computational fluid dynamics as a tool for investigating separated flow in river bends the detailed three dimensional structure of the flow patterns in river bends with separated flow and factors controlling the existence and characteristics of these flow patterns are unclear at present it is shown here firstly how a computational fluid dynamics cfd program may be used to reproduce the qualitative features of the mean flow in a real river bend to allow testing of the model s capabilities it is then shown how the cfd code may be used to construct hypothetical channel bends which allow the experimentation necessary to investigate the controls on the extent of the separated flow

highly siderophile element depletion in the moon abstract coupled 187 os 188 os and highly siderophile element hse os ir ru pt pd re abundance data are reported for apollo 12 12005 12009 12019 12022 12038 12039 12040 apollo 15 15555 and apollo 17 70135 mare basalts along with mare basalt meteorites la paz icefield lap 04841 and miller range mil 05035 these mare basalts have consistently low hse abundances at 2 10 5 to 2 10 7 the chondritic abundance the most magnesian samples have broadly chondrite relative hse abundances and chondritic measured and calculated initial 187 os 188 os the lower abundances and fractionated hse compositions of more evolved mare basalts can be reproduced by modeling crystal liquid fractionation using rock melt bulk partition coefficients of 2 for os ir ru pt and pd and 1 5 for re lunar mare basalt bulk partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions leading to increased hse compatibility the chondritic relative abundances and chondritic 187 os 188 os of the most primitive high mgo mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface mare basalt compositions are best modeled as representing 5 11 partial melting of metal free sources with low os ir ru pd 0 1 ng g 1 pt 0 2 ng g 1 re 0 01 ng g 1 and s 75 g g 1 with sulphide melt partitioning between 1000 and 10 000 apollo 12 olivine pigeonite and ilmenite normative mare basalts define an imprecise 187 re 187 os age of 3 0 0 9 ga with an initial 187 os 188 os of 0 107 0 010 this age is within uncertainty of 147 sm 143 nd ages for the samples the initial os isotopic composition of apollo 12 samples indicates that the source of these rocks evolved with re os within 10 of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted similarity in absolute hse abundances between mare basalts from the apollo 12 15 and 17 sites and from unknown regions of the moon la paz mare basalts mil 05035 indicates relatively homogeneous and low hse abundances within the lunar interior low absolute hse abundances and chondritic re os of mare basalts are consistent with a late accretion addition of 0 02 wt of the moon s mass to the mantle prior to the formation of the lunar crust late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation 4 4 ga to enable efficient mixing and homogenization within the mantle low lunar hse abundances are consistent with proportionally 40 times more late accretion to earth than the moon disproportional late accretion to the two bodies is consistent with the small 182 w excess 21 28 ppm measured in lunar rocks compared to the silicate earth

the case for an imbrium origin of the apollo thorium rich impact melt breccias abstract mafic th rich impact melt breccias most of which are identified with the composition known as low k fra mauro lkfm are the most common rock type in the nonmare regoliths of the apollo lunar landing sites the origin of mafic impact melt breccias bears on many lunar problems the nature of the late meteoroid bombardment cataclysm the spatial distribution of kreep both near the surface and at depth the ages of the major basins and the composition of the early crust of the nearside lunar highlands thus it is crucial that the origin of mafic impact melt breccias be accurately understood because of both intra and intersite differences in compositions of mafic impact melt breccia samples apparent differences in crystallization age and differences in siderophile element ratios previous studies have argued that either 1 most mafic impact melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that 2 they are all from the imbrium apollos 14 and 15 nectaris apollo 16 and serenitatis apollo 17 basins here we reconsider the hypothesis that virtually all of the th rich mafic impact melt breccias from the apollo missions are products of the imbrium impact ejecta deposit modeling based on modern crater scaling indicates that the imbrium event produced ejecta deposits that average hundreds of meters thick or more at all apollo highland sites which is thicker than some previous estimates substantial amounts of imbrium ejecta should have been sampled at every apollo highland site we suggest that the mafic impact melt breccias may be the principal form of those ejecta the imbrium projectile impacted into th rich material that we regard as part of a unique mafic lunar geochemical province we call the high th oval region based on the surface distribution of th only basins within the high th oval region excavated th rich material the th concentrations of the highlands as observed by the apollo orbiting ray experiments are consistent with the estimates from ejecta modeling of the younger basin forming impacts only imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact melt breccias in the apollo sampled regolith the high th oval region still may have been molten or hot at shallow depths 4 ga ago when the imbrium projectile struck we reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances we argue that siderophile element fingerprints of mafic impact melt breccias are not inconsistent with production of all common types by a single projectile we suggest that the narrow range of ages of 3 7 4 0 ga for all successfully dated mafic impact melt breccias may reflect a single event whose age is difficult to measure precisely rather than a number of discrete impact events closely spaced in time such that reported age variations among mafic impact melt breccias reflect the ability to measure 40ar 39ar ages with greater precision than the accuracy with which measured portions of mafic impact melt breccias have recorded the time of their formation

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

volcanic gas composition metal dispersion and deposition during explosive volcanic eruptions on the moon abstract the transport of metals in volcanic gases on the moon differs greatly from their transport on the earth because metal speciation depends largely on gas composition temperature pressure and oxidation state we present a new thermochemical model for the major and trace element composition of lunar volcanic gas during pyroclastic eruptions of picritic magmas calculated at 200 1500 c and over 10 9 10 3 bar using published volatile component concentrations in picritic lunar glasses we have calculated the speciation of major elements h o c cl s and f in the coexisting volcanic gas as the eruption proceeds the most abundant gases are co h 2 h 2 s cos and s 2 with a transition from predominantly triatomic gases to diatomic gases with increasing temperatures and decreasing pressures hydrogen occurs as h 2 h 2 s h 2 s 2 hcl and hf with h 2 making up 0 5 0 8 mol fractions of the total h water h 2 o concentrations are at trace levels which implies that h species other than h 2 o need to be considered in lunar melts and estimates of the bulk lunar composition the cl and s contents of the gas control metal chloride gas species and sulfide gas and precipitated solid species we calculate the speciation of trace metals zn ga cu pb ni fe in the gas phase and also the pressure and temperature conditions at which solids form from the gas during initial stages of the eruption elemental gases are the dominant metal species as the gas loses heat chloride and sulfide species become more abundant our chemical speciation model is applied to a lunar pyroclastic eruption model with isentropic gas decompression the relative abundances of the deposited metal bearing solids with distance from the vent are predicted for slow cooling rates 2 o co 2 and cl rich volcanic gas from erta ale volcano ethiopia as an analogy for more oxidized planetary eruptions in the terrestrial cl rich gas the metals are predominantly transported as chlorides as opposed to metallic vapors and sulfides in the lunar gas due to the presence of cl species metal transport is more efficient in the volcanic gas from erta ale compared to the moon

apollo 12 breccia 12013 impact induced partial pb loss in zircon and its implications for lunar geochronology abstract apollo 12 breccia 12013 is composed of two portions one grey in colour the other black the grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals in this study u pb analyses of ca phosphate and zircon grains were conducted in both portions of the breccia the zircon grains within the grey portion yielded a large range of ages 4154 7 to 4308 6 ma 2 and show decreasing u and th concentrations within the younger grains moreover some grains exhibit recrystallisation features and potentially formation of neoblasts the latter process requires high temperatures above 1600 1700 c leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts as a result of the high temperatures the u pb system of the remaining original zircon grains was most likely open for pb diffusion causing partial resetting and the observed range of 207pb 206pb ages the event that led to the pb loss in zircon could potentially be dated by the u pb system in ca phosphates which have a weighted average 207pb 206pb age across both lithologies of 3924 3 ma 95 conf this age is identical within error to the combined average 207pb 206pb age of 3926 2 ma that was previously obtained from ca phosphates within apollo 14 breccias zircon grains in apollo 12 impact melt breccias and the lunar meteorite sau 169 this age was interpreted to date the imbrium impact the zircon grains located within the black portion of the breccia yielded a similar range of ages 4123 13 to 4328 14 ma 2 to those in the grey portion given the brecciated nature of this part of the sample the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry similarly the currently available zircon data set for all lunar samples may be distorted by partial pb loss resulting in meaningless and misleading age distribution patterns therefore it is crucial to fully understand and recognize the processes and conditions that may lead to partial resetting of the u pb system in zircon in order to better constrain the magmatic and impact history of the moon

computational fluid dynamics modelling of boundary roughness in gravel bed rivers an investigation of the effects of random variability in bed elevation results from a series of numerical simulations of two dimensional open channel flow conducted using the computational fluid dynamics cfd code fluent are compared with data quantifying the mean and turbulent characteristics of open channel flow over two contrasting gravel beds boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra grid scale roughness elements e g particle clusters and small bedforms results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed typically at y h 0 1 0 3 this is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed use of the random elevation model to represent supra grid scale roughness also allows a reduction in the height of the near bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness despite these benefits the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution increasing the horizontal mesh resolution results in an increase in the near bed velocity gradient and turbulent kinetic energy effectively roughening the bed varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles but results in substantial changes to the shape of the turbulent kinetic energy profile these findings have significant implications for the application of cfd within natural gravel bed channels particularly with regard to issues of topographic data collection roughness parameterization and the derivation of mesh independent solutions copyright 2001 john wiley sons ltd

targeting the impactors siderophile element signatures of lunar impact melts from serenitatis abstract highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner solar system melt breccias associated with the 3 89 ga serenitatis impact basin resolve at least two separate impact events kreep rich melt breccias representing the apollo 17 poikilitic suite are enriched in highly siderophile elements 3 6 15 8 ppb ir with ci normalized patterns that are elevated in re ru and pd relative to ir and pt the restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an eh chondrite asteroid probably as melt sheet deposits from the serenitatis basin one exceptional sample a split from melt breccia 77035 has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites indicating a discrete impact event the recognition of multiple impact events and the clear signatures of specific types of meteoritic impactors in the apollo 17 melt breccias shows that the lunar crust was not comprehensively reworked by prior impacts from 3 9 to 4 5 ga an observation more consistent with a late cataclysm than a smoothly declining accretionary flux late accretion of enstatite chondrites during a 3 8 4 0 ga cataclysm may have contributed to siderophile element heterogeneity on the earth but would not have made a significant contribution to the volatile budget of the earth or oxidation of the terrestrial mantle siderophile element patterns of apollo 17 poikilitic breccias become more fractionated with decreasing concentrations trending away from known meteorite compositions to higher re ir and pd pt ratios the compositions of these breccias may be explained by a two stage impact melting process involving 1 deep penetration of the serenitatis impactor into meteorite free lower crust followed by 2 incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet trends to higher re ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component or a non chondritic late accretionary veneer in the pre serenitatis upper crust

phosphate ages in apollo 14 breccias resolving multiple impact events with high precision u pb sims analyses abstract the u pb systems of apatite and merrillite grains within four separate apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry no systematic difference was identified between the 207 pb 206 pb ages of the apatites and merrillites a combined 207 pb 206 pb age of 3927 2 ma 95 conf is determined for three of these samples 14305 103 3926 4 ma 14306 150 3926 6 ma 14314 13 3929 4 ma by combining these data with the ages previously obtained for zircons in apollo 12 impact melt breccia fragments and the lunar meteorite sau 169 a weighted average age of 3926 2 ma 95 conf is obtained which is attributed to the formation of the imbrium basin an age of 3943 5 ma is determined for the fourth breccia 14321 134 which is similar to ages of 3946 15 ma and 3958 19 ma obtained from several older phosphates in 14305 103 and 14314 13 the weighted average of these three older ages is 3944 4 ma 95 conf this is indistinguishable to the age 3938 4 ma 2 obtained for a different apollo 14 impact melt breccia in a previous study after investigating likely sources for this older 3940 ma age we conclude that the humorum or serenitatis basin forming events are likely candidates the potential identification of two large impact events within 15 myrs has important implications for the rate of lunar bombardment around 3 95 3 92 ga this study demonstrates the importance of high precision age determinations for interpreting the impact record of the moon as documented in lunar samples

rhodium gold and other highly siderophile element abundances in chondritic meteorites abstract the abundances of the highly siderophile elements hse re os ir ru pt rh pd and au and 187os 188os isotope ratios have been determined for a set of carbonaceous ordinary enstatite and rumuruti chondrites using an analytical technique that permits the precise and accurate measurement of all hse from the same digestion aliquot concentrations of re os ir ru pt and pd were determined by isotope dilution icp ms and n tims analysis the monoisotopic elements rh and au were quantified relative to the abundance of ir differences in hse abundances and ratios such as re os 187os 188os pd ir and au ir between different chondrite classes are further substantiated with new data and additional rh and au data including new data for ci chondrites systematically different relative abundances of rh between different chondrite classes are reminiscent of the behaviour of re carbonaceous chondrites are characterized by low average rh ir of 0 27 0 03 1s which is about 20 lower than the ratio for ordinary 0 34 0 02 and enstatite chondrites eh 0 33 0 01 el 0 32 0 01 r chondrites show higher and somewhat variable rh ir of 0 37 0 07 well defined linear correlations of hse in particular for bulk samples of ordinary and el chondrites are explained by binary mixing and or dilution by silicates the hse carriers responsible for these correlations have a uniform chemical composition indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components excepting rumuruti chondrites and au in carbonaceous chondrites these correlations also suggest that metamorphism alteration and igneous processes had negligible influence on the hse distribution on the bulk sample scale depletion patterns for rh pd and au in carbonaceous chondrites other than ci are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites fractionated hse abundance patterns of ordinary enstatite and rumuruti chondrites however are more difficult to explain fractional condensation combined with the removal of metal phases at various times and later mixing of early and late formed metal phases may provide a viable explanation planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the hse abundance patterns of chondrite groups hse abundances of some but not all rumuruti chondrites may be consistent with solid sulphide liquid sulphide fractionation processes during impact induced melting

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

lunar rock rain diverse silicate impact vapor condensates in an apollo 14 regolith breccia abstract apollo 14 regolith breccia 14076 long known to be uniquely endowed with high alumina silica poor hasp material of evaporation residue origin has been found to contain a diverse suite of complementary condensates dubbed gasp gas associated spheroidal precipitates gasp occurs in two forms as glassy or extremely fine grained quenched melt spheroids mostly less than 5 m across and as quenched textured clasts up to 200 m across in two of the clasts origin by aggregation of spheroidal gasp is confirmed by the presence of relict spheroids gasp is distinctively depleted in the same refractory major oxides that are characteristically enriched in hasp al 2 o 3 and cao among the larger gasp spheroids al 2 o 3 is seldom 1 wt among the clasts excluding two instances of apparent contamination by na and k rich substrate derived melt bulk al 2 o 3 averages 0 3 wt depletion of al 2 o 3 and cao is also manifested by pyroxene compositions in some clasts e g in the largest clast en 82 wo 0 45 with 0 07 wt al 2 o 3 although gasp bulk compositions are nearly pure sio 2 mgo feo they are nonetheless highly diverse spheroid compositions range in mg from 7 to 84 mol and in feo sio 2 weight ratio from 0 002 to 0 67 bulk compositions and textures of many gasp spheroids suggest that liquid immiscibility occurred prior to quenching implying that these materials were some time after condensation at temperatures of 1680 c textural evidence for immiscibility includes lobate boundaries between silicic and mafic domains and a general tendency for quenched mafic silicates to be concentrated into a few limited patches rather than evenly dispersed the parent melt of the largest clast s pyroxene is inferred to have formed as a partial melt within the parent aggregation of gasp matter compositionally near the pyroxene cristobalite melt eutectic and thus at 1500 c a few gasp spheroids show possible signs of in flight collision coalescence but aggregation of the much larger clasts probably took place in mushy puddles on the lunar surface little mixing took place between these gasp puddles and the related hasp probably because gasp condensation did not commence until after an intermediate stage during which while neither net evaporation nor net condensation took place expansion of the vapor cloud carried the eventual gasp matter well apart from the hasp considering the characteristic length scale of lunar regolith mixing the concentration of both gasp and hasp into this single unique regolith sample 14076 is most consistent with a parent crater size diameter of 10 100 km i speculate that the 14076 regolith may have been unusually situated almost directly uprange from an unusually oblique large impact mercurian analogs of the 14076 impact condensates may have significant implications for remote sensing

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

characterization of the dominant impactor signature for apollo 17 impact melt rocks abstract concentrations of the highly siderophile elements hse re os ir ru pt and pd and 187 os 188 os isotopic compositions are reported for seven apollo 17 impact melt rocks these data are used to examine the dominant chemical signature of the impactor that formed the melts six of the samples 72355 72435 72535 76035 76055 and 76135 have poikilitic textures one sample 73235 has an aphanitic texture data for the samples define linear correlations when ir is plotted versus other hse concentrations with y intercepts indistinguishable from zero for most hse in most rocks scatter about some of the trends and occasional trends with positive y intercepts indicate either mixing of additional components that are heterogeneously distributed within several rocks or modest fractionation of some hse by volatilization crystal fractionation or other processes during formation and evolution of the melt sheet there is no statistical difference between the aphanitic and poikilitic samples in terms of hse ratios after visible granulitic clasts were removed from aphanite 73235 hence earlier speculations that the two types of impact melt rocks at this site may have been generated by different impactors are not supported by our data most apollo 17 samples examined here and in prior studies are characterized by very similar hse signatures consistent with a common impactor these samples are characterized by elevated ru ir pd ir and re os relative to most chondrites collectively the data indicate that the impactor was characterized by the following hse ratios 2 re ir 0 093 0 020 os ir 1 03 0 28 ru ir 1 87 0 30 pt ir 2 36 0 31 pd ir 1 85 0 41 and present day 187 os 188 os of 0 1322 0 0013 the results most likely mean that the impactor was a body with a bulk composition that was just outside the range of meteoritic compositions currently sampled on earth

u pb sims ages of apollo 14 zircon identifying distinct magmatic episodes u pb ages of zircon in four different apollo 14 breccias 14305 14306 14314 and 14321 were obtained by secondary ion mass spectrometry some of the analyzed grains occur as cogenetic poikilitic zircon grains in lithic clasts revealing magmatic events at 4286 ma 4200 4220 ma and 4150 ma the age distribution of the crystal clasts in the breccias exhibits a minor peak at 4210 ma which can be attributed to a magmatic event as recorded in zircon grains located in noritic clasts an age peak at 4335 ma is present in all four breccias as well as zircon grains from different apollo landing sites enhancing the confidence that these grains recorded a global zircon forming event the overall age distribution among the four breccias exhibits minor differences between the breccias collected farther away from the cone crater and the ones collected within the continuous ejecta blanket of the cone crater a granular zircon grain yielded a 207pb 206pb age of 3936 8 ma which is interpreted as an impact event a similar age of 3941 5 ma n 17 mswd 0 89 p 0 58 was obtained for a large zircon grain 430 340 m in size this grain might have crystallized in the same impact melt sheet which formed the granular zircon or the age is representative of the final extrusion of kreep magma the majority of zircon grains however occur as isolated crystal clasts within the matrix and their ages cannot be correlated with any real events impact or magmatic nor can the possibility be excluded that these ages represent partial resetting of the u pb system

microbially catalyzed dolomite formation from near surface to burial abstract over the past century a number of models have been developed to explain the vast stratigraphic distribution of authigenic dolomite these models include hydrogeological geochemical and geomicrobiological interpretations that shed light on kinetic constraints on low temperature dolomite formation and in particular how certain microbes can overcome such limitations in this review we focus on the direct and indirect mechanisms by which microbes facilitate the nucleation and shallow burial diagenetic stabilization of dolomite in marine sediments we also consider the likely effects of local and global scale biogeochemical perturbations over the distribution and activities of subsurface microbes and thus potentially over the stratigraphic distribution of dolomite considering the many advances that have been made in the fields of geomicrobiology and biogeochemistry during the past 20 years investigating the important role microbes play in sedimentary systems will undoubtedly contribute to a more complete interpretation of dolomite formation in the rock record

osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks abstract the concentrations of highly siderophile elements hse re os ir ru pt rh pd au and 187os 188os isotope compositions have been determined for 67 subsamples of six lunar impact rocks from the apollo 14 16 and 17 landing sites and the lunar meteorite dar al gani dag 400 using inductively coupled plasma mass spectrometry icp ms and negative thermal ionization mass spectrometry n tims we report the first re os isochron age on a lunar impact melt rock 187re 187os isotope systematics for apollo 16 sample 67935 define an isochron age of 4 21 0 13 ga mswd 1 5 which is interpreted to reflect localized partitioning processes between solid metal liquid metal as this rock melted the new age adds further constraints on the significance of pre 4 0 ga basin forming impacts on the moon and possible mixing of ancient impactor compositions in lunar impact rocks linear correlations displayed by subsamples of a given impact rock in plots of hse versus ir concentrations are explained by dilution processes through essentially hse free anorthositic lunar crustal target rocks or binary mixing between a high hse meteoritic end member and a low hse end member composition slope derived hse ratios and 187os 188os of the meteoritic component in granulitic impactites 67915 67955 and 79215 are similar to slightly volatile element depleted carbonaceous chondrites suprachondritic ratios of ru ir pt ir rh ir and pd ir for apollo 14 impact melt rock 14310 are similar to ratios observed for other apollo 14 samples and apollo 17 poikilitic impact melt rocks apollo 16 poikilitic and subophitic impact melt rocks 60315 and 67935 show slightly subchondritic os ir and suprachondritic ratios of 187os 188os ru ir pt ir rh ir pd ir and au ir their strongly fractionated hse compositions are similar to some members of the iva iron meteorite group and provide further evidence for an iron meteorite impactor component in apollo 16 impact melt rocks the range of chondritic to suprachondritic ratios of 187os 188os ru ir pt ir pd ir and chondritic to slightly subchondritic os ir of apollo 17 and 14 impact melt rocks may be explained by variable mixing of a carbonaceous chondrite like hse composition as in granulitic impactites with a suprachondritic iron meteorite like impactor composition as inferred from 67935 and 60315 and subsequent homogenization by younger impact events the inferred dispersal of these two components across the lunar nearside and their antiquity 4 2 ga suggest at least two basin forming impacts that predate the 3 9 3 8 ga basin forming era the hse composition of the silicate earth can be matched by a late veneer comprised of 80 volatile depleted carbonaceous chondrite like material and 20 differentiated meteoritic metal similar to what has been identified in ancient lunar impact rocks

rhenium osmium systematics of calcium aluminium rich inclusions in carbonaceous chondrites the re os isotopic systematics of calcium aluminium rich inclusions cais in chondrites were investigated in order to shed light on the behavior of the re os system in bulk chondrites and to constrain the timing of chemical fractionation in primitive chondrites cais with relatively unfractionated rare earth element ree patterns groups i iii v vi define a narrow range of 187 re 188 os 0 3764 0 4443 and 187 os 188 os 0 12599 0 12717 and high but variable re and os abundances 3209 41 820 ppb os in contrast cais that show depletions in highly refractory elements and strongly fractionated ree patterns group ii also show a much larger range in 187 re 188 os 0 409 0 535 and 187 os 188 os 0 12695 0 13770 and greater than an order of magnitude lower re and os abundances than other groups e g 75 7 680 2 ppb os sixteen bulk cais and cai splits plot within analytical uncertainty of a 4558 ga reference isochron as is expected for materials of this antiquity eight samples however plot off the isochron several possible reasons for these deviations are discussed data for multiple splits of one cai indicate that the nonisochronous behavior for at least this cai is the result of re os reequilibration at approximately 1 6 ga thus the most likely explanation for the deviations of most of the nonisochronous cais is late stage open system behavior of re and os in the asteroidal environment the 187 os 188 os os systematics of cais are consistent with previous models that indicate group ii cais are mixtures of components that lost the bulk of their highly refractory elements in a previous condensation event and a minor second component that provided refractory elements at chondritic relative proportions the high re os of group ii cais relative to other cais and chondrite bulk rocks may have been caused by variable mobilization of re and os during medium to low temperature parent body alteration 4 5 ga ago this model is favored over nebular models which pose several difficulties the narrow range of 187 os 188 os in group i iii v and vi bulk cais and the agreement with 187 os 188 os of whole rock carbonaceous chondrites suggest that on a bulk inclusion scale secondary alteration only modestly fractionated re os in these cais the average of 187 os 188 os for group i iii v and vi cais is indistinguishable from average ci chondrites indicating a modern solar system value for 187 os 188 os of 0 12650 corresponding to a 187 re 188 os of 0 3964 copyright 2001 elsevier science ltd

coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification draken formation upper riphean svalbard the draken formation 120 250 m of northeast spitsbergen svalbard forms part of a thick upper proterozoic carbonate platform succession it consists predominantly of intraformational dolomitic conglomerates with excellent textural preservation six main lithofacies were recognized in the field quartz sandstones stromatolitic mats conglomerates with silicified intraclasts dolostone conglomerates with desiccated mudrocks oolitic pisolitic grainstones and fenestral dolostones a series of five main gradational biofacies were recognized from silicified and rare calcified microfossils biofacies 1 represents low energy subtidal benthos erect filaments and plankton acritarchs and vase shaped microfossils whereas biofacies 2 to 5 are microbial mat assemblages with filamentous mat builders and associated dwellers and washed in plankton ranging from basal intertidal to high intertidal supratidal colour values a measure of the lightness of the colour shade of sawn rock samples were quantified using a munsell chart and exhibit a pronounced variation means of major groups varying from 4 0 to 5 95 across the spectrum of subtidal to supratidal sediments as inferred from other criteria the lightening in progressively more exposed sediments is related to lowering of organic carbon contents probably mainly by oxidation six types of early cement have been recognized calcite microspar type 1 is common as a subtidal cement in many proterozoic formations whereas types 2 subtidal isopachous fringes 3 subtidal hardground dolomicrite and 4 intertidal meniscus dolomicrite are very similar to phanerozoic examples except for their dolomitic mineralogy types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena they characterize the fenestral lithofacies and compare with modern supratidal cements consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching dolomitization silicification were all significant syndepositional processes altering the original metastable carbonates the data set provides evidence for a spectrum of peritidal environments including ooid shoals protected subtidal tidal sandflats and protected carbonate mudflats different sections show a preponderance of particular facies the coastal lithofacies continuum was completely dolomitized unlike offshore to ooid shoal facies of adjacent formations dolomitization thus bears a relationship to depositional bathymetry although hydrodynamics clearly have a role the potential importance of whiting precipitation in raising mg ca in marginal marine environments is also stressed

osmium isotope and highly siderophile element systematics of the lunar crust abstract coupled 187os 188os and highly siderophile element hse os ir ru pt pd and re abundance data are reported for pristine lunar crustal rocks 60025 62255 65315 ferroan anorthosites fan and 76535 78235 77215 and a norite clast in 15455 magnesian suite rocks mgs osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks previously identified as pristine contain meteorite derived hse low hse abundances in fan and mgs rocks are consistent with derivation from a strongly hse depleted lunar mantle at the time of formation the lunar floatation crust represented by fan had 1 4 0 3 pg g 1 os 1 5 0 6 pg g 1 ir 6 8 2 7 pg g 1 ru 16 15 pg g 1 pt 33 30 pg g 1 pd and 0 29 0 10 pg g 1 re 0 00002 ci and re os ratios that were modestly elevated 187re 188os 0 6 to 1 7 relative to ci chondrites mgs samples are on average characterised by more elevated hse abundances 0 00007 ci compared with fan this either reflects contrasting mantle source hse characteristics of fan and mgs rocks or different mantle crust hse fractionation behaviour during production of these lithologies previous studies of lunar impact melt rocks have identified possible elevated ru and pd in lunar crustal target rocks the new results provide no supporting evidence for such enrichments if maximum estimates for hse in the lunar mantle are compared with fan and mgs averages crust mantle concentration ratios d values must be 0 3 such d values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle with the notable exception of re given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust the potential similarities in crust mantle hse partitioning for the earth and moon are somewhat surprising low hse abundances in the lunar crust coupled with estimates of hse concentrations in the lunar mantle implies there may be a missing component of late accreted materials as much as 95 to the moon if the earth moon mass flux estimates are correct and terrestrial mantle hse abundances were established by late accretion

identifying impact events within the lunar cataclysm from 40ar 39ar ages and compositions of apollo 16 impact melt rocks abstract lunar impact melt breccias provide a unique record of the timing and frequency of collisional events during the early history of the inner solar system prior to the development of a significant rock record on earth the predominance of ages clustering between 3 8 and 4 0 ga was a major unexpected discovery obtained from geochronological studies of lunar impact melts and is the basis of the concept that a cataclysmic bombardment of large planetesimals struck the earth and moon and possibly the entire inner solar system about 3 85 0 10 billion years ago as a test of the cataclysm hypothesis we measured high resolution 20 50 steps 40 ar 39 ar age spectra on 25 samples of apollo 16 impact melt breccias using a continuous laser heating system on sub milligram fragments twenty one of these 25 breccias produced multi step plateaus that we interpret as crystallization ages with 20 of these ages falling in the range 3 75 3 96 ga we propose that at least four different melt producing impact events can be distinguished based on the ages bulk compositions and petrographic characteristics of apollo 16 melt breccias the recognition of multiple impact events within the apollo 16 melt breccia suite shows that numerous impact events occurred on the lunar surface within a relatively narrow time interval providing additional evidence of a heavy bombardment of the moon during the early archean

ancient crustal components in the fra mauro breccias texturally pristine clasts preserve primary petrographic relationships and mineral compositions yielding insights into igneous processes of the early lunar crust that cannot be gained from highly shocked and brecciated chemically pristine samples the use of texture as a prime criterion allows for expansion of the data base derived solely from chemical criteria and provides complementary data texturally pristine clasts from the apollo 14 site studied here include anorthosite troctolites gabbronorites and basalts alkali anorthosites are plagioclase orthocumulates and may form by flotation in mg suite plutons ferroan anorthosite was cataclastically deformed and metamorphosed to granulite facies troctolites include both 01 plg and 01 en plg cumulates major and trace element analyses of two troctolites reveal eastern geochemical affinities that contrast other western troctolites gabbronorites are plg plg sp cumulates whose parent magmas may range from high al to intermediate ti mare basalt at least three varieties of mare basalt are found at apollo 14 high al low ti low al intermediate ti and low al ti vhk basalt vhk very high potassium basalt is a new variety indigenous to apollo 14

osmium isotope and highly siderophile element systematics of lunar impact melt breccias implications for the late accretion history of the moon and earth abstract to characterize the compositions of materials accreted to the earth moon system between about 4 5 and 3 8 ga we have determined os isotopic compositions and some highly siderophile element hse re os ir ru pt and pd abundances in 48 subsamples of six lunar breccias these are apollo 17 poikilitic melt breccias 72395 and 76215 apollo 17 aphanitic melt breccias 73215 and 73255 apollo 14 polymict breccia 14321 and lunar meteorite nwa482 a crystallized impact melt plots of ir versus other hse define excellent linear correlations indicating that all data sets likely represent dominantly two component mixtures of a low hse target presumably endogenous component and a high hse presumably exogenous component linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all hse except for ru and pd in two samples the slopes of the linear regressions are insensitive to target rock contributions of ru and pd of the magnitude observed thus the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks the 187 os 188 os and regression derived elemental ratios for the apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites the hse in the apollo 17 aphanitic melt breccias however might partially or entirely reflect the hse characteristics of hse rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation in this case the hse characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias the impactor components in the apollo 17 poikilitic melt breccias and in the apollo 14 breccia have higher 187 os 188 os pt ir and ru ir and lower os ir than most chondrites these compositions suggest that the impactors they represent were chemically distinct from known chondrite types and possibly represent a type of primitive material not currently delivered to earth as meteorites

the effect of 1 9 and 1 4 ga impact events on 4 3 ga zircon and phosphate from an apollo 15 melt breccia 1 zircon and phosphate grains from matrix and quartz monzodiorite qmd clasts in two thin sections of apollo 15 impact melt breccia 15405 were investigated using optical microscopy scanning electron microscopy raman spectroscopy and ion microprobe u pb analyses u pb results for zircon grains with well defined cathodoluminescence zoning define the primary i e magmatic crystallization age as 4330 6 ma 2 one zircon consists of a preserved inner part surrounded by a porous polycrystalline granular mixture of zircon and baddeleyite indicating incomplete reaction of the zircon with melt previous work showed that this microstructure could form at pressures above 60 gpa and a temperature close to 1700 c and is evidence of an impact related melting event the u pb system of this grain indicates a resetting event at 1940 10 ma interpreted as the age of this impact impact 1 other zircon and phosphate grains also have disturbed u pb systems showing an even younger reset event impact 2 at 1407 57 ma evidence of impact is supported by microstructures of zircon and baddeleyite such as secondary rims these impacts are tentatively identified as those having formed autolycus and aristillius craters

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

early diagenetic recrystallization of holocene 3000 years old peritidal dolomites ambergris cay belize three peritidal carbonate crusts and associated intercrust sediments total thickness of 30cm aged 3000 years bp on ambergris cay belize contain 32 100 calcian dolomite x 72 5 dolomite ranging in composition from 40 to 46 mol mgco3 x 43 3 dolomite replaced high mg calcite foraminiferal muds penecontemporaneously with sedimentation forming partially dolomitized sediments and lithified crusts dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth 0 4 m near the top of the section to 1 0 m near the base of the dolomitized section this size increase is not accompanied by any significant decrease in porosity crystal size distributions appear to be log normal and become increasingly broad and flat with depth rietveld x ray pattern fitting structure refinements indicate increasing ca and mg concentrations on their respective sites cation ordering as a function of increasing depth most of the ordering occurs within the first 15 cm of the surface stoichiometry does not increase with depth indicating no relationship between the ca mg ratio and cation ordering strong geochemical trends were observed down section in the dolomite including 1 increasing mn content 44 to 274 ppm and 2 decreasing 13c values 0 9 to 5 5 pdb oxygen isotope values range from 18o 1 3 pdb in the upper part of the section to 2 6 pdb in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend down section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy driven dissolution reprecipitation process ostwald ripening the observed trends in carbon isotopes and mn content probably result from geochemical re equilibration during recrystallization and reflect reducing conditions and an isotopically light organically derived carbon source oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level

compositional variation in apollo 16 impact melt breccias and inferences for the geology and bombardment history of the central highlands of the moon abstract high precision data for the concentrations of a number of lithophile and siderophile elements were obtained on multiple subsamples from 109 impact melt rocks and breccias mostly crystalline from the apollo 16 site compositions of nearly all apollo 16 melt rocks fall on one of two trends of increasing sm concentration with increasing sc concentration the eastern trend lower sm sc mg fe and sm yb ratios consists of compositional groups 3 and 4 of previous classification schemes these melt rocks are feldspathic poor in incompatible and siderophile elements and appear to have provenance in the descartes formation to the east of the site the western trend higher sm sc mg fe and sm yb ratios consists of compositional groups 1 and 2 these relatively mafic kreep bearing breccias are a major component 35 of the cayley plains west of the site and are unusual compared to otherwise similar melt breccias from other sites in having high concentrations of fe ni metal 1 2 the metal is the carrier of the low ir au 0 3 chondritic siderophile element signature that is characteristic of the apollo 16 site four compositionally distinct groups 1m 1f 2db and 2nr of western trend melt breccias occur that are each represented by at least six samples compositional group 1 of previous classification schemes the poikilitic or lkfm melt breccias can be subdivided into two groups group 1m represented by six samples including 60315 is characterized by lower al2o3 concentrations higher mgo and alkali concentrations and higher mg fe and cr sc ratios than group 1f represented by fifteen samples including 65015 group 1m also has siderophile element concentrations averaging about twice those of group 1f and ir au and ir ni ratios that are even lower than those of other western trend melt rocks ir au 0 24 0 03 ci normalized at the mafic extreme of group 2 vha melt breccias the melt lithology occurring as clasts in feldspathic fragmental breccias from north ray crater group 2nr is compositionally distinct from the melt lithology of dimict breccias from the cayley plains group 2db in having higher concentrations of sc cr and heavy rare earth elements and lower concentrations of siderophile elements the distinct siderophile element signature high absolute abundances low ir au ratio suggest that the four groups of mafic melt breccia are all somehow related ratios of some lithophile elements also suggest that they are more closely related to each other than they are to melt breccias from other apollo sites however none of the breccia compositions can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials thus the origin of the four groups of mafic melt breccia is enigmatic if they were produced in only one or two impacts then a mechanism exists for generating regimes of impact melt breccia in a single impact that are substantially different from each other in composition for various reasons including the problem of delivering large volumes of four different types of melt to the apollo 16 site it is unlikely that any of these breccias were produced in basin forming impacts if they were produced in as many as four crater forming impacts then the unusual siderophile element signature is difficult to explain possible explanations are 1 1 the four groups of melt breccia all contain metal from a single earlier impact 2 2 they were each formed by related metal rich meteoroids or 3 3 some common postimpact process has resulted in metal of similar composition in each of four melt pools within a compositional group most intrasample and intersample variation in lithophile element concentrations is caused by differences among samples in the proportion of a component of normative anorthosite or noritic anorthosite in most cases this compositional variation probably reflects variation in clast abundance for group 2db and probably 2nr differences in abundance of a component of ferroan anorthosite estimated al2o3 32 accounts for the compositional variation for groups 1m and 1f the anorthositic component is more mafic estimated al2o3 26 some group 2 samples may be related by a troctolitic component of varying abundance

sulfur isotope homogeneity of lunar mare basalts abstract we present a new set of high precision measurements of relative 33s 32s 34s 32s and 36s 32s values in lunar mare basalts the measurements are referenced to the vienna canyon diablo troilite v cdt scale on which the international reference material iaea s 1 is characterized by 33s 0 061 34s 0 3 and 36s 1 27 the present dataset confirms that lunar mare basalts are characterized by a remarkable degree of sulfur isotopic homogeneity with most new and published sf6 based sulfur isotope measurements consistent with a single mass dependent mean isotopic composition of 34s 0 58 0 05 33s 0 008 0 006 and 36s 0 2 0 2 relative to v cdt where the uncertainties are quoted as 99 confidence intervals on the mean this homogeneity allows identification of a single sample 12022 281 with an apparent 33s enrichment possibly reflecting cosmic ray induced spallation reactions it also reveals that some mare basalts have slightly lower 34s values than the population mean which is consistent with sulfur loss from a reduced basaltic melt prior to eruption at the lunar surface both the sulfur isotope homogeneity of the lunar mare basalts and the predicted sensitivity of sulfur isotopes to vaporization driven fractionation suggest that less than 1 10 of lunar sulfur was lost after a potential moon forming impact event

the great lunar hot spot and the composition and origin of the apollo mafic lkfm impact melt breccias thorium rich mafic impact melt breccias from the apollo 14 17 missions that is those breccias identified with the composition known as lkfm are regarded largely as products of basin forming impacts that penetrated the feldspathic crust and sampled underlying mafic material and magma ocean residuum carrying the compositional signature of kreep potassium rare earth elements phosphorous despite considerable compositional variation among such breccias compositions of all of them correspond to mixtures of only four components 1 a norite with composition generally similar to that of apollo 15 basalt mean abundance 58 range 30 95 2 fo 90 dunite mean 13 range 1 27 3 feldspathic upper crust mean 29 range 4 50 and feni metal 0 1 1 7 petrographic evidence has shown that much of the feldspathic component but none of the kreep component is clastic this observation and the high proportion of kreep norite component in the breccias suggest that the melt zone of the impact or impacts forming the breccias contained little feldspathic material but consisted predominantly of material with the average composition of kreep norite the dunite component probably derives ultimately from the upper mantle these conclusions support the hypothesis that the breccias were not formed in typical feldspathic crust but instead by one or more impacts into what is designated here the great lunar hot spot that is the anomalous th rich terrane in the imbrium procellarum area identified by the apollo and lunar prospector gamma ray spectrometers the lkfm composition is a special product of the great lunar hot spot and is not the average composition of the lower crust in typical feldspathic highlands similarly mg suite and alkali suite plutonic rocks of the apollo collection are likely all differentiation products of the hot spot not of plutons that might occur in typical feldspathic crust

using the sulfide replacement petrology in lunar breccia 67915 to construct a thermodynamic model of s bearing fluid in the lunar crust abstract in this work we investigate the compositions and origin of metasomatic fluids responsible for the formation of sulfide replacement textures in mg suite lithologies of lunar samples 67915 149 and 67915 150 we have constructed a quantitative thermodynamic model of the composition of the metasomatic fluid using f o 2 f s 2 and temperature constraints derived from a thermodynamic analysis of the metasomatic fluid mineral reactions and the measured compositions of the phases in the sulfide assemblages results from this modeling indicate that the metasomatic fluid responsible for the formation of the sulfide replacement textures was likely dominated by a combination of h 2 and ch 4 with minor abundances of h 2 o co and h 2 s the modeling indicates that h 2 s was by orders of magnitude the dominant s species present in the metasomatic fluid and s isotopes in the replacement sulfides suggest that the fluid experienced significant removal of h 2 s by sulfide precipitation the calculated h 2 and h 2 o contents of the metasomatic fluid are consistent with those that might be expected for the late stage degassing of shallowly emplaced intrusive magma bodies

slope induced changes in channel character along a gravel bed stream the allt dubhaig scotland changes in channel character along a small river in the scottish highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0 02 to o ooo15 because of local baselevel control this decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed the channel pattern alters progressively rather than abruptly in the sequence 1 near braided 2 meandering with active point bar chutes 3 meandering with active outer bank talweg 4 stable equiwidth sinuous the changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical cdncepts than by purely empirical discriminant or trend equations measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload it is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions are typical of many streams in mountainous areas and can be understood in terms of slope induced changes in hydraulic properties

on the history of the early meteoritic bombardment of the moon was there a terminal lunar cataclysm abstract this work revisits the hypothesis of the so called lunar terminal cataclysm suggested by tera et al 1974 tera et al 1973 as a strong peak in the meteorite bombardment of the moon around 3 9 ga ago according to the hypothesis most of the impact craters observed on the lunar highlands formed during this short time period and thus formed the majority of the lunar highland impact breccias and melts the hypothesis arose from the observation that the ages of highland samples from all the lunar missions are mostly grouped around 3 9 4 0 ga since those missions however radiometric dating techniques have progressed and many samples both old and new have been re analyzed nevertheless the debate over whether there was a terminal cataclysm persists to progress in this problem we summarized results of 269 k ar datings mostly made using the 40ar 39ar technique of highland rocks represented by the apollo 14 15 16 17 and luna 20 samples and 94 datings of clasts of the highland rocks from 23 lunar meteorites representing 21 localities on the lunar surface and considered them jointly with the results of our modelling of the cumulative effect of the impact gardening process on the presence of impact melt of different ages at the near surface of the moon the considered results of k ar dating of the apollo luna samples of lunar highland rocks confirmed a presence of strong peak centered at 3 87 ga but since the time when the hypothesis of terminal cataclysm was suggested it has become clear that this peak could be a result of sampling bias it is the only prominent feature at the sites with an apparent domination of imbrium basin ejecta apollo 14 and 15 and the age pattern is more complicated for the sites influenced not only by imbrium ejecta but also that of other basins nectaris at the apollo 16 site and serenitatis at the apollo 17 site our modelling shows that the cataclysm if it occurred should produce a strong peak in the measured age values but we see in the considered histograms and relative probability plots not only the 3 87 ga peak due to imbrium basin but also several secondary peaks caused by the formation of other basins distributed between 3 87 and 4 25 ga the lunar terminal cataclysm hypothesis is in disagreement with the distribution of k ar ages for the highland rocks of the lunar meteorites the population of lunar meteorites representing localities randomly distributed over the lunar surface and thus free from the mentioned sampling bias shows no 3 9 ga peak as it should if the cataclysm did occur we conclude that the statistics of sample ages contradict the terminal cataclysm scenario in the bombardment of the moon we also see evidence for the formation of several impact basins between 3 87 and 4 25 ga which is likewise incompatible with the hypothesis of a short interval cataclysm there remain other basins including the largest south pole aitken the ages of which should be determined in future studies to further clarify the impact history sample return missions targeted to date several key basins need to be planned and the continued study of lunar meteorites may also bring new details to the general view of the impact bombardment of the moon

petrology and geochemistry of lunar dimict breccia 61015 dimict breccia 61015 consists dominantly of granulated anorthosite and impact melt rock of vha very high alumina basalt composition the anorthosite is a typical apollo 16 ferroan anorthosite except that it is slightly more augite rich than most other ferroan anorthosites the breccia probably formed in the floor of an expanding crater cavity by the following process impact melt generated within the cavity was injected into underlying anorthositic bedrock and solidified rapidly and the anorthosite and melt rock were then fragmented by deformation in the later stages of the impact all the apollo 16 dimict breccias except perhaps 61016 appear to have formed in the same impact the source crater cannot yet be identified but it may be an intermediate sized 50 150 km diameter crater within a few hundred kilometers of the apollo 16 site in the direction of the imbrium basin of either pre nectarian or post nectarian pre imbrian age the breccia has been affected by at least one and perhaps as many as four recent small impacts the most recent was probably the south ray impact the recent impact s produced internal shock induced melting of the breccia and injected and coated the rock with externally derived impact melts the coating glass is heterogeneous its average composition is similar to the compositions of other apollo 16 coating glasses except that its tio2 content is lower the vein glass is unusually mafic and has very low tio2 for its al2o3 content the tio2 contents of these glasses establish that they cannot be melted soils their compositions require a mafic low tio2 protolith component that has not yet been identified among the large rock samples perhaps a very mafic member of the ferroan anorthosite suite

diverse impactors in apollo 15 and 16 impact melt rocks evidence from osmium isotopes and highly siderophile elements abstract concentrations of highly siderophile elements hse and 187 os 188 os isotopic compositions for eleven impact related rocks from the apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins data for the samples all define linear trends on plots of hse versus ir concentrations whose slopes likely reflect the relative hse compositions of the dominant impactors that formed the rocks the inferred imbrium basin impactor that generated apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187 os 188 os ru ir pt ir and pd ir ratios diverse impactor components are revealed in the apollo 16 impact melt rocks the 187 os 188 os and hse ir ratios of the impactor components in melt rocks 60635 63595 and 68416 with reported ages 4 0 ga by contrast the impactor components in melt rocks 60235 62295 and 67095 with reported ages of 3 9 ga are characterized by suprachondritic 187 os 188 os and hse ir ratios similar to the apollo 15 impact melt rocks and may also sample the imbrium impactor three lithic clasts from regolith breccias 60016 and 65095 also with 3 9 ga ages contain multiple impactor components of which the dominant composition is considerably more suprachondritic than those implicated for imbrium and serenitatis apollo 17 impactors the dominant composition recorded in these rocks was most likely inherited from a pre imbrium impactor consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend virtually all lunar impact melt rocks sampled by the apollo missions as well as meteorites are characterized by 187 os 188 os and hse ir ratios that when collectively plotted define linear trends ranging from chondritic to fractionated compositions the impact melt rocks with hse signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had hse compositions similar to known chondrite groups by contrast the impact melt rocks with non chondritic relative hse concentrations could not have been made by mixing of known chondritic impactors these signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections alternately they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core pre 4 0 ga ages for at least some impactor components with both chondritic and fractionated hse raise the possibility that the bulk of the hse were added to the lunar crust prior to the later stage basin forming impacts such as imbrium and serenitatis as proposed by fischer godde and becker 2012 for this scenario the later stage basin forming impacts were more important with respect to mixing prior impactor components into melt rocks rather than contributing much to the hse budgets of the rocks themselves

sedimentary textures of recent belizean peritidal dolomite abstract recent surface and buried dolomite crusts and massive dolomite beds on ambergris cay belize have formed on topographically low relief peritidal flats whereon the hydrologic system is characterized by a dominant phreatic diagenetic overprint as a consequence of their low subaerial exposure index such dolomites and associated undolomitized surface sediments lack many of the textural and microfabric features that are considered diagnostic of the peritidal environment the progressive dolomitization of precursor sandy fossiliferous sediments wackestones to packstones is accompanied by a textural change to dolowackestones with allochem ghosts and a porosity system that resembles but is distinct from syndepositional fenestral fabrics formed in unconsolidated or partly consolidated peritidal sediments the microcrystalline dolomite cements in these rocks are predominantly of phreatic aspect and only rare examples of possible vadose diagenetic fabrics are observed these texturally anomalous peritidal dolomites characterized by a markedly subtidal appearance may have fossil counterparts in many ancient carbonate tidal flat sequences

numerical simulation of hydrodynamics and bank erosion in a river bend we present an integrated analysis of bank erosion in a high curvature bend of the gravel bed cecina river central italy our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations the results identify two mechanisms that explain how most bank retreat usually occurs after rather than during flood peaks first in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases reducing sidewall boundary shear stress and fluvial erosion at peak flow stages second bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events

numerical simulation of three dimensional flow hydraulics in a braided channel results are presented from a numerical simulation of three dimensional flow hydraulics around a mid channel bar carried out using the fluent uns computational fluid dynamics cfd software package fluent uns solves the three dimensional reynolds averaged form of the navier stokes equations turbulence closure is achieved using a rng k model simulated flow velocities are compared with measured two dimensional velocities downstream and cross stream obtained using an electromagnetic current meter ecm the results of the simulation are qualitatively consistent with the flow structures observed in the field quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two r 0 88 and a mean difference of 0 09 m s 1 deviations between simulated and measured velocities may be identified that are both random and systematic the former may reflect a number of factors including subgrid scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ecm orientation model mesh configuration roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities these results illustrate the potential for using cfd software to simulate flow hydraulics in natural channels with complex configurations they also highlight the need for detailed spatially distributed datasets of three dimensional flow variables to establish the accuracy and applicability of cfd software copyright 1999 john wiley sons ltd

subophitic lithologies in kreep rich poikilitic impact melt rocks from cayley plains apollo 16 remnants of a volcanic highland crust abstract kreep rich poikilitic impact melt rocks 65777 11 65015 88 and 62235 66 are the only mafic impact melt rocks from cayley plains stations apollo 16 from which areas of subophitic texture can be reported the bulk chemistry of these unique subophitic areas and the surrounding poikilitic matrices as well as mineral compositions olivine plagioclase pyroxene fe ni metal were determined by electron microprobe analysis all subophitic areas could be undoubtedly identified as impact melt rock clasts inclusion 65777 11 ii is of uniquely kreep rich composition 62235 66 ii can be classified as anorthositic therefore our attempt to identify pristine volcanic basement rocks of the cayley regions among these inclusions of basaltic texture failed however the absence of pristine volcanic target rock fragments and the existence of kreep rich and anorthositic impact melt clasts in kreep rich impact melt rocks from cayley plains favors the theory that the cayley plains formation is part of the ejecta blanket from a large basin type impact crater imbrium which is underlain by anorthositic material nectaris ejecta and has been reworked by local impacts in post imbrian times

regolith mixing by impacts lateral diffusion of basin melt abstract impact cratering has been the primary process to alter the distribution of lunar highland material since the formation of a crust this impact history is recorded in the radiogenic clocks of impact melts which are accessible for study on lunar samples and meteorites however primary impact melt is exposed to a long time gardening process i e re melting excavation burial and re excavation by subsequent impacts resulting in a complex spatial distribution of materials representing specific impact events to investigate the diffusion behavior of impact melt a model tracing the evolving distribution of melt laterally and with depth was built using a monte carlo approach given scaling laws concerning melt production and ejecta distribution the size frequency distribution of impact craters and the rate function for crater formation we examine the evolution of melt component occurrence of different ages three mid to late forming basins serenitatis crisium and imbrium are chosen as a case study for the diffusion of melt from major basin forming events the survival probability of basin melt occurrence at the apollo and luna sampling spots is derived it is expected to find abundant imbrium and crisium melt at the apollo and luna sampling sites consistent with the k ar radiometric dates of highland samples whereas the older serenitatis melt was subjected to the later long term gardening strongly influenced by later local impacts and thus is less abundant understanding the diffusion of impact melt is helpful for interpretation of radiometric ages of lunar samples and can be used to predict the distribution of differently aged melts at future landing sampling sites such as the chinese chang e 4 ce 4

the application of computational fluid dynamics to natural river channels three dimensional versus two dimensional approaches this paper addresses the recent trend in fluvial geomorphology towards using computational fluid dynamics to explore the adjustment between flow processes sediment transport and river channel morphology it aims to evaluate the extent to which three dimensional 3d models improve predictive ability and prediction utility compared to two dimensional 2d applications this is achieved through comparing the predictions of both 3d and 2d models with high quality field data identical boundary conditions obtained from a confluence within a gravel bed river system with high relative roughness are defined for each model evaluation of the 3d model suggests that there is a fundamental limitation upon model predictive ability due to problems of specifying topographic complexity however comparison with the 2d model shows that the 3d model has a higher predictive ability particularly if the 2d model is not corrected for the effects on flow structure of secondary circulation further the 3d model provides more reliable estimates of bed shear stress and other more useful information such as the three dimensional flow field important for mixing processes this suggests that there is significant merit in the move towards 3d models but that research is required to incorporate methods developed in other fields for dealing with boundary condition uncertainties

formation of apollo 16 impactites and the composition of late accreted material constraints from os isotopes highly siderophile elements and sulfur abundances fe ni metal schreibersite troilite intergrowths in apollo 16 impact melt rocks and new highly siderophile element hse and s abundance data indicate that millimeter scale closed system fractional crystallization processes during cooling of impactor derived metal melt droplets in impact melts are the main reason for compositional variations and strong differences in abundances and ratios of hse in multiple aliquots from apollo 16 impact melt rocks element ratios obtained from linear regression of such data are therefore prone to error but weighted averages take into account full element budgets in the samples and thus represent a more accurate estimate of their impactor contributions modeling of solid metal liquid metal partitioning in the fe ni s p system and hse patterns in impactites from different landing sites suggest that bulk compositions of ancient lunar impactites should be representative of impact melt compositions and that large scale fractionation of the hse by in situ segregation of solid metal or sulfide liquid in impact melt sheets most likely did not occur the compositional record of lunar impactites indicates accretion of variable amounts of chondritic and non chondritic impactor material and the mixing of these components during remelting of earlier ejecta deposits the non chondritic composition appears most prominently in some apollo 16 impactites and is characterized by suprachondritic hse ir ratios which increase from refractory to moderately volatile hse and exhibit a characteristic enrichment of ru relative to pt large scale fractional crystallization of solid metal from sulfur and phosphorous rich metallic melt with high p s in planetesimal or embryo cores is currently the most likely process that may have produced these compositions similar materials or processes may have contributed to the hse signature of the bulk silicate earth bse