mechanisms by which gulf of guinea and eastern north atlantic sea surface temperature anomalies can influence african rainfall abstract the sensitivity of precipitation over west africa to sea surface temperature anomalies sstas in the gulf of guinea and the eastern north atlantic is studied using a gcm results from nine perpetual july simulations with various imposed sstas are presented and analyzed to reveal associations between the precipitation and sst fields via large scale circulation and atmospheric moisture anomalies rainfall increases over the guinean coast and decreases over the congo basin when warm sstas are present in the gulf of guinea these precipitation perturbations are related to the forcing of a kelvin and a rossby wave the former is associated with a weakening of the walker circulation while the latter strengthens the west african monsoon rainfall over west africa is less sensitive to cold sstas than to warm anomalies three contributing factors are identified as follows 1 latitude of the sst forcing 2 background flow and 3 nonlinearity of the clausius clapeyron equation no more than a 20 effec

u pb dating of prograde and retrograde titanite growth during the scandian orogeny u pb ages of metamorphic titanite from high grade gneisses of the nasafjallet window in the north central scandinavian caledonides record growth over an 20 m y interval during the late silurian to early devonian biotite amphibolite gneisses located near the basement cover boundary contain metamorphic titanite in two microtextural settings one as lenticular grains distributed throughout a recrystallized matrix of metamorphic phases and one as plate like grains enclosed within biotite grains these distinct morphological populations exhibit core overgrowth relationships with compositionally similar cores and overgrowths but the plate like titanite grains have a much higher proportion of overgrowth and give slightly younger ages cores and overgrowths in both types are inferred to have formed along the prograde ca 413 ma and retrograde ca 395 ma paths respectively of a single metamorphic cycle associated with the burial and exhumation of a slice of baltic basement during the caledonide orogeny baltica laurentia collision the existence of polygenic u pb systematics in metamorphic titanite demonstrates the potential usefulness of titanite to reveal multiple metamorphic and deformational episodes and to constrain the duration of metamorphic events

trace elements in titanite a potential tool to constrain polygenetic growth processes and timing abstract the petrological information preserved in the trace element signature of titanite is a valuable complement to in situ u pb geochronology and can be used to refine interpretations on the growth of this mineral we present trace element and u pb isotopic compositions of titanite grains in five amphibolite to granulite facies samples from the east albany fraser orogen of western australia chondrite normalised rare earth element ree abundance patterns discriminate between titanite populations and correlate with backscatter electron bse zonation in two samples titanite ree composition correlates with proximity to garnet and in one sample titanite composition correlates with gneissic compositional banding the dy yb ratio can be used as a geochemical indicator to link titanite growth to garnet growth titanite that crystallised in a garnet bearing assemblage is hree depleted with dy yb 2 whereas titanite that crystallised in a garnet free assemblage typically has flat hree patterns with dy yb by integrating the ree signatures with u pb data the five new titanite u pb ages can be linked to a range of processes magmatic titanite crystallisation metamorphic titanite growth garnet growth and or cooling through the closure temperature for pb diffusion in titanite

the seasonal cycle in the lower troposphere over west africa from sounding observations in situ sounding observations from selected sites in west and central africa were used to document the seasonal cycle in the lower troposphere associated with the annual migration of precipitation the main purpose of this exercise is to provide a description of the seasonal cycle free of the errors and biases of numerical models which can be substantial in this region many features in the circulation typically associated with the west african monsoon wam exist through almost the entire seasonal cycle in the region the low level easterly jet known as the african easterly jet during the monsoon season remains prominent and closely related to the rain band in other phases of the seasonal cycle even when the rain band is located near the equator the surface southerly monsoon onshore flow penetrates through the rain band in the entire season cycle the depth of the southerly surface monsoon flow undergoes a seasonal variation and is greatest during the peak season of the monsoon a unique feature is a low level 700 hpa northerly flow penetrating through the rain band in various phases of the seasonal cycle risking possible biases of land bound sounding data these observations are synthesized into a schematic describing the salient features in the low level circulation associated with the rain band in different phases of the seasonal cycle this synthesized circulation pattern in west africa is in contrast to the traditional conceptual circulation pattern associated with the marine itcz and other monsoon systems the results from this study call for more research attention to the low level meridional circulation for a full understanding of the dynamics of the wam this study also advocates the study of the wam to be conducted in a broad context of the seasonal cycle in the region and in comparison to the marine itcz copyright 2006 royal meteorological society

u pb geochronology of zircon and polygenetic titanite from the glastonbury complex connecticut usa an integrated sem empa tims and shrimp study abstract u pb ages for zircon and titanite from a granodioritic gneiss in the glastonbury complex connecticut have been determined using both isotope dilution thermal ionization mass spectrometry tims and the sensitive high resolution ion microprobe shrimp zircons occur in three morphologic populations 1 equant to stubby multifaceted colorless 2 prismatic dark brown with numerous cracks and 3 elongate prismatic light tan to colorless cathodoluminescence cl imaging of the three populations shows simple concentric oscillatory zoning the zircon tims age weighted average of 207pb 206pb ages from group 3 grains 450 5 1 6 ma mswd 1 11 and shrimp age composite of 206pb 238 u age data from all three groups 448 2 2 7 ma mswd 1 3 are interpreted to suggest a relatively simple crystallization history titanite from the granodioritic gneiss occurs as both brown and colorless varieties scanning electron microscope backscatter bse images of brown grains show multiple cross cutting oscillatory zones of variable brightness and dark overgrowths colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning electron microprobe analysis empa clearly distinguishes the two populations brown grains contain relatively high concentrations of fe2o3 ce2o3 up to 1 5 wt nb2o5 and zr cerium concentration is positively correlated with total ree y concentration which together can exceed 3 5 wt oscillatory zoning in brown titanite is correlated with variations in ree concentrations in contrast colorless titanite both as discrete grains and overgrowths on brown titanite contains lower concentrations of y ree fe2o3 and zr but somewhat higher al2o3 and nb2o5 uranium concentrations and th u discriminate between brown grains typically 200 400 ppm u all analyses but one have th u between about 0 8 and 2 and colorless grains 10 60 ppm u th u of 0 0 17 in contrast to the zircon u pb age results shrimp u pb data from titanite indicate multiple growth episodes in brown grains oscillatory zoned cores formed at 443 6 ma whereas white in bse cross cutting zones are 425 9 ma colorless grains and overgrowths on brown grains yield an age of 265 8 ma using the total pb method or 265 5 ma using the weighted average of the 206pb 238u ages however empa chemical data identify zoning that suggests that this colorless titanite may preserve three growth events oscillatory zoned portions of brown titanite grains are igneous in origin white cross cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite colorless titanite replaced and grew over the magmatic titanite during the permian alleghanian orogeny these isotopic data indicate that titanite like zircon can contain multiple age components coupling shrimp microanalysis with empa and sem results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic data to reveal the complexities of igneous crystallization and metamorphic growth

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

in situ la icp ms u pb dating of metavolcanics of norrbotten sweden records of extended geological histories in complex titanite grains in situ u pb dating of a variety of mineral phases is an important goal in petrology this study reports data chiefly from titanite but also from rutile and apatite obtained using the laser ablation la icp ms methodology on polished thick sections in order to retain as much petrologic information as possible and allowing trace element analyses from adjacent areas to the u pb analyses the samples analysed come from svecofennian intermediate to acid volcanic rocks of the porphyry group within the major iron ore province of norrbotten northern sweden where titanite is a common phase associated with the mineralisation using a laser beam of 30 45 m in diameter accurate and relatively precise 2 3 2 on 206pb 238u ratios data can be obtained on titanite similar precision on pb u ratios can be obtained in rutile but accuracy cannot be assessed because of the lack of well characterised standards however the data are a priori accurate as they are geologically reasonable apatite shows reverse discordance which could be explained by a number of scenarios however the 207pb 206pb ages are fairly precise 1 3 2 and the calculated age is geologically reasonable suggesting that the data may be accurate the titanite grains studied show complex internal structures rare earth element ree analysis by la icp ms demonstrates that the core and rim zones are distinct and supports a model for two stage evolution in two out of three samples with the rims in these samples being enriched in total ree and particularly in lree in one sample and displaying a positive y anomaly in contrast to the cores in situ u pb analysis reveals that core zones from all samples retain distinct older ages of c 2050 ma whereas the reworked rims have either distinct younger or strongly reset ages the older cores are interpreted as representing the first alteration metamorphism of the volcanic pile titanite rims record a u pb age of 1870 24 ma 2 at luossavaara and 1826 15 ma 2 at the smaller fe oxide apatite body at gruvberget in both cases the ages are consistent with previous age determinations from the fe oxide apatite deposits a sample from malmberget records a slightly different scenario where u pb ages spread along concordia from c 1920 to 1708 ma we attribute this pattern to metamorphic reworking of primary metasomatic titanite rutile ages from inclusions within the core of gruvberget titanite essentially mimic the ages obtained from the surrounding titanite and suggest that they have undergone the same history apatite inclusions within titanite at malmberget record a younger age of 1584 12 ma 2 which we attribute to later events known to have affected the area these data indicate that the porphyry group volcanics must be at least c 2050 ma in age in contrast to previous age determinations from zircon which suggested a deposition age of c 1960 to 1880 ma the large grain size of the titanites studied coupled with the ability for the first time to obtain in situ u pb ages from small areas has allowed probing of this hitherto unrecognised event smaller zircon grain size coupled with extremely saline hot pressurised high p t metasomatic fluids relating to regional magnetite apatite mineralisation in the period 1900 to 1800 ma may have resulted in resetting of zircon u pb systematics 2007 elsevier b v all rights reserved

retrograde deformation within the carthage colton zone as recorded by fluid inclusions and feldspar compositions tectonic implications for the southern grenville province the carthage colton zone ccz located in the northwestern portion of the proterozoic adirondack terrane forms the boundary between the adirondack lowlands and highlands and is characterized by textures indicative of ductile deformation samples from three rock types have been collected from this zone 1 pyroxene bearing syenite gneisses of the diana igneous complex 2 paragneiss samples from near the northwestern boundary of the diana complex 3 a quartz rich metasediment feldspar geothermometry performed on the diana metasyenites shows that porphyroclasts are relict igneous grains that formed att 900 c while dynamic recrystallization occurred at temperatures as low as 470 to 550 c as shown by the compositions of feldspar neoblasts all samples examined in this study from the ccz contain a suite of co2 rich fluid inclusions that are distinctive both texturally and in their microthermometric behavior th 27 7 to 7 1 c as compared to co2 rich adirondack fluid inclusions that do not lie within this zone th 45 9 to 31 0 c the results from fluid inclusion microthermometry and feldspar geothermometry restrict the conditions of dynamic recrystallization to temperatures and pressures of 400 to 550 c and 3 to 5 kbar the retrograde pressure temperature path must pass through these conditions similar fluid inclusion results have been obtained from the parry sound shear zone pssz which is a grenvillian shear zone that is located in southern ontario lamb and moecher 1992 however the inferred retrogradep t paths for these two areas the ccz and the pssz are different and this difference may be a result of late deformation along shear zones that are located between the two areas

a revised picture of the structure of the monsoon and land itcz over west africa this article presents an overview of the land itcz intertropical convergence zone over west africa based on analysis of ncar ncep reanalysis data the picture that emerges is much different than the classic one the most important feature is that the itcz is effectively independent of the system that produces most of the rainfall rainfall linked directly to this zone of surface convergence generally affects only the southern sahara and the northern most sahel and only in abnormally wet years in the region a second feature is that the rainbelt normally assumed to represent the itcz is instead produced by a large core of ascent lying between the african easterly jet and the tropical easterly jet this region corresponds to the southern track of african easterly waves which distribute the rainfall this finding underscores the need to distinguish between the itcz and the feature better termed the tropical rainbelt the latter is conventionally but improperly used in remote sensing studies to denote the surface itcz over west africa the new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift which in turn is controlled by the characteristics of the african easterly jet and tropical easterly jet rather than by moisture convergence this new picture also includes a circulation feature not generally considered in most analyses of the region this feature a low level westerly jet termed the african westerly jet plays a significant role in interannual and multidecadal variability in the sahel region of west africa included are discussions of the how this new view relates to other aspects of west africa meteorology such as moisture sources rainfall production and forecasting desertification climate monitoring hurricanes and interannual variability the west african monsoon is also related to a new paradigm for examining the interannual variability of rainfall over west africa one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north south displacements of this feature the new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the african easterly waves the disturbances that are linked to rainfall over west africa and spawn hurricanes over the atlantic and with our knowledge of the prevailing synoptic and dynamic features this article demonstrate a new aspect of the west africa monsoon a bimodal state with one mode linked to dry conditions in the sahel and the other linked to wet conditions the switch between modes appears to be linked to an inertial instability mechanism with the cross equatorial pressure gradient being a critical factor the biomodal state has been shown for the month of august only but this month contributes most of the interannual variability this new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability but also to the multidecadal variability evidenced in the region between the 1950s and 1980s

paleomagnetism and geochronology of the ecstall pluton in the coast mountains of british columbia evidence for local deformation rather than large scale transport 1 samples for geochronologic geobarometric and paleomagnetic analyses were collected across the northern portion of the ecstall pluton southeast of prince rupert british columbia al in hornblende geobarometry indicates pressures from 740 10 to 840 30 mpa corresponding to crystallization depths of 25 to 30 km u pb analyses of zircons from western central and eastern localities within the pluton yield crystallization ages of 91 5 1 0 ma 90 8 1 0 ma and 90 5 1 0 ma respectively rock magnetic experiments reflected light microscopy and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low ti titanohematite unblocking temperatures of the characteristic remanent magnetization chrm are dominantly in the 560 c to 630 c range with age of magnetization approximated by the 40ar 39ar hornblende ages of 84 2 0 10 ma on the western margin and 76 4 0 6 ma in the center of the pluton site mean chrm directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at 340 azimuth chrm directions from the central portion of the pluton are concordant with the expected cretaceous magnetic field direction while chrm directions from the western margin are discordant by 70 folding of the ecstall pluton either during late cretaceous west directed thrust transport above the convex upward prince rupert shear zone or during younger deformation of the pluton and underlying shear zone can account for the paleomagnetic data and is consistent with the geochronologic geobarometric and structural geologic observations

generation of the african easterly jet and its role in determining west african precipitation abstract an examination of analyses and model simulations is used to show that the african easterly jet forms over west africa in summer as a result of strong meridional soil moisture gradients in a series of gcm experiments the imposition of realistic surface wetness contrasts between the sahara and equatorial africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere the associated easterly shear in the atmosphere is strong enough to establish easterly flow the african easterly jet above the monsoon westerlies at the surface positive temperature gradients associated with the summertime distributions of solar radiation ssts or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients a thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet while moisture converges throughout the lower troposphere over east africa moisture divergenc

resetting of oxybarometers and oxygen isotope ratios in granulite facies orthogneisses during cooling and shearing adirondack mountains new york the petrography petrology and oxygenisotope geochemistry of granulite facies granitic and syenitic orthogneisses of the diana and stark complexes adirondack mountains new york show that the extent and nature of resetting of isotopic and mineralogic systems is highly variable there is a strong correlation between retrogression and shearing and the rocks may be divided texturally into 1 unsheared lithologies that preserve little retrogressed pyroxene or hornblendebearing peak metamorphic mineralogies and 2 sheared rocks that underwent retrogression marked by the growth of late biotite in centimetre to metre wide shear zones after the peak of metamorphism oxygen fugacities in the unsheared lithologies were estimated for reintegrated mineral compositions from magnetiteilmenite mt ilm and ferrosilite magnetic quartz fs mt qtz equilibria mt ilm yields logfo2mt ilm values of 15 9 to 17 6 0 6 to 1 3 log units below the fayalite magnetite quartz buffer fmq and temperatures of 670 745 c that agree with those from other geothermometry and phase equilibria studies these data suggest that aside from oxyexsolution of ilmenite from magnetite the fe ti system underwent only minor resetting during cooling and the fe ti oxides yield good estimates of peak metamorphic temperatures and fo2 in unsheared ilmenite magnetite orthopyroxene quartz assemblages values of logfo2mt ilm are lower than logfo2fs mt qtz by an average of 0 6 when the orthopyroxene activity model of sack and ghiorso is used minor resetting of the fe ti oxides analytical errors and errors in the placement of end member reactions probably account for this relatively small difference in fo2 values whole rock 18o values of unsheared diana and stark lithologies range from 4 0 to 10 3 reflecting pre regional metamorphic oxygen isotope ratios peak metamorphic minerals preserve high temperature oxygen isotope fractionations and in many samples the effective diffusion of oxygen in minerals ceased at higher temperatures than predicted from wet experimental diffusion data these data suggest that the rocks did not contain an aqueous fluid phase during cooling the combination of petrologic isotopic and textural data also permits a detailed study of shearing and retrogression ilmenites in the sheared lithologies underwent greater degrees of hematite loss than in the unsheared rocks resulting in logfo2mt ilm values as low as 24 1 3 1 log units below fmq and mt ilm temperatures that are up to 175 c below regional estimates sheared rocks also have higher 18o values up to 13 3 during shearing 18o values of biotite k feldspar and magnetite reset readily while the degree of isotopic resetting of quartz correlates with the intensity for recrystallization

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

effect of metamorphic reaction history on the u pb dating of titanite drill core samples of garnet clinopyroxene granulite at tirschheim and a reference sample at waldheim saxon granulite massif germany endured the same p t conditions but developed variable mineral assemblages due to differences in bulk chemistry reaction progress deformation and retrogression titanite formed during peak metamorphic conditions of 22 24 kbar and 1020 1050 c dating titanite from the various samples should yield the same age for all the observed age variation which exceeds the duration of the entire metamorphic cycle originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post peak re equilibration 1 pb inheritance observed in some peak metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization instead metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral 2 titanite that formed at peak metamorphic conditions is characterized by high a1 contents and x f 0 8 1 texturally comparable titanite that re equilibrated during cooling reduced al contents and x f yields too young u pb ages the age of such re equilibrated titanite does not correspond to the age of the event indicated by the texture

shallow meridional circulations in the tropical atmosphere abstract a shallow meridional circulation smc in the tropical atmosphere features a low level e g 700 hpa flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above representations of the smc in three global reanalyses show both similarities and astonishing discrepancies while the smc over west africa appears to be the strongest it also exists over the eastern atlantic and eastern pacific oceans and over the indian subcontinent with different strength and structure all smcs undergo marked seasonal cycles the smcs are summarized into two types one associated with the marine itcz and the other with the summer monsoon the large scale conditions for these two types of smcs are similar a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection the processes responsible for these conditions are different for the two types of smcs as are their str

the carthage colton mylonite zone adirondack mountains new york the site of a cryptic suture in the grenville orogen u pb ages were determined on metamorphic sphenes and monazites from the late proterozoic adirondack highlands and lowlands in the vicinity of the carthage colton mylonite zone monazites were extracted from metapelites and sphenes were separated from marbles calc silicate gneisses and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone monazite ages from the lowlands range from 1171 1137 ma sphene ages in the lowlands range from 1156 1103 ma those from the highlands immediately to the east of the mylonite zone range from 1050 982 ma the ages indicate that the last high grade metamorphism in the highlands is ca 100 m y younger than in the lowlands and that both terranes had separate cooling histories at least until ca 1000 ma sphenes from within the carthage colton mylonite zone yield ages of about 1098 ma which are distinct from sphene ages on either side of the shear zone the mineral

combined oxygen isotope and u pb zoning studies of titanite new criteria for age preservation this material is based upon work supported by the u s national science foundation award ear 0838058 the u s department of energy office of basic energy sciences under award number de fg02 93er14389 and geological society of america student research grant 9224 10 wiscsims is partially supported by nsf ear 1053466 and 1355590

dating shear zones with plastically deformed titanite new insights into the orogenic evolution of the sudbury impact structure ontario canada abstract the sudbury structure is a mineralized impact crater that hosts different families of ore controlling shear zones with poorly known orogenic affinities discriminating whether these deformation events relate to the 1 85 ga crater modification stage or later regional tectonism that collapsed the impact structure is important both for crustal and mineral exploration studies we have combined underground mapping with isotopic and microstructural analysis of titanite and host minerals in a benchmark ore controlling mylonitic shear zone of the mining camp the six shaft shear zone from the creighton mine three growth stages of chemically and microstructurally characterised titanite grains were identified related with the pre syn and late deformation stages in situ u pb age dating of the syndeformational grains demonstrates that a shearing event took place at 1645 54 ma during the mazatzalian labradorian orogeny 1 7 1 6 ga this event led to the plastic deformation and local scale remobilization of primary ni cu pge sulphides in creighton mine sudbury south range the adopted novel petrochronological approach can reveal the age significance of syn deformational processes and holds promise for the untangling of complex syn orogenic processes in precambrian terranes globally

the west african monsoon dynamics part i documentation of intraseasonal variability intraseasonal variability in the west african monsoon is documented by using daily gridded datasets of rainfall and convection and reanalyzed atmospheric fields over the period 1968 90 rainfall and convection over west africa are significantly modulated at two intraseasonal timescales 10 25 and 25 60 day leading to variations of more than 30 of the seasonal signal a composite analysis based on the dates of the maximum minimum of a regional rainfall index in wet dry sequences shows that these sequences last on average 9 days and belong to a main quasiperiodic signal of about 15 days a secondary periodicity of 38 days is present but leads to a weaker modulation during a wet dry sequence convection in the itcz is enhanced weakened and its northern boundary moves to the north south while the speed of the african easterly jet decreases increases the speed of the tropical easterly jet increases decreases and the monsoon flow becomes stronger weaker all these features being similar to the ones associated with interannual variability characterizing wet and dry years this modulation of convection at intraseasonal timescales is not limited to west africa but corresponds to a westward propagating signal from eastern africa to the western tropical atlantic an enhanced monsoon phase is associated with stronger cyclonic activity in the low levels over the sahel associated with stronger moisture advection over west africa five days before the full development of the wet phase a stronger cyclonic circulation at 20 e induces enhanced southerly winds along 25 e where convection enhances while another westwardpropagating cyclonic circulation is located at 0 this atmospheric pattern is linked to the enhancement of the subsiding branch of the northern hadley cell at 35 n northerly advection of drier air over west africa and to increased dry convection in the heat low at 20 n it propagates westward leading to a zonally extended area of enhanced monsoon winds over west africa consistent with the occurrence of the wet phase

a physical basis for the interannual variability of rainfall in the sahel a major factor in rainfall variability over sahelian west africa is the latitudinal location of the tropical rainbelt when it is displaced abnormally far northward the sahel experiences a wet year an anomalous southward displacement results in drought in this paper we examine the question of what controls the location during the boreal summer hypothesizing that inertial instability plays a role an analysis of surface pressure and temperature fields wind fields divergence and vertical motion show that the criteria for inertial instability are satisfied in wet augusts but not in dry ones the key determinant appears to be the surface pressure gradient between the continent and the equatorial atlantic when this is large inertial instability results in the development of a low level westerly jet the presence of this jet enhances the horizontal and vertical shear and displaces the african easterly jet northwestward associated with this situation is strong vertical motion over the sahel and subsidence over the guinea coast producing dry conditions over the latter the result is a rainfall dipole one of two major modes of variability over west africa important factors include sea surface temperatures ssts in the equatorial atlantic and pressure in the south atlantic the first of these factors suggests a link with the atlantic nino mode of tropical atlantic variability while the second suggests a possible link with the pacific and the extratropical south atlantic overall our study relates the well known sst influence on sahel rainfall to atmospheric dynamics over the continent copyright 2007 royal meteorological society

u pb systematics and trace element characteristics in titanite from a high pressure mafic granulite abstract simultaneous acquisition of u pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic high pressure granulite using la icp ms split stream analysis has enabled evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high temperature melt present environments although the titanite studied here have a comparatively low initial pb pb 0 component pb 0 pb the pb 0 is highly radiogenic relative to model crustal values indicating inheritance from u bearing accessory minerals consumed in the melt titanite forming reactions additionally titanite crystals typically exhibit core rim decreases in pb 0 pb as defined by 204 pb 206 pb calculated 206 pb 0 206 pb t and uncorrected 206 pb 238 u spot date profiles near the margins this is clearly dominated by local u enrichment but in the uniformly low u interiors outwardly decreasing pb 0 pb appears to reflect decreasing pb 0 concentrations during growth the positive correlation among pb 0 and sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation given the high crystallization temperatures 800 c and likely slow cooling rates 5 c our data support slow pb diffusivity in titanite even at high temperature conditions as has been proposed in a number of recent studies along the outer 50 100 m u th zr and ree concentrations are variably elevated relative to the crystal interiors with profiles taking one of two forms 1 sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge or 2 gradually increasing to highest concentrations nearest the crystal edge high contrast bse imaging shows that the former profiles are associated with 1 2 m wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is or formerly was in contact with matrix feldspar i e crystallized melt and are inferred to represent trace element enriched dissolution precipitation reaction fronts the latter profiles are associated with diffuse locally wispy brightness gradients adjacent to resorbed crystal boundaries subgrain boundaries or thicker bright bands formed in contact with matrix hornblende crystals and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro porosity pathways at some point in the recrystallization process

variability of the intertropical front itf and rainfall over the west african sudan sahel zone abstract the intertropical front itf is a fundamental feature of the atmospheric circulation over west africa it separates the wedge of warm moist southwesterly monsoon flow off the tropical atlantic from much hotter and very dry northeasterly wind from the sahara desert here the daily temperature humidity and rainfall data for 1974 2003 are analyzed to document the northward advance and southward retreat of the itf between boreal spring and autumn and assess its role in sudan sahel 10 20 n rainfall patterns using largely dekadal 10 day and monthly resolutions analyses are performed for the 30 yr average seasonal time scale and sets of extreme years with a major focus on concurrent monthly itf rainfall relations the seasonal rainfall predictive potential of the early season itf latitude is also investigated as is the secular variation of itf latitude weather system rainfall associations during 1974 2003 the northward advance of the itf across the sudan sahel from april to early august i

redistribution of hfse elements during rutile replacement by titanite titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon we investigated an amphibolite sample from the tromso eclogite facies terrain in northern norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite patchy compositional zoning of al ti and f in titanite relates to the presence of a fluid with variable ti al and or f during its growth laser ablation icp ms and electron microprobe data for high field strength elements hfse nb zr ta and hf of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal 8 mm and a systematic decrease towards uniform hfse contents in the large core hfse contents of smaller rutile grains 0 5 mm and rutile inclusions 100 m in the titanite overgrowth are similar or higher than in the rims of large rutile crystals element profiles from the rim inward demonstrate that hfse enrichment in rutile is controlled by diffusion hfse ratios in diffusion altered rutile show systematic variations compared with the uniform core composition of the large rutile modelling of zr and nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems variations of hfse contents in the newly formed titanite show no systematic spatial distribution hfse ratios in titanite and the rims of rutile are different indicating different solid fluid distribution coefficients in these minerals element fractionation by diffusion into the relict rutile and during fluid mediated growth of new titanite could substantially change the hfse budget of these minerals and could affect their use for geochemical tracing and other applications such as zr based geothermobarometry

mesoscale convective systems in relation to african and tropical easterly jets abstractthis paper documents the interaction processes between mesoscale convective systems mcs the tropical easterly jet tej and the african easterly jet aej over west africa during the monsoon peak of 2006 observed during the african monsoon multidisciplinary analyses amma project the results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle a systematical study reveals that intense and long lived mcss correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of tej and in exit regions of aej a case study on a particularly well documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence favoring the persistence of mcss

trace element incorporation in titanite constraints from experimentally determined solid liquid partition coefficients titanite liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite saturated liquids equilibrated at 1 5 gpa and 850 c starting from lamproitic compositions the high compatibility observed for ree hfse sr v and sc and the strong incompatibility observed for actinides large ion lithophile and light elements are discussed in terms of available crystal chemical mechanisms for incorporation and crystal structure control the exchange vectors na1ca 1 and al1ti 1 allow local charge balance to be achieved after incorporation of ree3 and r5 at the ca and ti site respectively the significant amounts of h measured are also relevant in this regard the exchange mechanism being oh 1o 1 at the o1 site the incorporation of u4 and th4 at the ca site is scarce and is likely balanced by that of mg2 at the ti site both these substitutions are responsible for strong structural strain titanite can thus be considered an important repository for ree and hfse in metamorphic and igneous rocks and its role must be accounted for when modelling trace element residence during metamorphic reactions and late magmatic crystallisation history due to the measured differences in compatibility titanite crystallisation increases the values of nb ta and lree hree ratios in residual liquids the similar compatibility of u and pb makes titanite suitable for u pb geochronology of igneous rocks only after common pb correction finally this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal chemical constraints and that titanite based waste forms should contain high amounts of na and mg2

high resolution p t t paths from δ18o zoning in titanite a snapshot of late orogenic collapse in the grenville of new york we characterize oxygen isotope zoning within single titanite crystals from the carthage colton mylonite zone ccmz adirondack mountains new york state united states by ion microprobe smooth gradients of 18 o up to 0 6 over 90 m resulted from diffusive exchange of oxygen during cooling from peak metamorphic temperatures of 650 700 c modeling of the observed profile indicates punctuated cooling rates of 30 60 c m y along the ccmz set within long periods of much slower cooling these results indicate a previously unrecognized period of rapid cooling along the ccmz that is interpreted to result from the post ottawan collapse of the grenville mountain belt and exhumation of the central adirondack highlands at ca 1050 ma

molybdenum sources and isotopic composition during early stages of pedogenesis along a basaltic climate transect abstract molybdenum mo is an essential micronutrient and redox sensitive trace metal that has the potential to be a tracer of pedogenic processes globally riverine 98mo values based on 98mo 95mo relative to nist srm 3134 are heavier relative to bedrock suggesting weathering processes preferentially retain light mo isotopes however the mechanisms governing this process in soils are poorly understood we investigated soil mo abundance and isotopic composition as a function of climate along a well constrained climosequence on a 10 kyr lava flow in hawaii receiving 660 to 2100 mm year 1 mean annual precipitation map we assessed mo abundance and isotopic composition as a function of soil organic matter om content short ranged ordered sro iron fe oxyhydr oxide abundance and mo loss gain we find net accumulation of mo across all soils up to 139 gain that is positively correlated with increasing map the highest mo gains are in surface soil horizons and are correlated with high om content the isotopic composition of soil mo deviates from underlying basaltic bedrock which ranges from 98mo values of 0 11 to 0 26 soil mo isotopic values are lightest at the dry sites 98mo 0 63 and become heavier with increasing precipitation up to 98mo 0 03 samples with the heaviest 98mo values are from horizons with the largest net gains of mo relative to bedrock and have higher om content in order to further constrain mo fluxes into and out of the soil system we measured mo isotope ratios in local rainwater average 98mo 1 11 groundwater average 98mo 0 14 and vegetation 98mo values between 0 18 and 0 64 based on these data we propose that mo in these soils is substantially augmented by additions from precipitation volcanic fog and potentially anthropogenic inputs of mo and that retention of these inputs within soils is likely related to mo om interactions the large atmospheric inputs of mo strongly modulate the 98mo values of dissolved mo fluxes from soil to values only slightly heavier than bedrock these patterns illustrate the potential for mo as a tracer of atmospheric inputs and pedogenic processes in soils and help elucidate the mechanisms that drive heavy 98mo values in the global riverine mo flux

fluid driven resetting of titanite following ultrahigh temperature metamorphism in southern madagascar abstract la icp ms u pb dates and epma trace element maps of titanite were collected from calc silicate gneisses of the ikalamavony peak t 750 800 c and anosyen peak t 900 950 c domains in southern madagascar to evaluate how titanite responds to high temperature metamorphism cooling and retrogression fluid mediated replacement of precursor titanite by titanite domains of different composition interface coupled dissolution precipitation icdr was the primary mechanism by which titanite was reset following high grade metamorphism comparison of titanite u pb dates 530 490 ma with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300 500 c apparent zr temperatures temperatures calculated assuming titanite quartz zircon equilibrium in altered titanite are less than or equal to the metamorphic peak but higher than the inferred alteration temperature implying that zr was removed but that titanite quartz zircon equilibrium was not achieved during alteration although evidence for icdr was observed in titanite samples separated by 10 s to 100 km differences in u pb dates among samples and even among titanite grains in the same thin section suggest that alteration at any given time was localized

preserved zr temperatures and u pb ages in high grade metamorphic titanite evidence for a static hot channel in the himalayan orogen abstract titanite grains from 800 c gneisses of the c 6 km thick greater himalayan sequence of central nepal were analyzed for zr in titanite temperatures and u pb ages to investigate the formation and evolution of a former weak mid crustal channel in the himalaya laser ablation icp ms depth profiles and spot analyses of titanite zr and u pb isotopic compositions are inconsistent with diffusional reequilibration on scales greater than 1 2 m and instead data appear to faithfully record both temperature and age titanite records protracted heating from 700 750 c at 37 ma to 775 c at 24 ma and possibly slight cooling to 765 c at 20 ma such temperatures exceed partial melting reactions in associated metapelites and imply profoundly weak rheologies i e a hot channel yet predate initiation of the bounding main central thrust and south tibetan detachment systems by as much as 15 myr evidently thick but weak crustal channels may remain stationary with respect to the convergence direction for 10 myr even during one of earth s biggest continent continent collisions

lead isotope analyses of leached feldspars constraints on the early crustal history of the grenville orogen abstract lead isotopic data on leached k feldspars plagioclases and muscovite provide constraints on the premetamorphic evolution of the grenville orogen in ontario and new york a boundary between reworked archean crust to the northwest and new mantle derived crust generated during mid proterozoic times to the southeast is identified within the northern portion of the gneiss belt of ontario the identification of juvenile mid proterozoic crust at the southeastern margin of the orogen is consistent with previous investigations based on neodymium model ages however near the grenville front lead isotopes indicate the presence of reworked archean crust rather than primary early proterozoic crust postulated on the basis of neodymium model ages this discrepancy between the lead and neodymium isotopic terranes underscores the necessity of using a multidisciplinary approach to identify possible terrane boundaries only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major orogenic cycle and crust that represents a mixture of old with new mantle derived material the combined neodymium and lead data imply that the grenville orogen in ontario consists essentially of reworked archean crust and juvenile mantle derived material generated around 1 5 ga the grenville orogen involved archean crust that was depleted in thorium and uranium several hundred ma before high grade metamorphism at ca 1 1 ga such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the gneiss belt

geochemical and u pb age constraints on the occurrence of polygenetic titanites in uhp metagranite in the dabie orogen abstract relict magmatic titanite was identified in the core of a few titanite grains with the overgrown rim of metamorphic titanite in uhp metagranite in the dabie orogen la icpms u pb dating gave neoproterozoic ages for the magmatic titanite but triassic ages for the metamorphic titanite the magmatic and metamorphic titanites are clearly distinct by differences in petrological and geochemical compositions the magmatic titanite occurs as residual cores that show bright bse the presence of allanite and quartz inclusions low contents of cao al2o3 and tio2 but high contents of fe2o3 and mgo in trace elements the magmatic titanite exhibits high ree and hfse contents distinctly negative eu anomalies with flat mree hree patterns and high th u ratios in contrast the metamorphic titanite occurs as rims and grains of homogeneously dark bse that contain inclusions of epidote quartz k feldspar rutile biotite and phengite and have relatively high contents of cao al2o3 and tio2 but low contents of fe2o3 and mgo and relatively low ree and hfse contents slightly negative eu anomalies with hree depletion relative to mree and low th u ratios the zr in titanite thermometry yields 727 to 877 c at 0 5 to 1 0 gpa for the magmatic titanite and 729 to 870 c at 1 5 to 2 0 gpa for the metamorphic titanite the neoproterozoic u pb chronometric system of magmatic titanite survived the triassic continental subduction zone hp uhp metamorphism this suggests a relatively high closure temperature of 800 c for the titanite u pb system the metamorphic titanite is principally a product of retrograde metamorphism during decompression exhumation at the transition from hp eclogite facies to amphibolite facies therefore titanite holds a great potential to geochronological and petrogenetic studies of continental subduction zone metamorphic rocks

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

sm nd and u pb systematics of single titanite grains abstract combined sm nd and u pb isotopic systems in single titanite grains from archean rocks may be used to obtain nd isotope composition at the time of titanite growth the closed sm nd system requirement is applied to titanite and its integrity is monitored using the u pb system in a similar way as u pb in zircon is used to interpret lu hf data the nd isotope composition at the time of titanite formation can be used to infer initial nd isotopic values of the rocks at the time of zircon crystallization this approach does not involve assumption that the sm nd system in the rocks remained closed since their formation whereas such assumption underlies all initial nd determinations based on whole rock analyses titanite is a common accessory mineral in felsic rocks and a major concentrator of ree and uranium titanite populations are in many cases heterogeneous and resolving the isotopic characteristics of old and young titanite populations requires isotopic analysis of multiple single grains here i report sm nd and u pb data from single titanite grains from four early archean gneisses previously studied for lu hf and u pb systematics of zircon gneisses from the isua area contain heterogeneous populations of titanite which form roughly linear arrays in the u pb concordia diagram with ca 3600 ma upper intercepts and ca 2600 2700 ma lower intercepts or contain individual concordant grains of the latter age two of these gneisses also contain titanite grains of a high u variety which plot below these discordia lines probably as a result of post archean loss of radiogenic pb titanite from a gneiss from the nuuk area is concordant at 2520 2600 ma sm nd analyses of several single titanite grains from the isua area gneiss ggu163263 yielded an isochron age of 3555 150 ma identical to the upper intercept u pb age of 3601 15 ma the e nd t tit values of these grains calculated using their 207 pb 206 pb ages are uniform for the oldest titanite fractions with nd content 1000 ppm and give the average e nd t tit 0 32 0 20 at 3601 ma extrapolation from this value to 3708 25 ma an average of the ages of two zircon populations using an average crustal 147 sm 144 nd 0 12 0 04 yields e nd t zir 1 4 0 6 at the time of crystallization zircon from the same rock has average e hf t zir 1 9 1 0 titanite grains from two other gneisses from the isua area show more complex sm nd patterns predominant pale brown to colorless populations have variable 147 sm 144 nd ratios reaching as high as 0 7 and do not form isochrons suggesting that recrystallization of titanite was possibly accompanied by modification of the sm nd system in the whole rock no reliable initial nd value has been obtained from these gneisses the sm nd analyses of titanite fractions from the gneiss from the nuuk area yielded an average e nd t 9 00 0 37 extrapolation to the zircon crystallization age of 3610 10 ma yields an e nd t value of 1 8 5 4 which has low precision due to extrapolation over a billion year period the results presented here show that precise and tenable initial nd values for early archean gneisses can be obtained from analyses of best preserved titanite grains in favorable cases even if these grains represent a small fraction of an otherwise young or altered titanite population

intragrain oxygen isotope zoning in titanite by sims cooling rates and fluid infiltration along the carthage colton mylonite zone adirondack mountains ny usa oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen bearing minerals and the dependence of mineral 18 o 16 o ratios on temperature and fluid composi tion in this study subtle intragrain oxygen isotope zoning in titanite is resolved at the 10 lm scale by secondary ion mass spectrometry the patterns of d 18 o zoning differ depending on microstructural context of individual grains and reflect multiple processes including diffusive oxy gen exchange partial recrystallization grain size reduction and grain growth using the chrono logical framework provided by structural relations these processes can be related to specific events during the grenville orogeny titanite was sampled from two outcrops within the car thage colton mylonite zone ccmz a long lived shear zone that ultimately accommodated exhumation of the adirondack highlands from beneath the adirondack lowlands during the ott awan phase 1090 1020 ma of the grenville orogeny titanite is hosted in the diana metasyenite complex which preserves three sequentially developed fabrics an early nw dipping protomylonitic fabric s1 is crosscut by near vertical ultramylonitic shear zones s2 which are locally reoriented by a nnw dipping mylonitic fabric s3 texturally early titanite pre s2 shows diffusion domi nated d 18 o zoning that records cooling from peak ottawan granulite facies conditions numerical diffusion models in the program fast grain boundary yield good fits to observed d 18 o profiles for cooling rates of 50 20 cm a 1 which are considerably faster than the 1 5 cm a 1 cooling rates previously inferred for the adirondack highlands from regional thermochronology high cool ing rates are consistent with an episode of rapid shearing and exhumation along the ccmz during gravitational collapse of the ottawan orogen at c 1050 ma texturally later titanite syn s2 h as higher overall d 18 o and shows a transition from diffusion dominated to recrystallization dominated d 18 o zoning indicating infiltration of elevated d 18 o fluids along s2 shear zones and continued shear ing below the blocking temperature for oxygen 500 c for grain sizes at the study site the tex turally latest titanite post s3 h as growth dominatedd 18 o zoning consistent with porphyroblastic grain growth following cessation of shearing along the harrisville segment of the ccmz

u pb isotopic dating of titanite microstructures potential implications for the chronology and identification of large impact structures identifying and dating large impact structures is challenging as many of the traditional shock indicator phases can be modified by post impact processes refractory accessory phases such as zircon while faithful recorders of shock wave passage commonly respond with partial u pb age resetting during impact events titanite is an accessory phase with lower pb closure temperature than many other robust chronometers but its potential as indicator and chronometer of impact related processes remains poorly constrained in this study we examined titanite grains from the sudbury ontario canada and vredefort south africa impact structures combining quantitative microstructural and u pb dating techniques titanite grains from both craters host planar microstructures and microtwins that show a common twin host disorientation relationship of 74 about in the vredefort impact structure the microtwins deformed internally and developed high and low angle grain boundaries that resulted in the growth of neoblastic crystallites u pb isotopic dating of magmatic titanite grains with deformation microtwins from the sudbury impact structure yielded a 207pb 206pb age of 1851 12 ma that records either the shock heating or the crater modification stage of the impact event the titanite grains from the vredefort impact structure yielded primarily pre impact ages recording the cooling of the ultra high temperature ventersdorp event but domains with microtwins or planar microstructures show evidence of u pb isotopic disturbance despite that the identified microtwins are not diagnostic of shock metamorphic processes our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record

la q icp ms apatite u pb geochronology using common pb in plagioclase examples from layered mafic intrusions apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks the la icp ms technique is widely applied to u pb geochronology using various minerals apatite u pb geochronology in contrast to e g zircon is compromised by variable amounts of common pb incorporated into the crystal during growth magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age if this is not the case the initial pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite two approaches are common one being the estimation of common pb from a pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate pb but not u e g feldspar most recent studies applying la icp ms to the analysis of pb isotopes in feldspar utilize either multicollector or magnetic sector mass spectrometers in this study we first evaluate the application of quadrupole mass spectrometry for apatite u pb geochronology combined with pb isotopic measurements in feldspar and compare the results with modeled initial pb isotopic compositions the resulting age information is accurate and precise despite using plagioclase rather than k feldspar as is normally used to define initial pb isotope compositions we apply this method to apatite bearing gabbroic rocks from layered intrusions bushveld bjerkreim sokndal hasvik and skaergaard ranging in age from ca 2 ga to ca 55 ma and generate metamorphic cooling ages generally consistent with the known geologic history of these intrusions

modern and last glacial maximum eolian sedimentation patterns in the atlantic ocean interpreted from sediment iron oxide content eolian dust derived from the desert regions of north africa is blown far into the tropical atlantic ocean by persistent easterly and northeasterly winds in this paper we demonstrate that the iron oxides hematite and goethite are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical atlantic iron oxides are identified by diffuse reflectance spectrophotometry a technique capable of identifying these minerals in concentrations as low as 0 01 by weight we analyze samples from both the modern and last glacial maximum lgm synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the atlantic ocean to determine the relative contribution of the iron oxides we factor analyzed the modern and lgm levels as a single data set the iron oxide factor explains about 25 of the variance in the combined core top and lgm data set mapped factor scores for the lgm and modern ocean indicate high iron oxide values are present in just two regions one off eastern north america and the other off northwest africa in the region off eastern north america iron oxide occurs primarily during the lgm as the previously noted brick red lutite a unique sediment type derived from the erosion of permo carboniferous red beds in atlantic canada a larger lobe shaped area of iron oxide rich sediment is present off northwest africa in both the modern and lgm levels the modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships satellites and analysis of air samples during the lgm iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration increases

synorogenic collapse a perspective from the middle crust the proterozoic grenville orogen structural petrological and geochronological studies of the middle to late proterozoic grenville orogen in ontario canada indicate that a major extensional fault developed synchronously with late thrusting this fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers the temporal and spatial relations among faulting metamorphism and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology comparison of grenville with recent himalayan orogenic activity suggests that during proterozoic times physiographic height crustal thickness and crustal strength were similar to modern conditions in orogenic belts

identification of processes driving low level westerlies in west equatorial africa abstractthis paper investigates and characterizes the control mechanisms of the low level circulation over west equatorial africa wea using four reanalysis datasets emphasis is placed on the contribution of the divergent and rotational circulation to the total flow additional focus is made on analyzing the zonal wind component in order to gain insight into the processes that control the variability of the low level westerlies llw in the region the results suggest that the control mechanisms differ north and south of 6 n in the north the llw are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the west african monsoon system this northern branch of the llw is well developed from june to august and disappears in december february south of 6 n the seasonal variability of the llw is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator where as

laser ablation icp ms titanite u th pb dating of hydrothermal ore deposits a case study of the tonglushan cu fe au skarn deposit se hubei province china abstract titanite from the tonglushan cu fe au skarn deposit daye district eastern china has been successfully dated by laser ablation inductively coupled plasma mass spectrometry la icp ms hydrothermal titanite from both skarn ores and calcite veins cutting the ore bodies are characterized by having much lower ree and th u and lu hf ratios compared to their magmatic equivalents from the ore related yangxin quartz diorite intrusion and a late stage albitite dike titanite from the yangxin quartz diorite and an albitite dike which intrudes both the quartz diorite intrusion and ore bodies yield weighted mean 206pb 238u ages of 136 0 1 5 ma and 120 6 2 3 ma 2 respectively in agreement within analytical uncertainty to their 208pb 232th ages in situ analysis of epidote enclosed hydrothermal titanite in thin sections of a skarn ore sample yields a mean 206pb 238u age of 135 9 1 3 ma and 208pb 232th age of 138 2 4 5 ma whereas titanite in calcite from a calcite dominated vein cross cutting the skarn ore body has consistent 206pb 238u and 208pb 232th ages of ca 121 ma the data thus reveal two independent hydrothermal events the earlier event was responsible for the known cu fe au skarn mineralization at tonglushan and throughout the daye district and was caused by coeval magmatism that has resulted in the emplacement of the yangxin quartz diorite the later event previously unrecognized was associated with a second magmatic pulse as represented by the albitite dikes and provides potential for discovering new mineral deposits at tonglushan and possibly over the daye district this study represents the first direct dating of skarn deposits using titanite u th pb geochronometer and signifies the utilization of titanite for precise dating of hydrothermal ore formation

west african monsoon dynamics and eastern equatorial atlantic and pacific sst anomalies 1970 88 abstract the laboratoire de meteorologie dynamique atmospheric gcm is used to investigate relationships between west african monsoon dynamics and sst anomalies in the eastern equatorial atlantic and pacific for the period 1970 88 positive sst anomalies in the eastern equatorial pacific mainly associated with a larger east west divergent circulation over the tropical atlantic are found to coincide with negative rainfall anomalies over west africa this is the case for the composite enso warm episodes of 1972 1976 1982 and 1983 by contrast positive sst anomalies in the eastern equatorial atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the sahel and positive rainfall anomalies in the guinean region this was the case in 1987 the enso warm event during this year had apparently no significant impact on west african monsoon dynamics a zonal atmospheric coupling associated with differences of sst anomalies between the eastern

global equatorial variability of 850 and 200 hpa zonal winds from rawinsondes between 1963 and 1989 the longitude height time variability of 3 month averaged zonal wind anomalies at 850 and 200 hpa over the equatorial area 5 degrees n 5 degrees s is analyzed using a three dimensional dataset constructed from rawinsonde data 1963 1989 the first mode closely related to the southern oscillation index suggests a strong vertical coupling associated with a horizontal out of phase pattern between the central western pacific and the remainder of the equatorial belt the vertical coupling appears to be phase locked to the annual cycle with strongest intensities found over south america and near the maritime continent early in the calendar year and over the pacific basin and africa during the second half of the year this mode of variability can be viewed as a standing pattern superimposed with an eastward migrating component coherent with the annual cycle this westerly moving mode originates near the maritime continent during the northern autumn and tends to precede el nino southern oscillation events in the central and eastern pacific ocean variance is mainly concentrated in the 3 8 year low frequency time scale throughout the tropics and in the 2 3 year quasi biennal band from 110 degrees 120 degrees e to 180 degrees 24 refs 4 figs

early history of the carthage colton shear zone grenville province northwest adirondacks new york u s a abstract the adirondack mountains expose two distinct tectonic elements of the proterozoic grenville province of northeastern north america the adirondack lowlands and highlands the lowlands are located along the eastern edge of the metasedimentary belt and the highlands form the western portion of the granulite terrane the two are separated by the carthage colton shear zone ccsz u pb titanite and 40ar 39ar hornblende ages in the lowlands are ca 100 m yr older than in the highlands across the ccsz while both the lowlands and the highlands record a history of metamorphism during the elzevirian orogeny ca 1150 ma only the highlands record evidence of a major phase of grenvillian thermotectonic activity ottawan orogeny at ca 1090 1030 ma proposed tectonic models require that the lowlands were either laterally separated from the highlands or were at structurally higher levels during this metamorphism new u pb and 40ar 39ar ages from titanite and hornblende bearing mylonites in the dana hil

common lead corrected laser ablation icp ms u pb systematics and geochronology of titanite u pb geochronology using laser ablation icp ms is a fairly recently introduced technique and thus far its development has centred on zircon dating using titanite as an example the application of laser ablation icp ms to u pb dating of common pb bearing accessory phases is presented a simple analytical set up comprising a 213 nm nd yag laser coupled to a quadrupole based icp ms instrument is used it is demonstrated that accurate and precise pb u ratios 2 3 at 2 for 206pb 238u can be obtained on small volumes of analysed material using the zircon geostandard 91500 as an external calibration standard a robust common pb correction necessitating the measurement of the stable 204pb isotope is detailed the isobaric 204hg background interference in icp ms is lowered to a level where it does not dominate the total 204 amu signal using in line gold traps on the carrier gas lines the remaining 204hg is stripped from the 204pb signal mathematically by applying a natural hg isotope ratio common pb correction using this method is demonstrated to work well in analyses with elevated u pb signal intensity where this correction fails an alternative method is required and a novel approach using three dimensional u pb concordia is adopted this approach is shown to be extremely powerful in assessing age precision and common pb composition correction the technique is applied to the well characterised fish canyon tuff demonstrating that accurate 7 8 at 2 compared to reference age and precise 2 4 at 2 on 206pb 238u ratios u pb age data can be obtained from titanite even as young as oligocene

u pb systematics of the mcclure mountain syenite thermochronological constraints on the age of the 40 ar 39 ar standard mmhb recent advances in u pb geochronology allow unprecedented levels of precision in the determination of geological ages however increased precision has also illuminated the importance of understanding subtle sources of open system behavior such as pb loss inheritance intermediate daughter product disequilibria and the accuracy of the model assumptions for initial pb deconvolution of these effects allows a much richer understanding of the power and limitations of u pb geochronology and thermochronology in this study we report high precision id tims u pb data from zircon baddelleyite titanite and apatite from the mcclure mountain syenite from which the 40 ar 39 ar hornblende standard mmhb is derived we find that excess 206 pb in zircon due to inclusions of high th minerals and ele vated th u in titanite and apatite jeopardize the utility of the 238 u 206 pb system in this rock strongly air abraded zircons give dates that are younger than chemical abraded zircons which yield a statistically robust 207 pb 235 u date of 523 98 0 12 ma that is interpreted as the crystallization age we explore the best method of pbc correction in titanite and apatite by analyzing the u pb isotopes of k feldspar and using 2 d and 3 d regression methods the latter of which yields the best results in each case however the calcu lated compositions of pbc for titanite apatite and k feldspar are different implying that using a single pbc correction for multiple u pb thermochronometers may be inaccurate the u pb thermochronological results are used to predict a closure time for ar in hornblende of 522 98 1 00 ma widely cited k ar and 40 ar 39 ar dates overlap with the u pb date and relatively large errors make it impossible to verify whether u pb dates are systematically 1 older than k ar and 40 ar 39 ar dates

unravelling complex geologic histories using u pb and trace element systematics of titanite abstract unravelling the spatio temporal evolution of orogenic terranes requires a comprehensive understanding of the duration and extent of metamorphic events and hydrothermal alteration commonly used minerals such as zircon and monazite may not fully record geological histories in complex tectonic settings because their elemental constituents do not react under many metamorphic and metasomatic conditions here we complement the current geochronological record of the capricorn orogen western australia with titanite u pb geochronology and geochemistry of felsic intrusive rocks to draw conclusions about the use of titanite in understanding the evolution of orogenic terranes because titanite usually incorporates common pb and may be variably reset by multiple metamorphic and hydrothermal events a workflow is provided here for the systematic and robust interpretation of titanite u pb data the addition of trace element data in titanite is particularly effective for differentiating whether a grain is igneous recrystallized or metamorphic we have developed several petrogenetic indices to differentiate these three types of titanite using zr in titanite temperature th u th pb al al fe light to heavy rare earth element ratio and eu anomalies the addition of trace element geochemistry can also highlight anomalously radiogenic 207pb 206pb reservoirs utilization of our workflow in the capricorn orogen reveals that titanite ages from the same samples as published zircon u pb data range from coeval to several hundreds of myr of age difference between the two minerals titanite geochronology and trace element geochemistry indicates 20 myr of previously unrecognized prolonged cooling for the capricorn orogeny to ca 1750 ma the spatial extent of the ca 1210 1170 ma part of the mutherbukin tectonic event is also broadened significantly farther north and south than previously recognized incorporating titanite geochronology and trace geochemistry with more commonly used techniques e g zircon and monazite petrochronology extends our ability to resolve the complete history of large scale orogenic terranes

wind field coherence and its variations over west africa abstract the wind field coherence over west africa is investigated during two contrasted periods 1953 63 abnormally wet and 1963 75 abnormally dry in the sahel zonal and meridional components of the monthly mean wind are computed with radiosonde data relative to the 950 100 hpa layer a descriptive analysis reveals the main features of the west african atmospheric circulation and a correlation analysis points out their dynamical coherence the exiwnce of hadley type cells of northern and southern hemispheres are confirmed and a mutual interaction in the lower layers is depicted the functioning of a walker type cell defined by a coupling between the zonal components of the monsoonal flow and of the tropical easterly jet tej is shown during northern summer over the whole of west africa there is no correlation between the tej velocity and the african easterly jet aej velocity over the sahelian belt during the dry period the coherence of the cell circulations vanishes in concomitance with a

interdecadal variation of the west african summer monsoon during 1979 2010 and associated variability this paper addresses the interdecadal variation of the west african summer monsoon wasm along with its background of atmospheric circulation and possible physical mechanism over the past 32 years 1979 2010 it is indicated that the wasm starts to strengthen from 1998 as the rainfall begins to increase over western west africa on the whole which shows a new interdecadal variation in this interdecadal variation the strengthened ascending motion corresponding to enhanced divergence convergence movement on the upper lower troposphere is prone to develop the local circulation of the monsoon moreover the strengthened southwestern eastern wind on the lower upper level leads to more moisture from the atlantic and the gulf of guinea transported to the west african continent in addition the summer subtropical high over the north atlantic and western west africa is strong and northward and the tropical east wind is also strong statistically the weaker stronger the spring north atlantic oscillation nao is the stronger weaker the tropical easterly is and then the wasm is also stronger but the effect of the nao on the decadal variation of the wasm is not so significant from the north atlantic anomaly sensitivity simulation with a single model this is also an indication that the relationship between the wasm and nao is complicated in an interdecadal time scale and is needed further study in terms of sea surface temperature sst variation the tendency is toward warming in the subtropical north pacific the south pacific and north atlantic numerical simulation experiments and data analysis show that the sst variation in the north pacific plays an important role in the latest interdecadal strengthening of the wasm during the past 32 years while the influences of the south pacific and the north atlantic sst anomalies are not so significant to the associated atmospheric circulation changes

ti al zoning of experimentally grown titanite in the system cao al2o3 tio2 sio2 nacl h2o f evidence for small scale fluid heterogeneity chemical zoning according to the substitution al oh f ti o is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions we synthesized titanite which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile visible in element distribution maps as a patchy irregular pattern run conditions in the system cao al 2 o 3 tio 2 sio 2 nacl h 2 o caf 2 were 600 c 0 4 gpa 1 to 107 days duration natural rutile crystals were placed in a perforated pt tube within an outer au capsule containing wollastonite al 2 o 3 optionally powdered caf 2 and a nacl brine to simulate conditions where rutile is transformed into titanite by a hydrous ca si al f fluid driven by a chemical potential gradient between inner and outer capsule spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 m large crystals in a reaction rim in the long time runs titanite is strongly zoned in al ti and shows up to 0 5 al per formula unit pfu in experiments containing f without f substitution is limited to 0 25 al pfu the range of the ti al exchange is already large in short run times and the same heterogeneity is observed in long time runs the al ti distribution with and without f in compositional domains up to several tens of micrometers is patchy and irregular no relation of the zoning with time core rim or preferred growth directions was observed in the case of fluid mediated growth mineral growth zoning represents a disequilibrium at supersaturated conditions the concentration of ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of al ca and si titanium is not transported in significant amounts into the outer capsule indicating low mobility in contrast to al ca and si which are transported into the inner capsule assuming buffered al concentration from dissolution of the al source we speculate that the ti al of the fluid at the precipitation site was controlled by the local ti concentration which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning

element and sr isotope signatures of titanite as indicator of variable fluid composition in hydrated eclogite abstract the trace element signature sr common pb u th ree and hfse of titanite gives important information about the formation process of metamorphic titanite we investigated titanite from different textural positions of a strongly retrogressed eclogite from the tromso norway high p area it shows characteristic titanite rims around partially absorbed rutile interpreted as grown during retrogression and hydration of the high p assemblage the titanite rims around rutile are polycrystalline and in contact with the plagioclase amphibole matrix large idioblastic single crystal titanite grew in veins without relics of precursor rutile compositional zoning of al ti f in titanite of both textural positions is irregular and not related to core or rim position in the crystals there is no evidence for systematic growth sequences or diffusion reflecting changing p t conditions or systematic changes of fluid composition trace element patterns of titanite la icp ms data however show systematic variations in the coupling and decoupling behaviour of strongly fluid mobile sr u moderately fluid mobile ree and little fluid mobile nb ta zr hf elements the trace element signatures of titanite from the two different fabric positions reflect contrasting characteristic fluid compositions single crystal titanite equilibrated with fluid that was channelled in the vein polycrystalline titanite on rutile records the composition of a fluid that had migrated along grain boundaries in the amphibole plagioclase matrix fluid from the matrix also contributed to the geochemical signature of vein titanite whereas influence of fluid from the vein on the composition of polycrystalline titanite in the matrix is not evident 87sr 86sr isotope ratios of the different titanite types support the inference from trace elements of spatially and temporally variable fluid sources

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane