biotic geochemical and environmental changes through the early sheinwoodian wenlock silurian carbon isotope excursion escie buttington quarry wales abstract palaeontological sedimentological and geochemical data particularly from la icpms analyses of pyrite from a 12 4 m section through the lower sheinwoodian trewern brook mudstone formation of buttington quarry wales are combined to reveal the environmental changes taking place during the early wenlock epoch of the silurian during the early part of the escie early sheinwoodian carbon isotope excursion the uppermost cyrtograptus murchisoni through to lower monograptus riccartonensis biozones show increased nutrient levels and peaks in palynomorph and graptolite abundance at this time graptolite diversity globally was declining dramatically possibly associated with metal particularly zinc concentrations attaining toxic levels and probably with the base of the euphotic zone being higher in the water column resulting from blooms of phytoplankton causing those graptolites that specialized in feeding on phytoplankton that were outcompeted in this diminished volume of illuminated surface waters to decline in numbers and eventually become extinct correlation of the graptolite and conodont biozonations enables the levels of the main stratigraphically higher ireviken event datum points to be proposed for the buttington section and this in turn allows events recognised elsewhere intervals of malformed palynomorphs and re organization of polychaete faunas together with an oceanic anoxic event to be correlated with the lower part of the studied buttington section the commencement of the oae identified on gotland at ireviken event datum point 4 appears to correlate rather well with a rapid transition marking the top of the butterley mudstone member at buttington from heavily bioturbated mudstones to laminated graptolitic mudstones surprisingly the most rapid change in 13ccarb values within the buttington section from 0 6 at 8 19 8 25 m to 2 09 at 8 50 8 55 m is not associated with any major faunal palynological abundance or geochemical changes these are all coincident with the initial rise in 13ccarb values up to about the 5 m level in the buttington section another unexplained feature of the graptolite record is a sudden change from monograptus dominated to pristiograptus dominated assemblages about half way through the studied section

u pb and th pb dating of apatite by la icpms abstract apatite is a common u and th bearing accessory mineral in igneous and metamorphic rocks and a minor but widespread detrital component in clastic sedimentary rocks u pb and th pb dating of apatite has potential application in sedimentary provenance studies as it likely represents first cycle detritus compared to the polycyclic behavior of zircon however low u th and radiogenic pb concentrations elevated common pb and the lack of a u th pb apatite standard remain significant challenges in dating apatite by la icpms and consequently in developing the chronometer as a provenance tool this study has determined u pb and th pb ages for seven well known apatite occurrences durango emerald lake kovdor mineville mud tank otter lake and slyudyanka by la icpms analytical procedures involved rastering a 10 m spot over a 40 40 m square to a depth of 10 m using a geolas 193 nm arf excimer laser coupled to a thermo elementxr single collector icpms these raster conditions minimized laser induced inter element fractionation which was corrected for using the back calculated intercept of the time resolved signal a tl u bi np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias external standards plesovice and 91500 zircon nist srm 610 and 612 silicate glasses and stdp5 phosphate glass along with kovdor apatite were analyzed to monitor u pb th pb u th and pb pb ratios common pb correction employed the 207pb method and also a 208pb correction method for samples with low th u the 207pb and 208pb corrections employed either the initial pb isotopic composition or the stacey and kramers model and propagated conservative uncertainties in the initial pb isotopic composition common pb correction using the stacey and kramers 1975 model employed an initial pb isotopic composition calculated from either the estimated u pb age of the sample or an iterative approach the age difference between these two methods is typically less than 2 suggesting that the iterative approach works well for samples where there are no constraints on the initial pb composition such as a detrital sample no 204pb correction was undertaken because of low 204pb counts on single collector instruments and 204pb interference by 204hg in the argon gas supply age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common pb corrected ages a tera wasserburg concordia intercept age and a tera wasserburg concordia intercept age anchored through common pb the samples in general yield ages consistent at the 2 level with independent estimates of the u pb apatite age which demonstrates the suitability of the analytical protocol employed weighted mean age uncertainties are as low as 1 2 for u and or th rich palaeozoic neoproterozoic samples the uncertainty on the youngest sample the cenozoic 31 44 ma durango apatite ranges from 3 7 7 6 according to the common pb correction method employed the accurate and relatively precise common pb corrected ages demonstrate the u pb and th pb apatite chronometers are suitable as sedimentary provenance tools the kovdor carbonatite apatite is recommended as a potential u pb and th pb apatite standard as it yields precise and reproducible 207pb corrected 232th 208pb and common pb anchored tera wasserburg concordia intercept ages

