depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

the origin and evolution of low δ18o magma recorded by multi growth zircons in granite we for the first time reported low 18o granites within craton in bengbu uplift southeast margin of north china craton integrated ion microprobe study of 18o and u pb age on single zircon grains and zircon populations gives direct evidence for the origin and later magmatic evolution of the low 18o granites three types of zircon domains are recognized inherited magmatic core type i inherited metamorphic core mantle type ii and overgrown magmatic rim type iii type i zircons were formed in neoproterozoic and have the average 18o values of 5 3 0 6 around mantle value triassic type ii zircons have extremely low and highly heterogeneous 18o values ranging from 9 4 to 8 6 consistent with the values and variations exhibited by metamorphic zircons from dabie sulu orogen this feature argues for a triassic metamorphic origin for the 18o depletion signature of zircons from ultrahigh pressure metamorphic rocks in dabie sulu orogen jurassic type iii zircons have isotopically homogeneous 18o values of 3 3 0 5 collectively zircon populations recorded the 18o variations from source to low 18o granite the primary protolith of jurassic granites in bengbu uplift is a neoproterozoic granitic intrusion in south china block later high temperature meteoric hydrothermal alteration has lowered the whole rock 18o values in various degrees the low 18o protolith was then buried to the middle lower crust by triassic continental subduction and jurassic anatexis produced the low 18o granitic magma moreover 18o values of type iii zircons are isotopically lighter than previously reported values of magmatic garnets in jurassic granites indicating later magmatic evolution of low 18o magma toward high 18o values probably as a result of infiltration of high 18o fluid from wall rocks

oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast china it has been long recognized from nd and sr isotopes that depleted mantle sources consist of recycled oceanic materials but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrother mal alteration zircon is expected to preserve this signature because it is resistant to high temperature hydrothermal alteration this effect is illustrated by a combined sm nd and oxygen isotope study of whole rock and mineral samples from a mesozoic a type granite at nianzishan in northeastern china the sm nd isotope results show positive nd t val ues of 10 86 to 14 27 with young nd model ages of 569 846 ma manifesting a significant input of newly mantle derived material the zircon d 18 o values of 3 12 4 19 are significantly lower than the d 18 o value of 5 3 6 0 3 for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust the combined nd o isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction but also demonstrate that the granitic magmas can result from partial melting of mantle derived rocks that were subjected to seawater hydrothermal alteration before magma generation disequilibrium oxygen isotope frac tionations are observed between common rock forming minerals with significantly lower d 18 o values for alkali feldspar than seawater corresponding to meteoric hydrothermal alteration after magma crystallization

forced gravity waves and the tropospheric response to convection we present theoretical work directed toward improving our understanding of the mesoscale influence of deep convection on its tropospheric environment through forced gravity waves from the linear hydrostatic non rotating incompressible equations we find a two dimensional analytical solution to prescribed heating in a stratified atmosphere which is upwardly radiating from the troposphere when the domain lid is sufficiently high we interrogate the spatial and temporal sensitivity of both the vertical velocity and potential temperature to different heating functions considering both the near field and remote responses to steady and pulsed heating we find that the mesoscale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves which are in turn dependent upon the temporal and spatial structure of the source and the assumed stratification we find a 50 reduction in tropospherically averaged vertical velocity when moving from a trapped i e low lid to upwardly radiating i e high lid solution but even with maximal upward radiation we still observe significant tropospheric vertical velocities in the far field 4 hours after heating ends we quantify the errors associated with coarsening a 10 km wide heating to a 100 km grid in the way a general circulation model gcm would observing a 20 reduction in vertical velocity the implications of these results for the parameterisation of convection in low resolution numerical models are quantified and it is shown that the smoothing of heating over a grid box leads to significant in grid box tendencies due to the erroneous rate of transfer of compensating subsidence to neighbouring regions further we explore a simple time dependent heating parameterisation that minimises error in a parent gcm grid box albeit at the expense of increased error in the neighbourhood

the west african squall line observed on 23 june 1981 during copt 81 mesoscale structure and transports abstract durirg the night of 23 24 june 1981 new korhogo ivory coast a squall line passed over the instrumented area of the copt 81 experiment observations were obtained with a dual doppler radar system a sounding station and 22 automatic meteorological surface stations data from these instruments and from satellite pictures were analyzed to depict the kinematic and thermodynamic structure of the squall line composite analysis techniques were used to obtain a vertical cross section of the reflectivity structure and of the wind field relative to the line the redistributions of air moisture and thermodynamic energy by the convection wet calculated through averaged two dimensional wind fields from a dual doppler radar system the method also allowed the evaluation of the exchanges that were occurring between the convective and the stratiform regions this squall line had many similarities with tropical squall lines previously described by others the leading convective part composed of intense updr

