isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis abstract diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute hf the weakened interface breaks when grains are agitated ultrasonically subsequent particle size separation is an effective means of concentrating the diagenetic component

phase based dispersion analysis for acoustic array borehole logging data a phase based dispersion analysis method for velocity slowness extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented the technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component the approach is nonparametric and completely data driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes results are free of the aliases and spurious modes which are characteristic of some prior approaches examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools results are compared with those from the weighted spectral semblance wss and amplitude and phase slowness estimation apes methods to demonstrate the effectiveness and utility of the proposed method

mapping formation shear velocity variation by inverting logging while drilling quadrupole wave dispersion data abstractthe logging while drilling lwd quadrupole wave is a dispersive wave mode guided along the borehole with a drill collar the wave is sensitive to the formation alteration caused by drilling we inverted lwd quadrupole wave dispersion data to estimate a radial shear velocity profile away from the wellbore we also explored the nonuniqueness of the inverse problem and solved it by using a constrained inversion method this was done by constraining the high frequency portion of the model dispersion curve with another curve calculated using the near borehole velocity the constraint condition is based on the physical principle that a high frequency lwd quadrupole wave has a shallow penetration depth and is therefore sensitive to the near borehole shear velocity particularly we found that a monotonically continuous velocity profile can be well approximated using a one zone equivalent model allowing for a drastic simplification of the inversion process we used theoretical modeling and real data exa

oxygen isotope study of diagenetic quartz overgrowths from the upper proterozoic quartzites of western mali taoudeni basin implications for conditions of quartz cementation abstract the upper proterozoic quartzites from the taoudeni basin contain up to 27 quartz cement occurring as overgrowth on detrital grains an oxygen isotope study of the quartz cement in the upper proterozoic quartzites of western mali souroukoto group so2 formation permitted an assessment of the nature and origin of the cementing fluids as well as the source of cement the 18o values measured for the quartz overgrowths range from 15 9 to 17 7 per mil smow these data combined with petrological and geological evidence suggest that quartz cementation took place relatively early after deposition at shallow depth and low temperature from meteoric fluids the markedly 18o depleted with respect to seaw ter character of the parent fluids 18o values between 25 and 11 per mil smow is directly related to the cold climatic conditions prevailing on the west african craton during late proterozoic times mass transfer paleogeographic and paleoclimatologic considerations suggest that the bulk of the silica necessary to form the observed volume of quartz cement was internally derived

oxygen isotope analyses of fine silica grains using laser extraction technique comparison with oxygen isotope data obtained from ion microprobe analyses and application to quartzite and silcrete cement investigation the laser fluorination technique reported here for analyzing the oxygen isotope composition d 18 o of fine quartz size fractions 50 20 20 10 10 5 5 2 2 1 and 1 lm has been validated by comparison with the ion microprobe technique it yields accurate d 18 o data with an external precision better than 0 15 this is a significant methodological improvement for isotopic studies dealing with materials such as soil or biogenic oxides and silicates particles are often too small and recovered in insufficient amount to be easily handled for ion microprobe analysis both techniques were used to investigate d 18 o composition of a cretaceous quartzite and silcrete sequence from the southeast of france quartzite cements average 31 04 1 93 they formed from mid cretaceous seawater higher in the series silcretes cements average 26 66 1 36 they formed from upper or post upper cretaceous soil water and groundwater oxygen isotope data show that the silicification steps from one mineralogical phase to another and from one layer to another including from an upper pedogenic silcrete to a lower groundwater silcrete occurred in a closed or weakly evaporating hydrological system

a starfish bed in the middle miocene grand bay formation of carriacou the grenadines west indies a starfish bed in the middle miocene grand bay formation of carriacou the grenadines west indies

