space weathering of asteroid surfaces abstract visible and near infrared spectra of reflected sunlight from asteroid surfaces exhibit features that hold the promise for identifying surface mineralogy however the very surfaces that are observed by remote sensing are also subject to impingement by micrometeoroids and solar wind particles which are believed to play the dominant role in space weathering which is the time dependent modification of an asteroid s reflectance spectrum such space weathering has confused the interpretations of telescopic spectra of asteroids especially concerning the possible association of common ordinary chondritic meteorites with so called s type asteroids recent spacecraft studies of asteroids especially of eros by near shoemaker have documented aspects of space weathering processes but we still do not understand the physics of space weathering well enough to confidently assay mineralogy of diverse asteroids by remote sensing a review of the intellectual history of this topic reveals the complexity of

paleoclimatic and paleomagnetic constraints on the paleozoic reconstructions of south china north china and tarim abstract paleomagnetic and paleoclimatic data provide the most useful latitudinal constraints for plate reconstructions distributions through the paleozoic of five types of climatically sensitive sediments coals evaporites reefs dolomites and limestones for south china north china and tarim are shown on 15 maps that include 1578 reliable data points these paleoclimatic data agree reasonably well with available paleomagnetic directions although significant divergence between the two exists for the early paleozoic these data indicate the following 1 1 south china was in low latitudes during the entire paleozoic with a subtropical position in the cambrian 2 2 north china also remained near the equator in the early and late paleozoic except for the ordovian and the late permian when extensive evaporites suggest slightly higher latitudinal positions while its middle paleozoic position is uncertain due to the missing stratigraphie record 3 3 in south china local tectonics appears to have played a dominant role in determining paleogeography and therefore marine sedimentation especially after the late ordovician early silurian because the areal coverage of marine sediments through time is distinctly different from what would be expected from published global sea level curves 4 4 paleoclimatic and paleomagnetic data are compatible with biogeographic data which suggest that south china was part of eastern gondwana in the early paleozoic but was widely separated from gondwana in the late paleozoic and the split between the two probably happened in the devonian giving rise to a major break up unconformity in central south china

multiple dolomitization and later hydrothermal alteration on the upper cambrian lower ordovician carbonates in the northern tarim basin china abstract thick upper cambrian lower ordovician carbonates were deposited on a shallow marine platform in the northern tarim basin which were extensively dolomitized particularly for the upper cambrian carbonates the resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites based on petrographic textures matrix dolomites consist of very finely to finely crystalline nonplanar a to planar s dolomite md1 finely to medium crystalline planar e s dolomite md2 and finely to coarsely crystalline nonplanar a dolomite md3 minor cement dolomites include finely to medium crystalline planar s e dolomite cd1 and coarsely crystalline nonplanar saddle dolomite cd2 which partially or completely fill dissolution vugs and fractures these cements postdate matrix dolomites but predate later quartz and calcite infills origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography isotopic geochemistry o c and sr and fluid inclusion microthermometry md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines e g evaporated seawater in near surface to very shallow burial settings whereas md2 dolomite was formed from connate seawater in association with burial dissolution and localized mg concentration or cannibalization in shallow burial conditions md3 dolomite however was likely the result of intense recrystallization or neomorphism upon previously formed dolomites e g md1 or md2 dolomite as the host carbonates were deeply buried and influenced by later hydrothermal fluids subsequent cement dolomite and quartz crystals precipitated from higher temperature hydrothermal fluids which were contributed more or less by the extensive permian large igneous province lip activity in tarim basin as evidenced by less radiogenic sr in the cement and parts of matrix dolomites this extensive abnormal hydrothermal activity could also have resulted in recrystallization or neomorphism on the previous matrix dolomites faults fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths however the basin uplift triggered by the late hercynian orogeny from the late permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids resulting in precipitation of later calcites in which lighter c and more radiogenic sr components demonstrate such a switch of fluid properties this study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the permian lip activity within tarim basin and elsewhere

the mineralogy of ordinary chondrites and implications for asteroid spectrophotometry abstract the normative mineralogies of 94 ordinary chondrites have been calculated from high quality bulk chemical analyses relative proportions of olivine orthopyroxene and metal are more variable within the h l and ll classes than has been recognized previously mineralogy within each chondrite class varies systematically with oxidation state these data have important implications for the design of reflectance spectroscopy studies that match asteroid and meteorite spectral features and for quantitative spectral interpretations of asteroids they also provide a tool with which to predict mineral abundances in unsampled asteroids that formed under different redox conditions

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

carbonate platform margin architecture and its influence on cambrian ordovician reef shoal development tarim basin nw china abstract six types of carbonate platform marginal slope have been identified in the cambrian ordovician of tarim basin these include a progradational carbonate ramp platform a progradational carbonate rimmed platform an aggradational rimmed carbonate platform a retrogradational rimmed carbonate platform an aggradational margin on an isolated carbonate platform and a retrogradational margin on a submerged carbonate platform various platform margin architectures and reef shoal characteristics occur in different areas of the tarim basin a progradational carbonate ramp platform margin began in the early cambrian in the tarim basin a progradational rimmed platform margin developed in the middle cambrian time in the northern tarim basin whereas contemporaneously a progradational ramp platform margin developed in the southwestern and central tarim basin a retrogradational rimmed carbonate platform developed in the northern tarim basin whereas an aggradational rimmed carbonate platform occupied the central tarim basin in the early ordovician time an aggradational margin on an isolated platform and a retrogradational margin on a submerged platform developed mainly during the late ordovician in the central tarim basin platform margin slopes migrated continually throughout the cambro ordovician the broad platform area western tarim platform that occupied most of the northwestern and central tarim basin in the early cambrian time was split by an east west oriented shelf seaway that began to develop along the eastern and western margins of the platform in the early ordovician time and completely transected the platform by late ordovician time the resultant central tarim basin platform area was much reduced elongate east west around the central tarim paleo uplift and terminated eastward towards region of the late ordovician basinal deposition across the eastern tarim basin affected by platform margin structure climate and paleodepth algal reefs and shoals developed mainly in the cambrian and intraclastic shoals developed mainly in the early ordovician followed by reef building of framestone or bafflestone in the late ordovician

global survey of color variations on 433 eros implications for regolith processes and asteroid environments abstract high resolution imaging acquired with the near earth asteroid rendezvous shoemaker near shoemaker spacecraft is used to elucidate the spectral properties and spatial distribution of color units on asteroid 433 eros previous workers mapped four distinct types of color units on the surface bright streaks dark soils ponded materials average regolith these units exhibit albedo and color boundaries but there is no evidence to indicate they represent distinct rock types rather the units are thought to show evidence of complex regolith transport and sorting processes here we report the results of a comprehensive study of all viable color multispectral imager msi data to identify and characterize the distribution and nature of color units across the whole asteroid due to a spacecraft upset that resulted in contamination of the msi optics color images are affected with a scattered light problem that hampers interpretation of subtle color contrasts even after a rigorous remediation to constrain interpretations of the msi color data we characterize this residual scattered light and demonstrate how complete correction would affect derived color ratios results of our comprehensive study are consistent with previous mapping confirming that bright streaks average regolith and dark soils fall on a mixing line consistent with space weathering effects we find that the ponded deposits do not fall on this putative mixing line the color and reflectance of the ponded deposits are consistent with some combination of compositional grain size and maturity variations from the average regolith additionally we show that spectral separation of the four units on ratio plots would only increase with full removal of residual scattered light especially for features that are small in terms of pixels global analysis of the eros color units illustrates complex regolith processes and grain sorting that may hold clues to understanding space weathering processes and the link between asteroids and meteorites

an igneous fragment from cluster idp l2011 21 an analog for the source of pyrrhotite and taenite in comet 81p wild 2 captured in stardust aerogel

magnesium isotopes in high temperature saddle dolomite cements in the lower paleozoic of canada abstract mg isotopes are used to better understand the genesis of hydrothermal saddle dolomite cements in lower paleozoic successions in canada these cements occur in fault bounded dolostones that overlay lithologically diverse basement rocks ordovician dolomite lies over the precambrian craton whereas the silurian and devonian dolomites overlay a succession of tectonically accreted sedimentary volcanic and ultramafic units of cambrian to ordovician age lower silurian saddle dolomites have the most negative 26mgdsm3 values of our dataset 3 25 to 1 13 and plot in two distinct groups a strongly negative subset that characterizes higher temperature 175 c dolomites and a less negative subset for lower temperature 153 c dolomites upper ordovician saddle dolomites precipitated at significantly lower temperatures 102 c and their 26mgdsm3 values range from 1 26 to 0 71 lower devonian saddle dolomites formed at very high temperature 350 c and have 26mgdsm3 values ranging from 1 29 to 0 78 no experimental data on high temperature 100 350 c fluid dolomite mg isotope fractionation factors have been published and recent research suggests that no significant fractionation occurs between diagenetic fluids and dolomites at high temperatures in closed to semi closed diagenetic systems our results indicate that the isotopic signature of diagenetic fluid is the primary control for the 26mgdsm3 values in these high temperature dolomites

