the role of tropical atlantic sst anomalies in modulating western north pacific tropical cyclone genesis the connection between north tropical atlantic nta sea surface temperature sst anomalies and tropical cyclone tc genesis over the western north pacific wnp and associated physical mechanisms are investigated in this study we demonstrate a remarkable negative correlation of wnp tc genesis frequency with the preceding boreal spring nta sst anomalies our analysis suggests that major factors for tc genesis including distributions of large scale vorticity and midtropospheric humidity are rendered unfavorable by remote teleconnections while barotropic energy conversion from the large scale flow is suppressed as shown in recent studies the remote teleconnection from the atlantic is sustained and enhanced throughout the typhoon season through local air sea interactions these results suggest that boreal spring nta sst anomaly could be a new predictor for the seasonal wnp tc activity

identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids abstract volcanogenic sediments are typically rich in fe and mn bearing minerals that undergo substantial alteration during early marine diagenesis however their impact on the global biogeochemical cycling of fe and mn has not been widely addressed this study compares the near surface 0 20 cm below sea floor cmbsf aqueous no 3 and solid phase reactive fe distributions between two volcanogenic sediment settings 1 a deep sea tephra rich deposit neighbouring the volcanically active island of montserrat and 2 mixed biosiliceous volcanogenic sediments from abyssal depths near the volcanically inactive crozet islands archipelago shallow penetration of o2 aq into montserrat sediments was observed 5 cmbsf and largely controlled by the oxidation of organic matter dissolved fe and mn distributions in montserrat pore waters were lowest in the surface oxic layer 0 3 m fe 32 m mn with maxima 20 m fe 200 m mn in the upper 1 15 cmbsf unlike montserrat fe and mn in crozet pore waters were ubiquitously partitioned between 0 2 m and 0 02 m filtrations indicating that the pore water distributions of fe and mn in the traditionally termed dissolved size fraction are dominated by colloids with respective mean abundances of 80 and 61 plausible mechanisms for the origin and composition of pore water colloids are discussed and include prolonged exposure of crozet surface sediments to early diagenesis compared to montserrat favouring nano particulate goethite formation and the elevated dissolved si concentrations which are shown to encourage fine grained smectite formation in addition organic matter may stabilise authigenic fe and mn in the crozet pore waters we conclude that volcanogenic sediment diagenesis leads to a flux of colloidal material to the overlying bottom water which may impact significantly on deep ocean biogeochemistry diffusive flux estimates from montserrat suggest that diagenesis within tephra deposits of active island volcanism may also be an important source of dissolved mn to the bottom waters and therefore a source for the widespread hydrogenous mnox deposits found in the caribbean region

assessing the origin of old apparent ages derived by pb stepwise leaching of vein hosted epidote from mount isa northwest queensland australia epidote metasomatism affected large areas of tholeiitic metabasalts of the 1 780 ma eastern creek volcanics in the western fold belt of the proterozoic mount isa inlier hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant s2 fabrics which formed during the regional metamorphic peak at 1 570 ma associated with the isan orogeny previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin with an evaporitic component application of the pb stepwise leaching technique to the epidote does not separate radiogenic pb4 and common pb2 generating little spread in 206pb 204pb between 16 0 and 30 5 the causes for this relatively low range are twofold there is little radiogenic pb in the epidotes the most radiogenic steps account for 1 of pb released and both pb2 and uranogenic pb4 substitute into the same site in the epidote crystal lattice consequently age regressions using the pb stepwise leaching data give ages between 150 and 1 500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak these old ages are attributed to chemical inheritance from the host metabasalts via radiogenic pb release by breakdown of phases such as zircon monazite titanomagnetite and ilmenite during metamorphism this idea is supported by trace element data and chrondrite normalized rare earth element patterns that are similar to both the metabasalts and epidotes except for a variable eu anomaly in the latter relatively high fo2 during vein formation fe3 dominates in the epidote crystal lattice would allow the incorporation of th4 and exclusion of u6 and would explain elevated th u ratios up to 12 in epidote compared with the host metabasalts non incorporation of u would explain the relatively low u pb ratios and non radiogenic character of the epidote this process may provide a source of metal for the small u deposits around mount isa and may also suggest a relationship between u mineralization and regional cu mobilization during the isan orogeny our work suggests that non conventional geochronometers should be used only if additional geological information and geochemical data e g mineral chemistry trace elements are available to evaluate any resulting age calculations

changes in the influence of the western pacific subtropical high on asian summer monsoon rainfall in the late 1990s the year to year variability of the western pacific subtropical high wpsh is primarily controlled by atmosphere ocean interaction aoi between the wpsh and the indo pacific warm pool dipole sst anomalies aoi mode and the anomalous sst forcing from the equatorial central pacific the cp forcing mode in this study we show that the impacts of the wpsh variability on asian summer monsoon rainfall have changed after the late 1990s before the late 1990s the pre epoch the wpsh primarily affects east asian summer monsoon easm and had little influence on indian summer monsoon ism whereas after the late 1990s the post epoch the wpsh has strengthened its linkage to the ism while weakened its relationship with the easm this epochal change is associated with a change in the leading circulation mode in the asia wp region during the pre post epoch the wpsh variation is mainly controlled by the aoi cp forcing that mainly affects easm ism the epochal change of the leading mode may be attributed to the change of the enso properties in late 1990s the cp types of el nino become a leading enso mode in the post epoch this work provides a new perspective for understanding decadal changes of the enso monsoon relationship through subtropical dynamics

fission track dating of zircon zircons from mont blanc granite french italian alps zircon population in granite rocks are most often heterogeneous with respect to uranium concentrations which can vary from a few hundred ppm to 1 and the typology of the grains for example in the type of dominant prism 100 or 110 making them difficult to date using fission track ft dating the mont blanc zircons which vary greatly in both uranium content and zircon type are typically difficult to date by the fission track method to obtain information about fission track retention characteristics of these minerals experimental isothermal annealing was performed on grains free of alpha damage i e annealing of freshly induced tracks uranium poor grains with a dominant 100 prism were found to be more stable a step etching method sem was used to date the zircons uranium poor zircons with a dominant 100 prism yielded higher fission track ages than did uranium rich ones with a 110 dominant prism thus etching conditions temperature and etching time and type of prism chosen

u pb isotopic dating of fluid infiltration and metasomatism during dalradian regional metamorphism in connemara western ireland a metasomatic diopside rock occurs at the top of the dolomitic connemara marble formation of western ireland and contains titanite and k feldspar in addition to around 90 diopside xmg 0 90 0 97 u pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium rich and isotopically concordant with the result that a precise u pb age of 478 2 5 ma can be determined the age is identical within error to a less precise rb sr age of diopside k feldspar of 483 6 ma petrological evidence indicates that the assemblage crystallized at c 620 c close to or below the closure temperature of titanite the age thus provides a precise estimate of the time of metamorphism this age is 11 3 ma younger than the 490 ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age this difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism the success of this approach is demonstrated by the results from connemara

textural geochronological and chemical constraints from polygenetic titanite and monogenetic apatite from a mid crustal shear zone an integrated epma sims and tims study abstract titanites extracted from a 520 9 ma syn orogenic syenite at otjimbingwe damara orogen namibia show shape colour and u pb systematics that vary with position in the syenite relative to a major shear zone most titanite fractions from syenite cropping out outside the shear zone are characterized by a medium brown anhedral core overgrown by a pale brown subhedral rim empa and sims analyses show that the cores are enriched in al fe and sometimes na mg and mn but also in ree nb and zr relative to the rim tims u pb ages from these titanites gave relatively imprecise 207pb 206pb ages between 680 21 ma and 695 20 ma as well as 570 8 ma to 604 9 ma mainly due to their low 206pb 204pb ratios combined th u pb relationships suggest that these titanites contain a component similar to ca 715 750 ma old titanite populations observed so far only in neoproterozoic syenites from the northern part of the orogen titanites from syenites within the shear zone are dark brown mostly euhedral to subhedral and display only occasionally a pale brown rim these titanites show decreasing concentrations of na mg al and fe and increasing concentrations of mn from core to rim interpreted to reflect growth zonation abundances of mree hree y nb and zr are higher in the rim than in the core 207pb 206pb ages obtained on these titanites range from 470 2 ma to 538 3 ma for most samples only the samples with the highest 206pb 204pb ratios between 350 and 230 have u pb ages similar to the previously published u pb zircon age of 520 9 ma from the same complex other titanite fractions with lower 206pb 204pb ratios have slightly older ages than ca 520 ma indicating some inheritance inherited titanite survived multiple metamorphic episodes in the damara orogen between ca 750 ma and 470 ma and the igneous event that lead to the intrusion of the otjimbingwe syenite at 520 ma these data imply a high closure temperature of u pb in titanite of 650 700 c consistent with previous studies coeval apatite has no apparent inheritance low 206pb 204pb ratios between 55 and 170 and u pb ages similar to or only slightly younger than the u pb zircon or titanite age obtained on the titanite fractions with the highest 206pb 204pb ratios assuming a closure temperature for u pb of 900 c for zircon 700 c for titanite and 450 c for apatite initial fast cooling of the complex can be inferred our data demonstrate that titanite can contain multiple age domains preserving a chronological and chemical record of previous igneous events

sea surface temperature in the north tropical atlantic as a trigger for el nino southern oscillation events the el nino southern oscillation in the pacific ocean influences temperature in other tropical ocean basins reanalysis data and model simulations suggest that temperature anomalies in the north tropical atlantic may also influence the development of la nina events

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

indian ocean capacitor effect on indo western pacific climate during the summer following el nino significant climate anomalies persist through the summer june august after el nino dissipates in spring over the equatorial pacific they include the tropical indian ocean tio sea surface temperature sst warming increased tropical tropospheric temperature an anomalous anticyclone over the subtropical northwest pacific and increased mei yu baiu rainfall over east asia the cause of these lingering el nino effects during summer is investigated using observations and an atmospheric general circulation model gcm the results herein indicate that the tio warming acts like a capacitor anchoring atmospheric anomalies over the indo western pacific oceans it causes tropospheric temperature to increase by a moist adiabatic adjustment in deep convection emanating a baroclinic kelvin wave into the pacific in the northwest pacific this equatorial kelvin wave induces northeasterly surface wind anomalies and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone the gcm results support this kelvin wave induced ekman divergence mechanism in response to a prescribed sst increase over the tio the model simulates the kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest pacific an additional experiment further indicates that the north indian ocean warming is most important for the kelvin wave and northwest pacific anticyclone a result corroborated by observations these results have important implications for the predictability of indo western pacific summer climate the spatial distribution and magnitude of the tio warming rather than simply whether there is an el nino in the preceding winter affect summer climate anomalies over the indo western pacific and east asia

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

the extreme drought event during winter spring of 2011 in east china combined influences of teleconnection in midhigh latitudes and thermal forcing in maritime continent region abstractthe middle and lower reaches of the yangtze river mlryr in china experienced an extremely severe and persistent drought event from january to may of 2011 using both the observational data and ncep ncar reanalysis features of the drought event and the related circulation anomalies were investigated it is found that the precipitation during the investigated period of 2011 was deficient mostly along the yangtze river the water vapor diverged from mlryr southward into the bay of bengal south china sea and the philippines there were two factors facilitating the drought event one was the quasi stationary rossby wave related teleconnection which propagated eastward at midhigh latitudes from the north atlantic to east asia reinforcing the siberian high and the east asian trough henceforth resulting in the divergence anomalies in mlryr in the lower troposphere this quasi stationary wave train though originating from the north atlantic region was not essentially related to the north atlantic

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

u pb sphene dating of metamorphism the importance of sphene growth in the contact aureole of the red mountain pluton laramie mountains wyoming the relative importance of thermal diffusion versus new growth or recrystallization on u pb isotopic data from sphene is assessed through a study of amphibolites and granite gneisses within the contact aureole of the red mountain pluton laramie anorthosite complex wyoming samples were collected along a traverse approximately perpendicular to the margin of the intrusion over a distance of 0 13 to 2 65 km from the contact the 207pb 206pb ages of sphene from amphibolite samples range between 1 43 ga the intrusive age of the red mountain pluton to 1 78 ga the age of regional metamorphism the 207pb 206pb ages of sphene in rocks metamorphosed above 700 c are within error of the intrusive age of the pluton and appear to have resulted from diffusional resetting of preexisting sphene and the metamorphic growth of additional sphene at 1 43 ga at greater distance from the contact the 207pb 206pb ages range from 1 45 to 1 72 ga this 300 million year spread in ages is interpreted to result from two periods of sphene growth one produced during regional metamorphism at 1 78 ga and another generation of newly grown or recrystallized sphene that formed during contact metamorphism at 1 43 ga these two age populations may be identified on the basis of petrographic textures the morphologies and color differences of grain separates as well as by the u pb systematics in rocks metamorphosed to temperatures less than 700 c sphene growth was the dominant process controlling the response of the u pb isotope system to contact metamorphism sphene grew well outside the zone of obvious contact metamorphism the u pb sphene ages were reset by diffusion only at high temperatures supporting the experimentally determined closure temperature estimates for the u pb system in sphene of around 650 c cherniak 1993 this study demonstrates that u pb ages of sphene can be used to date metamorphism not only in areas with a simple geologic history such as igneous intrusion or single metamorphic or deformational events but also to date multiple events so long as different generations of sphene can be identified and separated

migmatization by metamorphic segregation at subsolidus conditions implications for nd pb isotope exchange abstract we have carried out major and trace element analyses as well as nd and pb isotope measurements on a stromatic migmatite in the phikwe complex botswana in order to assess the effect of high grade metamorphic event leading to migmatization on apparent nd model ages the phikwe complex considered as an entity of the central zone of the limpopo belt underwent a high grade metamorphic event at 2 0 ga leading to migmatite formation migmatites in this area present a very good layering of migmatitic components leucocratic layers were segregated from mafic layers dominated either by hornblende or biotite on a centimeter to millimeter scale accessory minerals such as monazite apatite and zircon were not identified in hornblende rich and quartzo feldspathic layers the combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and or hornblende dehydration reactions or by metamorphic segregation at subsolidus conditions ree modeling applying batch fractional and disequilibrium melting equations allows to discriminate between these two processes the modeled ree results of the quartzo feldspathic layers match those observed via metamorphic segregation at subsolidus conditions the end values at 2 0 ga range from 0 3 to 8 9 with lower values in the ree rich migmatitic layers thus the distribution of major and trace elements as well as the scatter nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components the positive end value of hornblende rich layers suggests hornblende crystallization from a mantle derived source and sm nd fractionation at protolith formation age apparent nd model ages ranging from 2 3 to 3 1 ga do not reflect the crustal evolution in southern africa in archean to proterozoic time based on the geochemical features and the pb pb wr isochron at 2 0 ga mass conservation during migmatization can be assumed therefore chemical and nd isotopic compositions of the protolith prior to migmatization can be calculated the results indicate a si and k poor diorite as protolith moreover calculated protolith compositions yield a two point sm nd isochron indicating an age of 2 7 ga and very similar nd model ages of 2 73 2 75 ga those ages constrain the timing of an archean crust formation episode in the phikwe complex

contrasting eastern pacific and central pacific types of enso abstract surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of el nino southern oscillation enso in the tropical pacific an eastern pacific ep type and a central pacific cp type an analysis method combining empirical orthogonal function eof analysis and linear regression is used to separate these two types correlation and composite analyses based on the principal components of the eof were performed to examine the structure evolution and teleconnection of these two enso types the ep type of enso is found to have its sst anomaly center located in the eastern equatorial pacific attached to the coast of south america this type of enso is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical indian ocean in contrast the cp type of enso has most of its surface wind sst and subsurface anomalies confined in the central pacific and tends to onset develop and decay in situ

a 40ar 39ar and u pb isotopic study of the ilimaussaq complex south greenland implications for the 40k decay constant and for the duration of magmatic activity in a peralkaline complex abstract magmatism in the gardar province south greenland is related to two main rifting events at 1280 ma and 1180 to 1140 ma little is known about the duration of the magmatic activity in a specific complex the ilimaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types which intruded and fractionated in three successive magmatic events as the intrusion contains some of the most evolved incompatible element rich rocks on earth it was chosen for a detailed geochronological study to quantify the duration of melt production intrusion fractionation and cooling amphiboles which are abundant in all rock types including pegmatites and late magmatic veins were dated using the 40 ar 39 ar technique since the solidus temperature of the most evolved melts is below the closure temperature of amphibole and no later heating event occurred the 40 ar 39 ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation the 40 ar 39 ar plateau ages range between 1142 6 2 2 ma and 1152 3 3 7 ma using the steiger and jager steiger r h and jager e 1977 subcommision on geochronology convention on the use of decay constants in geo and cosmochronology earth planet sci lett 36 359 362 40 k decay constant these ages are younger than an u pb age of 1160 5 ma on baddeleyite from the first magma batch our results indicate that the 40 k decay constant of steiger and jager may be too high a lambda similar to that proposed by kwon et al kwon j min k bickel p and renne p r 2002 statistical methods for jointly estimating decay constant of 40 k and age of a dating standard math geol 34 457 474 is required to make the 40 ar 39 ar match the u pb age

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

40ar 39ar age gradients in micas from a high temperature low pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra estimates of the time temperature history of geologic samples are often based on 40 ar 39 ar ages obtained through incremental heating experiments on minerals with different ar retentivities laser incre mental heating of single crystals of muscovite and biotite from an 1700 ma monzogranite from a low pressure high temperature metamorphic terrain in central arizona suggested an essentially uniform distribution of radiogenic 40 ar in each crystal and yielded ages of 1412 5 ma for the muscovite and 1410 10 ma for the biotite the similarity in these ages implies rapid 25 k m y cooling of the monzogranite after a reheating episode slightly before 1412 ma in contrast detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40 ar gradients indicative of very slow 400 m y evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the early middle proterozoic lithosphere in addition inconsistency between the laser mapping and incremental heating results for these micas suggests caution when interpreting incremental heating data for slowly cooled samples

u pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites abstract u pb isotope analyses on major and accessory minerals from pelitic schists in of southwestern connecticut are used to date metamorphism and deformation most of the major minerals have 206 pb 204 pb ratios less than 100 however u pb mineral isochron and mineral whole rock isochrons constructed using aliquots of the same mineral yield concordant ages with uncertainties as low as 5 ma staurolite kyanite and garnet are older than metamorphic monazites and zircons these data are summarized below 5 mineral 206 pb 204 pb system the straits schist trap falls fm monazite zircon 207 pb 206 pb 379 384 ma 380 2 ma staurolite 48 623 238 u 206 pb 394 400 ma 398 5 ma garnet 22 46 238 u 206 pb 383 ma 414 10 ma 235 u 207 pb 418 ma 207 pb 206 pb 623 ma kyanite 21 27 238 u 206 pb 399 ma chlorite anatase ilmenite 20 27 238 u 206 pb 350 370 ma apatite quartz kspar 18 19 transposed pegmatitic intrusions in the straits schist yield monazites with u pb ages analytically indistinguishable from those of metamorphic monazites and zircons in the schists growth of monazite and zircon in the schists may be related to high t fluid influx associated with pegmatite intrusion age discordance in the garnets from the straits schist is most consistent with continuous loss of intermediate daughter products of the 238 u decay chain from about 420 ma to 380 ma garnets from the adjacent trap falls formation do not display this behavior

migration of lead in non metamict zircon abstract the problem of activation energy calculations of lead loss from zircon and the determination of the lead diffusion coefficient in zircon based on the kinetics of diffusion is dealt with the experimental technique for the evaluation of the diffusion parameters used in this study is thermal ionization mass spectrometry thims the results of experiments on the kinetics of lead loss from several zircons under linear heating step heating and isothermal heating regimes are considered the study of effects of grain size on the kinetics of lead loss is performed under all regimes a comparison of various mechanisms of lead diffusion in zircon is also made the experimental results may be much better explained by the principles of volume diffusion than by those of first order reactions the values of activation energies calculated from volume diffusion are in the range 7 8 to 9 5 ev for crystalline zircons the suitability of thims technique for lead migration studies is considered

geochronological constraints on tourmaline formation in the western fold belt of the mount isa inlier australia evidence for large scale metamorphism at 1 57 ga boron metasomatism during two deformational events resulted in tourmaline formation in parts of late palaeoproterozoic eastern creek volcanics of the western fold belt of the mount isa inlier despite low u pb ratios 1 as measured by laser ablation icp ms when the step leaching process was applied to the tourmaline it process extracted pb fractions with 206 pb 204 pb up to 76 illustrating the method s ability to efficiently separate radiogenic and common pb that are hosted in different crystal lattice sites tourmaline from two syn d2 metamorphic hydrothermal veins at anderson s lode and eldorado yield similar pb step leaching ages of 1573 12 and 1577 48 ma respectively tourmaline from a post d2 but pre d3a vein at king was dated at 1528 51 ma although there is evidence to suggest some contribution from u rich inclusions i e zircon and monazite to some leach steps the isochrons are thought to provide robust ages for hydrothermal tourmaline formation during two discrete thermal events this is supported by estimates of pb closure temperatures in tourmaline derived from ionic porosity models the 1575 ma ages for d2 also reflect the main metamorphic event in the western fold belt but are 40 myr older than accepted age estimates for this episode the revised timing for peak metamorphism tectonism in the western fold belt corresponds with the age of metamorphism in the eastern fold belt suggesting broadly coeval tectonic histories across the entire inlier from at least 1600 ma

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

structure and dynamics of the summertime pacific japan teleconnection pattern convective activity over the tropical western pacific is known to influence the extratropical circulation over east asia in the boreal summer in the form of teleconnection called the pacific japan pj pattern but its structure and dynamics have not yet been studied in depth in this study a composite analysis is performed for 32 monthly events of enhanced convection observed to the east of the philippines the composited monthly mean vorticity anomalies associated with the pj pattern are elongated zonally with a distinct poleward tilt with height this structure differs fundamentally from a combination of the first baroclinic mode in the tropics and the barotropic structure in midlatitudes as has widely been accepted as a conceptual model of the pj pattern a wave activity flux points polewards only in the lower troposphere indicating that rossby wave teleconnection occurs primarily through a low level south westerly jet those tilted anomalies over the western pacific can effectively gain kinetic energy in the exits of the mean jet streams in the upper and lower troposphere and available potential energy ape in the presence of the vertically sheared jets the enhanced convection can generate ape effectively and the associated low level anomalous circulation acts to increase moisture supply into the convective region while enhancing evaporation from the pre warmed ocean surface it is thus hypothesized that the pj pattern may be regarded as a dynamical mode that can be effectively excited in the zonally asymmetric baroclinic mean flow associated with the asian summer monsoon with an efficient self sustaining mechanism through moist processes copyright 2006 royal meteorological society

pacific east asian teleconnection how does enso affect east asian climate observational evidence is presented to show a teleconnection between the central pacific and east asia during the extreme phases of enso cycles this pacific east asian teleconnection is confined to the lower troposphere the key system that bridges the warm cold events in the eastern pacific and the weak strong east asian winter monsoons is an anomalous lower tropospheric anticyclone cyclone located in the western north pacific the western north pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures the anomalies persist until the following spring or early summer causing anomalously wet dry conditions along the east asian polar front stretching from southern china northeastward to the east of japan kuroshio extension using atmospheric general circulation and intermediate models the authors show that the anomalous philippine sea anticyclone results from a rossby wave response to suppressed convective heating which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central pacific warming the development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling both the anticyclone and the cooling region propagate slowly eastward the development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades the rapid establishment of the philippine sea wind and sst anomalies implies the occurrence of extratropical tropical interactions through cold surge induced exchanges of surface buoyancy flux the central pacific warming plays an essential role in the development of the western pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone sst interaction and midlatitude tropical interaction in the western north pacific

effect of summer arctic sea ice on the reverse august precipitation anomaly in eastern china between 1998 and 2016 abstractthe 1997 98 and 2015 16 el nino episodes are regarded as two super el nino events and have exerted profound influence on eastern china summer rainfall as expected however on the subseaso

inherited pb components in magmatic titanite and their consequence for the interpretation of upb ages abstract the occurrence of inherited radiogenic pb components in 35 ma old magmatic titanite is reported here for the first time the about 810 ma age of the inherited components is identical to that contained in zircon from the same amphibole bearing syenite sample in both minerals the precambrian components were not totally reset during magmatic new growth at 35 ma this age pattern substantiates that the u pb chronometer in titanite is resistant to temperatures in excess of 712 c as is already known for zircon and monazite in consequence u pb ages from crystalline titanites have to be interpreted as the age of mineral formation or re crystallization rather than the time of u pb system closure during cooling of the rock

recent changes in enso teleconnection over the western pacific impacts the eastern china precipitation dipole abstractthe eastern china precipitation dipole ecpd features an out of phase relationship between boreal summer precipitation over the middle and lower reaches of the yangtze river and the hetao region to its northwest the precipitation dipole is strongly influenced by enso teleconnections over the western tropical pacific here it is shown that a pronounced weakening of both the rainfall variability over eastern china as well as the precipitation dipole structure occurred around the mid 1990s the changes have been analyzed by considering two epochs one during 1979 95 and the other during 1996 2009 the characteristic feature of the circulation anomaly during the first epoch is the well known east asia pacific pacific japan eap pj pattern a quasi meridional teleconnection pattern emanating from the western tropical pacific on the other hand during the latter epoch eastern china precipitation variability occurs as an integral part of the circulation anomalies over the western pacific in contrast

the unusual summer of 1994 in east asia iod teleconnections 1 an extremely hot and dry summer of 1994 was reported in east asian countries using observational data we have demonstrated that the indian ocean dipole iod is at least one possible cause of the abnormal east asian summer climate an anomalous cyclonic circulation over the western pacific and the southern china weakened the monsoonal northward flow in the lower troposphere an anomalous anticyclonic circulation with the equivalent barotropic structure around japan korea and the northeastern part of china caused the hot and dry summer of 1994 this accumulation of the lower potential vorticity in the far east is related to the wave activity from the mediterranean sahara region the monsoon desert mechanism connects a rossby wave source with the iod induced diabatic heating around the bay of bengal another rossby wave train pattern was generated in the upper troposphere and propagates northeastward from the southern china both the rossby wave patterns influenced the circulation changes over east asia

ree diffusion in calcite chemical diffusion of four rare earth elements la nd dy and yb has been measured in natural calcite under anhydrous conditions using rare earth carbonate powders as the source of diffusants experiments were run in sealed silica capsules along with finely ground calcite to ensure stability of the single crystal samples during diffusion anneals rutherford backscattering spectroscopy rbs was used to measure diffusion profiles the following arrhenius relations were obtained over the temperature range 600 850 c dla 2 6 10 14 exp 147 14 kj mol 1 rt m2 s 1 dnd 2 4 10 14 exp 150 13 kj mol 1 rt m2 s 1 ddy 2 9 10 14 exp 145 25 kj mol 1 rt m2 s 1 dyb 3 9 10 12 exp 186 23 kj mol 1 rt m2 s 1 in contrast to previous findings for refractory silicates e g zircon differences in transport rates among the ree are not pronounced over the range of temperature conditions investigated in this study diffusion of the ree is significantly slower than diffusion of the divalent cations sr and pb and slower than transport of ca and c at temperatures above 650 c fine scale zoning and isotopic and ree chemical signatures may be retained in calcites under many conditions if diffusion is the dominant process affecting alteration

summer rainfall seesaw between hetao and the middle and lower reaches of the yangtze river and its relationship with the north atlantic oscillation abstractusing the ncep ncar reanalysis and other observational datasets the authors have investigated the relationship of summer rainfall variations between the hetao region of northern china and the middle and lower reaches of yangtze river mlryr the results have demonstrated that rainfall in hetao varies out of phase with that in mlryr on the interannual time scales this phenomenon is referred to as the hetao yangtze rainfall seesaw hyrs an hyrs index is defined to reveal both spatial and temporal features of hyrs it is found that the north atlantic oscillation nao affects the hyrs in years when the nao is in its positive phase anomalous divergences in the lower troposphere and anomalous convergences in the upper troposphere are observed in regions of the mediterranean and eastern europe the anomalous convergences in the upper troposphere occur as the positive rossby wave source excites a circumglobal teleconnection cgt in the midlatitudes exhibiting the eastward propagation of rossby w

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

two types of el nino events cold tongue el nino and warm pool el nino abstract in this study two types of el nino events are classified based on spatial patterns of the sea surface temperature sst anomaly one is the cold tongue ct el nino which can be regarded as the conventional el nino and the other the warm pool wp el nino the ct el nino is characterized by relatively large sst anomalies in the nino 3 region 5 s 5 n 150 90 w while the wp el nino is associated with sst anomalies mostly confined to the nino 4 region 5 s 5 n 160 e 150 w in addition spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of el nino events furthermore the difference in the transition mechanism between the two types of el nino is clearly identified that is the discharge process of the equatorial heat content associated with the wp el nino is not efficient owing to the spatial structure of sst anomaly as a result it cannot trigger a cold event it is also demonstrated that zonal advective feedback i e zonal adve

proterozoic transpressive deformation in the northern marginal zone limpopo belt zimbabwe a major tectono metamorphic event affected the southern half of the northern marginal zone of the limpopo belt at 2 0 ga structural analysis along a 43 km long profile perpendicular to strike shows extension inclined toward the northeast extension parallel to axes of folds that uplift the southeast of the section i e the central zone of the limpopo belt and dextral shear these deformation patterns developed coevally the lineations form distinct tightly clustered groups that occur together in the profile there is no evidence for superimposed deformation or deformed lineations nor are lineations observed cross cutting each other this single episode of deformation can therefore be called transpression petrographic and thermobarometric evidence support this kinematic picture in that decreases of both pressure 2 5 kb and temperature 150 c are documented from southeast to northwest across strike 39 ar 40 ar step heating of metamorphic hornblende from amphibolites has yielded ages of

dating a variscan pressure temperature loop with staurolite staurolite with a complex internal metamorphic and structural relationship occurs in metapelite from the silvretta thrust sheet eastern alps stepwise leaching of a core and rim fraction of a single staurolite crystal resulted in a total pb pb isochron of 308 28 ma the variscan metamorphism has been confirmed by a further single bulk staurolite pb pb age of 298 42 ma which is in accordance with u pb ages of 306 8 ma and 304 6 ma respectively obtained from combined u pb leaching of the same crystals direct dating of the staurolite in reaction isograd was therefore successful our results date the prograde part of a variscan counterclockwise pressure temperature path in the silvretta thrust sheet proximity with white mica rb sr and k ar cooling ages suggests that the whole variscan episode was fast and took place within a few million years

kinetics of pb release during the zircon evaporation technique abstract the release of silica and pb from zircon by the thermal evaporation technique is shown to relate to the conversion of zircon to a porous baddeleyite structure the zircon baddeleyite reaction occurs at a sharp reaction front which progressively migrates towards the centre from the grain margin significant reaction in non metamict zircon commences in the mass spectrometer at temperatures of the order of 1500 c the progressive migration of zircon baddeleyite transformation towards grain interiors explains why the evaporation technique with its inherent advantage of not requiring chemical separation of pb and u progressively samples pb in zones towards the centre of the grain and gives 207 pb 206 pb ages as accurate as conventional u pb dating methods the rates of pb and silica loss are primarily determined by the rate of advance of this de silicification reaction front towards the interior of the grain most estimates of pb diffusion coefficients in zircon imply that diffusion distances in the unreacted zircon are negligible 10 2 m at the temperatures and duration of the pb evaporation experiments isothermal experiments demonstrate the rate of zircon baddeleyite transformation with time and temperature and provide an estimate of 130 10 1 kcal mol 1 for the activation energy of the process significantly higher than estimates of pb diffusion activation energy

from nanometer to megameter isotopes atomic scale processes and continent scale tectonic models abstract earth sciences deal with physical entities spanning 15 orders of magnitude from nanometer crystal lattice unit cells to megameter faults and orogens the understanding of megameter scale processes is greatly improved by the study of the nanometer scale processes the growth of crystals their deformation under shear and the transport of isotopes used for dating from a physical point of view the transport of atoms across a crystal with high potential barriers and no interatomic voids is not accurately described by fick s equations which were developed for dilute solutions with unhindered particle mobility a variety of more general descriptions have been recently proposed all of which share features such as complex temperature dependence as opposed to linear arrhenian behaviour from a petrological point of view the ubiquitous recognition of microstructures in chemical and isotopic disequilibrium underscores the fact that diffusive reequilibration is a much slower process than creating the structures themselves diffusion in an inert matrix fickian diffusion is observed to be negligible with respect to the growth of discrete mineral generations as the latter frequently involves aqueous fluids isotope geoscientists should follow petrologists and stress the geohygrometric properties of minerals dissolution reprecipitation the fact that mineral reactions and microstructure development do not solely depend on temperature may explain why some thermochronological models from the literature have made predictions whose field tests have failed the demise of one class of models might initially seem unfortunate actually it is a great benefit because it shifts our attention to the reality of petrogenesis and to the sequence of events that any mineral assemblage records

crystal chemistry of the monazite and xenotime structures monazite and xenotime the re p04 dimorphs are the most ubiquitous rare earth re minerals yet accurate structure studies of the natural phases have not been reported here we report the results of high precision structure studies of both the natural phases and the synthetic re p04 phases for all individual stable rare earth elements monazite is monoclinic p2 n and xenotime is isostructural with zircon space group 14 amd both atomic arrangements are based on 001 chains of intervening phosphate tetrahedra and re polyhedra with a reogpolyhedron in xenotime that accommodates the heavy lanthanides tb lu in the synthetic phases and a re09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements la gd as the structure transforms from xenotime to monazite the crystallographic properties are comparable along the 00i chains with structural adjustments to the different sizes of re atoms occurring principally in 00i there are distinct similarities between the structures that are evident when their atomic arrangements are projected down 001 in that projection the chains exist in 100 planes with two planes per unit cell in monazite the planes are offset by 2 2 a along 010 relative to those in xenotime in order to accommodate the larger light re atoms the shift of the planes in monazite allows the re atom in that phase to bond to an additional 02 atom to complete the re09 polyhedron

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

contrasting p t t paths thermochronologic evidence for a late paleozoic final assembly of the avalon composite terrane in the new england appalachians strongly contrasting pressure temperature time paths for the avalon composite terrane and the structurally overlying putnam nashoba zone in eastern new england obtained from thermochronologic and thermobarometric data are best explained by a late paleozoic underthrusting of cover rocks by the avalon composite terrane we present new ar and u pb thermochronologic data that show that in the southern hope valley zone permian 280 ma anatectic metamorphic conditions of 700 c and 6 kbar were quenched by relatively rapid cooling 12 c m y and exhumation 0 5 km m y for 40 m y in contrast peak metamorphic conditions in the putnam nashoba zone predate silurian intrusions and slower cooling 3 5 c m y began at about 400 ma one dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the permian metamorphism of the avalon composite terrane because of the absence of high grade alleghanian metamorphism in rocks overlying the avalon terrane we conclude that high grade alleghanian metamorphism in the avalon terrane occurred east of rocks now overlying it and that significant motion between avalon and this cover occurred after peak alleghanian metamorphism similarly contrasting metamorphic histories between avalon inliers willimantic window massabesic complex gneiss pelham dome and their cover rocks reveals the regional significance of this boundary the core rocks all show permian cooling but the cover rocks show post acadian cooling ages decreasing from east to west to the pelham area where hornblende ages in avalon and cover differ by only 35 rather than 80 m y model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years consequently underthrusting of avalon is constrained to be middle mississippian or younger because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover core rocks of the pelham dome must have been relatively close to this leading edge thus carboniferous to permian underplating from a generally eastward direction best explains these thermochronologic relationships

relationship between the tibetan plateau heating and east asian summer monsoon rainfall 1 this study reveals the close correspondence between the interannual variability of the dominant east asian summer rainfall pattern and the diabatic heating over the tibetan plateau in both spring and summer the heating fluctuation over the tibetan plateau is associated with two wave like circulation patterns which bear the characteristics of forced rossby wave emanating away from the deep heating the wave like pattern in turn affects the east asian summer rainfall because of the persistent heating over the tibetan plateau from spring to summer and its possible effect on the surrounding areas the heating index defined in this study can be used as a good predictor for the jja heating and precipitation distributions evidences shown suggest that external conditions other than the sst anomaly must be considered to understand the interannual variability of the east asian summer rainfall

el nino modoki and its possible teleconnection 1 using observed data sets mainly for the period 1979 2005 we find that anomalous warming events different from conventional el nino events occur in the central equatorial pacific this unique warming in the central equatorial pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly ssta on both sides along the equator empirical orthogonal function eof analysis of monthly tropical pacific ssta shows that these events are represented by the second mode that explains 12 of the variance since a majority of such events are not part of el nino evolution the phenomenon is named as el nino modoki pseudo el nino modoki is a classical japanese word which means a similar but different thing the el nino modoki involves ocean atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution analogous to the southern oscillation in the case of el nino hence the total entity is named as el nino southern oscillation enso modoki the enso modoki events significantly influence the temperature and precipitation over many parts of the globe depending on the season the impacts over regions such as the far east including japan new zealand western coast of united states etc are opposite to those of the conventional enso the difference maps between the two periods of 1979 2004 and 1958 1978 for various oceanic atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline this appears to be a cause of more frequent and persistent occurrence of the enso modoki event during recent decades

basement complexes in the wasatch fault utah provide new limits on crustal accretion new and reinterpreted isotopic data for crystalline rocks ex posed in the wasatch range require a reevaluation of precambrian crustal boundaries in utah crystalline rocks of the santaquin com plex underwent metamorphism prior to ca 1670 ma consistent with sr and nd isotope data mafic to intermediate rocks have major element trace element and isotope ratios indicative of derivation in an arc accreted to the archean craton in proterozoic time requiring the crustal suture to be north of the santaquin complex farther north the farmington canyon complex has been considered ar chean based on published nd model ages and discordant u pb zir con ages however nd model ages and zircons could be inherited from sedimentary protoliths u pb and electron microprobe ages of monazite have a mode at 1650 to 1700 ma concordant with the santaquin complex and lack inheritance we propose that the farmington canyon complex was first cratonized from archean derived sediments in the proterozoic requiring a crustal suture to be north of it as well accretion ages of arc terranes in southeastern wyoming are 60 100 m y older than in utah thus a serious reevaluation of basement architecture in utah is needed and a pre viously unrecognized temporal complexity of accretion is indicated

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

cooperative effects of tropical pacific and atlantic sst forcing in southern china winter precipitation variability this study investigates the variability of winter precipitation over southern china sc and the impacts of sea surface temperature sst forcing in the tropical pacific and atlantic oceans the sc winter precipitation displays two major spatial modes a same sign mode with the maximum center located in southeastern china and a north south dipole mode with opposite variations between the lower reach of the yangtze river and the southeastern coast of china both modes are associated with the variation of the western north pacific wnp anticyclone the same sign mode is associated with canonical el nino through the pacific east asian teleconnection in the lower troposphere the north south dipole mode is subject to the impact of the western pacific warm pool associated with the central pacific el nino and equatorial atlantic forcing the influence of equatorial atlantic warming is through an upper tropospheric zonal wave pattern from the tropical atlantic to the mid latitudes of east asia both the tropical pacific and atlantic oceanic forcing induce a north south dipole pattern of winter precipitation anomalies over sc by affecting the northward extension of the wnp anticyclone moreover the observed statistical relationship between the dipole mode and the equatorial atlantic sst anomalies is present in hindcasts forecasts of the ncep climate forecast system version 2 cfsv2 confirming a potential source of the predictability of the north south dipole like variation of the winter precipitation in sc

influence of f oh 1 substitution on the relative mechanical strength of rock forming micas microtextural and experimental studies have yielded conflicting data on the relative mechanical strengths of muscovite and biotite wilson and bell 1979 kronenberg et al 1990 mares and kronenberg 1993 we propose a crystal chemical resolution to this conflict namely that 001 dislocation glide in biotite is rate limited by its fluorine content significant f oh 1 substitution and concomitant removal of hydroxyl h directed into the interlayer cavity potentially increases mechanical strength of biotite in two ways 1 it eliminates k h repulsion thereby strengthening the interlayer bonds and 2 it allows k to sink deeper into the interlayer cavity the resultant geometry being less favorable to basal slip in testing this hypothesis we analyzed the naturally deformed biotite studied by wilson and bell 1979 and documented its very low f content xf 0 02 compared to that of the biotite experimentally deformed by kronenberg et al 1990 our model and the comparative xf data explain why the biotite of wilson and bell 1979 deformed more easily in nature than its coexisting muscovite whereas the biotite of kronenberg et al 1990 was mechanically stronger than muscovite similarly deformed by mares and kronenberg 1993 our reconciliation of these otherwise conflicting results provides a framework for predicting mechanical strength of natural micas based upon the extent of their f oh 1 substitution our synthesis highlights the potential role of crystal chemistry in determining mechanical behavior in multicomponent mineral families further testing of crystal chemical effects on rheology will require mineral specimens of both appropriate composition and sufficient size

interdecadal changes in the major modes of asian australian monsoon variability strengthening relationship with enso since the late 1970s abstract the present paper develops an integral view of the year to year variability across the entire asian australian monsoon a am system which covers one third of the global tropics between 40 and 160 e using season reliant empirical orthogonal function s eof analysis the authors identified two major modes of variability for the period 1956 2004 the first exhibits a prominent biennial tendency and concurs with the turnabout of el nino southern oscillation enso providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation the second mode leads enso by one year the remote el nino forcing the monsoon warm pool ocean interaction and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode the monsoon ocean interaction is characterized by a positive feedback between the off equatorial convectively coupled rossby waves and the underlying sea surface temperature sst dipole anoma

a metamorphic history from micron scale 207pb 206pb chronometry of archean monazite ion microprobe measurements of pb isotope ratios in monazites have been obtained in situ from thin sections using the cambridge isolab 120 molecular interferences are sufficiently resolved at an rp of 6500 to allow 2 7pb z 6pb dating of monazite with precisions as low as 4 5 ma 2 r the results presented here provide important information on the chronological history of the late archean metamorphism of the wind river range wyoming usa matrix monazites and monazite inclusions in garnets from a metapelite from the northern wind river range have been analysed by sims in a previous study peak metamorphic conditions t 800 c p 8 1 kb were estimated using inclusion assemblages in garnets from this same sample isolated monazite inclusions in garnet yield 2 7pb z 6pb age estimates of 2781 6 to 2809 10 ma those along fractures yield lower ages 2603 2687 ma which are similar to tims and sims ages of matrix monazites a single large 500 xm monazite grain locally preserves growth zoning but has a recrystallised core and a resorbed recrystallised rim age estimates for these three regions are 2788 9 ma 2663 4 and 2523 6 ma respectively thus the inclusion assemblages of sharp and essene may record peak metamorphic conditions at ca 2 8 ga and indicate a phase of metamorphism that predates by over 100 ma the emplacement of the bridget batholith the major lithologic component of the northern wind river range the analysed monazite grains appear to preserve ca 300 ma history even within a single grain monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth even in high grade terranes where most chronometers are reset the age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure instead a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

ellenbergerite a new high pressure mg al ti zr silicate with a novel structure based on face sharing octahedra ellenbergerite occurs as purple millimetre size grains associated with talc kyanite clinochlore rutile and zircon in composite inclusions within decimetre large pyrope crystals 90 98 mole percent end member in the quartzite layer of the dora maira massif western alps from which coesite has been recently reported chopin 1984 it is hexagonal a 12 255 8 c 4 932 4 a z 1 space group p63 mohs hardness 6 5 dmes 3 15 dcal 3 10 no cleavage uniaxial negative and vividly pleochroic colourless e colourless to deep lilac with colour zoning the intensely coloured variety has 1 6789 5 e 1 670 1 microprobe analysis yields sio2 39 1 p2o5 0 45 al2o3 25 1 tio2 4 0 mgo 22 2 feo 0 20 sum 99 05 wt including h2o 8 0 coulometrically the formula calculated on a o28 oh 10 basis implying 7 5 wt h2o is mg6 71 fe0 03 ti0 61 al6 00 si7 92 p0 08 o28 oh 10 the colour zoning is due to nearly complete ti zr substitution in addition ellenbergerite may contain more than 8 wt p2o5 with strictly correlated changes of si mg al and ti zr contents over 80 of which represent the sial pmg substitution

zircon and sphene as fission track geochronometer and geothermometer a reappraisal the data published earlier on zircon and sphene fission track ages and annealing are discussed in the light of different etching conditions used for age determination and annealing experiments in order to explain the age discordances of some zircon and sphene pairs as well as numerous closing temperatures obtained for individual minerals using the new set of simple etching conditions zircon koh melt and sphene hf hcl the annealing experiments indicate that tracks in sphene are annealed more easily than in zircon the closing temperature of zircon and sphene have been calculated at 300 and 250 c respectively