite the pelitic migmatite and the syn tectonic granite is zoned with respect to ree the cores are enriched by a factor of 2 3 relative to the rims for the garnets from the garnet bearing metapelite equilibrium distribution following a simple rayleigh fractionation is responsible for the decreasing concentrations in ree from core to rim garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for ti v cr and ree from core to rim these features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open system behaviour in these rocks at a time when the garnet originally nucleated in the metamorphic environment was incorporated into the melt the garnet from the syn tectonic granite shows stepwise decreasing concentrations in ree from core to rim a ree rich core can be distinguished from a broad ree depleted rim notably from core to rim an inflection of the yb er and yb dy ratios is visible whereas the decrease of hree abundance in the core region of the garnet from the syn tectonic granite may arise from equilibrium partitioning during garnet growth the inflection can be interpreted as a result of partial melting garnet cores with high yb er and yb dy 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower yb er and yb dy

evolution and dynamics of the flow through herald canyon in the western chukchi sea abstract the flow of summer and winter pacific water masses through herald canyon is investigated using data from a high resolution hydrographic velocity survey conducted in summer 2004 the survey was part of the russian american long term census of the arctic rusalca program and consisted of four cross canyon transects occupied over a 2 day period at the time of the survey dense winter water was entering the western side of the canyon from the chukchi sea flowing alongside a poleward jet of summer water on the canyon s eastern flank as the dense water progressed northward it switched sides of the canyon and underwent a sudden increase in layer thickness this coincided with vertical mixing near the interface of the winter and summer water producing a new water mass mode exiting the canyon all of these features are consistent with the notion of hydraulic activity occurring in the canyon a three layer hydraulic theory is applied to the flow which suggests that it is supercritical and that hydraulic control is likely a lock exchange formulation accurately predicts the northward transport of the winter water the origin of the winter water and the manner in which it drains into the canyon is investigated using satellite ice concentration data atmospheric re analysis fields historical in situ data and a simple circulation model finally the fate of the pacific water exiting the canyon and its connection to the chukchi shelfbreak current is discussed

zoned cretaceous and cenozoic garnet and the timing of high grade metamorphism southern alps new zealand new garnet sm nd and lu hf and existing rb sr 40ar 39ar u pb and sm nd ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the alpine schist belt and southern alps orogen new zealand are diachronous and partly cenozoic in age the dominant metamorphic isograds in the alpine schist formed during crustal thickening at a previously unsuspected time at c 86 ma immediately prior to the opening of the tasman sea at c 84 82 ma obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism and fabric elements can be highly diachronous a detailed timing history for the growth of a single garnet is recorded by a sm nd garnet whole rock age of 97 8 8 1 ma for the inmost garnet core zone 1 lu hf ages of 86 2 0 2 ma and 86 3 0 2 ma for overgrowth zones 2 and 3 a step leach sm nd age of 12 37 ma for zone 4 and growth of the garnet rim zone 5 over the alpine fault mylonite foliation during the modern phase of oblique collision that began at c 5 6 ma plate convergence along the new zealand portion of the gondwana margin continued after c 105 ma almost certainly culminating in the oblique collision of a large oceanic plateau hikurangi plateau the metamorphism of the alpine schist at c 86 ma is evidence of that hit the mid to late cretaceous extension that is widespread elsewhere in the new zealand region is attributed to upper plate extension and slab roll back the effects of the collision with the hikurangi plateau may have contributed to the changing plate motions in the region leading up to the opening of the tasman sea at c 82 ma

secondary porosity in sandstones basic contributions of chepikov and savkevich the concept of secondary porosity in sandstones porosity caused primarily by dissolution has gained considerable attention recently the basic contributions made by chepikov and savkevich however have been ignored completely in north american publications chepikov et al not only developed the first set of criteria for recognizing secondary porosity but also attributed the dissolution of cements to the arrival of oil savkevich was the first to illustrate the trend of increasing porosity at considerable depths

anatexis during high pressure crustal metamorphism evidence from garnet whole rock ree relationships and zircon rutile ti zr thermometry in leucogranulites from the bohemian massif in situ mineral garnet zircon trace element and garnet rock ree distribution data obtained on leucogranulites from the bohemian massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration melting of muscovite bearing protoliths at high pressures during the variscan orogeny partial melting at p t conditions of 900 9408c and 1 6 1 8 gpa consumed mica and feldspar to produce peritectic kyanite and high ca 20 24 mol grossular high eu garnet along with leucogranitic melt dree grt rock values calculated from this garnet and its host rock are consistent with garnet melt equilibrium at these p t conditions ternary feldspar and rutile 2000 3200 ppm zr crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880 9208c these formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites this garnet is typified by strong depletion in eu and dree grt rock values consistent with garnet melt equilibrium only after the fractionation of apatite 50 3 wt and ternary feldspar 5 15 wt zircons in one leucogranulite are dominated by oscillatory zoned grains that yield pre variscan u pb ages 495 and 433 ma and crystallized from ti in zircon thermometry at 700 8108c in their original host magmas these xenocrystic grains are potentially preserved because of the short timespan of the high p t variscan event at 340 ma and the moderate amounts of leucogranitic partial melt present in the rocks estimated as at least 30 wt melt from zr mass balance the production of new zircon associated with the variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy ree patterns that approach ree equilibrium with garnet rims this post peak zircon was formed at minimum p t conditions of 8408c and 0 8 1 0 gpa along the decompression cooling path defined by previous work for the bohemian massif granulites the garnet whole rock ree relationships observation of eu depletion between early and later garnet growth phases and zr ti thermometric estimates indicating temperatures of48408c for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre variscan low p granitoids but support a model in which the bohemian leucogranulites were high p variscan age migmatites containing significant amounts of melt

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

a comparison of microstructural and chemical patterns in garnet from the fleur de lys supergroup newfoundland this study uses compositional analyses of garnet porphyroblasts to test a previously published microstructure based model of garnet growth in the fleur de lys supergroup x ray maps reveal significant compositional anomalies within garnet including zoning reversals and steepened compositional gradients these anomalies occur at the margin of the proposed first stage of garnet growth g1 and coincide with truncations of inclusion trails and changes in the inclusion assemblage intervals of reversed composition zoning and steepened compositional gradients across this boundary are interpreted to represent a hiatus in garnet growth possibly accompanied by garnet consumption during which changes in the garnet forming reaction p t conditions and deformation kinematics occurred the junction of the proposed second and third stages of garnet growth g2 and g3 coincides with the transition between successive crenulation cleavages without substantial microstructural truncations or changes in the inclusion assemblage the g2 g3 boundary is generally marked by uninterrupted normal zoning with subtle compositional anomalies in some samples this boundary may in fact record continuous garnet growth or alternatively mark a relatively short intra orogenic pause in garnet growth with minimal affect on zoning patterns individual porphyroblasts with contrasting inclusion trail microstructures also have different zoning patterns and this supports the previous recognition of contrasting growth histories between individual porphyroblasts a combined structural metamorphic model is presented that integrates the timing of garnet growth and foliation development reaction history and the p t t path in the fleur de lys supergroup