oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

site specific equilibrium isotopic fractionation of oxygen carbon and calcium in apatite the stable isotope composition of biogenic apatite is an important geochemical marker that can record environmental parameters and is widely used to infer past climates biomineralization processes dietary preferences and habitat of vertebrates in this study theoretical equilibrium isotopic fractionation of oxygen carbon and calcium in hydroxyapatite and carbonate bearing hydroxyapatite is investigated using first principles methods based on density functional theory and compared to the theoretical isotopic fractionation properties of calcite co 2 and h 2 o considering the variability of apatite crystal chemistry special attention is given to specific contributions of crystal sites to isotopic fractionation significant internal fractionation is calculated for oxygen and carbon isotopes in co 3 between the different structural sites occupied by carbonate groups in apatite typically 7 for both 18 o 16 o and 13 c 12 c fractionation at 37 c compared with calcite water oxygen isotope fractionation occurrence of a type substitution in apatite structure in addition to the main b type substitution could explain the larger temperature dependence of oxygen isotope fractionation measured at low temperature between carbonate in apatite and water theoretical internal fractionation of oxygen isotopes between carbonate and phosphate in b type carbonated apatite 8 at 37 c is consistent with experimental values obtained from modern and well preserved fossil bio apatites concerning calcium theoretical results suggest a small fractionation between apatite and calcite a0 17 at 37 c internal fractionation reaching 0 8 at 37 c occurs between the two ca sites in hydroxyapatite furthermore the ca isotopic fractionation properties of apatite are affected by the occurrence of carbonate groups which could contribute to the variability observed on natural samples owing to the complexity of apatite crystal chemistry and in light of the theoretical results measurements of site specific isotopic fractionation properties could improve our understanding and the interpretation of isotopic records in apatites

the ediacaran cambrian rise of siliceous sponges and development of modern oceanic ecosystems abstract the lack of unequivocal sponge fossils before the cambrian despite their probable deep origin during the cryogenian period has been a conundrum to geologists their impact on the dramatic evolution of ecosystems and the seawater silica cycle during the ediacaran cambrian e c transition is also speculative in this study abundant sponge spicules and spicule like structures that probably represent original sponge fossils were recovered from four sections of the e c boundary interval in the yangtze gorges south china the paleontological and geochemical data presented herein provides evidence for a continuous distribution of relatively abundant sponge spicules some even forming spiculites as well as the earliest biogenic deposition of silica by metazoans in the ediacaran cambrian boundary interval these results further confirm that a biological takeover of oceanic dissolved si co occurred with the evolution of silica biomineralization in the e c interval resulting in the widespread deposition of biogenic chert hydrothermal input in spicule bearing cherts is also observed such hydrothermal activity might have favored the blooming of sponges through significant nutrient supply these results provide the first conjunction of geochemical and paleontological proxies in support of previous models considering that sponges by ventilation filtration and oxygenation of seawater were important ecosystem engineers of the e c bioradiation event and of the related establishment of modern type ecosystems

pb diffusion in monazite constraints from a high t contact aureole setting abstract we report measurements of pb diffusion in natural monazite that was subjected to a static anhydrous high temperature overprint in the contact aureole of the makhavinekh lake pluton mlp northern labrador canada the 1850 ma monazite grains we studied were reheated to temperatures of 900 25 c during intrusion of mlp at 1322 ma ion microprobe depth profiling reveals the outermost 0 3 m of these grains to contain pb u gradients that can be modeled as a result of one dimensional concentration independent diffusion i e an error function profile at 1322 ma these gradients are combined with temperature time histories predicted by a numerical model of conductive heat flow in the contact aureole to calculate pb diffusion coefficients between 3 2 10 28 and 1 3 10 27 m2 s this range overlaps with the down temperature extrapolation of experimental pb diffusion data obtained under dry conditions confirming the slow diffusivity of pb in monazite at crustal temperatures and timescales

oxygen diffusion in quartz diffusional oxygen exchange between quartz and water was studied by determining 18o profiles in the quartz using an ion microprobe inverse error function plots of the data are linear as required for a diffusion mechanism one natural and two synthetic samples were used and although they had different impurity concentrations oh na and al per 106 si atoms are 7 4000 35 835 and 67 127 respectively the d values were the same within experimental uncertainty diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700 c and diffusion normal to the rhombohedron is intermediate the data indicate a change in slope on an arrhenius plot at the boundary at 100 mpa 1 kbar water pressure the preexponential factors do in square centimeters per second and activation energies q in kilojoules per mole for quartz 500 550 c are as follows do 190 and q 284 parallel to c and do 8 10 2 and q 238 normal to the rhombohedron for quartz 600 800 c do 4 10 7 and q 142 parallel to c do 9 10 7 and q 155 normal to the rhombohedron and do 1 10 4 and q 234 normal to c d increases with ph 2 o and between 25 and 350 mpa log d versus log h 2 o has a slope of approximately 1 1 these d values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water free systems

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

nondestructive chemical dating of young monazite using xrf 1 design of a mini probe age data for samples from the central alps and comparison to u pb tims data the analytical capabilities of an improved mini xrf instrument for trace element analysis are explored to obtain u th pb ages for single monazite grains as small as 50 m the detection limits for u th and pb with counting times of 10 min are 10 ppm counts collected for 40 min are sufficient to date single monazite grains as young as 15 ma the precision of the age data depends essentially on the counting statistics of the main x ray peak npb l 1 2 used to analyze for lead the age uncertainty is proportional to the age of the grain the total contents of th and u and the squared integration time the reliability of the chemical dating method is tested using monazite separates from amphibolite facies metamorphic monazite samples from the central alps for which the chemical ages are compared to mass spectrometric u pb tims ages we show that populations of analyses obtained using the new instrument on single grains as young as 25 30 ma agree within error with the tims data for monazite separates with complex age patterns such as polymetamorphic samples yielding discordant isotopic ages the statistical analysis of chemical age data obtained from populations of 26 50 grains is advantageous in that it may provide additional insight into the reasons for the discordancy chemical th u pb dating of monazite by the xrf method offers considerably improved accuracy and precision compared with chemical dating by electron microprobe which typically becomes impractical for samples less than 100 200 ma in age conversely the minimum grain size required for the new instrument 50 m is well above the spatial resolution of the electron microprobe 2 m a companion paper scherrer et al this volume describes further developments towards microanalysis by xrf with improved spatial resolution

armoured sponge spicules from panarea island italy implications for their fossil preservation abstract a variety of calcareous and siliceous skeletal components are associated with exceptional still active deposits of modern iron ooids off panarea one of the volcanic islands of the aeolian arc tyrrhenian sea italy both ooids and skeletal components occur as loose sediments influenced by submarine hydrothermal fluid vents whereas iron ooids are exclusively made of goethite laminae primarily nucleated on volcaniclastic material sponge spicules that represent most of the siliceous skeletal component develop laminated coatings concretions of varying sizes this partial or total coating consists of regularly banded fe rich layers exhibiting the same textural features and mineralogical composition goethite of the ooid cortex developed around inorganic cores spicules did not reveal any obvious attaching structure or interfingering with the surrounding coating and did not undergo bioerosion or any other evidence of biological intervention during their development a hydrothermal origin compatible with the general setting is therefore proposed for these armoured sponge spicules we believe that this unique modern case of iron concretions produced on siliceous spicules can contribute to explain other known fossil cases as well as to understand the taphonomy of this type of biogenic silica that seems rarely to have been preserved in such an extreme habitat

microprobe monazite geochronology understanding geologic processes by integrating composition and chronology abstractmonazite is a light rare earth element lree bearing phosphate mineral that is present in a wide variety of rock types has an extremely variable composition reflecting host rock conditions and is a robust geochronometer that can preserve crystallization ages through a long history of geological events monazite crystals typically contain distinct compositional domains that represent successive generations of monazite which in turn can provide a detailed record of the geologic history of its host rocks the electron microprobe can be used to characterize the geometry of compositional domains analyze the composition of each domain and when carefully configured determine the u th total pb age for domains as small as 5 m in width these data allow the monazite to be linked with and place timing constraints on silicate processes in the host rocks current applications span a broad range of geologic processes in igneous metamorphic hydrothermal and sedimentary rocks

palynological analysis of the late ordovician early silurian black shales in south china provides new insights for the investigation of pore systems in shale gas reservoirs abstract the late ordovician wufeng and early silurian lungmachi formations are primary targets for shale gas exploration in the southeastern marginal area of the sichuan basin southwest china in this research different groups of micro fossils large numbers of taxonomically unrelated organic and inorganic walled fossil groups from the two formations were analyzed in order to investigate their pore characters under the evaluation of shale gas reservoirs a large number of biological autologous micro nano pores in radiolarians sponge spicules acritarchs conodonts chitinozoans and scolecodonts were analyzed the results show that most pores are densely and randomly distributed with honeycomb or irregular shapes although the samples are over mature and were deeply buried micro fossils remain in a large number of biological micro nano pores pore diameter varies from 20 to 4000 nm but differs among micro fossil groups the widely distributed siliceous micro fossils such as radiolarians and sponge spicules could enhance the brittleness of shales they may promote the development of natural fractures during diagenesis or artificial fractures during stimulation when fractures are connected with micro nano pores in the shale they could provide effective storage space and improve the overall connectivity in most of the analyzed shale samples micro fossils show positive correlation with high toc strata considering the importance of micro fossils and their pores in the perspective of hydrocarbon generation connectivity accumulation and production of shale gas we suggest that micro fossil pores should be included in the classification of pore systems

carbon concentrations and isotopic ratios of eclogites from the dabie and sulu terranes in china abstract both concentration and isotope composition of bulk carbon in apatite and host eclogites from the dabie sulu ultrahigh pressure uhp terranes in china have been determined along with their oxygen isotope composition the results show significant 13 c depletion in the apatite 13 c 27 7 to 20 8 with the carbon concentrations of 0 59 to 1 65 wt co 2 despite a large variation in 18 o 6 5 to 9 5 the bulk carbon in 21 of the 24 eclogites has low 13 c values of 26 1 to 17 9 with low carbon content of 500 to 1000 ppm whereas the other three samples show high 13 c values of 7 1 to 2 8 with high carbon contents of 2400 to 4300 ppm noncarbonate carbon was measured by treating the all eclogites with 5 n hcl solution yielding uniformly low 13 c values of 27 9 to 24 2 and carbon contents of 200 to 300 ppm carbonate carbon is thus calculated by mass balance to have also low 13 c values of 25 6 to 15 1 for the 21 samples but high 13 c values of 4 3 to 1 2 for the remaining three samples secondary carbonate was identified in the three eclogites that are also depleted in 13 c primarily but subjected to overprint of 13 c rich co 2 bearing fluid subsequent to the uhp metamorphism the isotopically light carbon in both eclogite and apatite is interpreted to represent the isotope composition of carbon in eclogite precursors before plate subduction and thus has an origin of organic carbon from the earth s surface the uniformly low 13 c values of apatite suggest that the co 2 of metamorphic fluid in equilibrium with the host eclogites would be derived from the oxidation of organic carbon rather than the decarbonation of underthrust carbonates during progressive metamorphism protoliths of the eclogites are inferred to be of igneous origin which underwent more extensive interaction with organic matter than with meteoric hydrothermal fluid on the subsurface of the continental crust the break off of the subducted plate containing the isotopically light carbon and subsequent interactions with the surrounding mantle could produce the mafic and or silicic magmas that are significantly depleted in 13 c relative to the primary mantle carbon this may provide evidence for a linkage of the 13 c depleted mantle carbon to a surficial source via plate subduction

equilibrium oxygen hydrogen and carbon isotope fractionation factors applicable to geologic systems as demonstrated by the chapters in this short course stable isotope techniques are an important tool in almost every branch of the earth sciences central to many of these applications is a quantitative understanding of equilibrium isotope partitioning between substances indeed it was harold urey s 1947 thermodynamically based estimate of the temperature dependence of 18o 16o fractionation between calcium carbonate and water and a recognition of how this information might be used to determine the temperatures of ancient oceans that launched the science of stable isotope geochemistry the approach pioneered by urey has since been used to estimate temperatures for a wide range of geological processes e g emiliani 1955 anderson et al 1971 clayton 1986 valley this volume in addition to their geothermometric applications equilibrium fractionation data are also important in the study of fluid rock interactions including those associated with diagenetic hydrothermal and metamorphic processes baumgartner and valley this volume shanks this volume finally a knowledge of equilibrium fractionation is a necessary first step in evaluating isotopic disequilibrium a widespread phenomenon that is increasingly being used to study temporal relationships in geological systems cole and chakraborty this volume in the fifty four years since the publication of urey s paper equilibrium fractionation data have been reported for many minerals and fluids of geological interest these data were derived from 1 theoretical calculations following the methods developed by urey 1947 and bigeleisen and mayer 1947 2 direct laboratory experiments 3 semi empirical bond strength models and 4 measurement of fractionations in natural samples each of these methods has its advantages and disadvantages however the availability of a variety of methods for calibrating fractionation factors has led to a plethora of calibrations not all of which are in agreement in this chapter we evaluate the major methods for determining fractionation factors