neoproterozoic cap dolostone deposition in stratified glacial meltwater plume abstract neoproterozoic cap carbonates host distinctive geochemical and sedimentological features that reflect prevailing conditions in the aftermath of snowball earth interpretation of these features has remained contentious with hypotheses hinging upon timescale and synchronicity of deposition and whether or not geochemical signatures of cap carbonates represent those of a well mixed ocean here we present new high resolution sr and mg isotope results from basal ediacaran cap dolostones in south australia and mongolia least altered sr and mg isotope compositions of carbonates are identified through a novel incremental leaching technique that monitors the purity of a carbonate sample and the effects of diagenesis these data can be explained by the formation of these cap dolostones involving two chemically distinct solutions a glacial meltwater plume enriched in radiogenic sr and a saline ocean residue with relatively lower 87sr 86sr ratios model simulations suggest that these water bodies remained dynamically stratified during part of cap dolostone deposition most likely lasting for 8 thousand years our results can potentially reconcile previous conflicts between timescales estimated from physical mixing models and paleomagnetic constraints geochemical data from cap carbonates used to interpret the nature of snowball earth and its aftermath should be recast in terms of a chemically distinct meltwater plume

hydrogen and major element concentrations on 433 eros evidence for an l or ll chondrite like surface composition a reanalysis of near x ray gamma ray spectrometer xgrs data provides robust evidence that the elemental composition of the near earth asteroid 433 eros is consistent with the l and ll ordinary chondrites these results facilitated the use of the gamma ray measurements to produce the first in situ measurement of hydrogen concentrations on an asteroid the measured value 1100 1600 700 ppm is consistent with hydrogen concentrations measured in l and ll chondrite meteorite falls gamma ray derived abundances of hydrogen and potassium show no evidence for depletion of volatiles relative to ordinary chondrites suggesting that the sulfur depletion observed in x ray data is a surficial effect consistent with a space weathering origin the newfound agreement between the x ray gamma ray and spectral data suggests that the near landing site a ponded regolith deposit has an elemental composition that is indistinguishable from the mean surface this observation argues against a pond formation process that segregates metals from silicates and instead suggests that the differences observed in reflectance spectra between the ponds and bulk eros are due to grain size differences resulting from granular sorting of ponded material

spectral properties and geologic processes on eros from combined near nis and msi data sets from april 24 to may 14 2000 the near earth asteroid rendezvous near shoemaker mission s near infrared spectrometer nis obtained its highest resolution data of 433 eros high signal to noise ratio nis reflectance spectra cover a wavelength range of 8002400 nm with footprint sizes from 213 x 427 m to 394 x 788 m this paper describes improvement in instrument calibration by remediation of internally scattered light derivation of a pseudo channel for nis at 754 nm using multispectral imager msi eros approach maps at 951 and 754 nm synthesis of a 3127 spectrum high resolution data set with the improved calibration and expanded wavelength coverage and investigation of global and localized spectral variation with respect to mineralogy composition and space weathering of eros comparing the findings with previous analyses scattered light removal reduces the red slope of eros spectra though not to the level seen by telescopic observations the pseudo channel completes sampling of eros 1 micron band i absorption feature enabling direct comparison of nis data with other asteroid and meteorite spectra without additional scaling or correction following scattered light removal and wavelength range extension the spectral parameters of average eros plot well inside the s iv field of gaffey et al 1993 and are consistent with the l6 chondrite meteorite fields of gaffey and gilbert 1998 although eros shows no evidence of mineralogical heterogeneity modest spectral variations correlate with morphologically and geographically distinct areas of the asteroid eros bright to dark spectral ratios are largely consistent with laboratory space weathering experiment results and modeling of space weathering effects eros brightness variation unaccompanied by significant spectral variation departs from lunar type where band depths slopes and albedoes all correlate and ida type where significant spectral variation is unaccompanied by corresponding brightness variation the brightest areas on eros steep crater walls have lesser spectral slope and deeper band i consistent with exposure of fresher less space weathered materials bright crater slope materials have opx opx olv of 0 240 29 and may be more representative of the subsurface mineralogy than average eros which is probably affected by space weathering the floors of the large craters psyche and himeros have lower albedo and contain the most degraded or altered looking materials nis spectra retain a red spectral slope at greater than 2 microns the recalibrated and expanded nis spectra show better agreements with mixing models based on space weathering of chondritic mixtures

space weathering on airless bodies resolving a mystery with lunar samples using new techniques to examine the products of space weathering of lunar soils we demonstrate that nanophase reduced iron npfe0 is produced on the surface of grains by a combination of vapor deposition and irradiation effects the optical properties of soils both measured and modeled are shown to be highly dependent on the cumulative amount of npfe0 which varies with different starting materials and the energetics of different parts of the solar system the measured properties of intermediate albedo asteroids the abundant s type asteroids in particular are shown to directly mimic the effects predicted for small amounts of npfe0 on grains of an ordinary chondrite regolith this measurement and characterization of space weathering products seems to remove a final obstacle hindering a link between the abundant ordinary chondrite meteorites and common asteroids

features and effects of basement faults on deposition in the tarim basin the tarim basin as the largest inland oil producing basin in china is a large scale composite basin which formed from a palaeozoic craton basin that was overprinted by a mesozoic cenozoic foreland basin based on comprehensive analyses of gravitational data aeromagnetic data and seismic data from the basin 38 basement faults can be identified demonstrating that the tarim basin has undergone several tectonic cycles according to their distributional locations in the basin basement faults can be divided into marginal and internal basin basement faults moreover on the basis of the distribution characteristics in the plane and controlling effects on the basin the basement faults can be divided into 20 primary basement faults and 18 secondary basement faults combined with the interpretations of seismic profiles we can determine the relationship between the basement faults and sedimentary cover faults considering the section balance of seismic profiles we find that some basement faults are syn depositional with continuous long term activity during the basin s evolution and control the deposition in the basin by counting fault growth indexes of the main sections it is obvious that different parts of the same fault have different growth indexes but show similar activity within the same geological period the faults stress states remain consistent with their geological history although the growth indexes of different faults are inconsistent

quantifying the effect of diagenetic recrystallization on the mg isotopic composition of marine carbonates abstract the mg and sr isotopic compositions 26mg and 87sr 86sr of pore fluids and bulk carbonates from ocean drilling project site 1171 south tasman rise 2148 2 m water depth are reported in order to evaluate the potential of diagenesis to alter carbonate based geochemical proxies in an open marine system given the trace amounts of mg in marine carbonates relative to coexisting pore fluids diagenesis can alter carbonate 26mg a promising proxy for seawater 26mg that may help elucidate long term changes in the global mg cycle constraints on the effect of diagenetic recrystallization on carbonate 26mg are therefore critical for accurate proxy interpretations this study provides context for assessing the fidelity of geochemical proxy reconstructions using the primary components i e foraminiferal tests and nannofossils of bulk carbonate sediments we find that pore fluid 26mg values on the dsm3 scale at site 1171 increase systematically with depth from 0 72 to 0 39 in the upper 260 m while the 26mg of bulk carbonates decrease systematically with depth from 2 23 to 5 00 in the upper 260 m this variability is ascribed primarily to carbonate recrystallization with a small proportion of the variability due to down hole changes in nannofossil and foraminiferal species composition the inferred effect of diagenesis on bulk carbonate 26mg correlates with down core changes in mg ca sr ca na ca and 87sr 86sr a depositional reactive transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate 26mg model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work assuming calcite recrystallization rates of 7 ma constrained by sr geochemistry in addition either partial dissolution or a distinctly different previous diagenetic regime must be invoked in order to explain aspects of the elemental chemistry and 87sr 86sr of relatively deep sediments from holes a and c this study indicates that the dynamics of a given sedimentary system can significantly alter bulk carbonate geochemistry and presents a framework for considering the potential impact of such alteration on picked archives such as foraminiferal tests and nannofossils ultimately this study contributes to the development of 26mg as a proxy for seawater 26mg by quantifying the susceptibility of carbonate 26mg to diagenetic alteration particularly in sediments in open marine systems this study suggests that because of the sensitivity of carbonate 26mg to diagenetic recrystallization it can in certain systems be used to quantify the impact of diagenesis on carbonate based geochemical proxies

oxygen and asteroids hundreds of thousands of asteroids have been discovered in the asteroid belt and in near earth space oxygen is an abundant element in meteorites and presumably in most asteroids spectral reflectance measurements of asteroids in the visible and near infrared can identify oxygen bearing minerals such as those found in the olivine pyroxene and serpentine groups due to their distinctive absorption features interpretation of the mineralogy of asteroids is complicated by the effects of space weathering which tends to redden and darken the surfaces of asteroids asteroids are primarily classified into a number of taxonomic classes and subclasses according to their spectral properties in the visible wavelength region however asteroids with similar spectral properties in the visible may have different spectral properties in the near infrared and therefore different interpreted mineralogies definite trends in the abundances of different taxonomic classes versus heliocentric distance are apparent these trends appear to be a function of both composition differences and degree of heating however dynamical processes have significantly affected these trends close observation of asteroids by spacecraft such as near shoemaker to 433 eros and hayabusa to 25143 itokawa are currently the best way to discern the mineralogies of individual asteroids

dolomitization by penesaline sea water in early jurassic peritidal platform carbonates gibraltar western mediterranean peritidal carbonates of the lower jurassic liassic gibraltar limestone formation which form the main mass of the rock of gibraltar are replaced by fine and medium crystalline dolomites replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an 460 m thick platform succession the fine crystalline dolomite has d 18 o values either similar to or slightly higher than those expected from early jurassic marine dolomite and d 13 c values together with 87 sr 86 sr ratios that overlap with sea water values for that time indicating that the dolomitizing fluid was early jurassic sea water absence of massive evaporitic minerals and or evaporite solution collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation the occurrence of peritidal facies a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions salinity 72 199 the medium crystalline dolomite has some d 18 o and d 13 c values and 87 sr 86 sr ratios similar to those of early jurassic marine dolomites which indicates that ambient sea water was again a likely dolomitizing fluid however the spread of d 18 o d 13 ca nd 87 sr 86 sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow 500 m burial or that dolomitization was multistage these data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation taking earlier models into consideration it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities normal penesaline to hypersaline provided that there is a driving mechanism for fluid migration the gibraltar dolomites confirm other reports of significant early jurassic dolomitization in the western tethys carbonate platforms

petrological and geochemical constraints on diagenesis and deep burial dissolution of the ordovician carbonate reservoirs in the tazhong area tarim basin nw china abstract deeply buried 4500 7000 m ordovician carbonate reservoirs in the tazhong area tarim basin nw china show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27 8 in sour dolostones from which economically important oils sour gas and condensates are currently being produced petrographic features c o sr isotopes were determined and fluid inclusions were analyzed on diagenetic calcite dolomite and barite from ordovician reservoirs to understand controls on the porosity distribution ordovician carbonate reservoirs in the tazhong area are controlled mainly by initial sedimentary environments and eo genetic and near surface diagenetic processes however vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction filling and cementation in some locations or wells especially zg5 zg7 oilfield nearby zg5 fault burial diagenesis e g thermochemical sulfate reduction tsr probably played an important role in quality improvement towards high quality reservoirs c2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures ths from 86 to 113 c from 96 to 128 c and from 128 to 151 c respectively we observed petrographically corroded edges of these high temperature minerals with oil inclusions indicating the dissolution must have occurred under deep burial conditions the occurrence of tsr within ordovician carbonate reservoirs is supported by c3 calcite replacement of barite and the association of sulfur species including pyrite anhydrite or barite and elemental sulfur with hydrocarbon and 12c rich as low as 7 2 v pdb c3 calcite with elevated ths 135 153 c the tsr may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations in contrast no significant burial dissolution occurred in limestone reservoirs and non tsr dolostone reservoirs the deeply buried sour dolostone reservoirs may therefore be potential exploration targets in tarim basin or elsewhere in the world

spectral properties and rotational spectral heterogeneity of 433 eros composite spectra of different rotational aspects of the s asteroid 433 eros were assembled from previous studies covering the wavelength range 0 33 2 5 m these data allow us to measure significant attributes of eros s spectrum its mafic mineral absorptions and the degree and nature of compositional heterogeneity of its surface we also evaluated previous s asteroid taxonomies using independent methods and from this test determined that the taxonomy of gaffey et al 1993 provides a sound basis for relating eros to other s asteroids on average eros resembles the s iv asteroids which bear the closest resemblance to ordinary chondrites in mafic mineralogy however eros also exhibits distinct rotational spectral variations comparable to the greatest yet recognized on an s asteroid one side of the asteroid has a shorter wavelength 1 m absorption and a stronger 2 m absorption than the global average and is interpreted to be richer in pyroxene the opposite face has a longer wavelength 1 m absorption and a weaker 2 m absorption and is interpreted to be richer in olivine the pyroxene rich side coincides with a facet like face identified in radar studies and the olivine rich side coincides with a convex shaped face each side of the asteroid exhibits spectral affinities with a different spectral class of s asteroids demonstrating that such subdivisions of s asteroids represent different parts of a compositional continuum high spatial resolution measurement of the mineralogic and elemental compositions and especially the geologic settings of these different materials by the near earth asteroid rendezvous mission will provide a great advance in the understanding of the origin and evolution of a candidate ordinary chondrite parent body

space weathering on airless bodies space weathering refers to alteration that occurs in the space environment with time lunar samples and to some extent meteorites have provided a benchmark for understanding the processes and products of space weathering lunar soils are derived principally from local materials but have accumulated a range of optically active opaque particles oaopq that include nanophase metallic iron on in rims formed on individual grains imparting a red slope to visible and near infrared reflectance and larger iron particles which darken across all wavelengths such as are often found within the interior of recycled grains space weathering of other anhydrous silicate bodies such as mercury and some asteroids produce different forms and relative abundance of oaopq particles depending on the particular environment if the development of oaopq particles is minimized such as at vesta contamination by exogenic material and regolith mixing become the dominant space weathering processes volatile rich bodies and those composed of abundant hydrous minerals dwarf planet ceres many dark asteroids outer solar system satellites are affected by space weathering processes differently than the silicate bodies of the inner solar system however the space weathering products of these bodies are currently poorly understood and the physics and chemistry of space weathering processes in different environments are areas of active research

the composition of 433 eros a mineralogical chemical synthesis abstract the near earth asteroid rendezvous near mission carried x ray gamma ray spectrometers and multi spectral imager near infrared spectrometer instrument packages which gave complementary information on the chemistry and mineralogy respectively of the target asteroid 433 eros synthesis of these two data sets provides information not available from either alone including the abundance of non mafic silicates metal and sulfide minerals we have utilized four techniques to synthesize these data sets venn diagrams which examine overlapping features in two data sets suggest that the best match for 433 eros is an ordinary chondrite altered at the surface of the asteroid or perhaps a primitive achondrite derived from material mineralogically similar to these chondrites normalized element distributions preclude feo rich pyroxenes and suggest that the x ray and gamma ray data can be reconciled with a common silicate mineralogy by inclusion of varying amounts of metal normative mineralogy cannot be applied to these data sets owing to uncertainties in oxygen abundance and lack of any constraints on the abundance of sodium matrix inversion for simultaneous solution of mineral abundances yields reasonable results for the x ray derived bulk composition but seems to confirm the inconsistency between mineral compositions and orthopyroxene clinopyroxene ratios a unique solution does not seem possible in synthesizing these multiple data sets future missions including a lander to fully characterize regolith distribution and sample return would resolve the types of problems faced in synthesizing the near data

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

near at eros imaging and spectral results eros is a very elongated 34 kilometers by 11 kilometers by 11 kilometers asteroid most of the surface of which is saturated with craters smaller than 1 kilometer in diameter the largest crater is 5 5 kilometers across but there is a 10 kilometer saddle like depression with attributes of a large degraded crater surface lineations both grooves and ridges are prominent on eros some probably exploit planes of weakness produced by collisions on eros and or its parent body ejecta blocks 30 to 100 meters across are abundant but not uniformly distributed over the surface albedo variations are restricted to the inner walls of certain craters and may be related to downslope movement of regolith on scales of 200 meters to 1 kilometer eros is more bland in terms of color variations than gaspra or ida spectra 800 to 2500 nanometers are consistent with an ordinary chondrite composition for which the measured mean density of 2 67 0 1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent

grosvenor mountains 95 howardite pairing group insights into the surface regolith of asteroid 4 vesta regolithic howardites are analogs for the surface materials of asteroid 4 vesta recently mapped by the dawn spacecraft rigorously evaluating pairing of howardites recovered in 1995 in the grosvenor mountains gro 95 antarctica enables an examination of a larger more representative regolith sample previous work on two of the howardites studied here concluded that gro 95602 and gro 95535 are solar wind rich surface regolith samples and that they are not paired with each other leading to uncertainty regarding pairing relationships between the other gro 95 howardites based on petrology cosmic ray exposure history and terrestrial age four gro 95 howardites are paired the paired howardites gro 95534 95535 95574 95581 were from a meteoroid with radius of 10 15 cm a preatmospheric size comparable to that of kapoeta the largest known regolithic howardite the paired gro 95 howardites contain clasts of at least 18 separate hed lithologies providing evidence they were assembled from diverse source materials the total eucrite diogenite mixing ratio ratio of all eucrite lithologies to all diogenite lithologies in the paired gro 95 howardites is 2 1 petrographically determined basaltic eucrite cumulate eucrite ratios in regolithic howardites studied here and previously vary more widely than total eucrite diogenite ratios relative to eucritic pyroxene plagioclase is depleted in these howardites which provides evidence that plagioclase is preferentially comminuted in the vestan regolith the extent of plagioclase depletion could be an indicator of regolith maturity

hydrogen isotopic composition of water from fossil micrometeorites in howardites we have measured the hydrogen isotopic composition d h ratios of the water from 13 carbonaceous chondritic microclasts ccms size 1 mm trapped in two howardites kapoeta and yamato 793497 early in the evolution of solar system the division into tochilinite rich magnetite rich olivine poor magnetite rich olivine rich ccm types is corroborated by the hydrogen isotopic compositions both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite rich ccms represent cm2 chondritic matter in contrast there is no good match between the isotopic and mineralogical properties of the magnetite rich ccms and the known groups of carbonaceous chondrites suggesting that magnetite rich ccms represent a new kind of chondritic matter not yet sampled in meteorite collections this demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete the study of micro clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids the average hydrogen isotopic composition of water belonging to ccms d h 152 0 4 8 x 10 6 1 sigma m is similar to that of antarctic micrometeorites amms d h 161 2 3 8 x 10 6 1 sigma m the similarity in terms of mineralogy and hydrogen isotopic composition between ccms and amms demonstrates that the composition of the micrometeorites has not been modified over the whole history of the solar system it indicates that the composition of the micrometeorite flux onto earth has been and is dominated by a mixture of cm2 like magnetite rich olivine poor magnetite rich olivine rich carbonaceous chondritic matter exemplified by ccms found in howardites because ccms have not suffered atmospheric entry they provide an abundant source of pristine micrometeorites the average d h ratio of the whole population of ccms is identical within errors to that of the earth 149 3 x 10 6 the match between the ccms d h ratio and that of the earth is especially remarkable because 1 three different populations of ccms are needed to make the d h ratio of the earth 2 there is no single carbonaceous chondrite group for which a similar match exists this observation suggests that ccms population might be representative of the late veneer agent s that delivered water to the earth

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

early diagenetic dolomitization and dedolomitization of late jurassic and earliest cretaceous platform carbonates a case study from the jura mountains nw switzerland e france early diagenetic dolomitization is a common feature in cyclic shallow water carbonates throughout the geologic record after their generation dolomites may be subject to dedolomitization re calcification of dolomites e g by contact with meteoric water during emersion these patterns of dolomitization and subsequent dedolomitization frequently play a key role in unravelling the development and history of a carbonate platform on the basis of excellent outcrops detailed logging and sampling and integrating sedimentological work high resolution sequence stratigraphic interpretations and isotope analyses o c conceptual models on early diagenetic dolomitization and dedolomitization and their underlying mechanisms were developed for the upper jurassic lower cretaceous jura platform in north western switzerland and eastern france three different types of early diagenetic dolomites and two types of dedolomites were observed each is defined by a distinct petrographic isotopic signature and a distinct spatial distribution pattern different types of dolomites are interpreted to have been formed by different mechanisms such as shallow seepage reflux evaporation on tidal flats and microbially mediated selective dolomitization of burrows depending on the type of dolomite sea water with normal marine to slightly enhanced salinities is proposed as dolomitizing fluid based on the data obtained the main volume of dolomite was precipitated by a reflux mechanism that was switched on and off by high frequency sea level changes it appears however that more than one dolomitization mechanism was active pene contemporaneously or several processes alternated in time during early diagenesis percolating meteoric waters obviously played an important role in the dedolomitization of carbonate rocks that underlie exposure surfaces cyclostratigraphic interpretation of the sedimentary succession allows for estimates on the timing of early diagenetic de dolomitization these results are an important step towards a better understanding of the link between high frequency probably orbitally forced sea level oscillations and early dolomitization under mesozoic greenhouse conditions

hydrothermal dolomitization of the bekhme formation upper cretaceous zagros basin kurdistan region of iraq record of oil migration and degradation abstract the common presence of oil seepages in dolostones is widespread in cretaceous carbonate successions of the kurdistan region of iraq this integrated field petrographic chemical stable c o and sr isotopes and fluid inclusion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the upper cretaceous bekhme carbonates flux of hot basinal hydrothermal brines which is suggested to have occurred during the zagros orogeny resulted in dolomitization and cementation of vugs and fractures by coarse crystalline saddle dolomite equant calcite and anhydrite the saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot 81 115 c basinal nacl mgcl2 h2o brines with salinities of 18 22 wt nacl eq the equant calcite cement which surrounds and hence postdates saddle dolomite has precipitated during oil migration from cooler 60 110 c nacl cacl2 h2o brines 14 18 wt nacl eq the yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had api gravity of 15 25 composition which is lighter than present day oil in the reservoirs api of 10 17 this difference in oil composition is attributed to oil degradation by the flux of meteoric water which is evidenced by the low 13c values 8 5 to 3 9 vpdb as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement this study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration degradation and reservoir evolution

meteoritic and other constraints on the internal structure and impact history of small asteroids studies of the internal structure of asteroids which are crucial for understanding their impact history and for hazard mitigation appear to be in conflict for the s type asteroids eros gaspra and ida spacecraft images and geophysical data show that they are fractured coherent bodies whereas models of catastrophic asteroidal impacts family and satellite formation and studies of asteroid spin rates and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble these conflicting views may be reconciled if 10 50 km s type asteroids formed as rubble piles but were later consolidated into coherent bodies many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration metamorphism igneous and impact processes ordinary chondrites which are the best analogs for s asteroids are commonly breccias some may have formed in cratering events but many appear to have formed during disruption and reaccretion of their parent asteroids some breccias were lithified during metamorphism and a few were lithified by injected impact melt but most are regolith and fragmental breccias that were lithified by mild or moderate shock like their lunar analogs shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together and by friction and pressure welding of silicate and metallic fe ni grains we suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble internal voids would be partly filled with regolith by impact induced seismic shaking consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body fractures on ida that were created by antipodal impacts and are concentrated in and near large craters and small positive gravity anomalies associated with the psyche and himeros craters on eros are consistent with this concept spin data suggest that smaller asteroids 0 6 6 km in size are unconsolidated rubble piles c type asteroids which are more porous than s types and their analogs the volatile rich carbonaceous chondrites were probably not lithified by shock

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

mineralogical interpretation of reflectance spectra of eros from near near infrared spectrometer low phase flyby high signal to noise near infrared spectrometer nis spectra acquired during the low phase flyby of the near earth asteroid rendezvous near mission to 433 eros are analyzed to determine mineral chemistry and proportions of mafic silicates across the asteroid s surface at 2 68 5 50 km spatial resolution spectral band parameters are derived and compared with those of laboratory samples of known mineral composition grain size distribution and terrestrial meteoritic and lunar pyroxene spectral properties the nis derived band parameters are consistent with ordinary chondrite meteorites we invoke the presence of a clinopyroxene component in the spectra which is consistent with ordinary chondrite mineralogy and or some degree of partial melting of ordinary chondritic material spectra measured across the surface of eros can reveal small but real spectral variations most relative spectra are uniform to within 1 2 some areas suggest compositional variations of a few percent spectral slope variations of a few percent are seen indicating a non uniform distribution of materials affecting the slope parameter but with no resolved absorption bands we find no correlation of slope with viewing geometry or compositional variation the band parameter values do not consistently indicate a specific ordinary chondrite class but eros is definitely undifferentiated with possible compositional variations of no more than 1 2

porosity and density of ordinary chondrites clues to the formation of friable and porous ordinary chondrites densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors we report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study based on our measurements we find that no significant correlation exists between porosity and petrologic grade chemical group sample mass bulk and grain density or shock level to investigate variations in porosity and density between pieces of a meteorite we examined stones from two showers holbrook and pultusk examination of nine samples of holbrook suggests relative homogeneity in porosity and density between pieces of this shower measurements of three samples of pultusk show homogeneity in bulk density in contrast to wilkison and robinson 2000 a study that reported significant variations in bulk density between 11 samples of pultusk finally examination of two friable ordinary chondrites bjurbole and allegan reveal variability in friability and porosity among pieces of the same fall we suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

geochemistry of 4 vesta based on hed meteorites prospective study for interpretation of gamma ray and neutron spectra for the dawn mission asteroid 4 vesta believed to be the parent body of the howardite eucrite and diogenite hed meteorites will be investigated by the dawn orbiting spacecraft dawn carries a gamma ray and neutron detector grand that will measure and map some major and trace element abundances drawing on hed geochemistry we propose a mixing model that uses element ratios appropriate for the interpretation of grand data because the spatial resolution of grand is relatively coarse the analyzed chemical compositions on the surface of vesta will likely reflect mixing of three endmember components diogenite cumulate eucrite and basaltic eucrite reliability of the mixing model is statistically investigated based on published whole rock data for hed meteorites we demonstrate that the mixing model can accurately estimate the abundances of all the grand analyzed major elements as well as of minor elements na cr and mn not analyzed by this instrument we also show how a similar mixing model can determine the modal abundance of olivine and we compare estimated and normative olivine data for olivine bearing diogenites by linking the compositions of well analyzed hed meteorites with elemental mapping data from grand this study may help constrain the geological context for hed meteorites and provide new insight into the magmatic evolution of vesta

carbon elemental and isotopic composition in mantle xenoliths from spain insights on sources and petrogenetic processes abstract the carbon elemental concentration c wt and isotopic 13c composition of mantle xenoliths from the tallante and calatrava volcanic occurrences in south east and central spain respectively have been investigated to identify carbon sources and processes occurring in distinct geodynamic settings of the iberian peninsula the peridotitic mantle xenoliths from calatrava show elemental c ranging from 0 11 to 2 87 wt which is coupled with a continuous isotopic variation from very negative values 13c 26 1 to typical mantle values 13c 5 9 on the other hand the tallante mantle xenolith suite displays lower c contents 0 06 0 15 wt showing a tighter variation with 13c depleted values ranging between 20 1 and 23 7 higher elemental c up to 0 41 wt displaying distinctly less negative isotopic values 13c between 13 8 and 11 9 have been recorded in veins crosscutting tallante peridotites plausibly representing the product of metasomatic reactions the data from the two investigated xenolith suites invariably display a good correlation between elemental and isotopic composition suggesting a mantle origin for carbon and rayleigh type fractionation as the process responsible for the observed c 13c variation however the correlation between the carbon isotopic data with other isotopic tracers e g 87sr 86sr 3he 4he used to identify distinct mantle components and metasomatic reactions indicates systematic differences between the two xenolith suites suggesting that beneath the betic cordillera where tallante is located the deep c cycle involves recycling via subduction preceding accompanying continental collision of crustal components back in the mantle coherently geochemical trends observed in the tallante xenoliths seem to be influenced by metasomatic agents generated by melting of crustal lithologies that according to the analysis of a metasedimentary xenolith can contain c up to 1 2 wt having 13c of ca 18 0

variability in magnesium carbon and oxygen isotope compositions of brachiopod shells implications for paleoceanographic studies abstract this study presents the carbon oxygen and magnesium isotope compositions of two modern brachiopods terebratalia transversa and frieleia halli and one fossil specimen 2 3 ma terebratula scillae the aim of this study is to investigate the variability of these isotopic compositions and to evaluate the potential of brachiopods as a proxy of past seawater 26mg values the two investigated brachiopod shells present the same range of 26mg variation up to 2 this variation cannot be ascribed to changes in environmental parameters temperature or ph as has been previously observed the primary layer of calcite shows the largest degree of oxygen and carbon isotope disequilibrium relative to seawater in contrast the 26mg value of the primary layer is comparable to that of the secondary calcite layer value in both t scillae and t transversa negative trends are observable between magnesium isotopic compositions and oxygen and carbon isotopic compositions these trends can be explained by kinetic effects linked to changes in growth rate during the brachiopod life the innermost calcite layer of t transversa is in isotopic equilibrium for both oxygen and magnesium and could therefore be the best target for reconstructing past 26mg values of seawater

the hazard of near earth asteroid impacts on earth near earth asteroids neas have struck the earth throughout its existence during epochs when life was gaining a foothold f4 ga the impact rate was thousands of times what it is today even during the phanerozoic the numbers of neas guarantee that there were other impacts possibly larger than the chicxulub event which was responsible for the cretaceous tertiary extinctions astronomers have found over 2500 neas of all sizes including well over half of the estimated 1100 neas 1 km diameter neas are mostly collisional fragments from the inner half of the asteroid belt and range in composition from porous carbonaceous chondrite like to metallic nearly one fifth of them have satellites or are double bodies when the international telescopic spaceguard survey which has a goal of discovering 90 of neas 1 km diameter is completed perhaps as early as 2008 nearly half of the remaining impact hazard will be from land or ocean impacts by bodies 70 600 m diameter comets are expected to contribute only about 1 of the total risk the consequences of impacts for civilization are potentially enormous but impacts are so rare that worldwide mortality from impacts will have dropped to only about 150 per year averaged over very long durations after the spaceguard goal has presumably ruled out near term impacts by 90 of the most dangerous ones that is in the mid range between very serious causes of death disease auto accidents and minor but frightening ones like shark attacks differences in perception concerning this rather newly recognized hazard dominate evaluation of its significance the most likely type of impact events we face are hyped or misinterpreted predicted impacts or near misses involving small neas d 2004 elsevier b v all rights reserved

mg and ca isotope signatures of authigenic dolomite in siliceous deep sea sediments abstract authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the c 13 of the global dic dissolved inorganic carbon reservoir but their contributions to ancient sedimentary sections are difficult to assess in this study of authigenic dolomite from the miocene age monterey formation of offshore california mg and ca isotopes are shown to vary with stratigraphic depth as a result of early diagenetic processes the dolomite is a pre compaction authigenic phase that occurs as beds and nodules with c 13 ranging from 16 to 9 light c 13 values were likely acquired from the sedimentary zone of microbial sulfate reduction while heavy c 13 values were acquired from the zone of methanogenesis mg and ca isotopes are roughly anti correlated with intervals of negative c 13 associated with low mg 26 and higher ca 44 40 values the variability is observed over a wide range of length scales from 10 2 meters within individual authigenic beds nodules to 10 2 meters over the entire stratigraphic column and can be understood as the consequence of dolomite precipitation in pore fluids where mg supply is limited by diffusive transport the relationship of mg 26 and ca 44 40 to the more common stable isotope measurements of c 13 and o 18 represents a new diagenetically robust geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record

the mg isotopic composition of cenozoic seawater evidence for a link between mg clays seawater mg ca and climate abstract cooling of earth s climate over the cenozoic has been accompanied by large changes in the magnesium and calcium content of seawater whose origins remain enigmatic the processes that control these changes affect the magnesium isotopic composition of seawater rendering it a useful tool for elucidating the processes that control seawater chemistry on geologic timescales here we present a cenozoic magnesium isotope record of carbonate sediments and use a numerical model of seawater chemistry and the carbon cycle to test hypotheses for the covariation between cenozoic seawater chemistry and climate records are consistent with a 2 3 increase in seawater mg ca and little change in the mg isotopic composition of seawater these observations are best explained by a change in the cycling of mg silicates we propose that mg ca changes were caused by a reduction in removal of mg from seawater in low temperature marine clays though an increase in the weathering of mg silicates cannot be excluded we attribute the reduction in the mg sink in marine clays to changes in ocean temperature directly linking the major element chemistry of seawater to global climate and providing a novel explanation for the covariation of seawater mg ca and climate over the cenozoic

collisional evolution of the asteroid belt abstract a new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than 1 km size are strengthless gravitational aggregates rubble piles near found eros not to be a rubble pile but a shattered collisional fragment with a through going fracture system and an average of about 20 m regolith cover of four asteroids visited by spacecraft none appears likely to be a rubble pile except perhaps mathilde nevertheless current understanding of asteroid collisions and size dependent strength and the observed distribution of rotation rates versus size have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles is eros the best observed asteroid highly unusual because it is not a rubble pile is mathilde if it is a rubble pile like most asteroids what would be expected for the small asteroid itokawa the muses c sample return target an asteroid size distribution is synthesized from the minor planet center listing and results of the sloan digital sky survey an infrared space observatory survey the small main belt asteroid spectroscopic survey and the infrared astronomical satellite survey a new picture emerges of asteroid collisional evolution in which the well known dohnanyi result that the size distribution tends toward a self similar form with a 2 5 index power law is overturned because of scale dependent collision physics survival of a basaltic crust on vesta can be accommodated together with formation of many exposed metal cores the lifetimes against destruction are estimated as 3 gyr at the size of eros 10 gyr at ten times that size and 40 gyr at the size of vesta eros as a shattered collisional fragment is not highly unusual the new picture reveals the new possibility of a transition size in the collisional state where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions in this case well defined families would be found in asteroids larger than about 5 km size but for smaller asteroids families may no longer be readily separated from a background population moreover the measured boulder size distribution on eros is re interpreted as a sample of impactor size distributions in the asteroid belt the regolith on eros may result largely from the last giant impact and the same may be true of itokawa in which case about a meter of regolith would be expected there even a small asteroid like itokawa may be a shattered object with regolith cover

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

433 eros problems with the meteorite magnetism record in attempting an asteroid match the magnetometer experiment mag onboard the near shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2 1x10 6 am 2 kg 1 for asteroid 433 eros this is in sharp contrast with the estimated magnetization of other s class asteroids gaspra 2 4 x 10 2 am 2 kg 1 braille 2 8 x 10 2 am 2 kg 1 and is below published values for all types of ordinary chondrites this includes the l ll types considered to most closely match 433 eros based on preliminary interpretations of near remote geochemical experiments the ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid meteorite match based on magnetic property measurements nrm natural remanent magnetization intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the h l ll groups all well above the 2 1 x10 6 am 2 kg 1 level for 433 eros the rem values ratio of the nrm to the sirm saturation remanent magnetization range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between nrm and the amount of magnetic material levels of magnetic noise in chondrite meteorites can be as much as 70 or more of the nrm consequently published values of the nrm should be considered suspect unless careful evaluation of the noise sources is done goddard studies of per unit mass intensities in large 10 000gm and small down to 1 gm samples from the same meteorite demonstrate magnetic intensity decreases as size increases this would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples this would then argue for some level of demagnetization of large objects such as an asteroid the possibility that 433 eros is an ll chondrite cannot be discounted

impact of diagenetic alteration on brachiopod shell magnesium isotope δ26mg signatures experimental versus field data due to their thermodynamically stable low mg calcite mineralogy the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the phanerozoic in marine waters consequently traditional and non traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties this paper tests the sensitivity of brachiopod shell magnesium isotope 26mg data to diagenetic alteration we apply a dual approach by i performing hydrothermal alteration experiments using meteoric marine and burial reactive fluids and ii comparing these data to naturally altered ancient brachiopod shells the degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy the absence of luminescence might indicate both well preserved shell material but also the secondary enrichment of quenching elements such as iron along diagenetic pathways complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells brachiopod shell magnesium isotope values respond to differential fluid temperature chemistry and experiment durations the patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals generally the range in bulk 26mg from experimentally altered 1 52 and that of bulk samples from ancient diagenetically altered brachiopod valves 1 53 exceed the geochemical variability of 26mgbrachiopod bulk values of most recent specimens 1 26 in the lower and upper range more 26mg enriched 0 8 and more 26mg depleted 0 7 values respectively are found in altered shells in comparison to unaltered ones the data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives consequently we propose tentative guidelines for magnesium isotope research applied to ancient carbonates

behavior of mg isotopes during dedolomitization in the madison aquifer south dakota abstract to constrain how mg isotopes behave during chemical interactions and physical transport in carbonate rich settings we measured 26 mg values of surface water groundwater and dolomite samples from the madison aquifer south dakota groundwater in the madison aquifer chemically evolves by dedolomitization during transport along a 236 km flow path surface streams recharging the aquifer have 26 mg values of 1 08 and 1 18 following recharge groundwater 26 mg values vary between 1 10 and 1 63 up to a distance of 20 km between 20 and 189 km 26 mg values remain nearly constant at 1 40 and a final sample at 236 km shows an increase to 1 09 dolomite exhibits a wide range of 26 mg values between 2 21 and 1 27 reactive transport modeling and isotope mixing calculations employing previously published major ion mass balances 87 sr 86 sr ratios and 44 ca values were used to determine whether dedolomitization reactions namely dolomite dissolution calcite precipitation and mg for na ion exchange fractionate mg isotopes we tentatively attribute the final 26 mg value to preferential uptake of 24 mg during mg for na ion exchange otherwise we find little evidence of isotopic fractionation and observe instead that 26 mg conservatively traces lithologic and hydrologic sources either isotope exchange between dolomite and water with a fractionation factor of 0 or mixing between different water sources establishes the 26 mg value of 1 40 at 20 km this value remains unchanged for the next 169 km of water transport because dolomite adds mg with an average 26 mg value near 1 40 and no other processes cause fractionation calcite precipitation is unimportant either because calcite is not a significant sink for mg or because mg uptake during calcite precipitation under conditions of chemical equilibrium does not fractionate mg isotopes this study suggests mg isotopes undergo conservative transport in carbonate rich settings where waters are in chemical equilibrium with respect to major sources and sinks of mg

depositional environments and paleogeography of the cretaceous gypsum horizon in west central jordan abstract in the upper cretaceous upper cenomanian gypsum horizon of west central jordan the lithofacies of gypsum limestone and dolomite as well as their sedimentary structures and petrography indicate a coastal intertidal supratidal sabkha environment very much like the persian gulf model 1500 km 2 in area and elongated n s parallel to the dead sea rift fact that this is the second time in the cenomanian of the mujib area that total emergence has happened is taken as an example of synsedimentary tectonics in an unstable area along the not yet existing dead sea rift

anatomy of a partially differentiated asteroid a near sighted view of acapulcoites and lodranites with the insertion of the near spacecraft into orbit around the asteroid 433 eros a new stage of asteroid exploration has begun with the return of spatially resolved mineralogical geological and chemical information from specific asteroids in this paper we outline the features of a partially differentiated asteroid using the acapulcoite and lodranite meteorites as a guide such a body would likely contain a variety of lithologies including chondritic rocks largely unaltered by melting residues of partial melts and basaltic and fe ni fes rocks crystallized from the partial melts the presence or nature of the regolith remains largely unconstrained mineralogical and isotopic heterogeneities preserved in these rocks suggest spatial separation of these rock types by distances in excess of 200 m spacecraft based remote sensing instrumentation should detect substantial spectral heterogeneity most prominently manifested in the band area ratio and chemical heterogeneity observation of these various lithologies in situ would provide important new constraints on the processes involved during the earliest differentiation of asteroids

the geology of 433 eros the global high resolution imaging of asteroid 433 eros by the near earth asteroid rendezvous near shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small s type asteroid eros displays a variety of surface features and evidence of a substantial regolith large scale facets grooves and ridges indicate the presence of at least one global planar structure directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object as with other small objects impact craters dominate the overall shape as well as the small scale topography of eros depth diameter ratios of craters on eros average 0 13 but the freshest craters approach lunar values of 0 2 ejecta block production from craters is highly variable the majority of large blocks appear to have originated from one 7 6 km crater shoemaker the interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of phobos this lack of mechanical boundaries and the abundant evidence of regolith in nearly every high resolution image suggests a gradation in the porosity and fracturing with depth the density of small craters is deficient at sizes below 200 m relative to predicted slopes of empirical saturation this characteristic which is also found on parts of phobos and lunar highland areas probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith eros displays a variety of regolith features such as debris aprons fine grained ponded deposits talus cones and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith these processes serve to mix materials in the upper loose fragmental portion of the asteroid regolith in the instance of ponded materials and crater wall deposits there is evidence of processes that segregate finer materials into discrete deposits the near observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the near shoemaker spacecraft

magnesium isotope fractionation during low mg calcite precipitation in a limestone cave field study and experiments abstract the chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings despite intensive research including modelling and novel approaches speleothem data remain difficult to interpret a possible way foreword is to apply a multi proxy approach including non conventional isotope systems for the first time we here present a complete analytical dataset of magnesium isotopes 26 mg from a monitored cave in nw germany bunker cave the data set includes 26 mg values of loess derived soil above the cave 1 0 0 5 soil water 1 2 0 5 the carbonate hostrock 3 8 0 5 dripwater in the cave 1 8 0 2 speleothem low mg calcite stalactites stalagmites 4 3 0 6 cave loam 0 6 0 1 and runoff water 1 8 0 1 in the cave respectively magnesium isotope fractionation processes during weathering and interaction between soil cover hostrock and solute bearing soil water are non trivial and depend on a number of variables including solution residence times dissolution rates adsorption effects and potential neo formation of solids in the regolith and the carbonate aquifer apparent mg isotope fractionation between dripwater and speleothem low mg calcite is about 1000ln mg cc mg aq 2 4 a similar mg isotope fractionation 1000ln mg cc mg aq 2 1 is obtained by abiogenic precipitation experiments carried out at aqueous mg ca ratios and temperatures close to cave conditions accordingly 26 mg discrimination during low mg calcite formation in caves is highly related to inorganic fractionation effects which may comprise dehydration of mg 2 prior to incorporation into calcite surface entrapment of light isotopes and reaction kinetics relevance of kinetics is supported by a significant negative correlation of mg isotope fractionation with the precipitation rate for inorganic precipitation experiments

constraining dolomitization by mg isotopes a case study from partially dolomitized limestones of the middle cambrian xuzhuang formation north china the dolomite problem refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record as well as failure of laboratory inorganic dolomite precipitation at near earth surface temperature novel mg isotope systematics provides a promising tool in resolving the dolomite problem here we develop a protocol to place constraints on the dolomitization process by using mg isotopes in this study we measured mg isotopic compositions 26mg of two batches of partially dolomitized limestone samples from the middle cambrian xuzhuang formation in north china 26mg varies between 0 55 and 3 18 and shows a negative linear correlation with 1 mg suggesting that 26mg can be described by a binary mixing between the calcite and dolomite components mg isotopic composition of the dolomite component 26mgdol for the lower sample set that is collected from a 4 m stratigraphic interval containing three high frequency ribbon rock packstone cycles is 1 6 while 26mgdol for the upper sample set from a thick sequence of ribbon rock is significantly higher 0 3 however neither mineralogical and elemental compositions carbon and oxygen isotopes nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples 26mgdol of the xuzhuang limestone is simulated by the advective flow af and the diffusion advection reaction dar models the af model assumes that mg is transported by advective fluid flows while the dar model simulates a contemporaneous seawater dolomitization process in which mg is delivered by diffusion the af modeling result indicates that 26mg of the dolomitization fluid is 0 4 and 1 7 for the lower and upper sample sets respectively these values are significantly higher than modern and cenozoic seawater mg isotopic composition suggesting that the dolomitization fluid is not contemporaneous seawater the af model also predicts spatially heterogeneous 26mgdol with progressive enrichment in 26mg along the fluid flow pathway in the dar model both dolomite content and 26mgdol of the lower sample set can be simulated by using seawater mg isotopic composition of 0 75 thus contemporaneous seawater dolomitization may explain 26mgdol of the xuzhuang limestone furthermore the dar model demonstrates spatially homogeneous 26mgdol to differentiate the af and dar models samples from multiple sections are required nevertheless this study implies that mg isotope might be a useful tool in the study of dolomitization

chemical multispectral and textural constraints on the composition and origin of rocks at the mars pathfinder landing site rocks at the mars pathfinder site are probably locally derived textures on rock surfaces may indicate volcanic sedimentary or impact generated rocks but aeolian abration and dust coatings prevent unambiguous interpretation multispectral imaging has resolved four spectral classes of rocks gray and red which occur on different surfaces of the same rocks pink which is probably soil crusts and maroon which occurs as large boulders mostly in the far field rocks are assigned to two spectral trends based on the position of peak reflectance the primary spectral trend contains gray red and pink rocks maroon rocks constitute the secondary spectral trend the spatial pattern of spectral variations observed is oriented along the prevailing wind direction the primary spectral trend arises from thin ferric coatings of aeolian dust on darker rocks the secondary spectral trend is apparently due to coating by a different mineral probably maghemite or ferrihydrite a chronology based on rock spectra suggests that rounded maroon boulders constitute the oldest petrologic unit a flood deposit succeeded by smaller cobbles possibly deposited by impact and followed by aeolian erosion and deposition nearly linear chemical trends in alpha proton x ray spectrometer rock compositions are interpreted as mixing lines between rock and adhering dust a conclusion supported by a correlation between sulfur abundance and red blue spectral ratio extrapolations of regression lines to zero sulfur give the composition of a presumed igneous rock the chemistry and normative mineralogy of the sulfur free rock resemble common terrestrial volcanic rocks and its classification corresponds to andesite igneous rocks of this composition may occur with clastic sedimentary rocks or impact melts and breccias however the spectral mottling expected on conglomerates or breccias is not observed in any apxs analyzed rocks interpretation of the rocks as andesites is complicated by absence of a 1 m pyroxene absorption band plausible explanations include impact glass band masking by magnetite or presence of calcium and iron rich pyroxenes and olivine which push the absorption band minimum past the imager s spectral range the inferred andesitic composition is most similar to terrestrial anorogenic icelandites formed by fractionation of tholeiitic basaltic magmas early melting of a relatively primitive martian mantle could produce an appropriate parent magma supporting the ancient age of pathfinder rocks inferred from their incorporation in hesperian flood deposits although rocks of andesitic composition at the pathfinder site may represent samples of ancient martian crust inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted

space weathering on eros constraints from albedo and spectral measurements of psyche crater we present combined multi spectral imager msi 0 95 m and near infrared spectrometer nis 0 8 2 4 m observations of psyche crater on s type asteroid 433 eros obtained by the near earth asteroid rendezvous near shoemaker spacecraft at 5 3 km in diameter psyche is one of the largest craters on eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material at spatial scales of 620 m spectrum psyche crater wall materials exhibit albedo contrasts of 32 40 at 0 946 m associated spectral variations occur at a much lower level of 4 8 2 4 we report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends grain size differences olivine pyroxene and troilite variations and optical surface maturation we find that the albedo contrasts in psyche crater are not consistent with a cause due solely to variations in grain size olivine pyroxene or lunar like optical maturation a grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed olivine and pyroxene variations would produce strong band correlated variations that are not observed a simple lunar like optical maturation effect would produce strong reddening that is not observed the contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite a dark spectrally neutral component simulating optical effects of shock and lunar like optical maturation these results suggest that space weathering processes may affect the spectral properties of eros materials causing surface exposures to differ optically from subsurface bedrock however there are significant spectral differences between eros proposed analog meteorites ordinary chondrites and or primitive achondrites and eros freshest exposures of subsurface bright materials after accounting for all differences in the measurement units of our reflectance comparisons we have found that the bright materials on eros have reflectance values at 0 946 m consistent with meteorites but spectral continua that are much redder than meteorites from 1 5 to 2 4 m most importantly we calculate that average eros surface materials are 30 40 darker than meteorites

detection of temperature dependent spectral variation on the asteroid eros and new evidence for the presence of an olivine rich silicate assemblage data obtained by the near infrared spectrometer carried by the near shoemaker spacecraft show that the spectral properties of the asteroid eros vary with temperature the manner in which they vary demonstrates that the mineral olivine is a major constituent of the surface the near ir temperature dependent spectral properties of eros in the northern hemisphere and for two individual regions on the surface show clear evidence of the presence of the mineral olivine and are a close match to the temperature spectral behavior of ll type ordinary chondrite meteorites while the presence of other olivine rich meteorites cannot be excluded h type ordinary chondrites are clearly too pyroxene rich to be permitted as a major surface component of eros the results of the thermal spectral analysis are consistent with results from analysis of conventional reflectance spectra of the asteroid and contribute unambiguous detection of olivine to the understanding of the surface composition of eros

evidence for cross formational hot brine flow from integrated 87sr 86sr ree and fluid inclusions of the ordovician veins in central tarim china abstract rare earth elements 34s 87sr 86sr and 18o were determined and fluid inclusions were analyzed of fracture filling barite anhydrite fluorite calcite and quartz from upper ordovician carbonates in central tarim china the aim was to assess the origin evolution and flow of fluid in the fractures there was mixing of relatively hot ba rich and 87sr rich fluid with in situ basinal water and fresh water which was relatively cool and more radiogenic the hotter fluid is hydrothermal as indicated by homogenization temperatures hts about 20 c higher than the formation and was most likely derived from precambrian to lower cambrian clastic rocks or and from the basement through faults this is because almost all of 87sr 86sr ratios of late stage fracture fillings mainly 0 70889 0 71036 and present brines are higher than that of upper ordovician limestones the hydrothermal fluid was enriched in f eu2 ba2 sr2 and zn2 during migration as indicated by the positive relationships of eu to sr ba and zn when the hydrothermal fluid mixed with local basinal water it precipitated isotopically normal s anhydrite fluorite and calcite with positive eu anomalies the precipitation of these ca minerals was followed by thermochemical so4 reduction tsr of dissolved sulphates by hydrocarbons it was likely that only part of the so 4 2 was reduced and the generated h2s leaked out thus barite deposited from residual so 4 2 has abnormally high 34s values from 42 to 47 and the generated h2s is isotopically light from 15 to 18 subsequently hydrothermal fluid was likely to have mixed with relatively cool and more 87sr rich freshwater resulting in precipitation of quartz and fluorite with salinities as low as 0 5 wt nacl equiv and 87sr 86sr ratios up to 0 71036

the effects of diagenesis and dolomitization on ca and mg isotopes in marine platform carbonates implications for the geochemical cycles of ca and mg abstract the ca mg o and c isotopic and trace elemental compositions of marine limestones and dolostones from odp site 1196a which range in depth 58 to 627 mbsf and in depositional age 5 and 23 ma are presented the objectives of the study are to explore the potential for non traditional isotope systems to fingerprint diagenesis to quantify the extent to which geochemical proxies are altered during diagenesis and to investigate the importance of diagenesis within the global ca and mg geochemical cycles the data suggest that ca which has a relatively high solid to fluid mass ratio can be isotopically altered during diagenesis in addition the alteration of ca correlates with the alteration of mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks bulk carbonate 44 ca values vary between 0 60 and 1 31 srm 915a scale the average limestone 44 ca is 0 97 0 24 1sd identical within error to the average dolostone 1 03 0 15 1sd magnesium isotopic compositions 26 mg dsm 3 scale range between 2 59 and 3 91 and limestones 3 60 0 25 and dolostones 2 68 0 07 are isotopically distinct carbon isotopic compositions 13 c pdb scale vary between 0 86 and 2 47 with average limestone 1 96 0 31 marginally offset relative to average dolostone 1 68 0 57 the oxygen isotopic compositions 18 o pdb scale of limestones 1 22 0 94 are substantially lower than the dolostones measured 2 72 1 07 the isotopic data from 1196a suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories numerical modeling supports the contention that such trends can be interpreted as diagenetic and suggests that the appropriate distribution coefficient k mg associated with limestone diagenesis is 1 to 5 10 3 distinctly lower than those values 0 015 reported in laboratory studies with respect to mg isotopes the modeling also suggest that diagenetic fractionation factors of 0 9955 4 5 and 0 9980 2 are appropriate for limestone diagenesis and dolomitization respectively

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

distribution and interpretation of rare earth elements and yttrium in cenozoic dolostones and limestones on cayman brac british west indies abstract the cenozoic carbonate succession on cayman brac a small island 19 km long and 1 5 to 3 km wide located in the caribbean sea has been dolomitized to varying degrees the rare earth element ree and yttrium y concentrations ree y of 134 carbonate samples 127 from cayman brac and 7 supplementary samples from grand cayman vary from 0 2 to 7 5 ppm average 2 8 ppm n 134 the shale normalized ree y patterns of the carbonate samples akin to those of oxygenated seawater are characterized by 1 lree depletion relative to hree average dy n sm n 1 7 n 125 2 positive la anomalies 3 negative ce anomalies average ce ce 0 4 n 133 and 4 superchondritic y ho molar ratios average y ho 85 n 86 the lack of correlation between the dolomite content of the samples and their ree y dy n sm n la n nd n ce ce or y ho indicates that dolomitization did not have a major impact on ree y signatures and that dolomitization was probably mediated by seawater like fluids the shale normalized patterns e g dy n sm n la n nd n ce ce and y ho vary from formation to formation for samples from the cayman formation the pedro castle formation and the ironshore formation there are subtle but gradual changes in dy n sm n la n nd n ce ce with depth in contrast there are marked changes in la n nd n ce ce y ho and sm nd across the brac unconformity which forms the boundary between the lower oligocene brac formation and the overlying miocene cayman formation variations in the ree y patterns reflect diagenetic processes e g dy n sm n and possibly secular changes in the ree y composition of seawater e g la n nd n y ho and sm nd

dolomitization of the late cretaceous paleocene platform carbonates golkoy ordu eastern pontides ne turkey abstract late cretaceous paleocene agillar formation in the eastern pontide zone ne turkey is composed of shallow marine platform carbonates that were buried until uplift in late paleocene time the formation which is represented by 100 m thick reefal limestones around golkoy is completely dolomitized on the basis of petrographic and geochemical features four different replacive and cement dolomite phases are present the replacive dolomites which are cut by low amplitude stylolites include 1 fine crystalline planar s dolomite rd1 and 2 coarsely crystalline planar s e dolomite rd2 two types of dolomite cements were developed in minor amounts 1 a coarse dolomite cement cd1 that mostly fills molds vugs and fractures in rd1 dolomites and 2 rim dolomite cement cd2 that commonly develops on the space facing surfaces of rd2 dolomite the replacive dolomites are nearly stoichiometric ca 49 51 mg 48 50 and have 18 o values of 3 5 to 1 6 vpdb 13 c values of 1 1 to 0 8 vpdb and 87 sr 86 sr ratios of 0 707104 to 707458 sr na mn and fe concentrations of replacive dolomites are 102 171 ppm bdl 371 ppm 244 9941 ppm and 2643 8951 ppm respectively petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from late cretaceous paleocene seawater and or seawater slightly modified by water rock interaction process and then recrystallized by meteoric waters at increasing burial depths and temperatures compared with replacement dolomites cd1 dolomites yield lower 18 o values 4 9 to 5 3 vpdb similar 13 c values 1 2 to 0 5 vpdb and higher 87 sr 86 sr ratios 0 707660 0 707609 the cd1 dolomites overlie the low amplitude stylolites and have homogenization temperatures between 94 and 112 c indicating that they were deposited from basinal brines although replacive dolomites and dolomite cements have similar na bdl 413 ppm mn 259 6990 ppm and fe 2184 6690 ppm concentrations the cements have lower sr contents 63 114 ppm like replacive dolomites cd1 dolomite is nearly stoichiometric ca 49 50 mg 47 49 but cd2 dolomite is slightly ca rich ca 51 52 mg 47 48 the cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

stable carbon isotope stratigraphy in the ordovician of south china abstract about 500 samples from 11 ordovician sections in south china have been analysed for their 13 c carb composition four sections are located in the yangtze platform region the remaining seven sections in the jiangnan slope region the sections span more or less the entire ordovician but the sample resolution in its lower part tremadocian to dapingian is lower than in its upper part darriwilian to hirnantian overall the data show a good correlation with the published global isotope curve at the cambrian ordovician boundary positive values are observed in the succeeding tremadocian the values decrease and remain negative into the early floian in the late floian the values increase to slightly positive the correlation of dapingian rocks between the yangtze and the jiangnan regions is somewhat problematic which might be due to high siliciclastic input in the yangtze section in the darriwilian a small positive excursion is observed in the yangtze platform area however it is missing in the jiangnan slope area this excursion probably correlates with the mid darriwilian excursion mdice reported from baltoscandia both chemostratigraphy and conodont biostratigraphy indicate a possible gap between the shihtzupu formation and the pagoda formation i e most of the sandbian is probably not represented in the upper yangtze region the earliest katian is characterised by a pronounced positive excursion correlating with the well known global guttenberg excursion gice in the late katian at least one additional positive excursion was determined the precise biostratigraphy however is not clear and therefore it is not known which of the known four to five remaining katian 13 c excursions identified in baltica correlate s with the one s observed in the present study and further biostratigraphic study is required the lower part of the hirnantian excursion hice is probably not preserved with only small remnants of the hice observed near the base of the kuanyinchiao bed

ordovician stable carbon isotope stratigraphy in the tarim basin nw china abstract the tarim basin located in nw china represents one of china s major palaeoplates its palaeogeographic position in the ordovician however is debated in this study we present biostratigraphically controlled stable carbon isotope data from some 600 samples spanning almost the entire ordovician in order to provide a tool that can be used for future chemostratigraphic correlations the data correlate well with results from south china and several major global 13c excursions such as the mdice dw2 3 saice sa2 gice ka1 and whitewater excursion ka4 are documented for the first time from the tarim basin major sedimentary gaps are detected in the lower floian between the penglaiba and yinshan formations in the upper darriwilian to lower sandbian between the dawangou and saergan or kanling formations and in the latest sandbian between the kanling and qilang formations as well as between the tumuxiuke and lianglitag formations three scenarios for the depositional history of the late darriwilian to early sandbian saergan formation which represents an important hydrocarbon source rock in the tarim basin are discussed with respect to their bio and chemostratigraphic implications the mid upper ordovician rocks recording high 13c values have been deposited during transgressive or highstand intervals whereas the regressive intervals are mostly missing this results in a stacking of several positive excursions and thus chemostratigraphic correlation without biostratigraphic control might produce misleading results especially for shallow water carbonates

experimental calibration of mg isotope fractionation between dolomite and aqueous solution and its geological implications abstract hydrothermal experiments at 220 160 and 130 c were performed to calibrate the mg isotope fractionation factor between dolomite and aqueous mg hydrothermal experiments included synthesis of dolomite using different starting materials as well as exchange experiments that used poorly ordered proto dolomite the morphology of synthesized dolomite was dependent on starting mineralogy suggesting that dolomite was synthesized by different pathways hydrothermally synthesized dolomite was initially fine grained disordered or poorly ordered dolomite that with time recrystallized to coarser grained ordered dolomite isotopic exchange was monitored using 87 sr 86 sr ratios and 25 mg tracers and these indicated near complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments the mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology suggesting attainment of isotopic equilibrium combining results from synthesis and exchange experiments the temperature dependent mg isotope fractionation factor for ordered dolomite is 26 mg dolo aq 0 1554 0 0096 10 6 t 2 where t is in kelvin in contrast poorly ordered dolomite has a 26 mg dolo aq fractionation factor that is up to 0 25 lower than that of ordered dolomite and this is attributed to longer mg o bonds in imperfectly ordered dolomite the experimentally calibrated 26 mg dolo aq fractionation factors lie between those calculated by schauble 2011 and rustad et al 2010 the 26 mg dolo aq fractionation factor extrapolated to lower temperatures using the 26 mg t function of this study matches the 26 mg dolo aq fractionation factor obtained by modeling of mg isotope compositions of odp drill core samples this study shows that significant mg isotope fractionation occurs during dolomite precipitation these results collectively demonstrate that mg isotopes in dolomite are a useful tool for studying mg global cycling and dolomitization

conservative behavior of mg isotopes in massive dolostones from diagenesis to hydrothermal reworking abstract mg isotopes in syndepositional dolomite have been suggested to be a potential proxy for understanding seawater chemistry however it is argued that the 26mg values of dolomite could be complicated by the effects of early diagenesis and later hydrothermal activities further investigations into the behaviors of mg isotopes in dolomitization systems are needed to resolve this controversy in the present study we investigated early ordovician dolostones from the tarim basin including diagenetically altered dolomites hydrothermally altered dolomites and well preserved dolostones that precipitated in the slope margin and interior of a carbonate platform different types of dolomites bulk dolostone and limestone were sampled by microdrilling for analyses of c o mg isotope compositions and ree concentrations the dolostone 13c values match those of coeval seawater and the ree distribution patterns in the dolostones are comparable with those in the limestones indicating that the dolostones originated from syndiagenetic dolomitization the 26mg values of the various syndiagenetic dolomites that formed in the slope margin and interior of the carbonate platform are similar averaging approximately 2 06 0 20 no stratigraphic variability in the dolomite mg isotopes can be discerned which implies that the mg isotope compositions of the porewater were homogeneous during massive dolomitization and remained in equilibrium with seawater additionally the 26mg values of the altered dolostone do not show correlations with diagenetic and hydrothermal signals demonstrating that dolomite mg isotopes are insensitive to postdepositional alteration given these facts we propose that mg isotopes in dolostones have conservative behaviors during diagenesis and late stage hydrothermal reworking

magnesium isotope and ree compositions of lower ordovician carbonates from eastern laurentia implications for the origin of dolomites and limestones abstract carbonates representing different depositional and diagenetic settings near surface to deep burial including modern sabkha were collected from the lower ordovician st george group in eastern laurentia western newfoundland canada based on petrographic examination three dolomite phases were identified d1 dolomicrite crystals ranging from 4 to 50 m d2 dolomite 50 to 150 m and d3 saddle dolomite 500 m they occur as replacements and cements and exhibit dull for d1 and d3 to zoned for d2 luminescence the occurrence of near micritic dolomites d1 suggests that dolomitization started at low temperatures under near surface conditions during an early stage of diagenesis whereas microthermometric studies of the d2 and d3 dolomites confirmed their formation at higher temperatures on average of 112 19 c and 153 30 c respectively under deeper burial conditions the d1 dolomicrite yields an average 26 mg value of 1 92 0 30 dsm3 which is slightly more negative than those of the d2 dolomite with 1 75 0 34 dsm3 and d3 saddle dolomite with 1 58 0 31 dsm3 the slightly more positive 26 mg values of the higher temperature dolomites d2 and d3 suggest insignificant kinetic fractionation and slight overprinting by 26 mg enriched diagenetic fluids recirculating in siliciclastic and 26 mg rich crustal rocks under closed to semi closed diagenetic conditions the sr isotope signatures support the formation of d1 dolomicrite in an early diagenetic stage and the ree compositions of the investigated dolomites favor their formation in semi closed to closed diagenetic systems with fluid compositions evolving by circulation through crustal rocks with progressive burial furthermore the mg isotopes suggest that the lime mudstones of the st george group had an algal origin whereas the d2 dolomite was sourced and subsequently altered mostly from the d1 dolomicrites of the succession

a model for impact induced lineament formation and porosity growth on eros abstract we investigate the impact history of the near earth asteroid nea eros 433 using a new material model for brittle materials such as rocks where initial flaw distributions within the rock are explicitly defined to match what is known about flaw size distributions in rocks these simulations are limited to the initial impact phase of the crater formation process and use a very crude approximation for the effect of the gravitational overburden pressure given these approximations our simulations of this numerical approximation of eros suggest that the current observed bulk porosity of about 25 could be consistent with the porosity generated by the formation of the three largest craters observed on eros indicating that eros could have started as an intact shard from a prior impact event further we investigate the consequences of two possible internal flaw distributions for the asteroid a strong flaw distribution with shorter crack lengths that are more difficult to activate during cratering and a weak flaw distribution with longer flaws the strong distribution produces localized deformation regions lineaments that are resolved by the simulations while the weak distribution does not produce resolved localized features for either distribution of internal flaws the initial impact assumed to be the himeros forming impact shatters but does not disrupt the body implying that simulations of asteroid mitigation approaches should assume that asteroids will behave like rubble piles subsequent impact events activate linear features created by prior impacts but only change the orientation of the lineament structure near the impact site