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

zircon hf o isotopic constraints on the origin of late mesozoic felsic volcanic rocks from the great xing an range ne china abstract the voluminous late mesozoic magmatism was related to extensive re melting of juvenile materials that were added to the central east asia continent in phanerozoic time the most favoured magma generation mechanism of late mesozoic magmas is partial melting of underplated lower crust that had radiogenic hf nd isotopic characteristics but this mechanism faces difficulties when interpreting other isotopic data the tectonic environment controlling the generation of the late mesozoic felsic magmas is also in dispute in this study we obtained new u pb ages and geochemical and isotopic data of representative jurassic 154 4 1 5 ma and cretaceous 140 2 1 5 ma felsic volcanic samples the jurassic sample has inherited zircon cores of permian age with depleted mantle like ehf t of 7 4 8 5 which is in contrast with those of the magmatic zircons ehf t 2 4 0 7 whereas the inherited cores and the magmatic zircons have identical mantle like 18o composition ranges 4 25 5 29 and 4 69 5 54 respectively these hf o isotopic characteristics suggest a mixed source of enriched mantle materials rather than ancient crustal components and a depleted mantle source represented by the inherited permian zircon core this mechanism is manifested by the eruption of jurassic alkaline basalts originated from an enriched mantle source the cretaceous sample has high ehf t of 7 0 10 5 suggesting re melting of a mafic magma derived from a depleted mantle source however the sub mantle zircon 18o values 3 70 4 58 suggest the depleted mantle derived mafic source rocks had experienced high temperature hydrothermal alteration at upper crustal level therefore the cretaceous felsic magma if not all could be generated by re melting of down dropped supracrustal volcanic rocks that experienced high temperature oxygen isotope alteration the two processes enriched mantle contribution and supracrustal juvenile material re melting are new generation mechanisms of the late mesozoic magmas from central east asia rift settings may have controlled these processes throughout crustal and mantle levels

hydrogen and oxygen isotope geochemistry of a type granites in the continental margins of eastern china hydrogen and oxygen isotope studies of both whole rock and mineral separates were carried out for five representative atype granites in eastern china from north to south the whole rock d d and d 18 o values vary from 2145 to 282 and from 21 2 to 5 0 for nianzishan pluton from 2135 to 2110 and from 1 6 to 6 7 for shanhaiguan pluton and from 2145 to 2111 and from 1 5 to 9 1 for laoshan massif respectively the highest whole rock dd and d 18 o values are observed in suzhou pluton and range from 281 to 259 and from 3 5 to 9 2 respectively homogeneous d d and d 18 o values are found in kuiqi massif ranging from 2113 to 299 and from 5 6 to 8 5 respectively the results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution such as magma degassing fractional crystallization as well as post magmatic alteration by local meteoric water under subsolidus conditions the initial d d and d 18 o values of the a type granites in eastern china are constrained to be about 250 5 and 7 5 1 0 respectively this is consistent with mantle like protoliths for granitoid rocks and thus precludes the possibility that the a type granites is derived from the anatexis of the 18 o rich upper crust however recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the a type granitic magmas the variation in the ddepletion of the granites with latitude implies that the paleogeographic localities of the a type granites have not changed significantly since the mesozoic q 2000 elsevier science b v all rights reserved

a three dimensional simulation of a tropical squall line convective organization and thermodynamic vertical transport abstract three dimensional convective scale simulations of an african squall line observed during the french copt 81 experiment are presented three simulations with different representations of large scale forcing are performed on a domain of 50 km along the line by 80 km across the line they exhibit a similar circulation pattern characteristic of a squall line but differ in intensity the first simulation supposes an unperturbed environment and produces a slow moving squall line 7 m s 1 with weaker total precipitation rate then observed 25 the second one includes a representation of observed thermodynamic and dynamic environment modifications and produces a fast moving squall line 10 m s 1 still weaker than observations 50 or the rain rate the third simulation takes into account the forcing induced by the rear inflow jet as depicted by smull and houze and observed on that day it allows the system to reach an intensity in agreement with observations the convective region 30 km wid