stable isotopic calibration of the earliest skeletal fossil assemblages in eastern siberia precambrian cambrian boundary sections from the eastern margin of the siberian platform provide important reference sections for stable isotopic correlation of the newly defined precambrian cambrian boundary level since they contain some of the earliest pre tommotian shelly fossil assemblages known and are amenable to 13 c stratigraphy stable isotopes are examined from the dvortsy and ulakhan sulugur sections river aldan where sparse pre tommotian assemblages occur and from the uchur river region where the putative anabarites trisulcatus and purella antiqua zones of the nemakit daldynian stage are well developed diagenetic resetting of 18 o and 13 c must be taken into account when comparing such sections simple corrections allow for stable isotopic correlation of basal nemakit daldynian and tommotian strata across eastern siberia

ediacarian sponge spicule clusters from southwestern mongolia and the origins of the cambrian fauna carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments with anabarites from southwestern mongolia are of ediacarian age spicule morphologies include bundles of oxeas arranged in three dimensional quadrules linked together at junctions by tetracts pentacts hexacts or polyactines all are referred to the phylum porifera class hexactinellida these sponge spicules provide the oldest remains that can be assigned without question to an extant phylum and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record it is suggested that siliceous and phosphatic members of the cambrian fauna may have had their origins in eutrophic and outer shelf facies of the late proterozoic

palaeoceanographic redox environments for the lower cambrian hetang formation in south china evidence from pyrite framboids redox sensitive trace elements and sponge biota occurrence abstract the lower cambrian hetang formation a black shale sequence contains a stone coal a flammable organic rich mudstone unit abundant pyrite framboids occur in this unit with the average mean sizes falling in a narrow range from 4 6 to 5 4 m and the maximum diameters being around 10 m the size distribution pattern of the pyrite framboids indicates a euxinic depositional environment the redox sensitive trace metals of the stone coal samples reveal a large enrichment of mo 10 180 times v 4 40 times u 10 60 times and ni 2 20 times compared to the average upper continental crust value consistent with an anoxic environment for their deposition the redox sensitive element ratios th u v v ni v cr indicate that the depositional environment for the lower part of the stone coal unit was the most anoxic and euxinic in contrast a sponge biota including eleven species of demosponges and hexactinellids and two undetermined forms indicate an oxic or dysoxic environment to reconcile these two facts we propose that although an anoxic euxinic environment predominated during the deposition of the lower cambrian hetang formation black shales occasional currents may also have brought free oxygen to the bottom water column to allow the growth of the sponges on the sea floor

petrology and geochronology of rutile rutile tio2 is an important accessory mineral that when present offers a rich source of information about the rock units in which it is incorporated it occurs in a variety of specific microstructural settings contains significant amounts of several trace elements and is one of the classical minerals used for u pb age determination here we focus on information obtainable from rutile in its original textural context we do not present an exhaustive review on detrital rutile in clastic sediments but note that an understanding of the petrochronology of rutile in its source rocks will aid interpretation of data obtained from detrital rutile for further information on the important role of rutile in provenance studies the reader is referred to previous reviews e g zack et al 2004b meinhold 2010 triebold et al 2012 coarse rutile is the only stable tio2 polymorph under all crustal and upper mantle conditions with the exception of certain hydrothermal environments smith et al 2009 as such we will focus on rutile rather than the polymorphs brookite anatase and ultrahigh pressure modifications in this chapter we first review rutile occurrences trace element geochemistry and u pb geochronology individually to illustrate the insights that can be gained from microstructures chemistry and ages then in the spirit of petrochronology we show the interpretational power of combining these approaches using the ivrea zone italy as a case study finally we suggest some areas of future research that would improve petrochronologic research using rutile rutile is a characteristic mineral in moderate to high pressure metapelitic rocks in high pressure metamorphosed mafic rocks and in sedimentary rocks e g force 1980 frost 1991 zack et al 2004b triebold et al 2012 rutile also occurs rarely in magmatic rocks e g anorthosites as well as in some hydrothermal systems coarse grained

nutrient enriched waters and the early skeletal fossil record this paper examines the role of stratified oxygen depleted and nutrient enriched water masses new in transforming the fossil record over the precambrian cambrian boundary interval the following may serve as markers for the encroachment of new over low latitude cratons trace element enriched black shales stone coals and hydrocarbon source rocks phosphorites phosphatized shells and organic matter sponge spicule cherts early appearance of silica and phosphatic skeletons appearance of anabarites and coleoloides biofacies peak diversity of phosphatic species occurrence of black shale lagerstatten and light 13 c values of carbonates nutrient depleted waters ndw are inferred over inner cratonic areas and nutrient starved waters nsw may well have developed over deeper waters at times through the later cambrian related to the effects of sea level rise and salinity stratification carbon isotopic fluctuations are suggested to record the history of these water masses with major changes in carbon burial and palaeoproductivity the latter may well imply episodes of greenhouse climate with globally raised levels of carbon dioxide and relatively high ocean temperatures

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

sponge spicules from the lower cambrian in the yanjiahe formation south china the earliest biomineralizing sponge record abstract sponges are the earliest metazoans and according to molecular clocks have a deep history back into the cryogenian however the record of their biomineralized elements below the ediacaran cambrian boundary has been controversial here we present the first and earliest indisputable record of hexactinellid spicules in the lowest cambrian terreneuvian below the small shelly fossil anabarites trisulcatus protohertzina anabarica assemblage zone zone i spicules recovered from cherts of the lowest yanjiahe formation in the yichang hubei province south china include monaxons diaxons triaxons and that are all siliceous in composition this earliest record of silica biomineralization by filter feeding metazoan suggests sponges have contributed to the cycling of silica in the oceanic system as early as the beginning of the phanerozoic these results along with a review of previously reported sponge occurrences suggest a two step evolution on sponges of the early cambrian