oxygen isotope systematics in a multiphase weathering system in haiti abstract physical and chemical partial dissolution techniques have been applied to a suite of young karst bauxite and laterite samples from the southern peninsula of haiti the 18o values have been obtained for nine mineral species and range from 1 0 for anatase rutile to 33 4 for authigenic quartz on one hand results for quartz calcite kaolinite gibbsite and boehmite compare favourably with 18o values predicted from accepted mineral water fractionation factors assuming modern temperatures 25 2 c and water 18o values 3 1 0 5 on the other hand the 18o values measured for anatase illite smectite chlorite smectite and some iron oxide samples do not compare favourably with predicted values departures from expected values for these minerals may be related to crystallization from water with a different 18o value than modern water an imprecise knowledge of some mineralwater fractionation factors under surficial conditions or nonattainment of isotopic equilibrium between mineral and water during formation partial dissolution techniques hold considerable promise for obtaining quantitative 18o values of individual minerals in single samples of complex fine grained regolith material

oxygen isotope analyses of fine grained minerals and rocks using the laser extraction technique abstract the 18o values of fine grained materials made with the laser extraction technique are often lower than those obtained conventionally we propose a simple method to overcome this problem that involves the mixing of samples with a lif binder and pressing the powder into pellets prior to evacuation and analysis the presence of the lif allows for rapid fluorination of all minerals or whole rock samples to be made thus eliminating potential problems related to loss of partially reacted ejecta slow laser heating and incidental fluorination accurate isotope determinations of fine grained materials can be made with this method

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

diversity patterns and biostratigraphy of cenozoic echinoderms from florida abstract fossil echinoderms are an important faunal component in cenozoic rocks of florida and the southeastern coastal plain of the usa that provide useful information regarding diversity patterns and paleoecology for the formations in which they were deposited the research presented in this paper significantly improves what is known about the stratigraphic distribution and diversity patterns of echinoids comatulid crinoids asteroids and ophiuroids from the middle eocene through pleistocene strata of florida biostratigraphic distribution of echinoderms specifically the echinoids has not been synthesized on a large scale for many years and the first compilation of these taxa from the state is provided in this study echinoderms now are known from 25 formations that are exposed throughout the state the echinoids are better documented and described than the other classes of echinoderms yet even the echinoids were poorly known from some epochs most notably the miocene previously published information showed echinoid diversity decreasing significantly from the eocene into the oligocene and continuing to decline into the miocene our research shows the echinoid diversity does not drop from the oligocene into the miocene but instead increases by at least five species this marked change in alpha diversity during the miocene is a result of careful examination of smaller size fractions of sediments which contained fragmented components of echinoids such as test plates and spines and the diligent collecting of moldic samples which produced additional taxonomic and stratigraphic records the record of other echinoderms is still poor but fine fraction sediments also have produced disarticulated ossicles of crinoids asteroids and ophiuroids from several formations that had no previous record of such echinoderms the diversity pattern of echinoderms from florida is a reflection of both intrinsic and extrinsic factors that affect in varying degrees the echinoderm bearing formations from the paleogene through the neogene one of the most influential appears to be collector bias against recovering incomplete echinoderms many of the new species or new records of taxa from stratigraphic units in the state are based on fossils that are poorly preserved or incomplete other biases include over splitting of taxonomic groups stratigraphic unit revisions taphonomy and preservation potential variation in carbonate versus siliciclastic environments significant refinement of the echinoderm biostratigraphy and diversity patterns has occurred due to detailed examination of field samples museum collections and literature reviews as well as accounting for biases inherent in the fossil record of florida

geochemical mass balance and oxygen isotope constraints on silcrete formation and its paleoclimatic implications in southern australia abstract two geographically distinct silcrete associations are present in southern australia inland and eastern these were sampled in central south australia and central victoria respectively at each site both silicified and immediately adjacent unsilicified parent material were collected analytical data from these pairs were used to construct isocons assuming zr immobility and to calculate the volume change and amount of silica introduced during silicification these results together with whole rock oxygen isotope compositions were used to determine the 18o of the introduced silica the results show that the eastern silcretes in central victoria are probably linked genetically to the associated basalts weathering of which supplied the introduced silica this conclusion is based on the close spatial connection between the two as well as the substantial amount of introduced silica in the silcretes greater than in the inland silcretes resulting in volume increases in some eastern silcretes oxygen isotopic calculations for the silcretes indicate that the silica precipitated from groundwaters at temperatures slightly higher than present conditions silcrete formation apparently occurred during the miocene and pliocene basalts in victoria younger than pliocene lack associated silcrete and may reflect the much wetter climate in southeastern australia at that time the inland silcretes of central south australia can be divided into pedogenic the most common and groundwater varieties the pedogenic silcretes which show typical soil features like columnar and nodular textures contain moderate amounts of introduced silica that precipitated by evaporation from saline groundwaters for the groundwater silcretes which have massive textures and formed at or close to the water table insufficient data are available to determine the mode of formation the inland pedogenic silcretes have probably been forming from the eocene miocene to the present implying that conditions of seasonally high evaporation have occurred in central australia during this time period thus silcrete formation depends on a complex interplay between climate and silica supply and it is impossible to generalize that the presence of silcrete is indicative of a particular climate likewise the elemental composition of silcretes particularly ti content is not necessarily of climatic significance nevertheless detailed geochemical and oxygen isotopic studies of a silcrete and its parent material can elucidate the mechanisms of silcrete formation and if evaporation is indicated as a major factor in silcrete formation then the climate at the time was likely to have been at least seasonally arid

determining formation s wave transverse isotropy from borehole flexural wave dispersion data abstractmany earth formations are characterized as transversely isotropic ti media in acoustic logging through a vertical borehole the s wave ti property has traditionally been determined from borehole monopole stoneley wave measurements but the feasibility of shear ti estimation from dipole flexural waves has not been fully investigated we have developed a methodology to determine the ti parameters from borehole dipole flexural wave data our analysis shows that the stoneley wave is sensitive to the ti property mainly in an acoustically slow formation and the sensitivity diminishes when the formation becomes faster the advantage of the flexural wave over the stoneley wave is that the former wave is sensitive to the ti property in the slow and fast formations provided the wave measurement is made in a broad frequency range in which the flexural wave dispersion characteristics from low to high frequencies can be used by calculating the theoretical flexural wave dispersion curve for the ti formati

in situ calibrated velocity to stress transforms using shear sonic radial profiles for time lapse production analysis abstract borehole acoustic waves are affected by near and far field stresses within rocks that exhibit stress sensitivity typically in medium to high porosity formations nonlinear or third order elastic constants are obtained from the inversion of borehole sonic shear radial profiles with an elastic wellbore stress model the stress to velocity relationship determined from these profiles in the elastic region surrounding the wellbore is used for calibration to compare with empirical laboratory data traditionally used in time lapse seismic feasibility studies to assess simulated production this analysis enables rock physicists to use the wellbore as a laboratory and to examine the stress dependence of the acoustic velocities from in situ field data in their zone of interest laboratory experiments on core samples can yield both empirical and mathematical rock physics models to describe the relationship between stress and velocity to link rock properties to in situ measurements of acoustic data seis

evolution of caribbean echinoderms during the cenozoic moving towards a complete picture using all of the fossils abstract in the antillean region collecting bias towards complete tests has engendered a lop sided view of the diversity of the echinoderms through time crinoids asteroids and ophiuroids as well as certain echinoid groups are widespread as fossils but are only preserved as disarticulated or broken fragments that have commonly been ignored in studies of faunal diversity utilization of these disarticulated remains and collection of specimens from units that have hitherto been considered to lack common fossil echinoderms such as the plio pleistocene of many islands is resulting in a more consistent synthesis of their regional diversity during the cenozoic both echinoids and stalked crinoids show distinctive faunal changes following the eocene oligocene extinction events with the roots of their modern components appearing in the region at that time at least at the generic level in contrast distinctive asteroid species that are recognised from the oligocene do not appear to have persisted into the later cenozoic of the region incorporation of data from disarticulated elements commonly indicates that regular echinoids were at least as diverse as irregular echinoids some taxa that are common in the caribbean at the present day such as diadematoids that apparently lacked a recognisable fossil record due to taphonomic factors are recognised from throughout the cenozoic on the basis of distinctive disarticulated ossicles

