hafnium isotopes in zircon a tracer of fluid rock interaction during magnetite apatite kiruna type mineralization abstract in situ analyses of hafnium isotopes in zircon combined with u pb zircon ages and trace element data provide new information on the characterization and evolution of magnetite apatite kiruna type deposits and the tectonic environments in which they occur the lyon mountain granite in the adirondack mountains of new york is the host to numerous zircon bearing magnetite apatite deposits hafnium isotopic compositions and rare earth element contents in individual zircon crystals were measured in situ in both the host granites and the ore bodies by laser ablation inductively coupled plasma mass spectrometry hafnium isotopic compositions in the ore zircon can be divided into two groups those that have initial ehf t values that are indistinguishable from those of the host granites e g ehf t less than 7 and are typical of relatively juvenile proterozoic crust and those that have extremely radiogenic ehf t values as high 40 two models are proposed to explain the observed ehf t values in the zircon crystals 1 early formed ore bodies containing magnetite apatite and clinopyroxene were remobilized by secondary fluid alteration releasing zr and hf for the crystallization of new zircon or alternatively 2 fluids responsible for ore formation have interacted with garnet bearing rocks during retrograde metamorphism scavenging rare earth elements and radiogenic hf previous work done to determine the u pb ages from the same zircon crystals which were analyzed for the hf isotopic composition in this study revealed that ore bodies record a mineralizing event that is 20 to 60 m y younger than the age of granite emplacement this age discrepancy plus the highly radiogenic ehf t values in the ore zircon crystals suggests that the fluids responsible for this younger event could not have been derived from the granite hosts these data argue that magnetite apatite deposits in the lmg have multiple mineralizing events superimposed upon one another and that early formed deposits may be reworked modified and redeposited by fluids subsequent to magma crystallization

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

fluvial changes of the guadalquivir river during the holocene in cordoba southern spain this research was funded by a grant from proyectos de cordoba siglo xxi coordinated by prof r jose roldan canas universidad de cordoba

prediction of concentrated flow width in ephemeral gully channels empirical prediction equations of the form w d aq b have been reported for rills and rivers but not for ephemeral gullies in this study six experimental data sets are used to establish a relationship between channel width w m and flow discharge q m 3 s 1 for ephemeral gullies formed on cropland the resulting regression equation w d 2 51 q 0 412 r 2 d 0 72 n d 67 predicts observed channel width reasonably well owing to logistic limitations related to the respective experimental set ups only relatively small runoff discharges i e q 0 02 m 3 s 1 were covered using field data where measured ephemeral gully channel width was attributed to a calculated peak runoff discharge on sealed cropland the application field of the regression equation was extended towards larger discharges i e 5 10 4 m 3 s 1 q 0 1m 3 s 1 comparing w q relationships for concentrated flow channels revealed that the discharge exponent b varies from 0 3 for rills over 0 4 for gullies to 0 5 for rivers this shift in b may be the result of i differences in flow shear stress distribution over the wetted perimeter between rills gullies and rivers ii a decrease in probability of a channel formed in soil material with uniform erosion resistance from rills over gullies to rivers and iii a decrease in average surface slope from rills over gullies to rivers the proposed w q equation for ephemeral gullies is valid for sealed cropland with no significant change in erosion resistance with depth two examples illustrate limitations of the w q approach in a first example vertical erosion is hindered by a frozen subsoil the second example relates to a typical summer situation where the soil moisture profile of an agricultural field makes the top 0 02 m five times more erodible than the underlying soil material for both cases observed w values are larger than those predicted by the established channel width equation for concentrated flow on cropland for the frozen soils the equation w d 3 17 q 0 368 r 2 d 0 78 n d 617 was established but for the summer soils no equation could be established copyright 2002 john wiley sons ltd

implications of bank failures and fluvial erosion for gully development field observations and modeling 1 gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop field observations attribute the headward growth and widening of many gully systems to gravitational mass wasting processes of oversteepened sidewalls soil saturation groundwater sapping and tension crack development contribute to the instability recent landscape evolution models treat such mass failures as slope dependent continuous sediment transport processes sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient in this study we first present an explicit physically based theory for the stability analysis of gully heads and walls the theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face then we test the theory against field data collected in our field site in colorado and against other published data second the theory is implemented in a one dimensional hillslope profile development model and the three dimensional channel hillslope integrated landscape development child to study the effects of soil cohesion erosion thresholds and stochastic climate on the tempo of gully development and morphology preliminary results indicate that wider and shallower gullies develop and integrate forming wide valleys when soil cohesion is small as soil cohesion increases erosion slows down gullies become deeper with vertical walls and episodic mass failures occur differences in storm intensity duration characteristics and erosion thresholds are predicted to have a significant impact on gully development vertical gully walls develop rapidly and gullies enlarge by slab failures in a climate characterized by high intensity short duration storm pulses however under low intensity long duration storms gullies quickly stabilize and vertical walls are eliminated and rounded forming diffusion dominated hilltops erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction lowering the erosion threshold enhances gully widening by slab failures gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls

zircon geochronology and ca 400 ma exhumation of norwegian ultrahigh pressure rocks an ion microprobe and chemical abrasion study understanding the formation and exhumation of the remarkable ultrahigh pressure uhp rocks of the western gneiss region norway hinges on precise determination of the time of eclogite recrystallization we conducted detailed thermal ionization mass spectrometry chemical abrasion analysis and sensitive high resolution ion microprobe analysis of zircons from four ultrahigh and high pressure hp rocks ion microprobe analyses from the flatraket eclogite yielded a broad range of apparently concordant caledonian ages suggesting long term growth in contrast higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi step chemical abrasion yielded moderate pb loss from the first lowest temperature abrasion step possible minor pb loss or minor growth at 400 ma from the second step and a 407 404 ma cluster of slightly discordant 206pb 238u ages most likely free from pb loss from the remaining abrasion steps we interpret the latter to reflect zircon crystallization at 405 400 ma with minor discordance from inherited cores zircon crystallization occurred at eclogite facies possibly post peak conditions based on compositions of garnet inclusions in zircon as well as nearly flat hree profiles and lack of eu anomalies in zircon fractions subjected to chemical abrasion these ages are significantly younger than the 425 ma age often cited for western norway eclogite recrystallization implying faster rates of exhumation 2 5 8 5 km myr and coeval formation of eclogites across the uhp portion of the western gneiss region

hadean diamonds in zircon from jack hills western australia this paper reports the discovery of micro diamond inclusions in zircon from the jack hills western australia which is up to 4 252 million years old and includes the oldest known diamonds found in terrestrial rocks the spread of ages indicates that either the conditions required for diamond formation were repeated several times during early earth history or that there was significant recycling of ancient diamond

the cycle of instability stress release and fissure flow as controls on gully head retreat gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation sapping or piping the empirical evidence to substantiate these conceptual models is sparse however and often contradictory this paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology stability and elastic deformation of a marl gully complex in granada province south east spain the hydrological and slope stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head and that vertical or subvertical heads will be stable unless saturation is achieved owing to the high unsaturated strengths of marl measured in this research failure in unsaturated conditions is possible only where the gully head wall is significantly undercut head retreat thus requires the formation of either a tension crack or an undercut hollow finite element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base and a second stress anomaly just upslope of the head wall although tension cracks on gully heads have often been attributed to slope unloading this research provides strong evidence that the so called sapping hollow commonly found in the gully headwall base is also a function of stress release although further research is needed it seems possible that pop out failures in river channels may be caused by the same process the hydrological analysis shows that once a tension crack has developed throughflow velocity in the gully headwall will increase by an order of magnitude promoting piping and enlargement of this weakened area it is therefore possible to envisage a cycle of gully expansion in which erosion channel incision or human action unloads the slope below a gully head leading to stress patterns that account for the tension crack and a stress release hollow the tension crack promotes faster throughflow encouraging hollow enlargement and piping which undercut the gully head the tension crack permits the development of positive pore water pressures behind the gully head leading either to failure or contributing to toppling finally the debris may be eroded by fluvial action unloading a new section of slope and completing the cycle of gully head retreat copyright 2001 john wiley sons ltd

testing the ephemeral gully erosion model egem for two mediterranean environments few models can predict ephemeral gully erosion rates e g creams egem the ephemeral gully erosion model egem was specifically developed to predict soil loss by ephemeral gully erosion although egem claims to have a great potential in predicting soil losses by ephemeral gully erosion it has never been thoroughly tested the objective of this study was to evaluate the suitability of egem for predicting ephemeral gully erosion rates in mediterranean environments an egem input data set for 86 ephemeral gullies was collected detailed measurements of 46 ephemeral gullies were made in intensively cultivated land in southeast spain guadalentin study area and another 40 ephemeral gullies were measured in both intensively cultivated land and abandoned land in southeast portugal alentejo study area together with the assessment of all egem input parameters the actual eroded volume for each ephemeral gully was also determined in the field a very good relationship between predicted and measured ephemeral gully volumes was found r2 0 88 but as ephemeral gully length is an egem input parameter both predicted and measured ephemeral gully volumes have to be divided by this ephemeral gully length in order to test the predictive capability of egem the resulting relationship between predicted and measured ephemeral gully cross sections is rather weak r2 0 27 therefore it can be concluded that egem is not capable of predicting ephemeral gully erosion for the given mediterranean areas a second conclusion is that ephemeral gully length is a key parameter in determining the ephemeral gully volume regression analysis shows that a very significant relation between ephemeral gully length and ephemeral gully volume exists r2 0 91 accurate prediction of ephemeral gully length is therefore crucial for assessing ephemeral gully erosion rates copyright 2001 john wiley sons ltd

