conflicting sm nd whole rock and u pb zircon ages for archean lavas from newton township abitibi belt ontario abstract archean komatiitic and tholeiitic lavas from newton township ontario have a sufficient range of sm nd ratios to define a well constrained line on the normal 143 nd 144 nd vs 147 sm 144 nd isochron plot the data give an isochron age of 2826 64ma and an initial e nd of 2 65 0 26 however u pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite tholeiite suite give an age of 2697 1 1ma there is strong evident that the zircon age is the eruption age suggesting that the older sm nd age is incorrect and probably results from mixing between isotopically distinct mantle sources at the time of eruption the sources had e nd values from about 4 2 to 1 6 indicating that the archean mantle in this area was markedly heterogeneous and not uniformly depleted

compositional heterogeneity of archean mantle estimated from sr and nd isotopic systematics of basaltic rocks from north pole australia and the isua supracrustal belt greenland abstract compositional variability found in modern mid ocean ridge basalt morb and ocean island basalt oib reflects differentiation processes associated with material recycling in the mantle crust system to investigate the timing at which this recycling system was established and how it transformed into the present day system we present geochemical analyses of the archean basalts from north pole np in the east pilbara craton western australia and the isua supracrustal belt isb southern west greenland these rocks represent archean accretionary complexes with ages of 3 5 ga and 3 7 3 8 ga respectively we analyzed the trace element contents including rare earth elements rees and sr and nd isotopic compositions of the basalts which may represent morbs and oibs from np and isb their trace element compositions are broadly similar but show distinct geochemical characteristics particularly with respect to rees that probably reflect differences in both the source mantle and degree of melting such differences are also evident in their initial nd isotopic compositions which were estimated based on equilibrium partitioning of rees and well defined isochron ages in contrast the sr isotopic compositions of the np and isb basalts are highly variable and their isochron ages are inconsistent with previous studies furthermore the partitioning of rb and sr in the np basalts indicates disequilibrium suggesting that the rb sr system has been disturbed by post igneous alteration and metamorphism based on these observations we propose the following model to explain the temporal variations in the geochemical composition of the archean mantle i 3800 ma recycling of plate material and melting occurred quite readily and therefore morbs and oibs were produced from differentiated mantle sources ii 3460 ma to 3800 ma mantle crust mixing occurred as the result of an extreme event such as mantle overturning reducing the compositional variation of the mantle and iii after 3460 ma mantle heterogeneity gradually developed in the material recycling system re establishing the compositional differences between morbs and oibs this model requires an extreme event to drive the homogenization during stage ii which may provide new insights into the evolution of the crust mantle system

vertical diabatic heating structure of the mjo intercomparison between recent reanalyses and trmm estimates abstractcapitalizing on recently released reanalysis datasets and diabatic heating estimates based on tropical rainfall measuring mission trmm the authors have conducted a composite analysis of vertical anomalous heating structures associated with the madden julian oscillation mjo because diabatic heating lies at the heart of prevailing mjo theories the intention of this effort is to provide new insights into the fundamental physics of the mjo however some discrepancies in the composite vertical mjo heating profiles are noted among the datasets particularly between three reanalyses and three trmm estimates a westward tilting with altitude in the vertical heating structure of the mjo is clearly evident during its eastward propagation based on three reanalysis datasets which is particularly pronounced when the mjo migrates from the equatorial eastern indian ocean eeio to the western pacific wp in contrast this vertical tilt in heating structure is not readily seen in the three trmm product

zircon age data from a greenstone of the archaean yilgarn block australia mid proterozoic heating or uplift a new age determination has been made on the greenstones of the agnew jones creek area western australia by use of the zircon u pb dating procedure late stage granophyre from the differentiated kathleen valley doleritic sill provided zircons which are otherwise unobtainable from mafic rock suites an upper intercept age of 2 795 38 ma extends the known length of the otherwise tightly established late archaean tectonic cycle at agnew from 250 ma to 320 ma a lower intercept of age 1 345 153 ma is interpreted to record either a hitherto unknown very low temperature heating or a hitherto unknown tectonic uplift and cover removal in the mid proterozoic the existance of a postulated ancient unconformity of craton size dimensions may be being confirmed and dated apart from some inconclusive olivine and pyroxene rb sr isotopic data from 190km to the west the only evidence known that could support either possibility is found outside the yilgarn block itself minor perturbations to a simple two stage discordia pattern imply further small amounts of lead loss at later times studies of this nature could eventually provide information on the later history of stabilized cratons

realistic initiation and dynamics of the madden julian oscillation in a coarse resolution aquaplanet gcm 1 the main mechanisms for the initiation and propagation of the madden julian oscillation mjo are still widely debated the capacity of operational global climate models gcms to correctly simulate the mjo is hindered by the inadequacy of the underlying cumulus parameterizations here we show that a coarse resolution gcm coupled to a simple multicloud model parameterization mimicking the observed dynamics and physical structure of organized tropical convection simulates the mjo in an idealized setting of an aquaplanet without ocean dynamics we impose a fixed nonhomogeneous sea surface temperature replicating the indian ocean western pacific warm pool this results in a succession of mjos with realistic phase speed amplitude and physical structure each mjo event is initiated at a somewhat random location over the warm pool and dies sometimes near the eastern boundary of the warm pool and sometimes at a random location way beyond the warm pool also occasionally the mjo events stall at the center of maximum heating this is reminiscent of the fact that in nature some mjos stall over the maritime continent while others reach the central pacific ocean and beyond the initiation mechanism in the model is believed to be a combination of persistent intermittent convective events interacting with observed large scale flow patterns and internal tropical dynamics the large scale flow patterns are associated with planetary scale dry kelvin waves that are triggered by preceding mjo events and circle the globe while congestus cloud decks on the flanks of the warm pool are believed to force rossby gyres which then funnel moisture toward the equatorial region

investigating complex isochron data using mixture models the isochron technique for dating geological events is widely used in many fields of isotope geochemistry however data sets can be complex and many samples may not behave as closed systems resulting in excess scatter around a regression line in certain data sets however excess scatter may result when geological samples are partially reset or when samples are comprised of multiple components recording different geological processes separated in time and or initial isotope composition here we introduce a new approach for investigating evidence of such multicomponent mixtures within isochron space we provide a mathematical framework to resolve the number of mixture components without requiring any prior knowledge of this number crucially the model reports the proportion of each individual sample that belongs to each mixture component allowing the user to return to the samples and independently test model outputs with other techniques it is important to distinguish accurate and meaningful ages and initial daughter isotope compositions from the meaningless values that are caused by mismatch between geological processes and the models that we use to describe them first we demonstrate our new model on a synthetic data set to show that it can successfully separate distinct isochrons in a complex mixture we further validate our approach with a previously published data set using the 187re 187os system in bitumen and 147sm 144nd system in magmatic rocks we identify discrete components in these data sets and using complementary geological observations discuss the implications of treating these scattered data as recording multiple geological processes

mjo initiation in the real time multivariate mjo index abstractmadden julian oscillation mjo initiation in the real time multivariate mjo rmm index is explored through an analysis of observed case studies and composite events specific examples illustrate that both the dates of mjo initiation and the existence of the mjo itself can vary substantially among several well known mjo indices depending on whether the focus is on convection or circulation composites of primary mjo initiation events in which the rmm index rapidly increases in amplitude from a non mjo state to an mjo state are presented and are supplemented by two case studies from the 1985 86 winter season results illustrate that for primary mjo initiation events in the indian ocean rmm phase 1 slowly eastward propagating 850 hpa 200 hpa easterly westerly anomalies over the indian ocean precede the amplification of the rmm index by at least 10 days while suppressed convection over the western pacific ocean precedes the amplification by 5 days these local eastern hemispheric prede

a case study of the mechanics of extratropical influence on the initiation of the madden julian oscillation abstract mechanisms for extratropical influences on the initiation of the madden julian oscillation mjo are investigated using numerical simulations and a global reanalysis product previous simulations by a tropical channel model captured the timing and gross features of the initiations of two mjo events and suggested that the initiations were due to influences from the extratropics in this study latitudinal transport of momentum from the extratropics is found to be crucial in generating the lower tropospheric westerlies in the tropics associated with the mjo initiation the diagnoses of the zonal momentum budget for the mjo initiation region revealed that the advection by meridional winds could be important prior to the initiation of the mjo the time evolution of the wave activity identifies its source over the southern indian ocean where it grows by extracting kinetic energy from the mean flow the time scale of the lateral boundary conditions that is responsible for the mjo initiation is also inv

double thresholds in scour fill processes and some implications in channel adjustment abstract through an analysis of data collected from the yellow river and its tributaries on the loess plateau of china the phenomenon of double thresholds in scour fill processes of wide range water sediment two phase flows has been shown thresholds located in non hyperconcentrated flows may be called the lower threshold and that in hyperconcentrated flows the upper threshold this double threshold phenomenon leads to complicated sediment transport behavior of heavily sediment laden rivers with an increase in suspended sediment concentration the channel sediment delivery ratio increases initially and becomes higher than 1 followed by a decrease and finally becomes lower than 1 again controlled by the double thresholds in the scour fill processes channel adjustment of the lower yellow river is non linear and complex when the suspended concentrations were lower than the lower threshold or higher than the upper threshold scour or bed downcutting was the dominant channel forming process channel shape tends to be narrower and deeper and the channel thalweg became more sinuous when the suspended concentrations lay between the lower and upper thresholds deposition of sediment was the dominant channel forming process channel shape tended to be shallower and wider and channel thalweg became less sinuous

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

a numerical case study on the initiation of the madden julian oscillation a mesoscale tropical channel model is used to study the long standing problem of the initiation of the madden julian oscillation mjo with initial and lateral boundary conditions provided by a global reanalysis this model is able to reproduce the initiation and gross features of two observed mjo events up to 2 months after the start of simulations this leads to a conjecture that these two mjo events are generated by the influences from the lateral boundaries this conjecture is supported by a series of sensitivity tests these sensitivity tests demonstrate that the simulated mjo initiation does not critically depend on detailed characteristics of sea surface temperature varying versus constant in time mean distribution from boreal spring versus winter initial conditions within a 10 day period the latitudinal location of the lateral boundaries 218 388n and s or even latent heating and moist processes the only factor found critical to the reproduction of the mjo initiation is time varying lateral boundary conditions from the reanalysis when such lateral boundary conditions are replaced by time independent conditions the model fails to reproduce the mjo initiation these results support the idea that extratropical influences can be an efficient mechanism for mjo initiation implications of these results are discussed

composition age and tectonic setting of amphibolites in the central bushmanland group western namaqua province southern africa abstract the western namaqua province wnp of southern africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid proterozoic 2 0 1 0 ga ago the bushmanland group bg has been the subject of intense study recently because of its considerable strategic metal potential hosting several giant stratiform base metal deposits e g gamsberg and aggeneys the bg contains amphibolites which represent metamorphosed tholeiitic basalts with mgo 9 4 and enriched in ni relative to modern basalts a feature reminiscent of archaean tholeiites post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics however the amphibolites have yielded a sm nd isochron age of 1649 90 ma nd t 0 48 0 89 which is interpreted as the time of extrusion of the basalt precursors and by inference a minimum sedimentation age for the underlying metalliferous bg sequence it follows that the bg sequence evolved during an intermediate period in the history of the namaqua province after major crustal stabilisation around 2 0 1 9 ga and before the last major tectonothermal event around 1 2 1 1 ga results of a parallel rb sr and u th pb study of the bg amphibolites indicate strong metamorphic resetting 1 2 1 1 ga ago thereby confirming earlier work on the wnp characteristics of the source region to the bg basalts have been inspected with morb normalised trace element abundances with the salient result being the bg basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones such a process is currently preferred over crustal contamination since the sm nd isochron for the bg amphibolites is remarkably linear but independent confirmation of the sm nd age is still required before this alternative model can be discounted to reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin we suggest that back are continental extension could provide a satisfactory tectonic setting as such an environment is intrinsically linked to subduction

relationships between calc alkaline acidic and basic mantle derived magmas in late archaean composite dykes kambalda goldfield western australia abstract composite dykes from the north dome area kambalda comprise basic spessartite hornblende plagioclase lamprophyre 56 sio 2 with mg numbers cr and ni contents typical of primitive mantle derived rocks and more acidic plagioclase phyric quartz andesite 66 sio 2 together with felsic net veins and hornblendic patches suggesting local differentiation the spessartites can be regarded chemically as volatile lile and incompatible element enriched versions of siliceous high magnesian basalts from the local greenstone sequence a pb pb isochron on one dyke yields an age of 2636 27 ma making this the oldest composite dyke so far dated field and geochemical evidence including identical pb initial ratios for the constituent rock types virtually indistinguishable ree spectra and tight linear trends on plots such as y versus ti and ce cr versus ti cr suggests that the two magmas lamprophyric and acidic were genetically related and were emplaced contemporaneously into the same fracture quantitative least squares modelling of whole rock and mineral compositions suggests that the acidic components resulted mainly from fractionation of hornblende from parental spessartite magmas together with a possible contribution from mixing of the same spessartite magma with crustally derived acidic e g trondjhemitic magma the latter is represented by separate dykes and stocks in the kambalda area close similarities between the kambalda composite dykes and archaean tertiary composite dykes in other parts of the world imply a persistent role for mantle derived magmas such as lamprophyres in the genesis of the dominant intermediate acidic e g granitoid magmas in many calc alkaline provinces

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

large scale distinctions between mjo and non mjo convective initiation over the tropical indian ocean abstractin this study the authors seek large scale signals that may distinguish mjo from non mjo convective events before they start over the indian ocean three such signals were found low level easterly anomalies extend from the surface to the midtroposphere and move from the western to eastern indian ocean surface pressure anomalies exhibit a zonal structure of wavenumber 1 with an equatorial low pressure surge penetrating eastward from africa through the indian ocean and reaching the maritime continent negative temperature anomalies in the middle to upper troposphere start over the indian ocean and move eastward all of them emerge 20 days before convective initiation of the mjo and move eastward at speeds close to that of the mjo without any direct connection to mjo convection they are not obviously related to the extratropics in any discernible way or any preceding mjo events they are absent in non mjo convective events these signals provide useful information for forecasting mjo initiation o

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

an approximation of the sediment budget of the lower mississippi river prior to major human modification a sediment budget is constructed for the lower mississippi river prior to the introduction of most human modifications components of the budget are derived from historical data including mississippi river commission hydrographic survey maps for the period from 1877 to 1924 a quantitative estimate of major sediment sources sinks and storage locations in the lower mississippi valley suggest that the most important sediment source was from bank caving and that at least 50 per cent more sediment was deposited in short term channel storage than in longer term overbank storage the budget indicates that the proportion of sediment transported as bedload was significantly greater than previously estimated the magnitude and variability of these sediment budget components are discussed the results of the study suggest that prior to modifications the river was aggrading over much of its length

mjo initiation processes over the tropical indian ocean during dynamo cindy2011 abstracta multination joint field campaign called the dynamics of mjo cooperative indian ocean experiment on intraseasonal variability in year 2011 dynamo cindy2011 took place in the equatorial indian ocean io in late 2011 during the campaign period two strong mjo events occurred from the middle of october to the middle of december referred to as mjo i and mjo ii respectively both the events were initiated over the western equatorial indian ocean wio around 50 60 e using multiple observational data products era interim the ecmwf final analysis and nasa merra the authors unveil specific processes that triggered the mjo convection in the wio it is found that 10 days prior to mjo i initiation a marked large scale ascending motion anomaly appeared in the lower troposphere over the wio the cause of this intraseasonal vertical motion anomaly was attributed to anomalous warm advection by a cyclonic gyre anomaly over the northern io the mjo ii initiation was preceded by a low level specifi

vertical mode decompositions of 2 day waves and the madden julian oscillation abstract vertical structures of 2 day waves and the madden julian oscillation mjo are projected onto vertical normal modes for a quiescent tropical troposphere three modes capture the gross tropospheric structure of 2 day waves while only two modes are needed to represent most of the baroclinic structure of the mjo deep circulations that project onto the first baroclinic mode are associated with deep cumulonimbus and stratiform rainfall shallow circulations that project onto higher wavenumber modes are associated with precipitating shallow cumulus and congestus and stratiform rainfall for both disturbances the horizontal divergence contributed by shallow modes is an important factor in the column integrated moist enthalpy budget these modes converge moist static energy for a time prior to when deep circulations export moist static energy these results highlight the importance of properly representing the effects of shallow cumulus congestus and stratiform precipitation in theories of convective

primary and successive events in the madden julian oscillation conventional analyses of the mjo tend to produce a repeating cycle such that any particular feature cannot be unambiguously attributed to the current or previous event we take advantage of the sporadic nature of the mjo and classify each observed madden julian mj event as either primary with no immediately preceding mj event or successive which does immediately follow a preceding event 40 of mj events are primary events precursor features of the primary events can be unambiguously attributed to that event a suppressed convective anomaly grows and decays in situ over the indian ocean prior to the start of most primary mj events an associated mid tropospheric temperature anomaly destabilises the atmosphere leading to the generation of the active mj event hence primary mj events appear to be thermodynamically triggered by a previous dry period although stochastic forcing may also be important other theories predict that boundary layer convergence humidity propagation of dynamical structures around the equator sea surface temperatures and lateral forcing by extratropical transients may all be important in triggering an event although precursor signals from these mechanisms are diagnosed from reanalysis and satellite observational data in the successive mj events they are all absent in the primary mj events hence it appears that these apparent precursor signals are part of the mjo once it is established but do not play a role in the spontaneous generation of the mjo the most frequent starting location of the primary events is the indian ocean but over half of them start elsewhere from the maritime continent to the western pacific copyright 2008 royal meteorological society

barrier effect on mjo propagation by the maritime continent in the mjo task force gewex atmospheric system study models abstractthe simulated madden julian oscillation mjo events in 27 general circulation models gcms are identified using an mjo tracking method the results suggest that the occurrence frequencies

pb isotopic evidence for u th pb behaviour in a prograde amphibolite to granulite fades transition from the lewisian complex of north west scotland implications for pb pb dating abstract two end member types of u pb fractionation behaviour are recognised type i in which pb isotopic homogenisation occurs and type ii in which pb isotopic homogenisation is incomplete or absent type ii can be further subdivided into three categories depending upon the relationship of the post fractionation 3 238 u 204 pb value to prefractionation 2 in type iia 3 is unrelated to 2 in type iib 3 k 2 where k is a constant in type iic 3 values are uniform each of these types of behaviour results in a predictable effect upon observed uranogenic pb isotope systematics which may lead to an incorrect geochronological interpretation an example of the relationship between these specific types of u pb behaviour and metamorphic grade is provided from the late archaean lewisian complex of n w scotland this complex was metamorphosed during the 2660 ma badcallian event three suites of tonalitic granodioritic grey gneisses have been analysed pyroxene granulite facies from scourie hornblende granulite facies from gruinard bay and amphibolite facies from the northern region the present day pb isotopic composition of these suites can be interpreted as representing a transition from type lib behaviour in the lowest grade rocks resulting in a rotated isochron through type ha in the hornblende granulites which yield a scatter of data points to type i in the pyroxene granulites which yield an isochron corresponding to the badcallian event this transition presumably reflects increasing u depletion with metamorphic grade and ultimately at the highest grade pb mobility and or depletion in addition a consideration of the thorogenic pb isotopic data reveals a systematic variation during prograde metamorphism an attempt to test the model of 2660 ma initial ratio inhomogeneity by analysis of k feldspars from the amphibolite facies gneiss suite reveals 1700 ma laxfordian resetting of their pb isotope systematics this study has implications for pb pb geochronological data particularly in lower grade metamorphic terrains it suggests that reliable isochrons may only be generated by type i behaviour or by the imposition of high 3 values during type iia behaviour at higher grades of metamorphism

evolution of depleted mantle the lead perspective abstract isotopic data have established that compared to estimated bulk earth abundances the sources of oceanic basaltic lavas have been depleted in large ion lithophile elements for at least several billions of years various data on the tertiary mesozoic gorgona komatiite and cretaceous oka carbonatite show that those rocks also sample depleted mantle sources this information is used by analogy to compare pb isotopic data from 2 6 billion year old komatiite and carbonatite from the suomussalmi belt of eastern finland and munro township ontario that are with associated granitic rocks and ores that should contain marked crustal components within experimental error no differences are detected in the isotopic composition of initial pb in either of the rock suites these observations agree closely with sr and nd data from other laboratories showing that depleted mantle could not have originated in those areas more than a few tenths of billions of years before the rocks were emplaced on a world wide basis the pb isotope data are consistent with production of depleted mantle by continuous differentiation processes acting over approximately the past 3 billion years the data show that pb evolution is more complex than the simpler models derived from the rb sr and sm nd systems the nature of the complexity is still poorly understood

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

ocean rossby waves as a triggering mechanism for primary madden julian events the madden julian oscillation mjo is sporadic with episodes of cyclical activity interspersed with inactive periods however it remains unclear what may trigger a madden julian mj event which is not immediately preceded by any mjo activity a primary mj event a combination of case studies and composite analysis is used to examine the extent to which the triggering of primary mj events might occur in response to ocean dynamics the case studies show that such events can be triggered by the arrival of a downwelling oceanic equatorial rossby wave which is shown to be associated with a deepening of the mixed layer and positive sea surface temperature sst anomalies of the order of 0 5 1 c these sst anomalies are not attributable to forcing by surface fluxes which are weak for the case studies analysed furthermore composite analysis suggests that such forcing is consistently important for triggering primary events the relationship is much weaker for successive events due to the many other triggering mechanisms which operate during periods of cyclical mjo activity this oceanic feedback mechanism is a viable explanation for the sporadic and broadband nature of the mjo additionally it provides hope for forecasting mj events during periods of inactivity when mjo forecasts generally exhibit low skill copyright c 2011 royal meteorological society

long term morphodynamic changes of a desert reach of the yellow river following upstream large reservoirs operation abstract china s yellow river has experienced its dramatically decreasing trend for the flow discharge since the construction and operation of large reservoirs located upstream this low flow regulation has triggered a severe aggradation of the ulan buh desert channel of the yellow river because the declining flow exhibits no capability to scour and carry away large amount input of desert sands from the ulan buh desert twenty monitoring cross sections documented the ulan buh desert channel has experienced its increasing aggradational trend in conjunction with its lateral migration decreasing trend from 1966 to 2005 which is opposite to the normal pattern of aggradation with deepening or symmetrical infilling for a channel located downstream of a reservoir the channel aggradation can also be identified two stages slow aggradation and rapid aggradation slow aggradation is characterized by the channel bed elevation rising 9 5 cm on average between 1968 and 1985 which responded to the operation of the liujiaxia reservoir during this period the flow discharge was similar to pre dam flow conditions but the sediment transport reduced to half of its pre dam value because of about 0 24 108 t of desert sands entering the channel from the ulan buh desert annually this dilute flow indicated not to scour the channel as expected but contrarily to cause the channel aggraded rapid aggradation followed completion of the longyangxia reservoir with the channel bed elevation rising by 73 cm on average between 1986 and 2005 in this period the combined regulation of liujiaxia and longyangxia reservoirs has caused the flow discharge decreasing dramatically which is more beneficial for accumulation of the desert sands 0 19 108 t yr 1 on average in the desert channel and led to the channel aggradation rate accelerated rapidly

petrogenetic evolution of metabasalts and metakomatiites of the lower onverwacht group barberton greenstone belt south africa abstract a well preserved sequence by archean standards of mantle derived metabasalts and metakomatiites forms large parts of the lower onverwacht group of the barberton greenstone belt south africa to elucidate the origin of mafic and ultramafic rocks from this 3 55 to 3 45 ga sequence we present a comprehensive geochemical dataset including major and trace elements as well as lu hf and sm nd isotope compositions for a variety of metavolcanic rocks these include metabasalts of the amphibolite facies sandspruit and theespruit formations as well as metabasalts and metakomatiites of the lower greenschist facies komati formation based on their incompatible trace element patterns the basalts of the sandspruit and theespruit formations can be subdivided into a light rare earth element lree depleted group a lree undepleted group and a lree enriched group positive ehf t and end t values of ca 3 to 4 and 0 to 2 respectively together with depletions in th and lacn ybcn indicate derivation of the lree depleted basalts from a depleted mantle source however chondritic ehf t and end t values combined with positive th and lacn ybcn of the lree enriched samples indicate a contribution from older granitoid crust in the petrogenesis of these samples trace element patterns of komatiites and basalts of the komati formation are generally flat relative to primitive mantle with slight depletions in heavy rare earth elements and th and overall positive ehf t of 2 5 3 5 2 s d and end t of 0 5 2 2 2 s d the coherence in trace element characteristics suggests a common magmatic origin for basalts and komatiites this study reveals that the two lavas were derived from the same mantle plume i e komatiites were formed by high degrees of melting of a depleted mantle source containing residual garnet and the basalts were formed by moderate degrees of partial melting in shallower regions of the mantle based on the current dataset combined with published data we propose a geodynamic model for the oldest units of the barberton greenstone belt that describes the development from a submerged continental setting for the sandspruit and theespruit formations to a submarine plateau setting for the komati formation as a consequence of continental rifting

the development of upper tropospheric wind over the western hemisphere in association with mjo convective initiation abstractthis study examines the structure and driving mechanisms of upper tropospheric intraseasonal zonal wind anomalies over the western hemisphere wh during the convective initiation of the madden julian oscillation mjo over the indian ocean using composite and budget analyses the initiating mjo convection is more often associated with wh upper tropospheric intraseasonal easterly wind anomalies and when it is it tends to develop a stronger and zonally broader envelope of enhanced convection than events associated with westerly wind anomalies the wh upper tropospheric zonal wind anomaly associated with the mjo is often described as a dry kelvin wave radiated from mjo convection but the results show that both the structure and driving mechanisms are different from the ones of theoretical kelvin waves unlike the theoretical kelvin wave the zonal wind anomaly is not driven mainly by the zonal pressure gradient force and it is strongly coupled with rotational wind associated with subtropical and

age and lead isotope systematics of uranium enriched cobalt mineralization in the modum complex south norway abstract in the modum complex in the precambrian kongsberg sector of south norway cobalt sulphides and arsenides chalcopyrite and pyrite occur in stratiform bands and lenses the cobalt ore minerals are intimately associated with thorian uraninite both on a regional and on a microscopic scale due to the presence of coeval inclusions and intergrowths of uraninite in the cobalt minerals hand picked separates of ore minerals show a large range of u pb ratios this has led to a very large range of present day lead isotopic composition 206 pb 204 pb 18 1 to 277 207 pb 204 pb 15 6 to 36 208 pb 204 pb 37 to 57 two groups of samples can be discerned group 1 defines an imperfectly fitted pb pb lead isochron with an age of 1434 29 ma group 2 defines a pb pb correlation line with an age of 1146 66 ma these results indicate that the uranium bearing cobalt ore was deposited during the orogenic interlude commonly thought to separate the gothian kongsbergian ca 1 5 ga and sveconorwegian 1 2 0 9 ga orogenies in the baltic shield multi stage lead modelling indicates that the initial isotopic composition of the lead in the ores is indistinguishable from that of lead in the country rock metasediments suggesting that it was extracted from such a source the group 2 leads were affected by a strongly localized hydrothermal reworking process during the sveconorwegian orogeny which led to small scale partial pb isotopic homogenization and to strong fractionation of uranium thorium and lead the 1 43 ga age for primary mineralization defines a definite lower age limit for the onset of clastic sedimentation in the kongsberg sector the crustal precursor of the sediments must have retained a high lile upper continental crust character for several hundred million years prior to sedimentation

the onset of convection in the madden julian oscillation abstract an observational study of the onset of convection in the madden julian oscillation mjo was performed composites of radiosonde data from the comprehensive aerological reference data set were constructed for an ensemble of tropical stations located in the indian ocean maritime continent and western pacific ocean the composites suggest that for the off equatorial stations used in this study the mjo period may be set by the buildup and discharge of the low level moist static energy this result supports the discharge recharge hypothesis of blade and hartmann mjo events appear to begin when the off equatorial atmosphere has been destabilized through a combination of low level moist static energy buildup and concurrent drying of the middle troposphere by subsidence in the wake of the previous cycle of mjo convection the low level moist static energy buildup is controlled by a corresponding increase in low level moisture the increase in low level moisture is not caused by the 1000 mb convergen

conflicting mineral and whole rock isochron ages from the late archaean lewisian complex of northwestern scotland implications for geochronology in polymetamorphic high grade terrains we report major and trace element including rare earth element ree geochemistry and whole rock smnd geochronology on two suites of mafic ultramafic inclusions in an agmatite complex within a trondhjemitic host at gruinard bay in the late archaean lewisian complex of northwest scotland a suite of medium grained equigranular amphibolites main amphibolite suite mas has broadly chondritic high field strength element hfse ratios ti zr ca 115 ti y ca 318 y cr ca 0 39 flat heavy ree patterns at five to fifteen times chondrites usually with mild light ree enrichment lan 50 and small eu anomalies this is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite trondhjemite granodiorite ttg suite magmas as argued previously by rollinson and fowler 1987 a smnd regression line mswd 3 4 obtained from the mas yields an age of 2 943 0 091 ga with an initial end value of 2 69 0 62 a suite of coarser grained hornblendites and metagabbros hornblendite metagabbro suite hms displays significant geochemical differences from the mas exemplified by significantly light ree enriched patterns in which the hornblendites have the highest total ree with lan ndn 1 and large negative eu anomalies and the most felsic metagabbros have low total ree and large positive eu anomalies samples from the hms yield a smnd isochron mswd 0 52 age of 2 846 0 073 ga with an initial end value of 1 10 0 72 the hms geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local ttg magmas combining the new data with previously published data from the host trondhjemites at gruinard bay whitehouse 1989a yields a smnd isochron mswd 1 09 age of 2 795 0 028 ga with an initial end value of 0 66 0 38 the relatively low initial end values for the hms and combined hms trondhjemite isochrons compared to contemporaneous depleted mantle end 2 8 ca 2 1 to 3 3 is consistent with derivation of a parental magma from light ree enriched components of a mafic basaltic precursor which was itself derived from a depleted mantle source approximately 100 200 ma previously the mas suite at gruinard bay may be representative of such a mafic precursor our new geochronological data differ significantly from the ca 3 3 ga mineral smnd and pbpb regression ages presented recently for two members of the hms by burton et al 1994 which if correct require radical revision of the early geological history of the lewisian complex comparison of the relatively old mineral ages with our new whole rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole rock systematics however we argue that extensive retrogression evident in all lithologies at gruinard bay makes them less than ideal samples for a mineral isotopic study and it is likely that the ca 3 3 ga ages may be spurious the product of disturbance of isotope systematics during the laxfordian on the basis of the geochronology presented here evolution of the lewisian closely follows generally accepted models for late archaean crust formation

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

geochronologic significance of lead lines from old cratons abstract lead isotope data from geologically mixed suites of archaean gneisses often yield secondary isochrons that have statistically better fits than might be expected from the geological constraints on the range of samples used accepted as true isochrons these pb pb lines are commonly used both to determine the age of the rock suite and as evidence that all the rocks that yield data points within error of the isochron were formed at the same time from the same source using within geologically reasonable limits random data for initial lead and secondary lead growth we demonstrate that heterogeneous sample sets may yield statistically valid secondary isochrons this implies that the fit to a common secondary isochron does not necessarily mean that the samples used have the same age or an isotopically similar source numerical modelling of artificial data sets suggests that high correlation between 1 and 2 values is the single most important factor that enhances the statistical fit of the samples about a lead line and smoothes the initial scatter in contrast initial scatter will be preserved in data sets with a random relation between 1 and 2 values the 1 value estimated from the lead line will be biased towards the sample with the lowest 2 value

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

unscrambling the lead model ages abstract a linear relation is derived for the secular evolution of 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios that permits tests to be made for open system evolution on each system independently application of the method to conformable ore bodies of various geological age indicates that the available data do not demand an open system evolution for the last 3 8 b y 238 u 204 pb and 232 th 204 pb of 9 66 0 15 and 37 65 1 14 respectively fit best the data for this time interval a single stage evolution since 4 5 b y is unlikely however and the major events of continent formation postdate the earth accretion by at least 400 m y the larger scatter of 207 pb 204 pb data about the evolution line relative to the other isotopic ratios is also interpreted as resulting from a series of continental differentiation events taking place at 3 85 0 15 b y

isotopic peculiarities of an archaean pegmatite union mine mica south africa abstract an isotopic investigation rb sr u pb pb pb and sm nd of the archaean union mine pegmatite northern province south africa is reported the intrusion of this pegmatite devoid of zircon and monazite has been dated at 2912 2 6 ma by pb pb step leaching of magmatic garnet this age is supported by an identical two garnet apatite sm nd isochron of 2917 27 ma a rb sr k feldspar garnet albite isochron of 2023 11 ma corresponds to a later hydro thermal overprint isotope resetting during this overprint was incomplete and revealed that various parent daughter isotope pairs within minerals of the union mine pegmatite behaved in exceedingly different manners under the same physico chemical conditions sr isotope resetting was incomplete and resulted in open system behaviour of the pegmatite k feldspar almost quantitatively lost its radiogenic sr which had accumulated between the time of intrusion and the overprint sr loss from muscovite was variable and incomplete as indicated by a wide range in ages from 2088 74 ma to 2744 13 ma only insignificant portions of this sr were incorporated into adjacent albite as revealed by a micro drilled rb sr profile through a muscovite albite grain boundary the 87 sr 86 sr initial ratio is 0 8301 11 for the proterozoic isochron which is far less radiogenic than the calculated value of 2 24 for a closed system it is therefore concluded that sr was lost from the system and that the rb sr ratio of the whole rock was changed apatite on the other hand seems to have behaved as a closed system and retains a low initial sr ratio of 0 7125 u pb analyses of apatite reveal that this mineral has behaved as an almost closed system for pb diffusion as well but was open for u significant loss during the overprint this has resulted in high reverse discordance of apatite u pb analysis and in a geologically meaningless secondary pb pb isochron the data demonstrate that both preferential loss of daughter sr over parent rb and parent u over daughter pb isotopes can occur in some minerals of one and the same rock this process altered the parent daughter ratios to such an extent that in the case of apatite a secondary pb pb isochron with an erroneous age of 4119 66 ma was established it appears likely that preferential loss of daughter or parent isotopes is the reason for spurious secondary whole rock isochrons reported and reviewed by moorbath and taylor 1986 and moorbath et al 1986 such losses far exceeding predictions based on volume diffusion experiments may occur by means of fast transport diffusion which in part may depend on the micro textures of the host minerals geochemical studies of early archaean rocks require substantial back correction for radiogenic decay and the example of the union mine pegmatite shows that changes of parent daughter isotope ratios may occur long after rock formation however such changes may go unnoticed in high precision geochronology i e garnet dating in this study but will lead to erroneous correction for radiogenic decay and hence to erroneous conclusions regarding the origin of the studied samples

fluvial sediment budget of a mediterranean river the lower tordera catalan coastal ranges ne spain abstract the lower tordera river is a representative case of water and sediment dynamics and uses in a large river in the western mediterranean region along the study reach the fluvial regime of the river changes shifting from a perennial water and sediment circulation in the upper parts to an ephemeral flow pattern in the lower sections the fluvial sediment budget of the lower tordera was estimated based on field measurements of suspended and bedload and a survey of cross sections field data were obtained between january 1997 and june 1999 a period that was dryer than the average most sediment is transported as bedload more than 3 4 of the total load both at the entrance section of the river reach and at the outlet section close to the mediterranean sea a total of 156 700 tonnes of sediment 62 680 tonnes year entered the system mostly during discharges below bankfull while 107 000 tonnes 42 800 tonnes year were exported to the sea almost exclusively during floods the net accumulation of sediment was 14 600 tonnes per year as bedload especially during the dry years of 1998 and 1999 when small floods circulated through the upper sections but did not reach the lowermost part of the river as a consequence a mean aggradation of the riverbed of around 6 8 mm year has occurred a figure that has been corroborated after the analysis of a series of cross sections residence time of sediment in the perennial reach calculated at a long term ranges from 6 years for sediments in the active base flow channel to 164 years for sediment placed above bankfull level which is only entrained during high flows residence time of sediment in the ephemeral reach calculated for the period 1997 1999 is only slightly higher than that of the perennial part of the river this fact can be related to the current adjustment of the river system to historical mining the riverbed is slowly adjusting to the new conditions and important sedimentation is taking place in many upstream sections while erosion still occurs in the lowermost reaches thus causing the relatively low residence time of sediment

sediment storage in the reach of the middle yellow river located in the fenwei graben china fenwei graben is a famous sediment sink the longmen sanmexia sediment sink of middle yellow river is located in the middle part using the sediment budget based on annual data from the period 1920 2006 and flood event data from 154 flood events from the period 1950 1985 the variations in sediment storage release and transport have been analysed data from different methods and sources indicate that during an 1800 year period the variation of sedimentation rate in this sink has undergone a cycle from increase to decline the cause for this can be found in the changes in the manner and intensity of human activities over 87 years sediment storage in this sink can be separated into four stages which showed different trends depending on changing human activities such as reservoir construction soil and water conservation and water diversion stepwise multiple regression shows that the runoff and sediment yield from three major source areas have differing influences on sediment storage in the sink copyright c 2012 john wiley sons ltd

dating of subduction and differential exhumation of uhp rocks from the central dabie complex e china constraints from microfabrics rb sr and u pb isotope systems abstract the correlation of deformation fabrics and metamorphic reactions with geochronologic data of uhp metamorphic rocks demonstrate that the multistage subduction and exhumation history of the central dabie complex requires rapid subduction and rapid initial exhumation moreover these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems thus our age data do not simply reflect a thermal cooling history in the investigated section the maximum age for uhp is given by the 244 3 ma 2 u pb age of a pre uhp titanite phenocryst that survived uhp metamorphism and subsequent tectonometamorphic events a minimum age for uhp is set by the 238 1 ma 2 238 u 206 pb mineral isochron age of titanite and cogenetic epidote these minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust in the period of ca 238 218 ma the uhp terrain records ht metamorphism local partial melting and extensive pervasive strain below the eclogite jd grt stability field exhumation was polyphase with a first phase of fast exhumation succeeded by episodes of ht metamorphism and concomitant deformation at deep mid crustal level between 238 and 218 ma slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca 209 204 ma the resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep which was the dominant deformation mechanism in quartz feldspar rocks in combination with fluid influx

the geology of the kambalda nickel field western australia the kambalda nickel field contains the major concentration of volcanic peridotitc associated nickel ores in the archean of australia it occupies an area of 400 km 2 underlain by two sequences of ultramafic mafic and felsic volcanics and sedimentary rocks which can be broadly correlated with previously erected regional stratigraphic sections most ores occur in the lower kambalda sequence at the base of the kambalda ultramafic rocks the ultramafic rocks of the upper bluebush sequence contain only minor mineralization four phases of deformation occurred over an interval of more than 300 m y resulting in a dominant north northwest structural trend faulting was active during original volcanism and continued intermittently until after mafic felsic dike intrusion the ultramafic rocks have been serpentinized and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism the kambalda ultramafic rocks are of variable thickness and in places exceed 1 000 m the lower member of the ultramafic rocks occupying approximately one third of the sequence comprises thicker high magnesium flows i e generally 36 volatile free mgo commonly separated by sulfidic quartz albite sediments thinner lower magnesium flow units characterize much of the upper member the ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few grouped ribbonlike original orebodies eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt ultramafic contact these troughs are thought to be original volcanic depressions which focused later deformation the remainder is hanging wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow hanging wall orebodies occasionally grade into stratigraphically equivalent interflow sediments the ores contain varying proportions of massive matrix and disseminated sulfides the hypogene mineral assemblage is pyrrhotite pentlandite pyrite chalcopyrite but millerite is locally significant the massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position metamorphism has caused recrystallization of the ores and some redistribution of elements there is commonly marked variation of mineralogy and s ni ratios between contact and overlying hanging wall ores and within and between original ore surfaces within shoots but the relative contributions of primary composition and metamorphism to this variation are uncertain in ore environments the footwall basalt ultramafic contact is more deformed than in non ore environments the basal ultramafic flows are thicker and richer in magnesium the trend toward lower magnesium flows upward through the ultramafic formation is less well developed and there is better textural development and compositional differentiation within flows non ore environments usually contain interflow sediments whereas there is generally a sediment free prism of ultramafic rocks above ore environments most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor volcanic exhalations probably contributed to minor ore in sulfidic sediments

role of crustal contamination in heterogeneous archean volcanics from the leonora region western australia abstract archean supracrustal volcanic successions in the leonora laverton region of the eastern goldfields province western australia are subdivided into two stratigraphic associations 1 an early succession ten km thick containing homogeneous high ti tholeiite basalts displaying nearly flat chondrite normalized incompatible lithophile element patterns with la sm n ratios ranging from 0 88 to 1 12 and 2 an unconformably overlying heterogeneous succession four km thick consisting of a early siliceous low ti tholeiites and associated intermediate calc alkaline volcanics grading to b high ti tholeiite similar to those of association 1 and c late siliceous high mg spinifex textured basalts fractionation models and trace element distribution in high ti basalts suggest that all the flows were derived from similar but not identical parental magmas and that subvolcanic gabbroic fractionation was characteristic basaltic end members of the siliceous low ti suite had identical fractionation patterns but hornblende joined the assemblage in contaminated intermediate calc alkaline members producing low hree fractionates the siliceous basalts exhibit variable enrichment in large ion lithophile lile and light rate earth lree elements with a continuum of la sm n ratios up to 2 75 they also have marked negative anomalies for nb p and ti several samples also exhibit a positive anomaly for zr on normalized distribution diagrams low sr concentrations are inconsistent with mantle enrichment processes and compositions of siliceous volcanics are explained instead by up to 30 contamination of high ti tholeiite liquids by archean crust composed of hornblende tonalite gneiss geochemistry of the volcancis is consistent with eruption in a continental back arc basin tectonic setting

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

emplacement and metamorphism of archaean mafic volcanics at kambalda western australia geochemical and isotopic constraints abstract extensive exploration and mining at kambalda western australia has revealed a conformable mafic ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units geochemical and rb sr u pb isotopic analyses have been carried out on two 70 140 m sections of drill core from the metabasalts and u pb isotopic analyses have been made on sulphide ore samples greenschist metamorphism of the metabasalts is dated at 2610 30 m y and was approximately isochemical except for addition of h 2 o and co 2 and a differential mobilization of k and rb within the units the addition of h 2 o and co 2 is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism the later 2600 m y event is correlated with a high thermal regime also detected at kalgoorlie 50 km to the north a pb pb isochron age of 2720 105 m y may represent the metamorphic event or the time of emplacement of the metabasalts new sm nd data c hauvel et al c laque l ong et al suggest the mafic volcanics are 3200 m y old if true large scale homogenization of pb isotopes is required alternatively the mafic sequence may be only slightly older than 2800 m y and the sm nd data does not have time significance

the late archaean to early proterozoic lead isotopic evolution of the northern baltic shield of norway sweden and finland abstract the late archaean rocks of the northern baltic shield comprise high grade gneiss terrains and granite greenstone terrains the high grade gneiss terrains consist mainly of older upper crustal material which became metamorphosed during the lopian orogeny ca 2 9 2 6 ga in contrast the 2 7 ga old granite greenstone terrains comprise mainly mantle derived materials at the southern and western edge the archaean craton became partially remobilized during the svecofennian orogeny 2 0 1 75 ga this orogeny represents a major crust forming event in the baltic shield during which mantle material was added to the crust and older material was remobilized whole rock galena and sulfide trace lead are used to outline lead provinces on the northern baltic shield the archaean lead from the baltic shield defines three secondary lead isochrons at 2 65 ga with different initial lead compositions which reflect average pre 2 65 ga 1 ratios of 8 0 8 1 and 8 3 respectively the variation of the initial lead composition of the high grade gneisses and tonalites lofoten and koitelainen area probably indicates 1 variable crustal residence time and or 2 different degrees of crustal contamination assimilation before the 2 65 ga metamorphic event and the lead from the eastern finland greenstone belts indicates a heterogeneous mantle with respect to u pb before 2 65 ga lead in proterozoic rocks of the different lead provinces defines sets of subparallel mixing fields which can be modelled by a 2 65 ga lead component and a 1 8 ga lead component the parallel offset of these fields reflects the pre 2 65 ga 1 ratio of these terrains i e it reveals the pre 2 65 ga 1 of the involved archaean rocks proterozoic terrains with involvement of archaean lower crustal lead show a poor correlation in the 206 pb 204 pb 208 pb 204 pb diagram whereas the other proterozoic terrains show a good correlation in the 206 pb 204 pb 208 pb 204 pb diagram

origins of nd sr pb isotopic variations in single scheelite grains from archaean gold deposits western australia accessory gangue scheelite cawo4 from the archaean mt charlotte lode au deposit can be divided into two types with different rare earth element ree signatures in some scheelite grains specific ree signatures are reflected by different cathodoluminescence colours which can be used to map their often complex oscillatory intergrowths domains with specific ree contents from two grains were sampled for sm nd rb sr and pb isotopic analyses using a micro drilling technique type i scheelite is strongly enriched in middle ree mree and eu anomalies are either absent or slightly positive four fragments collected from type i regions of two crystals have initial 87sr 86sr and end values ranging from 0 70141 to 0 70163 and 2 5 to 3 5 respectively and pb isotope ratios reflecting the composition of greenstone sequence this may indicate that nd and pb have their source either locally or regionally in the greenstones basic greenstone lithologies have 87sr 86sr 0 7015 and the radiogenic sr signatures indicate that part of the sr originated from felsic lithologies located either within or beneath the host greenstone pile alternatively the sr signature may have evolved from preferential leaching of a rb rich mineral during hydrothermal alteration of the greenstone the ree patterns of type ii scheelite are either flat or mree depleted and have strong positive eu anomalies three fragments collected from type ii regions of the same two crystals have initial 87sr 86sr ratios and end values between 0 70130 and 0 70146 and 1 1 to 2 6 respectively and pb isotope signatures that are once again similar to that of the greenstone this implies that 87sr 86sr ratios in type ii fluids were closer to those of the host dolerite 0 7008 0 7013 due to more extensive fluid interaction with the dolerite a positive correlation between na and ree suggests that ree3 are accommodated by the coupled substitution ree3 na 2 ca2 into both type i and type ii scheelite this is consistent with a fractional crystallisation model to explain the change in ree patterns from type i to type ii but not with a model involving different coupled substitutions and fluids from different origins we propose that the complex ree and isotopic signatures of scheelite at mt charlotte are related to small m to medium km scale processes involving mixing between fresh batches of hydrothermal fluid with fluids that had already been involved in extensive wall rock alteration the very high end values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high sm nd ratios however tiny 20 m grains of secondary hydroxyl bastnasite were found within micro fractures of one scheelite grain containing an extremely high end signature the hydroxyl bastnasite probably formed during recent ree redistribution within the scheelite as a result of meteoric fluid circulation the scale of this cryptic low temperature alteration is sufficient to explain the anomalously high endi values observed in scheelite from western australia

the distribution of 3 0 ga and 2 7 ga volcanic episodes in the yilgarn craton of western australia abstract the results of a conventional zircon u pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the yilgarn craton of western australia indicate that volcanism took place during two distinct episodes at ca 3 0 and ca 2 7 ga each episode spans a time period of about 75 ma and the two ages of volcanism are spatially separated 3 0 ga volcanic rocks are found in greenstone belts in the northern part of the western gneiss terrain and in the murchison province whereas 2 7 ga volcanic rocks are present in the southern part of the western gneiss terrain and in the southern cross and eastern goldfields provinces there is no evidence of inheritance in the zircon u pb systems which would support an older crustal origin for these rocks present results together with previous studies of the ages of granites and gneisses suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism it is suggested that the evolution of the yilgarn craton took place in three main stages the first was the development of a stable 3 4 ga granitic nucleus which incorporated earlier generations of granitoid a 3 7 ga layered complex and possibly 4 2 ga primitive crust this was followed by the two separate granite greenstone episodes at ca 3 0 and ca 2 7 ga involving the addition of significant amounts of new crust each episode was characterised by the accretion of extensive granite greenstone terranes at the margins of the stable craton the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems

the role of shallow cloud moistening in mjo and non mjo convective events over the arm manus site abstractobservations from the atmospheric radiation measurement program arm site at manus island in the western pacific and re analysis products are used to investigate moistening by shallow cumulus clouds and by the circulation in large scale convective events large scale convective events are defined as rainfall anomalies larger than one standard deviation for a minimum of three consecutive days over a 10 10 domain centered at manus these events are categorized into two groups madden julian oscillation mjo events with eastward propagation and non mjo events without propagation shallow cumulus clouds are identified as continuous time height echoes from 1 min cloud radar observations with their tops below the freezing level and their bases within the boundary layer daily moistening tendencies of shallow clouds estimated from differences between their mean liquid water content and precipitation over their presumed life spans and those of physical processes and advection from re analysis

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

disequilibrium induced initial os isotopic heterogeneity in gram aliquots of single basaltic rock powders implications for dating and source tracing abstract the re os isotopic heterogeneity of mantle derived rocks has been well documented however it is unclear whether and how the os isotopic heterogeneity of a mantle source can influence re os isotopic dating and source tracing of mantle derived melts here we report that replicate analyses of gram aliquots of single basaltic powders one of reference material bhvo 2 and three of the hatu basalts from the western junggar region china show large variations in both os concentrations and isotopic ratios more importantly these replicate data define good apparent re os isochron correlations yielding ages significantly older than their formation ages and display remarkable correlations between the 187 os 188 os ratio and the reciprocal of the common os concentration i e 1 192 os this indicates that the initial os isotopic composition in a basaltic magmatic system may be heterogeneous and cannot be homogenized by pulverizing theoretical and mathematical deduction demonstrates that the observed apparent re os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation primarily due to limited diffusional exchange of os isotopes between refractory os bearing inclusions and host minerals under mantle conditions the regressed initial os isotopic composition may bias the true value of the mantle source pulsed mantle melting and magma mixing during ascent are potential processes responsible for such initial os isotopic heterogeneity in basaltic rock systems thus when using the re os isotopic system to date and to trace the source of basaltic rocks or other rocks with relatively low os concentrations it is necessary to consider whether the initial os isotopic composition was heterogeneous or whether it had reached complete isotopic equilibrium

pb isotopes of gorgona island colombia isotopic variations correlated with magma type abstract lead isotopic results obtained on komatiites and basalts from gorgona island provide evidence of large isotopic variations within a restricted area 8 2 5 km the variations are correlated with differences in volcanic rock type the highest isotopic ratios 206 pb 204 pb 19 75 correspond to tholeiites which make up most of the island the lowest ratios 18 3 correspond to the komatiites of the west coast of the island other rock types komatiites of the east coast k tholeiites picrites and tuffs have isotopic characteristics intermediate between these two extreme values these results are explained by the existence of two distinct mantle source regions and by mixing or contamination between them

the development of upper tropospheric geopotential height anomaly in the western hemisphere during mjo convective initiations this study uses geopotential height budget and composite analysis of reanalysis data to examine the driving dynamics and structural evolution of intraseasonal upper tropospheric circulation over the western hemisphere wh prior to and during the convective initiation of the madden julian oscillation mjo over the indian basin previous studies by the authors showed that during most of mjo convective initiation events over the indian basin upper tropospheric equatorial easterly wind anomaly initially develops in association with subtropical and midlatitude rossby ridges over the east pacific basin as the easterly wind continues to amplify and propagate farther eastward a negative upper tropospheric geopotential height anomaly develops over tropical south america and couples with the easterly wind anomaly forming a structure consistent with theoretical kelvin waves this study shows that the development of the negative geopotential height anomaly over south america is driven by net tropospheric adiabatic cooling induced by updrafts the convection associated with the updrafts is triggered by the intrusion of a midlatitude wave train from the southern hemisphere suggesting that the interaction with midlatitude circulation is a part of the dynamics driving the formation of upper tropospheric kelvin wave structure over the wh during some mjo convective initiation events the upper tropospheric zonal wind anomaly is positive over the east pacific basin and the formation of kelvin wave structure is weak or absent over south america those events subsequently develop weaker mjo convective envelopes over the indian basin apparently due to the absence of upper tropospheric divergence induced by the incoming kelvin wave circulation that enhances convective updrafts over the region of initiating convection the results suggest that the formation of the upper tropospheric kelvin wave over the wh partly determines the subsequent development of mjo convection over the indian basin

structural setting and shape analysis of nickel sulfide shoots at the kambalda dome western australia implications for deformation and remobilization integrated three dimensional structural studies have led to new insights into the geologic controls on the distribution of magmatic nickel sulfide ore shoots associated with metakomatiites at the archean kambalda dome western australia exploration models emphasize volcanic channels in deep embayments trough structures modified by thermal erosion at the base of turbulent lava flows however analysis of three dimensional models of the kambalda exploration database reveals ore shoot scale controls consistent with the regional deformation sequence d1 to d4 ore shoots in d1 settings e g ken shoot are transposed into the north northwest d1 trend and have asymmetric trough structures with reentrant updip margins indicating a sense of movement opposite to that on the kambalda dome ore shoots in d2 settings e g lunnon shoot have asymmetric trough structures bound by low angle thrusts indicating a sense of movement typical of that on the dome with thrust movement of ore into the talc and carbonated hanging wall rocks lacking primary igneous features e g mcmahon shoot ore shoots in d3 settings are offset by normal faults which host gold mineralization at the south end of the kambalda dome e g hunt shoot or by major reverse faults at the north end of the dome e g otter juan shoots ore shoots in d4 settings fisher shoot are transposed from the north northwest trend into the north to north northeast d4 trend in contrast ore shoots associated with shallow poorly defined trough structures with or without serpentinized hanging wall showing relict igneous features e g durkin shoot indicate preserved volcanic control the spectrum of volcanic and structural controls indicates a continuum of kambalda ore shoots from a volcanic controlled end member 3 shoots in shallow trough structures with relict igneous features through structurally modified 7 shoots in deep trough structures without relict features to a structurally controlled end member 1 shoot tectonically emplaced in the hanging wall with deformation features shape analysis of the three dimensional models indicates that the structurally modified and controlled ore shoots have prolate shapes and two of the volcanic controlled ore shoots have oblate shapes comparison of the prolate shapes with the shapes of strain indicators reveals the importance of constrictional d1 deformation in elongation of the ore shoots particularly at the north and south ends of the kambalda dome the oblate volcanic controlled end members reflect low strain settings adjacent to major fault zones comparisons to previous experimental systems suggest that the ore shoots and metakomatiite deformed preferentially by ductile flow the importance of deformation controls on nickel sulfide distribution at the kambalda dome means that exploration for kambalda style mineralization should take into consideration the deformation history of terranes that host ore

