numerical simulation of diffusion controlled nucleation and growth of porphyroblasts this article presents the conceptual basis for a new numerical model of diffusion controlled nucleation and growth of porphyroblasts describes its implementation and illustrates the range of crystallization behaviours encompassed by it the model differs from previous efforts principally in its ability to track explicitly the evolution in time and space of the chemical affinity for reaction in the intergranular medium which provides a more accurate characterization of nucleation suppression in the vicinity of pre existing crystals and of growth suppression due to competition for nutrients among neighbouring crystals critical new features of these numerical simulations include maintenance of local equilibrium for fluid in contact with reactants or products persistence of reactants until they are eliminated by dissolution due to reaction progress and local diffusive flux nucleation rates that vary as the local reaction affinity evolves complex initial distributions of reactants if desired and the flexibility to encompass any porphyroblast forming reaction for which changes in free energy as a function of time and temperature are specified model results reveal that radial growth rates remain proportional to the square root of time in diffusion systems buffered by persistent reactants they document the interchangeable effects of diffusivity porosity and solubility on material fluxes and thus growth rates and they illustrate the offsetting textural effects of rates of diffusion nucleation and heating the initial distribution of reactants is found to exert a first order effect on crystal size distributions confirming their limited utility for diagnosing crystallization mechanisms these numerical simulations yield novel and rigorous confirmation of the textural effects of nucleation site saturation and variation in interconnected porosity and reproduce with high fidelity much of the textural and chemical information gathered from natural specimens

overbank sediments from central mexico an evaluation of their use in regional geochemical mapping and in studies of contamination from modern and historical mining abstract overbank sediment sequences in central mexico display vertical changes in chemistry which can be related to both anthropogenic contamination and natural geological sources and processes they also show significant lateral chemical variation at both local and regional levels which makes the design of a sampling strategy for regional geochemical mapping or contamination studies difficult this variation is particularly pronounced in drainage basins which have been contaminated by mining activity and limits the use of overbank sediment as a systematic regional geochemical mapping medium to be used with confidence overbank sediments require detailed studies of fluvial geomorphology and history accompanied by accurate age dating active drainage sediments are a more viable alternative in areas of historical mining activity but must be examined carefully in the light of the regional background where levels of contamination are low or the contaminants are buried within the river floodplain

the spatial distribution of grains and crystals in rocks characterisation and analysis of the spatial distribution pattern sdp of grains or crystals in rocks is potentially a powerful technique which can be used to constrain the processes which operate in the formation of rocks a method to quantify the sdp of grains in thin section is presented the distance betwen the centre of a grain and the centre of its nearest neighbour is calculated for all the grains in the sample area to produce a distribution of distances that characterises the spatial pattern of grains in the rock this distribution is then normalised to a random distribution of points with the same population density to give a descriptive value r values of r for rock samples are plotted against porosity modal abundance of other phases in igneous and metamorphic rocks to characterise the sdp the sdp of randomly packed distributions of equal size spheres varies systematically with porosity producing a line on a porosity versus r plot termed the random sphere distribution line rsdl rocks which plot above the rsdl have an ordered sdp and those that plot below a clustered sdp the effects of variation in grain packing order grain sorting compaction and random crystallisation overgrowth on determined r values were investigated using a combination of 3 d sphere models and 2 d texture models the maximum possible value of r is 2 148 corresponding to a perfect section through hexagonal cubic close packing of grains the minimum value of r is dependent on the proportion of grains in the sample volume and may be as low as 1 2 for a sample volume with 30 grains which are clustered variations in size sorting can cause r to vary by approximately 0 25 mechanical compaction of a loose framework of grains results in a higher packing order and an increase in r continued compaction creates a fabric in the texture and r decreases as cluster patterns are developed perpendicular to the principal stress the overgrowth of grains in a touching framework alters the location of the geometric grain centre as the crystal shape changes leading to an increase in r as porosity decreases results reveal that grain sorting compaction and grain overgrowth produce different r versus porosity relationships which can be identified in and therefore used to constrain the relative importance of these processes in rock samples

heavy metal storage in near channel sediments of the lahn river germany abstract heavy metal pollution in urban industrial and mined watersheds of europe is well documented but less is known about metal contamination in agrarian watersheds or those with no history of mining along a 75 km reach of the lahn river central germany near channel flood plain sediments

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

metamorphic rates in collisional orogeny from in situ allanite and monazite dating the prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates in individual samples from the northern lepontine central alps allanite is partly replaced by monazite at 560 580 c relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions 430 450 c coexisting monazite and allanite were dated in situ to delimit in time successive stages of the barrovian metamorphism in situ sensitive high resolution ion microprobe shrimp u th pb dating of allanite 31 5 1 3 and 29 2 1 0 ma and monazite 18 0 0 3 and 19 1 0 3 ma constrains the time elapsed between 430 450 c and 560 580 c which implies an average heating rate of 8 15 c m y combined with new fission track ages zircon 10 9 ma apatite 7 5 6 5 ma metamorphic rates of the entire orogenic cycle from prograde to final cooling can be reconstructed

heavy metal contamination of an arid river environment gruben river namibia abstract the dispersal of metalliferous pollution from several spoil heaps produced during 20th century copper mining in the gruben river valley namibia is examined the gruben river flows through an extremely arid environment with an average rainfall of 25 mm p a the potential for physical and chemical remobilisation of cu zn and ni is assessed by examining the spatial and temporal distribution of metal contaminated sediment deposited within the confines of the channel the relationships between metal content grain size geomorphic environment and the downstream distribution of metals are discussed the phase specific heavy metal concentrations of sediments collected as part of the downstream sampling programme are also examined using progressively more aggressive sequential acid extractions in addition metal concentrations are compared with dutch guidelines for soil contamination to ascertain the extent of environmental risk total metal concentrations show that the gruben valley is highly contaminated particularly with respect to cu and ni concentrations which exceed dutch target values for cu 36 ppm in 94 7 and ni 35 ppm in 90 5 of samples respectively zn concentrations are much lower with only 6 3 of the samples exceeding the target value 140 ppm as might be expected the metal sediment concentrations of cu are the most highly elevated with a peak value of 10 500 ppm being recorded from material collected from suspended sediment transported during a minor flow event that occurred in march 1999 lower energy and fine grained sedimentary environments are shown to be clearly associated with higher metal concentrations sequential extractions of metals show that of the three principal elements considered in this study only a negligible proportion of cu 0 41 is held in the exchangeable phase ni and zn were below detection limits although the total metal concentrations in the gruben river valley sediments are extremely high and are in themselves a concern the small percentage of metals held in the exchangeable phase and the low potential for remobilisation under the arid conditions would suggest that they pose only a minor risk to the environment

a geochemical record of flooding on the upper river severn uk during the last 3750 years abstract projected future changes in climate call for more definite information on the magnitude frequency relationships of major floods than can be obtained from short instrumental or historical records alone floodplains represent a sedimentary archive of unrecorded flood events which can potentially be used to extend existing flood records by centuries or millennia in this study a c 3750 year flood record was produced using ln zr rb profiles from two 4 m deep cores through floodplain silts at the roundabout in the upper severn catchment uk sediment geochemical profiles were obtained using high resolution 500 m xrf core scanning ages were assigned to the flood record using 14c dating and contaminant pb profiles comparison of sediment ln zr rb with sediment grain size showed that ln zr rb increases with increasing grain size and indicates that it can be used as a grain size proxy at this site within the ln zr rb flood record two floods that occurred at c 235 and c 10 cal bc were probably the largest that have occurred at the site during the past 3750 years while the historical flood of a d 1795 appears to be the largest since c ad 200 the record shows that during the past c 3750 years multicentennial periods characterized by the occurrence of high magnitude floods return periods c 30 years have alternated with periods of similar length without such floods these periods correspond to large scale variations in hydroclimate recorded in the north atlantic region however the incidence of the highest magnitude floods appears to be unaffected by changes in catchment land use this study provides information about flood magnitude frequency relationships and their controls at a local level that could be used to help catchment managers in assessing future flood risk at a time of rapidly changing climate

beyond the equilibrium paradigm how consideration of kinetics enhances metamorphic interpretation figure 1 the equilibrium model of prograde metamorphism in which rocks are regarded as departing only negligibly from equilibrium states as they recrystallize has generated a wealth of petrologic insights but mounting evidence from diverse approaches and observations over a range of scales has revealed that kinetic impediments to reaction may prevent metamorphic rocks from attaining rock wide chemical equilibrium along their prograde crystallization paths to illustrate the resulting potential for inaccurate interpretation if kinetic factors are disregarded we briefly review several case studies including out of sequence metastable and displaced isograds in contact aureoles paragenetic sequences documenting overstepped disequilibrium reaction paths patterns of compositional zoning in garnet demonstrating partial chemical equilibrium petrologic incongruities between observation and thermodynamic prediction and inhibited reaction progress revealed by petrologically constrained numerical simulations of garnet crystallization while the equilibrium model provides an indispensable framework for the study of metamorphic systems these examples emphasize that all reactions require departures from rock wide equilibrium so all rocks must traverse kinetically sensitive reaction paths during recrystallization mindfulness of the potential significance of kinetic influences opens new avenues for petrologic investigation thereby enhancing both analysis and interpretation 1 pending yes

global emissions of helium 3 by subaerial volcanism the emission rate of helium 3 by subaerial volcanism is re evaluated by coupling 3he co2 ratios in volcanic gases with new estimate of the flux of carbon dioxide from both arc and non arc volcanoes steady volcanic plume emissions are computed to supply 240 310 mole 3he yr most of which is due to non arc volcanoes major eruptions represent minor additions on average whereas diffuse 3he emanations through volcanic piles may be important sources although not including this contribution the above figure is equivalent to 20 40 of the 3he flux from mid ocean ridges and thus suggests that primordial 3he degassing from subaerial volcanic regions may be larger than expected

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

the chronology and stratigraphy of the alluvial terraces of the river dane valley cheshire n w england three groups of alluvial terraces together with the modern floodplain mark the postglacial development of the middle part of the dane valley cheshire these are a high terrace group of late pleistocene age a middle terrace group of late pleistocene to early holocene age a low terrace of mid late holocene age and a modern post ca 1840 ad floodplain a chronology of erosion deposition and landform development since mid holocene times is established in this paper on the basis of terrace morphology stratigraphy sedimentology soil analysis magnetic mineral analysis and four radiocarbon dates after dissection of the middle terrace during the early to mid holocene a long period of lateral activity by the river was followed by a major aggradation phase which formed the low terrace surface this was followed by dissection during the last ca 300 years and the development of the modern floodplain since ca 1840 ad various explanations for the changes during the holocene are considered the low terrace aggradation appears to be related to a major phase of mediaeval soil erosion

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

two and three dimensional quantification of lead contamination in alluvial soils of a historic mining area using field portable x ray fluorescence fpxrf analysis abstract two and three dimensional element distributions in soils and sediments may be important for the identification of geomorphic processes and the reconstruction of landscape evolution field portable x ray fluorescence fpxrf analysers provide new possibilities in the detection and quantification of these element distributions results from a research project studying the heavy metal contamination of alluvial soils in a historic mining area at the vils river near freihung eastern bavaria germany exemplify how fpxrf can be applied to quantify two and three dimensional element distributions in a floodplain environment on different scales the results show that two dimensional lead distribution of topsoil bulk samples 0 to 20 cm depth clearly depends on the geomorphology of the study area concentrations of more than 1000 mg kg 1 lead are restricted to the river floodplain on a fluvial terrace lead concentrations of the topsoils are consistently far below 500 mg kg 1 within the floodplain environment the lead contamination is not homogeneously distributed but forms areas of higher and lower lead concentrations micro fluvial structures channels ridges are polluted on different levels depending on the composition of the sediments or the water contents this demonstrates that accurate assessments of floodplain metal contamination can only be made if small scale spatial variability has been considered analysis of the three dimensional lead distribution shows that concentrations of the in situ measurements range from 500 6000 mg kg 1 subject to depth geomorphology and distance from the vils river although the absolute contents are higher three dimensional lead distribution derived from laboratory measurements is identical with the plotted results of the in situ investigations by calculating a correction factor for further investigations water content as one of the main causes for differing results between field and laboratory analyses can be diminished in situ fpxrf analysis as a robust technique could be used to investigate metal contamination on floodplain soils at a range of spatial scales it allows detailed sampling which can yield important information on the geomorphological controls on metal contamination in such environments

large scale facies sequences in alluvial overbank environments abstract gradational coarsening upwards or fining upwards sequences order of meters thick in overbank environments can be produced by progressive crevasse splay levee progradation or abandonment while the channel belt remains unchanged in position in contrast meter scale facies sequences related to periodic channel bell avulsion may show relatively abrupt changes in facies and paleocurrent direction simulation models suggest that the number of such avulsion related facies changes in any overbank sequence bounded above and below by channel belt deposits is on the order of 1 to 10 this is confirmed using an ancient example in this example the average thickness of each avulsion bounded overbank facies sequence was 3 m assuming an average recurrence interval for avulsions of about 1 000 ye rs the mean overbank deposition rate was about 3 mm per year

multiple discriminant analysis of the drava river alluvial plain sediments three discriminant function models are raised and cross compared in order to distinguish geochemical patterns characteristic for the drava river floodplain sediments based on data representing total element concentrations in samples collected from alluvium a terrace t and unconsolidated bedrock b at the border of a floodplain four element clusters emerged accounting for discrimination between the referred groups of sediments the most prominent is contaminant carbonate cluster characteristic for alluvium the other two are silicate cluster typical for unconsolidated geological substrate neogene sedimentary rocks and naturally dispersed heavy metal cluster separating terrace from the former two groups models introducing depth intervals and single profiles as grouping criteria reveal identical sediment heavy metal matrices the second important issue of this paper is possibility of reclassification of samples originally assigned to one of the a priori defined groups of sediments based on established geochemical pattern the mapped geological units can be reconsidered by the post hoc assignments to a different group if geological border between alluvium and terrace or between terrace and bedrock can not be established geologically with absolute certainty

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part i methods and techniques macroscopic textures resulting from different atomic scale mechanisms for metamorphic crystallization display different degrees of order clustering intergrowth and relative isolation of porphyroblasts data on the sizes and locations of thousands of crystals in a three dimensional volume are required to identify reliably the mechanisms governing nucleation and growth of porphyroblasts from these textural features these data can now be acquired by means of high resolution computed x ray tomography numerical models that simulate porphyroblast formation governed by either interface controlled or diffusion controlled reaction mechanisms indicate that quantitative textural analysis can discriminate between these possibilities these numerical models also allow a comparison between textures predicted for different crystallization mechanisms and textures measured in natural samples from which inferences can be drawn concerning the relative importance of these mechanisms in nature an independent test of the validity of such inferences is possible for porphyroblasts such as garnet that may preserve prograde growth zoning and allow the examination of normalized radius rate relations

vapor phase cristobalite as a durable indicator of magmatic pore structure and halogen degassing an example from white island volcano new zealand vesicles in volcanic rocks are physical records of magmatic degassing however the interpretation of their textures is complicated by resorption coalescence and collapse we discuss the textural significance of vesicle hosted vapor phase cristobalite high t low p sio2 polymorph and its utility as a complement to textural assessments of magmatic degassing using a representative dacite bomb erupted from white island volcano new zealand in 1999 imaging in 2d sem and 3d ct shows the bomb to have 56 bulk porosity almost all of which is connected 99 and devoid of sio2 phases the remaining 1 of porosity is in isolated sub spherical vesicles that have corroded walls and contain small 30 m across prismatic vapor phase cristobalite crystals 98 4 0 4 wt sio2 with diagnostic laser raman spectra halogen degassing models show vapor phase cristobalite to be indicative of closed system chlorine and fluorine partitioning into h2o rich fluid in isolated pores at white island this occurred during shallow 100s of meters ascent and extensive 50 groundmass crystallization associated with slow cooling in a volcanic plug pristine textures in this white island bomb demonstrate the link between pore isolation and vapor phase cristobalite deposition we suggest that because these crystals have higher preservation potential than the bubbles in which they form they can serve as durable qualitative textural indicators of halogen degassing and pre quench bubble morphologies in slowly cooled volcanic rocks e g lava flows and domes even where emplacement mechanisms have overprinted original bubble textures

thermogenic hydrocarbons from the offshore calypso hydrothermal field bay of plenty new zealand hydrothermal gases from shallow seafloor vents in the bay of plenty new zealand contain co2 ch4 and the higher gaseous hydrocarbons up to i n c4h10 the gases are similar to those discharged at fumaroles on the nearby white island carbon isotope compositions for co2 fall between 3 4 and 5 5 pdb and reflect a shallow magmatic carbon source the 13c values of ch4 range from 24 6 to 28 9 pdb and the d values vary between 122 and 135 smow the ch4 isotope values and the presence of the higher hydrocarbon compounds such as c2h6 and c3h8 with 13c values near 20 pdb suggest hydrocarbon production by high temperature maturation of sedimentary organic matter and mixing 1 1 of the thermogenic ch4 with abiogenic ch4 long chained hydrocarbons occur in dredged samples close to the active vents their n alkane distribution has a high to moderate odd even predominance and an extensive hopane series indicative of higher land plant waxes and prokaryotic membranes in the source substantial amounts of unsubstituted polynuclear aromatic hydrocarbons pah mark the transition from aliphatic to aromatic dominated bitumens consistent with extensive source maturation resulting from thermal stress the bitumens are interpreted as pyrolysates derived from buried near coastal vegetation and terrestrial detritus under various thermal regimes mixed with immature seafloor organic matter

the use of overbank sediment for geochemical mapping and contamination assessment results from selected english and welsh floodplains abstract overbank sediment profiles from floodplains in england and wales contain a record of both natural geochemical patterns and those showing the influence of man s activities it has been suggested that this characteristic can be used to allow maps to be compiled which show human impact on the fluvial geochemical environment studies reviewed in this paper however show that a single overbank profile very rarely spans the period from before anthropogenic disturbance through to the industrial revolution and later significant lateral variations in metal concentrations occur also over a relatively small area in overbank sediments of the same general age these and the nature of vertical changes in chemistry make the choice of sample sites and sampling interval within a profile difficult even sediments which appear uncontaminated may record anthropogenic influences from activities such as deforestation and agriculture a means of dating the sediment and an appreciation of river erosion and sedimentation histories are shown to be essential in order to ensure that maps intended to depict natural geochemical variations are based on material deposited before disturbance of the catchment by human activity these considerations and associated costs may render overbank sediment non viable as a regional geochemical mapping medium

soil co2 flux baseline in an urban monogenetic volcanic field the auckland volcanic field new zealand the auckland volcanic field avf is a dormant monogenetic basaltic field located in auckland new zealand though soil gas co2 fluxes are routinely used to monitor volcanic regions there have been no published studies of soil co2 flux or soil gas co2 concentrations in the avf to date or many other monogenetic fields worldwide we measured soil gas co2 fluxes and soil gas co2 concentrations in 2010 and 2012 in varying settings seasons and times of day to establish a baseline soil co2 flux and to determine the major sources of and controlling influences on auckland s soil co2 flux soil co2 flux measurements varied from 0 to 203 g m 2 day 1 with an average of 27 1 g m 2 day 1 higher fluxes were attributed to varying land use properties e g landfill using a graphical statistical approach two populations of co2 fluxes were identified isotope analyses of 13co2 confirmed that the source of co2 in the avf is biogenic with no volcanic component these data may be used to assist with eruption forecasting in the event of precursory activity in the avf and highlight the importance of knowing land use history when assessing soil gas co2 fluxes in urban environments

using sedimentary archives to reconstruct pollution history and sediment provenance the ohře river czech republic abstract in industrialised areas rivers and their floodplains are often highly polluted the historic contamination of floodplains offers an important archive of past polluting activities and fluvial sediment dynamics it can also present a significant threat to contemporary aquatic ecosystems and water quality resulting from the remobilisation of sediment associated pollutants from floodplain secondary sources therefore knowledge of the type and level of pollutants present and their spatial distribution in floodplains is essential for supporting measures to mitigate future impacts here we outline an approach for reconstructing pollution history and extent by using floodplains as archives of historic chemical contamination coupled with geochemical and geomorphological analyses to account for variations in sediment provenance and depositional processes the study is situated in the oh e river total catchment area of 5606 km 2 which provides an excellent test case given its complex pollution history heterogeneous geology and variable deposition patterns most severe pollution in this river system has originated from a chemical factory in marktredwitz germany mainly hg and uranium processing in nejdek the czech republic mainly u historical lead mining and modern coal combustion have also played a role the geological geochemical variability of the oh e catchment required extensive sampling along the river course variations in several major lithogenic elements in particular al k rb si and ti allowed the river course to be subdivided to geochemical river reaches using simple scatter plots ti against al si ratio and rb against k or pca there was no need to sieve the sediment samples before analyses or perform other discrimination of sediment lithology the geochemical river reaches reflect catchment geology and hence they exhibit distinct background functions for cu pb and zn and varying background concentrations of u titanium was the best performing reference element the oh e river has deposited most historical pollution in former abandoned or laterally shifting channels rather than in overbank fine grained materials the former channel sediments can be recognised by a specific surface topography vegetation cover and sediment lithology the use of geographic information systems evaluating historical maps aerial photographs and dtm by lidar is indispensable in the study of the depositional patterns the narrow floodplain limited floodplain deposition and has probably allowed particulates to travel through the channel for tens of kilometres in the case of u pollution in

soil co2 emissions as a proxy for heat and mass flow assessment taupō volcanic zone new zealand the quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems here we use high resolution measurement of carbon dioxide flux co2 and heat flow at the surface to characterize the mass co2 and steam and heat released to the atmosphere from two magma hydrothermal systems our soil gas and heat flow surveys at rotokawa and white island in the taup volcanic zone new zealand include over 3000 direct measurements of co2 and soil temperature and 60 carbon isotopic values on soil gases carbon dioxide flux was separated into background and magmatic hydrothermal populations based on the measured values and isotopic characterization total co2 emission rates co2 of 441 84 t d 1 and 124 18 t d 1 were calculated for rotokawa 2 9 km2 and for the crater floor at white island 0 3 km2 respectively the total co2 emissions differ from previously published values by 386 t d 1 at rotokawa and 25 t d 1 at white island demonstrating that earlier research underestimated emissions by 700 rotokawa and 25 white island these differences suggest that soil co2 emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches combining the magmatic hydrothermal sourced co2 emission constrained using stable isotopes with reservoir h2o co2 mass ratios and the enthalpy of evaporation the surface expression of thermal energy release for the rotokawa hydrothermal system 226 mwt is 10 times greater than the white island crater floor 22 5 mwt

effective bulk composition changes due to cooling a model predicting complexities in retrograde reaction textures diffusive processes are a strong function of temperature thus during cooling of rocks mineral grains may develop zoning profiles as successively larger parts of the grain close to the diffusive exchange with the rock one of the consequences of this process is that during cooling successively larger parts of zoned minerals depending on grain size are effectively removed from the reacting part of the rock volume thus the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size because the sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition this process may have the consequence that two rocks of identical overall bulk composition but of different grain size may experience a different sequence of reactions qualitatively identical peak paragenesis may therefore react to form qualitatively different retrograde reaction textures the model is applied to examples in the pelitic system there garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition it is shown that during cooling of pelitic rocks from amphibolite facies conditions typical aluminous peak parageneses of garnet muscovite kyanite biotite may react to form either staurolite chlorite or muscovite or different combinations thereof depending on grain size during cooling from the granulite facies aluminous peak parageneses of garnet cordierite sillimanite may form biotite either on the expense of cordierite or garnet also depending on grain size the two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature

pb isotope evidence for contaminant metal dispersal in an international river system the lower danube catchment eastern europe abstract lead isotope signatures 207pb 206pb 208pb 206pb 208pb 204pb 206pb 204pb determined by magnetic sector icp ms in river channel sediment metal ores and mine waste have been used as geochemical tracers to quantify the delivery and dispersal of sediment associated metals in the lower danube river catchment due to a diverse geology and range of ore body ages pb isotope signatures in ore bodies within the lower danube river catchment show considerable variation even within individual metallogenic zones it is also possible to discriminate between the pb isotopic signatures in mine waste and river sediment within river systems draining individual ore bodies lead isotopic data along with multi element data were used to establish the provenance of river sediments and quantify sedimentary contributions to mining affected tributaries and to the danube river data indicate that mining affected tributaries in serbia and bulgaria contribute up to 30 of the river channel sediment load of the lower danube river quantifying relative sediment contributions from mining affected tributaries enables spatial patterns in sediment associated metal and as concentrations to be interpreted in terms of key contaminant sources combining geochemical survey data with that regarding the provenance of contaminated sediments can therefore be used to identify foci for remediation and environmental management strategies

rates of sulfur dioxide and particle emissions from white island volcano new zealand and an estimate of the total flux of major gaseous species airborne correlation spectrometry cospec was used to measure the rate of so2 emission at white island on three dates i e november 1983 1230 300 t d november 1984 320 120 t d and january 1985 350 150 t d t metric tons the lower emission rates are likely to reflect the long term emission rates whereas the november 1983 rate probably reflects conditions prior to the eruption of december 1983 the particle flux in the white island plume as determined with a quartz crystal microbalance cascade in november 1983 was 1 3 t d unusually low for volcanic plumes the observed plume particles as shown from scanning electron microscopy include halite native sulfur and silicates and are broadly similar to other volcanic plumes

volcanic emissions of metals and halogens from white island new zealand and erebus volcano antarctica determined with chemical traps abstract volcanic emission rates of as sb pb hg se cl and f were determined at erebus volcano antarctica and white island new zealand using chemical traps the trace metal fluxes were determined by combining the species to s ratios in the solutions with so2 emission rates measured by correlation spectrometry at the two volcanoes at erebus volcano fluxes for the metals pb and hg were 2 0 10 4 and 8 1 10 6 kg s 11 respectively fluxes for cl f as sb and se 0 35 0 15 2 5 10 4 1 2 10 5 and 4 5 10 6 kg s 1 respectively agreed within error limits for values determined previously by the lioh impregnated filter method zreda gostynska g kyle p finnegan d prestbo k 1997 volcanic gas emissions from mount erebus and their impact on the antarctic environment journal of geophysical research 102 b7 15039 15055 demonstrating the utility of the chemical trap method a fall in the as s ratio from 7 10 4 in 1997 1999 to 3 10 4 in 2000 at erebus coincided with a change in the frequency and style of eruptive activity that may have been due to injection of magma into the system at white island chemical trap data indicated fluxes of cl 0 90 f 0 0079 pb 2 7 10 4 hg 1 1 10 5 as 1 3 10 4 sb 1 9 10 5 and se 1 5 10 5 kg s 1 samples collected 600 m downwind of the active crater were comparable to samples collected adjacent to the main gas vent showing that this method can still be used at some distance from a degassing vent

major and rare gases at white island volcano new zealand origin and flux of volatiles analyses of fumarolic gases at white island new zealand are reported with special emphasis on rare gases he ne ar kr and xe normalization of major species to rare gases have lead to the identification of several sources air saturated meteoric or marine water a magma body the crust and or sediments when compared to mid ocean ridge discharges the white island gas discharge represents about 3 0 7 0 1 and 0 05 of the global water carbon sulfur and helium 3 discharges respectively this balance shows that except for 3he which derives from the mantle practically all rare gases water and carbon and part of sulfur derive from originally superficial sources

spiral growth of grossular under hydrothermal conditions grossular grs 99 adr 01 crystals were synthesized in hydrothermal experiments by the reaction wollastonite calcite anorthite grossular co 2 400 mpa 695 750 c x co 2 0 1 solid fluid 2 1 using natural minerals as starting materials growth spirals are visible due to silica decorations the crystals are bounded by 110 and 211 if crystallized at t 730 c they are birefringent and show sector twinning grown at t 730 c they are optically isotropic the crystal growth rates in the experiments ranged from about 1 to 100 m d the edges of the growth terraces moved by 5 200 nm s the morphology of the growth steps is interpreted against the background of supersaturation with respect to grossular in the fluid the highest supersaturation occurs during the initial stage of the reaction 1 h ultimately at the wollastonite surfaces in this initial stage grossular crystallizes with only 111 faces present covered by round growth spirals when a steady state with respect to dissolution and precipitation rates is reached after 24 h all grossular crystals show 110 and 211 with polygonal spirals on 110 and parallel growth steps on 211 all straight growth step edges parallel 111 when the solid phases assume equilibrium with the fluid after calcite in calcite deficient experiments is used up completely round growth spirals appear on 211 the spirals are elongated parallel to 111 and compete against parallel steps in that same direction the surface morphology of the crystals synthesized under steady state conditions mimicks that of natural grandite crystals and their growth rates equal the estimated growth rates of grandite crystals from natural hydrothermal systems

chemical and isotopic compositions of volcanic gases from the east sunda and banda arcs indonesia the easternmost sunda arc and the banda arc represent a continent arc collision zone where magma genesis is influenced by subducted continent derived material chemical and isotopic studies of volcanic gas samples from this environment provide information on the sources of volatiles in arc magmas these volcanic gases some of which last equilibrated at magmatic temperatures are characterized by anomalous low sup 3 he sup 4 he values but by common arc values of c s approx 2 4 delta sup 13 c approx minus 3 per thousand and delta sup 34 s sub tot approx 5 per thousand abundant helium and high he ar ratios are consistent with the subduction of terrigenous components in local sediments or slivers of continental crust although individual concentration and isotope ratios of volatile components may be explicable by complex fractionation in the recycling process the combined data are in agreement with an important role of subducted sedimentary source components comparison of the authors results with volcanic gas data from other arcs indicates that the carbon and sulfur signals in arc gases are relatively insensitive to the amount and nature of sediment on the subsiding plate hence a contribution to arc volcanic gases from subducted altered oceanic crust cannot be excluded

terraced fills of pleistocene and holocene age in the rheidol valley wales the legacy of repeated pleistocene glaciations has endowed many welsh river valleys with locally thick successions of glacial and alluvial sediments investigations of a well preserved flight of terraced sediments with good exposures at capel bangor on the afon rheidol mid wales has allowed its quaternary valley fill stratigraphy to be examined in detail study has revealed five terraced fills consisting of seven distinct sedimentary units these range from late devensian ice contact and ice marginal deposits to holocene high sinuosity stream sediments with episodes of man induced accelerated deposition of fine grained alluvium and to aggradation and subsequent incision associated with historic metal mining examination of general sedimentary properties e g granulometry sedimentary structures terrace surface morphology show both differences in the pattern and controls of deposition and also progressive changes over late devensian and holocene times the sediments of the rheidol valley record the response and subsequent recovery of a drainage basin to glaciation and the increasing influence of man on sediment yields channel processes and sediment quality

high resolution interpretative geomorphological mapping of river valley environments using airborne lidar data the availability of airborne lidar data provides a new opportunity to overcome some of the problems associated with traditional field based geomorphological mapping such as restrictions on access and constraints of time or cost the combination of airborne lidar data and gis technology facilitates the rapid production of geomorphological maps of floodplain environments however unfiltered lidar data which include vegetation and buildings are currently more suitable for geomorphological mapping than data that have been filtered to remove these features classification of lidar data according to elevation in a gis enables the user to identify and delineate geomorphological features in a manner similar to field mapping but it is necessary to use a range of classification intervals in order to map the various types of feature that occur within a single reach comparison of a lidar derived geomorphological map with an independently produced field geomorphological map showed a high degree of similarity between the results of the two methods although ground truthing is essential in cases where a high degree of accuracy is required ground truthing of a lidar derived geomorphological map showed that around 80 of features mapped using both methods were identified from the lidar data suggesting that the method is suitable for applications such as production of base maps for use in field mapping and selection of sites for detailed investigation copyright 2007 john wiley sons ltd

evolution of the taupo hikurangi subduction system abstract the present day taupo hikurangi subduction system is a southward extension of the tonga kermadec arc system into a sediment rich continental margin environment it consists of a shallow structural trench the hikurangi trough a 150 km wide imbricate thrust controlled accretionary borderland the continental slope shelf and coastal hills of eastern north island a frontal ridge the main greywacke ranges of north island and a volcanic arc and marginal basin the taupo volcanic zone structural elements become progressively more elevated and subduction more oblique towards the south the whole nne trending system is truncated at a largely strike slip transform boundary that extends along the southwestern part of the hikurangi trough and the hope fault of south island to the main alpine fault the volcanic arc is 200 270 km from the structural trench and comprises a nne trending chain of andesite dacite volcanoes extending along the eastern side of the taupo volcanic zone most of the andesites are olivine bearing and have been erupted within the last 50 000 years it is suggested the taupo hikurangi margin has evolved by rotation of accretionary elements from an original nw trending subduction system north of new zealand the older elements of the prism were associated with subduction of a re entrant of the pacific plate and perhaps the south fiji basin in mid tertiary times they subsequently became separated from their nw trending volcanic arc by dextral strike slip movement along curved faults east of the main greywacke ranges during the plio pleistocene oblique subduction and accretion intensified as the taupo hikurangi margin rotated into line with the nne trending kermadec system and a marginal basin was developed along a similar trend to form the taupo volcanic zone within the last 50 000 years olivine bearing andesite volcanism has commenced along the eastern side of the taupo volcanic zone

sediment transfer and transformation of an alluvial valley floor the river south tyne northumbria u k the amounts of alluvial storage and the mechanisms responsible for the dispersal of sediment associated with historic metal mining are examined for a 115 year timespan on a 22 km reach of the river south tyne northern england analysis of lateral and vertical channel change over this period the extent of actively reworked gravels and identification of metals in alluvial units show five sedimentation zones separated by more stable reaches aggradation in the late nineteenth century was followed by a period of storage and local reworking and then incision some reaches show short term storage of sediment injected from tributaries which may be dispersed by floods there is some evidence of transfer of sediment bodies downvalley within sedimentation zones and of a sediment wave movement between zones in lower parts of the south tyne investigated present day incision the variation in the area of actively reworked gravel spreads and the widespread dispersal of fine grained toxic metal mining wastes all have practical implications the intricate response of the channel system to historic changes in sediment supply is not readily described by either a complex response model or a simple translatory wave

heavy metal contamination in overbank sediments of the geul river east belgium its relation to former pb zn mining activities overbank sediments of the geul river east belgium are highly contaminated by the heavy metals pb zn and cd due to former pb zn mining activities in the drainage basin geochemical variations in vertical overbank sediment profiles sampled 1 km north of the mine tailings of plombieres allow metal fluxes back to the 17th century to be reconstructed the vertical profiles are subdivided into three major units corresponding to different industrial periods based on sedimentological criteria as well as on the distribution of contaminants alluvial sediments with the highest heavy metal concentrations correspond to the major period of mining activity of the 19th century the fact that zn mining at the la calamine open mine started before large scale mining of the pbs zns subsurface exploitations is reflected in the vertical profiles by an increase in zn content before a marked increase in pb and cu the regional extent of contamination in the alluvial deposits was evaluated on the basis of the geochemical analysis of sediments at depths of the 0 20 cm and 80 100 cm most of the upper samples are extremely contaminated significant local variations in heavy metal concentration in the lower samples are interpreted in terms of which overbank sediment horizon has been sampled at a depth of 80 100 cm this indicates that blind sampling of overbank sediments to characterize the degree of contamination in shallow boreholes can give very erratic results

porphyroblast crystallization kinetics the role of the nutrient production rate the mechanisms that govern porphyroblast crystallization are investigated by comparing quantitative textural data with predictions from different crystallization models such numerical models use kinetic formulations of the main crystallization mechanism to predict textural characteristics such as grain size distributions in turn data on porphyroblast textures for natural samples are used to infer which mechanism dominated during their formation whereas previous models assume that the rate limiting step for a porphyroblast producing reaction is either transport or growth the model advanced in this study considers the production of nutrients for porphyroblasts as a potentially rate limiting factor this production reflects the breakdown of metastable reactants which at a specific pressure p and temperature t depends on the bulk composition of the sample the production of nutrients that potentially contribute to the formation of porphyroblasts is computed based on thermodynamic models the conceptual model assumes that these nutrients feed into some intergranular medium and products form by nutrient consumption from that medium with rates depending on reaction affinity for any sequence of p t conditions along a p t t path the numerical model first computes an effective supersaturation reff of the product phase s then an effective nucleation rate j and finally the amount of porphyroblast growth as a result the model is useful in investigating how the textural characteristics of a sample of given bulk composition depend on the p t t path followed during porphyroblast crystallization the numerical model is tested and validated by comparing simulation results with quantitative textural data for garnet porphyroblasts measured in samples from the swiss central alps

the timing of mineral growth across a regional metamorphic sequence if regional metamorphism occurs in response to crustal thickening then low grade rocks are likely to record earlier thermal conditions than will high grade rocks in the same exhumed terrain1 4 at sulitjelma north norway regional metamorphism during the caledonian orogeny created a type sequence of progressive metamorphic zones5 7 garnet and coexisting minerals from the high grade part of this terrain yield precise and concordant sm nd u pb and rb sr ages which relate directly to the pressure temperature conditions of mineral growth8 here we present pressure temperature time p t t information for lower grade rocks occurring at the garnet isograd taken together these data provide a precise estimate of the timing of mineral equilibration across a prograde metamorphic terrain and indicate that peak metamorphic conditions at the garnet isograd at 458 20 c were attained 8 3 3 0myr t sm nd or 8 7 3 2myr t u pb before the peak in the high grade sample at 544 16 c these results clearly illustrate that the exhumed p t array recorded in such terrains cannot be used to deduce any single geotherm that existed during metamorphism

the variability of heavy metals in floodplain sediments a case study from mid wales abstract a series of vertical alluvial profiles from the upper river severn floodplain at welshpool ranging in age from late pleistocene to the present have been examined for their ni cu zn cd ba and pb concentrations the floodplain deposits have been variably contaminated with heavy metals following extensive metalliferous mining in the upper catchment during the mid 19th century the data are used to establish whether vertical increases in concentrations can be used to indicate a younging upwards of the sediments the data show that grain size and organic content do not influence heavy metal concentrations although some of the most recent alluvial profiles contain the highest levels of contamination a great deal of variation exists not only between the general vertical distribution of individual metals but also between the absolute values themselves there was no consistent age concentration relationship in the vertical profiles pb displays the least complex picture with predominantly increasing values towards the top of the alluvial profiles in the historically sedimented sites indicating a younging upwards rapid sedimentation and post depositional processes such as leaching particle translocation or the lateral migration of groundwaters enriched in heavy metals result in different vertical heavy metal distributions at superficially similar sites it is recommended that sampling strategies for contaminated floodplain sediments utilise a multiple profile approach including the whole range of sediment types ages and depositional forms that are known to exist at a study site

the spatial distribution of heavy metal contaminated sediment across terraced floodplains abstract deposits of fine alluvial sediment contaminated by heavy metals derived from mining provide an opportunity to reconstruct the alluvial histories of mineralised catchments field data were collected to identify the spatial distribution of heavy metal contamination in surficial floodplain sediments and establish the relationships between terrace height terrace age and metal concentrations floodplain terrace morphology was mapped and terrace heights were measured for three separate study reaches in the upper sevem basin mid wales concentrations of pb ba and zn were determined in 97 samples taken from fine overbank units capping these terrace surfaces between the three study reaches element concentrations showed wide ranges pb 11 964ppm ba 141 1118 ppm zn 73 1134 ppm they also exhibited significant spatial variation across the floodplain at each study reach e g at llandinam pb 36 964ppm ba 141 1118 ppm and zn 173 1134 ppm peak metal concentrations at morfodion pb 399 ppm ba 474 ppm and welsh pool pb 149 ppm ba 558 ppm were found on terrace surfaces 1 5 2 5m and 2 0 3 0 m above present river level respectively lower concentrations reflect natural background levels of contaminants higher terraces and post mining reworking of spoil heaps and floodplains lower terraces in general the spatial distribution of floodplain contaminants is more significantly correlated with terrace height than with terrace age different flooding mechanisms and styles of channel instability in relation to the timing and mode of sedimentation have resulted in different patterns of floodplain contamination focused deposition particularly in palaeochannels is also an important mechanism for creating strongly contaminated sites within the floodplain environment

a holocene alluvial sequence in the lower tyne valley northern britain a record of river response to environmental change palaeoenvironmental investigations have been made of late quaternary sediments infilling the lower tyne valley at scotswood newcastle upon tyne 18 km upstream from the mouth of the river attention has focused on the holocene alluvial sedimentary sequence using sedimentological geochemical and palynological analyses holocene valley floor development from c 7500 bp has been largely dominated by a tendency towards river aggradation with some limited lateral and vertical reworking of sediments four alluvial fills have been identified and dated to 1 c 7500 4800 bp 2 from c 2590 bp up to the early first millennium ad and possibly as late as medieval times 3 between the medieval period and the early seventeenth century and 4 between the seventeenth and nineteenth centuries episodes of incision are dated to c 4800 2590 bp and to the period following the early seventeenth century marked variations in the rates and nature of sedimentation and erosion appear to coincide with rising post glacial

quantitative analysis of crystal grain sizes and their distributions in 2d and 3d abstract we review methods to estimate the average crystal grain size and the crystal grain size distribution in solid rocks average grain sizes often provide the base for stress estimates or rheological calculations requiring the quantification of grain sizes in a rock s microstructure the primary data for grain size data are either 1d i e line intercept methods 2d area analysis or 3d e g computed tomography serial sectioning these data have been used for different data treatments over the years whereas several studies assume a certain probability function e g logarithm square root to calculate statistical parameters as the mean median mode or the skewness of a crystal size distribution the finally calculated average grain sizes have to be compatible between the different grain size estimation approaches in order to be properly applied for example in paleo piezometers or grain size sensitive flow laws such compatibility is tested for different data treatments using one and two dimensional measurements we propose an empirical conversion matrix for different datasets these conversion factors provide the option to make different datasets compatible with each other although the primary calculations were obtained in different ways in order to present an average grain size we propose to use the area weighted and volume weighted mean in the case of unimodal grain size distributions respectively for 2d and 3d measurements the shape of the crystal size distribution is important for studies of nucleation and growth of minerals the shape of the crystal size distribution of garnet populations is compared between different 2d and 3d measurements which are serial sectioning and computed tomography the comparison of different direct measured 3d data stereological data and direct presented 2d data show the problems of the quality of the smallest grain sizes and the overestimation of small grain sizes in stereological tools depending on the type of csd

the use of heavy metal concentrations and dendrochronology in the reconstruction of sediment accumulation mala panew river valley southern poland abstract heavy metal concentrations were investigated in overbank sediments of the mala panew river southern poland samples were collected from seven vertical profiles located within channel infills of a 20th century floodplain at three sites each up to 50 m wide in each profile 15 24 samples were collected and analysed for ba cd cu pb and zn sequential extraction of these elements was carried out in the 0 063 mm fraction of selected samples additionally the age of the oldest trees growing close to the profiles has been used to estimate the initiation of sediment accumulation there ba cu and pb which occur mostly in less mobile moderately reducible and residual fractions were used for sediment dating zn and cd which in 50 75 occur in the mobile exchangeable fraction were not suitable for dating correlation of ba cu and pb concentrations in vertical profiles with changes in the load of effluents discharged to the river showed abrupt changes in the thickness of the strongly polluted sediments across the floodplains a comparison of the relative changes between heavy metal peaks in sediments of similar age in the different profiles suggests a variable rate of downward metal migration in general none of the heavy metals investigated seems to have been mobilised within the stratigraphic layers above the water table in layers located at stratigraphically lower levels the zn and cd peaks seem to migrate several centimetres to several decimetres down in the profile in profiles inundated for several weeks every year zn and cd as well as the relatively less mobile ba cu and pb have migrated downward by several decimetres the investigation shows that frequent fluctuations of the water table have blurred the original depositional metal patterns of metal concentrations within a period of less than 40 years

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part ii application to natural samples three dimensional quantitative textural analysis coupled with numerical modelling has been used to assess the dominant mechanisms governing crystallization of garnet porphyroblasts in rocks from diverse regional metamorphic environments in every case spatial dispositions crystal size distributions and compositional zoning patterns of porphyroblasts indicate the dominance of diffusion controlled nucleation and growth mechanisms nine samples from three geological areas were studied a suite of semi pelitic rocks from the picuris mountains new mexico usa a suite of mafic samples from the llano uplift texas usa and a kyanite schist from mica dam british columbia canada the semi pelitic suite exhibits post deformational garnet growth whereas garnet in the mafic suite and in the kyanite schist grew synkinematically in rocks displaying weak and strong penetrative fabrics respectively for each sample the centres and radii of thousands of garnet crystals were located and measured in three dimensions using images produced by high resolution computed x ray tomography statistical measures of the degree of ordering and clustering of nucleation sites and estimates of crystal isolation for each porphyroblast were then computed from the measured spatial dispositions these measures can be reproduced in simple numerical models only by diffusion controlled nucleation and growth mechanisms normalized radius rate relations computed from compositional zoning patterns in the garnets require thermally accelerated diffusion controlled growth providing independent confirmation of the conclusions based on textural analysis the unexpected similarity of results from all samples indicates that diffusion controlled nucleation and growth mechanisms may govern porphyroblast crystallization in many metamorphic regimes

geochemical investigations on fluvial sediments contaminated by tin mine tailings cornwall england tin mine tailings containing high concentrations of sn cu zn fe mn as and w are discharged into the red river of cornwall england and are then transported into st ives bay under normal flow conditions most of the tin bearing particles in the fluvial sediments are smaller than 170 m but tin bearing composite grains or mineral grains with tin interspersed in the crystal lattices also occur in coarser size fractions tin distribution in the sediments is controlledby 1 the distance from the source of the tailings and 2 the concentration processes operating on the river bed suspended sediment and sediment transported by saltation filtered from river water samples also showed high concentrations of metals although in contrast to the bottom sediments they vary within a narrow range distributions of cu zn fe as and pb in the filtered sediments probably are related to the physical and chemical behavior of their sulphide minerals during fluvial transportation

pressure temperature paths from p t pseudosections and zoned garnets potential limitations and examples from the zanskar himalaya nw india the extraction of p t histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny while continued re equilibration frequently obliterates early information garnet zonation and inclusion assemblages can often surmount this problem the task is more difficult in high variance assemblages or if inclusions are not preserved but one approach is to use pseudosections that are specific to the bulk composition of a given rock in the latter case the compositions and abundances of all the minerals are fixed at a given p t point such that if the effective bulk composition is known the garnet composition alone can be used to reconstruct the history here we explore this approach using examples from the zanskar himalaya nw india pseudosections have been calculated for four pelitic to semipelitic rocks from the zanskar himalaya and have been contoured for garnet composition the calculations adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite is found as inclusions within garnet moreover the pseudosections successfully model the garnet core compositions with all three independent compositional contours overlapping at a single pressure and temperature this occurs at 550 c and at pressures varying from 3 7 kbar for the four rocks studied we have been less successful however at modelling garnet compositions beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself however in this case a combination of the pseudosection and conventional thermobarometry using fe ti inclusions and matrix phases allows us to reconstruct the entire p t history the resulting p t paths record burial of 3 5 kbar without significant temperature increase followed by isobaric heating of 50 100 c this evolution is consistent with himalayan collision in the early tertiary but a combination of the p t data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the oligocene in addition the data demonstrate that no extra heat source is required to cause melting of the himalayan crust in the miocene while melting could have occurred both by dehydration during decompression or in the presence of a fluid the lack of garnet resorption does suggest decompression was rapid and followed quickly by cooling this scenario favours melting by decompression

cumulative effects of channel correction and regulation on floodplain terrestrialisation patterns and connectivity abstract one of the main drivers of overbank fine deposition and floodplain formation is the hydrological connectivity between the channel and the floodplain channel correction i e groyne field construction within the main flow channel and secondary channel disconnections and flow regulation can typically lead to a disconnection of riverine floodplains and disturbances that directly affect terrestrialisation channel correction and flow regulation can sometimes occur successively and it is challenging to distinguish the roles of each this work attempts to assess the respective effects of two phases of channel regulation correction versus flow regulation on floodplain terrestrialisation by comparing three bypassed reaches of the rhone france pierre benite peage de roussillon and donzere mondragon we applied a transversal methodology coupled with gis analysis old maps orthophotos dem s etc to understand processes of channel planform evolution conducted a sediment survey metal rod to assess floodplain terrestrialisation and performed sediment sampling manual auger to obtain surface sediment metal content levels x ray fluorescence and inductively coupled plasma mass spectrometry we found a general trend of channel narrowing within the three reaches among which approximately 40 was found to be associated with correction works while 20 was attributed to flow lowering caused by channel bypassing the number of flowing channels in all sections declined significantly and local anabranching reaches evolved into very stable single thread channels overbank sedimentation declined significantly over the period with very high sedimentation levels observed immediately after correction works and with very low sedimentation levels observed after diversion we also found overbank flooding in the number of days per year decreased while fine sediment thickness increased similarly the highest concentrations of metals zc pb and cu were found to be associated with a low connection frequency and vice versa when similar 2 staged terrestrialisation patterns are observed in all three reaches they differ in chronology and driving factors because of their longitudinal positioning and specific local conditions

island arc tectonics of new zealand manifested in helium isotope ratios abstract thirty gas samples of various types volcanic gas geothermal gas ch4 rich natural gas and petroleum gas were collected in and around the north island new zealand 3 he 4 he and 4 he 20 ne ratios were measured using a mass spectrometer the observed 3 he 4 he and 4 he 20 ne ratios of the samples varied from 0 08 10 6to 8 75 10 6 and from 4 8 to 2100 respectively based on a 3 he 4 he 4 he 20 ne diagram he in the samples can be explained by mixing of a subduction type magmatic a radiogenic and an air component the geographical distribution of 3 he 4 he ratios over the island indicates a clear contrast between forearc and volcanic arc regions high in the former and low in the latter a he boundary can be drawn from the taupo volcanic zone to egmont volcano in terms of the 3 he 4 he profile this boundary agrees well with that of high q lowq in the upper mantle as well as with the geographical distribution of terrestrial heat flow values the high 3 he 4 he ratios in the volcanic arc region are attributed to subduction type he associated with an uprising magma from the upper mantle and the lower ratios in the forearc region to radiogenic he derived from u and th in basement and sedimentary rocks this tendency is very similar to that found in northeast japan and is understood to be a general signature of the he isotope ratio in an island arc system

storage of sediment associated heavy metals along the channelized odra river poland the importance of long term storage of heavy metals in groyne fields functioning over 150 years is investigated for the river odra oder western poland construction of groynes along the odra preceded rapid development of heavy industrialization in the largest coal mine districts in poland and the czech republic that resulted in persistent riverine pollution the 187 km long middle odra reach was repeatedly channelized from the first half of the eighteenth century to the turn of the twentieth century during which time partially filled groyne fields were dissected by new bank lines and groyne systems with older groyne fields partially keyed into the floodplain consequently concentrations of zinc lead cadmium and copper within historically deposited groyne field sediments exceed local geochemical background levels by more than 60 40 15 and 10 times respectively sediments contaminated with heavy metals occur within three distinctive geomorphic zones zone i is up to 250 m wide and furthest from the present channel comprising decimeter thick polluted sediments overlying eighteenth century sand and gravel bars zone ii represents the former nineteenth century groyne fields with widths between 10 and 100 m filled with as much as 3 m of polluted sediments zone iii represents the twentieth century groyne fields which are several to a dozen metres wide and filled with polluted sediments averaging depths of more than 2 m this investigation indicates that large and extensive sediment quantities of moderately polluted sediments are stored immediately along the banks of the river odra these sediments could be a significant secondary pollution source and therefore careful maintenance of contemporary bank protection structures is required copyright 2009 john wiley sons ltd

equilibration and disequilibration between monazite and garnet indication from phase composition and quantitative texture analysis the integration of information which can be gained from accessory i e age t and rock forming minerals i e temperature t and pressure p requires a more profound understanding of the equilibration kinetics during metamorphic processes this paper presents an approach comparing conventional p t estimate from equilibrated assemblages of rock forming minerals with temperature data derived from yttrium garnet monazite ygm and yttrium garnet xenotime ygx geothermometry such a comparison provides an initial indication on differences between equilibration of major and trace elements regarding this purpose two migmatites two polycyclic and one monocyclic gneiss from the central alps switzerland northern italy were investigated while the polycyclic samples exhibit trace element equilibration between monazite and garnet grains assigned to the same metamorphic event there are relics of monazite and garnet obviously surviving independent of their textural position these observations suggest that surface processes dominate transport processes during equilibration of those samples the monocyclic gneiss on the contrary displays rare isolated monazite with equilibration of all elements despite comparably large transport distances with a nearly linear crystal size distribution of the garnet grain population growth kinetics related to the major elements were likely surface controlled in this sample in contrast to these completely equilibrated examples the migmatites indicate disequilibrium between garnet and monazite with a change in ree patterns on garnet transects the cause for this disequilibrium may be related to a potential disequilibrium initiated by a changing bulk chemistry during melt segregation while migmatite environments are expected to support high transport rates i e high temperatures and melt presence the evolution of equilibration in migmatites is additionaly related to change in chemistry as a key finding surface controlled equilibration kinetics seem to dominate transport controlled processes in the investigated samples this may be decisive information towards the understanding of age data derived from monazite

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

an evaluation of spatial correlation functions in textural analysis of metamorphic rocks spatial correlation functions which quantify spatial relationships among porphyroblasts over a range of length scales can be used in combination with other techniques of quantitative textural analysis to constrain crystallization mechanisms in metamorphic rocks the utility reliability and robustness of these functions however depend critically upon correct methods of calculation and application to geological samples application of the l function pair correlation function and mark correlation function stoyan and stoyan 1994 to artificial arrangements of crystals yields results consistent with their predetermined ordering and clustering qualities these results serve as a foundation for the interpretation of more complex simulated and natural crystal arrays analysis of artificial and simulated crystal arrays in which ordering signals are obscured in various ways displacing crystals in an ordered array by increasing amounts reducing the number of crystals and increasing the sample s aspect ratio demonstrates that these scale dependent functions are robust indicators of effects diagnostic of certain crystallization mechanisms even in complex circumstances the effects of clustering of nucleation sites however can strongly obscure any underlying signal that might reveal crystallization mechanisms the l function and the pair correlation function are sensitive to short range ordering of crystals which may reflect suppression of nucleation in the vicinity of growing porphyroblasts the mark correlation function is sensitive to size isolation correlations which may reflect retardation of growth among crystals competing for nutrients interpretation of these functions however requires careful attention to proper calculation of monte carlo simulations which are used to identify values of the functions that constitute a null hypothesis region for comparison to samples with unknown ordering and clustering characteristics to yield functional values commensurate with those calculated for a particular natural rock specimen each simulation must match as closely as possible several critical features of the natural rock including the set of crystal radii limitations on the observability of crystals and the shape and size of the bounding surface of the sample crystallization mechanisms in seven previously studied garnetiferous rocks from three localities carlson et al 1995 denison and carlson 1997 have been reassessed using both scale dependent correlation functions and single valued spatial statistics both evaluated by comparison to rigorously computed null hypothesis regions the results confirm previous inferences that the nucleation and growth rates of the garnet porphyroblasts in these specimens were governed by rates of diffusion through the intergranular medium

river system recovery following the novaţ rosu tailings dam failure maramures county romania the river viseu catchment in maramures county northwestern romania has a long history of base and precious metal mining between 1994 and 2003 waste from mining activity at baia borsa was stored in the nova rosu tailings pond in the upper viseu catchment however in march 2000 the tailings dam failed releasing approximately 100 000 m3 of contaminated water and 20 000 t of mineral rich solid waste which was routed downstream through the rivers nova vaser and viseu into the river tisa following the accident metal cd cu pb zn concentrations in river water and river channel sediment were assessed in samples collected annually july 2000 2001 2002 and 2003 from 29 sites in the viseu catchment downstream of the tailings pond additionally the speciation of sediment associated metals was established using a 4 stage sequential extraction procedure sep and pb isotope analysis 206 204pb and 207 204pb was carried out to establish the provenance of contaminated sediments metal concentrations in river water were found to comply with eu directive target values within four months of the failure however the impact of the spill upon river channel sediments was found to be much longer lasting with evidence of the delayed downstream remobilization of tailings stored within the narrow nova valley following the dam failure as well as continued inputs of contaminated sediment to the river viseu from the river tisla another mining affected tributary comparison with data from other recent tailings dam failures indicates that river system recovery rates depend upon local geomorphological conditions hydrological regimes and the nature and scale of post spill clean up operations

the effect of metamorphic fluid flow on the nucleation and growth of garnets from troms north norway a spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from troms north norway the size distribution of quartz veins reflects the flow regime of metamorphic fluids the hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals in regions where fluid flow was interpreted as pervasive acceleration parameters for nucleation were high whereas in regions where fluid flow was interpreted as channelled acceleration parameters for growth were high accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins this spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and or mass transfer in the study area rhodochrosite mnco3 veins provide evidence for mass transfer of mn by fluid flow an influx of mn would expand the stability field of garnet to lower temperatures the resulting thermal overstep could accelerate nucleation and or growth of garnets the corollary of this study is that size distributions and chemical zoning of garnets or other porphyroblast phases can be used to study metamorphic fluid flow