application of the egem model to predict ephemeral gully erosion in sicily italy abstract recent research has shown the importance of ephemeral gully eg erosion in the context of global erosion and at the same time the lack of adequate models to estimate it at present the ephemeral gully erosion model egem is the only conceptual model specifically developed for ephemeral gully erosion estimation the main aim of the paper is to evaluate the performance of egem model adapted to local conditions compared with that of simple empirical equations a data set of 92 ephemeral gullies formed between 1995 and 2000 in a wheat cultivated study area in sicily italy was used for the purpose in order to improve egem performance two important adaptations were made both the rain distribution type and the causative rainfall of the hydrological component of egem were modified to reproduce local conditions the egem assumption of a constant depth for the entire length of the ephemeral gully was also modified egem performance was tested with two different rain distribution types three different causative rainfalls and two different eg depths the adaptation of the egem hydrological component improves the volume prediction with respect to the standard version of egem the improvement is mainly due to the rain distribution type whereas causative rainfall seems to be less important the use of adjusted eg depths is however essential to minimise the errors between measured and predicted volumes the capability of egem to predict the ephemeral gully cross section is weak and non significant relationships were found between measured and estimated width a regression analysis between ephemeral gully eroded volume and different morphological topographical and hydrological parameters confirms the great importance of eg length to explain eroded volume r 2 0 64 for the simple correlation between volume and length considering that the adaptation of the hydrological component and the measurement of eg depths are necessary for the eg volumes estimated by egem to be reliable it seems more simple as suggested by other authors to use empirical relations between eroded volume and length performed in different environments until more precise physically based models are developed

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

the role of sidewall processes in gully development some n s w examples little attention has been paid to the role of sidewall processes in gully development simple estimates of linear incision sidewall erosion ratios at two localities in new south wales australia suggest that sidewall erosion is responsible for more than half the gully volume studies at three localities where overland flow and throughflow are limited to only one sidewall indicate that these processes are responsible for 10 30 per cent of gully volume these observations have important implications for gully management

an experimental study of trace element partitioning between zircon and melt as a function of oxygen fugacity abstract the partitioning of p sc ti sr y nb the rare earth elements ree hf ta th and u between zircon and a synthetic andesitic melt was determined as a function of oxygen fugacity f o 2 over a range of fourteen log units from qfm 8 to qfm 6 where qfm is the quartz fayalite magnetite oxygen buffer at 1300 c and 1 atm using sims and la icp ms the partition coefficients of ce and eu were found to vary systematically with f o 2 relative to those of the other ree producing ce and eu anomalies similar to those that are characteristic of natural igneous zircon these anomalies coexist at terrestrial f o 2 s and were used to quantify eu 3 eu 2 eu 3 and ce 4 ce 3 ce 4 in the melt the partition coefficients of the redox invariant trivalent cations are in excellent agreement with lattice strain theory the values of d ree are related to those determined for natural samples by the expected dependence on temperature the incorporation of ree 3 in zircon was independent of the presence of p d u d th was found to vary systematically with f o 2 indicating the presence of u 4 u 5 and u 6 in the melt at terrestrial conditions d ti was independent of f o 2 despite an expectation of significant ti 3 in the most reduced experiments

radiation effects in zircon the widespread distribution of zircon in the continental crust its tendency to concentrate trace elements particularly lanthanides and actinides its use in age dating and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies because zircon is highly refractory it also has important industrial applications including its use as a lining material in high temperature furnaces however during the past decade zircon has also been proposed for advanced technology applications such as a durable material for the immobilization of plutonium or when modified by ion beam irradiation as an optic waveguide material in all of these applications the change in properties as a function of increasing radiation dose is of critical importance in this chapter we summarize the state of knowledge on the radiation damage accumulation process in zircon

factors controlling the morphology and volume v length l relations of permanent gullies in the northern ethiopian highlands small scale aerial photographs and high resolution satellite images available for ethiopia since the second half of the twentieth century as for most countries allow only the length of gullies to be determined understanding the development of gully volumes therefore requires that empirical relations between gully volume v and length l are established in the field so far such v l relations have been proposed for a limited number of gullies environments and were especially developed for ephemeral gullies in this study v l relations were established for permanent gullies in northern ethiopia having a total length of 152 km in order to take the regional variability in environmental characteristics into account factors that control gully cross sectional morphology were studied from 811 cross sections this indicated that the lithology and the presence of check dams or low active channels were the most important controls of gully cross sectional shape and size cross sectional size could be fairly well predicted by their drainage area the v l relation for the complete dataset was v 0 562 l 1 381 n 33 r2 0 94 with 34 9 of the network having check dams and or being low active producing such relations for the different lithologies and percentages of the gully network having check dams and or being low active allows historical gully development from historical remote sensing data to be assessed in addition gully volume was also related to its catchments area a and catchment slope gradient sc this study demonstrates that v l and v a sc relations can be very suitable for planners to assess gully volume but that the establishment of such relations is necessarily region specific copyright 2013 john wiley sons ltd

overwash processes along a gravel beach in south east ireland recent overwashing and to a more limited extent overtopping has occurred on a gravel dominated beach just west of carnsore point in south east ireland at relatively low points swash excursions have breached the duneline and extensive gravel fans have formed on landward facing slopes also at interbreach positions swash ramps have been built up in front of previously eroding dune cliffs allowing overtopping at heights of up to 5 m above mhwost spreading a veneer of sediment over the adjacent dune ridge in both cases events appear to be preceded by erosion and backbeach sand and gravel deposition stratigraphic textural and fabric analyses assist in identifying environmental conditions that prevailed during overwash throat and fan and overtopping ramp sedimentation

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

exsolution of thortveitite yttrialite and xenotime during low temperature recrystallization of zircon from new caledonia and their significance for trace element incorporation in zircon recrystallized zircon grains in a phengite epidote chlorite schist from north eastern new caledonia contain as inclusions a mineral assemblage consisting of celadonite kaolinite quartz fe oxy hydroxide smectite chlorite xenotime y thortveitite yttrialite and allanite ce this assemblage formed during low temperature 100 c seafloor alteration of a plagioclase rich mafic rock and represents the first documented evidence of this alteration event in the high p belt of northeastern new caledonia the survival of this low temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions thortveitite sc2si2o7 and yttrialite y2si2o7 inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized the altered zircon cores generally have lower trace element contents than the pristine igneous zircon cores and we suggest that thortveitite yttrialite and xenotime y formed as a result of trace element expulsion from zircon during low temperature recrystallization trace element concentrations in the zircon cores indicate that trivalent cations ree y sc in zircon cannot be charge balanced by xenotime substitution alone pentavalent cation concentrations nb as are also insufficient for charge balance the presence of thortveitite sc2si2o7 and yttrialite y2si2o7 suggests that trivalent cations in zircon might be charge balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as h occurring interstitially in the zircon lattice

linking long term gully and river channel dynamics to environmental change using repeat photography northern ethiopia abstract in the highlands of northern ethiopia gully occurrence is linked to poverty driven unsustainable use of the land in a vulnerable semi arid and mountainous environment where intensive rainfall challenges the physical integrity of the landscape trends in gully and river channel erosion and their relation to triggering environmental changes can proffer valuable insights into sustainable development in northern ethiopia in order to assess the region wide change in gully and river channel morphology over 140 years a set of 57 historical photographs taken in tigray and clearly displaying gully cross sections were precisely repeated from 2006 till 2009 ninety two percent of the gully and river sections n 38 increased in cross sectional area during the studied period especially after 1975 two repeatedly photographed catchments of lake ashenge and atsela allowed a detailed study of gully development from 1936 until 2009 a conceptual hydrogeomorphic model was devised for these catchments and validated for the northern ethiopian highlands three major phases can be distinguished in the hydrological regime of the catchments in the first phase between 1868 or earlier and ca 1965 the relatively stable channels showed an oversized morphology inherited from a previous period when external forcing in environmental conditions had caused the channels to shape in the second phase ca 1965 ca 2000 increased aridity and continued vegetation clearance accelerated the channel dynamics of the gully and river system the third phase ca 2000 present started after the large scale implementation of soil and water conservation measures in 2009 23 of the gully and river sections were stabilizing this paper validates previous research indicating severe land degradation in the second half of the 20th century additionally it demonstrates that the recent erosive cycle started around 1965 and that at the present time improved land management stabilizes headwater streams

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

zircon solubility in alkaline aqueous fluids at upper crustal conditions field evidence and thermodynamic data at ambient conditions suggest that complexing of zr with hydroxyl ions at high ph enhances the solubility of zircon we tested this hypothesis by measuring the solubilities of the assemblages zircon zrsio 4 baddeleyite z b and zircon quartz z q in neutral to alkaline fluids at 0 2 gpa and 450 750 c in neutral to alkaline fluids 0 0 1 1m naoh zircon dissolved incongruently in quartz undersaturated fluids to form baddeleyite at 450 c various na zr silicates precipitated from fluids saturated and undersaturated in quartz we observed no significant dependence of solubility on temperature the measured solubilities of zircon in pure h 2o at 600 and 750 c are only slightly higher than the procedural blank of 3 3 10 7 m zr however the measured solubility of zircon increases with increasing aqueous silica suggesting that zirconium complexes with silica in the fluid at 600 c linear regression of experimental results yielded ln zr 0 25 2 9 ln si where terms in parentheses represent molal concentrations zircon solubility also seems to increase with increasing hydroxyl concentration in the fluid enhanced solubility in silica rich alkaline fluids may cause dissolution reprecipitation of zircon and resetting of its isotopic clocks suggesting that some zircon u pb ages may correspond to fluid events

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

discussion on coesite bearing eclogite breccia implication for coseismic ultrahigh pressure metamorphism and the rate of the process by yang et al contrib mineral petrol 2014a 167 1013 recently yang et al contrib mineral petrol 167 1013 2014a proposed that cataclasites breccias and pseudotachylytes of eclogites at yangkou in the chinese sulu uhp metamorphic belt are formed by a compression stress wave of earthquake they suggested that the intergranular coesite resulted from a rapid cooling from about 700 c to below 375 400 c and a sudden pressure release of seismic wave from 3 3 to 1 2 gpa in some hours however the earthquake induced uhp metamorphism under fluid deficient conditions proposed by these authors remains inconclusive and inconsistent with the available data from the uhp metamorphic belt the coesite inclusions within garnet zircon and omphacite and intergranular coesite grains between these minerals can be preserved by the presence of the pressure vessel effect the lack of fluid infiltration and the presence of a low temperature and nonhydrostatic deformation environment during rapid exhumation

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

linking growth episodes of zircon and metamorphic textures to zircon chemistry an example from the ultrahigh temperature granulites of rogaland sw norway in situ u th pb analyses by ion microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon this approach helps a to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the rogaland anorthosite norite intrusive complex and b to constrain the age of m 2 ultrahigh temperature uht metamorphism and the subsequent retrograde m 3 event all samples yield magmatic inherited zircon of c 1035 ma some an additional group at c 1050 ma this suggests that loss of pb by volume diffusion in non metamict zircon is not an important factor even under extreme crustal conditions furthermore the identical inheritance patterns in aluminous garnet cordierite osumilite bearing migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite results for the m 1 metamorphic event at c 1000 ma bp are consistent in all samples including those from outside the orthopyroxene in isograd the latter do not show evidence for zircon growth during the m 2 metamorphic episode zircon intergrown with or included within m 2 metamorphic minerals magnetite spinel orthopyroxene give an age of 927 7 ma 2 n 20 the youngest observed results are found in zircon outside m 2 minerals some overgrown by m 3 mineral assemblages late garnet coronas garnet quartz and orthopyroxene garnet symplectites and yield a slightly younger pooled age of 908 9 ma 2 n 6 these textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the uht event these youngest age groups are consistent with the emplacement age of the rogaland intrusive complex and the last magmatic activity tellnes dyke intrusion respectively this is direct and conclusive evidence for uht metamorphism in the regional aureole being caused by the intrusions and corrects earlier notions that the events are not linked trace element behaviour of zircon tb u and y content has been tracked through time in the samples and shows variations both within and between samples this heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of rules of thumb in the interpretation of zircon chemistry but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism

formation of planar deformation features pdfs in zircon during coseismic faulting and an evaluation of potential effects on u pb systematics abstract zircons from pseudotachylyte and cataclasite veins in the svarthumlevatnet metagabbro sw norway are faulted brecciated and display multiple sets of planar deformation features pdfs that we assign to coseismic deformation in the middle crust during caledonian continent collision the zircons give a u pb upper intercept age of 1507 4 ma interpreted as a minimum age of the gabbro and a lower intercept age of 968 31 ma likely indicating the time of granulite facies metamorphism but they do not record the caledonian coseismic faulting even when the fault planes are healed by thin veins of low u zircon the work indicates that pdfs are not exclusive to meteorite impacts but may also develop during seismic faulting

low temperature alteration of monazite fluid mediated coupled dissolution precipitation irradiation damage and disturbance of the u pb and th pb chronometers abstract low temperature alteration of monazite is documented in three centimeter sized monazite crystals from norway arendal madagascar ambato and sri lanka the three crystals have different chemical compositions especially in their u th y and pb contents and have 208 pb 232 th ages ranging from 491 to 900 ma optical microscope om scanning electron microscope sem and transmission electron microscope tem images and electron microprobe analyses epma show that all three preserve a similar patchy texture this texture is interpreted as resulting from an alteration reaction in which unaltered monazite mnz1 reacts to form a secondary th u y depleted high th u monazite mnz2 accompanied by thorite huttonite thsio 4 thorianite tho 2 and xenotime ypo 4 the proportions of which are dependent upon the initial composition of the monazite mnz1 images reveal variably intense internal fracturing with cracks filled with th rich fe rich phases monazite xenotime thermometry demonstrates that the pristine monazites mnz1 interacted with a low temperature fluid the alteration process is interpreted to follow a mechanism of fluid mediated coupled dissolution precipitation chemical dating with the electron microprobe shows no th u pb age differences between primary and secondary monazites except in the case of the ambato monazite in which altered domains yield older 750 ma apparent ages than the pristine mnz1 domains u pb and th pb isotope dating using la icp ms yields ages consistent with electron probe dates for pristine mnz1 zones however disturbance of these systems in the altered monazite domains leads to variable age results for these depending on sample in the case of sri lanka and arendal only 208 pb 232 th dates provide a reasonable estimate of the age of alteration which are constrained to be 450 and 864 ma respectively u pb systems are disturbed due to common pb contamination up to 40 and u fractionation relative to th during alteration responsible for depletion of u in altered monazites and increase of th u in contrast for the ambato monazite both the u pb and th pb systems were affected and yield inconsistent older dates for altered zones this is attributed to significant common pb contamination up to 80 which affects all pb isotopes and explains why electron probe ages are erroneous th u silicate contamination during measurement resulting from the presence of a numerous nano phases and nano fractures filled with th u silicates that are visible only under tem also contributes to the anomalously old ages for these disturbed mnz2 domains these results demonstrate the important role of radiation damage effects in particular swelling induced fracturing and the essential role of porosity and cracks which allow fluid charged with elements migration through monazite during low temperature alteration

predicting gully width and widening rates from upstream contribution area and rainfall a case study in sw spain abstract gully width w is usually characterized by having a power relation with discharge however calculating runoff discharge in ungauged basins where these gullies typically form is inherently difficult and associated with large uncertainties this paper examines a simplified approach to relating gully width directly to the runoff contributing area a in a cultivated area in sw spain the effect of rainfall and land use on this w a relation was also analysed gully width was measured in 46 sections through the analysis of 10 historical orthophotos between 1956 and 2013 in a 21 km2 catchment in s spain these were validated by field measurements in 2013 widening rates varied strongly over time at between 0 and 2 19 m year 1 no significant differences in the gully widening rates were found between the two land uses present in the study zone olive groves and herbaceous crops the data obtained show a significant power relation of the form w a for all time periods analysed except for 1980 when many gullies were filled in artificially due to a change in land use in the study area the power relation coefficient varied at between 0 2 and 0 3 a good correlation was obtained between the number of days with daily rainfall of above 13 mm and the increase in the coefficient over time the results of this study on gully width dynamics provide new insights into improving the estimation of gully volume for example in combination with gully headcut retreat models and including the effects of different climate scenarios

u pb dates and trace element geochemistry of zircon from migmatite western gneiss region norway significance for history of partial melting in continental subduction article i nfo thewesterngneissregion wgr norway isdominatedbymigmatiticgneissthatcontainsinclusionsofeclogite some of which contain evidence for ultrahigh pressure metamorphism to evaluate geochemical and age relation ships between host migmatite and eclogite we obtained la icp ms u pb dates and trace element analyses for zircon from a variety of textural types of leucosome from layer parallel to crosscutting zircon textures euhedral oscillatory and sector zone grains indicate a likely magmatic origin of the leucosomes caledonian u pb zircon dates from zircon rim and near rim regions are as old as 410 406 ma coeval with previously determined ages of high and ultrahigh pressure metamorphismof wgr eclogite trace elementanalysesobtained simultaneously with u pb dates indicate crystallization of zircon under garnet present conditions in the majority of leucosomes other zircons including those from crosscutting pegmatite yield younger ages as young as 385 ma coinciding with dates determined for amphibolite facies retrogression of eclogite trace element analyses suggest that these zircons grew under plagioclase present garnet absent conditions combined age and trace element data for leucosome zircons record the transition from high pressure garnet present plagioclase absent crys tallization to lower pressure plagioclase present crystallization if the euhedral zircons that yield ages coeval with peak or near peak uhp metamorphism represent crystallization from anatectic leucosomes these results combined with field and petrographic observations of eclogite migmatite relationships are consistent with the presence of partially molten crust in at least part of the wgr during continental subduction the decreased viscosity and increased buoyancy and strain weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths

mobility of trace elements and oxygen in zircon during metamorphism consequences for geochemical tracing this study addresses the mobility of the rare earth elements ree o u and pb during the formation of metamorphic zircon by overgrowth and coupled dissolution reprecipitation for this purpose the texture and ree 18o and u pb compositions of zircon crystals from a metabasite an anatectic granite and a metapelite sampled on naxos island greece have been characterised the behaviour of o and ree during the formation of the zircon metamorphic domains shows that two end members can be distinguished i closed or partially closed system behaviour for oxygen and trace elements in zircon formed by coupled dissolution reprecipitation metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements such zircons have to be used with caution for geochemical tracing when re equilibration is not complete the u pb system is also not completely reset and the metamorphic domains yield old scattered ages ii open system behaviour for oxygen and trace elements in zircon formed by overgrowth these metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals because equilibrium between the metamorphic zircon and the matrix was thus favoured zircon crystallisation can be more confidently related to the p t history and fluid circulation events by geochemical tracing

gully head erosion processes on a semi arid valley floor in kenya a case study into temporal variation and sediment budgeting a three year monitoring programme of gully head retreat was established to assess the significance of sediment production in a drainage network that expanded rapidly by gully head erosion on the low angled alluvio lacustrine njemps flats in semi arid baringo district kenya this paper discusses the factors controlling the large observed spatial and temporal variation in gully head retreat rates ranging from 0 to 15 m a 1 the selected gullies differed in planform and in runoff contributing catchment area but soil material and land use were similar the data were analysed at event and annual timescales the results show that at annual timescale rainfall amount appears to be a good indicator of gully head retreat while at storm event timescale rainfall distribution has to be taken into account a model is proposed including only rainfall p and the number of dry days dd between storms which explains 56 per cent of the variation in retreat rate of the single headed gully of lam1 a detailed sediment budget has been established for lam1 and its runoff contributing area rca by measuring sediment input from the rca the sediment output by channelized flow and linear retreat of the gully head for nine storms it can be seen that erosion shifts between different components of the budget depending on the duration of the dry period dd between storms sediment input from the rca was usually the largest component for the smaller storms the erosion of the gully head occurred as a direct effect of runoff falling over the edge ghwaterfall and of the indirect destabilization of the adjacent walls by the waterfall erosion and by saturation ghmass storage the latter component ghmass storage was usually much larger that the former ghwaterfall the sediment output from the gully was strongly related to the runoff volume while the linear retreat because of its complex behaviour was not overall the results show that the annual retreat is the optimal timescale to predict retreat patterns more detailed knowledge about relevant processes and interactions is necessary if gully head erosion is to be included in event based soil erosion models copyright 2001 john wiley sons ltd

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

anatomy of zircon growth in high pressure granulites sims u pb geochronology and lu hf isotopes from the jiaobei terrane eastern north china craton abstract granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction collision settings here we investigate zircons in the paleoproterozoic high pressure hp granulites from jiaobei terrane in the eastern north china craton that experienced a multi stage metamorphic evolution we present sims zircon u pb ages for zircons from garnet amphibolites mafic granulites and pelitic granulites our data show four age peaks at 2 5 ga 1 92 ga 1 87 1 85 ga and 1 83 ga zircons belonging to the latter three age groups show similar characteristics of low th and u contents low th u ratios 176 lu 177 hf ratios and low 176 hf 177 hf ratios of zircons from the protoliths the youngest u pb age of ca 1899 ma is interpreted as the maximum estimate of the time of peak metamorphism zircons formed through intergranular melt growth during low to medium pressure retrogression display homogeneous lu hf isotope compositions low 176 lu 177 hf and high 176 hf 177 hf ratios the u pb age of 1870 1850 ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low to medium pressure granulite facies retrogression some zircon grains with prismatic morphology and low 176 lu 177 hf and 176 hf 177 hf ratios display heterogeneous lu hf isotopic compositions possibly related to coupled dissolution reprecipitation the u pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage zircons formed through subsolidus growth show large variations in lu hf isotopic compositions and their ca 1833 ma age represents the timing of amphibolite facies retrogression the hp granulite facies metamorphism in jiaobei is thus constrained between 1870 and 1899 ma with low to medium pressure granulite facies retrogression at ca 1850 1870 ma followed by amphibolite facies retrogression at ca 1833 ma our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites for the pelitic granulites the older zircons are detrital grains of magmatic origin the hf crustal model ages of these zircons that show peaks at ca 2 7 ga and 2 9 ga are consistent with the timing of mafic magmatism in the region as identified in previous studies the lu hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith the zircon hf crustal model age of ca 2 5 2 7 ga from the pelitic granulites is consistent with that of the basement rocks in the jiaobei terrane and the zircon hf model ages t dm of ca 2 0 ga and 2 2 2 3 ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination the geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the jiaobei terrane are comparable with the tectonic history of the central and western north china craton and provide insights into the complex tectonothermal events associated with the neoarchean paleoproterozoic crustal evolution history of the craton also the pervasive 2 5 ga zircons in a variety of rocks seem to suggest that the jiaobei terrane was already part of a coherent tectonic unit at such time

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

an o isotope profile through the hp lt corsican ophiolite france and its implications for fluid flow during subduction abstract subduction zones are important sites for fluid generation as the downgoing slab undergoes progressive high pressure metamorphism these fluids are thought to play an important role in the generation of arc magmas but their affect on the subducted slab itself is less clear in this study the stable isotope geochemistry of the corsican ophiolite which is variably affected by subduction related metamorphism was investigated to assess the impact of subduction zone fluid flow on the downgoing slab a total of 170 stable isotope analyses oxygen and carbon showed the following features 1 overall the ophiolite preserves the same oxygen isotope profile with depth as unmetamorphosed ophiolites 2 the variation in oxygen isotope values for different areas does not correlate with metamorphic grade 3 oxygen and carbon isotope values for calcite in the ophiolite are not in equilibrium with the ophiolite but are similar to those of the overlying calc schists schistes lustres and 4 hydrogen isotopes indicate that serpentinitisation of ultramafics occurred during sub seafloor hydrothermal alteration and probably also during tectonic emplacement of the ophiolite these features indicate that large scale fluid flow did not affect the corsican ophiolite during subduction zone metamorphism calcite in the ophiolite was probably precipitated from co2 bearing fluids derived from decompression dehydration reactions affecting the overlying schistes lustres during exhumation of the ophiolite bearing schistes lustres nappes given that ocean crust must undergo at least some dehydration during prograde subduction zone metamorphism the evolved fluids must be either highly channelled or exit the slab relatively quickly so as to not interact with it the results of this study indicate that the important fluid pathways on corsica have yet to be identified but are probably the shear zones and fault systems that bound different slices of the high pressure nappe stack

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

an empirical investigation of gully widening rates in upland concentrated flows abstract soil erosion in its myriad forms devastates arable land and infrastructure as an integral landscape feature gully erosion is a complex system as its evolution is controlled by upstream migration of a gully head headcut face incision of the gully bed plunge pool and gravitational mass movement widening on gully channel sidewalls bed incision is often limited by the presence of a less erodible soil layer when erosion reaches such a layer the gully typically widens creating a wide shallow cross section once a gully is initiated transport and deposition of the eroded soil and widening of the channel further govern its evolution our knowledge of these processes within agricultural regions however is still quite limited with theory largely borrowed from river hydraulics experiments were conducted to examine gully expansion widening as a function of channel slope and overland flow discharge packed soil beds were subjected to simulated rainstorms followed by clear water overland flow as bed slope and discharge increased channel widening increased an empirical equation based upon measured channel width from constant discharge experiments was developed in terms of slope and discharge by further quantifying key aspects of gully erosion processes results from this experimental campaign will significantly improve soil erosion prediction technology for use in agricultural regions

the oxygen isotope composition of karoo and etendeka picrites high δ18o mantle or crustal contamination oxygen isotope compositions of karoo and etendeka large igneous province lip picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation olivine and orthopyroxene phenocrysts from lavas and dykes mg 64 80 from the tuli and mwenezi nuanetsi regions of the ca 180 ma karoo lip have 18o values that range from 6 0 to 6 7 they appear to have crystallized from magmas having 18o values about 1 1 5 higher than expected in an entirely mantle derived magma olivines from picrite and picrite basalt dykes from the ca 135 ma etendeka lip of namibia and karoo age picrite dykes from dronning maud land antarctica do not have such elevated 18o values a range of 18o values from 4 9 to 6 0 and good correlations between 18o value and sr nd and pb isotope ratios for the etendeka picrites are consistent with previously proposed models of crustal contamination explanations for the high 18o values in tuli mwenezi picrites are limited to 1 alteration 2 crustal contamination and 3 derivation from mantle with an abnormally high 18o previously a variety of models that range from crustal contamination to derivation from the enriched mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as k and average end and esr values of 8 and 16 in mwenezi nuanetsi picrites however the primitive character of the magmas mg 73 combined with the lack of correlation between 18o values and radiogenic isotopic compositions mgo content or mg is inconsistent with crustal contamination thus an 18o enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated high 18o values are accompanied by negative nb and ta anomalies consistent with the involvement of the mantle lithosphere whereas the high 18o themselves are consistent with an eclogitic source magma 18o values about 1 higher than expected for mantle derived magma are also a feature of the bushveld mafic and ultramafic magmas and the possibility exists that a long lived 18o enriched mantle source has existed beneath southern africa a mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during archaean subduction

metamorphic growth and recrystallization of zircons in extremely 18o depleted rocks during eclogite facies metamorphism evidence from u pb ages trace elements and o hf isotopes abstract a combined in situ sims and la mc icpms study of u pb ages trace elements o and lu hf isotopes was conducted for zircon from eclogite facies metamorphic rocks in the sulu orogen the two microbeam techniques sampled various depths of zircon domains revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique this enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization magmatic cores with u pb ages of 769 9 ma have positive 18o values of 0 1 10 1 high th u and 176lu 177hf ratios high ree contents and steep mree hree patterns with negative eu anomalies they are interpreted as crystallizing from positive 18o magmas during protolith emplacement in contrast newly grown domains have concordant u pb ages of 204 4 to 252 7 ma and show negative 18o values of 10 0 to 2 2 low th u and 176lu 177hf ratios low ree contents and flat hree patterns with weak to no eu anomalies they are interpreted as growing from negative 18o fluids that were produced by metamorphic dehydration of high t glacial hydrothermally altered rocks during continental subduction zone metamorphism differences in 18o between different domains within single grains vary from 0 8 to 12 5 suggesting different degrees of o isotope exchange between the positive 18o magmatic core and the negative 18o metamorphic fluid during the metamorphism the magmatic zircons underwent three subtypes of metamorphic recrystallization depending on their accessibility to negative 18o fluids the zircons recrystallized in solid state maintained positive 18o values and ree and lu hf isotopes of protolith zircon but their u pb ages are lowered the zircons recrystallized through dissolution exhibit negative 18o values similar to the metamorphic growths almost completely reset u pb ages and partially reset ree systems the zircons recrystallized through replacement show variably negative 18o values and partially reset ree and u pb and lu hf isotopic systems therefore this study places robust constraints on the origin of metamorphic zircons in eclogite facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision