re os geochronology of the lacustrine green river formation insights into direct depositional dating of lacustrine successions re os systematics and paleocontinental weathering abstract lacustrine sedimentary successions provide exceptionally high resolution records of continental geological processes responding to tectonic climatic and magmatic influences these successions are therefore essential for correlating geological and climatic phenomena across continents and furthermore the globe producing accurate geochronological frameworks within lacustrine strata is challenging because the stratigraphy is often bereft of biostratigraphy and directly dateable tuff horizons the rhenium osmium re os geochronometer is a well established tool for determining precise and accurate depositional ages of marine organic rich rocks lake systems with stratified water columns are predisposed to the preservation of organic rich rocks and thus should permit direct re os geochronology of lacustrine strata we present re os systematics from one of the world s best documented lacustrine systems the eocene green river formation providing accurate re os depositional dates that are supported by ar ar and u pb ages of intercalated tuff horizons precision of the green river formation re os dates is controlled by the variation in initial 187 os 188 os and the range of 187 re 188 os ratios as also documented in marine systems controls on uptake and fractionation of re and os are considered to relate mainly to depositional setting and the type of organic matter deposited with the need to further understand the chelating precursors of re and os in organic matter highlighted in addition to geochronology the re os data records the 187 os 188 os composition of lake water 1 41 1 54 at the time of deposition giving an insight into continental runoff derived from weathering of the geological hinterland of the green river formation such insights enable us to evaluate fluctuations in continental climatic tectonic and magmatic processes and provide the ability for chemostratigraphic correlation combined with direct depositional dates furthermore initial 187 os 188 os values can be used as a diagnostic tool to distinguish between lacustrine and marine depositional settings when compared to known oceanic 187 os 188 os values

re os geochronology of the hitachi volcanogenic massive sulfide deposit the oldest ore deposit in japan the hitachi volcanogenic massive sulfide vms deposit in the hitachi belt is one of the largest cu zn sulfide deposits in japan however its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems e g k ar ages on wall rocks further metamorphism has greatly reduced the fossil record which has limited the use of biostratigraphy to establish the ore deposit age we applied re os isotope dating to samples from two representative sulfide deposits of the greater hitachi vms deposit the fudotaki and fujimi deposits re os isotope data from the fudotaki deposit yields a model 3 age of 533 13 ma with an initial 187 os 188 os ratio of 0 35 0 20 in contrast re os isotope data from the fujimi sulfide ore did not exhibit a linear correlation suggesting that the re os isotope system there has been disturbed by regional metamorphism of higher grade than the fudotaki deposit as well as contact metamorphism during intrusion of cretaceous granitic rocks although the fudotaki sulfide ores have been metamorphosed to lower epidote amphibolite facies the well defined re os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor some orebodies of the greater hitachi vms deposit have been estimated to be early carboniferous based on the scarce fossil records and considered to be distributed in the daioin formation but our results revealed that the fudotaki deposit were formed during the cambrian and occurs in the upper akazawa formation which strongly supports the idea that the cambrian beds are widely distributed in the hitachi belt and that a significant unconformity exists between the cambrian and early carboniferous above the fudotaki deposit the cambrian re os date classifies the hitachi vms deposit as the oldest dated ore deposit in japan and in turn provides an insight to understand the geologic framework and genesis of the proto japanese islands and the east asian region

marine os isotopic fluctuations in the early eocene greenhouse interval as recorded by metalliferous umbers from a tertiary ophiolite in japan abstract the mineoka umbers occur within the tertiary accretionary complex of central japan immediately overlying 53 7 ma tholeiitic basalts the associated tholeiitic basalts exhibit typical normal mid ocean ridge basalt n morb type geochemical signatures and are interpreted to be a remnant of tertiary oceanic crust centimeter resolution bulk rock chemical analyses were performed to cover the entire section of the 35 cm thick umber deposit there are striking similarities in major element trace element re and os contents between the umbers and modern metalliferous sediments along the east pacific rise these results demonstrate that the mineoka umbers are ancient counterparts of mor metalliferous sediments that are plume fall out ferromanganese precipitates scavenging various seawater derived elements including p v y rees and os and confirm that the umbers subaerially exposed in the accretionary complex generally preserve their primary geochemical signatures at the time of deposition the marine 187 os 188 os ratios inferred from the mineoka umbers gradually increase up section with moderate fluctuations the 187 os 188 os composition ranging from 0 47 to 0 54 is significantly higher than that of the prior period post paleocene eocene thermal maximum petm 187 os 188 os 0 38 this was probably the result of an increase in the chemical weathering rate of continental crust caused by the long duration of noticeably warm climate during the early eocene greenhouse interval several fluctuations observed in the marine 187 os 188 os record during this interval are most likely related to petm like brief hyperthermals supporting the suggestion that the intensity of continental crustal weathering controlled by climatic changes can fluctuate on timescales of 10 5 yr

antigorite induced seismic anisotropy and implications for deformation in subduction zones and the tibetan plateau the present study which is a follow up of the journal of geophysical research paper by ji et al 2013a provides a new calibration for both seismic and fabric properties of antigorite serpentinites comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high t serpentinite which is essentially controlled by the antigorite c axis fabric is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock extrapolation of the experimental data with both pressure and temperature suggests that vp anisotropy decreases but shear wave splitting vs and vp vs increase with increasing pressure in either cold or hot subduction zones for a cold steeply subducting slab antigorite is most likely deformed by nearly coaxial flattening or trench parallel movements forming trench parallel seismic anisotropy for a hot shallowly subducting slab however antigorite is most likely deformed by simple shear or transpression trench normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30 the geophysical characteristics of the tibetan plateau such as strong heterogeneity in vp vs and attenuation shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks hydrated zones of subducting lithospheric mantle and the crust mantle boundary if the temperature is below 700 c in the region of interest

geochemistry of metalliferous hydrothermal deposits in the aeolian arc tyrrhenian sea abstract in shallow water areas of the submerged volcanic complex around the island of panarea aeolian archipelago hydrothermal precipitation of both low temperature fe oxyhydroxide rich red muds and crusts and high temperature sediment hosted massive sulfides was discovered during an integrated high resolution survey iron rich crusts were also found on the bathymetric high of secca del capo north of salina island the exhalative iron rich sediments occur in small closed depressions or in proximity to faults and scarps at water depths ranging from 55 to 285 m the principal chemical characteristics of these deposits are high but variable fe content ranging from 12 2 to 45 and low contents of the transition elements mn cu zn ni and co the low contents of cu ni and co suggest a hydrothermal origin the fe oxyhydroxide deposits are enriched in light rare earth element ree 35 110 chondrite compared to heavy ree 10 25 chondrite their ree patterns are similar to those of associated calc alkaline volcanics negative slope of light ree and a horizontal distribution of the heavy ones this contrasts with the pattern for iron and manganese oxides of hydrogenous origin which have tilted trends of heavy ree paralleling that of seawater the mineralogy of the polymetallic sulfide deposits consists of galena sphalerite pyrite and barite in the form of silt sand grains and decimeter sized fragments disseminated in clay 30 cm below the seabed at a waterdepth of 80 m the chemistry of the aeolian iron rich low temperature deposits and of the high temperature ba rich pb and zn sulfides suggests that they are genetically analogous to respectively the kuroko type iron formation tetsusekiei and the black ore exposed in the miocene hokuroku district of japan

ages and geodynamic settings of xilamulun mo cu metallogenic belt in the northern part of the north china craton abstract the xilamulun belt along the northern part of the north china craton is located in eastern segment of the central asian orogenic belt and has great economic potential for mo cu mineralization more than ten medium to large ore deposits have been discovered in this region in the recent years the major types of mineralization type include porphyry chehugou mo cu kulitou mo cu xiaodonggou mo and jiguanshan mo quartz vein nianzigou mo xinjing mo epithermal hongshanzi mo u and alteration assemblage liulingou mo the timing of mineralization was previously thought to be yanshanian 208 290 ma however indosinian 260 208 ma ages for intrusions and mineralization have been recognized in recent years based on geochronologic data and regional geological evidence it is suggested that the mineralization in the xilamulun belt was formed during multiple events the mineralization processes are related to a post collisional extension stage 258 210 ma with the generation of the porphyry molybdenum copper deposit a tectonic stress transformation from ns to ew 185 150 ma that gave rise to vein or porphyry molybdenum deposit and a lithospheric thinning stage 140 110 ma with porphyry molybdenum deposit

high resolution records of the late paleocene thermal maximum and circum caribbean volcanism is there a causal link two recently drilled caribbean sites contain expanded sedimentary records of the late paleocene thermal maximum a dramatic global warming event that occurred at ca 55 ma the records document significant environmental changes including deep water oxygen deficiency and a mass extinction of deep sea fauna intertwined with evidence for a major episode of explosive volcanism we postulate that this volcanism initiated a reordering of ocean circulation that resulted in rapid global warming and dramatic changes in the earth s environment

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

deformation and melt transport in a highly depleted peridotite massif from the canadian cordillera implications to seismic anisotropy above subduction zones abstract seismic anisotropy in subduction zones results from a combination of various processes although it depends primarily on the orientation of olivine in response to flow the presence of water and melt in the wedge may modify the deformation of olivine the melt distribution also influences anisotropy direct observations of the deformation and melt rock interactions in a strongly depleted spinel harzburgite massif from the cache creek terrane in the canadian cordillera allow evaluating the relative contribution of each process structural mapping shows that this massif has recorded high temperature low stress deformation high degrees of partial melting and synkinematic melt rock interaction at shallow depths

seismic velocities anisotropy and shear wave splitting of antigorite serpentinites and tectonic implications for subduction zones 1 antigorite the high temperature ht form of serpentinite is believed to play a critical role in various geological processes of subduction zones we have measured p and s wave velocities vp and vs anisotropy and shear wave splitting of 17 serpentinite samples containing 90 antigorite at pressures up to 650 mpa the new results combined with data for low temperature lt lizardite and or chrysolite reveal distinct effects of lt and ht serpentinization on the seismic properties of mantle rocks at 600 mpa vp 5 10 and 6 68 km s vs 2 32 and 3 67 km s and vp vs 2 15 and 1 81 for pure lt and ht serpentinites respectively above the crack closure pressure 150 mpa the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 c should be at least twice that of previous estimates based on lt serpentinization the presence of seismic anisotropy high pressure fluids or partial melt is also needed to interpret ht serpentinized mantle with vp 1 81 the intrinsic anisotropy of the serpentinites 3 8 16 9 with an average value of 10 5 for vp and 3 6 18 3 with an average value of 10 4 for vs is caused by dislocation creep induced lattice preferred orientation of antigorite three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics s l and ls tectonites formed by three categories of strain geometry i e coaxial flattening coaxial constriction and simple shear respectively our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide

chemostratigraphic evidence of deccan volcanism from the marine osmium isotope record continental flood basalt cfb volcanism is hypothesized to have played a causative role in global climate change and mass extinctions uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of cfb volcanism our results document a 25 decline in the marine 187 os 188 os record that predates the cretaceous tertiary boundary ktb and coincides with late maastrichtian warming we argue that this decline provides a chemostratigraphic marker of deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of deccan volcanism was a transient global warming event of 3 to 5 c that is fully resolved from the ktb mass extinction

distribution and mineral assemblages of bedded manganese deposits in shikoku southwest japan implications for accretion tectonics abstract numerous bedded manganese and manganese iron ore deposits occur in shikoku southwest japan the mineralogy wall rock characteristics and fossil age data of the deposits are synthesized in this paper their mineral assemblages correlate well with the progressive metamorphic grade of the accretionary complexes in shikoku based on the plate tectonic framework of subduction accretion process the deposits are considered to have formed by metamorphism of mn fe nodule bearing siliceous sediments on the ocean floor

basaltic explosive volcanism but no comet impact at the paleocene eocene boundary high resolution chemical and isotopic records from egypt spain and denmark in the search for a triggering mechanism for the paleocene eocene p e boundary event 55 ma centimeter resolution chemical e g ir os pt and isotope e g os o 187 os 188 he 3 he 4 sr 87 sr 86 records across this boundary have been established for six uplifted marine sections in egypt spain and denmark the sections studied represent some of the stratigraphically most complete records across the onset of the carbon isotopic excursion cie and associated benthic foraminifera extinctions that mark the paleocene eocene boundary high sensitivity analyses failed to uncover evidence of extraterrestrial element or isotope enrichments in the six sections refuting the hypothesis of a major comet impact at the boundary preliminary searches for other impact indicative features such as spherules or shocked quartz also gave negative results in the danish section studied three basaltic ir rich ash layers occur at the paleocene eocene boundary but no similar ashes were found in egypt or spain the three ashes represent the earliest known manifestation of an unusual i to 2 million year long phase of explosive basaltic volcanism in the faero greenland region this volcanism is synchronous with major flood basalt effusions in east greenland and is associated with prominent paleogeographic changes in the high latitude north atlantic region discharge of mantle derived os to seawater during this volcanism may explain a small decrease in os o 187 os o 188 ratio at the paleocene eocene boundary in the zumaya section in spain the environmental perturbations at the paleocene eocene boundary appear to have been triggered by basaltic volcanism but any model for the detailed causal relation remains speculative c 2004 elsevier b v all rights reserved less

seismic properties of subducting oceanic crust constraints from natural lawsonite bearing blueschist and eclogite in sivrihisar massif turkey abstract investigating the seismic properties of natural lawsonite lws bearing blueschist and eclogite is particularly important for constraining the seismic interpretation of subducting oceanic crust based on seismological observations to achieve this end we analyzed in detail the mineral fabrics and seismic properties of foliated lws blueschist and lws eclogites from sivrihisar massif in turkey in both blueschists and eclogites the lawsonite fabric is characterized by three different patterns 0 0 1 axes aligning sub normal to foliation and 0 1 0 axes aligning sub parallel to lineation normal type 0 0 1 axes aligning sub parallel to lineation and 1 0 0 axes aligning sub normal to foliation with a girdle sub normal to lineation abnormal type and 0 0 1 axes aligning both sub normal to foliation and sub parallel to lineation 0 1 0 axes aligning sub parallel to lineation and 1 0 0 axes aligning sub normal to foliation transitional pattern in contrast glaucophane and omphacite mostly present consistent axial fabrics with the 0 0 1 axes aligning to lineation these mineral fabrics produce whole rock seismic anisotropies with similar patterns however the variations in seismic anisotropies are mainly controlled by the rock type to a lesser extent are determined by the lawsonite fabric type and to only a small extent are affected by mineral fabric strength despite the constructive abnormal type lawsonite fabric on whole rock seismic anisotropies because of their weaker mineral fabric strength or deformation degree the abnormal type lws blueschist still exhibit comparatively lower seismic anisotropies than those normal type lws blueschist from other localities based on the calculated seismic anisotropies and velocities we estimated that when oceanic crust transforms from lws blueschist to lws eclogite with increasing subduction depth 1 p wave and max s wave polarization anisotropies reduce about 70 and 40 respectively and 2 variations of vp and vs contrasts relative to mantle peridotites are about 7 to 3 and 8 to 6 respectively these results corroborate the important roles of lws bearing blueschist and eclogite in interpreting the existence and gradual weakening of low velocity layers in subducting oceanic crust during the subduction process

the ronda peridotite spain a natural template for seismic anisotropy in subduction wedges the ronda peridotite spain a natural template for seismic anisotropy in subduction wedges

late quaternary hydrothermal sedimentation adjacent to the central lau spreading center the partition geochemistry of 10 sediment cores collected adjacent to the southward propagating central lau spreading center clsc has been studied the hydrothermal oxide component in these sediments is mainly formed of mn oxides containing other hydrothermally associated elements e g fe zn cu mo and pb the mn oxides represent distal plume fall out derived from hydrothermal vents hydrothermal mn accumulation rates are between 2 6 and 74 4 mg cm 2 ka 1 during the 0 30 ka period and between 2 1 and 12 8 mg cm 2 ka 1 during the 30 120 ka period recent 30 ka hydrothermal input to the sediments adjacent to the clsc has been more variable than in the older sediments from the area this may be attributed to localised hydrothermal activity occurring behind the propagator and possibly to magma chamber development periods of enhanced hydrothermal input from the clsc at ca 30 ka and 175 ka appear to be contemporaneous along the clsc however an enrichment in mn at ca 112 ka probably represents distal hydrothermal plume fall out from increased activity on the valu fa ridge in the southern lau basin hydrothermal mn input derived from the clsc is below that from the vfr

geochemistry of the margi umbers cyprus and the os isotope composition of cretaceous seawater the major element rare earth element and platinum group element chemistry of the margi umbers have been investigated in order to evaluate these sediments as recorders of past variations in the os isotopic composition of seawater because of the compositional similarity of the margi umbers to metalliferous sediments and hydrothermal plume particles from the modern ocean we conclude that these supraophiolite sediments record the os isotope composition of cretaceous seawater this conclusion requires that the margi umbers have in large part retained their compositional integrity throughout burial diagenesis and subsequent uplift that the 187 os 188 os ratio 0 52 0 55 of ca 90 ma seawater is similar to or more radiogenic than seawater throughout the paleogene provides evidence of decoupling of the marine sr and os isotope records beyond constraining the 187 os 188 os of cretaceous seawater these results are significant in that they provide a set of chemical criteria that can be used to evaluate the potential of other metalliferous sediment sequences as recorders of the 187 os 188 os ratio of ancient seawater establishing these criteria represents an important step toward extending the marine os isotope record into the mesozoic and paleozoic

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode oceanic anoxic events oaes were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen deficient bottom waters oae2 occurring at the cenomanian turonian boundary about 93 5 myr ago is the most widespread and best defined oae of the mid cretaceous although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios the actual trigger mechanism corresponding closely to the onset of these episodes of increased carbon sequestration has not been clearly identified it has been postulated that large scale magmatic activity initially triggered oae2 refs 4 5 but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of oae2 has not yet been found here we report seawater osmium isotope ratios in organic rich sediments from two distant sites we find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of oae2 using a simple two component mixing equation we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin a approximately 30 50 fold increase relative to pre oae conditions furthermore the magmatic osmium isotope signal appears slightly before the oae2 as indicated by carbon isotope ratios suggesting a time lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial which may reflect the reaction time of the global ocean system our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of oae2 which may have triggered the subsequent deposition of large amounts of organic matter

variations of the187os 186os ratio of seawater over the past 28 million years as inferred from metalliferous carbonates abstract the os concentration and isotopic composition of metalliferous carbonates deposited on the east pacific rise over the past 28 ma are reported with complimentary sr isotope data variations in the os isotopic composition of these samples are interpreted as a record of past changes in the os isotopic composition of seawater these results are consistent with isotopic analyses of leachable os in pelagic clays which have also been interpreted as a record of the187os 186os ratio of seawater through time 1 the metalliferous carbonate record clearly shows that seawater os and sr isotope systems are partially decoupled from one another over the past 28 ma accelerated weathering of ancient organic rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the187os 186os ratio of seawater over the past 15 ma this rapid increase suggests that the seawater os record can potentially be used as a stratigraphic tool in some neogene marine deposits

interelement relationships in ferromanganese crusts from the central pacific ocean their implications for crust genesis the fine scale chemical variations in ferromanganese fe mn crusts from the central pacific ocean were investigated using a variety of techniques in order to elucidate their genesis thirty seven major minor and trace elements were determined by icp oes and icp ms in sub samples collected at approximately 2 mm intervals throughout the crust sequences an inverse relationship is observed between the logarithm of the elemental distribution coefficients log cmncsw and the mean residence time tr of elements in seawater these associations provide evidence supporting the strong link between seawater and fe mn crust chemistry results of factor analysis reveal that elements in crusts can generally be partitioned into four major groups with selected elements displaying associations with several groups these statistical associations are interpreted to represent hydrogenetic mn ni co cu ce zn biogenic ba zn cu pt fe detrital al si fe and carbonate fluorapatite ca p ree groups a comparison between elemental associations in fine scale data from individual crust sequences and bulk compositional data from samples collected in separate locations reveals very similar elemental associations the similar elemental associations in bulk samples and in fine scale subsamples from individual crusts are consistent with the notions that hydrogenetic crusts exhibit no significant post depositional remobilization of elements and that elemental distributions within crusts reflect the chemistry of oceanic water during the period in which they accreted chemical paleotracks are proposed based on factor scores derived from chemical variations as a function of depth in individual crusts our results show that there is internal consistency between the chemical evolution previously proposed seamount paleotracks and nanofossil biostratigraphy dating results

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

natural examples of olivine lattice preferred orientation patterns with a flow normal a axis maximum tectonic plate motion is thought to cause solid state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals the mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction this has been explained by the existence of an olivine preferred orientation with an a axis maximum parallel to the induced mantle flow direction in subduction zones however the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion which has usually been ascribed to localized complex mantle flow patterns recent experimental work suggests an alternative explanation under conditions of high water activity a b type olivine preferred orientation may form with the a axis maximum perpendicular to the flow direction natural examples of such b type preferred orientation are however almost entirely unknown here we document widespread b type olivine preferred orientation patterns from a subduction type metamorphic belt in southwest japan and show that these patterns developed in the presence of water our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought

seismic anisotropy and implications for mantle deformation beneath the ne margin of the tibet plateau and ordos plateau the northeastern tibet and the ordos plateau connects east china and west china which are dominated by horizontal extension and compression respectively knowledge of seismic anisotropy beneath this transitional region can provide important constraints on deformation pattern of the crust and lithosphere mantle during an orogeny process we measured sks wave splitting parameters from the new installed regional networks and obtained a detailed anisotropy map for 119 broadband stations beneath the ne margin of tibet the observed wnw fast polarization direction is parallel to the surface geological features the coherence between the observed geodetic motion of the crust and fast axis direction suggests that the vertical coherent deformation of the lithosphere is the dominate source for the observed seismic anisotropy further east to the qinling orogenic belt the fast axis direction tracks change in strike of the faults with the fast direction aligning almost ew we suggest that both the lithosphere mantle and east extrusion of mantle material contribute to the observed anisotropy small value of delay time is observed interior the thick rooted ordos plateau suggesting a rigid and stable lithosphere with little deformation the deviation of fast polarization direction beneath the adjacent rifting regions might be caused by the edge flow induced by sharp changes in the thickness of lithosphere our results propose that the upper crust and mantle lithosphere beneath the northeastern tibet are at least partly coupled or subjected to the same boundary conditions which is inconsistent with the channelized lower crustal flow model

marine osmium isotope record across the triassic jurassic boundary from a pacific pelagic site the triassic jurassic t j boundary ca 200 ma represents one of the major mass extinction events of the phanerozoic however the cause of this event remains controversial because of a paucity of geological evidence in this study we present an isotopic record of osmium extracted from a bedded chert succession across the t j boundary in the kurusu section of japan deposited within a paleo pacific panthalassa deep basin the data show a gradual decrease in seawater 187 os 188 os values during the rhaetian followed by a sharp increase in the latest rhaetian and a subsequent stable phase across the t j boundary the decreasing trend of 187 os 188 os values during the rhaetian indicates a gradual increase in the relative supply rate of unradiogenic os from the mantle associated with emplacement of the central atlantic magmatic province the subsequent shift toward radiogenic values reflects an increased supply of radiogenic os due to enhanced continental weathering this interval marks more negative isotopic values of organic carbon the onset of radiolarian faunal turnover and conodont extinctions indicating that the rapid increase in continental weathering rate was closely linked to the perturbation of the carbon cycle and the t j biotic crisis

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

effect of temperature on the b to c type olivine fabric transition and implication for flow pattern in subduction zones abstract the effect of temperature on the b to c type fabric transition in olivine was investigated by large strain shear deformation experiments a new series of deformation experiments was conducted at 1273 1373 k 2 0 gpa and strain rates ranging from 3 8 10 4 to 3 2 10 5 s 1 under water saturated conditions and combined with previous results at 1473 k the influence of temperature on deformation fabrics was investigated two types of olivine fabrics were identified one in which the olivine 0 0 1 axis is subparallel to the shear direction and the 0 1 0 plane is parallel to the shear plane b type and another one in which the 0 0 1 axis is parallel to the shear direction and the 1 0 0 plane is parallel to the shear plane c type the b type olivine fabric occurs at higher stress than the c type but the stress magnitude at which the b to c type fabric transition occurs decreases significantly with decreasing temperature the temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study when the experimental results are extrapolated to lower temperatures the fabric transition between b and c type is predicted to occur at 1 mpa and 1000 k this prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate convergent regions the present study indicates that when some amounts of water 200 ppm h si are present the b type fabric will dominate in low temperature regions of the upper mantle whereas the c type or e type fabric will dominate at high temperatures the complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables such as temperature and water content rather than by complexities in mantle wedge flow pattern

microstructural and seismic properties of the upper mantle underneath a rifted continental terrane baja california an example of sub crustal mechanical asthenosphere the gulf of california rift is a young and active plate boundary that links the san andreas strike slip fault system in california to the oceanic spreading system of the east pacific rise the xenolith bearing lavas of the san quintin volcanic area provide lower crust and upper mantle samples from beneath baja california peninsula the microstructures crystallographic preferred orientations cpo and petrology of the san quintin xenoliths suggest that the continental lithosphere in this region has undergone several stages of deformation recrystallisation and melt rock interaction melt rock interactions have led to enrichment in olivine while fine grained microstructures suggest intense deformation in an active shear zone in the shallow upper mantle in this study we highlight the effect of the fine grained mylonitic shear zone development in the upper mantle as an important process of weakening of continental lithosphere the results of the microstructural study show a reduction in cpo strength with increasing grain size reduction most cpos are consistent with dominant slip on the 0kl 100 system as a consequence corresponding seismic anisotropies decrease for both p and s waves with increasing grain size reduction the shallow crystallographic fabric can be related to active shear zones which accommodate the relative motion between the northern baja terrane and the pacific plate estimates of the strain rate stress and viscosity indicate that the shallow mantle beneath northern baja is thermally and chemically lithospheric but mechanically has similar viscosity as the asthenosphere the northern baja terrane is an interesting case of continental crust lying directly on low viscosity upper mantle 2012 elsevier b v all rights reserved

miocene seawater 187os 188os ratios inferred from metalliferous carbonates abstract seawater 187os 188os ratios for the middle miocene were reconstructed by measuring the 187os 188os ratios of metalliferous carbonates from the pacific dsdp 598 and atlantic dsdp 521 oceans atlantic and pacific 187os 188os measurements are nearly indistinguishable and are consistent with previously published os isotope records from pacific cores the atlantic data reported here provide the first direct evidence that the long term sedimentary 187os 188os record reflects whole ocean changes in the os isotopic composition of seawater the pacific and the atlantic os measurements confirm a long term 0 01 myr increase in marine 187os 188os ratios that began no later than 16 ma the beginning of the os isotopic increase coincided with a decrease in the rate of increase of marine 87sr 86sr ratios at 16 ma a large increase of 1 in benthic foraminiferal 18o values interpreted to reflect global cooling and ice sheet growth began approximately 1 million years later at 14 8 ma and the long term shift toward lower bulk carbonate 13c values began more than 2 myr later around 13 6 ma the post 16 ma increase in marine 187os 188os ratios was most likely forced by weathering of radiogenic materials either old sediments or sialic crust with a sedimentary protolith we consider two possible miocene specific geologic events that can account for both this increase in marine 187os 188os ratios and also nearly constant 87sr 86sr ratios 1 the first glacial erosion of sediment covered cratons in the northern hemisphere 2 the exhumation of the australian passive margin new guinea arc system the latter event offers a mechanism via enhanced availability of soluble ca and mg silicates in the arc terrane for the maintenance of assumed low co2 levels after 15 ma the temporal resolution three samples myr of the 187os 188os record from site 598 for which a stable isotope stratigraphy was also constructed is significantly higher than that of previously published records these high resolution data suggest oscillations with amplitudes of 0 01 to 0 02 and periods of around 1 myr although variations in the 187os 188os record of this magnitude can be easily resolved analytically this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent however the short term 187os 188os variations may correlate inversely with short term benthic foraminiferal 18o and bulk carbonate 13c variations that reflect glacioeustatic events

recognition of the latest permian to early triassic cu mo mineralization on the northern margin of the north china block and its geological significance abstract in this paper we present newly obtained molybdenite re os ages for the chehugou copper molybdenum deposit and the yuanbaoshan molybdenum mineral locality from the chifeng area on the northern margin of the north china block combined with u pb zircon ages of the host granites the re os isochron age of six molybdenite samples from the chehugou deposit is 257 5 2 5 ma and one molybdenite sample from the yuanbaoshan locality yields a re os model age of 248 0 2 7 ma these ages clearly indicate the signature latest permian to early triassic cu mo or mo mineralization that has not been previously recognized zircon la icp ms u pb dating shows that the host granites of the chehugou deposit and yuanbaoshan locality formed in the late devonian 376 3 ma and late permian 269 3 ma respectively and is not of jurassic age as previously thought the latest permian to early triassic cu mo mineralization is likely related to extension due to final closure of the paleo asian ocean and amalgamation of the mongolian arc terranes with the northern north china block the establishment of a latest permian to early triassic age for mineralization should encourage renewed exploration aimed at the discovery of new mo or cu mo resources along the northern margin of the north china block

deposition of osmium and other platinum group elements beneath the ultramafic hosted rainbow hydrothermal plume osmium and other platinum group elements pges have been measured in sediments accumulating directly under the rainbow hydrothermal plume at 36 deg n on the mid atlantic ridge 187os 188os and pge concentrations in the sediments of four cores taken 2 25 km from the active vent site reveal evidence of both radiogenic os scavenged from seawater and importantly unradiogenic os released by hydrothermal venting the majority of analyses n 16 yield 187os 188os ratios that fall in a narrow range 0 99 1 06 close to that of modern seawater in these samples os concentrations correlate linearly with hydrothermal v enrichments which are also derived from seawater scavenging a smaller subset of samples characterized by unusually high concentrations of ni and cr derived from ultramafic rock fragments exhibit distinctly lower 187os 188os ratios their ir content is also higher and their pge ratios are more similar to mantle material than the pge patterns of the larger set of samples dominated by seawater scavenged os although the two most proximal cores lack evidence for any similar ultramafic detritus in their surficial sediments 187os 188os ratios in these samples are also lower than that of ambient seawater suggesting they are likely to have been influenced by unradiogenic os from vent fluid incorporated into the plume samples from the most distal core by contrast exhibit 187os 188os ratios indistinguishable from that of seawater compared with background ne atlantic sediments os is enriched four fold in the seawater dominated samples while pd pt and ir are twice the background value burial fluxes of os scavenged by hydrothermal fe in these sediments are 200 400 pg cm 2 kyr 1 up to twice that found on the epr and several times the background bg of 50 pg cm 2 kyr 1 for pt burial fluxes are 1800 3000 pg cm 2 kyr 1 bg ca 1000 pg cm 2 kyr 1 and for pd 2700 4600 pg cm 2 kyr 1 bg ca 800 pg cm 2 kyr 1

the marine osmium isotope record over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research several analytical developments particularly in the past few years have significantly increased our ability to study the behaviour of osmium in the surficial environment the 187os 188os and osmium concentration of seawater river water rain and hydrothermal vent fluids have been measured directly recently the behaviour of osmium in estuaries critical for estimating the marine residence time of osmium has been studied our knowledge of the surficial osmium cycle has thus significantly improved in addition reconstructions of past variations in the marine 187os 188os recently have been extended back into the mesozoic this review attempts to summarize our current understanding of the marine osmium system present and past the 187os 188os of seawater during the cenozoic to first order mimics the marine 87sr 86sr record it is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic tectonic processes however the marine osmium isotope system differs fundamentally from the marine strontium isotope system this review emphasizes three important differences first large impacts are capable of resetting the 187os 188os to unradiogenic values without significantly affecting the marine strontium system second organic rich sediments are characterized by high 187re 188os resulting 187os 188os ingrowth trajectories are similar to the average slope of the cenozoic 187os 188os seawater record trends towards more radiogenic 187os 188os seawater therefore can be caused by weathering of organic rich sediments at a constant rate third the marine residence time of osmium is sufficiently short to capture short periodic glacial interglacial fluctuations that are inaccessible to the buffered marine strontium isotope system this offers the opportunity to discriminate between high frequency climatic and low frequency tectonic forcing

geochemistry of late permian to early triassic pelagic cherts from southwest japan implications for an oceanic redox change major trace and rare earth element abundances were determined for the southwestern japanese pelagic chert sequence from the early late permian to early early triassic to investigate a redox change in deep sea pelagic environments before and at the permo triassic boundary ptb 251 ma the sequence was primarily deposited in the deep sea of the superocean panthalassa and then was accreted to japan in the middle jurassic a remarkable lithostratigraphic change from red chert to siliceous carbonaceous claystone through gray chert is observed in this sequence constituent elements for these sedimentary rocks are essentially derived from two sources 1 an ancient seawater via biogenic mainly radiolarian hydrothermal and authigenic materials and 2 an average shale like terrigenous material the present measurement demonstrates significant stratigraphic changes of the ce cenon ter value estimated ce anomaly value of non terrigenous component recalculated by subtraction of terrigenous rees from bulk rees and the mn te sample mn te paas value excess mn component other than terrigenous one te terrigenous elements including ti al nb hf th paas post archean average australian shale in the permian chert the ce cenon ter values increase from 0 2 to 1 and the mn te sample mn te paas values decrease up section suggesting that the redox condition of deep sea open ocean changed from oxic to suboxic in an interval of approximately 10 myr the fe te sample fe te paas and mn te sample mn te paas values of carbonaceous claystone near or at the ptb are less than unity suggesting that reductive dissolution of iron and manganese occurred under an anoxic condition this supports the idea of the ptb oceanic anoxia in the superocean panthalassa the present data suggest that the anoxic condition prevailed in the deep sea pelagic regions for an extremely long period much more than 10 myr from the middle late permian to early early triassic this long term development of widespread oceanic anoxia may have been linked to the greatest mass extinction of the phanerozoic

climate correlated variations in seawater 187 os 188 os over the past 200 000 yr evidence from the cariaco basin venezuela abstract a high resolution record of seawater osmium isotopic composition 35 samples over the past 191 000 yr has been obtained from sediments from ocean drilling program odp leg 165 site 1002 in the cariaco basin venezuela strong correlations are observed between 187os 188os and both the local and global oxygen isotopic records with less radiogenic values observed during glacial intervals in conjunction with previous analyses of sediments from the east pacific rise the santa barbara basin and the japan sea the data demonstrate conclusively that there have been ocean wide changes in seawater 187os 188os over the pleistocene global seawater excursions to relatively unradiogenic 187os 188os indicating decreased continental inputs occur during cold intervals times of increased aridity and increased ice cover of old radiogenic continental masses the data thus indicate a direct climatic control on chemical weathering rates and or composition of weathering products on a glacial interglacial timescale

major and trace element geochemistry and os isotopic composition of metalliferous umbers from the late cretaceous japanese accretionary complex 1 metalliferous umbers and red shales occur as unique products of the kula pacific ridge forearc collision in the late cretaceous shimanto supergroup an accretionary complex in japan these umbers are closely associated with greenstones of mid ocean ridge basalt morb origin and are regarded as hydrothermal metalliferous precipitates related to mor type volcanism the umbers and red shales were deposited in the trench area where both terrigenous detritus from land and hydrothermal metalliferous particulates from a mor were supplied simultaneously besides a predominance of fe and mn the umbers exhibit remarkable enrichments in p v co ni zn y mo rare earth elements rees and os relative to continental crustal abundances the x fe x mn p v co ni zn y and rees ratios and paas normalized ree patterns of the umbers are very similar to those of modern hydrothermal plume fallout precipitates deposited on flanks of mor this indicates that the umbers preserve primary geochemical signatures of hydrothermal metalliferous sediments that scavenged seawater derived elements and thus can be used as a proxy for late cretaceous seawater the marine 187os 188os ratios reconstructed from the late maastrichtian umbers range from 0 42 to 0 56 and are very consistent with recent data obtained from the pacific and atlantic pelagic carbonates that record an abrupt decline from 0 55 to 0 4 during the period between 67 0 ma and 65 7 ma

tourmaline as a recorder of magmatic hydrothermal evolution an in situ major and trace element analysis of tourmaline from the qitianling batholith south china four types of tourmaline in the qitianling granitic batholith south china were identified in this study euhedral disseminated tourmaline dt type in the early stage of granite consolidation typical interstitial nodular tourmaline nt type formed late in the crystallization history radial tourmaline veinlet rt type and vein tourmaline vt type of hydrothermal origin we performed major and trace element analysis by in situ electron microprobe and laser ablation inductively coupled plasma mass spectrometry on these four tourmaline types compositionally these tourmalines fall into the alkali group and schorl dravite solid solution series there is clear correlation between trace and major elements in vt type tourmalines likely related to co variations of these elements in the hydrothermal fluid from which the tourmaline precipitated tourmaline from granites displays low ree abundances and negative lree trends positive eu anomalies and hree below or close to their detection limits the dt type tourmalines show a positive eu anomaly which is probably due to a preferential incorporation of eu2 over eu3 ree depletion in nt type tourmalines may reflect co crystallization of ree rich minerals the positive eu anomaly in nt type tourmalines is due to late stage release of eu2 caused by tourmaline replacement of early feldspar vein tourmalines rt and vt type display similar slopes from la to nd without an eu anomaly the chondrite normalized ree patterns in tourmalines from veins display a concave upward shaped mree depleted pattern with minimum values at ho increasing steadily to lu and may reflect the ree characteristics of the precipitated medium hree enrichment in hydrothermal fluid can be attributed to the formation of ree fluoride complexes during the internal fluid evolution

hydrothermal scavenging at the mid atlantic ridge modification of trace element dissolved fluxes abstract we have carried out an icp ms study of the distributions of trace elements fe ca mg al mn cu zn v as cr co pb y mo ag sn cd and be in particulate material collected from the tag hydrothermal plume 26 n mid atlantic ridge to examine the extent to which fluxes of dissolved trace elements emitted from hydrothermal vent fluids are balanced by the precipitation of associated particulate phases comparison of particulate trace element distributions with particulate fe concentrations within the neutrally buoyant plume reveals that the distribution of trace elements within the plume is controlled by the following processes i co precipitation from vent fluids ii scavenging from seawater and iii preferential settling from the plume and oxidative dissolution within the water column for y and be continuous scavenging from seawater appears to be important whereas for v as and cr it appears that conditions suitable for uptake into particles may be restricted to the buoyant portion of the hydrothermal plume from a comparison of trace element fe ratios in the particles and associated mid atlantic ridge vent fluids it appears that for many elements e g cu co ag pb al cd be and zn the flux of dissolved material from vent fluids is matched closely by the precipitation of hydrothermal particulate phases of the various elements studied here hydrothermal activity only appears to act as a significant source for dissolved ca and dissolved mn

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

variable microstructure of peridotite samples from the southern mariana trench evidence of a complex tectonic evolution abstract we retrieved samples of peridotite from a dredge haul kh92 1 d2 collected during cruise kh92 1 undertaken by the research vessel r v hakuho in 1992 at the landward trench slope of the southern mariana trench 11 41 16 n 143 29 62 e depth 6594 7431 m which is the deepest ocean in the world ten of 30 retrieved samples possessed both a foliation and lineation as assessed from 46 thin sections of various orientations and observations of hand samples the samples showed marked variation in microstructure ranging from coarse 5 mm equigranular and intensely elongated textures to finer

arc parallel flow within the mantle wedge evidence from the accreted talkeetna arc south central alaska 1 residual mantle exposures in the accreted talkeetna arc alaska provide the first rock analog for the arc parallel flow that is inferred from seismic anisotropy at several modern arcs the peridotites exposed at the base of the jurassic talkeetna arc have a moho parallel foliation and indicate dislocation creep of olivine at temperatures of 1000 to 1100 c slip occurred chiefly on the 001 100 slip system which has only rarely been observed to be the dominant slip system in olivine stretching lineations and olivine 100 slip directions are subparallel to the long axis of the talkeetna arc for over 200 km indicating that mantle flow was parallel to the arc axis the alignment of the olivine 100 axes yields a calculated s wave anisotropy with the fast polarization direction parallel to the arc thus 1 the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog 2 arc parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones fluid filled pockets or glide on the 010 001 h2o induced slip system 3 seismic anisotropy beneath modern arcs may be due to slip on 001 100 with a horizontal foliation rather than slip on 010 100 with a vertical foliation and 4 the observed dominance of the 001 100 slip system may be due to high h2o concentrations suggesting that strain in the oceanic upper mantle may be accommodated dominantly by 001 100 olivine slip

variations in seawater osmium isotope composition since the last glacial maximum a case study from the japan sea abstract concentrations of re and os and the isotopic composition of os have been measured in the japan sea sediments to assess the response of the japan sea to glacial interglacial climate change and associated weathering fluxes the osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg g and 187os 188os from 0 935 to 1 042 only 187os 188os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of re and os and with 80 hydrogenous os are explained in terms of seawater composition lower 187os 188os were observed for sediments deposited during the last glacial maximum lgm when planktonic foraminifera from the japan sea recorded lighter oxygen isotopic composition decrease in dissolved os fluxes from continents and or change in the composition of the dissolved load to the japan sea are suggested as the driving mechanisms for the observed lower lgm 187os 188os the results of this study coupled with lower 187os 188os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates suggest that this trend is characteristic of the global oceans data from this study show that the japan sea recorded higher 187os 188os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values this is explained in terms of preferential release of 187os during deglacial weathering and or higher continental os flux driven by warm and wet climate this study demonstrates that os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition in addition 187os 188os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi enclosed basins such as the japan sea

re os geochronology of the iimori besshi type massive sulfide deposit in the sanbagawa metamorphic belt japan abstract besshi type cu deposits are strata bound volcanogenic massive sulfide deposits usually associated with mafic volcanic rocks or their metamorphic equivalents although there are numerous besshi type deposits in the sanbagawa metamorphic belt japan their tectonic settings and depositional environments remain controversial because of a lack of depositional age constraints we report re os data for the iimori deposit one of the largest besshi type deposits in western kii peninsula in order to examine the robustness of the re os isotope system for dating sulfide minerals in the high p t metamorphic belt and to elucidate the primary depositional environment of the iimori sulfide ores an 11 point re os isochron plot yields an age of 156 8 3 6 ma as this re os isochron age is significantly older than the timing of the sanbagawa peak metamorphism 110 120 or 90 ma and a well defined isochron was obtained the re os age determined here is most likely the primary depositional age despite high grade metamorphism at up to 520 25 c and 8 9 5 kbar the re os isotope system of the iimori sulfides was not disturbed hence we consider that the whole rock re os closure temperature for the iimori sulfide ores was probably higher than 500 c as the accretion age of the sanbagawa metamorphic belt is considered to be 120 130 or 65 90 ma on the basis of radiolarian and radiometric ages we estimated the time from the iimori sulfide deposition on the paleo seafloor to its accretion at the convergent plate boundary to be greater than 25 myr consequently the depositional environment of the iimori sulfide ores was not in the marginal sea but was truly pelagic

the osmium riverine flux and the oceanic mass balance of osmium the osmium concentration os and isotopic composition were determined in a set of 17 of the largest rivers of the world os varies between 4 6 and 52 1 pg kg and the 187os 188os ratio varies between 0 64 and 2 94 measurement of rainwater samples shows that there is no input of oceanic os to the continent through rain assuming a negligible anthropogenic os input in the dissolved load the natural average river water has an os concentration of 7 9 pg kg and a mean 187os 188os ratio of 1 54 the total riverine flux of os to the ocean is estimated to be 295 kg yr the dissolved os flux from island arcs and oceanic islands represents less than 5 of the total riverine flux and is not further considered the continental os flux to the ocean is then represented by the riverine flux as dissolved os from eolian dust and glacial sediments is negligible assuming steady state it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg yr cosmic dust or mantle derived and an oceanic residence time between 2 5 104 and 5 4 104 with a mean of 3 5 104 year the estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater os would be 14 of the contribution of the unradiogenic component which means that the mantle derived flux should contribute a major part the first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and or of the ultramafic exposures contributes dominantly to the input of unradiogenic os to the seawater we show that it would be necessary to leach 1 3 of the os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic os to the ocean in which case the riverine input represents 94 of the total dissolved flux to the ocean instead of 70 the modern global dissolved os flux to the ocean would then have a 187os 188os ratio of 1 44 instead of 1 06 and the system would be far from steady state

orbital climate forcing of δ13c excursions in the late paleocene early eocene chrons c24n c25n 1 high resolution stable carbon isotope records for upper paleocene lower eocene sections at ocean drilling program sites 1051 and 690 and deep sea drilling project sites 550 and 577 show numerous rapid 40 60 kyr duration negative excursions of up to 1 we demonstrate that these transient decreases are the expected result of nonlinear insolation forcing of the carbon cycle in the context of a long carbon residence time the transients occur at maxima in earth s orbital eccentricity which result in high amplitude variations in insolation due to forcing by climatic precession the construction of accurate orbital chronologies for geologic sections older than 35 ma relies on identifying a high fidelity recorder of variations in earth s orbital eccentricity we use the carbon isotope records as such a recorder establishing a robust orbitally tuned chronology for latest paleocene earliest eocene events moreover the transient decreases provide a means of precise correlation among the four sites that is independent of magnetostratigraphic and biostratigraphic data at the 105 year scale while the eccentricity controlled transient decreases bear some resemblance to the much larger amplitude carbon isotope excursion cie that marks the paleocene eocene boundary the latter event is found to occur near a minimum in the 400 kyr eccentricity cycle thus the cie occurred during a time of minimal variability in insolation the dominant mechanism for forcing climate change on 104 year scales we argue that this is inconsistent with mechanisms that rely on a threshold climate event to trigger the paleocene eocene thermal maximum since any threshold would more likely be crossed during a period of high amplitude climate variations

p wave velocities of polymineralic rocks comparison of theory and experiment and test of elastic mixture rules abstract we compare the p wave velocities vp of 696 dry samples measured at pressures up to 0 6 1 0 gpa with values calculated from the volume fraction and room pressure elastic constants of each constituent mineral using 16 averaging methods the exceptional large number of samples covers almost all common lithologic types of igneous and metamorphic rocks the calculated vp data are in good agreement with laboratory values measured at about 300 mpa even though elastic constants of only 22 common minerals are used in the computation the mean vp of a polymineralic rock is exclusively controlled by the volume fractions of its constituent minerals while grain shape and crystallographic preferred orientations anisotropy and other perturbations have minimum effects the mean vp can be fairly well predicted as long as a relevant mixture rule is used and the volume fraction is accurately determined for each mineral however none of the mixture rules can simultaneously produce the best fit to measured p wave velocities for all the lithologic types one method may work well for one lithology but poor for other lithology even for a given lithology an averaging method may yield good agreement at moderate pressure but poor agreement at high pressure applications of an inappropriate mixture rule will potentially cause the misinterpretation of the crust and mantle vp data in terms of mineralogical compositions and structures

seismic anisotropy beneath the southern ordos block and the qinling dabie orogen china eastward tibetan asthenospheric flow around the southern ordos abstract sks wave splitting analysis is performed to estimate the seismic anisotropy in the upper mantle using teleseismic data recorded by a temporary seismic array of 180 stations called sosarray deployed in the southern ordos block and the qinling dabie orogen the most important finding is that large delay times with nw se fast polarization directions in the northeastern tibet are continuous across the boundary into the southwestern part of the ordos block where the sks wave splitting results are significantly different from those in the rest of the ordos block based on our sks wave splitting results in addition to the results from previous studies we propose an asthenospheric flow model for the eastward extrusion of the tibetan upper mantle the model consists of two corner flows around the southwestern corner and the southeastern corner of the ordos block and the eastward flow along the weihe graben and the qinling dabie orogen for the escaping tibetan upper mantle finally the clockwise turning flow of the asthenosphere around the southwestern corner of ordos block has currently extended laterally into the interior of the ordos block suggesting that the thick cold lithospheric root of the southwestern ordos block there is currently being replaced with hot tibetan asthenosphere at depths that is we observed an on going process of thermal erosion of a cratonic lithosphere by lateral hot asthenospheric flow

uptake and fractionation of rare earth elements on hydrothermal plume particles at 9 45 n east pacific rise abstract particulate samples 0 45 m from a neutrally buoyant hydrothermal plume at 9 45 n on the northern east pacific rise were collected using large volume in situ filtration and analyzed for fe al mn ni and fourteen rare earth elements ree the sm fe ratio a proxy for overall ree fe and nd er light heavy ree fractionation increased moderately with decreasing particulate fe chemically the sense of these relationships matched that documented previously in the tag plume on the mid atlantic ridge german et al 1990 although particulate fe was about 10 fold lower at 9 45 n spatial trends relative to the vent source however were opposite of expectation because slow fe ii oxidation and fe iii colloid aggregation over this interval led to increased particulate fe 10 26 nm with distance from source field and sherrell submitted after subtraction of non plume background particle composition plume particles at 9 45 n and tag had indistinguishable ranges of light ree enriched fractionation relative to ambient seawater and had very similar sm fe therefore kd for fe oxyhydroxides demonstrating that plume particles in both oceans reflect to a first degree the local seawater ree composition within plume ree variations at 9 45 n were investigated using a simple mixing model which accounts for the bulk fe al mn variations in the plume using two endmembers fresh hydrothermal oxyhydroxide precipitates and ridge crest background particles composed largely of locally resuspended sediment sm fe and nd er plot linearly with mixing ratio r 0 96 implying that the observed ree trends result from mixing of these two endmembers extrapolation to the composition of pure hydrothermal precipitates suggests that nd er is fractionated relative to seawater by a factor of 1 8 during adsorption onto fresh fe oxyhydroxide particles the ridge crest background particles are 5 fold higher in sm fe and nd er is 2 49 relative to seawater partly a result of enriched terrigenous component in the resuspended matter a reinterpretation of ree at tag reveals that positive curvature in ree vs fe plots argued previously to reflect continuous ree uptake i e increasing kd german et al 1990 may result from local depletion of the dissolved ree pool by partitioning onto fe particles at fe 100 nm similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater source particle reactive species in plumes that are sampled at high particulate fe concentration in sum ree behavior in hydrothermal plumes is more consistent with equilibrium adsorption and mixing of distinct particle types than with kinetic uptake control precise measurements of rees in modern ridge crest metalliferous sediments could be compared to the endmember composition calculated from the plume data to evaluate long term changes in ree of the hydrothermal component

an osmium isotope excursion associated with the late paleocene thermal maximum evidence of intensified chemical weathering in the latest paleocene an abrupt shift to more negative 13c values has been documented at numerous marine and terrestrial sites bralower et al 1997 cramer et al 1999 kaiho et al 1996 kennett and stott 1991 koch et al 1992 stott et al 1996 thomas and shackleton 1996 zachos et al 1993 this carbon isotope event cie is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4 6 c kennett and stott 1991 and more moderate warming in the subtropics thomas et al 1999 here we report 187os 188os isotope records from the north atlantic and indian oceans which demonstrate a 10 increase in the 187os 188os ratio of seawater coincident with the late paleocene cie this excursion to higher 187os 188os ratios is consistent with a global increase in weathering rates the inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate which acts to stabilize earth s climate walker et al 1981 estimates of the duration of late paleocene cie bains et al 1999 bralower et al 1997 norris and rohl 1999 rohl et al 2000 in conjunction with the os isotope data imply that intensified chemical weathering in response to warm humid climates can occur on timescales of 104 105 years this interpretation requires that the late paleocene thermal maximum os isotope excursion be produced mainly by increased os flux to the ocean rather than a transient excursion to higher 187os 188os ratios in river runoff although we argue that the former is more likely than the latter we cannot rule out significant changes in the 187os 188os ratio of rivers

multiple early eocene hyperthermals their sedimentary expression on the new zealand continental margin and in the deep sea the paleocene eocene thermal maximum petm ca 55 5 ma was a geologically brief interval characterized by massive influx of isotopically light carbon extreme changes in global climate and profound variations in earth system processes an outstanding issue is whether it was an isolated event or the most prominent example of a recurring phenomenon recent studies of condensed deep sea sections support the latter but this finding remains uncertain here we present and discuss lithologic and carbon isotope records across two lower eocene outcrops on south island new zealand the petm manifests as a marl rich horizon with a significant negative carbon isotope excursion cie above in sediment deposited between 54 and 53 ma are four horizons with similar though less pronounced expressions marl beds of all five horizons represent increased terrigenous sedimentation presumably linked to an accelerated hydrological cycle five corresponding clay rich horizons and cies are found in deep sea records although the lithologic variations represent carbonate dissolution rather than siliciclastic dilution the presence of five intervals with similar systemic responses in different environments suggests a mechanism that repeatedly injected large masses of 13 c depleted carbon during the early eocene

geochemical evidence for volcanic activity prior to and enhanced terrestrial weathering during the paleocene eocene thermal maximum abstract the re os and major element geochemistry of an expanded gray shale sequence from the central basin in spitsbergen svalbard archipelago norway over the paleocene eocene thermal maximum petm is presented the data suggest that a significant volcanic episode as indicated primarily by a large drop in 187 os 188 os occurred just prior to or coincident with the onset of the petm the distinct geochemistry of the inferred ash component is consistent with the geochemistry of contemporaneous ashes previously sampled in denmark and persists in the sedimentary record for 8 ka based on the substantial size of the 187 os 188 os decrease 187 os 188 os 0 23 the volcanic event must have involved widespread ash deposition in the local catchment the terrestrially deposited ash weathered rapidly shifting the 187 os 188 os of the basin water column to 187 os 188 os to more radiogenic values and depositing clastic materials with higher chemical index of alteration and lower na ti persistently anoxic sedimentary pore fluids interpreted to reflect bottom water anoxia initiated close to the onset of the petm anoxic mainly euxinic conditions persisted for 50 ka ending abruptly 55 77 ma anoxia appears related to enhanced high latitude hydrological cycling and consequent basinal stratification during the petm

reos dating of molybdenites from ore deposits in japan implication for the closure temperature of the reos system for molybdenite and the cooling history of molybdenum ore deposits abstract rhenium and osmium abundances were determined on molybdenites from eighteen ore deposits in japan fifteen vein type a skarn type a greisen type and a pegmatite type deposit re os ages obtained for molybdenites from northeastern central and southwestern japan are 100 130 ma 50 75 ma and ca 10 ma respectively reflecting the ages of regional igneous activities in individual areas for vein type ore deposits re os ages for molybdenites agree with rb sr whole rock ages of host granitoids while they are 3 12 ma older than k ar mineral ages of the host rocks on the other hand re os ages of molybdenites from skarn pegmatite and greisen type ore deposits agree with k ar ages of the host rocks the comparison of re os ages for molybdenites with rb sr and k ar ages of host rocks in vein type deposits suggests that re os closure temperature for vein molybdenite is close to whole rock rb sr closure temperature for host granitic rocks i e roughly estimated to be around 500 c and higher than k ar closure temperature for host granodiorites i e 230 370 c if the thermal history of vein molybdenites is essentially equivalent to that of host rocks one possible explanation for age discrepancies of 3 12 ma between re os ages for molybdenites and k ar ages for the wall rocks observed in vein type molybdenum deposits is that later thermal events after formation of molybdenum ores may reset k ar ages hydrothermal alteration might have occurred in such a temperature condition as to disturb k ar mineral systems in wall rocks and made them younger but not the re os system in molybdenite the other explanation might be that the age discrepancies correspond to a cooling interval from ca 500 to 300 c of host granitoid on the other hand the agreement of re os ages of molybdenites and k ar ages of host rocks in skarn pegmatite and greisen type deposits in this study indicates no later disturbance for k ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits

seismic anisotropies of the songshugou peridotites qinling orogen central china and their seismic implications abstract though extensively studied the roles of olivine crystal preferred orientations cpos or fabrics in affecting the seismic anisotropies in the earth s upper mantle are rather complicated and still not fully known in this study we attempted to address this issue by analyzing the seismic anisotropies e g p wave anisotropy avp s wave polarization anisotropy avs radial anisotropy and rayleigh wave anisotropy g of the songshugou peridotites dunite dominated in the qinling orogen in central china based on our previously reported olivine cpos the seismic anisotropy patterns of olivine aggregates in our studied samples are well consistent with the prediction for their olivine cpo types and the magnitude of seismic anisotropies shows a striking positive correlation with equilibrium pressure and temperature p t conditions significant reductions of seismic anisotropies avp max avs and g are observed in porphyroclastic dunite compared to coarse and fine grained dunites as the results of olivine cpo transition from a d type in coarse grained dunite through ag type like in porphyroclastic dunite to b type like in fine grained dunite and strength variation weakening a d type ag type like strengthening ag type like b type like during dynamic recrystallization the transition of olivine cpos from a d type to b ag type like in the forearc mantle may weaken the seismic anisotropies and deviate the fast velocity direction and the fast s wave polarization direction from trench perpendicular to trench oblique direction with the cooling and aging of forearc mantle depending on the size and distribution of the peridotite body such as the songshugou peridotites b and ag type like olivine cpos can be an additional despite minor local contributor to the orogen parallel fast velocity direction and fast shear wave polarization direction in the orogenic crust such as in the songshugou area in qinling orogen

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

changes in magma storage conditions following caldera collapse at okataina volcanic center new zealand large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera forming eruptions although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as yellowstone and long valley much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse we present 238u 230th age trace element and hf isotopic data from individual zircon crystals from four eruptions from the okataina volcanic center taupo volcanic zone new zealand in order to assess changes in trace element and isotopic composition of the reservoir following the 45 ka caldera forming rotoiti eruption our data indicate that 1 mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir 2 while the basic processes of melt generation likely did not change significantly between pre and post caldera rhyolites post caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse and 3 post caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption these data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir

did paleogene north atlantic rift related eruptions drive early eocene climate cooling abstract the delivery of volcanogenic sulphur into the upper atmosphere by explosive eruptions is known to cause significant temporary climate cooling therefore phreatomagmatic and phreatoplinian eruptions occurring during the final rifting stages of active flood basalt provinces provide a potent mechanism for triggering climate change during the early eocene the northeast atlantic margin was subjected to repeated ashfall for 0 5 m y this was the result of extensive phreatomagmatic activity along 3000 km of the opening northeast atlantic rift these widespread predominantly basaltic ashes are now preserved in marine sediments of the balder formation and its equivalents and occur over an area extending from the faroe islands to denmark and southern england these ash bearing sediments also contain pollen and spore floras derived from low diversity forests that grew in cooler drier climates than were experienced either before or after these highly explosive eruptions in addition coeval plant macrofossil evidence from the bighorn basin wyoming usa also shows a comparable pattern of vegetation change the coincidence of the ashes and cooler climate pollen and spore floras in northwest europe identifies volcanism as the primary cause of climate cooling estimates show that whilst relatively few phreatomagmatic eruptive centres along the 3000 km opening rift system could readily generate 0 5 1 c cooling on an annual basis only persistent or repeated volcanic phases would have been able to achieve the long term cooling effect observed in the floral record we propose that the cumulative effect of repeated balder formation eruptions initiated a biodiversity crisis in the northeast atlantic margin forests only the decline of this persistent volcanic activity and the subsequent climatic warming at the start of the eocene thermal maximum allowed the growth of subtropical forests to develop across the region

inferring upper mantle flow from seismic anisotropy an experimental perspective abstract patterns of mantle flow are most directly inferred from observations of seismic anisotropy which is mainly caused by the crystallographic preferred orientation cpo of olivine the most abundant mineral in the upper mantle cpo is generated by high temperature ductile deformation which often yields predictable relationships between the elastic or seismic properties of a material and the kinematics of flow over the last 15 years there has been a wealth of new data describing the how olivine cpo forms and evolves as a function of deformation conditions and strain magnitude in this review we explore the relationships between deformation the evolution of cpo and the development of seismic anisotropy from the perspective of experimental rock mechanics we first review the experimental basis for the study of olivine cpo evolution from the formative studies in the early nineteen sixties through recent advances we then review some emerging complications to the study of cpo evolution such as the long lived transient cpos that arise from changes in deformation kinematics mechanisms and conditions finally we discuss the origins of seismic anisotropy and the challenges of interpreting seismic anisotropy in terms of mantle flow

seismic anisotropy and global geodynamics for many years seismic anisotropy was often neglected mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves the usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media where new phenomena come up such as birefringence or shear wave splitting or difference between directions of propagation of phase velocity and of group velocity consequently geophysicists often claimed that it was a second order effect and considered the earth as isotropic this hypothesis was assumed to be a good approximation because of the random orientation of crystals in most parts of the earth and of the random sampling of anisotropic regions by seismic rays this assumption furthermore made easier the description of wave propagation as well as the parameterization of media in inverse problems an isotropic elastic medium can be described by two independent elastic parameters and lame parameters but the simplest anisotropic medium transverse isotropy with a vertical symmetry axis requires 5 independent parameters love 1927 anderson 1961 to date seismic observations have been explained in terms of isotropic and often thermal lateral heterogeneities ignoring manifestations of anisotropy however since the 1960s it was recognized that most parts of the earth are not only laterally heterogeneous but also anisotropic and that seismic anisotropy provides a simple explanation of different observational data seismic anisotropy contrary to isotropy is reflecting some inherent

himalayan uplift and osmium isotopes in oceans and rivers previous studies have shown that 187 os 188 os in seawater has become increasingly radiogenic over the last 40 ma in a manner analogous to strontium this rapid rise in the marine 187 os 188 os over the last 17 ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the himalayas and or selective weathering and erosion of highly radiogenic organic rich ancient sediments the key test of this hypothesis is the 187 os 188 os and the total osmium concentration of the himalayan rivers we report the concentration and isotopic composition of osmium in the ganges the brahmaputra and the indus rivers the 187 os 188 os of the ganges close to its source at kaudiyal 30 05 n 78 50 e is 2 65 and os 45 fm kg a second sample of the lower reaches of the ganges at patna 25 30 n 85 10 e gives 187 os 188 os 1 59 and os 171 fm kg the 187 os 188 os of the brahmaputra at guwahati 26 10 n 91 58 e is 1 07 and os 52 fm kg a sample of the indus besham 34 55 n 72 51 e has a 187 os 188 os of 1 2 and os 59 fm kg we infer that the himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans the overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the himalayas themselves appear not to be the dominant source

deformation fabrics of olivine in val malenco peridotite found in italy and implications for the seismic anisotropy in the upper mantle abstract an exhumed huge body of ultramafic rocks is preserved in val malenco italy in this study petrofabrics and deformation microstructures of olivine in the val malenco peridotite were examined and new observations of lattice preferred orientation lpo of olivine such as type b and type e lpo in the peridotite are reported three types depending on the grain size of olivine fabrics were observed type a fabric exhibiting the 010 100 slip system in the large relic grain area and type b and type e fabrics exhibiting the 010 001 and 001 100 slip systems respectively in the widely distributed recrystallized grain area the slips occurred mainly on 010 001 and the 001 100 slip system has only rarely been reported to be the dominant slip system of olivine found in natural rocks further seismic velocities and seismic anisotropies were calculated for the three types of fabrics found the seismic anisotropies in the upper mantle can be attributed to the type b type e or type a lpos of olivine moreover the anomaly in seismic low velocity in the mantle wedge can be attributed to the serpentinization as well as the mixed fabrics of the type a type b and type e

quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone the influence of pyroxene on upper mantle viscosity remains unclear but may occur either through its difference in strength relative to olivine or through its effect on olivine microstructure to determine the role of pyroxene in moderating peridotite viscosity we analyzed microstructures in paired harzburgites and dunites from a natural shear zone exposed in the josephine peridotite olivine crystallographic textures evolve similarly in harzburgites and dunites with increasing strain indicating the operation of similar deformation mechanisms in olivine the mean olivine grain sizes are 1 5 times larger in dunites than in harzburgites whereas the mean olivine subgrain sizes are a factor of 1 2 smaller in dunites than in harzburgites the average stresses in olivine estimated with a subgrain size piezometer are inversely correlated with pyroxene volume fraction and directly correlated with the mean olivine grain size the calculated ratio of pyroxene viscosity to olivine viscosity for each harzburgite dunite pair varies from 1 2 to 3 3 notably our data indicate that olivine viscosity is non newtonian with a finite grain size sensitivity we suggest that at the conditions of the shear zone pyroxene is more viscous than olivine but that the increase in aggregate strength with larger pyroxene fractions is offset by correspondingly smaller olivine grain sizes thus major rheological weakening associated with pyroxene content may only be possible if 1 temperatures are higher than those characterizing this shear zone increasing the viscosity contrast between olivine and pyroxene or 2 stresses are high enough that grain size reduction promotes a transition to a deformation mechanism with extreme grain size sensitivity such as diffusion creep

lithologic controls on osmium isotopes in the rio orinoco abstract the seawater 187 os 188 os ratio has been increasing over the last 50 ma and at present is 1 06 reflecting that about 80 of os in the oceans is derived from the continents that provide os with an average 187 os 188 os ratio of about 1 26 whether the past variations in seawater os isotopes record changes in the bulk os flux with an average isotopic composition or the changes in the average 187 os 188 os ratio of the continental inputs or a combination of these two pathways is not clear the fundamental importance of lithology in controlling continental weathering rates and dissolved ion yields has been well established for the major cations it is dramatically manifested in the sr isotopes of rivers that are affected by selective dissolution of carbonates accordingly for os isotopes it has been proposed that disproportionate contributions of os with extremely high 187 os 188 os ratio come from weathering of organic rich black shales here we examine the effect of the weathering of specific lithologies on osmium isotopic composition and concentration in water samples of the orinoco river basin a major river basin draining both black shales exposed in the northern andes and granites of the precambrian guyana shield the northern andes was uplifted 17 ma which corresponds to the time of a dramatic rise in 187 os 188 os ratio the osmium isotopic composition and concentrations vary considerably in the tributaries and the main stem of the orinoco the highest os concentrations are observed in the rivers draining the andes however waters draining the andes are not highly radiogenic 187 os 188 os 187 os 188 os ratio of seawater during the last 17 million years

the marine 187 os 188 os record of the eocene oligocene transition the interplay of weathering and glaciation abstract osmium os isotope analyses of bulk sediments from the south atlantic equatorial pacific and the italian apennines yield a well dated and coherent pattern of 187os 188os variation from the late eocene to the early oligocene the resulting composite record demonstrates the global character of two prominent features of the low resolution ll44 gpc3 os isotope record pegram and turekian geochim cosmochim acta 63 1999 4053 4058 these are 1 a pronounced minimum in 187os 188os 0 22 0 27 in the late eocene between 34 and 34 5 ma and 2 a subsequent rapid increase in 187os 188os to approximately 0 6 by 32 ma an ultramafic weathering event and an increased influx of extraterrestrial particles to the earth are discussed as alternative explanations for the late eocene 187os 188os minimum comparison of the 187os 188os to benthic foraminiferal oxygen isotope records demonstrates that the nearly three fold increase in 187os 188os from the late eocene minimum coincides with the growth and decay of the first large ice sheet of the oligocene oi1 miller et al j geophys res 96 1991 6829 6848 the fine structure of the os isotope record indicates that enhanced release of radiogenic os unrelated to the recovery from late eocene minimum lagged the initiation of the oi1 event by roughly 0 5 myr this record in conjunction with weathering studies in modern glacial soils blum in w f ruddiman ed tectonic uplift and climate change plenum press new york 1997 pp 259 288 peucker ehrenbrink and blum geochim cosmochim acta 62 1998 3193 3203 suggests that exposure of freshly eroded material during deglaciation following oi1 enhanced chemical weathering rates and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide the improved temporal resolution and age control of the refined eocene oligocene os isotope record also makes it possible to illustrate the late eocene os isotope excursion as a tool for global correlation of marine sediments

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

os isotopes as tracers in seafloor hydrothermal systems metalliferous deposits from the tag hydrothermal area 26 n mid atlantic ridge abstract os isotopic analyses of a suite of samples from the tag hydrothermal area 26 n on the mid atlantic ridge reveal evidence of release of ocean crust derived os to seawater and removal of seawater derived os to the seafloor 187 os 186 os ratios of sulfide samples from the active tag mound vary between 8 7 and 1 3 the highest ratios are indistinguishable from the inferred 187 os 186 os ratio of seawater the incorporation of seawater derived os in these samples is interpreted as evidence of both entrainment of seawater into hydrothermal fluids and the addition of seawater os to samples during sulfide oxidation several sulfide samples from the active tag mound have 187 os 186 os ratios lower than the inferred seawater value with the lowest ratio approaching values expected for pristine oceanic crust these low 187 os 186 os ratios indicate that os derived from alteration of the oceanic crust is released to seawater in association with venting of high temperature hydrothermal fluids precipitates from low temperature hydrothermal fluids fe and mn oxide deposits from the east wall of the axial rift valley also have low 187 os 186 os ratios 4 2 6 8 relative to seawater these data suggest os isotopes may be sensitive tracers of low temperature hydrothermal circulation on the seafloor and that supply of ocean crust derived os to seawater is not strictly limited to high temperature hydrothermal vents the os concentration and isotopic composition of metalliferous sediment samples from a push core collected 125 m from the base of the active tag mound are also reported in this core metalliferous sediment horizons derived from mass wasting of altered sulfides exhibit variable 187 os 186 os ratios 4 8 8 7 reflecting mixing of seawater derived and ocean crust derived os in contrast sediments in which the metalliferous component is derived from fall out of hydrothermal material from the neutrally buoyant hydrothermal plume have 187 os 186 os ratios which vary over a narrow range 8 6 8 7 and appear to accurately record the os isotopic composition of seawater

the earliest mantle fabrics formed during subduction zone infancy harzburgites obtained from the oldest crust mantle section in the philippine sea plate 52 ma along the landward slope of the southern izu ogasawara trench preserve mantle fabrics formed during the infancy of the subduction zone that is during the initial stages of pacific plate subduction beneath the philippine sea plate the harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations and show inequigranular interlobate textures and crystal preferred orientation patterns in olivine 001 100 and opx 100 001 the harzburgites have the characteristics of residual peridotites whereas the dunites obtained from the same location as the harzburgites provide evidence for the earliest stages of arc volcanism during the inception of subduction we propose that the 001 100 olivine patterns began forming in immature fore arc mantle with an increase in slab derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc

protracted fabric evolution in olivine implications for the relationship among strain crystallographic fabric and seismic anisotropy abstract crystallographic fabrics in olivine rich rocks provide critical information on conditions and mechanisms of deformation as well as seismic properties of earth s upper mantle previous interpretations of fabrics produced in laboratory experiments were complicated by uncertainty as to whether the steady state fabric was attained to examine the systematics of the evolution of olivine crystallographic fabrics at high strain we conducted torsion experiments on olivine aggregates to shear strains of up to 20 our results demonstrate that a steady state fabric is not reached until a shear strain 10 a much higher value than previously thought necessary fabrics characterized by girdles of 010 and 001 axes or by clusters of 010 and 001 axes are both observed until now these fabrics were associated with either two different deformation mechanisms or two different sets of deformation conditions here we establish that both fabrics are in fact part of the same evolutionary process an eigenvalue analysis allows the fabric shape to be quantitatively correlated with the magnitude of shear strain misorientation analysis suggests that the observed fabric evolution results from the competition of the two easiest slip systems in olivine 010 100 and 001 100 our results open up the possibility of using olivine crystallographic fabrics or seismic anisotropy to quantitatively evaluate strain histories in both field studies and geophysical investigations of upper mantle rocks

a re os study of molybdenites from the lanjiagou mo deposit of north china craton and its geological significance article i nfo the lanjiagou mo deposit is located in the eastern part of the north china craton rhenium and osmium isotopes in molybdenites from the lanjiagou porphyry mo deposit have been used to determine the timing of mineralization molybdenite was analyzed mainly from granite porphyry which is characterized by moderate to strong silicification rhenium concentrations in molybdenite samples are between 33 and 48 g g analysis of eleven molybdenite samples yields an isochron age of 181 6 6 5 ma 2 based on the geological history and spatio temporal distribution of the granitoids it is proposed that the mo deposits in the eastern part of the north china craton were related to the subduction of the paleo pacific plate during jurassic time 2009 published by elsevier b v on behalf of international association for gondwana research

accumulation rates chemical partitioning and q mode factor analysis of metalliferous sediments from the north fiji basin abstract in order to assess recent hydrothermal contributions to pelagic sediments from active back arc spreading surface samples collected from the north central north fiji basin and the vanuatu archipelagic apron were chemically and mineralogically analyzed including phase and size partitioning and q mode factor analysis of the chemical data sediment cores were analyzed for accumulation rates by the excess 230 th method using alpha track autoradiography the north central north fiji basin sediments are largely carbonate oozes in which the non carbonate material is dominated by volcanic detritus in the 2 m fraction and by fe montmorillonite amorphous ferromanganese oxides and fine grained volcanic ash in the m fraction phase and size partitioning indicate that metal concentrations in the m fraction approach those for metalliferous sediments from the eastern pacific and that the metalliferous material in the majority of these sediments is concentrated in the oxide portion of the m fraction suggesting potential widespread colloidal dispersion from the source areas iron accumulation rates are similar to those of metalliferous sediments from the east pacific rise the mean mn accumulation rate is about twice the global authigenic rate whereas al is accumulating about ten times faster than in eastern and equatorial pacific pelagic sediments because of volcanic ash contributions from the vanuatu island arc and possible intra basin volcanism q mode factor analysis of the compositional data produced three major factors hydrothermal authigenic and detrital which together account for 90 of the sample variance in both size fractions of the non carbonate material the aerial distribution of the m fraction factor components reveals a major hydrothermal source associated with the south pandora ridge spreading center in general agreement with the mn and as accumulation rate patterns two smaller hydrothermal sources were revealed by the factor analysis which are not associated with any known active tectonic features suggesting that back arc rifting and associated hydrothermal activity in this portion of the north fiji basin may follow both ordered and complex regional modes

on the equilibrium profile of river beds 1 despite the wide spectrum of perturbations of flow and sediment transport experienced by rivers as a result of hydrologic variations the paradigm of morphodynamic equilibrium has long been present in the geomorphological literature where it is traditionally associated with the semiempirical notion of formative discharge whereby the unsteady forcing is taken as morphologically equivalent to some effective steady forcing here we investigate the mechanisms responsible for maintaining a quasi equilibrium bed profile of a river reach sufficiently short to have no significant tributary inputs more importantly we assume the channel banks to be fixed hence the case we have in mind is that of rivers protected by levees which cannot respond to hydrologic forcing by changing their width like natural rivers employing a 1 d model of river morphodynamics we first determine the equilibrium profile of the river reach for given steady forcing conditions and discuss the capability of this approach for interpreting bed profiles observed in the field by applying it to the terminal reach of the magra river italy field observations turn out to be reasonably well fitted by the equilibrium profile associated with a steady effective discharge which however differs from the typical formative discharge mean annual flood for natural channels with erodible banks finally we clarify how fluctuations of the hydrodynamic forcing associated with the recorded historical sequence of hydrologic events of variable intensities have acted to maintain the river equilibrium

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

transient scour and fill the case of the pilcomayo river abstract transient scour and fill refers to the general scour and fill in a riverbed due to flood passage it was given much attention in times of leopold and maddock 1953 and colby 1964 from records of outstanding case studies field data from a station in a contracted long reach of the large sand bed pilcomayo river 1192 newly typed in forms are presented with sediment concentration in the database up to 60 kg m 3 mostly wash load water surface rises and falls during a flood event whereas streambed evolves in reverse it renders the analogy to an accordion streambed scouring accounts for 36 of the change in flow area during floods and data reveal scouring up to 4 m from the streambed elevation when flood starts velocity data display large hysteresis loops inexplicable by unsteady flow only once streambed starts to fall river bed scour is self sustaining water depth gradient appears to be a crucial variable in a novel equation for transient scour and fill derived from exner equation it turns out that flow non uniformity is relevant to explain transient scour and fill in contrast to flow unsteadiness which is not quasi steady gradually varied flow equation solved along the contracted reach is the base for a model that captures well the recorded bed evolution the model explains most of the transient scour and fill by the downstream expansion but the non uniform flow and the bed material suspended load also play a role the model fails on the days of highest sediment concentrations

the marine187os 186os record of the past 80 million years we report new187os 186os data and re and os concentrations in metalliferous sediments from the pacific to construct a composite os isotope seawater evolution curve over the past 80 m y analyses of four samples of upper cretaceous age yield187os 186os values of between 3 and 6 5 and187re 186os values below 55 mass balance calculations indicate that the pronounced minimum of about 2 in the os isotope ratio of seawater at the k t boundary probably reflects the enormous input of cosmogenic material into the oceans by the k t impactor s following a rapid recovery to187os 186os of 3 5 at 63 ma data for the early and middle part of the cenozoic show an increase in187os 186os to about 6 at 15 ma variations in the isotopic composition of leachable os from slowly accumulating metalliferous sediments show large fluctuations over short time spans in contrast analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data these results together with sediment leaching experiments indicate that dissolution of non hydrogenous os can occur during the hydrogen peroxide leach and demonstrate that os data from pelagic clay leachates do not always reflect the os isotopic composition of seawater new data for the late cenozoic further substantiate the rapid increase in the187os 186os of seawater during the past 15 ma we interpret the correlation between the marine sr and os isotope records during this time period as evidence that weathering within the drainage basin of the ganges brahmaputra river system is responsible for driving seawater sr and os toward more radiogenic isotopic compositions the positive correlation between87sr 86sr and u concentration the covariation of u and re concentrations and the high dissolved re u and sr concentrations found in the ganges brahmaputra river waters supports this interpretation accelerating uplift of many orogens worldwide over the past 15 ma especially during the last 5 ma could have contributed to the rapid increase in187os 186os from 6 to 8 5 over the past 15 ma prior to 15 ma the marine sr and os record are not tightly coupled the heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic os and sr to the oceans and account for the periods of decoupling of the marine sr and os isotope records

hematite formation by oxygenated groundwater more than 2 76 billion years ago geoscientific drilling in the marble bar area of the pilbara craton western australia resulted in the discovery of locally abundant hematite in archean basalts 200 m below the present land surface the hematized basalts occurring along a bedding parallel shear zone are cross cut by pyrite veinlets 3 mm in width and contain euhedral pyrite grains 10 500 m in diameter with sharp crystal edges indicating that the hematite formed before the pyrite we have dated the pyrite in the veinlets at 2 763 0 016 ga using the re os method therefore the hematite formed prior to 2 763 ga the basalts containing the hematite belong to the apex basalt of the warrawoona group and were erupted onto the archean seafloor at 3 46 ga due to 2 9 ga orogenic deformation and subsequent deep erosion the apex basalt was exposed at the surface of a continental landmass prior to 2 77 ga sometime in the period between 2 9 ga and 2 77 ga the basalt section we describe was less than 200 m below the late archean land surface and within range of groundwater percolation through the shear zone in the basalts geological mineralogical and geochemical lines of evidence strongly suggest that the infiltration of o2 rich groundwater through the bedding parallel shear in the basalts formed hematite prior to 2 76 ga and hence oxygenated surface environments at least localized and or short lived emerged more than 300 million years before the widely accepted great oxidation event during 2 45 and 2 32 ga

re os geochronology of organic rich sediments an evaluation of organic matter analysis methods rhenium and osmium in organic rich sedimentary rocks are dominantly hydrogenous but any nonhydrogenous component will influence the accuracy and precision of the re os date obtained to minimize the influence of any nonhydrogenous re and os we evaluate analysis of isolated organic matter from the whole rock together with whole rock analysis using a cro3 h2so4 digestion medium instead of inverse aqua regia for a black shale unit of the exshaw formation canada this unit previously returned a whole rock re os date of 358 10 ma model 3 geochim cosmochim acta 2002 using inverse aqua regia dissolution organic matter isolated from the whole rock matrix using the hf bf3 technique org geochem 20 1993 249 yields scattered data and a re os date of 449 220 ma model 3 mswd 616 the organic matter analyses show similar 187os 188os values but significantly lower 187re 188os values in comparison to the whole rock analyses we show that the re os systematics of organic matter are altered during chemical isolation and as such we suggest that the hf bf3 method should not be used for re os analysis of organic matter whole rock re os analysis using a cro3 h2so4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method and the re os data identify two distinct initial 187os 188os values for the sample set separate regressions of these data yield precise dates 366 1 9 6 mswd 2 2 and 363 4 5 6 ma mswd 1 6 model 3 which are indistinguishable from the age constraints for this formation 363 4 0 4 ma u pb monazite comparison of the re os dates obtained from aqua regia and cro3 h2so4 methods suggests that the former may contain nonhydrogenous re and os whereas the cro3 h2so4 method dominantly liberates hydrogenous re os from organic matter allowing for better stratigraphic age determinations and evaluation of the os isotope composition of seawater

variations in the osmium isotope composition of sea water over the past 200 000 years the first osmium isotope data are presented for marine sediments spanning the past 200 000 years two full glacial cycles and establish that there have been ocean wide variations in sea water osmium isotope composition over this time while the osmium isotope composition of sea water has been relatively invariant over much of this interval 187os 186os of 8 5 to 8 6 excursions to lower isotope ratios 187os 186os 8 1 occur late in the last two glacial periods at 20 ka and 160 ka these are times of high global aridity low global temperature and maximum ice extent while various interpretations are possible a likely explanation is that the low sea water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity there is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation the response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12 000 years