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold

tectonic and climate control on allochthonous bauxite deposition example from the mid cretaceous villeveyrac basin southern france karst bauxite deposits represent 11 of the global quantity of bauxite bardossy and aleva 1990 karst bauxite is defined as bauxite deposits overlying carbonate rocks above a more or less karstified surface allochtonous karst bauxites result from two successive processes 1 intense weathering and formation of a lateritic profile involving the development of bauxitic and duricrust horizons and 2 erosion of the lateritic profile and deposition above a carbonate unit located downstream in the villeveyrac basin southern france lower to mid albian allochtonous karstic bauxites are deposited on a jurassic karstified bedrock and are overlain by a late albian marine siliciclastic cover a drillcore sampling the substratum the bauxites and their cover is analyzed and offers the opportunity to investigate the depositional conditions and interactions of climate and tectonic during the mid to late cretaceous through the analysis of mechanical and chemical weathering evidence sedimentological study allowed the definition of four bauxite facies and three siliciclastic cover facies overprinted by four different pedofacies mineralogical and geochemical analyses allowed the distinction of four mineralogical units controlled by climate related weathering processes the bauxite deposits emplaced by mud flow debris flow gravity currents are composed by boehmite hematite and goethite the top of the bauxite presents a progressive upward decrease in al2o3 and tio2 associated with the occurrence of kaolinite the base of the cover that overlies the bauxite interval corresponds to a paralic environment dominated by kaolinite with a sub ferricrete paleosol the following paralic sequence presents a clay rich suite dominated by smectite associated with mica illite and quartz which suggests aridification of climate around the latest albian we interpret this sedimentary succession as the result of erosion of an autochthonous laterite cover induced by major tectonic uplift in the upstream drainage area which led to a significant increase in slope gradient under a wet tropical climate mechanical erosion of autochthonous laterite bauxite in the upstream drainage led to a reverse stacking of the reworked autochthonous lateritic profile in the depositional basin a continuous transgressive trend was responsible for trapping of the allochtonous karst bauxite this was followed by stacking of siliciclastic paralic sequences which record progressive erosion of the bedrock in the upstream drainage area under dry tropical conditions our results thus suggest that dismantling of the autochthonous laterite cover was initiated by albian uplift and this ended with a change towards drier and more seasonal climate at the albian cenomanian transition

ichnofacies distribution in the eocene early miocene petra tou romiou outcrop cyprus sea level dynamics and palaeoenvironmental implications in a contourite environment ichnological analysis of the carbonate contourite drift at the petra tou romiou outcrop southern cyprus reveals a relationship between sea level dynamics shoreline position sedimentation rate flow hydrodynamics and trace fossil assemblages the base of the outcrop is composed of chalky deposits showing the zoophycos ichnofacies attributes the transition to the first contouritic interval beds is marked by the gradual disappearance of zoophycos and dominance of horizontal traces this change is caused by a gradual sea level fall determining an increase in proximity and energy conditions in the environment that favoured the development of distal cruziana ichnofacies upper contouritic interval beds contain abundant burrow systems with vertical components increased ichnodiversity particularly among vertical morphologies coincides with the appearance of teichichnus gyrolithes and rare ophiomorpha revealing a transition to the proximal cruziana ichnofacies related with the maintenance of the previous trend toward more proximal and higher energy settings the differentiated ichnofacies in the petra tou roumiou carbonate drifts are controlled by complex interactions between variable environmental factors with sea level dynamics playing a major role

genesis of the jinding zn pb deposit northwest yunnan province china constraints from rare earth elements and noble gas isotopes abstract the giant sediment hosted jinding zinc lead deposit is located in the lanping basin northwestern yunnan province china the genesis of the deposit has long been debated and the sources of the ore forming fluids and metals are controversial this study presents rare earth element ree and noble gas isotope data that constrain the origins of the ore fluids and the heat source driving the hydrothermal circulation the early stage sulfides are enriched in light rees and have high ree values 30 8 94 8 ppm and weakly negative eu eu 0 85 0 89 and ce anomalies ce 0 84 0 95 suggesting that the fluids were likely derived from dissolution of upper triassic marine carbonates with input of rees from aluminosilicate rocks in the basin in contrast the late stage sulfides have irregular ree patterns generally low ree values 0 24 10 8 ppm and positive eu eu 1 22 10 9 and weakly negative ce anomalies ce 0 53 0 90 which suggest that the ore forming fluids interacted with evaporite minerals the 3 he 4 he 0 01 0 04 r a and 40 ar 36 ar values 301 340 of the ore forming fluids indicate crustal and atmospheric origins for these noble gases these findings are in agreement with the published fluid inclusion microthermometry data and the results of h o c s pb and sr isotope studies our data in combination with published results support a two stage hydrothermal mineralization model involving early stage basinal brines and late stage meteoric water that acquired metals and heat from crustal sources

detrital rutile ages can deduce the tectonic setting of sedimentary basins abstract sedimentary sequences contain a detailed record of the evolution of the earth s surface detrital mineral grains provide for example evidence of source redox conditions insights into the age geochemistry and petrology of basement sources and can also be used to elucidate tectonic environments detrital zircon has been used as a means of analysing the tectonic setting of host sedimentary successions but with potentially ambiguous results it is important to find additional ways to discriminate depositional settings particularly in precambrian sequences where other proxies are either not available or have been lost in this contribution we provide a new way to discriminate between different sedimentary tectonic environments using the mineral rutile we present a large compilation of detrital rutile data to show that the u pb age distribution is sensitive to the tectonic setting of the basin in which the host sediments were deposited we then apply this new approach to two case studies where the depositional setting and age are well constrained siliciclastic units of nw scotland which were deposited in the neoproterozoic foreland of the grenville orogen and on the cambrian passive margin of the iapetus ocean and the brazilian sabara basin located within the palaeoproterozoic foreland of the minas orogen we compare the detrital rutile and zircon age distributions of these successions showing that in some cases they are different and that rutile is most sensitive to the youngest metamorphic events affecting the sources reinforcing the applicability of this tool by testing this new method on polyphasally deformed successions sabara basin we show that detrital rutile can still inform the tectonic setting even at medium grades of metamorphism