towering sponges in an early cambrian lagerstatte disparity between nonbilaterian and bilaterian epifaunal tierers at the neoproterozoic cambrian transition epifaunal suspension feeding bilaterian animals in the cambrian lived close to the sediment water interface and hence their ecological tiering levels were low 10 cm here we report an early cambrian diandongian or probably tommotian atdabanian lagerstatte from the hetang formation in anhui province south china the hetang biota is characterized by high tiering to 50 cm sponges and small 0 5 cm bilaterians including orthothecid hyoliths and bivalved arthropods nonbilaterian suspension feeders sponges cnidarians and archaeocyathids as high tiering animals and bilaterian suspension feeders as low tiering animals also characterize other neoproterozoic cambrian assemblages such as the ediacaran chengjiang burgess shale and sinsk biotas these data are consistent with medium to high tiering levels in neoproterozoic cambrian epifaunal communities but suggest that nonbilaterians achieved such tiering levels long before bilaterian suspension feeders did so in the early ordovician the disparity between bilaterian and nonbilaterian tierers during the neoproterozoic cambrian transition and the delayed appearance of high tiering bilaterians demand phylogenetic and ecological explanations the cambrian substrate revolution may have triggered a cascade of ecological evolution including the rise of bilaterian animals in high tiering levels during the ordovician radiation of the paleozoic fauna

neoproterozoic to early cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the yangtze platform china abstract small shelly fossils ssfs occur widely on the yangtze platform and have great potential for biostratigraphic subdivision of pre trilobitic early cambrian strata based on the ssf record of the shallow water realm five biozones can be recognized for the meishucunian stage in ascending order the biozones are anabarites trisulcatus protohertzina anabarica assemblage zone paragloborilus subglobosus purella squamulosa assemblage zone watsonella crosbyi assemblage zone formerly heraultipegma yunnanensis zone poorly fossiliferous interzone sinosachites flabelliformis tannuolina zhangwentangi assemblage zone in addition one ssf biozone is recognized for the overlying qiongzhusian stage pelagiella subangulata taxon range zone the formerly used siphogonuchites triangularis paragloborilus subglobosus zone and heraultipegma yunnanensis zone are discussed and redefined approximately 80 species of ssfs were screened for their spatial and temporal distribution on the yangtze platform variations in lithofacies and biofacies can be recognized throughout the yangtze platform extending over an area of 2000 900 km in a deeper water shelf setting the first zone is represented by the protohertzina anabarica kaiyangites novilis assemblage zone while younger zones are not recognized in the southern region at the northern platform margin the qiongzhusian is represented by the ninella tarimensis cambroclavus fangxianensis assemblage zone and the rhombocorniculum cancellatum taxon range zone the southeastern shaanxi western hubei region followed a slightly different lithological and faunal development than the rest of yangtze platform indicating a stronger similarity with parts of east gondwana some taxa such as w crosbyi p subangulata r cancellatum microdictyon effusum a trisulcatus protohertzina unguliformis and p anabarica occur nearly worldwide and support an international correlation of early cambrian sequences between the yangtze platform and smaller west gondwanan blocks siberia newfoundland and australia the six investigated zones of the yangtze platform comprise an interval spanning the early nemakit daldynian to the late atdabanian early botoman stage on the siberian platform palaeobiogeographic analysis revealed a strong taxic similarity between the yangtze platform and the tarim platform a smaller number of species are shared with other west gondwanan platform fragments such as india and iran palaeobiogeographic results do not support the previously reported position of the south china block between australia and siberia during the early cambrian

tracing sources and cycling of phosphorus in peru margin sediments using oxygen isotopes in authigenic and detrital phosphates abstract many bio geochemical processes that bring about changes in sediment chemistry normally begin at the sediment water interface continue at depth within the sediment column and may persist throughout the lifetime of sediments because of the differential reactivity of sedimentary phosphate phases in response to diagenesis dissolution precipitation and biological cycling the oxygen isotope ratios of phosphate 18op can carry a distinct signature of these processes as well as inform on the origin of specific p phases here we present results of sequential sediment extraction sedex analyses combined with 18op measurements aimed at characterizing authigenic and detrital phosphate phases in continental margin sediments from three sites sites 1227 1228 and 1229 along the peru margin collected during odp leg 201 our results show that the amount of p in different reservoirs varies significantly in the upper 50 m of the sediment column but with a consistent pattern for example detrital p is highest in siliciclastic rich layers the 18op values of authigenic phosphate vary between 20 2 and 24 8 and can be classified into at least two major groups authigenic phosphate precipitated at near the sediment water interface in equilibrium with paleo water oxygen isotope ratios 18ow and temperature and phosphate derived from hydrolysis of organic matter porg with subsequent incomplete to complete re equlibration and precipitated deeper in the sediments column the 18op values of detrital phosphate which vary from 7 7 15 4 suggest two possible terrigenous sources and their mixtures in different proportions phosphate from igneous metamorphic rocks and phosphate precipitated in source regions in equilibrium with 18ow of meteoric water more importantly original isotopic compositions of at least one phase of authigenic phosphates and all detrital phosphates are not altered by diagenesis and other biogeochemical changes within the sediment column these findings help to understand the origin and provenance of p phases and paleoenvironmental conditions at near the sediment water interface and to infer post depositional activities within the sediment column

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

metamorphic zircon trace elements and neoarchean metamorphism in the ca 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract the nuvvuagittuq supracrustal belt nsb in northwestern quebec ranks as one of the oldest granitoid gneiss complexes thus far discovered emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz magnetite schists at the porpoise cove outcrops of the nsb cluster at 3 75 ga as with all other pre 3 7 ga terranes thus far documented the nsb has been metamorphosed and multiply deformed in a combined approach to explore the thermal history of the nsb conventional garnet biotite and plagioclase hornblende geothermometry was coupled with zircon u th pb depth profiles ree partitioning and ti in zircon thermometry to show that neoarchean metamorphism in the belt reached upper amphibolite facies conditions 640 c this metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the northeast superior province and the initiation of widespread igneous activity in the vicinity of the nsb an earlier resolvable metamorphic episode from a zircon depth profile recorded at 3 62 ga probably corresponds to the intrusion of the granitoids which envelope the nsb