intraseasonal variability in a dry atmospheric model abstract a long integration of a primitive equation dry atmospheric model with time independent forcing under boreal winter conditions is analyzed a variety of techniques such as time filtering space time spectral analysis and lag regressions are used to identify tropical waves it is evident that oscillations with intraseasonal time scales and a kelvin wave structure exist in the model tropical atmosphere coherent eastward propagations in the 250 hpa velocity potential and zonal wind are found with a speed of about 15 m s 1 the oscillation is stronger in the eastern hemisphere than in the western hemisphere interactions between the tropical and extratropical flows are found to be responsible for the simulated intraseasonal variability wave activity flux analysis reveals that a tropical influence occurs in the north pacific region where a northeastward wave activity flux is found associated with the tropical divergent flow in the western and central pacific in the north atlantic sector on the ot

the relationship between the itcz and mjo initiation over the indian ocean abstractunderstanding convective initiation of the madden julian oscillation mjo remains an unmet challenge mjo initiation has been perceived as a process starting from a convectively suppressed

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

the pipestone pond complex central newfoundland complex magmatism in an eastern dunnage zone ophiolite ophiolitic rocks are preserved in both the notre dame and exploits subzones in the dunnage zone of the newfoundland appalachians ophiolites in the exploits subzone are generally less well preserved and exposed than their notre dame subzone counterparts and consequently have received less attention in the literature the pipestone pond complex is an exploits subzone ophiolitic sequence which outcrops on the western side of a structural window through the exploits subzone into the underlying gander zone it includes a basal harzburgite which passes upwards into a cumulate pyroxenite and gabbro sequence and thence into isotropic gabbro intruded by pegmatitic gabbro diabase and plagiogranite there is no sheeted dyke unit pillow lava occurs at the top of the sequence but is not observed to be in stratigraphic contact with the intrusive rocks the ophiolitic rocks are structurally disrupted and no single cross section traverses the complete ophiolitic stratigraphy although the stratigraphic sequence of

zonal and vertical structure of the madden julian oscillation abstract a statistical study of the three dimensional structure of the madden julian oscillation mjo is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered outgoing longwave radiation olr data mjo convection is generally preceded by low level convergence and upward motion in the lower troposphere while subsidence cooling and drying prevail aloft this leads to moistening of the boundary layer and the development of shallow convection followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection after the passage of the heaviest rainfall a westerly wind burst region is accompanied by stratiform precipitation where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion the evolution of the heating field leads to a temperature structure that favors the growth of the mjo the analysis also reveals distinct differences in the vertical structure

erosion caused by hyperconcentrated flow on the loess plateau of china abstract hyperconcentrated flows are widely observed in the middle yellow river basin they occur as a result of erosion but once they form on slopes they operate as a strong eroding and sediment carrying force and play an important role in drainage basin erosion and sediment production processes since the energy expenditure is less and sediment carrying capacity is greater hyperconcentrated flows have very high eroding and transporting abilities there exists a strong coupling relation between hyperconcentrated flow transport and gravitational erosion which ensures a rapid removal of mass wasted materials hyperconcentrated flows are an important factor responsible for the high intensity erosion and sediment yield in the middle yellow river basin many previous studies have indicated that the sediment delivery ratio on the loess plateau approaches 1 this can be explained by the action of hyperconcentrated flows as hyperconcentrated flows exhibit some particular behaviors in erosion and sediment production processes the present study suggests that the erosion by hyperconcentrated flows be regarded as a special type of erosion accordingly some countermeasures are proposed to control hyperconcentrated flows and the resulting erosion

revised sm nd systematics of kambalda greenstones western australia sm nd isotope systematics in mafic ultramafic lavas from the norseman wiluna greenstone belt western australia have been reinvestigated a previous age based on mixing lavas and granites is not confirmed instead a sm nd age of 3 262 44 myr is indicated for the lavas 400 myr older than intruding granites and predating gneisses that have been proposed as basement rocks

origin of granitic magma by crustal remobilisation rb sr and pb pb geochronology and isotope geochemistry of the late archaean qorqut granite complex of southern west greenland abstract rb sr and pb pb whole rock isochrons on the qorqut granite complex yield ages of 2530 30 myr initial 87 sr 86 sr 0 7081 0 0008 and 2580 80 myr respectively a model relating initial sr and pb isotopic compositions of the qorqut granites to the sr and pb isotopic compositions of the amitsoq gneisses ca 3700 myr and nuk gneisses ca 2900 myr at 2550 myr ago as well as sr and pb contents of the gneiss units suggests that between 40 and 50 of the qorqut granite magma was generated by partial melting of amitsoq gneisses and the remainder by partial melting of nuk gneisses

systematic decomposition of the madden julian oscillation into balanced and inertio gravity components we present a new method for the three dimensional multivariate decomposition of the mjo into balanced and inertio gravity ig components the method analyzes global fields with no filtering involved and it provides a quantitative comparison between the contribution of the rossby kelvin and other balanced and ig modes to the mjo circulation and its teleconnections results based on the era interim reanalysis data and the multivariate mjo index show that the rossby mode with the lowest meridional index is the largest contributor to the mjo circulation over the pacific a smaller role of the kelvin mode is diagnosed over the indian ocean and the maritime continent the mjo teleconnections in the polar stratosphere appear associated with the leading balanced vertical modes the presented method shows new ways of evaluating the mjo structure and its global impacts in weather and climate models

production storage and output of coarse upland sediments natural and artificial influences as revealed by research catchment studies rates of sediment production in uk river basins are generally low however upland source areas control both runoff and sediment volumes to he routed through larger basins and there are indications that development pressures on land in upland areas have far reaching implications output rates for bedload from semi natural grassland and commercially afforested catchments are now available for a ten year period at the plynlimon catchments mid wales the influence of forestry drainage practice is to increase bedload yields sixfold suspended load yields are doubled in an attempt to predict yields and to investigate the operation of storage and transport processes on bedload a regression analysis was conducted using the weight of bedload trapped as a dependent variable and selected climatic supply and transport factors as independent variables the climatic supply factors chosen are shown to have little influence on yield relative to transport factors despite evidence of supply limited yields tracer studies indicate that channel storage of material exercises a critical control on yield a factor which is not accommodated by the climate based supply indices implications for field measurement techniques are discussed

evolution of the madden julian oscillation in two types of el nino abstractevolution characteristics of the madden julian oscillation mjo during the eastern pacific ep and central pacific cp types of el nino have been investigated mjo activities are strengthened over the western pacific during the predeveloping and developing phases of ep el nino but suppressed during the mature and decaying phases in contrast mjo activities do not show a clear relationship with cp el nino before their occurrence over the western pacific but they increase over the central pacific during the mature and decaying phases of cp el nino lag correlation analyses further confirm that mjo activities over the western pacific in boreal spring and early summer are closely related to ep el nino up to 2 11 months later but not for cp el nino ep el nino tends to weaken the mjo and lead to a much shorter range of its eastward propagation anomalous descending motions over the maritime continent and western pacific related to el nino can suppress convection and moisture flux convergence the

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

structural evolution in heating profiles of the mjo in global reanalyses and trmm retrievals abstract diabatic and latent heating profiles from four global reanalyses and three tropical rainfall measuring mission trmm algorithms were compared first generally for the tropics and then in the context of the madden julian oscillation mjo most of them exhibit three heating maxima corresponding to known convection centers over south america africa and the indian western pacific warm pool but they still differ substantially in many ways most noticeably a double peak vertical structure with one peak in the upper and the other in the lower troposphere and relative weak heating over the maritime continent in comparison to heating over the indian and western pacific oceans are clearly seen in some heating data but absent in others heating profiles associated with the mjo were diagnosed through composites and case studies the composites were constructed as functions of mjo phases at three longitudes representing the indian ocean maritime continent and western pacific respectively four mjo ev

geochronology and isotopic variation of the early archaean amitsoq gneisses of the isukasia area southern west greenland abstract relatively unaltered samples of the amitsoq gneiss complex in the isukasia area were identified and analysed despite the effects of metamorphism and local proterozoic hydrothermal alteration and contamination the parents of the three lithologies that make up the amitsoq gneisses of the area were emplaced over about 320 ma taking a weighted average of dates from u pb on zircons rb sr on wr sm nd on wr and pb pb on wr the most probable ages of intrusion are grey gneisses at 3690 50 ma white gneisses at 3590 40 ma and pegmatitic gneisses at 3370 60 ma the rb sr isochron initial 87 sr 86 sr ratios for the grey white and pegmatitic gneisses are 0 7006 1 0 7016 1 and 0 8248 3 respectively the corresponding average nd t values are 1 0 0 7 and 0 2 since the ree chemistry of the rocks that were the direct parents of the grey gneisses is not known the 1 0 average nd t of the grey gneisses need not imply generation from a depleted mantle the sr initial ratio and nd t values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses the progressive evolution of nd with t for the amitsoq gneisses in the early archaean shows that values of nd t near zero in granitoids do not necessarily indicate derivation from the undepleted mantle chur

la ce dating of lewisian granulites to constrain the 138la β decay half life the isotope 138la makes up 0 089 of natural lanthanum and exhibits a branched decay by to 138ce and electron capture to 138ba the la ce isotope scheme is a potentially powerful adjunct to the sm nd method for petrogenetic studies but the best determination by counting of the decay half life of la 3 02 x 1011 yr 1 is in conflict with geological half life determinations based on la ce and sm nd mineral isochrons2 4 to test these alternatives i determined a six point la ce isochron on whole rock samples of lewisian granulite facies archaean gneiss from nw scotland here i report based on a half life of 3 02 1011yr a la ce age of 2 990 400 myr 2 which falls very well within the error of a whole rock sm nd date on lewisian granulites of 2 895 100 myr 2 this half life equivalent to a la decay constant of 2 30 10 12 yr 1 is therefore recommended for geological application of the ce isotope method

age and origin of early proterozoic dolerite dykes in south west greenland samples of three dolerite dykes from south west greenland give a pooled weighted mean rb sr whole rock isochron age of 2 130 65 ma initial 87sr 86sr ratios are 0 70155 0 00018 for one of the dykes naujat and 0 70277 0 00012 for the other two torssut and akinaq a fourth dyke qaqarssuaq did not yield an isochron torssut samples have significantly lower initial 207pb 204pb ratios than samples from the naujat dyke indicating that the torssut dyke has been affected by crustal contamination

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

the 1985 86 intraseasonal oscillation and the role of the extratropics abstract an intraseasonal oscillation that occurred in the 1985 86 northern winter is documented in this study the tropical convection of this event is dominated by the mixture of a standing oscillation over the maritime continent and an eastward moving feature from the indian ocean into the central pacific the time evolution of the upper tropospheric circulation patterns instead of propagating eastward along the equator as suggested in the existing composites of the intraseasonal oscillation is characterized by a series of wave patterns in the northern hemisphere and does not complete the cycle around the globe the familiar moist kelvin wave explanation for the intraseasonal oscillation receives little support from diagnosis of this event using zonal wind height field streamfunction and potential vorticity only in the lower troposphere near the date line is the convincing evidence for its existence found a scenario for the intraseasonal oscillation which is suggested by the analysis includes

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

isotopically heterogeneous initial pb and continuous 222 rn loss in fossils the u pb systematics of brachiosaurus brancai abstract diagenetic incorporation of uranium into bone material establishes a u pb system that provides a minimum age for the fossil loss of rn and ra from the 238u decay series recent mobility of u and pb and incorporation of radiogenic pb from an old basement which was available during diagenesis disturbed the u pb system all data fall in a 206pb 238u 207pb 235u concordia diagram to the right of the concordia while samples that do not experience redistribution of u pb 222rn and 226ra should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic pb loss of 222rn and 226ra has led to displacement of the present data set to lower 206pb 238u and therefore to younger intercept ages and to older 207pb 206pb ages the 207pb 204pb 208pb 204pb systematics demonstrates that 1 three out of 19 samples experienced significant postdiagenetic fractionation of u and pb and that 2 the diagenetic pb addition was isotopically heterogeneous the 235u 207pb 206pb 207pb and 206pb 238u 207pb 235u systematics combined demonstrate that 3 the samples experienced 5 to 18 222rn loss for a 1 9 ga old source of the diagenetically introduced pb and correspondingly more for a younger pb source and that 4 there is no unique solution for the age of the source of diagenetic pb since pb from a younger source appears in the 235u 207pb 206pb 207pb diagram in the same way as higher 222rn loss no precise age can be determined from the data although the most consistent solution falls in the range of 140 to 150 ma addition of radiogenic pb to the fossil bone is obvious only through its anomalous highly radiogenic character and the isotopic heterogeneity in 208pb 204pb no th i e no in situ 208pb growth the effect of unrecognized diagenetic additions of radiogenic pb is an overestimation of the age whereas unrecognized 222rn loss results in a too young discordant age since fossil material formed at times for which the discordia only shows a small curvature anomalies originating from diagenetic pb additions and 222rn loss may not always be recognized

a review of structural patterns and melting processes in the archean craton of west greenland evidence for crustal growth at convergent plate margins as opposed to non uniformitarian models abstract the archean craton of west greenland consists of many fault bounded eoarchean to neoarchean tectonic terranes crustal blocks these tectonic terranes are composed mainly of tonalite trondhjemite granodiorite ttg gneisses granitic gneisses metavolcanic dominated supracrustal belts layered anorthositic complexes and late to post tectonic granites rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs consisting mainly of ttg plutons tholeiitic to calc alkaline basalts boninites picrites and cumulate layers of ultramafic rocks gabbros leucogabbros and anorthosites with minor sedimentary rocks the structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics suggesting that the archean craton of west greenland grew at convergent plate margins several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms the terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds geometrically these structures are similar to those occurring on regional scales suggesting that the archean craton of west greenland can be interpreted as a continental scale accretionary complex such as the paleozoic altaids melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of ttgs non uniformitarian models proposed for the origin of archean terranes have no analogs in the geologic record and are inconsistent with structural lithological petrological and geochemical data collected from archean terranes over the last four decades the style of deformation and generation of felsic rocks on outcrop scales in the archean craton of west greenland and the mesozoic sulu orogenic belt of eastern china are similar consistent with the formation of archean continental crust by subduction zone processes

the relationship between archaean gold mineralization and spatially associated minor intrusions at the kambalda and norseman gold camps western australia lead isotope evidence many archaean mesothermal gold deposits are spatially associated with felsic to lamprophyric minor intrusions and it has been suggested that magmatic processes related to such intrusions may be important in the genesis of these deposits a comparison of the pb isotopic signature of gold related galenas from kambalda and norseman with that of spatially associated minor intrusions at the time of mineralization indicates that the ore fluid pb cannot have been derived solely from the intrusions or their source regions for both study areas the galena pb isotopic compositions are bracketed by those of local volcanic mafic and intrusive largely felsic rock types this is consistent with the ore fluid having derived metallic components from the crust or crustally derived granitic rocks and the mantle or mantle derived rocks of the greenstone succession via metamorphic dewatering or mantle crustal degassing interaction of granite derived magmatic fluids with greenstone lithologies could plausibly produce a similar array of pb isotopic signatures the norseman data as a whole are more radiogenic than the kambalda data for broadly synchronous mineralization reflecting the greater abundance of older granitic rocks with respect to mafic ultramafic rocks in the norseman district the provinciality exhibited by the pb isotopic composition of the ore fluid indicates that the gold mineralizing process formed galena whose pb isotopic composition was very sensitive to local variations in crustal pb isotopic composition either within the source region of the fluid or along fluid conduits

lead ages reset rubidium strontium ages and implications for the archaean crustal evolution of the diemals area central yilgarn block western australia abstract the pb pb whole rock geochronology of archaean granitic and gneissic rocks from the diemals area in the central yilgarn granite greenstone terrain provides important constraints on crustal evolution the regionally extensive banded gneisses previously considered as candidates for basement to the greenstones give a pb pb whole rock age of 2700 97 ma 2 errors this is within error of previously published rb sr and sm nd gneiss ages and also within error of the sm nd ages on the greenstones in the eastern goldfields province two synkinematic plutons give pb pb whole rock ages 2737 62 ma and 2700 100 ma and pb isotopic compositions consistent with the hypothesis based on field and geochemical relations that these plutons were derived by partial melting of the precursors to the banded gneisses assuming this the combined data date the melting event at 2723 25 ma with a model source value of 8 18 0 02 this source value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than 200 ma a post kinematic pluton with a whole rock pb pb age of 2685 26 ma and value of 8 26 0 02 puts a younger limit on this relatively short lived crustal accretion differentiation event comparison of pb pb and rb sr whole rock dates for the plutons suggests that the latter became closed systems up to 200 ma after the pb pb ages and that the plutons gained or lost rb or sr at this time

the early archaean nulliak supracrustal assemblage northern labrador the nulliak supracrustal assemblage the remains of ca 3800 ma succession of volcanic and sedimentary rocks was broken up by intrusion of the protoliths of the early archaean uivak orthogneisses and then deformed metamorphosed and variably metasomatised several times under upper amphibolite to granulite facies conditions in the archaean amphibolites of komatiitic basalt and tholeiitic chemical affinity are the most important nulliak assemblage lithologies high al2o3 metagabbroic rocks and anorthosites also occur interlayered with the amphibolites are marbles calc silicate rocks and banded iron formation interpreted as chemical sediments that were probably laid down in a shallow water environment also found are felsic rocks probably derived by reworking of penecontemporaneous felsic volcanic rocks and garnet and sillimanite bearing paragneisses derived from pelites all these lithologies are randomly interlayered on a scale down to 1 m or less the occurrence of 3850 3900 ma cores for zi