ecologic interpretation of deep sea trace fossil communities abstract trace fossil types and their vertical arrangement tiering are closely related to the biotope and hence can provide information on ecological conditions especially on oxygenation benthic food content sedimentation rate and substrate consistency however to use the full potential of trace fossils for environmental analyses various ichnologic criteria are necessary in addition to trace fossil taxonomy based on a taxonomically oriented ichnologic evaluation of modern deep sea environments six ichnologic aspects to ecologic changes have been found to be of major importance 1 1 penetration depth of burrows is influenced by oxygenation benthic food availability and substrate consistency 2 2 the number and vertical extension of tiers varies due to environmental changes a decrease in the vertical extension of tiers telescoping indicates a decrease in oxygenation benthic food content and or sedimentation rate if shallow penetrating traces disappear oxygen may be depleted and if the deepest tiers disappear the benthic food content may be lowered 3 3 the size diameter of specific trace fossils can indicate oxygen and benthic food availability 4 4 degree of bioturbation is related to sedimentation rate and benthic food content and the diversity of the whole ichnocoenoses or within a specific tier is affected by oxygenation benthic food content sedimentation rate and substrate consistency 5 5 the type of burrow wall unlined mucus lined pellet lined mainly refers to substrate consistency and subordinately to pore water oxygenation 6 6 the type of sediment displacement by body appendages used for excavation ingestion sideward pressing or outside pumping is related to substrate consistency and grain size

where does the time go assessing the chronostratigraphic fidelity of sedimentary geological outcrops in the pliocene pleistocene red crag formation eastern england it is widely understood that earth9s stratigraphic record is an incomplete record of time but the implications that this has for interpreting sedimentary outcrop have received little attention here we consider how time is preserved at outcrop using the neogene quaternary red crag formation england the red crag formation hosts sedimentological and ichnological proxies that can be used to assess the time taken to accumulate outcrop expressions of strata as ancient depositional environments fluctuated between states of deposition erosion and stasis we use these to estimate how much time is preserved at outcrop scale and find that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600 800 kyr of crag time much of the apparently missing time may be accounted for by the parts of the formation at subcrop rather than outcrop stratigraphic time has not been lost but is hidden the time completeness of the red crag formation at outcrop appears analogous to that recorded in much older rock units implying that direct comparison between strata of all ages is valid and that perceived stratigraphic incompleteness is an inconsequential barrier to viewing the outcrop sedimentary stratigraphic record as a truthful chronicle of earth history supplementary material further details of the regional geology and specific information on outcrops are available at https doi org 10 6084 m9 figshare c 4561001

apatite triple dating and white mica 40ar 39ar thermochronology of syntectonic detritus in the central andes a multiphase tectonothermal history we applied apatite u pb fission track and u th he triple dating and white mica 40 ar 39 ar thermochronology to syntectonic sedimentary rocks from the central andean puna plateau in order to determine the source area geochronology and source sedimentary basin thermal histories and ultimately the timing of multiple tectonothermal events in the central andes apatite triple dating of samples from the eocene geste formation in the salar de pastos grandes basin shows late precambrian devonian apatite u pb crystallization ages eocene apatite fission track aft and eocene miocene u th he ca 8 47 ma cooling ages double dating of cobbles from equivalent strata in the arizaro basin documents early eocene 46 2 3 9 ma and cretaceous 107 6 7 6 109 5 7 7 ma aft and eocene oligocene ca 55 30 ma u th he ages thermal modeling suggests relatively rapid cooling between ca 80 and 50 ma and reheating and subsequent diachronous basin exhumation between ca 30 ma and 5 ma the 40 ar 39 ar white mica ages from the same samples in the salar de pastos grandes area are mainly 400 350 ma younger than apatite u pb ages suggesting source terrane cooling and exhumation during the devonian early carboniferous together these data reveal multiple phases of mountain building in the paleozoic and cenozoic basin burial temperatures within the plateau were limited to

sediment routing and basin evolution in proterozoic to mesozoic east gondwana a case study from southern australia abstract sedimentary rocks along the southern margin of australia host an important record of the break up history of east gondwana as well as fragments of a deeper geological history which collectively help inform the geological evolution of a vast and largely underexplored region new drilling through cenozoic cover has allowed examination of the cretaceous rift related madura shelf sequence bight basin and identification of two new stratigraphic units beneath the shelf the possibly proterozoic shanes dam conglomerate and the interpreted palaeozoic southern officer basin unit the decoration sandstone recognition of these new units indicates an earlier basinal history than previously known lithostratigraphy of the new drillcore has been integrated with that published from onshore and offshore cores to present isopach maps of sedimentary cover on the madura shelf new palynological data demonstrate progression from more localised freshwater brackish fluvio lacustrine clastics in the early cretaceous foraminisporis wonthaggiensis valanginian to barremian to widespread topography blanketing fully marine glauconitic mudrocks in the mid cretaceous endoceratium ludbrookiae albian geochronology and hf isotope geochemistry show detrital zircon populations from the madura shelf are comparable to those from the southern officer basin as well as cenozoic shoreline and palaeovalley sediments in the region the detrital zircon population from the shanes dam conglomerate is defined by a unimodal 1400 ma peak which correlates with directly underlying crystalline basement of the madura province peak ages of 1150 ma and 1650 ma dominate the age spectra of all other samples indicating a stable sediment reservoir through much of the phanerozoic with sediments largely sourced from the albany fraser orogen and musgrave province directly and via multiple recycling events the madura shelf detrital zircon population differs from published data for the upper cretaceous ceduna delta to the east indicating significant differences in sediment provenance and routing between the ceduna sub basin and central bight basin

ichnology of american chalks the selma group upper cretaceous western alabama chalk facies within the selma group most of them impure crop out only in a relatively small area of western alabama and eastern mississippi and grade laterally into calcareous terrigenous clastics body fossils and skeletal borings described in a separate paper are diverse and abundant in some units especially the impure chalks major burrow forms also distributed unequally among these units include chondrites sp cylindrichnus pustulosus n sp planolites beverleyensis skolithos sp teichichnus rectus teichichnus zigzag n sp thalassinoides paradoxicus thalassinoides suevicus trichichnus linearis zoophycos sp and mineralized burrows minor forms include palaeophycus sp rosselia spp and mantled burrows characteristic trace fossil distributions especially include the striking dominance of thalassinoides paradoxicus in chalk hardgrounds of the arcola member of the mooreville chalk and successive patterns of bioturbation in the lower unnamed member of the demopolis chalk in purer chal

the ichnogenus rhizocorallium classification trace makers palaeoenvironments and evolution rhizocorallium is one of the oldest known trace fossils with wide distribution through the phanerozoic and all over the world originally introduced from the epicontinental triassic of central germany its high morphological lability gave reason for the subsequent erection of about twenty ichnospecies the study of newly collected material from the type area and many specimens from various collections permits the conclusion that rhizocorallium jenense and rhizocorallium commune are the only valid ichnospecies of rhizocorallium the type ichnospecies r jenense is a comparatively small inclined and heavily scratched firmground burrow with passive fill while r commune consists of extensive more or less horizontal burrows with occasionally scratched marginal tubes and an actively filled spreite between the tubes the faecal pellets coprulus oblongus are typically associated with r commune morphological variations of r commune are captured in ichnosubspecies and varieties of this ichnospecies and can aid a refined reconstruction of palaeoenvironments a review of more than 180 records from the literature reveals the common confusion of both ichnospecies which has consequences for the application of rhizocorallium in facies interpretations the end members of both ichnospecies may be linked by transitional forms which suggests the same kind of trace maker polychaetes are the most likely producers of marine rhizocorallium based on their long ranging occurrence morphological features appearance of faecal pellets associated soft body remains and modern analogues r commune occurs from early cambrian to holocene while r jenense just appears after the end permian mass extinction probably as a consequence of an adapted firmground burrowing lifestyle of its producer fluvial r jenense are probably produced by mayfly larvae in homology to marine polychaete burrows a consequent application of the newly established classification scheme allows for a more rigorous application of rhizocorallium in the reconstruction of palaeoenvironments thus palaeozoic and mesozoic r commune are restricted to the cruziana ichnofacies of shallow marine environments while in cenozoic time similar forms are also found in deep marine deposits r jenense on the other hand is a constituent of the widespread glossifungites ichnofacies and aside from continental environments occurs in peritidal to deep marine deposits several studies have demonstrated the value of rhizocorallium for interpreting sequence stratigraphical surfaces current directions and fluctuations in salinity and oxygen

tracing element sources of hydrothermal mineral deposits ree and y distribution and sr nd pb isotopes in fluorite from mvt deposits in the pennine orefield england fluorite from mississippi valley type mvt deposits in the south pennine orefield england displays significantly different distributions of rare earths and yttrium rey compared to fluorite from similar mvt deposits in the north pennine orefield samples from the south pennine orefield display negative ce and positive gd and y anomalies but lack any eu anomaly indicating that the rey were mobilized from relatively pure marine sedimentary carbonates in marked contrast fluorite from the north pennine orefield lacks any ce and gd anomalies but shows a pronounced positive eu anomaly suggesting that the rey were provided by different source rock s that the mineralizing hydrothermal fluid had experienced higher temperatures prior to fluorite precipitation and that it was derived from deeper crustal levels in the north compared to the south the isotopic composition of sr in blue john fluorite from the south pennine orefield suggests that sr was mobilized from lower carboniferous tournaisian limestones whereas pb isotopes suggest that in contrast to rey and sr pb was derived from aluminosilicate rocks neither nd nor sr or pb isotopes can be used to radiometrically date the formation of blue john fluorite all isotope systems studied indicate that the limestone host rock of this fluorite mineralization did not contribute to the trace element budget of the hydrothermal fluid our results show that different solutes in a natural water hydrothermal fluid groundwater etc may be derived from different sources and that the study of a small set of elements or isotope ratios may not provide full insight into the genesis or history of a mineralization or a hydrothermal fluid our data provide evidence for the uncoupling of sr nd and pb during fluid rock interaction and fluid migration and show that the use of plots such as 87sr 86sr vs end to learn about mixing relationships as is commonly done in igneous geochemistry is unreliable when applied to natural waters and their precipitates

diagenetic signatures of stratal surfaces in the upper jurassic fulmar formation central north sea ukcs sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried upper jurassic fulmar formation south central graben ukcs as a result of the bioturbated nature of the shallow marine strata variations in ichnofabrics and cross cutting relationships between trace fossils are important in identifying and correlating stratal surfaces two transgressive and two regressive surfaces sequence boundaries were studied using petrographical mineralogical and geochemical techniques cements including early diagenetic pyrite framboids grain rimming apatite microcrystalline dolomite and ankerite occur at above and immediately below both transgressive and forced regressive surfaces sequence boundaries breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time enhancing diagenetic reactions at and below the surfaces the distribution of ankerite despite being a deep burial cement was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces the diagenetic character of the forced regressive surfaces sequence boundaries is more complex than that of the transgressive surfaces both in terms of mineralogy and paragenesis this is attributed to the superimposition of diagenetic assemblages as a result of erosion re sedimentation and sediment by passing although the diagenetic signature of the surfaces has been modified by dissolution replacement reactions during deep burial early diagenetic signatures can still be distinguished

the role of trace fossil ichnofabric analysis in the development of depositional models for the upper jurassic fulmar formation of the kittiwake field quadrant 21 ukcs

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

trace fossils and the aftermath of the permo triassic mass extinction evidence from northern italy abstract the duration of early triassic anoxia in the carbonate ramp settings of the werfen formation dolomites northern italy has been constrained using ichnofabric and sedimentological data this includes fine lamination and abundant pyrite framboids commonly reworked into lags by storm events the anoxic event spanned the entire griesbachian stage and extended from deep waters into extraordinarily shallow water settings evidence for oxygen deficiency can be found in mid and inner ramp facies the extent and duration of this event may at least partially explain the magnitude of the permo triassic mass extinction normal benthic conditions returned rapidly in the dienerian stage and pervasively bioturbated distal ramp facies testify to the ventilation of the deeper water areas of the region in post griesbachian times a post mass extinction radiation is not particularly well displayed in either trace fossil or body fossil diversity compilations from the dolomites moderate low levels of trace fossil diversity are maintained throughout the post griesbachian werfen formation whilst the body fossil data shows a moderate increase from the mid smithian stage there is thus a curious lag period spanning the dienerian to mid smithian when environmental conditions were apparently normal but the marine fauna failed to show any recovery

facies characteristics and depositional models of highly bioturbated shallow marine siliciclastic strata an example from the fulmar formation late jurassic uk central graben abstract an extensive core database from the late jurassic fulmar formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated heterolithic shallow marine siliciclastic strata twelve major sedimentary facies 1 12 are defined on the basis of lithology grain size visually estimated sand silt content and the presence or absence of primary sedimentary structures ichnofabric characteristics are also defined and have proved to be not always facies specific for purposes of depositional modelling the 12 facies types are resolved into six facies associations a f each representative of distinct shoreline shallow marine environments from these associations three broad depositional models have been constructed storm influenced shoreface model 1 bioturbated shoreface model 2 and a speculative bioturbated shelf model model 3 stratigraphical and palaeontological criteria suggest that models 1 and 2 are shoreline attached sedimentological data indicate that they occur as end members to a spectrum of shoreline settings constrained by incident wave energy bands for shorefaces subject to significant storm influence well preserved event beds occur seaward of the upper shoreface in the bioturbated shoreface model however the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking the bioturbated shelf model displays no clear evidence of shoreline connection with the resultant facies often exhibiting highly abundant siliceous sponge spicules together with other open marine indicators such as belemnites and rare ammonites unlike the shoreface sand bodies which received sediment from basin margin fluvial systems shelf sand bodies in the fulmar formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone this sediment may have undergone a long transit residence period on the shelf prior to eventual burial in hanging wall depocentres strongly influenced by zechstein salt withdrawal this study demonstrates that an ichnofabric approach as opposed to one of simple ichnodiversity is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata in the fulmar formation ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels water depth sedimentation rates erosion rates substrate consistency and dissolved oxygen levels evidence of these fluctuations is often apparent at intra facies scale but would largely go unnoticed using a conventional sedimentological approach to data gathering based on ichnodiversity at best depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling

constraints on the u pb systematics of metamorphic rutile from in situ la icp ms analysis abstract in situ laser ablation icp ms u pb dating of metamorphic rutile from granulite facies metapelitic rocks of the archaean pikwitonei granulite domain manitoba canada provides constraints on pb diffusion and characterizes the closure behavior of rutile the analysis of transects of 35 m spots across 15 rutile grains having a size of 120 to 280 m yielded concordant ages with core ages of ca 2450 ma and core to rim younging towards 2280 ma age profiles indicate that volume diffusion of pb occurs in rutile implying that the ages represent cooling ages to investigate the closure behavior of pb in rutile closure temperature profiles t c x were constructed based on different models combined with experimentally determined diffusion parameters the classical t c x model of dodson 1986 mat sci forum 7 145 154 indicates a rapid decrease of t c in the rims of grains providing unrealistic estimates for the cooling rate when combined with u pb ages a new t c x model was constructed based on the analyzed age profiles that are described by an error function this model shows a more steady decrease in t c throughout the grain from ca 640 c in the core depending on grain size to a rim intercept t c rim of 490 c 7 2 which is interpreted to be the extrapolated theoretical absolute temperature of insignificant pb diffusion in rutile the new model provides a better description of the relation between age and t c for the analyzed grains however both t c x models demonstrate that even in small grains the variations of t c can be significant making it impossible to derive one representative t c for pb in rutile the error function based t c x model allows the determination of cooling rates which show a decrease over time from ca 2 2 to 0 4 c ma agreeing well with previous estimates for the pikwitonei granulite domain this consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by la icp ms u pb dating of rutile providing constraints on the cooling history of a metamorphic terrane the slow cooling rates imply that exhumation was slow

infaunal holothurian distributions and their traces in the fraser river delta front and prodelta british columbia canada abstract holothurians sea cucumbers constitute one of the largest and most abundant infaunal groups inhabiting the fraser river delta front 0 to 150 m water depth and prodelta greater than 150 m water depth the spatial distribution and density of animals reveal their preferred substrate grain sizes and their tolerance or sensitivity to various stress factors including salinity fluctuations available food resources sedimentation rate and pollution in the delta front below 25 m water depth below storm wave base holothurians are asymmetrically distributed with respect to the main distributary channel population densities primarily reflect asymmetry in grain size distributions across the delta front holothurians are absent or occur in low density in the sand dominated southern delta front sdf whereas the mud dominated northern delta front ndf is more heavily colonized sedimentation rate and nutrient content are equally important in holothurian distribution population densities of certain holothurians show significant increase where there are moderate sedimentation rates 2 4 cm yr 1 and river derived nutrient supply several subsurface traces and their surface expressions are attributed to holothurians subsurface traces include diminutive and robust artichnus thalassinoides and u shaped bio deformation structures on the sediment surface bio topographic structures such as mounds and funnel shaped depressions are produced by the activity of sea cucumbers of the holothurian generated traces only artichnus can conclusively be attributed to sea cucumbers based on the distributions of the trace makers of artichnus we hypothesize that in the rock record this trace fossil can be used as evidence of stable euhaline salinity conditions and sediment deposition and colonization below storm wave base artichnus occurs mainly in mud dominated 55 mud substrates

u pb and u th sm he double dating of detrital apatite by laser ablation a critical evaluation abstract paired u pb and u th sm he dating of individual detrital apatite crystals has potential as a valuable tool for constraining the high and low temperature thermal evolution of sediment source terrains here we present the results of exploratory applications of the laser ablation double dating ladd method originally developed for detrital zircon research to the widely available durango fluorapatite standard in order to evaluate the practicality of the method sixty two laser ablation analyses of a single large crystal of durango fluorapatite yielded an inverse variance weighted mean 206pb 238u date of 31 46 0 48 ma and an inverse variance weighted mean u th sm he date of 31 75 0 60 ma both of which are in good agreement with previously published conventional dates while these results are encouraging several factors suggest that ladd may be less useful for detrital apatites than for detrital zircons given widely available analytical instrumentation these principally reflect the propensity for apatites to have comparatively lower u th concentrations and thus lower radiogenic he and pb concentrations as well as high concentrations of common pb these factors contribute to substantially higher analytical imprecision for most ladd u pb dates for apatite occasionally too high for the dates to be geologically useful reasonably precise laser ablation u th sm he dating of detrital apatites requires relatively large crystal sizes 100 m in the shortest dimension with the minimum useful size increasing with decreasing u th sm he apparent age in contrast to the geological interpretation of ladd datasets for detrital zircons the interpretation of datasets for detrital apatites is less straightforward in particular researchers should consider carefully the possibility that u th sm he apparent age distributions are biased by the need to analyze only larger apatites

a recipe for the use of rutile in sedimentary provenance analysis abstract rutile has received considerable attention in the last decade as a valuable petrogenetic indicator mineral based on both new and previously published data we carve out advantages and pitfalls regarding tio 2 minerals in sedimentary provenance analysis this results in a recipe for the use of rutile in provenance studies the main points are rutile geochemistry from different grain size fractions does not differ systematically and hence rutiles should be extracted from the fraction containing the most rutile grains usually 63 200 m similarly different magnetic susceptibility of rutile does not systematically imply different trace element composition before interpretation of tio 2 mineral data it is important to determine the polymorph type rutile anatase and brookite appear to differ systematically in trace element composition as an alternative to raman spectroscopy chemical classification according to nb cr sn fe v and zr concentrations can be applied for rutile a new host lithology discrimination scheme based on cr nb systematics is introduced x 5 nb ppm 500 cr ppm which leads to better classification results than previously published discrimination methods according to this equation metamafic rutiles have negative values of x while metapelitic rutiles have positive values evaluation of the growth temperature calculations of metamorphic rutile after different authors shows that the equations given by tomkins et al 2007 should be applied to both metamafic and metapelitic rutiles although there is a pressure effect on the zr incorporation in rutile the pressure range for most rutiles of 5 15 kbar introduces an uncertainty in calculated temperature of no more than 35 c the distribution of calculated temperatures from detrital rutiles is crucial only well defined temperature populations should be used for thermometry interpretation

new trace fossil evidence for an early recovery signal in the aftermath of the end permian mass extinction abstract new ichnological data from the lower triassic werfen formation dolomites italy revealed an unexpectedly diverse and complex ichnofauna in mixed siliclastic carbonate shelf sediments of the western tropical tethys shortly after the end permian mass extinction event common elements are thalassinoides palaeophycus and planolites other associated ichnogenera include spongeliomorpha rhizocorallium lockeia catenichnus helminthopsis and taenidium one structure is tentatively determined as curvolithus biostratigraphic data conodonts and bivalves of the trace fossil interval lower seis siusi member suggest a late griesbachian age less than 1 ma after the end permian mass extinction event similarly diverse ichnofossil communities from this time interval have recently been described from the boreal realm but this is the first record of a diverse griesbachian ichnoassemblage from the tropics indicating that the early recovery of trace fossil producers was not latitudinally restricted as previously proposed accordingly relatively advanced recovery stages were reached on a global scale much earlier than the spathian as is commonly acknowledged the early and interregional peak in ichnofaunal recovery implies subsequent ecological setbacks in post griesbachian times that explain the overall delay of benthic recovery until the spathian rather than persistent catastrophic conditions during the griesbachian a global and synchronous succession of crises and relaxation phases after the griesbachian is proposed as an explanatory model for the recovery pattern of benthic ecosystems

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

yttrium and rare earth elements in fluids from various deep sea hydrothermal systems rare earth element ree and yttrium y concentrations were measured in fluids collected from deep sea hydrothermal systems including the mid atlantic ridge mar i e menez gwen lucky strike tag and snakepit the east pacific rise epr i e 13 n and 17 19 s and the lau vai lili and manus vienna woods pacmanus desmos back arc basins bab in the south west pacific in most fluids y is trivalent and behaves like ho chondrite normalized y ree y reen concentrations of fluids from mar epr and two bab sites i e vai lili and vienna woods showed common patterns with lree enrichment and positive eu anomalies ree analysis of plagioclase collected at lucky strike strengthens the idea that fluid ree contents are controlled by plagioclase phenocrysts other processes however such as ree complexation by ligands cl f so42 secondary phase precipitation and phase separation modify ree distributions in deep sea hydrothermal fluids ree speciation calculations suggest that aqueous ree are mainly complexed by cl ions in hot acidic fluids from deep sea hydrothermal systems ree concentrations in the fluid phases are therefore influenced by temperature ph and duration of rock fluid interaction unusual y reen patterns found in the pacmanus fluids are characterized by depleted lree and a positive eu anomaly the demos fluid sample shows a flat y reen pattern which increases regularly from lree to hree with no eu anomaly these manus basin fluids also have an unusual major element chemistry with relatively high mg so4 h2s and f contents which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation ree distribution in pacmanus fluids may stem from a subseafloor barite precipitation and the ree in demos fluids are likely influenced by the presence of sulfate ions

the ichnogenus teichichnus seilacher 1955 the ichnotaxonomy of the phanerozoic trace fossil teichichnus seilacher 1955 is revised and its ethology producers and environmental affiliation are re evaluated newly studied material reveals that teichichnus is a sub vertical spreite burrow with an arcuate shape containing stacked laminae and a passively filled terminal causative burrow funnel like extension of the terminal causative burrow and wall ornamentation may occur high variation of burrow morphology is common and has resulted in the erection of 18 ichnospecies of which only four are herein regarded as valid teichichnus rectus t zigzag t patens and t duplex these ichnospecies are characterised by their overall burrow shape spreite morphology and more subordinate occurrence of branching and sculpture as recommended ichnotaxobases combined modes of feeding become evident including deposit and suspension feeding suggesting that teichichnus is a dwelling trace rather than a feeding trace aside from the classical interpretation of polychaetes as producers many features accord with an interpretation of dwelling echiurans and holothurians this interpretation may apply for many post palaeozoic t rectus and t zigzag whereas t duplex and partly t patens are likely produced by arthropods i e trilobites and crustaceans bivalves are also considered for producing teichichnus like traces teichichnus is a marine trace fossil with predominance in shallow marine environments low diversity and high density occurrences of teichichnus characterise marginal marine paralic environments with reduced salinity i e brackish water and oxygen i e dysoxia whereas in shoreface and offshore settings teichichnus occurs in association with many other ichnotaxa and in low density deep marine occurrences of teichichnus are comparably sparse and isolated records may indicate dysoxic bottom conditions

heavy mineral analysis and detrital u pb ages of the intracontinental paleo yangzte basin implications for a transcontinental source to sink system during late cretaceous time

a clearer view of crustal evolution u pb sm nd and lu hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern china

substrate control of lower cambrian trace fossils from bornholm denmark abstract a ten meter thick sequence within the lower cambrian succession from snogebaek harbour to broens odde bornholm contains abundant trace fossils throughout the systematics of some of the ichnogenera are briefly discussed in particular buthotrephis and a new form of diplocraterion the trace fossils can be subdivided into four associations the skolithos diplocraterion rhizocorallium and teichichnus buthotrephis associations the recurrence of these associations can be correlated with changes in substrate in particular the diplocraterion association in the middle and upper part of the broens odde member exposed here occurs only in storm deposits

the clastic record of a wilson cycle evidence from detrital apatite petrochronology of the grampian taconic fore arc abstract detrital apatite deposited before during and after grampian taconic arc accretion in western ireland reveals dramatic changes in source provenance and provides a detailed detrital petrochronological record of a wilson cycle sited on the laurentian margin these determinations are made possible by recent improvements in source rock type diagnosis by apatite trace element analysis in addition by interrogating the proportion of apatite derived from peraluminous vs metaluminous melt and felsic vs mafic melt in detritus temporal changes can be seen in the relative proportions of these magma compositions over time we present combined trace element and u pb age data for detrital apatite in clastic formations from the south mayo trough a uniquely well preserved fore arc basin accreted to the laurentian margin during the grampian taconic orogeny 475 460 ma which records the early phase of closing of the iapetus ocean apatite deposited before collision is derived from the laurentian margin and mafic igneous rocks of the central iapetus magmatic province associated with the neoproterozoic break up of rodinia pannotia leading to the birth of the iapteus ocean following the onset of ocean closure in the cambrian igneous apatite derived from the nascent grampian taconic arc is detected apatite derived from felsic and peraluminous melts becomes progressively more dominant over apatite from more mafic and metaluminous sources as the arc evolves from an intra oceanic to continental arc setting after collision the subsequent grampian taconic unroofing history recorded by detrital apatite records the transition from ancient laurentian margin and intra oceanic arc derived detritus to the exhumation of the high grade metamorphic core of the grampian taconic orogen and the exposure of post collisional continental arc granitoids our results are in broad agreement with both the grampian fore arc provenance record heavy mineral analysis detrital zircon u pb and white mica ar ar and the laurentian crustal reworking record derived from the geochemistry of volcaniclastic units in the south mayo trough however we also obtain novel provenance information undetected by detrital zircon in the south mayo trough such as apatite derived from the central iapetus magmatic province critically this information is obtained from a single mineral system which eliminates the need to consider differing mineral stabilities during weathering transport and diagenesis the high preservation potential for apatite in similar tectonic environments illustrates the potential to employ detrital apatite as a record of earth s lithological composition in deep time

detecting magma poor orogens in the detrital record the clastic record is commonly interrogated by analysis of detrital heavy mineral assemblages with the bulk of modern detrital geochronological studies employing u pb dating of detrital zircon however the bias of zircon toward felsic igneous sources and the limited ability of the u pb system in zircon to record low to medium grade metamorphic events means that the u pb detrital zircon record is largely insensitive to magma poor orogens in this study u pb ages were obtained by laser ablation inductively coupled plasma mass spectrometry for apatite and rutile extracted from alluvium of the french broad river fbr in the southern appalachians southeastern united states in contrast to previously published fbr u pb zircon data sets which yield essentially no record of the most recent appalachian metamorphic events ca 320 ma associated with assembly of pangea the u pb detrital rutile and especially the u pb apatite systems together provide a complete record of complex polyphase appalachian orogenesis it is unexpected that the apatite and rutile u pb appalachian age populations differ significantly with probable low temperature breakdown of rutile biasing the rutile data set toward the most recent alleghanian metamorphic event these data make the fbr one of the most intensely studied river systems globally for multiproxy single grain u pb analysis clearly demonstrate dependence of provenance information on mineral proxy choice and emphasize the resolving power of multiproxy provenance studies

resolving mantle and crustal contributions to ancient hydrothermal fluids hear isotopes in fluid inclusions from dae hwa wmo mineralisation south korea abstract helium and argon isotopes from fluid inclusions in individual colour zones b c d and e of a large scheelite crystal from the 88 ma dae hwa w mo deposit south korea trace the source and history of the ore fluids a gradual decrease of the fluid 3 he 4 he 3 he 36 ar and 40 ar 36 ar from the core to the edge of the scheelite reflects the progressive dilution of a magmatic fluid by meteoric water and is consistent with the previously observed decrease of 18oh2o and fluid inclusion homogenisation temperatures th shelton et al 1987 the covariation of fluid inclusion he ar isotope systematics with 18o and th defines a magmatic component with 3 he 4 he 1 2 10 6 3 he 36 ar 0 01 and 40 ar 36 ar 1000 anomalously high helium and argon isotope ratios in zone d fluids represents undiluted magmatic noble gases this may reflect local variation in the magmatic gas flux or gas loss due to boiling of the hydrothermal fluids prior to mixing with magmatic gas helium and argon isotope systematics constrain mantle and crustal components in the hydrothermal fluids 40 ar 3 he 4 5 104 are close to the mid ocean ridge basalt value implying that 40ar is mantle in origin radiogenic isotope ratios of the mantle endmember 40 ar 4 he 0 69 0 06 are similar to contemporary geothermal fluids the coincidence of mantle derived he and ar in the fluids is strong evidence that mantle melting during late cretaceous subduction triggered the crystal melting responsible for granite formation 40 ar 4 he of the meteoric fluid 0 007 0 001 is far lower than the crystal production ratio 0 2 implying an origin in crust below 200 c

revised biochronology of the lower cambrian of the central iberian zone southern iberian massif spain trilobites from the upper unit of the lower cambrian pusa formation south central spain substantially antedate other iberian trilobites according to widely published correlation schemes and arguably would be among the earliest trilobites globally these trilobites previously only briefly mentioned in texts are here described and illustrated and their biochronological context examined the pusa formation trilobites are treated in open nomenclature but with suggested affinity to the genus abadiella they are associated with small shelly fossils including pelagiella sp chancellorid spicules and cupitheca sp and unidentfied archaeocyathans trace fossils from the upper unit of the pusa formation down section of the trilobites include dactyloidites isp and rusophycus isp the latter representing the lowest occurrence of this ichnogenus in the region this biostratigraphical context demonstrates that the pusa formation trilobites are substantially younger than had traditionally been thought in terms of iberian regional stages they are ovetian not corduban as previously thought as a consequence of the data presented here the definitions of iberian lower cambrian regional stages are discussed and a substantially revised correlation between key lower cambrian strata of the central iberian and ossa morena zones is proposed

the role of slabs and oceanic plate geometry in the net rotation of the lithosphere trench motions and slab return flow 1 absolute plate motion models with respect to a deep mantle reference frame e g hot spots typically contain some net rotation nr of the lithosphere global mantle flow models for the present day plate setting reproduce similarly oriented nrs but with amplitudes significantly smaller than those found in some high nr pacific hot spot reference frames it is therefore important to understand the mechanisms of nr excitation which we attempt here with two dimensional cylindrical models of an idealized pacific domain we study the influence of slab properties oceanic ridge position continental keels and a weak asthenospheric layer on nr and trench migration fast slab return flow develops in models with stiff slabs and moderate slab dips rapid nrs comparable to the high nr pacific hot spot reference frames are primarily induced by asymmetric slab dips in particular a shallow slab beneath south america and a steep slab in the western pacific a scaling relationship links the amplitude of nr to plate size slab dip angle and slab viscosity asymmetric ridge positions also promote nr through asymmetric plate sizes continental keels have less impact in contrast to what has been found in earlier global studies several models yield unidirectional pacific trench motions such as slab advance in the western pacific and simultaneously slab retreat in the eastern pacific our model provides a physical explanation for nr generation in the present day pacific setting and hints at mechanisms for the temporal evolution of the basin

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

spatial and temporal trends in exhumation of the eastern himalaya and syntaxis as determined from a multitechnique detrital thermochronological study of the bengal fan the bengal fan provides a neogene record of eastern and central himalaya exhumation we provide the first detrital thermochronological study apatite and rutile u pb mica ar ar zircon fission track of sediment samples collected during international ocean discovery program iodp expedition 354 to the mid bengal fan our data from rutile and zircon fission track thermochronometry show a shift in lag times over the interval 5 59 3 47 ma the oldest sample with a lag time of 6 m y has a depositional age of 5 59 4 50 ma and the zircon and rutile populations then show a static peak until 12 ma this interval from 5 59 4 50 ma to 12 ma is most easily interpreted as recording passive erosion of the greater himalaya however single grains with lag times of 4 m y but with high analytical uncertainty are recorded over this interval for sediments older than 10 ma these grains were derived from the greater himalaya which was exhuming rapidly until ca 14 ma in sediments younger than 10 ma these grains could represent slower yet still rapid exhumation of the syntaxial antiform to the south of the massif lag times 1 m y are again recorded from 14 5 ma to the base of the studied section at 17 ma reflecting a period of greater himalayan rapid exhumation mica 40ar 39ar and apatite u pb data are not sensitive to syntaxial exhumation we ascribe this to the paucity of white mica in syntaxial lithologies and to high levels of common pb resulting in u pb ages associated with unacceptably high uncertainties respectively

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure