chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

effects of experimental leaching on rbsr and kar isotopic systems and ree contents of diagenetic illite leaching with 1 n hcl 1 n nh4cl acetone humic acid extracts cation exchange resin and nh4 edta have negligible effects on the rb sr and k ar systems of different size fractions 0 4 and 0 8 2 m of a 1m diagenetic illite none of these treatments including the 1 n hcl treatment induced preferential removal of radiogenic 87sr and 40ar from mineral structures however the 1 n hcl treatment removed significant amounts of sr and ree as compared to the treatments by nh4 edta and cation exchange resin however much of these hcl derived elements resided in non clay minerals such as carbonate and fe oxide phases the k ar and rb sr ages of the samples were not affected by the hcl treatment any more than by the nh4 edta or the cation exchange resin because of potential loss of solid material during elutriation of differentially treated samples the hcl treatment is preferred over other methods allowing sufficient controls on the isotopic and mass balances between the leachate and the corresponding residue of an untreated sample for rb sr isotopic dating purposes of clay minerals from sedimentary and metasedimentary rocks leaching experiments give information about the ambient environment of crystallization analysis of both the leachate and residue under carefully controlled conditions is recommended

the impact of hydrothermal mineral replacement reactions on the formation and alteration of carbonate hosted polymetallic sulfide deposits a case study of the artemis prospect queensland australia abstract hydrothermal sulfidation of carbonate rocks and the reverse replacement of sulfides by carbonates are important processes responsible for the formation and subsequent alteration of ore deposits these processes have been recognized in the case study of the artemis zn cu au prospect which is located in the cloncurry district within the eastern succession of the mount isa inlier nw queensland australia the high grade polymetallic mineralization displays a complex association of massive sulfides and carbonates hosted in a vertical marble lens adjacent to garnet bearing schist staurolite muscovite schist and quartzo feldspathic psammite petrographic analyses of drill core specimen by optical and electron microscopy micro x ray florescence elemental maps electron probe microanalyses cathodoluminescence and bulk geochemistry suggested three major stages of mineralization i pre ore stage marble calcite 0 ii ore stage involving sulfidation of the marble and precipitation of calcite 1 and spherical sulfide inclusions in the first episode and the formation of calcite 2 and massive sulfide in the second episode and iii post ore stage involving replacement of sulfides by calcite 3 and garnet mineral replacement reactions are dominant processes in rocks of ore stage and post ore stage the ore stage involved the dissolution of coarse and pristine calcite 0 ca0 95mn0 3fe0 2co3 and the precipitation of sulfides and calcite 1 ca0 96mn0 3fe0 1co3 and calcite 2 ca0 96mn0 3fe0 1co3 the sulfide assemblage consists of pyrrhotite sphalerite chalcopyrite galena arsenopyrite cobaltite costibite bi cubanite ag sulfides and au in decreasing order of abundance in episode 1 calcite 1 contains spherical sulfide inclusions while in episode 2 calcite 2 envelopes calcite 1 the replacement of co rich arsenopyrite fe0 75co0 26as1 13s0 83 by relatively co poor arsenopyrite fe0 86co0 15as0 99s0 99 via a coupled dissolution precipitation reaction occurred in episode 2 based on arsenopyrite geothermometry the temperatures for the first and second episode are 650 50 and 450 70 c respectively micro xrf elemental maps of the garnet bearing schist adjacent to the sulfidized marble revealed the sulfidation front where pre ore garnet and amphibole were replaced by pyrrhotite chalcopyrite au biotite and muscovite in the post ore stage calcite 2 was partially replaced by siderite and calcite 3 ca0 94mn0 4fe0 2mg0 1co3 which shows sectoral and oscillatory zoning galena was replaced by calcite 3 as documented by embayed lobate grain boundaries between these minerals and pyrrhotite fe 1 x s x 0 091 0 119 was pervasively replaced by garnet mn1 2fe1 2ca0 6 al2si3o12 the microscale investigations of the mineralization processes in the artemis orebody document the impact of hydrothermal mineral replacement reactions on metal sequestration mineral replacement reactions facilitate the formation of ore deposits and subsequent alteration processes which may lead to the loss of economic value of the deposit

images from the surface of asteroid ryugu show rocks similar to carbonaceous chondrite meteorites the near earth asteroid 162173 ryugu is a 900 m diameter dark object expected to contain primordial material from the solar nebula the mobile asteroid surface scout mascot landed on ryugu s surface on 3 october 2018 we present images from the mascot camera mascam taken during the descent and while on the surface the surface is covered by decimeter to meter sized rocks with no deposits of fine grained material rocks appear either bright with smooth faces and sharp edges or dark with a cauliflower like crumbly surface close up images of a rock of the latter type reveal a dark matrix with small bright spectrally different inclusions implying that it did not experience extensive aqueous alteration the inclusions appear similar to those in carbonaceous chondrite meteorites

craters boulders and regolith of 101955 bennu indicative of an old and dynamic surface small kilometre sized near earth asteroids are expected to have young and frequently refreshed surfaces for two reasons collisional disruptions are frequent in the main asteroid belt where they originate and thermal or tidal processes act on them once they become near earth asteroids here we present early measurements of numerous large candidate impact craters on near earth asteroid 101955 bennu by the osiris rex origins spectral interpretation resource identification and security regolith explorer mission which indicate a surface that is between 100 million and 1 billion years old predating bennu s expected duration as a near earth asteroid we also observe many fractured boulders the morphology of which suggests an influence of impact or thermal processes over a considerable amount of time since the boulders were exposed at the surface however the surface also shows signs of more recent mass movement clusters of boulders at topographic lows a deficiency of small craters and infill of large craters the oldest features likely record events from bennu s time in the main asteroid belt

implications for ice stability and particle ejection from high resolution temperature modeling of asteroid 101955 bennu the finding by the osiris rex origins spectral interpretation resource identification and security regolith explorer mission that its target 101955 bennu is an active asteroid has raised questions as to whether the observed particle ejection events are driven by temperature to investigate sublimation of water ice and rock thermal fracture as possible temperature driven causes we modeled the global temperatures of bennu and searched for correlations with the identified ejection points on the asteroid surface we computed temperatures with the advanced thermophysical model and the 75 cm resolution global shape model of bennu derived by the osiris rex mission we find that 1856 m 2 of bennu s polar regions have orbit averaged temperatures that are sufficiently cold to enable water ice if buried within the top few meters of the surface to remain stable over geological timescales millimeter thick layers of surface water ice are also stable over 10 3 year timescales within polar centimeter scale cold traps however we do not find evidence of conditions enabling ice stability in the warmer equatorial regions where ejection events have been observed implying that sublimation of water ice is not the cause of particle ejection conversely rock thermal fracture remains a possible mechanism of particle ejection we find high amplitudes of diurnal temperature variation a proxy for the efficacy of thermal fracturing at all latitudes on bennu due to its extreme ruggedness therefore if rock thermal fracture is the mechanism particles could be ejected from any latitude which is consistent with the continued observations of particle ejection by osiris rex

secondary submicrometer impact cratering on the surface of asteroid 25143 itokawa abstract particle ra qd02 0265 returned by the hayabusa spacecraft from near earth asteroid 25143 itokawa displayed a unique abundance of submicrometer sized 500 nm impact craters which are rarely observed among the hayabusa samples the particle consists of intensely twinned diopside that was subjected to a large scale shock event before exposure to the space environment on the surface of 25143 itokawa intense sub micrometer scale impact cratering may suggest a long surface exposure and hence a long residence time of regolith material on the surface of small asteroids bearing implications for the dynamical evolution of these bodies however our combined fe sem and fib tem study shows that the degree of solar wind induced space weathering and the accumulation of solar flare tracks are not exceptionally different from other hayabusa particles with surface exposure ages estimated to be less than 1 ka a 500 nm wide crater on the surface of ra qd02 0265 exhibits microstructural damage to a depth of 400 nm below its floor and contains residues of fe ni metal excluding a formation by space craft exhausts or curatorial handling the geometrical clustering among the 15 craters is unlikely random and we conclude that the craters have formed through the impacts of secondary projectiles at least partially fe ni metal created in a nearby micro impact event besides structural damage by the solar wind and deposition of impact generated melts and vapors secondary impact cratering on the submicrometer scale is another potential mechanism to modify the spectral properties of individual regolith grains the lack of extensively exposed regolith grains supports a dynamic regolith on the surface of 25143 itokawa

ejecta from impact craters an important feature of impacts into solar system bodies is the fate of crater ejecta the near surface material launched during the highly dynamic crater formation process laboratory measurements of impact crater ejecta from 18 studies are summarized the data are examined and used to assess our understanding of how the ejecta velocity and mass distributions depend on the conditions of an impact event the effects of impact speed on the ejecta are reasonably well understood but the dependences on target properties such as strength and porosity are only poorly constrained a point source scaling model for the ejecta mass and velocity distributions is developed and fit to the data for several classes of materials distinguished by porosity

solute sources and chemical weathering in the owens lake watershed eastern california 1 chemical and strontium isotope analyses of solutes in a series of streams representing the bulk of the water reaching the 8500 km2 owens lake watershed provide a unique opportunity to evaluate factors that control stream chemistry on a regional scale the streams drain terrain that is dominated by mesozoic granitoids of the sierra nevada batholith but that also contains irregularly distributed roof pendants of paleozoic metasedimentary sequences mass balance modeling indicates that plagioclase feldspar weathering is the dominant contributor to stream chemistry and that carbonate dissolution is also a significant source of solutes for most streams the calculated ratio of plagioclase to carbonate is highly sensitive to the assumed weathering product of plagioclase kaolinite versus smectite and we suggest that the importance of carbonate could be exaggerated in cases where only the plagioclase to kaolinite reaction is considered strontium isotope 87sr 86sr ratios range from 0 70685 to 0 71958 with a flow weighted mean of 0 70911 to the extent that this value represents the water that would be flowing into owens lake were it not diverted by the los angeles aqueduct system the flow weighted mean 87sr 86sr ratio is consistent with ratios measured in owens lake core carbonates ranging in age up to 25 kyr the overall sr budget of the watershed is determined by the relative contributions of carbonate dissolution and plagioclase feldspar weathering and is dominated by reactions related to pyrite oxidation and hydrothermal fluid input from just two subbasins the impact of these processes needs to be considered when modeling regional and global riverine sr budgets climate related shifts in the relative importance of silicate mineral weathering reactions appear to be only second order effects on a regional scale

the effect of hydrothermal fluids on ordovician carbonate rocks southern ordos basin china abstract hydrothermal processes have an important influence on the spatial distribution and formation of carbonate reservoirs and play a critical role in certain deep reservoirs where hydrothermal fluid rock interaction may result in permeability creation due to carbonate dissolution or permeability destruction due to the deposition of cementing minerals this study analyzed certain hydrothermal indicator minerals quartz celestite multi stage calcareous fillings in carbonate rocks from the ordovician majiagou formation southern ordos basin china the petrological and geochemical characteristics of these indicator minerals imply that the majiagou formation was affected by two metasomatic events related to the hercynian indosinian and yanshanian orogenies and linked to magmatic hydrothermal fluids derived from basaltic melts most of the ce and eu values obtained from matrix dolomite samples were lower than one but u th and v c ratios showed positive anomalies the 13c and 18o values of the matrix dolomite were concentrated between 12 8 and 6 0 whereas 13c showed a bipolar distribution at the same depth elemental geochemistry results showed that the ordovician carbonate rocks were subjected to freshwater leaching followed by evaporation and widespread hydrothermal dissolution successively as a result of the multi episodic hydrothermal activity that affected the southern ordos basin and the path of migration channel a parental magmatic hydrothermal fluid fluid 1 that had been generated during the underplating of the southern north china craton by mesozoic alkaline basalt melts separated into two types of fluids with different chemical properties an acidic hydrocarbon bearing fluid fluid 2a mixed with cambrian oil and gas and a weakly to moderately alkaline brine fluid 2b not in contact with hydrocarbons both fluids 2a and 2b then interacted with the ordovician carbonate rocks with fluid 2a having undergone hydrothermal sulfate reduction with so42 in the layers in which gypsum was developed and convection of fluid 2a having resulted in brine formation the reaction products of h2s and co2 together with organic acid co dissolved carbonate minerals in the ordovician strata played a major role in the formation of a high porosity reservoir conversely owing to the progressive reduction of temperature and solubility during fluid ascent fluid 2b precipitated quartz calcite fillings in the residual pore spaces resulting in the localized destruction of the original reservoir permeability in summary this study analyzed the petrological and geochemical characteristics of 60 carbonate rock drill core samples established the likely formation mechanism of fluids 2a and 2b and determined the effects of these hydrothermal fluids on the ordovician majiagou formation carbonate reservoir

direct observation of microcrack development in marble caused by thermal weathering one of the properties that makes marble such an excellent construction and ornamental material is its low porosity it is very difficult for water or decay agents to penetrate the internal structure of materials with no or few pores so enhancing the durability of these materials however environmental temperature fluctuations bring about significant physical changes in marbles that result in an increase in porosity due to the appearance of new microcracks and the expansion of existing ones these cracks offer new paths into the marble which make it easier for solutions containing pollutants to penetrate the material thermal expansion tests were performed on three different types of marble known as white tranco and yellow macael almeria spain after which an increase in porosity from 17 to 73 depending on marble type was observed mainly due to crack formation the structural changes occurring during thermal expansion tests were more significant in the case of white macael samples a fact that is not only related to its mineralogical composition but also to the morphology of the grains grain boundaries and crystal size our research suggests that thermally weathered white macael marble could be more susceptible to decay by other contaminant agents than tranco or yellow macael the use of hot stage environmental scanning electron microscopy is proposed as a valid tool for observing both in situ and at high magnification changes in the fracture system of building stones induced by thermal stress

origin of the fule pb zn deposit yunnan province sw china insight from in situ s isotope analysis by nanosims the sichuan yunnan guizhou syg pb zn metallogenic province is one of the most productive areas of pb zn resources in china the fule deposit occurs in permian carbonate and contains pb zn reserves exceeding 1 mt to investigate the sulphur source in situ s isotopic analysis of sphalerite and pyrite was carried out using nanoscale secondary ion mass spectrometry the results show that the 34 s values of the sulphide minerals range from 16 1 to 23 0 higher than that of marine sulphates hosted in permian carbonate rocks 11 but similar to that of sulphates over a broader area 12 9 to 25 9 the sulphates in the regional rocks could therefore represent an important source of s for the fule deposit via thermochemical sulphate reduction the s source of the fule deposit is different from those of most other pb zn deposits in the syg pb zn mineralization province which were mainly derived from the ore bearing strata the 34 s values of the early to late generations and some single sulphide crystals from the cores to rims show a slight increasing trend implying that partial rayleigh fractionation took place in the fule deposit it is suggested that the fule sulphide precipitation resulted from the mixing of a metalliferous fluid with a h 2 s rich fluid derived from the regional strata combining the geology mineralogy and s isotope results with previous pb isotope studies it is suggested that the fule deposit should be attributed to a mississippi valley type deposit

lunar surface roughness derived from lro diviner radiometer observations abstract sunlit and shaded slopes have a variety of temperatures based on their orientation with respect to the sun generally greater slope angles lead to higher anisothermality within the field of view this anisothermality is detected by measuring changing emitted radiance as a function of viewing angle or by measuring the difference in brightness temperatures with respect to observation wavelength thermal infrared measurements from the lunar reconnaissance orbiter diviner radiometer were used to derive lunar surface roughness via two observation types 1 nadir multispectral observations with full diurnal coverage and 2 multiple emission angle targeted observations measurements were compared to simulated radiance from a radiative equilibrium thermal model and gaussian slope distribution model nadir observations most closely match a 20 rms slope distribution and multiple emission angle observations can be modeled using 20 35 rms slope distributions limited sampling of the lunar surface did not show any clear variation in roughness among surface units two dimensional modeling shows that surfaces separated by distances greater than 0 5 5 mm can remain thermally isolated in the lunar environment indicating the length scale of the roughness features non equilibrium conditions are prevalent at night and near sunrise and sunset preventing the use of the equilibrium thermal model for roughness derivations using data acquired at these local times multiple emission angle observations also show a significant decrease in radiance at high emission angles in both daytime and nighttime observations and hemispherical emissivity is lower than is apparent from nadir observations these observations and models serve as a basis for comparison with similar measurements of other airless bodies and as an initial template for the interpretation of tir measurements acquired under a variety of geometric conditions

thermally induced stresses in boulders on airless body surfaces and implications for rock breakdown abstract this work investigates the macroscopic thermomechanical behavior of lunar boulders by modeling their response to diurnal thermal forcing our results reveal a bimodal spatiotemporally complex stress response during sunrise stresses occur in the boulders interiors that are associated with large scale temperature gradients developed due to overnight cooling during sunset stresses occur at the boulders exteriors due to the cooling and contraction of the surface both kinds of stresses are on the order of 10 mpa in 1 m boulders and decrease for smaller diameters suggesting that larger boulders break down more quickly boulders 30 cm exhibit a weak response to thermal forcing suggesting a threshold below which crack propagation may not occur boulders of any size buried by regolith are shielded from thermal breakdown as boulders increase in size 1 m stresses increase to several 10 s of mpa as the behavior of their surfaces approaches that of an infinite halfspace as the thermal wave loses contact with the boulder interior stresses become limited to the near surface this suggests that the survival time of a boulder is not only controlled by the amplitude of induced stress but also by its diameter as compared to the diurnal skin depth while stresses on the order of 10 mpa are enough to drive crack propagation in terrestrial environments crack propagation rates in vacuum are not well constrained we explore the relationship between boulder size stress and the direction of crack propagation and discuss the implications for the relative breakdown rates and estimated lifetimes of boulders on airless body surfaces

cathodoluminescence and composition of calcite cement in the taum sauk limestone upper cambrian southeast missouri abstract study of calcite cements in the taum sauk limestone upper cambrian southeast missouri using cathodoluminescence petrography and microprobe analysis has shown that 1 distinct luminescent zones correspond to distinct compositional zones 2 the degree of luminescent intensity of a particular calcite zone appears to be controlled by the fe mn ratio of the zone not by the absolute concentrations of either cation 3 manganese may induce luminescence in calcite at concentrations as low as 300 ppm and 4 there may be no critical concentration of ferrous iron in calcite above which there can be no luminescene

constraints on the recent rate of lunar ejecta breakdown and implications for crater ages we present a new empirical constraint on the rate of breakdown of large ejecta blocks on the moon based on observations from the lunar reconnaissance orbiter lro diviner thermal radiometer we find that the rockiness of fresh crater ejecta can be quantified using the diviner derived rock abundance data set and we present a strong inverse correlation between the 95 th percentile value of the ejecta rock abundance ra 95 5 and crater age for nine craters with published model ages derived from crater counts on their continuous ejecta ra 95 5 decreases with crater age as age m y 0 46 this result implies shorter rock survival times than predicted based on downward extrapolation of 100 m crater size frequency distributions and represents a new empirical constraint on the rate of comminution of large rocks not previously analyzed experimentally or through direct observation in addition our result provides a new method for dating young lunar craters

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

latitudinal dependence of asteroid regolith formation by thermal fatigue abstract the latitudinal dependence of regolith formation by thermal fatigue is studied for variable solar declination and surface thermal inertia we find that regolith generation takes place in a surprisingly wide band around the equator and make predictions for the regolith distribution on the target asteroids of the upcoming hayabusa2 and osiris rex missions

calcite cement stratigraphy and cementation history of the burlington keokuk formation mississippian illinois and missouri abstract iron content and cathodoluminescence define six regionally extensive calcite cement zones over 100 000 sq km of study area in the burlington keokuk formation of illinois and missouri the regional cement stratigraphy is composed of mappable zones analogous to lithostratigraphic units which represent diagenetic episodes during which calcite with distinct cathodoluminescent properties precipitated in a given time interval relative to other diagenetic events such as cement fracturing and calcite dissolution individual cement zones may not be strictly isochronous throughout the study area but are not strongly diachronous two different but related cathodoluminescent sequences have been identified within pre pennsylvanian cements in the burlington keokuk formation 1 within lower strata early nonferroan cement consists of both nonluminescent and luminescent calcite zones ii iv 2 within upper strata early nonferroan cement consists of entirely luminescent calcite zone ii petrographic relationships indicate that zone ii is time equivalent with zones ii iv in lower burlington keokuk strata the occurrence of cement zones ii iv encased in pennsylvanian shale and quartz sand within pre pennsylvanian karst cavities constrains the age of early nonferroan calcite as pre pennsylvanian in ll cases pre pennsylvanian cements are followed by ferroan dull to moderately luminescing calcite zone v which in turn is overgrown by nonferroan moderate to brightly luminescing calcite zone vi cross cutting relations of burlington keokuk syntaxial calcite with mississippi valley type mineralization phases constrains the minimum age of cement zones v and vi as early permian pre pennsylvanian cements formed in a shallow meteoric aquifer characterized by vertically segregated chemical facies pre pennsylvanian cements within lower strata precipitated from waters of varying eh while those in upper strata formed from reduced waters post mississippian pre early permian cements precipitated under less than 1 km of overburden from reducing meteoric waters or subsurface brines

micro meteoroid seismic uplift and regolith concentration on kilometric scale asteroids abstract seismic shaking is an attractive mechanism to explain the destabilisation of regolith slopes and the regolith migration found on the surfaces of asteroids richardson j e melosh h j greenberg r 2004 science 306 1526 1529 http dx doi org 10 1126 science 1104731 miyamoto h et al 2007 here we use a continuum mechanics method to simulate the seismic wave propagation in an asteroid assuming that asteroids can be described by a cohesive core surrounded by a thin non cohesive regolith layer our numerical simulations of vibrations induced by micro meteoroids suggest that the surface peak ground accelerations induced by micro meteoroid impacts may have been previously under estimated our lower bound estimate of vertical accelerations induced by seismic waves is about 50 times larger than previous estimates it suggests that impact events triggering seismic activity are more frequent than previously assumed for asteroids in the kilometric and sub kilometric size range the regolith lofting is also estimated by a first order ballistic approximation vertical displacements are small but lofting times are long compared to the duration of the seismic signals the regolith movement has a non linear dependence on the distance to the impact source which is induced by the type of seismic wave generating the first movement the implications of regolith concentration in lows of surface acceleration potential are also discussed we suggest that the resulting surface thermal inertia variations of small fast rotators may induce an increased sensitivity of these objects to the yarkovsky effect

how did the foreland react yangtze foreland fold and thrust belt deformation related to exhumation of the dabie shan ultrahigh pressure continental crust eastern china during the triassic collision of the yangtze and sino korean cratons the leading edge of the yangtze crust subducted to mantle depths and was subsequently exhumed as a penetratively deformed coherent slab capped by a normal shear zone this geometry requires a reverse shear zone at the base of the slab and we suggest that the yangtze foreland fold and thrust belt constitutes this zone lower triassic rocks of the eastern foreland record nw se compression as the oldest compressional stress field onset of related deformation is indicated by middle triassic clastic sedimentation subsequent jurassic stress fields show a clockwise change of compression directions based on nearly coeval onset and termination of deformation and on a common clockwise change in the principal strain stress directions we propose that the foreland deformation was controlled by the extrusion of the ultra high pressure slab widespread cretaceous cenozoic reactivation occurred under regional extension to transtension which characteristically shows a large scale clockwise change of the principal extension directions during the lower cretaceous

physical weathering in arid landscapes due to diurnal variation in the direction of solar heating despite the prominent role of physical weathering in arid and semi arid landscapes there has been little study of the specifi c processes responsible for the rapid breakdown of subaerially exposed rocks for example many boulders and cobbles in deserts exhibit fi ne near vertical cracks although workers have hypothesized that these and other cracks are initiated by diurnal heating and cooling no convincing specifi c mechanism for their formation has been proposed we have characterized these cracks at eight sites on surfaces of different ages in the mojave sonoran and chihuahuan deserts and the high desert of central new mexico our data reveal four basic types of cracks longitudinal surfaceparallel fabric related and meridional the orientations of the fi rst three types are associated with clast shape and rock fabric the azimuths of meridional cracks however are preferentially aligned north south typically with a nonrandom multimodal distribution we propose that these cracks are caused by tensile stresses that arise in the interior of clasts due to strong radial gradients in temperature that evolve and rotate in alignment with the sun s rays we suggest that the multimodal nature of crack orientations may be in part attributable to the seasonally varying latitude dependent solar elevation angle over millennial time scales we suggest that this thermal cracking is an effi cient weathering process that together with cumulic soil epipedon development creates the key attributes of most desert pavements in addition to individual clasts exposed on desert surfaces this mechanism of cracking is potentially signifi cant in other climates and on other planets as well as for rock outcrops and for man made structures

karstification without carbonic acid bedrock dissolution by gypsum driven dedolomitization aggressive karstification can take place where dolomite and gypsum are in contact with the same aquifer gypsum dissolution drives the precipitation of calcite thus consuming carbonate ions released by dolomite lake banyoles in northeastern spain is a karst lake supplied by sublacustrine springs and karstic collapse is occurring in the immediate vicinity of the lake lake water is dominated by mg ca and so 4 hco 3 and is supersaturated with calcite that is actively accumulating in lake sediments water chemistry sulfur isotope composition local stratigraphy and mass balance modelling suggest that the primary karst forming process at at lake banyoles is dedolomitization of basement rocks driven by gypsum dissolution karstification takes place along the subsurface contact between the gypsiferous beuda formation and the dolomitic perafita formation this process is here recognized for the first time to cause karstification on a large scale this is significant because it proceeds without the addition of soil generated carbonic acid gypsum driven dedolomitization may be responsible for other karstic systems heretofore attributed to soil generated carbonic acid

234u 238u activity ratios in freshwaters as tracers of hydrological processes the strengbach watershed vosges france abstract 234u 238u and 87sr 86sr isotopic ratios as well as major and trace u ba sr rb element concentrations were analyzed in dissolved loads of the strengbach stream vosges france in order to constrain the sources of u isotopes 234u 238u to river waters the variations of the 234u 238u activity ratios along the stream indicate a clear dependence between the 234u 238u activity ratios of the surface waters and the different types of rocks forming the watershed with near equilibrium values for the waters draining granites and high 234u 238u disequilibria 1 4 for those flowing over carbonates the high 234u 238u disequilibria are related to a supply of 234u enriched groundwaters located within the carbonate rocks the 234u 238u activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology decrease from 1 02 to 0 96 when the discharge of the stream increases such a tendency requires mixing between a water body enriched in 234u which weathered the granitic bed rock at secular equilibrium and a water with a 234u 238u activity ratio below unity representing a mobilization of u from material that has already been weathered comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event reveals the involvement of two different weathered end members depending on the hydrological conditions during the year the dissolved u transported by the river originates from bed rock and deep horizons of the weathering profile whereas a significant part of u during the flood event is supplied by superficial horizons of soils probably complexed by organic colloids these results outline the potential of 234u 238u activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously

major and trace elements of river borne material the congo basin the congo river basin is the second largest drainage basin in the world after the amazon the materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the african continent this paper presents the chemical composition of the different phases carried in the congo rivers and is followed by a companion paper devoted to the modelling of major and trace elements the congo river between bangui and brazzaville as well as its main tributaries including a few organic rich rivers also called black rivers were sampled during the 1989 high water stage the three main phases suspended load dissolved load and bedload were analysed for twenty five major and trace elements concentrations normalized to the upper continental crust show that in each river suspended sediments and dissolved load are chemical complements for the most soluble elements ca na sr k ba rb and u while these elements are enriched in the dissolved loads they are considerably depleted in the corresponding suspended sediments this is consistent with their high mobility during weathering another type of complementarity is observed for zr and hf between suspended sediments and bedload related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads compared to other rivers absolute dissolved concentrations of ca na sr k ba rb and u are remarkably low surprisingly high dissolved concentrations are found in the congo waters for other trace elements e g rees especially in the black rivers on a world scale these concentrations are among the highest measured in rivers and are shown to be ph dependent for a number of dissolved trace elements the dissolved loads are systematically normalized to the suspended loads for each river in order to remove the variations of the element abundances owing to source rock variations normalized diagrams for rees are presented and extended to the other elements they strongly support the argument that the apparent higher solubility of trace elements in the congo waters is due to the presence in the dissolved load of a colloidal phase as a result of 0 2 m filtration an important result is that these colloids are strongly depleted in fe and al with respect to the other elements finally the comparison of the dissolved suspended and sandy transport fluxes of each element in the congo basin rivers shows that although the proportions of for example the rees in the dissolved loads of the majority of the congo basin rivers is close to 10 of the total transport flux up to 80 of the rees are transported by the so called dissolved load in the black rivers

geological features and origin of the huize carbonate hosted zn pb ag district yunnan south china abstract the huize zn pb ag district in the sichuan yunnan guizhou zn pb ag metallogenic region contains significant high grade zn pb ag deposits the total metal reserve of zn and pb exceeds 5 mt the district has the following geological characteristics 1 high ore grade zn pb 25 wt 2 enrichment in ag and a range of other trace elements ge in ga cd and tl with galena sphalerite and pyrite being the major carriers of ag ge cd and tl 3 ore distribution controlled by both structural and lithological features 4 simple and limited wall rock alteration 5 mineral zonation within the orebodies and 6 the presence of evaporite layers in the ore hosting wall rocks of the early carboniferous baizuo formation and the underlying basement fluid inclusion and isotope geochemical data indicate that the ore fluid has homogenisation temperatures of 165 220 c and salinities of 6 6 12 wt nacl equiv and that the ore forming fluids and metals were predominantly derived from the kunyang group basement rocks and the evaporite bearing rocks of the cover strata ores were deposited along favourable specific ore controlling structures the new laboratory and field studies indicate that the huize zn pb ag district is not a carbonate replacement deposit containing massive sulphides but rather the deposits can be designated as deformed carbonate hosted mvt type deposits detailed study of the deposits has provided new clues to the localisation of concealed orebodies in the huize zn pb ag district and of the potential for similar carbonate hosted sulphide deposits elsewhere in ne yunnan province as well as the sichuan yunnan guizhou zn pb ag metallogenic region

lunar regolith thickness deduced from concentric craters in the ce 5 landing area abstract lunar regolith is the layer of fragmented and unconsolidated rock material that is repeatedly stirred and overturned on the lunar surface it can provide critical information about lunar geologic processes e g impact volcanism and the space environment china s chang e 5 ce 5 mission which will be launched in 2019 will land on the lunar near side surface and return at least 2 kg of lunar regolith through surface collection and by boring a hole no

boulder size and shape distributions on asteroid ryugu abstract in 2018 the japanese spacecraft hayabusa2 arrived at the small asteroid ryugu the surface of this c type asteroid is covered with numerous boulders whose size and shape distributions are investigated in this study using a few hundred optical navigation camera onc images with a pixel scale of approximately 0 65 m we focus on boulders greater than 5 m in diameter smaller boulders are also considered using five arbitrarily chosen onc close up images with pixel scales ranging from 0 7 to 6 cm across the entire surface area 2 7 km2 of ryugu nearly 4400 boulders larger than 5 m were identified boulders appear to be uniformly distributed across the entire surface with some slight differences in latitude and longitude at 50 km 2 the number density of boulders larger than 20 m is twice as large as on asteroid itokawa or bennu the apparent shapes of ryugu s boulders resemble laboratory impact fragments with larger boulders being more elongated the ratio of the total volume of boulders larger than 5 m to the total excavated volume of craters larger than 20 m on ryugu can be estimated to be 94 which is comparatively high these observations strongly support the hypothesis that most boulders found on ryugu resulted from the catastrophic disruption of ryugu s larger parent body as described in previous papers watanabe et al 2019 sugita et al 2019 the cumulative size distribution of boulders larger than 5 m has a power index of 2 65 0 05 which is comparatively shallow compared with other asteroids visited by spacecraft for boulders smaller than 4 m the power index is even shallower and ranges from 1 65 0 05 to 2 01 0 06 this particularly shallow power index implies that some boulders are buried in ryugu s regolith based on our observations we suggest that boulders near the equator might have been buried by the migration of finer material and as a result the number density of boulders larger than 5 m in the equatorial region is lower than at higher latitudes

a global geochemical mass budget applied to the congo basin rivers erosion rates and continental crust composition rivers carry the products of continental denudation either in a dissolved form chemical erosion or in a solid form physical erosion we focus in this paper on the relationship between physical erosion and chemical erosion we establish the mass budget of the congo basin rivers using chemical complementarities between river suspended sediments sandy bedload and dissolved load of the congo basin rivers reported in a previous paper dupre et al 1995 a series of equations are presented assuming that the physical and chemical erosion processes are in a steady state during one year the total mass of river borne material dissolved and particulate transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time we show that the local continental crust on each drainage basin can be estimated and solve our steady state weathering model using an inversion procedure the very good agreement between modelled and measured values of the river suspended sediment concentrations validates the steady state hypothesis in this wet tropical area consequently in this area the sediment yield provide a good estimate of the rates of mechanical denudation this result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load erosion rates for the silicate upper crust and thus independent of the lithological variability silicates evaporites and carbonates of the drainage basins are calculated mechanical erosion rates and chemical erosion rates for the congo basin at brazzaville are 8 t km2 y and 5 t km2 y the corresponding consumption of atmospheric co2 by weathering process is estimated to 51 103 mol km2 an these weathering and consumption rates are low in spite of the severity of the weathering conditions of the high soil temperature and of the intensity of precipitations these conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering finally by estimating the local continental crust chemical composition before the onset of erosion processes especially for the most soluble elements we can test the model of taylor and mclennan

granular avalanches on the moon mass wasting conditions processes and features seven lunar crater sites of granular avalanches are studied utilizing high resolution images 0 42 1 3 m pixel from the lunar reconnaissance orbiter camera one in kepler crater is examined in detail all the sites are slopes of debris extensively aggraded by frictional freezing at their dynamic angle of repose four in craters formed in basaltic mare and three in the anorthositic highlands diverse styles of mass wasting occur and three types of dry debris flow deposit are recognized 1 multiple channel and lobe type with coarse grained levees and lobate terminations that impound finer debris 2 single surge polylobate type with sub parallel arrays of lobes and fingers with segregated coarse grained margins and 3 multiple ribbon type with tracks reflecting reworked substrate minor levees and no coarse terminations the latter type results from propagation of granular erosion deposition waves down slopes dominantly of fine regolith and it is the first recognized natural example dimensions architectures and granular segregation styles of the two coarse grained deposit types are like those formed in natural and experimental avalanches on earth although the timescale of motion differs due to the reduced gravity influences of reduced gravity and fine grained regolith on dynamics of granular flow and deposition appear slight but we distinguish for the first time extensive remobilization of coarse talus by inundation with finer debris the few sites show no clear difference attributable to the contrasting mare basalt and highland megaregolith host rocks and their fragmentation this lunar study offers a benchmarking of deposit types that can be attributed to formation without influence of liquid or gas

lunar crater ejecta physical properties revealed by radar and thermal infrared observations abstract we investigate the physical properties and changes through time of lunar impact ejecta using radar and thermal infrared data we use data from two instruments on the lunar reconnaissance orbiter lro the diviner thermal radiometer and the miniature radio frequency mini rf radar instrument together with earth based radar observations we use this multiwavelength intercomparison to constrain block sizes and to distinguish surface from buried rocks in proximal ejecta deposits we find that radar detectable rocks buried within the upper meter of regolith can remain undisturbed by surface processes such as micrometeorite bombardment for 3 gyr we also investigate the thermophysical properties of radar dark haloes comprised of fine grained rock poor ejecta distal to the blocky proximal ejecta using diviner data we confirm that the halo material is depleted in surface rocks but show that it is otherwise thermophysically indistinct from background regolith we also find that radar dark haloes like the blocky ejecta remain visible in radar observations for craters with ages 3 ga indicating that regolith overturn processes cannot replenish their block populations on that timescale

strontium isotope compositions of river waters as records of lithology dependent mass transfers the garonne river and its tributaries sw france the relation of lithology in the drainage basin to the dissolved load of the garonne river and its main tributaries in southwestern france was evaluated by determining 87sr 86sr ratios and concentrations of major and trace elements during a 2 year long survey in the upper drainage basin the garonne river waters were isotopically varied at two observation points 0 71131 0 00030 2 for 84 18 ppb 2 and 0 71272 0 00044 for 86 10 ppb respectively in the lower drainage basin the garonne river waters were isotopically identical at three observation points at 0 71020 0 00024 for 125 22 ppb by contrast the tributaries lot truyere aveyron arriege gers and salat are widely varied in their 87sr 86sr ratios and sr concentrations from 0 70836 0 00049 to 0 71058 0 00057 and from 18 8 to 280 116 ppb the sr isotope ratios and concentrations suggest a dominant supply of two reservoirs of sr one of which is with low 87sr 86sr ratios and high sr contents that is typically characteristic of carbonate rocks and the other with high 87sr 86sr ratios and low sr concentrations that is characteristic of felsic rocks locally as in the lot waters draining the massif central and within the pyrenees mountains a third source of sr from mafic rocks may be involved mass balance calculations based on the mean 87sr 86sr ratios and contents of the dissolved sr and on the mean discharges integrating the 2 year survey suggest that contribution of the silicate reservoir amounts 3 8 of the total dissolved sr flux mass balance calculations also suggest that variation in the supply of sr from either of the two major reservoirs does not exceed the analytical uncertainty at about 5 the 87sr 86sr ratios of hcl and nh4cl leachates of suspended loads of the garonne river are different from that of the associated dissolved sr this leaching related supplementary sr represents less than 10 of the total amount of sr transported by the garonne waters the sr isotope characteristics of the leachates are probably records of an intermediate pedogenic episode in the weathering erosion process occurring in the garonne drainage basin

model for asteroid regolith to guide simulant development abstract when creating asteroid regolith simulant it is necessary to have a model of asteroid regolith to guide and to evaluate the simulant we created a model through evaluation and synthesis of the available data sets including 1 the returned sample from itokawa by the hayabusa spacecraft 2 imagery from the hayabusa and near spacecraft visiting itokawa and eros respectively 3 thermal infrared observations from asteroids 4 the texture of meteorite regolith breccias and 5 observations and modeling of the ejecta clouds from disrupted asteroids comparison of the hayabusa returned sample with other data sets suggest the surficial material in the smooth regions of asteroids is dissimilar to the bulk regolith probably due to removal of fines by photoionization and solar wind interaction or by preferential migration of mid sized particles into the smooth terrain we found deep challenges interpreting and applying the thermal infrared data so we were unable to use those observations in the model texture of regolith breccias do not agree with other data sets suggesting the source regolith on vesta was coarser than typical asteroid regolith the observations of disrupted asteroids present a coherent picture of asteroid bulk regolith in collisional equilibrium unlike lunar regolith hed textures and the itokawa returned sample the model we adopt consists of power laws for the bulk regolith in unspecified terrain differential power index 3 5 representing equilibrium and the surficial regolith in smooth terrain differential power index 2 5 representing disequilibrium available data do not provide adequate constraints on maximum and minimum particle sizes for these power laws so the model treats them as user selectable parameters for the simulant

a new method to determine the grain size of planetary regolith abstract airless planetary bodies are covered by a dusty layer called regolith the grain size of the regolith determines the temperature and the mechanical strength of the surface layers thus knowledge of the grain size of planetary regolith helps to prepare future landing and or sample return missions in this work we present a method to determine the grain size of planetary regolith by using remote measurements of the thermal inertia we found that small bodies in the solar system diameter less than 100 km are covered by relatively coarse regolith grains with typical particle sizes in the millimeter to centimeter regime whereas large objects possess very fine regolith with grain sizes between 10 m and 100 m

meter scale thermal contraction crack polygons on the nucleus of comet 67p churyumov gerasimenko we report on the detection and characterization of more than 6300 polygons on the surface of the nucleus of comet 67p churyumov gerasimenko using images acquired by the osiris camera onboard rosetta between august 2014 and march 2015 they are found in consolidated terrains and grouped in localized networks they are present at all latitudes from north to south and longitudes head neck and body sometimes on pit walls or following lineaments about 1 5 of the observed surface is covered by polygons polygons have an homogeneous size across the nucleus with 90 of them in the size range 1 5 m and a mean size of 3 0 1 4 m they show different morphologies depending on the width and depth of their trough they are found in networks with 3 or 4 crack intersection nodes the polygons observed on 67p are consistent with thermal contraction crack polygons formed by the diurnal or seasonal temperature variations in a hard mpa and consolidated sintered layer of water ice located a few centimeters below the surface our thermal analysis shows an evolution of thermal contraction crack polygons according to the local thermal environment with more evolved polygons i e deeper and larger troughs where the temperature and the diurnal and seasonal temperature range are the highest thermal contraction crack polygons are young surface morphologies that probably formed after the injection of 67p in the inner solar system typically 100 000 years ago and could be as young as a few orbital periods following the decreasing of its perihelion distance in 1959 from 2 7 to 1 3 a u meter scale thermal contraction crack polygons should be common features on the nucleus of jupiter family comets

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

controls over the strontium isotope composition of river water abstract strontium concentrations and isotope ratios have been measured in river and ground waters from the ganges orinoco and amazon river basins when compared with major element concentrations the data set has allowed a detailed examination of the controls over the strontium isotope systematics of riverine input to the oceans in the following environments 1 1 typical drainage basins containing limestones evaporites shales and alumino silicate metamorphic and igneous rocks 2 2 shield terrains containing no chemical or biogenic sediments and 3 3 the floodplains that constitute the largest areas of many large rivers the strontium concentration and isotope composition of river waters are largely defined by mixing of strontium derived from limestones and evaporites with strontium derived from silicate rocks the strontium isotope composition of the limestone endmember generally lies within the phanerozoic seawater range which buffers the 87 sr 86 sr ratios of major rivers a major exception is provided by the rivers draining the himalayas where widescale regional metamorphism appears to have led to an enrichment in limestones of radiogenic strontium derived from coexisting silicate rocks the strontium isotope systematics of rivers draining shield areas are controlled by the intense transportlimited nature of the weathering reactions and thereby limits variations in the strontium flux from these terrains floodplains are only a minor source of dissolved strontium to river waters and precipitation of soil salts in some floodplains can reduce the riverine flux of dissolved strontium to the oceans the most effective mechanisms for altering the isotope ratio and flux of riverine strontium to the oceans are increased glaciation and large scale regional metamorphism of the type produced during continental collision both mechanisms provide a means for increasing the 87 sr 86 sr ratio of the global riverine flux

evidence for geologic processes on comets spacecraft missions have resolved the nuclei of six periodic comets and revealed a set of geologically intriguing and active small bodies the shapes of these cometary nuclei are dominantly bilobate reflecting their formation from smaller cometesimals cometary surfaces include a diverse set of morphologies formed from a variety of mechanisms sublimation of ices driven by the variable insolation over the time since each nucleus was perturbed into the inner solar system is a major process on comets and is likely responsible for quasi circular depressions and ubiquitous layering sublimation from near vertical walls is also seen to lead to undercutting and mass wasting fracturing has only been resolved on one comet but likely exists on all comets there is also evidence for mass redistribution where material lifted off the nucleus by subliming gases is deposited onto other surfaces it is surprising that such sedimentary processes are significant in the microgravity environment of comets there are many enigmatic features on cometary surfaces including tall spires kilometer scale flows and various forms of depressions and pits furthermore even after accounting for the differences in resolution and coverage significant diversity in landforms among cometary surfaces clearly exists yet why certain landforms occur on some comets and not on others remains poorly understood the exploration and understanding of geologic processes on comets is only beginning these fascinating bodies will continue to provide a unique laboratory for examining common geologic processes under the uncommon conditions of very high porosity very low strength small particle sizes and near zero gravity

timing of mineralization at the navan zn pb deposit a post arundian age for irish mineralization navan is the largest of the irish zn pb deposits and contains some of the most important evidence for the inferred early timing of mineralization in ireland clasts of ore above a middle carboniferous erosion surface we have examined diagenetic fabrics within the main ore pale beds and the overlying erosion surface boulder conglomerate in order to determine the timing of mineralization this diagenetic analysis and particularly the presence of premineralization dolomite in the boulder conglomerate has revealed that all of the mineralization postdates the erosion surface and therefore must be post arundian 345 ma in age furthermore the navan mineralization must be entirely epigenetic in origin i e similar to mississippi valley type deposits and is likely to be holkerian 343 ma in age or younger this conclusion disagrees with the previous model of at least partially syngenetic mineralization and has important implications for the age of other deposits in ireland and the nature of irish type deposits in general

in situ fragmentation of lunar blocks and implications for impacts and solar induced thermal stresses abstract this study deals with an aspect of blocks observed on many rocky planetary surfaces in situ fragmentation using lroc nac images we characterized the morphology morphometry and abundance of in situ fractured blocks observed on the rim of six large impact craters of known emplacement age on the moon the relative number of disrupted blocks increases with crater retention age of surfaces on which blocks are hosted consistent with fragmentation post emplacement due to impacts of small meteoroids the type of break up morphologies we observe appears to be independent of surface exposure age of the blocks the inferred flux and size frequency distribution of projectiles responsible for disrupting blocks is consistent with expected lunar impact fluxes block fragmentation due to insolation driven thermal stresses is subordinate to impacts the possible effects of thermal stresses are evident as meridional cracks which have preferred orientations in a young block population 4 ma and as loose material fillet developing on top of surviving blocks in old populations 800 ma

neodymium and strontium isotopic dating of diagenesis and low grade metamorphism of argillaceous sediments the behaviour of the rb sr and sm nd isotopic systems with increasing degree of hercynian metamorphic overprint was studied along a transect in cambrian shales of northwestern morocco clay fractions of 0 2 to 2 6 m size from five samples were investigated representing a range from nonmetamorphic to epizonal metamorphic conditions the samples were washed in cold l n hc1 prior to digestion to separate soluble exchangeable rb sr sm and nd from amounts of these elements fixed in the crystallographic sites of the minerals and to analyze both components separately the results reveal that the rb sr isotopic system is dominated by sr hosted by clay mineral phases both detrital and authigenic illite and chlorite and carbonate hosted soluble sr isotopic homogenization of sr occurred during hercynian metamorphism yielding ages between 309 and 349 ma the sm nd isotopic system on the other hand is dominated by cogenetic apatite and fe oxide hydroxide both having high contents of leachable rees the leachates yield a sm nd isochron age of 523 72 ma indicating diagenetic equilibrium between apatite and fe oxide hydroxide fine grained clay fractions of 0 2 m show inheritance of a detrital nd component the study demonstrates that the diagenesis of the investigated argillaceous sediments can be dated by the sm nd chronometer in authigenic cement phases the isotopic system of these minerals apatite fe hydroxide oxide was homogenized during authigenic mineral growth in a sediment that was flushed by diagenetic fluids and had abundant primary or secondary interconnected pore space the hercynian metamorphic overprint caused partial isotopic rehomogenization of the adsorbed and clay hosted portion of the sr as well as of the carbonate hosted sr the sm nd system in the cement phases survived this metamorphism this results in decoupling of the two isotopic systems and allows the dating of diagenesis on the one hand sm nd and metamorphism on the other hand rb sr

evidence for an asteroid comet continuum from simulations of carbonaceous microxenolith dynamical evolution abstract micrometeoroids with 100 and 200 m size dominate the zodiacal cloud dust such samples can be studied as micrometeorites after their passage through the earth atmosphere or as microxenoliths i e submillimetric meteorite inclusions microxenoliths are samples of the zodiacal cloud dust present in the asteroid main belt hundreds of millions years ago carbonaceous microxenoliths represent the majority of observed microxenoliths they have been studied in detail in howardites and h chondrites we investigate the role of carbonaceous asteroids and jupiter family comets as carbonaceous microxenolith parent bodies the probability of low velocity collisions of asteroidal and cometary micrometeoroids with selected asteroids is computed starting from the micrometeoroid steady state orbital distributions obtained by dynamical simulations we selected possible parent bodies of howardites vesta and h chondrites hebe flora eunomia koronis maria as target asteroids estimates of the asteroidal and cometary micrometeoroid mass between 2 and 4 au from the sun are used to compute the micrometeoroid mass influx on each target the results show that all the target asteroids except koronis receive the same amount within the uncertainties of asteroidal and cometary micrometeoroids therefore both these populations should be observed among howardite and h chondrite carbonaceous microxenoliths however this is not the case carbonaceous microxenoliths show differences similar to those existing among different groups of carbonaceous chondrites e g ci cm cr but two sharply distinct populations are not observed our results and the observations can be reconciled assuming the existence of a continuum of mineralogical and chemical properties between carbonaceous asteroids and comets

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

minor and trace element distributions in the bonneterre dolomite cambrian southeast missouri evidence for possible multiple basin fluid sources and pathways during lead zinc mineralization the sources and flow paths of basinal fluids that precipitated the dolomite hosted mississippi valley type orebodies of southeast missouri have historically been a subject of debate this study presents geochemical data for epigenetic dolomites of the bonneterre dolomite directly above the lamotte sandstone aquifer and from the viburnum trend orebodies samples of epigenetic replacement dolomite were collected at the bonneterre dolomite lamotte sandstone contact from 35 drill cores covering an area of more than 25 000 km 2 west of the st francois mountains there are distinct aerial trends in fe mn and sr contents of the dolomite from values of 3 47 wt 0 38 wt and 27 ppm in the south to 0 82 wt 0 12 wt and 54 ppm in the north respectively in and near the viburnum trend lead zinc subdistrict the distributions of fe mn and sr are reversed from values of 0 50 wt 0 10 wt and 105 ppm in the south to 3 15 wt 0 55 wt and 40 ppm in the north respectively fe and mn contents of gangue dolomite cement in the viburnum trend orebodies show a similar south to north enrichment ranging from 1 41 mole feco 3 and 0 11 mole mnco 3 in the south to 2 34 mole and 0 23 mole in the north respectively on the basis of fractionation of minor and trace elements between fluids and dolomites the data are interpreted to indicate a regional south to north flow of water from the arkoma basin through the lamotte sandstone a second fluid flow system with a northern source possibly the illinois basin precipitated the late phases of gangue dolomite cement in the viburnum trend and may have been active during the earlier precipitation of the dolomite at the lamotte bonneterre contact the inferred fluid flow paths suggest a more complex multiple basin fluid involvement than previously suggested for the mississippi valley type pb zn mineralization of southeast missouri

syn sedimentary hydrothermal dolomites in a lacustrine rift basin petrographic and geochemical evidence from the lower cretaceous erlian basin northern china

meteoroid impacts as a source of bennu s particle ejection events asteroid 101955 bennu a near earth object with a primitive carbonaceous chondrite like composition was observed by the origins spectral interpretation resource identification and security regolith explorer osiris rex spacecraft to undergo multiple particle ejection events near perihelion between december 2018 and february 2019 the three largest events observed during this period which all occurred 3 5 to 6 hr after local noon placed numerous particles 10 cm on temporary orbits around bennu here we examine whether these events could have been produced by sporadic meteoroid impacts using the national aeronautics and space administration s nasa meteoroid engineering model 3 0 most projectiles that impact bennu come from nearly isotropic or jupiter family comets and have evolved toward the sun by poynting robertson drag we find that 7 000 j impacts on bennu occur with a biweekly cadence near perihelion with a preference to strike in the late afternoon 6 pm local time this timing matches observations crater scaling laws also indicate that these impact energies can reproduce the sizes and masses of the largest observed particles provided the surface has the cohesive properties of weak porous materials bennu s ejection events could be caused by the same kinds of meteoroid impacts that created the moon s asymmetric debris cloud observed by the lunar atmosphere and dust environment explorer ladee our findings also suggest that fewer ejection events should take place as bennu moves further away from the sun a result that can be tested with future observations

population characteristics of submicrometer sized craters on regolith particles from asteroid itokawa abstract we investigated impact crater structures on regolith particles from asteroid itokawa using scanning electron microscopy we observed the surfaces of 51 itokawa particles ranging from 15 m to 240 m in size craters with average diameters ranging from 10 nm to 2 8 m were identified on 13 itokawa particles larger than 80 m we examined the abundance spatial distribution and morphology of approximately 900 craters on six itokawa particles craters with sizes in excess of 200 nm are widely dispersed with spatial densities from 2 6 m2 to 4 5 m2 a fraction of the craters was locally concentrated with a density of 0 1 m2 the fractal dimension of the cumulative crater diameters ranges from 1 3 to 2 3 craters of several tens of nanometers in diameter exhibit pit and surrounding rim structures craters of more than 100 nm in diameter commonly have melted residue at their bottom these morphologies are similar to those of submicrometer sized craters on lunar regolith we estimated the impactor flux on itokawa regolith forming craters assuming that the craters were accumulated during direct exposure to the space environment for 102 to 104 yr the range of impactor flux onto itokawa particles is estimated to be at least one order of magnitude higher than the interplanetary dust flux and comparable to the secondary impact flux on the moon this indicates that secondary ejecta impacts are probably the dominant cratering process in the submicrometer range on itokawa regolith particles as well as on the lunar surface we demonstrate that secondary submicrometer craters can be produced anywhere in centimeter to meter sized depressions on itokawa s surface through primary interplanetary dust impacts if the surface unevenness on centimeter to meter scales is a significant factor determining the abundance of submicrometer secondary cratering the secondary impact flux could be independent of the overall shapes or sizes of celestial bodies and the secondary impact flux could have similar values on itokawa and the moon

simulating weathering of basalt on mars and earth by thermal cycling physical weathering induced by heating and cooling may cause rock breakdown on mars and earth we report results from parallel weathering simulations on basalt blocks exposed to diurnal cycles representing mars like two simulation runs from 55 to 20 oc and 75 to 10 oc 1 100 relative humidity 4 8 mbar pressure co 2 atmosphere and hot arid earth 23 72o c 30 100 relative humidity conditions under earth conditions thermally pre stressed blocks showed measurable strength declines whilst salt pre treated blocks showed strength gains under mars like conditions pre stressed blocks recorded greater or similar strength declines and salt pre treated blocks showed more muted strength declines than under earth conditions the results imply that on earth and mars diurnal cycling of temperature alone can cause deterioration of basalt with a pre existing stress history the type of stress history is important with salt pre treatment affecting the response of thermally pre stressed blocks under both earth and mars conditions

the barringer award address presented 1996 july 25 berlin germany impact experiments related to the evolution of planetary regoliths impact induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales resulting in comminution products that range from fine grained surface soils to massive polymict ejecta deposits to collisionally fragmented objects within this wide range of comminution products we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock s in this paper we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows and we discuss some of their implications for asteroidal surfaces cratering experiments in both rock and noncohesive materials combined with photogeologic observations of the lunar surface demonstrate that craters 500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites the overall excavation rate and thus growth rate of the debris layer decreases with time because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock typical growth rates for a 5 m thick lunar soil layer are initially 3 ga ago a few mm ma and slowed to 1 mm ma at present the coarse grained crater ejecta are efficiently comminuted by collisional fragmentation processes and the mean residence time of a 1 kg rock is typically 10 ma the actual comminution of either lithic or monomineralic detritus is highly mineral specific with feldspar and mesostasis comminuting preferentially over pyroxene and olivine thus resulting in mechanically fractionated fines especially at grain sizes 20 m such fractionated fines also participate preferentially in the shock melting of lunar soils thus giving rise to agglutinate melts as a consequence agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil s compositionally homogeneous impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces reflecting lithic regolith components and associated mineral mixtures cumulatively experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock the addition of foreign exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils regoliths on asteroids are qualitatively different from those of the moon the modest impact velocities in the asteroid belt some 5 km s 1 are barely sufficient to produce impact melts also substantially more crater mass is being displaced on low gravity asteroids compared to the moon collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces these processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the moon impact melting does not seem to be a significant process under these conditions however the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past unfortunately the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of s type asteroids remain poorly understood at present despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution

evidence for slowly changing 87sr86sr in runoff from freshwater limestones of southern france abstract 87 sr 86 sr ratios have been determined in modern rivers and freshwater limestones dating from 65 ma to the present over an area extending from the pyrenees to the massif central southern france about 40 ma ago the alpine mountain range of the pyrenees was strongly uplifted by the alpine orogenic activity and its pre hercynian basement dominated by schists gneisses and granites was rapidly exposed from beneath its blanket of mesozoic marine limestones meanwhile the 87 sr 86 sr ratio of the rivers has remained remarkably constant in both space with the exception of the very few northernmost samples and time in this area extreme variations through the orogenic period are small with 87 sr 86 sr lying between 0 7073 and 0 7084 at 55 65 ma and 0 7077 0 7096 for the last 30 ma river sr is controlled by a nearly binary mixing of silicate 3 4 and 1 4 end members with negligible influence of evaporites it is inferred that the silicate sediment contribution ratio of the world average rivers to seawater sr changes at a very slow rate even taking major basaltic events into account does not alter this conclusion as their contribution to the river sr is very subordinated to other components it is inferred that the seawater 87 sr 86 sr secular main trend over the phanerozoic is controlled by hydrothermal alteration of oceanic basalts and slow variation of the limestone silicate contribution ratio to the riverine sr oceanic events changes in organic productivity and carbonate compensation depth ccd may produce intense short term variations but recovery of steady state seawater 87 sr 86 sr is delayed by 40 80 ma through diagenetic sr remobilization in carbonate sediments

trace elements and c o isotopes of calcite from carlin type gold deposits in the youjiang basin sw china constraints on ore forming fluid compositions and sources abstract the carlin type gold deposits in the youjiang basin sw china can be further classified into three sub types strata bound fault controlled and diabase associated the compositions and sources of the ore forming fluids for each deposit type are evaluated using the texture of calcite by cl cathode luminescence its chemistry by empa electron probe microanalysis and la icp ms laser ablation inductively coupled plasma mass spectrometry and its c o isotopic compositions by sims secondary ion mass spectroscopy the studied deposits are the stratabound zimudang deposit the fault controlled lannigou deposit and the diabase associated badu deposit calcite in the zimudang and lannigou deposits formed at three different stages pre ore stage pore filling and massive calcite cal 1 late ore stage vein calcite cal 2 associated with realgar and pyrite and post ore stage vein calcite cal 3 calcite in the badu deposit formed in two stages ore stage vein and replacement calcite cal 2 associated with pyrite and post ore stage calcites cal 3 calcite with brighter red cl color has higher mn and lower fe content than calcite with darker red cl color furthermore the higher the total ree content the brighter the cl color the host rocks are characterized by light ree rare earth element enrichments in contrast the ore stage calcite shows middle ree enrichments the ore stage calcite has higher concentrations of fe mn mg si and sr and lower concentrations of cu sb and zn than both the pre and post ore calcite the ranges of carbon pdb and oxygen smow isotope compositions of calcite are from 11 29 to 6 89 and from 16 49 to 22 60 for the zimudang deposit from 13 31 to 2 80 and from 9 12 to 29 71 for the lanningou deposit and from 11 94 to 0 49 and from 10 51 to 18 97 for the badu deposit generally the ore stage calcite has a wider range of 13c and 18o values than the other stage calcite the isotope data indicate that a deep crustal fluid was involved in the formation of all of these three deposits meteoric water containing abundant organic matter was also involved in the formation of the badu deposit as well as the zimudang deposit whereas a basin brine was also involved in the formation of the lanningou deposit

thermal fatigue as a driving mechanism for activity on asteroid bennu many boulders on 101955 bennu a near earth rubble pile asteroid show signs of in situ disaggregation and exfoliation indicating that thermal fatigue plays an important role in its landscape evolution observations of particle ejections from its surface also show it to be an active asteroid though the driving mechanism of these events is yet to be determined exfoliation has been shown to mobilize disaggregated particles in terrestrial environments suggesting that it may be capable of ejecting material from bennu s surface we investigate the nature of thermal fatigue on the asteroid and the efficacy of fatigue driven exfoliation as a mechanism for generating asteroid activity by performing finite element modeling of stress fields induced in boulders from diurnal cycling we develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events we find that crack spacing ranges from 1 mm to 10 cm and disaggregated particles have ejection speeds up to 2 m s exfoliation events are most likely to occur in the late afternoon these predictions are consistent with observed ejection events at bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest suggesting that events should be more energetic and more frequent when closer to the sun annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events

thermal fatigue as the origin of regolith on small asteroids space missions1 2 and thermal infrared observations3 have shown that small asteroids kilometre sized or smaller are covered by a layer of centimetre sized or smaller particles which constitute the regolith regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break up of boulders by micrometeoroid impact4 5 laboratory experiments6 and impact models4 however show that crater ejecta velocities are typically greater than several tens of centimetres per second which corresponds to the gravitational escape velocity of kilometre sized asteroids therefore impact debris cannot be the main source of regolith on small asteroids4 here we report that thermal fatigue7 8 9 a mechanism of rock weathering and fragmentation with no subsequent ejection is the dominant process governing regolith generation on small asteroids we find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts because thermal fragmentation is independent of asteroid size this process can also contribute to regolith production on larger asteroids production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near earth asteroids and may explain the observed lack of low perihelion carbonaceous near earth asteroids10

zinc sulfur and lead isotopic variations in carbonate hosted pb zn sulfide deposits southwest china abstract the sichuan yunnan guizhou pb zn metallogenic province in the western yangtze block southwest china contains more than four hundred pb zn deposits with more than 200 million tons of pb zn ores at mean grades of 5 wt pb and 10 wt zn these deposits are hosted in sinian ediacaran to permian carbonate rocks and are structurally controlled by thrust fault fold structures and are spatially associated with the late permian 260 ma emeishan flood basalts two representative low temperature hydrothermal pb zn sulfide deposits the tianqiao and banbanqiao deposits in the southeastern part of the sichuan yunnan guizhou pb zn metallogenic province are selected for zn s pb isotopic analyses sphalerite from the tianqiao deposit has 66 zn values ranging from 0 26 to 0 58 relative to the jmc 3 0749l zinc isotope standard whereas 66 zn values of sphalerite from the banbanqiao deposit range from 0 07 to 0 71 the zinc isotopic composition of sphalerite from both deposits increase from early to final mineralization stage in addition sphalerite from the center near to bottom part of the no 1 ore body in the tianqiao deposit has lower 66 zn values 0 01 to 0 43 than those 0 11 to 0 57 in the periphery near to top sinian to permian sedimentary rocks and permian emeishan flood basalts the potential zinc metal source rocks have 66 zn values range from 0 24 to 0 17 and from 0 32 to 0 44 respectively the majority of the hydrothermal sphalerite has heavier zinc isotope than the country rocks precluding the mixing of multiple zinc sources as the key factor controlling the spatial and temporal variations of zinc isotope therefore the increased 66 zn values from the early to late stage and from the center to top could be due to kinetic raleigh fractionation sphalerite from the tianqiao and banbanqiao deposits has 34 s values ranging from 10 9 to 14 8 and from 3 9 to 9 0 respectively lower than cambrian to permian marine sulfates 15 to 35 and sulfate bearing evaporates 15 to 28 in the devonian to permian carbonate host rocks sulfur of the pb zn ores from both deposits is interpreted as the result of thermal chemical sulfate reduction of evaporates in the sedimentary rocks most likely the host rocks sphalerite from the tianqiao deposit has pb isotope similar to that of age corrected devonian to permian carbonate host rocks whereas sphalerite from the banbanqiao deposit has pb isotope similar to that of age corrected underlying precambrian basement rocks therefore at least lead in the tianqiao and banbanqiao deposits was mainly originated from the host rocks and the underlying basements respectively zn s pb isotopic studies of sphalerite from both deposits indicate that sources of metals and sulfur in the hydrothermal fluid for the tianqiao deposit are the paleozoic carbonate host rocks whereas for the banbanqiao deposit the sources are the precambrian basements and the paleozoic carbonate host rocks respectively

breccias in mississippi valley type deposits the ore deposits in many mississippi valley type districts occur in brecciated carbonate host rocks in some districts the pattern of mineralization and the spatial relationship of ore to earlier karst breccias indicate that the ore solutions followed and enlarged preexisting caves in such cases dissolution of favored layers mainly limestone has allowed collapse and fragmentation of overyling beds usually dolostone the subsidence is long continued and much of it probably occurs while the ore solution is circulating in addition to gravity induced collapse there is evidence that tectonic compression and chemical brecciation working in concert with host rock dissolution may have contributed to the fragmentation other types of breccias found in some of the ore deposits are caused by penecontempo raneous sliding and by wave action breaking forcefully against algal reefs and producing talus economically these types of breccia are of much less importance

shape of 101955 bennu indicative of a rubble pile with internal stiffness the shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the solar system prior to the osiris rex origins spectral interpretation resource identification and security regolith explorer mission earth based radar imaging gave an overview of 101955 bennu s shape here we construct a high resolution shape model from osiris rex images we find that bennu s top like shape considerable macroporosity and prominent surface boulders suggest that it is a rubble pile high standing north south ridges that extend from pole to pole many long grooves and surface mass wasting indicate some low levels of internal friction and or cohesion our shape model indicates that similar to other top shaped asteroids bennu formed by reaccumulation and underwent past periods of fast spin which led to its current shape today bennu might follow a different evolutionary pathway with an interior stiffness that permits surface cracking and mass wasting near earth asteroid bennu has a top like shape with longitudinal ridges macroporosity prominent boulders and surface mass wasting suggesting that it is a stiff rubble pile according to early observations by the osiris rex mission

dolomite fronts and associated zinc lead mineralization usa this paper provides previously unpublished drill core and mapping evidence of the spatial relationship between regional dolomitization and mississippi valley type mvt mineralization in the midcontinent of the united states dolomitization is discussed for several key carbonate units and their contained mvt mineralization this more complete picture of the relationship between regional dolomitization and mineralization provides constraints on models for mvt mineralization and serves as an exploration guide for new deposits

strontium and its isotopes in canadian rivers fluxes and global implications systematic sampling of the 39 largest canadian rivers shows that the weighted average 87sr86sr ratio in the dissolved load is 0 7111 similar to previous measurements on such large rivers as the amazon and mississippi consequently we believe that the above estimate is likely representative of the global average this imposes a limit of 6 5 1011g yr 1 on the amount of sr exchanged with basalts in hydrothermal cells on mid oceanic ridges evaluation of geological information suggests that sr from silicate sources is of considerable importance for all but the largest canadian rivers the latter have chemical and isotopic composition consistent with 4 1 carbonate vs silicate derivation of sr but such interpretation is not unique in terms of their water discharge the 39 canadian rivers studied account for 4 2 of the world total and their weighted average concentrations for other dissolved solutes are tds 176 ppm ca 18 ppm cl 6 8 ppm and sr 84 ppb

origin and reservoir characteristics of upper ordovician trenton black river hydrothermal dolomite reservoirs in new york in the past decade more than 20 new natural gas fields have been discovered in laterally discontinuous dolomites of the upper ordovician black river group in south central new york the dolomites form around basement rooted wrench faults that are detectable on seismic data most fields occur in and around elongate fault bounded structural lows interpreted to be negative flower structures away from these faults the formation is composed of impermeable limestone and forms the lateral seal for the reservoirs in most cases the faults die out within the overlying trenton limestone and utica shale most porosity occurs in saddle dolomite coated vugs breccias and fractured zones matrix porosity is uncommon in the black river cores described for this study the patchy distribution around basement rooted faults and geochemical and fluid inclusion analyses supports a fault related hydrothermal origin for the saddle and matrix dolomites this play went for many years without detection because of its unconventional structural setting i e structural lows versus highs using the appropriate integrated structural stratigraphic diagenetic model more hydrothermal dolomite natural gas reservoirs are likely to be discovered in the black river of new york and in carbonates around the world

the ore forming process of the maoping pb zn deposit northeastern yunnan china constraints from cathodoluminescence cl petrography of hydrothermal dolomite abstract most mississippi valley type mvt pb zn deposits have a common host rock lithofacies sequence known as the hydrothermal dolomite htd facies however the spatial and temporal relationships between dolomitization and pb zn mineralization remain unclear in this paper we report a detailed cathodoluminescence cl study on the htd facies of the no ii orebody in the maoping pb zn deposit in northeastern yunnan province southwestern china the results indicate that the dolomite in the carboniferous limestone is part of a spectrum of hydrothermal mineral deposits and the diagenesis and mineralization processes in which the acidity of fluids played an important role can be divided into six steps a dolomite prototype formed during the first and second steps in the third step acid fluids produced by sulfide precipitation interacted with the dolomite prototype resulting in dissolution and alteration thus forming cavities the fluid that formed during the first three steps flowed along open tensional faults dolomitization in the first step covered most of the region of later mineralization and gradually extended toward the center of the orebody in the fourth and fifth steps vugs and fractures were the main pathways for fluids with only a small volume of fluid in the fifth step dedolomitization occurred in the sixth step and vugs were filled mostly by calcite however the fact that voids are present in only a small number of vugs is indicative of fluid exhaustion dolomite with growth zoning is characteristic of precipitation by ore forming fluids accompanied by pulsed filling there is no significant correlation between vugs and the locus of mineralization rather the vugs indicate that the ore forming fluids had passed through them and the locus of massive ore formation was the center of the migrating fluid body the saddle dolomite was deposited during pauses in sulfide precipitation cathodoluminescent zonation of htd can be used as an aid in understanding the ore forming process and for mineral prospecting in the maoping deposit and other mvt deposits

the efficiency of thermal fatigue in regolith generation on small airless bodies abstract regolith generation by thermal fatigue has been identified as a dominant mechanism for the breakdown of small cm sized rocks on certain airless bodies simple numerical models for thermal fatigue seemed to indicate that this breakdown occurs faster in the larger decimeter sized rocks which is in contrast to the predictions of disruption models through successive micrometeorite impacts the observation is justified by the existence of larger temperature gradient in bigger rocks but it is not clear that this conclusion can be extrapolated or scaled to meter sized boulders here we reveal a transition in the rock disaggregation rates by thermal fatigue when rock sizes rise above a critical length scale a simple analytic model is formulated to predict the time to fracture of rocks on small airless bodies we consider an uncoupled approach consisting of a one dimensional thermal model and a two dimensional fracture model the solution of the heat equation is used as input to the thermomechanical crack growth problem this new understanding could provide bounds on the survival rates of asteroidal rocks and may help in coupling thermal fatigue with a mechanical disruption model to obtain a multi mechanism view of regolith evolution in the solar system

impact produced seismic shaking and regolith growth on asteroids 433 eros 2867 steins and 25143 itokawa abstract of the several near earth and main belt asteroids visited by spacecraft to date three display a paucity of small craters and an enhanced number of smoothed and degraded craters 433 eros which has a deficit of craters 100 m in diameter 2867 steins which has a deficit of craters 500 m in diameter and 25143 itokawa which has a deficit of craters 100 m in diameter the purpose of this work was to investigate and model topographic modification and crater erasure due to impact induced seismic shaking as well as impact driven regolith production and loss on these asteroid surfaces to perform this study we utilized the numerical three dimensional cratered terrain evolution model ctem initially presented in j e richardson icarus 204 2009 which received a small body sb specific update for this work sbctem simulations of the surface of 433 eros correctly reproduce its observed cratering record for craters up to 3 km in diameter at a minimum main belt exposure age of 225 75 myr using a very weak rock target strength of 0 5 5 0 mpa and producing a mean regolith depth of 80 20 m which agrees with published estimates of an actual regolith layer tens of meters in depth sbctem simulations of the surface of 2867 steins correctly reproduce its heavily softened cratering record at a minimum main belt exposure age of 175 25 myr using a weak rock target strength of 2 5 5 0 mpa extending this simulation produces a mean regolith depth of 155 5 m at a minimum main belt exposure age of 475 25 myr in good agreement with the 145 35 m actual mean regolith depth estimated from two chains of drainage pit craters on the surface with both crater density and regolith depth in an equilibrium state sbctem simulations of the surface of 25143 itokawa correctly reproduce its extremely sparse cratering record at a minimum main belt exposure age of 20 5 myr using a soft soil target strength of 10 20 kpa and producing a mean regolith overturn depth of 5 0 1 0 m given that itokawa is composed entirely of regolith and boulders these simulations demonstrate the efficacy of impact induced seismic shaking to erase small craters on the surface of asteroids 25 km mean diameter correctly reproducing their observed cratering records while using a main belt impactor population and standard crater scaling relationships these simulations also show that the observed regolith layer on asteroids in the 5 50 km size range can be generated entirely through impact cratering processes and indicate that regolith depth can be used as a additional constraint on the surface age of a given asteroid

regional epigenetic dolomitization in the bonneterre dolomite cambrian southeastern missouri a dolomite bed averaging 6 m in thickness forms the base of a limestone and shale facies of the bonneterre dolomite cambrian southeastern missouri the dolomite immediately overlies the lamotte sandstone and covers an area of more than 16 600 km 2 it is proposed that this basal dolomite formed as a result of interaction with warm basin derived water circulating through the underlying lamotte sandstone and that the dolomitization event coincided with the emplacement of mississippi valley type sulfide ores in the nearby viburnum trend this hypothesis is supported by 1 petrographic evidence that suggests an origin at temperatures 50 c 2 cathodoluminescent microstratigraphy of dolomite cements which is correlative with that in gangue dolomite cements in the nearby orebodies and 3 stable oxygen and carbon isotope values that are consistent with a basinal origin of the dolomitizing water the basal dolomite bed may represent one of the most extensive epigenetic dolomites yet documented this implies that a basinal water can alter rocks over a wide area at a considerable distance from its source

nanomorphology of itokawa regolith particles application to space weathering processes affecting the itokawa asteroid abstract the morphological properties of 26 regolith particles from asteroid itokawa were observed using scanning electron microscopes in combination with an investigation of their three dimensional shapes obtained through x ray microtomography surface observations of a cross section of the ll5 chondrite and of crystals of olivine and pyroxene were also performed for comparison some itokawa particles have surfaces corresponding to walls of microdruses in the ll chondrite where concentric polygonal steps develop and euhedral or subhedral grains exist these formed through vapor growth owing to thermal annealing which might have been caused by thermal metamorphism or shock induced heating in itokawa s parent body most of the itokawa particles have more or less fractured surfaces indicating that they were formed by disaggregation probably caused by impacts itokawa particles with angular and rounded edges observed in computed tomography images are associated with surfaces exhibiting clear and faint structures respectively these surfaces can be interpreted by invoking different degrees of abrasion after regolith formation a possible mechanism for the abrasion process is grain migration caused by impact driven seismic waves space weathered rims with blisters are distributed heterogeneously across the itokawa regolith particles this heterogeneous distribution can be explained by particle motion and fracturing combined with solar wind irradiation of the particle surfaces the regolith activity including grain motion fracturing and abrasion might effectively act as refreshing process of itokawa particles against space weathered rim formation the space weathering processes affecting itokawa would have developed simultaneously with space weathered rim formation and regolith particle refreshment

episodes of particle ejection from the surface of the active asteroid 101955 bennu introduction active asteroids are small bodies in the solar system that show ongoing mass loss such as the ejection of dust which may be caused by large impacts volatile release or rotational acceleration studying them informs our understanding of the evolution and destruction of asteroids and the origin of volatile materials such as water on earth the osiris rex origins spectral interpretation resource identification and security regolith explorer spacecraft has rendezvoused with the near earth asteroid 101955 bennu the selection of bennu as the osiris rex target was partially based on its spectral similarity to some active asteroids observations designed to detect mass loss at bennu were conducted from earth and during the spacecraft s approach but no signs of asteroid activity were found however when the spacecraft entered orbit in january 2019 we serendipitously observed particles in the vicinity of bennu that had apparently been ejected from its surface rationale we analyzed the properties and behavior of particles ejected from bennu to determine the possible mechanisms of ejection and provide understanding of the broader population of active asteroids images obtained by the spacecraft indicate multiple discrete ejection events with a range of energies and resultant particle trajectories we characterized three large ejection events that respectively occurred on 6 january 19 january and 11 february 2019 tracking of individual particles across multiple images by means of optical navigation techniques provided the initial conditions for orbit determination modeling by combining these approaches we estimated the locations and times of ejection events and determined initial velocity vectors of particles we estimated the particle sizes and the minimum energies of the ejection events using a particle albedo and density consistent with observations of bennu results particles with diameters from 3 m s 1 estimated energies ranged from 270 mj for the 6 january event to 8 mj for the 11 february event the three events arose from widely separated sites which do not show any obvious geological distinction from the rest of bennu s surface however these events all occurred in the late afternoon between about 15 00 and 18 00 local solar time in addition to discrete ejection events we detected a persistent background of particles in the bennu environment some of these background particles have been observed to persist on temporary orbits that last several days in one case with a semimajor axis 1 km the orbital characteristics of these gravitationally bound objects make it possible to determine the ratio of their cross sectional area to their mass combined with their photometric phase functions this information constrains the parameter space of the particles diameters densities and albedos conclusion plausible mechanisms for the large ejection events include thermal fracturing volatile release through dehydration of phyllosilicates and meteoroid impacts the late afternoon timing of the events is consistent with any of these mechanisms bennu s boulder geology indicates that thermal fracturing perhaps enhanced by volatile release could occur on the asteroid surface smaller events especially those that occur on the night side of bennu could be attributable to reimpacting particles our observations classify bennu as an active asteroid active asteroids are commonly identified by major mass loss events observable with telescopes on scales much greater than we observed at bennu our findings indicate that there is a continuum of mass loss event magnitudes among active asteroids

structurally controlled hydrothermal dolomite reservoir facies an overview structurally controlled hydrothermal dolomite htd reservoir facies and associated productive leached limestones are major hydrocarbon producers in north america and are receiving increased exploration attention globally they include multiple trends in the ordovician locally silurian and devonian of the michigan appalachian and other basins of eastern canada and the united states and in the devonian and mississippian of the western canada sedimentary basin they also occur in jurassic hosts along rifted atlantic margins in the jurassic cretaceous of the arabian gulf region and elsewhere hydrothermal dolomitization is defined as dolomitization occurring under burial conditions commonly at shallow depths by fluids typically very saline with temperature and pressure t and p higher than the ambient t and p of the host formation the latter commonly is limestone proof of a hydrothermal origin for htd reservoir facies requires integration of burial thermal history plots fluid inclusion temperature data and constraints on timing of emplacement hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary exhalative lead zinc ore bodies and htd hosted mississippi valley type sulfide deposits all three hydrothermal deposits show a strong structural control by extensional and or strike slip wrench faults with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall transtensional sags above negative flower structures on wrench faults are favored drilling sites for htd reservoir facies saddle dolomite in both replacive and void filling modes is characteristic of htd facies for many reservoirs matrix replacive dolomite and saddle dolomite appear to have formed near contemporaneously and from the same fluid and temperature conditions the original host facies exerts a major influence on the lateral extent of dolomitization resultant textures pore type and pore volume breccias zebra fabrics shear microfractures and other rock characteristics record short term shear stress and pore fluid pressure transients particularly proximal to active faults high temperature hydrothermal pulses may alter kerogen in host limestones a process designated forced maturation basement highs underlying sandstone and or carbonate aquifers probably overpressured and overlying and internal shale seals and aquitards also may constrain or influence htd emplacement although many questions and uncertainties remain particularly in terms of mg and brine source and mass balance recognition and active exploration of the htd play continues to expand increasing use of three dimensional seismic imagery and seismic anomaly mapping combined with horizontal drilling oblique to linear trends defined by structural sags helps to reduce risk