numerical study of an observed orogenic mesoscale convective system part 2 analysis of governing dynamics abstract a detailed analysis of the dynamics and thermodynamics responsible for the structure growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made the process of scale interaction is addressed through fourier analysis and reynolds averaging analysis of representative predicted variables diabatic forcing and momentum acceleration terms additional dynamical analysis is accomplished through sensitivity experiments in which coriolis diabatic heating and ambient airflow are varied the general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems the heating scales range over a nearly continuous spectrum ranging from 10 250 km the heating occurs in response to primary advective gravity modes the larger scale gravity wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns the la

secondary initiation of multiple bands of cumulonimbus over southern britain ii dynamics of secondary initiation summary the convective storm initiation project csip pilot field programme took place in the summer of 2004 in southern england during this field campaign a case of a cold pool outflow initiating an arc of convection downwind of the generating storm the primary storm was observed three further arcs were initiated further downwind of this first arc these arcs all later gave significant rainfall over south east england results from the modelling studies described in this paper show that gravity waves generated by the primary storm may have been responsible for initiating the further three arcs of convective showers that were observed the modelled primary storm generates waves with a range of vertical wavelengths these are separated by dispersion with the higher order modes with the largest vertical wavelengths travelling fastest approximately 30 m s 1 the fastest two modes suppressed convection and later modes increased boundary layer depth and so initiated convection the multiple peaks from non hydrostatic effects in the third mode may have been responsible for the two main arcs observed and the fourth mode may have been responsible for the third arc the amplitude of the modelled waves at low levels was much larger downwind of the primary storm than upwind and only initiated convection there which agrees with the observations of the arcs forming downwind of the primary storm this asymmetry appears to be caused by the moving wave source and asymmetric wave trapping finally we discuss the implications for numerical weather prediction nwp reducing the resolution of the model and varying the magnitude of the wind speed used shows that a non hydrostatic high resolution 1k m nwp model should be able to capture such wave processes if the generating storm is forecast or analysed correctly

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

formation of positive end t granitoids from the alataw mountains xinjiang china by mixing and fractional crystallization implication for phanerozoic crustal growth abstract syn collisional granitic plutons intruded into devonian and carboniferous sedimentary and volcanic rocks along the southern slope of the alataw mountains in the north tianshan fold belt these intrusions are 307 290 ma in age granitoids in the eastern part of the mountain show characteristics similar to i type affinity while those in the western part show s type affinity carboniferous volcanic rocks andesite to rhyolite cover the eastern part of the region they share many common geochemical characteristics with the eastern granitoids intermediate and acid igneous rocks show positive e nd t values ranging from 7 to about 0 and initial sr isotopic ratios of 0 702 0 711 carboniferous basalts exposed in the region give 87 sr 86 sr t 0 701 0 704 and e nd t 7 12 country rock and basement gneisses show 87 sr 86 sr t 0 707 0 715 and e nd t 5 to 8 the positive e nd t suggests that mantle derived material play an important role in petrogenesis of the alataw granitoids a common mantle derived component is much likely to be involved in the petrogenesis of the granites from northern xinjiang since the positive e nd t granites with early paleozoic ages are widespread in the entire central asia mongolia granitic belt our data provide a constraint on the sr and nd isotopic ratios of this mantle derived mafic component which may be similar to those of the carboniferous basalts but can also have lower e nd t of 3 7 positive e nd t values suggest large mantle input representing a contribution of mantle derived materials to the continental crust it is very likely that this event of continental growth took place in very late proterozoic or early paleozoic

potential vorticity anomalies associated with squall lines abstract this study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines using data from the oklahoma kansas pre storm experiment we analyze potential vorticity fields near two squall lines one with and one without a trailing stratiform region from this observational analysis we suggest that squall lines with trailing stratiform regions can leave large positive midtropospheric potential vorticity anomalies in their wake to further interpret these observations we consider a two dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates which results in a simple potential pseudodensity inverse potential vorticity equation using apparent heat source fields that model those computed diagnostically from pre storm data we find that theory does indeed predict large midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines t

oxygen isotope ratios of zircon magma genesis of low δ18o granites from the british tertiary igneous province western scotland igneous zircons from granites of the british tertiary igneous province btip in western scotland preserve magmatic oxygen isotope ratios in spite of hydrothermal alteration and thus provide new evidence for the genesis of granites co2 laser and mass spectrometer based bulk 1 3 mg oxygen isotope analyses from individual plutons show remarkable homogeneity in zircon 18o regardless of zircon size and magnetism average reproducibility of 18o zrc in a single pluton is 0 2 in addition ion microprobe analysis shows that individual zircons all have the same 18o and thus bulk samples are homogeneous fourteen granite bodies from the isle of skye show extreme variability in zircon 18o values from pluton to pluton 18o zrc 0 6 5 3 vsmow all the granites record zircon 18o values less than would be in high temperature equilibrium with normal mantle 18o zrc 5 except for the beinn an dubhaich granite zircons from one intrusion in the mafic cuillins center also have low 18o 2 5 similar variability is observed in 18o amongst three intrusions on the isle of mull 18o zrc 3 4 5 9 both mull and skye are characterized by intense shallow paleohydrothermal systems and the low and variable 18o values of the magmas likely result from melting or interaction with hydrothermally altered country rock analysis of granites on the isle of arran where the percentage of low 18o country rocks is much less yields higher 18o zrc of 6 8 and 5 4 published trace element and radiogenic isotope data for btip mafic and felsic rocks have supported the hypothesis that the granites are formed primarily by differentiation from the mantle with magmas experiencing minimal crustal input dickin et al phil trans r soc london a 310 1984 755 780 however conservative estimates based on oxygen isotope compositions suggest that some of the btip granites can only be formed from normal magmas 18o wr 6 if they assimilated significant amounts of hydrothermally altered crustal material some granites require up to 40 bulk crustal input and are better explained as the result of wall rock melting by an adjacent mafic pluton whereas pb and sr suggest a general trend towards increasing input of shallow crust for the younger granites dickin et al phil trans r soc london a 310 1984 755 780 the correlation between age and 18o is less apparent suggesting that pb and sr isotopic compositions are decoupled from oxygen isotopes this would be expected of melted country rock that was hydrothermally altered in oxygen isotope ratios but not in pb and sr isotope ratios

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

low 18o silicic magmas why are they so rare low 180 silicic magmas are reported from only a small number of localities e g yellowstone and iceland yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization afc during magma genesis for nearly all silicic magmas the rarity of 10w l o magmas in intracontinental caldera settings is remarkable given the evidence of intense 10w l o meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems in the platoro caldera complex regional ignimbrites 150 1000 km3 have plagioclase 6180 values of 6 8 0 1 whereas the middle tuff a small volume est 50 100 km3 post caldera collapse pyroclastic sequence has plagioclase 8 80 values between 5 5 and 6 8 o on average the plagioclase phenocrysts from the middle tuff are depleted by only 0 3 0 relative to those in the regional tuffs at yellowstone small volume post caldera collapse intracaldera rhyolites are up to 5 5 o depleted relative to the regional ignimbrites two important differences between the middle tuff and the yellowstone 10w 180 rhyolites elucidate the problem middle tuff magmas reached water saturation and erupted explosively whereas most of the 10w l 80 yellowstone rhyolites erupted effusively as domes or flows and are nearly devoid of hydrous phenocrysts comparing the two eruptive types indicates that assimilation of 10w 180 material combined with fractional crystallization drives silicic melts to water oversaturation water saturated magmas either erupt explosively or quench as subsurface porphyrins bejiire the magmatic 180 can be dramatically lowered partial melting of low 180 subvolcanic rocks by near anhydrous magmas at yellowstone produced small volume 10w 180 magmas directly thereby circumventing the water saturation barrier encountered through normal afc processes

zircon u pb dating of water rock interaction during neoproterozoic rift magmatism in south china abstract a combined study of zircon u pb dating and mineral o isotope analysis was carried out for neoproterozoic granite gabbro and gabbrodiorite from the beihuaiyang zone that marks a rifted margin between the south china block and the supercontinent rodinia the results provide precise constraints on timing of water rock interaction resulting in unusual 18 o depletion in igneous minerals cl imaging reveals that zircons mostly show clear oscillatory and or sector zoning with or without thin alteration rims no metamorphic zircon occurs in the igneous intrusives although they suffered greenschist facies metamorphism during triassic continental collision la icpms zircon u pb dating yields consistent ages of 739 8 to 754 5 ma which are interpreted as timing of magma crystallization mineral separates have highly variable 18 o values of 0 6 to 6 8 for zircon 1 2 to 12 5 for quartz and 0 3 to 8 4 for plagioclase oxygen isotope heterogeneities exist within single intrusions and between different intrusions magmatic zircons with low 18 o values and a weighted mean u pb age of 748 6 ma crystallized from low 18 o magmas that were derived from melting of hydrothermally altered low 18 o rocks both low and normal 18 o intrusives in the beihuaiyang zone are shown to form at the same time contemporaneous with the rodinia breakup thus tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high t hydrothermal alteration and low 18 o magmatism such processes are a characteristic feature of high t water rock interaction in volcanic rift margins associated with localized water saturated melting while the occurrence of low 18 o magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting the existence of normal 18 o magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water rock interaction resulting in the unusual 18 o depletion in adjacent rocks the concurrent occurrence of both low and normal 18 o magmas appears to indicate highly heterogeneous high t water rock interaction during rift magmatism