the link between metazoan diversity and paleo oxygenation in the early cambrian an integrated palaeontological and geochemical record from the eastern three gorges region of south china abstract the link between environmental perturbations especially oceanic redox conditions and evolution during the cambrian biodiversification event is still highly controversial although this issue has attracted considerable attention for decades we here report on the fossil record of the early cambrian biota discovered in the uppermost yanjiahe and shuijingtuo formations that crop out in the luojiacun and qiaojiaping sections eastern three gorges area south china the uppermost member of the yanjiahe formation contains abundant benthic algae based on the fossil record four distinct intervals were recognized in the shuijingtuo formation i few sessile suspension feeders such as sponges were recovered from interval i ii interval ii is dominated by bivalved arthropods living in the water column in this interval are also found a few sponges and eodiscoid trilobites iii no fossils other than few sponge spicules were recovered from interval iii iv diverse benthic communities composed of sponges cnidarians brachiopods as well as bradoriids dominate interval iv comprehensive research on framboidal pyrite morphology whole rock geochemistry as well as in situ la icp ms analyses were carried out on samples from the luojiacun section in order to assess paleo redox conditions the results show that the overall evolution of the paleo redox conditions was characterized by a double episode of sharp anoxic events ae observed at the base of intervals i and iii followed by a gradual oxygenation event oe evidence of oxygenation fluctuations were detected especially within interval iv a case study is presented herein which brings together early cambrian changes in metazoan diversity and changes in environmental conditions it is suggested that periodical oxygenation of the water column increased the biological diversity in the restricted basins of the eastern three gorges thus four intervals of habitat succession can be tentatively reconstructed in the studied area for the cambrian age 2 age 3 interval indicating a transition from uninhabited sediments to well populated habitats finally both the temporal and spatial characteristics of the cambrian aes oes and the fossil record in south china are compared and links between them are discussed in the context of their specific paleo geographical setting

u pb isotopic behaviour of zircon during upper amphibolite facies fluid infiltration in the napier complex east antarctica understanding the factors that contribute to u pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of u pb systematics in zircon might be achieved under subsolidus conditions we present a field example from the napier complex east antarctica in which a palaeoproterozoic 2450 70 ma zircon population interacted locally with an early palaeozoic 498 1 7 ma aqueous fluid at upper amphibolite facies conditions conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of u and pb in zircons both normal and reverse discordance with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration in contrast ion probe drilling or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of u pb systematics to depths of 0 2 m with near surface ages consistent with the timing of fluid influx at 498 ma although zircon underwent some radiogenic pb redistribution during fluid interaction infiltrating fluids resulted in minimal grain scale isotopic modification of zircon based on ion probe depth profiling results we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses in this case is controlled by an analytical mixture of reset and or recrystallised zircon along penetrative micro fracture networks with that of adjacent unaffected zircon we also suggest that the observed reverse discordance is genuine resulting from localised intra grain net accumulations of radiogenic pb we conclude that the isotopic response of zircon in this case is controlled by the interaction of an aqueous metamorphic fluid of low to moderate salinity resulting in sub micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro fracture networks and that grain scale isotopic modification was negligible therefore we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper amphibolite facies conditions

assessing the veracity of precambrian sponge fossils using in situ nanoscale analytical techniques abstract paleontological inferences molecular clocks and biomarker fossils indicate sponges evolved in the cryogenian but precambrian sponge fossils are rare poorly substantiated and controversial spicule like microstructures slms hosted in phosphatized fossils from the ediacaran doushantuo formation 635 551 ma at weng an of south china have been interpreted as cylindrical siliceous monaxons and their hosting fossils as the oldest demosponges in the fossil record in order to assess their veracity as the oldest spiculate demosponges we utilize a suite of in situ nanoscale analytical techniques including scanning electron microscopy synchrotron x ray fluorescence mapping x ray absorption near edge structure xanes spectroscopy focused ion beam electron microscopy and transmission electron microscopy to evaluate the ultrastructures and elemental chemical and mineralogical compositions of the slms our data decisively shows that the slms are carbonaceous in composition and rectangular in transverse sections and therefore are not cylindrical siliceous spicules instead the slms may be microbial strands axial filaments of early hexactinellids or acicular crystals molded by organic matter regardless our new data invalidate the oldest and only precambrian demosponges with mineralized spicules these results indicate that interpretations of precambrian sponge fossils should be scrutinized with compositional mineralogical and ultrastructural data collected using in situ analytical techniques in addition our conclusions affirm that no unequivocal biomineralizing sponges occur below the ediacaran cambrian boundary if hexactinellids and demosponges did diverge in the cryogenian as suggested by molecular clocks and biomarkers they either evolved biomineralization long after their divergence or their biomineralized spicules were never preserved until after the ediacaran cambrian boundary in either case the dearth of biomineralizing sponge fossils in the precambrian and their abundance in the early cambrian must reflect a geobiologically significant aspect of the precambrian phanerozoic transition

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

source of silica and silicification of the lowermost cambrian yanjiahe formation in the three gorges area south china abstract the rise of siliceous organisms is generally considered to have driven the abrupt decrease of silica in the ocean during the ediacaran cambrian e c interval even if the exact timing and biological triggers of this event are still debated although ediacaran cambrian chert depositions are widely distributed multiple evidences converge to support their hydrothermal origin in this study we provide petrographic study supplemented by elemental concentration sem and raman spectral data on the siliceous rocks from the yanjiahe formation the three gorges area south china in order to ascertain the origin of included silica and discuss the involved processes of silica deposition contrary to most previous studies our analyses suggest that studied cherts mainly resulted from biogenic silica and subsidiary from abiotic precipitation influenced by surrounding hydrothermal activity and from detrital input the deposition model of silica includes direct deposition of siliceous bioclasts secondary replacement of carbonate by dissolved biosilica and microbial induced silicification from seawater the sink of silica recorded in the yanjiahe formation was characterized of a transition from typical precambrian to phanerozoic process in addition micropalaeontological and taphonomical analyses of the yanjiahe formation revealed two pulses of microfossil abundance and diversity in the fortunian and stage 2 respectively they can be correlated with a regression and an enrichment in dissolved phosphate that probably derived from enhanced detrital flux

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

monazite th pb age depth profiling the significant capabilities of the ion microprobe for thermochronometric investigations of geologic materials remain largely unexploited whereas sup 208 pb sup 232 th spot analysis allows approximately 10 mm scale imaging of pb loss profiles or overgrowths in sectioned monazite grains the spatial resolution offered by depth profiling into the surface region of natural crystals is more than two orders of magnitude higher the authors document here the ability of the high resolution ion microprobe to detect sup 208 pb sup 232 th age differences of 1 m y with better than 0 05 micro m depth resolution in the outer micron of tertiary monazites from the hanging wall of the himalayan main central thrust age gradients on this scale are inaccessible to ion microprobe spot analysis or conventional thermal ionization mass spectrometry interpretation of the near surface sup 208 pb distributions with available monazite pb diffusion data illustrates the potential of the approach for recovering continuous high temperature thermal history information not previously available

new and extraordinary early cambrian sponge spicule assemblage from china the fossil record of siliceous sponges compared with that of other skeleton secreting metazoa is poorly known based as it is on disarticulated spicules and sporadically preserved body fossils abundant spicules recovered from lower cambrian strata in shaanxi china essentially double the known morphological diversity of siliceous sponges for that interval of geologic time these fossils along with a comparable coeval fauna from south australia have a remarkably modern aspect thereby demonstrating that the principal siliceous sponge groups and styles of body architecture were established quickly in the earliest phanerozoic as part of the cambrian explosion and that they inhabited a variety of low energy relatively deep water settings the similarity of spicule shape and variation to that of younger assemblages reflects a conservative architecture for the siliceous sponges

interpreting late precambrian microfossils chia wei li et al describe microfossils found in doushantuo phosphate rocks from guizhou province south china that are about 580 million years old 1 1 well preserved macroscopic multicellular fossils associated with prokaryotic and eukaryotic microfossils from the same source rocks have been

nitrogen and organic carbon isotope stratigraphy of the yangtze platform during the ediacaran cambrian transition in south china abstract n and corg isotope results are presented from six sections along a west east transect in the south china basin scb covering both shallow and deeper domains in order to investigate biogeochemical cycling stratigraphic correlation and isotope systematics over the crucial ediacaran cambrian transition 15n bulk values range between 3 and 7 while 13corg values range between 21 and 39 similar isotopic trends have been identified for both these proxies across the basin although hiatuses differences in depositional setting and syn depositional bacterial fermentation may have caused some inconsistencies a trend towards negative n isotope values can be recognized above the pc c boundary in both shallow and deeper basin realms across the yangtze platform this negative 15n excursion is probably a response to photic zone anoxia and intense nitrogen fixation assimilation by diazotrophic cyanobacteria and green purple sulfur bacteria gsb and psb the xiaotan section and a composite section from the yangtze gorges area show meaningful similarities in their nitrogen isotope trends interpreted as chemocline fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform using carbon isotope stratigraphy we correlate boundary strata across the platform to test the wider significance of nitrogen isotopic variations increased bioturbation and food chain complexity across the ediacaran cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early cambrian reflecting the beginning of a new more biologically controlled era

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

an experimental calibration of the temperature dependence of oxygen isotope fractionation between apatite and calcite at high temperatures 350 800 c the fractionation of oxygen isotopes between apatite and calcite has been measured experimentally using the direct exchange technique in a piston cylinder apparatus at high temperature and pressure 350 800 c 1 1 gpa the fractional approach to equilibrium ranged from 20 to 80 and varies systematically with temperature and run duration isotopic xrd and sem analyses of the run products indicate that the exchange occurs dominantly by recrystallization in the early stages of the experiments the equilibrium fractionation over the temperature range of this study ranges from 5 5 to 2 but the data at 350 and 425 c have relatively large errors for t 500 c an error weighted least squares fit yields 1000 ln ap cc 0 68 1 60 0 26 106t 2 the magnitude of the temperature dependence is intermediate to that for felsic and mafic minerals to calcite mineral pairs involving apatite are potentially useful indicators of temperature cooling rates and equilibrium disequilibrium phenomena in igneous and metamorphic rocks

stable isotope composition of minerals in the belaya zima plutonic complex russia implications for the sources of the parental magma and metasomatizing fluids abstract the belaya zima alkaline carbonatite complex in the east sayan mountains russia is a multiphase concentric intrusion which comprises diverse alkaline silicate rocks and carbonatites melteigites are the earliest products of crystallization and they are followed by ijolites urtites nepheline syenites and finally calcite calcite dolomite and ankerite carbonatites in this paper we present new geochronological data ar ar method chemical and stable isotope h c and o compositions of the main rock forming minerals in carbonatites and the silicate rocks of the complex the ar ar dating of phlogopite from ankerite carbonatites is consistent with the age of syenite emplacement and implies that the silicate rocks and carbonatites are likely to be comagmatic the evolutionary trends of pyroxene fluorapatite amphibole and phlogopite are consistent with fractional crystallization from a single parental magma primary nb mineralization is represented by pyrochlore whereas columbite fe is a replacement product of pyrochlore at the post magmatic hydrothermal stage at that stage interaction of calcite carbonatites with hydrothermal fluids resulted in the formation of hydrothermal paragenesis comprising dolomite ankerite monazite ce ancylite ce minerals of burbankite group and ca ree fluorcarbonates the hydrothermal processes were also responsible for cation leaching from pyrochlore the replacements of phlogopite by tetraferriphlogopite and early ca rich pyroxene by aegirine the stable isotope data suggest that the rocks crystallized from a 18o depleted magma shifts toward higher 18o values relative to the primary compositions are commonly observed in minerals from carbonatites which is attributed to hydrothermal processes

microbial and animal evolution in relation to redox fluctuations in a deep water setting of south china during the ediacaran cambrian transition ca 551 523 ma abstract the ediacaran cambrian e c transition witnessed significant ecosystem perturbations of the oceanic environment e g oxygen concentrations of seawater and biological evolution however in spite of abundant ediacara type megafossils trace fossils small shelly fossils ssfs macroscopic algae and metazoans recovered from shallow water e c transition strata throughout south china the manifestation of geological events in deep water settings remains elusive here we present new paleontological and geochemical evidence from the 38 m thick deep water liuchapo formation ca 551 523 ma of the yangtou section northeastern guizhou province china which consists mainly of chert through the e c transition the liuchapo formation can be divided into two members based on biostratigraphy and redox proxies the lower member 0 32 m is characterized by low mo al 0 15 1 82 ppm and u al 0 47 2 51 ppm values that suggest deposition under suboxic conditions these deposits are dominated by horodyskia minor paleopascichnus jiumenensis spherical cyanobacteria myxococcoides and random acritarchs the upper member 32 38 m records a transition to anoxic non sulfidic conditions as suggested by marked increases in mo al 5 47 ppm and u al 8 810 ppm the upper member contains ssfs sponge spicules filamentous cyanobacteria multicellular algae and sheet like fossils the survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that marine redox fluctuations may limit the growth of late ediacaran organisms and provide the living space and elevated nutrients for early cambrian biota

experimental and geologic evaluation of monazite u th he thermochronometry catnip sill catalina core complex tucson az abstract monazite is a petrologically important and analytically promising target for u th he thermochronology previous studies have reported highly variable he diffusion results from monazite from a single sample and demonstrated that composition can significantly affect he diffusion parameters in this study we performed incremental heating of single monazite grains to experimentally determine the 4he diffusion properties of reference monazite 554 that occurs within a peraluminous two mica granite from the catnip sill within the catalina core complex arizona assuming that the grain size defines the diffusion geometry the six experiments yielded e a values of 212 to 238 5 kj mol 1 1 and d o values of 15 7 to 103 cm 2 s 1 with one value of 784 cm 2 s 1 monazite u th he data from five grains yielded closure temperatures of 291 to 262 c c 15 c and ages of 23 8 20 3 1 2 2 ma the weighted mean age is 21 8 0 73 mswd 1 83 n 5 and the weighted mean closure temperature is 282 6 c mswd 0 96 n 5 1 we tested the accuracy of these results by comparing our monazite thermochronology data with monazite th pb depth profiling results the 40ar 39ar thermal history for the catnip sill constrained using coexisting muscovite biotite and k feldspar and published regional zircon and apatite fission track results the monazite th pb data indicate emplacement of the catnip sill at 45 ma the 40ar 39ar muscovite and biotite data indicate cooling from 460 to 350 c from 27 to 26 ma k feldspar mdd modeling suggests cooling from 360 to 240 c from 26 to 24 ma zircon fission track data indicate cooling through 250 c between 29 and 20 ma additional cooling through 110 c is recorded by apatite fission track ages of 19 16 ma because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers our results 1 confirm the accuracy of the 4he diffusion kinetics from monazite 554 and 2 demonstrate that monazite can be an effective u th he thermochronometer provided that he diffusion properties are either measured or reasonably assumed for each sample

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

lowermost cambrian acritarchs from the yanjiahe formation south china implication for defining the base of the cambrian in the yangtze platform the asteridium heliosphaeridium comasphaeridium ahc acritarch assemblage is composed of common organic walled microfossils in the basal cambrian chert phosphorite units in south china indicating that the ahc assemblage can be a useful biostratigraphic tool for the ediacaran cambrian boundary successions in the yangtze platform to test the validity of the ahc acritarch assemblage as a biostratigraphic tool the stratigraphic range of the ahc acritarch assemblage was confirmed and its spatial and temporal relationships to other bio and chemostratigraphic tools were analysed in the yanjiahe formation yangtze gorges area south china the result shows that the ahc assemblage temporally correlates to the anabarites trisulcatus protohertzina anabarica assemblage zone and spatially correlates to the large negative carbon isotope anomaly of the lowermost cambrian bace in the yanjiahe formation this implies that the radiation of phytoplankton occurred slightly before the radiation of the small shelly fossils and the ahc acritarch assemblage can be another important chronological reference to the lowermost cambrian successions in south china and potentially to global correlations

increased productivity as a primary driver of marine anoxia in the lower cambrian abstract the relationship between metazoan evolution and ocean atmosphere oxygen levels has been extensively debated similarly there is no consensus on the factors controlling the evolution of the marine redox landscape the early cambrian is a particularly critical time interval to examine as there is a marked increase in metazoan body plan diversity and increased ecosystem complexity but few constraints on marine redox conditions during this seminal interval we present an assessment of early cambrian marine redox conditions from the luojiacun section in west hubei province the upper yangtze platform there are low trace elements enrichments u mo in the yanjiahe formation followed by an abrupt increase in the overlying shuijingtuo formation u mo systematics in the most metal enriched samples suggest deposition under a weakly restricted offshore basin which is consistent with previous studies there is correlation between excess ba baxs and total organic carbon toc suggesting a strong production control on organic matter concentration further higher ba excess in the shuijingtuo formation than the yanjiahe formation indicates dysoxic oxic conditions in the yanjiahe formation is linked to lower productivity and anoxic conditions in the shuijingtuo formation are linked to higher productivity although strong productivity controls on proterozoic and paleozoic marine redox conditions have been commonly invoked we provide some of the first empirical evidence for this control on marine redox structure more broadly we find that excess barium coupled with redox proxies may provide useful constraints on the interplay between preservation and production controls on organic matter abundances in the paleozoic successions

isotope geochemistry of oxygen and carbon in phosphate and carbonate of phosphorite francolite oxygen isotope ratios in phosphate delta sup 18 o sub p were determined in 219 samples of phosphoritic rocks from all major phosphogenic provinces of the world ranging in age from lower proterozoic to recent in 58 samples oxygen was analyzed in the two oxygen lattice sites of francolite both in phosphate and in structural carbonate delta sup 18 o sub c delta sup 13 c was also analyzed in the latter and in coexisting calcite for 10 of them both delta sup 18 o sub p and delta sup 18 o sub c decrease sharply from the recent to the cretaceous and decrease with a gentler slope to the precambrian the nature of the time variation is elucidated by means of a delta sup 18 o sub p vs delta sup 18 o sub c plot which suggests that a the main cause of the time trends is the increase in temperature between recent and the cretaceous b the pre cretaceous trend is overprinted by both low temperature nonequilibrium exchange between sup 18 o depleted water and the rocks and higher temperature exchange between phosphate and carbonate c the observed trend cannot be explained by sup 18 o depleted cretaceous and paleozoic oceans the calcite coexisting with apatite in phosphatic rocks is similar in both delta sup 18 o and delta sup 13 c to the structural carbonatemore of the apatite even though phosphate may exchange its oxygen with environmental water to a greater extent than the authors originally claimed it is probably still a very good isotopic recorder of sedimentary environments less

composition and tiering of the cambrian sponge communities abstract sponges are dominant sessile suspension feeders in the cambrian metazoan community thus sponge tiering and composition in the cambrian communities will provide important information for elucidating the evolutionary history of the epifaunal community based on more than two thousand articulated sponge specimens from the early cambrian chengjiang fauna the present study demonstrates that the chengjiang sponge community is dominated by demosponges with a well developed three level tiering consisting of 0 5 cm 5 15 cm and 15 30 cm tiers the majority of the sponges are restricted to the 0 15 cm tier suggesting rich nutrients in the bottom seawater of the cambrian ocean tiering in the cambrian sponge communities is consistent and similar to that of the ediacaran and phanerozoic epifaunal communities but the fourth tier 50 100 cm in these latter communities is absent in the cambrian communities demosponges dominate the shallow water sponge communities and occur in all tiers hexactinellids are relatively rare and limited in the lower tiers in the shallow water community but dominate in the deep water communities the composition variation in the cambrian sponge communities may be controlled by the differences in skeleton architecture between the demospongiae and hexactinellida

diffusion obstacles and opportunities in petrochronology many of the approaches in petrochronology are rooted in the assumption of equilibrium diffusion is an expression of disequilibrium the movement of mass in response to chemical potential gradients and isotopes in response to isotopic gradients it is extremely important that we be aware of how the effects of diffusion can place obstacles across our path towards petrochronologic enlightenment conversely the effects of diffusion also provide opportunities for understanding rates processes and conditions experienced by rocks the enormity of the field does not permit us to provide a comprehensive review of either the mathematics of diffusion or quantitative data that have been obtained relevant to the interpretation of diffusive processes in rocks and minerals many resources cover these topics including rimg volume 72 diffusion in minerals and melts zhang and cherniak 2010 and several textbooks crank 1975 glicksman 2000 particularly relevant to the discussion of petrochronology are summaries of the theory and controls on diffusion brady and cherniak 2010 zhang 2010 as well as diffusion rates in feldspar cherniak 2010a accessory minerals cherniak 2010b garnet ganguly 2010 mica pyroxene and amphibole cherniak and dimanov 2010 and melts zhang and ni 2010 zhang et al 2010 rather than duplicate that material our goal is to explore the obstacles and opportunities presented by the effects of diffusion as they inform the rates of petrologic processes to achieve this goal we emphasize key principles and illustrative examples quantitative interpretation of the effects of diffusion assumes predictability of numerous factors that may affect chemical or isotopic transport including temperature initial and boundary conditions water and oxygen fugacities activities of other components multiple mechanisms of diffusion and crystal chemistry coupling of the substitution of elements into different crystallographic sites additionally the extraction of meaningful ages durations of events and temperatures requires

terreneuvian small shelly faunas of east yunnan south china and their biostratigraphic implications the study discusses the detailed composition of terreneuvian small shelly fossil ssf assemblages of eastern yunnan province meishucun xianfeng and huize regions the described sections represent shallow water deposits of the yangtze platform that include a number of stratigraphical hiati the reported ssf assemblages can be assigned to the regional ssf biozones i iii anabarites trisulcatus protohertzina anabarica assemblage zone paragloborilus subglobosus purella squamulosa assemblage zone watsonella crosbyi assemblage zone zones ii and iii are slightly redefined based on new data two subzones of zone ii are proposed here namely annelitellus yangtzensis obtusoconus honorabilis subzone and maikhanella cambriana oelandiella korobkovi subzone claviconchella qianyii gen et sp nov and bubiites simplex gen et sp nov are reported here we also provide a critical revision of the ssf distribution for the well known meishucun section the correlation scheme for east yunnan documents the extent and duration of stratigraphical hiati the distribution of ssfs and the lithological characterization indicate three major events of exposure of the carbonate platform resulting in sedimentary breaks and possible karstification this prompts new consideration of the previously described stable isotope stratigraphy of shallow platform strata considering the complexity of the terreneuvian earth history as well as the uniqueness of ssf an integration of sedimentology geochemistry and paleontology is needed for future ssf study

u pb geochronology and trace element composition of zircon from the horseshoe lake greenstone belt superior province canada implications for the tectonic and metamorphic history1 depth profile techniques for u pb geochronology and rare earth element ree geochemistry were conducted on unpolished archean zircon with metasomatic rims from metasedimentary rocks within the horseshoe lake greenstone belt western superior province canada these zircon crystals are shown to have isotopically distinct rims typically 100 million years younger than the cores the 207pb 206pb rim ages can be temporally correlated with regional magmatism and with a later potentially au bearing hydrothermal event synchronous with greenschist facies metamorphism and regional deformation notably the zircons do not record evidence of local ca 2741 2715 ma magmatism manifested by the emplacement of a quartz feldspar porphyry dike swarm laser ablation inductively coupled plasma mass

germanium silicon of the ediacaran cambrian laobao cherts implications for the bedded chert formation and paleoenvironment interpretations sedimentary strata of the terminal ediacaran 635 542 ma to early cambrian 542 488 ma laobao liuchapo bedded cherts in the south china block include the ediacaran oxidation event and the cambrian explosion understanding the origin and depositional environment of the bedded cherts may provide insight into how the earth s surface environment changed between the proterozoic and phanerozoic we measured major and trace element compositions and ge si ratios of the laobao cherts from northern guangxi province the laobao cherts were deposited in the deep basinal environment of the south china block we show that the composition of the laobao cherts is determined by a mixture of four components quartz clay carbonate and pyrite iron oxide the quartz component is the dominant component of the laobao cherts the maximum estimated ge si of the quartz component is between 0 4 and 0 5 mol mol which is close to the ge si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids these ge si systematics suggest that normal seawater rather than mid ocean ridge hydrothermal fluids is the primary si source for the laobao cherts the ge si of the clay component varies between 1 and 10 mol mol which is comparable to the ge si of typical marine clays but 10 100 times lower than that of chert nodules from early ediacaran beds the doushantuo formation predating the terminal ediacaran laobao cherts studied here our observations indicate that the clay component ge si ratio decreased from the early ediacaran to the late ediacaran we speculate that high ge si ratios in clays reflect the preferential chelation of ge by dissolved organic compounds adsorbed onto clays if so this suggests that the decrease in ge si ratio of the clay component in the ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the ediacaran cambrian transition consistent with oxidation of the oceans during the late ediacaran finally the seawater origin of the laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation instead direct precipitation from seawater or early diagenetic silicification of calcareous sediments perhaps due to the emergence of si accumulation bacteria may have been responsible for the bedded laobao liuchapo chert formation in south china block

biostratigraphy of the vendian cambrian beds and the lower cambrian boundary in siberia general agreement exists that the position of the base of the cambrian precambrian cambrian boundary should coincide with a biostratigraphic marker that can be widely traced and is linked to the evolution of the small shelly fauna on the siberian platform the presence of distinct facies during the vendian cambrian interval has made biostratigraphic correlation based on palaeontological data highly contentious a solution to this problem appears to exist with the recognition of an almost uninterrupted profile in the aldan uchur watershed where key sections expose the eastern and transitional facial region on this basis we present a general biostratigraphic scheme for the vendian cambrian across the southern siberian platform that connects separate facies further correlation with key sections in northern siberia leads to the recognition of three biostratigraphic zones for the vendian cambrian interval these are anabarites trisulcatus zone purella antiqua zone aldanocyathus sunnaginicus zone the abundance of chinese type small shelly fossils in the region of the eastern facies suggest correlation of the a sunnaginicus zone with the siphogonuchites paragloborilus zone of china and the p antiqua zone with the circotheca anabarites protohertzina zone in places at the base of the circotheca anabarites protohertzina zone in china analogies to the a trisulcatus zone can be identified it is proposed that in siberia the base of the a sunnaginicus zone base of tommotian stage be taken as the base of the cambrian this would correspond with marker b of the meishucunian in china