sequential porphyroblast growth during deformation in a low pressure metamorphic terrain orrs island harpswell neck maine poikiloblastic index minerals in pelitic rocks from the orrs island harpswell neck area of coastal maine contain inclusion textures that indicate sequential growth of progressively higher grade metamorphic minerals during development of a near vertical crenulation foliation the sequence of zones in the field is garnet staurolite staurolite andalusite staurolite sillimanite and sillimanite inclusion fabrics characteristic of different stages in crenulation cleavage development indicate that index minerals nucleated and grew sequentially biotite began to grow before deformation garnet began to grow during early stages of crenulation cleavage development staurolite grew during intermediate stages and andalusite grew relatively late when transposition of the foliation was nearly complete muscovite pseudomorphs and sillimanite were mainly post kinematic the fact that metamorphic index minerals grew sequentially in individual rocks in the same order in which they appear across the field area indicates that the high temperature part of the pressure temperature path was similar to the metamorphic field gradient metamorphism in the orrs island harpswell neck area is consistent with the magmatic heating model that has been proposed for western maine sequential development of index minerals in pelitic rocks in the orrs island harpswell neck area apparently resulted from sequential nucleation after substantial overstepping of mineral forming reactions once nucleation of an index mineral had taken place initial growth was rapid and poikiloblasts preserved inclusion trails characteristic of the prevailing stage of crenulation cleavage development because nucleation of sillimanite may have required more overstepping of the andalusite sillimanite reaction than nucleation at dehydration reactions determination of metamorphic conditions for rapidly heated rocks such as these by comparison with a petrogenetic grid is problematic garnet zoning patterns in these rocks should reflect the fact that growth of garnet interiors occurred early during metamorphism in equilibrium with a low grade assemblage only garnet rims would be expected to record the subsequent pressure temperature path

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

three dimensional imaging of garnet porphyroblast sizes and chemical zoning nucleation and growth history in the garnet zone the three dimensional 3 d growth history of two garnet zone samples grt chl bt ms pl qtz ilm from southwestern maine was examined by serial sectioning and 3 d reconstructions of compositional zoning from backscatter images and x ray maps mn fe mg and ca zoning is broadly concentric the concentration of mn in garnet cores generally correlates with size d 50 to 750 micrometers indicating progressive nucleation in detail all elements show irregular patchy zoning in the cores assuming constancy of mn on the rims of all garnet crystals in a rock volume plus no subsequent diffusional modification mn concentration can be used as a time line for garnet growth examination of the evolution of individual garnet crystals reveals that multiple nuclei formed simultaneously in the core regions and that nuclei expanded by growth in amoeba shape forms along preexisting mineral grain boundaries primarily quartz and plagioclase dissolving the interior grains until the grains were either gone or encapsulated at which time dissolution ceased amoeba shaped garnet crystals coalesced as they grew and simultaneously new nuclei appeared in the nearby matrix the net result was a single garnet porphyroblast that formed by the growth and coalescence of multiple nuclei radius rate plots constructed by counting pixels in 2 d images reveal that crystals grew at the same radial rate regardless of size the observation of continuous nucleation in the vicinity of preexisting crystals plus the radius rate plots rule out diffusion over length scales on the order of the garnet radius or greater as the rate limiting step and are consistent with either diffusion over shorter length scales or interface control as rate limiting to garnet growth the strong clustering of garnet nuclei requires clustering of favorable nucleation sites which may have been caused by favorable orientation of garnet precursor minerals i e chlorite quartz

reconstruction of currents in the mid ordovician early silurian central baltic basin using geochemical and mineralogical indicators in the central part of the mid ordovician early silurian baltic basin two different transport pathways of terrigenous material can be recognized kaolinite indicates the south to north sediment influx and cr the western and northern sources a pathway from the ukrainian shield in the south was active in the floian darriwilian ordovician and in the llandovery silurian the influx from the western source was active in the sandbyan and influx from the northern side occurred in the katyan the sediment transport was carried out by water flows which turned into rapid currents from time to time these currents coming from the ocean surface waters were oxygenated facilitating red facies formation in the central baltic basin currents on the shelf were linked to the adjacent oceanic currents which changed in course with the drift of baltica from temperate to subtropical latitudes

an overgrowth model to explain multiple dispersed high mn regions in the cores of garnet porphyroblasts the central portions of garnet porphyroblasts from harpswell neck maine exhibit small discrete regions of high mn content dispersed in three dimensions orientation contrast imaging oci and quantitative crystallographic orientation measurements from electron backscatter diffraction ebsd patterns show that most garnet porphyroblasts contain no variations in crystallographic orientation it is difficult to reconcile these data with a model in which each high mn region represents a discrete garnet nucleus an alternative hypothesis invokes a model of garnet growth in which precursor phases rich in mn are overgrown and their mn is incorporated locally into the garnet structure although this model requires length scales for equilibration of mn during the early stages of garnet growth that are shorter than any previously documented that inference is consistent with the very low temperatures of garnet nucleation in these rocks and with compositional evidence for progressively larger scales of equilibrium as reaction progressed very short length scales for equilibration of mn identified in this study for the lowest grade portion of the history of the rocks call into question the common assumption of rock wide equilibrium for mn during garnet growth at low temperature the absence of rock wide equilibrium for mn at low grade has potential negative implications for all common interpretive methods based in phase equilibria including thermobarometric studies and thermodynamic analysis of the origin and meaning of zoning in low grade garnet

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

neodymium isotopic composition of cambrian ordovician biogenic apatite in the baltoscandian basin implications for palaeogeographical evolution and patterns of biodiversity biogenic apatite preserved in 148 samples of conodonts and organophosphatic shelled brachiopods from cambrian through ordovician successions of the baltoscandian basin baltica plate preserves a sensitive record of early palaeozoic sea water chemistry interpreted via neodymium isotope ratios consistent e nd t values of 9 6 to 8 3 for lower to middle cambrian samples suggest no significant lateral or temporal variation across the region average upper cambrian values are 7 2 to 7 7 sedimentary analysis suggests that the influence of continental weathering from baltica as a major source of radiogenic nd was negligible ordovician samples show a rise to 5 to 6 in the early arenig early mid llanvirn and late caradoc sea water mixing from the southeast iapetus ocean was a constant factor throughout cambrian ordovician times the rise reflects erosion of obducted volcanic arc complexes along the caledonian margin and probably also relates to pollution of the baltica sector of iapetus from the approaching avalonia plate patterns of evolutionary biodiversity and palaeobiogeographical linkages support the geochemical signatures in interpreting the tectonic history of the region extinction of lingulate brachiopod faunas in the tremadoc followed by subsequent recovery and emergence of benthic assemblages typical of the ordovician evolutionary fauna in the billingen early volkhov regional stages coincide with significant changes in geochemical characteristics of water masses across the baltoscandian basin the early and mid ordovician arenig to llandeilo brachiopod faunas of the north estonian confacies belt are characterized by high endemism and low turnover rates whereas increased immigration resulted in the extinction of a number of local lineages in the late llanvirn from the mid caradoc to mid ashgill when baltica was drifting on a course to collide eventually with avalonia and gradually approach laurentia brachiopod assemblages were characterized by higher turnover rates at the same time they gradually became more cosmopolitan and less influenced by the invasion of new faunas

sub millimeter heterogeneity of yttrium and chromium during growth of semi pelitic garnet x ray maps of yttrium and chromium in two garnets from a semipelitic schist from central nepal reveal parallel spiraling bands of high and low concentration 10 30 m wide this zoning mimics similar concentration bands defined by foliation in the matrix outside the garnets the garnets apparently incorporated the matrix zoning intact during growth at conditions up to lower amphibolite facies these observations and interpretations indicate that yand cr concentrations remained heterogeneous at the scale of tens of micrometers during breakdown of matrix minerals and growth of garnet this conclusion extends results of previous work which showed that y and cr do not homogenize at the centimeter to millimeter scale during metamorphism to amphibolite facies the width of the concentration bands in the garnets places a constraint on the maximum effective intracrystalline diffusion coefficient for both y and cr in garnet of about 10 21 cm 2 s for peak metamorphic conditions of about 9 kbar at 5708c different concentrations of yand cr at different positions around the rim of each garnet indicate disequilibrium for these elements between garnet and matrix phases the presence of an internal foliation defined by the yand cr concentration bands rules out pre kinematic garnet growth most useful in conjunction with other indicators zoning of these elements in garnet is thus a tool for deciphering the relative timing of deformation and garnet growth

al2o3 tio2 ratio of the clay fraction of late ordovician silurian carbonate rocks as an indicator of paleoclimate of the fennoscandian shield abstract in the fennoscandian shield the fluctuations in arid humid climate conditions are inferred from the al2o3 tio2 ratio of the clay fraction of estonian carbonate rocks of the baltoscandian sedimentary basin the late ordovician climate of the fennoscandian shield was mainly semi humid with several humid pulses in the sandbian mid katian and hirnantian the post hirnantian climate was fluctuating between humid and arid until stable humid conditions were established in the middle and early late llandovery starting from the latest llandovery the climate turned semi arid and arid the ordovician silurian long term trend toward aridity was caused by the equator ward drift of the baltica plate shorter term fluctuations of al2o3 tio2 reflect climate changes of the global extent

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

crystal plasticity of natural garnet new microstructural evidence scanning electron microscope sem orientation contrast images of mantle nodules show that garnets contain cellular domains of different crystallographic orientation electron backscatter diffraction shows small crystallographic mismatches

marine redox variability from baltica during extinction events in the latest ordovician early silurian abstract it is well documented that upper ordovician and silurian successions record multiple marine turnover events including the second largest mass extinction in the phanerozoic widespread glaciation and multiple global carbon cycle perturbations whereas causal mechanisms for the late ordovician major mass extinction event involving climate paleoceanographic variation and 13c records have been published similar records remain poorly constrained for subsequent extinction events in the early silurian here we present new organic matter carbon isotope 13corg chemostratigraphy and corresponding paleoredox proxies fe speciation mn v mo and pyrite sulfur isotopes from two organic rich drill core sections in sweden and latvia that span the upper katian through lower wenlock stages 446 431 ma pyritized fe and bulk sedimentary mn concentrations from upper ordovician strata in southern sweden suggest a local redox shift to more reducing conditions in the late hirnantian possibly including euxinic anoxic and sulfidic water column conditions that coincide with the second mass extinction pulse the new high resolution 13corg and 34spyr datasets from the late aeronian early silurian interval within both drill cores show positive excursions that are broadly coincident with the associated sandvika and sedgwickii extinction events independently fe speciation and bulk sedimentary trace metal data from this late aeronian interval record locally euxinic conditions in both the deep basinal sweden and distal shelf latvia settings before and during the late aeronian positive 13c excursion this multiproxy paleoredox dataset provides the first direct evidence for local to regional expansion of reducing marine conditions coincident with this early silurian late aeronian biotic event and positive 13c excursion additionally new 34spyr values spanning the llandovery wenlock boundary interval in the latvia core show a positive excursion coincident with fe speciation and trace metal enrichments that imply a local redox perturbation with intermittently euxinic bottom waters during the rising limb of the ireviken positive 13c excursion the combination of these geochemical data for local to regional scale more data required for global interpretations changes in marine redox conditions with paleobiological records and evidence for eustatic sea level rise indicate that environmental stresses related to an expansion of anoxic to euxinic conditions were a probable driver for several extinction events during the latest ordovician early silurian

three dimensional patterns of garnet nucleation and growth three dimensional compositional zoning patterns in garnet zone garnets from harpswell neck maine united states suggest complex patterns of nucleation and growth compositional zoning is often asymmetric and core compositions do not always coincide with the geometric center of the garnet in detail garnet zoning patterns show multiple isolated high mno areas that we interpret as sites of garnet nucleation if it is assumed that garnet growth is close to equilibrium and the mn zoning is not significantly modified by diffusion then the mno contours or isosurfaces can serve as a proxy for patterns of garnet nucleation and growth computer generated three dimensional animations of these patchwork mno zoning patterns reveal that many garnets contain multiple nuclei that grew and coalesced to form a single crystal new nuclei often appeared next to preexisting porphyroblasts and became incorporated into the growing crystal contrary to the widely held idea that garnets grow radially outward from a single nucleus we propose that many of the garnets examined in this study grew by the coalescence of multiple nuclei

fast diffusion along mobile grain boundaries in calcite experimental measurements of grain boundary diffusion are usually conducted on static boundaries despite the fact that grain boundaries deep in the earth are frequently mobile in order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44ca from a layer of 44ca enriched calcite powder during the static recrystallization of a single crystal of calcite at 900 c a region about 500 m wide adjacent to the powder layer is heterogeneously enriched in 44ca and complex zoning patterns including sharp steps in composition and continuous increases and decreases in 44ca content are developed in metamorphic rocks these would normally be interpreted in terms of changes in pressure or temperature rayleigh fractionation or episodic fluid infiltration these explanations cannot apply to our experiments and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate we have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width dgb to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer the value of dgb in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions in order to allow the scale of chemical equilibrium and hence textural evolution to be predicted under both experimental and geological conditions we present quantitative diffusion regime maps for static and mobile boundaries in calcite using both published values and our new values for grain boundary diffusion in mobile boundaries enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of earth materials for geochronology and for interpretations of the length and time scales of chemical mass transport moreover zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones rather than regions of anomalous mineralogy water or melt content as is generally suggested

palaeontological and isotope evidence for warm saline deep waters in ordovician oceans today s oceans are characterized by cold bottom waters produced at high latitudes here we present morphological and oxygen isotope data for brachiopods from the caradocian upper ordovician trenton group in new york which indicate that both temperature and salinity increased with depth in ordovician oceans production of warm saline deep waters in low to mid latitude evaporative seas and reduced production of cold deep water may have been favoured by high sea level warm global climate and fortuitous positioning of continents1 5 the resulting ocean circulation system was markedly different from that of today lower oxygen contents of warm saline deep waters may have contributed to the deposition of organic rich sediments which are abundant in caradocian strata6 7but rare in modern ocean sediments

microfacies analysis of the lower middle ordovician succession at xiangshuidong southwestern hubei province and the drowning and shelf ramp transition of a carbonate platform in the yangtze region abstract during the early middle ordovician the yangtze platform in south china was drowned as indicated by the replacement of pure grey grainstones and packstones of the hunghuayuan formation by purple red argillaceous limestone of the zitai formation and correlative units which are mixed siliciclastic and carbonate deposits the xiangshuidong section located in songzi city southwestern hubei province was selected to study the process in detail cluster analysis of point count groups of the hunghuayuan and the zitai formations indicates that the former contains six microfacies mf 1 peloidal grainstone mf 2 thrombolitic grainstone mf 3 oolitic grainstone mf 4 bioclastic grainstone mf 5 lithoclastic grainstone mf 6 interreef bioclastic grain packstone indicating a shelf setting before drowning while the latter contains seven microfacies mf 7 bioclastic grainstone mf 8 bioclastic grain to packstone mf 9 bioclastic packstone mf 10 bioclastic wackestone mf 11 argillaceous bioclastic wackestone mf 12 open marine bioclastic wackestone mf 13 calcimudstone indicating the ramp setting during and after the drowning the drowning was gradual and fluctuating starting as a marginal shoal through the inner ramp and becoming a stable ramp setting this drowning is mainly attributed to a global sea level rise and an increase of terrigenous supply that resulted from local tectonic movement i e the initiation of collision between the south china and cathaysia blocks after drowning red offshore marine facies were well developed along the platform margin indicating an oxic sea bottom environment during the first acme of gobe in south china the changes in environments associated with this transition may have provided new evolutionary opportunities that contributed to the middle ordovician diversification

charge contrast imaging of fine scale microstructure and compositional variation in garnet using the environmental scanning electron microscope gaseous secondary electron gse imaging of eclogite garnets under the environmental scanning electron microscope esem at low chamber gas pressures reveals detailed image contrast patterns charge contrast images cci that are not present in back scattered or secondary electron images image intensity is a function of the amount of surface charge accumulation successful acquisition of cci depends on frame size and beam scan rate at a given chamber gas pressure and beam current images are obtained in a few seconds and are stable and reproducible cci patterns do not correlate with cracks or grain boundaries but do correspond closely to variations in major element composition both in the form of concentric growth zoning and branching linear features interpreted as cracks that have been healed by new garnet growth causes of cci are not yet well understood but may be related to variations in lattice defect density and their influence on charge trapping and dissipation these in turn influence the rate of charge build up at or very close to the specimen surface one interesting possibility is that cci images detect vacancies related to non homovalent coupled substitutions involving for example ree and hydroxyl so the method offers a way of imaging the distribution of these trace species in garnets the cci images are rich in microstructural detail and offer the potential for rapid high resolution low noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets

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

ordovician reef and mound evolution the baltoscandian picture the widespread growth of reefs formed by a framework of biogenic constructors and frame lacking carbonate mounds began on baltica during ordovician time previously ordovician reef and mound development on baltica was considered to be sporadic and local a review of all known bioherm localities across the baltic basin reveals a more consistent pattern ordovician bioherms grew in a wide e w aligned belt across the baltic basin and occur in several places in norway substantial reef development began simultaneously across the region during the late sandbian early katian interval and climaxed during the late katian pirgu age the current spatiotemporal distribution of bioherms is a result of interdependent factors that involve original drivers of reef development such as relative sea level climate during the time of deposition and effects of post depositional erosion oceanographic conditions were likely more favourable during times of cooler global climates low sea level and glacial episodes at the same time the likelihood that bioherms are preserved from long term erosion is higher when deposited during low sea level in deeper parts of the basin a main factor controlling the timing of the reef and mound evolution was baltica s shift toward palaeotropical latitudes during late ordovician time the time equivalence between initial reef growth and the guttenberg isotope carbon excursion gice suggests that global climatic conditions were important

the origin of mn and y annuli in garnet and the thermal dependence of p in garnet and y in apatite in calc pelite and pelite gagnon terrane western labrador x ray compositional maps and zoning profiles for major and trace elements have been measured in garnet apatite and epidote group minerals from calc pelitic and pelitic schists from western labrador by electron microprobe and by laser ablation icp ms high y abundance in apatite adjacent to resorbed garnet and a decreasing modal abundance of apatite with garnet growth indicate that apatite participates in major rock forming mineral reactions phosphorus concentration in garnet porphyroblasts coexisting with apatite decreases smoothly from core to rim and depends moderately on metamorphic grade apatite coexisting with xenotime shows high y cores and the concentration of y in apatite increases with metamorphic grade many of the analyzed garnet porphyroblasts exhibit y annuli which provide information about different physicochemical processes operating during garnet growth we describe criteria to fingerprint three of these processes in the analyzed garnets 1 yttrium annuli produced by garnet resorption and regrowth are characterized by an asymmetric shape with a steeper slope on the inner side of the annulus and by a decoupled variation between garnet compatible and garnet incompatible elements at the resorption margin 2 y annuli produced by breakdown of y and ree rich trace phases such as xenotime monazite apatite and epidote group minerals are indicated by ree peaks in lree monazite allanite mree epidote or hree xenotime zircon 3 y annuli associated with diffusion controlled growth occur in garnet with oscillatory textural zoning defined by alternating inclusion rich and inclusion poor zones with increases in concentrations of garnet compatible elements such as y and hree correlating with inclusion free zones mn annuli in some garnets were produced by sporadic local breakdown of mn rich epidote and in several cases indicate that thin section scale equilibrium during annulus formation was not achieved raising questions concerning the use of mn concentration as a time marker in garnet growth modeling

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

carbon isotope event markers through the wenlock pridoli sequence at ohesaare estonia and priekule latvia abstract the ohesaare borehole in the northern part of the silurian baltic gulf penetrates shallow and deep shelf marlstones limestones and dolostones the priekule borehole in the deep shelf belt of the basin was drilled through marlstones and graptolitic mudstones most of the successions except the upper part are characterized by a high sr ca ratio close to marine calcites suggesting good preservation of carbonate material conodont alteration indices are low 1 2 carbon isotopes were measured using whole rock samples in the ohesaare core section two distinctive 13 c positive excursions were established one in the early wenlock riccartonensis and low rigidus ellesae standard graptolite zones 4 2 and the second in the late wenlock nassa zone 4 6 before and after the latter there are also intervals with low 13 c values the first just above the lundgreni zone 1 8 and another in the ludensis perhaps reaching into nilssoni zone 3 8 in the priekule section the early wenlock 13 c peak is well represented 3 1 but in the late wenlock early ludlow 13 c values show little variation from 2 0 to 1 in this section a high peak of 13 c values 5 9 was determined in the nova beds correlated with the topmost gorstian and lowermost ludfordian ludlow the carbon isotope positive excursions show good correlation with lows of the global sea level curve and with most of the biotic and supposed climatic events glaciation ocean state

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

oceanic circulation as an element in palaeogeographical reconstructions the arenig early ordovician as an example a new conceptual palaeo oceanographical model is outlined in this paper the model differs from previous models by using shifts in hadley circulation caused by orbital variations and the rotation rate of the earth to locate the position of the planetary oceanic low and high pressure systems around which the planetary ocean surface currents flow adapting the model to the arenig early ordovician the temperate low pressure zones were found to be located at 50 latitude and the subtropical high pressure zones at 25 latitude traditionally most palaeozoic palaeogeographical recon structions are reconstructed using palaeomagnetic data supplemented with data from climate sensitive lithofacies and palaeo biogeographical distributions however as a new approach in palaeogeographical reconstructions the con ceptual palaeo oceanographical model is combined with palaeobiogeographical data for the arenig series comple menting the palaeomagnetic data and resulting in a new refined palaeogeographical reconstruction

accretionary mechanisms and temporal sequence of formation of the boda limestone mud mounds upper ordovician siljan district sweden abstract late ordovician mud mounds in the boda limestone siljan area central sweden consist mainly of massive spiculitic stromatactis limestone similar to other well known paleozoic mud mounds the mound cores are predominantly formed by spiculite and ostracod rich mudstone and or wackestone and networks of spar filled syndepositional cavities the mounds of the boda limestone are specific in two ways 1 large parts of the lower mound cores are formed by a palaeoporella framestone and or bafflestone with signs of early recrystallization and cementation and 2 the stromatactis facies is accompanied by masses of limestone with non stromatactis cryptocrystalline patches cryptocrystalline patches are filled with massive fibrous low mg calcite have cloudy transitional margins contain randomly spaced skeletal grains and dolomitic micro inclusions and are interpreted as recrystallized patches of the original host lime mud models of formation of paleozoic mud mounds generally hypothesize accretionary processes that are based on sponge biomass degradation and associated diagenetic processes in the boda limestone palaeoporella skeleton disintegration early recrystallization and additionally cementation were essential for the mound formation the boda mounds exemplify that formation of paleozoic mud mounds reflects a spectrum of different essentially diagenetic accretionary mechanisms this diversity should be taken into account in future discussions about paleozoic mud mound formation the new name osmundsberget formation is given for a hirnantian succession of brachiopod rich limestone above the boda limestone the osmundsberget formation occurs only in direct geographical vicinity to the mud mounds

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

delineation of a complexly dipping temperate glacier bed using short pulse radar arrays we have defined the complex bed topography for a section of a small temperate glacier using 50 mhz monostatic short pulse radar data and a synthetic aperture array processing method the data were collected on a 100 m by 340 m array grid in the upper stem of gulkana glacier central alaska u s a the array processing was based on a modified three dimensional 3 d kirchhoff migration integral and implemented with a synthetic aperture approach that uses sequences of overlapping sub arrays to generate depth images in vertical planes typical sub array beam patterns are generally 5 at the 6 db level giving a flashlight like searching capability without distorting the wavelet shape the bed topography was constructed using normal reflections picked from 3 d array depth images in some instances reflections were imaged outside the data coverage area the bed surface dips steeply both parallel and transverse to the direction of ice flow the maximum observed depth is roughly 140 m the 3 d method resolved bed dips up to 45 in regions of steepest dip it improved depth accuracy by 36 compared with raw data and by 15 compared with standard two dimensional 2 d migration over 12 db of signal to noise improvement and improved spatial resolution was achieved compared to raw data and 2 d migration false bottom layering seen in the raw data and in 2 d migrations is not observed in the 3 d array results furthermore loss of bottom reflections is shown by the 3 d migration to be attributable to the dip and curvature of the reflector and not scattering losses or signal clutter from englacial inclusions

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

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

an integrated epma ebsd study of metamorphic histories recorded in garnet growth histories recorded in garnet grains in metasedimentary rocks from the sanbagawa belt in japan and the mogok belt in myanmar were analyzed using an effective combination of electron backscatter diffraction ebsd and electron probe microanalysis epma data garnet in the sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote amphibolite facies stages respectively based on epma element mapping this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation the ebsd data however document that the garnet grain is composed of four domains the misorientation angles of crystallographic orientations between the domains are as large as 59 and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4 the garnet grains formed via a three step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage inner zone resorption around garnet rims and along domain boundaries during exhumation crystallization of the outer zone and in the domain boundaries during the prograde epidote amphibolite facies stage the garnet porphyroblasts in the mogok pelitic gneisses which formed during prograde metamorphism to the upper amphibolite granulite facies 0 6 1 0 gpa 780 850 c are now separated into segments of various sizes by mosaic or symplectite aggregates of biotite plagioclase and quartz or monomineralic biotite veins the segment texture formed at about 0 3 0 4 gpa 610 650 c or lower grade conditions the ebsd analysis shows that most of the segments share the same crystallographic orientation with misorientation angles less than 4 and show no evidence of deformation and or rotation processes after segmentation these data suggest that the mogok sample did not experience dynamic deformation of the garnet grains after the resorption and segmentation stage and may have been exhumed under static conditions from depths of 9 12 km

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

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

the tornquist sea and baltica avalonia docking early ordovician late arenig limestones from the sw margin of baltica scania bomholm have multicomponent magnetic signatures but high unblocking components predating folding and the corresponding palaeomagnetic pole latitude 19degreesn longitude 051degreese compares well with arenig reference poles from baltica collectively the arenig poles demonstrate a midsoutherly latitudinal position for baltica then separated from avalonia by the tomquist sea tornquist sea closure and the baltica avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for avalonia and baltica during the mid late ordovician by the caradoc avalonia had drifted to palaeolatitudes compatible with those of sw baltica and subduction beneath eastern avalonia was taking place we propose that explosive vents associated with this subduction and related to andean type magmatism in avalonia were the source for the gigantic mid caradoc c 455 ma ash fall in baltica i e the kinnekulle bentonite avalonia was located south of the subtropical high during most of the ordovician and this would have provided an optimum palaeoposition to supply baltica with large ash falls governed by westerly winds in scama we observe a persistent palaeomagnetic overprint of late ordovician ashgill age pole latitude 4degreess longitude 012degreese the remagnetisation was probably spurred by tectonic derived fluids since burial alone is inadequate to explain this remagnetisation event this is the first record of a late ordovician event in scania but it is comparable with the shelveian event in avalonia low grade metamorphism in the north sea basement of ne germany 440 450 ma and sheds new light on the baltica avalonia docking c 2002 elsevier science b v all rights reserved less