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

isotopic studies bearing on the tectonics of the west junggar region xinjiang china isotopic investigations from the hercynian age fold belt between the kazakhstan and siberian cratons in the west junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region sphene from the leucogabbro phase of the tangbale ophiolite melange the oldest member of the ophiolite sequences in the fold belt yields an isotopic pb pb age of 523 2 7 2 ma zircon from a postcollision alkali granite yields slightly discordant isotopic u pb ages that indicate magma crystallization at 321 4 6 7 ma dating it in the lower carboniferous period radiometric dating thus documents a time span of circa 200 ma for igneous activity in the area petrogenetic studies were made to test whether precambrian crustal rocks might underlie the junggar sedimentary basin initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid ocean ridge basalts in pb isotope correlation diagrams sm nd data from sphene and apatite from one of the granites yield an initial end t 6 1 the granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks

oxygen isotopic composition of alkaline anorogenic granites as a clue to their origin the problem of crustal oxygen abstract the primary isotopic characteristics of alkaline granites are often obscured by secondary processes enhanced by their unusual chemical compositions this is true for radiogenic as well as for stable isotopes for example the 87 sr 86 sr ratios can vary drastically in closed systems because of very high rb sr ratios and can also be easily modified by direct or indirect interaction with continental crust because of low sr concentrations moreover the frequent occurrence of the granitic massifs as hypovolcanic complexes increases the probability of interaction with meteoric waters which is a common source of important isotopic variability the investigation of oxygen isotope systematics in alkaline acidic rocks from various environments shows the 18 o content of their quartz to be highly invariable and the 18 o values to be close to the mantle range of values this is due to the resistance of quartz to isotopic exchange which makes it a good tracer of primary isotopic composition if we eliminate the quartz 18 o values for which interaction with meteoric water is well documented five samples the total range of variation seventeen samples is from 6 0 to 7 3 relative to smow the values can easily be accounted for by and correspond to equilibrium with mantle type material in a temperature range of 1200 800 c if we consider possible effects of fractional crystallization this temperature range can probably be reduced to its lower limit which is much more likely for rocks of acidic composition the present oxygen isotope study strongly supports an origin for alkaline anorogenic granites from mantle dominated sources

hydrological processes in a small arid catchment scale effects of rainfall and slope length many studies have been concerned with the scale issue in geomorphology and hydrology most studies focus on the possibility of data transfer from small to large watersheds less attention has been given to the scale problem within very small watersheds where differences in slope length may seriously affect the spatial distribution and extent of runoff contributing areas to the channel it is usually assumed that runoff and erosion increase as slope length and angle increase however field observations in the negev highlands show positive relationships between slope length and deposition rates regardless of slope angles short hillslopes are devoid of colluvial mantle while thick colluvial deposits with well developed soil profiles indicative of long term stability are found at the base of long hillslopes the hypothesis advanced is that temporal variations in effective rainfall control the spatial structures of surface properties that further enhance the effects of temporal variations in rainfall long term monitoring of rainfall and runoff 1982 1998 conducted at spatial scales varying from few hundreds of m2 up to 0 3 km2 show a decrease in runoff at the hillslope scale the low efficiency of runoff and erosion processes on long hillslopes is because the concentration time required for continuous flow along such slopes is longer than the duration of most effective rain showers prevailing in the area field data lead to the notion that runoff and erosion models in which a positive relationship between slope length and angle and runoff and erosion rates is assumed should not be applied to arid and semiarid areas more attention should be given in latter areas to the complex hydrological relationships between rainfall scales and spatial scales the implications of data obtained regarding the spatial distribution of areas contributing to channel flow at a geological time scale and under changing climatic conditions are discussed

the development of organized convection in a simplified squall line model squall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration precipitation output and area of influence time dependent models have identified surface outflow boundaries and low level environmental wind shear as the crucial elements in squall line formation and or maintenance steady state models have shown how properties of the downdraught reservoir or cold pool must be matched to the far field wind and thermodynamic profiles to sustain the convection however the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain for help with these issues a time dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far field environments the convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by storm fronts forced or neutral ascent occurs at a surface gust front on the upwind side of the cold pool we distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts it is found that subsidence over the cold pool disorganizes the updraughts whereas a deep overturning circulation downshear from the gust front has an organizing effect hence factors which allow the initial subsidence to occur ahead of the gust front namely contrasting lower and upper level winds and large potential buoyancy favour squall line development it is argued that in cases with large convective potential energy the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught

the influence of convectively generated thermal forcing on the mesoscale circulation around squall lines abstract the dynamical processes that determine the kinematic and thermodynamic structure of the mesoscale region around 2d squall lines are examined using a series of numerical simulations the features that develop in a realistic reference simulation of a squall line with trailing stratiform precipitation are compared to the features generated by a steady thermal forcing in a dry simulation with no microphysical parameterization the thermal forcing in the dry simulation is a scaled and smoothed time average of the latent heat released and absorbed in and near the leading convective line in the reference simulation the mesoscale circulation in the dry simulation resembles the mesoscale circulation in the reference simulation and around real squall lines it includes an ascending front to rear flow a midlevel rear inflow a mesoscale up and downdraft an upper level rear to front flow ahead of the thermal forcing and an upper level cold anomaly to the rear of the thermal forcing it is also shown t

generation of fragment rich pseudotachylite bodies during central uplift formation in the vredefort impact structure south africa abstract target rocks underlying the central portions of large terrestrial impact structures are characterized by the pervasive presence of fragment rich pseudotachylite bodies debates regarding the formation of these bodies include the origin of pseudotachylitic melts i e friction versus shock induced melting melt mobility causes of target rock fragmentation and timing of fragmentation and melt emplacement with respect to stages of cratering comprehensive structural analysis of pre impact mineral fabrics and properties of fragment rich pseudotachylite in the vredefort dome suggests that melt is allochthonous and was emplaced at an advanced stage of cratering into tensional fracture zones within the crater floor both concentration of bending strains imparted on target rocks during central uplift formation and thermal stresses induced by the emplacement of allochthonous melt led to fragmentation of target rock tensional fracture zones formed in an overall dilational strain field towards the end of cratering likely during collapse of the central uplift and constitute low pressure zones into which melt was forcefully drawn melt may have been drained from the overlying impact melt sheet or from sites within the crater floor and transported into fragment rich dilation zones our field based analysis failed to identify the presence of bona fide shear faults that could potentially have generated in situ frictional melts and fragments rather target rock fragmentation and melt generation that resulted in fragment rich pseudotachylite bodies are processes separated in space and time during cratering

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

gravity waves compensating subsidence and detrainment around cumulus clouds abstract gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field we explore this role in a two dimensional numerical experiment on a simple moist convecting system consisting of an isolated long lasting nonprecipitating cloud in a calm atmosphere with no surface forcing the cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic the buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence unlike the circulations inside clouds this adjustment takes place without turbulent mixing hence the buoyancy adjustment time t1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the mixing time t2 which it takes a tracer initially concentrated at s

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

two dimensional convection in non constant shear a model of mid latitude squall lines numerical simulations of two dimensional deep convection are analysed using analytical models extended to include shallow downdraughts and non constant shear the cumulonimbus are initiated by low level convergence created by a finite amplitude downdraught these experiments have constant low level shear and differ only in the profile of mid and upper level winds quasi steady convenction is produced if the mid and upper level flow has small shear and the low level shear is large the surface precipitation ismaximized for no intial relative relative flow aloft if stationary this storm p o can give prodigious locilized rainfall p o is the two dimentisonal equivalent of the supercell these results are placed in context with previous two dimensional simulations attention is drawn to the similiarity with previous two dimensional simulations attention is drawn to the similarity with squall lines in central and eastern u s a storm p o is analysed by construction of time averaged fields of streamfunction vorticity teperature and height deviation the smoothness of these fields suggests a conceptual model of the storm dynamics which involves cooperation between distinct charcteristic flows an overturning updraught a jump type updraught a shallow downdraught a low level rotor and a boundary layer an idealized analytical model is described by solution of the equations for steady convection these solutions for the remote flow are derived from energy conversation mass continuity and a momentum budget and they give relationships between the non dimensional parameteres of the problem it is apparent that the convection is a high froude or low richardson number flow demanding the existence of a cross storm pressure gradient inherent in this idealized model is a vortex sheet between updraught and down draught and it is considered that the dynamical instability of this sheet is related to complexities in the numerical simulation furthermore these results show that in two dimensions both non constant shear and a shallow downdraught are necessary to maintain steady convection

diabatic divergence profiles in western pacific mesoscale convective systems abstract heating in the atmosphere can be expressed as diabatic divergence d which is nearly equal to the actual horizontal divergence in tropical convection high quality profile measurements from airborne doppler radar purls in ten mesoscale convective systems mcs observed during toga coare are examined and the mean profile is compared with rawinsonde array measurements young convective features have strong near surface convergence while older cells with better developed downdrafts and stratiform precipitation areas have their peak convergence aloft in the mean then surface flow is only weakly convergent or oven divergent so that the main convergence into mcss is deep and peaked aloft with a sharp melting convergence at 0 c divergence prevails above 10 km altitude but was undersampled by the radar unusual but well sampled observations in the purl dataset include a persistent widespread profile feature in one well sampled mcs a cyclone rainband oscillatory reverberations c

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

interactions between upper and lower tropospheric gravity waves on squall line structure and maintenance abstract using a simplified thermodynamic sounding and variable vertical wind shear we investigate the role of gravity waves on the structure and propagation of a simulated two dimensional squall line based on an observed squall line environment the modeled troposphere has been divided into three distinct thermodynamic layers these consist of an absolutely stable atmospheric boundary layer abl an elevated well mixed layer and an upper tropospheric layer of intermediate stability we find the mixed layer to have a dual role it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts and it provides an ideal layer to trap meso scale 20 200 km wave energy generated in the stable layers the generated waves thus have a significant and lasting impact on the simulation we also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long lived squall lines experiments that vary the vertical wind shear profile demonstrate

the two dimensional response of a tropical jet to propagating lines of convection this paper uses simple one and two dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet representative of the african easterly jet the results are used to infer changes in the environment of the forcing region which would in reality tend to influence the evolution of the storm through convective mechanisms which are not resolved here from linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing this result is confirmed in two dimensional nonlinear simulations when linear wave modes are computed in a basic state that includes the jet structure small sensitivities to the basic state jet are found the two dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing principally it is found that the modes of response to the forcing may be associated with characteristic changes in the basic state shear convective available potential energy cape and convective inhibition cin which would be expected to have significant influence on the convective system itself

gregarious tropical convection abstract a beat source with a vertical profile like that of observed tropical mesoscale convective systems mcss is shown to cause through inviscid gravity wave dynamics upward displacement at low levels in a mesoscale region surrounding the heating typical values are 10 30 area contraction at the surface everywhere within 270 km of the heating 6 h after it starts as a result conditions near an existing mcs but beyond the area of mcs outflow become more favorable for the development of additional convection this theory predicts that cloud clusters should be gregarious infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday supercluters

analysis of the african easterly jet using aircraft observations from the jet2000 experiment analyses of the african easterly jet aej are presented which are based on meridional transects of high resolution dropsonde observations made during jet2000 an aircraft campaign conducted in the last week of august 2000 the observations have confirmed that the aej is closely defined by geostrophic balance the baroclinicity between the extreme northern and southern profiles accurately determines the altitude of the jet core while the location and morphology of the jet core correspond to a locally defined geostrophic wind measure the potential vorticity pv structure has also been found to accord with theoretical expectations with distinctive positive and negative pv anomalies equatorward and poleward of the jet core respectively the thermodynamic structure of the aej environment can be categorized into coherent layers the monsoon layer is a humid zone connected to the land surface extending northwards into the sahel and increasing in depth towards the south this layer is affected by the land surface on diurnal time scales through the growing convective mixed layer and through shallow cumulus clouds above the monsoon layer is the saharan air layer sal which can be identified as a layer of low static stability and low pv the sal is deep where it merges with the saharan boundary layer in the north and becomes thinner toward the south it has been shown that the boundaries of the sal can be approximated to good accuracy as adiabatic surfaces meaning that the sal comprises air which is adiabatically connected to the land surface via the saharan boundary layer the upper region of the sal is identified as a layer of high relative humidity where altocumulus and stratocumulus layers are observed finally the troposphere above the sal is again almost pseudoadiabatic with small baroclinicity which determines the closure of the aej core aloft through inspection of thermodynamic tracers evidence of convective and lateral transport and exchange between these layers is also presented copyright 2005 royal meteorological society

low δ18o magma isle of skye scotland evidence from zircons abstract zircons in tertiary granitic rocks from the isle of skye scotland were resistant to measurable oxygen isotope exchange during intense hydrothermal activity in the subvolcanic environment five granite bodies from the western red hills complex were investigated four have 18o zrc 4 1 0 2 o vsmow while the fifth which intruded before major cauldron subsidence is 2 lower zircons from lewisian gneiss are visually and isotopically distinct indicating that zircons in the granite are not xenocrysts from the lewisian basement analysis of different magnetic and size fractions of zircon shows no significant correlation to 18o supporting the conclusion that these values represent the true magmatic compositions and that the western red hills granites intruded as low 18o magmas with a whole rock 18o 6 0 quartz separates from four granites are close to the calculated magmatic values in 18o but in the fifth values are 5 lower indicating variable exchange of quartz with low 18o heated meteoric waters these results might be explained by magmatic interaction with hydrothermally altered crust early in the evolution of the granites before crystallization of zircon followed by interaction with later hydrothermal fluids alternatively independent evidence for low 18o mafic magmatism in nw scotland suggests the presence of large quantities of low 18o subducted ocean crust in the subcontinental lithosphere that could contaminate or be the source of low 18o mantle derived melts

oxygen isotope ratios of archean plutonic zircons from granite greenstone belts of the superior province indicator of magmatic source zircons commonly preserve primary magmatic oxygen isotope ratios even through high grade metamorphic events unlike quartz which more readily loses its primary 18o value by subsolidus exchange intrusive bodies from the superior province canada have an average 18o zircon of 5 8 0 6 n 59 rocks those from granite greenstone belts have an average 18o zrc 5 8 0 6 n 55 in contrast to those from metasedimentary belts that average 18o zrc 6 7 0 2 n 4 the 18o zircon values from granite greenstone belts indicate a maximum allowable input of 14 supracrustal high 18o material into archean magmas of the superior province relative to mantle 18o values igneous bodies of the superior province have been separated into two groups based on geochemistry unenriched plutons and sanukitoids magmatic oxygen isotope ratios inferred from these zircons indicate up to a subtle but significant division of 1 between the two geochemical sets of rocks this distinction is apparent only in the 18o zrc and not in more commonly analyzed less refractory minerals such as quartz and feldspar oxygen isotope data correlate with geochemical modeling of sanukitoids the average 18o zrc for sanukitoids is 6 4 0 2 in comparison to 5 5 0 4 for unenriched plutons there is almost no overlap in values the elevation of sanukitoid 18o zrc indicates input from a high 18o source such as fluids from the subducting oceanic crust melts of subducted sediments or continental crust all of the plutons analyzed in this study have been dated by u pb zircon techniques and the combination of u pb ages and 18o zrc indicates little variability of 18o versus age during archean plutonism in the superior province therefore recycling of sediments and other high 18o sources was not a dominant process during crustal growth from 3003 to 2670 ma in the superior province

direct evidence for the origin of low 18o silicic magmas quenched samples of a magma chamber s partially fused granitoid walls crater lake oregon abstract partially fused granitoid blocks were ejected in the climactic eruption of mount mazama which was accompanied by collapse of crater lake caldera quartz plagioclase and glass in the granitoids have much lower 18 o values 3 4 to 4 9 than any fresh lavas of mount mazama and the surrounding region 5 8 to 7 0 oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at t 900 c following subsolidus exchange with hydrothermal fluids of meteoric origin assimilation of 10 20 of material similar to these granitoids can account for the o and sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber many of which have 18 o values 0 5 or more lower than comparable lavas of mount mazama the o isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids dominantly granodiorite are similar to those of erupted juvenile magmas the granitoid blocks from crater lake serve as direct evidence for the origin of 18 o depletion in large shallow silicic magma bodies

lower cretaceous alkali feldspar granites in the central part of the great xing an range northeastern china chronology geochemistry and tectonic implications the mingshui jilasitai suolun area located in the central part of the great xing an range is characterized by large volumes of alkali feldspar granites however the formation time and tectonic setting of these rocks remains controversial owing to a lack of precise geochronological and detailed geochemical data in this paper we report new sims u pb zircon ages and mineralogical petrographical and geochemical data for lower cretaceous alkali feldspar granites from the mingshui jilasitai suolun area the sims zircon dating results indicate that these granites formed at 133 6 135 9 ma the mineralogical petrographical and geochemical data show that these granitic rocks belong to highly fractionated i type granites combined with the regional geology data we propose that the formation of the lower cretaceous alkali feldspar granitic rocks was related to an extension induced by delamination of the lithosphere that arose from subduction of the palaeo pacific plate