antiphase hydrogen and oxygen isotope periodicity in chert nodules abstract oxygen and hydrogen isotope analyses were made of jurassic age chert nodules from the holy cross mountains se poland along radial transects at high spatial resolution there is a radial sigmoidal periodicity for both isotope ratios but the two are out of phase with high d values corresponding to low 18 o values periodicity for a 100 to 120 mm diameter nodule is approximately 16 mm increasing slightly toward the rim with amplitudes approaching 20 and 3 0 for hydrogen and oxygen respectively the combined hydrogen oxygen isotope data for one nodule fall on a published curve for chert forming in equilibrium with seawater knauth and epstein 1976 the range of delta values corresponds to temperature variations of 10 c data for a second chert fall on a subparallel d 18 o line with d values that are almost 50 lower the d 18 o patterns for the nodules cannot be explained by periodic mixing of meteoric and ocean water because the hydrogen and oxygen isotope data are out of phase two possible explanations for the antiphase periodicity are a cyclical temperature variations perhaps related to an unstable convection system e g bolton et al 1999 and b self organizing catalytic precipitation e g wang and merino 1990 the systematic isotopic variations are difficult to explain by diagenesis and strongly suggest that primary isotopic compositions are preserved the isotopic data provide important information on the thermal history of the sedimentary basin if temperature variations are the cause of the isotopic periodicity

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

methodological improvements for investigating silcrete formation petrography ft ir and oxygen isotope ratio of silcrete quartz cement lake eyre basin australia the oxygen isotopic composition 18o of the cement of a silcrete sample from the cordillo silcrete late eocene early oligocene of lake eyre basin australia was investigated the massive structure of the silcrete outcrop and the absence of pedogenic features suggest that this silcrete formed by precipitation from groundwater in order to avoid errors inherent in estimates of the cement 18o value from isotopic analyses of bulk material and mass balance considerations methodological improvements are described the silcrete cement is physically separated from the detrital phases its purity is checked using cathodoluminescence cl spectroscopy optical microscopy x ray diffraction xrd analyses and fourier transform infrared ft ir spectroscopy show the cement is made of three phases i overgrowth ii crypto crystalline quartz and iii micro crystalline quartz thus its 18o value must be interpreted as that of a mixture although the mixture consists mainly of crypto crystalline quartz as the amount of si oh exchangeable oxygen in the cement would bias the results by an amount less than the analytical error 0 09 it is unnecessary to go through an equilibration procedure prior to isotopic analysis the 18o value of the cement sample is between 24 8 and 25 8 relative to v smow assuming an eocene oligocene temperature of 15 20 c in agreement with published regional temperature estimates the inferred 18o value of the quartz forming solution was between 6 9 and 12 2 this is lower than the minimum 18o value of 6 70 estimated for the modern groundwater near the sample site a plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modern one from shallow groundwater frequently recharged in a setting of slow tectonic uplift

the rapid heating defocused beam technique a co2 laser based method for highly precise and accurate determination of δ18o values of quartz abstract we describe a laser fluorination technique specifically for quartz that allows both high accuracy and high precision determination of oxygen isotope ratios regardless of grain size the rapid heating defocused beam technique rhd utilizes a defocused 32 w co 2 laser at full power which delivers 17 w 20 w mm 2 to the sample to quickly react quartz in contrast other recent laser fluorination studies use a tightly focused laser beam and slow heating i e sharp and kirschner 1995 fouillac and girard 1996 kirschner and sharp 1997 and find laser analysis of fine grained quartz to be inaccurate three quartz standards nbs 28 qz bra and qz cwru were analyzed using the rhd technique at the university of wisconsin to test for grain size effects rhd analyses show no correlation between grain size and 18 o values and are in excellent agreement with 18 o values obtained using conventional fluorination techniques additional analyses of the same quartz standards performed at university of wisconsin using slow heating and a focused beam yield 18 o values that are both less precise and significantly up to 0 8 lower than the accepted values attempts to use the rhd technique with a 20 w co 2 laser which delivers 8 w 10 w mm 2 to the sample at brgm were unsuccessful probably due to insufficient power density of the defocused beam we conclude that the rhd technique yields excellent accuracy and precision but that power densities of 15 20 w mm 2 may be necessary across a large part of the sample surface we recommend the use of lasers with at least 30 w of power

geochemistry of a chert breccia a complex history of diagenetic interactions between a siliceous sediment seawater and fresh water is revealed by intraformational chert breccias chert breccias were formed in the campanian mishash formation in israel by practically contemporaneous fracturing of lithified cherty layers followed by silicification and lithification of the matrix pairs of fragments and matrix were compared with respect to their chemical ca sr na k mg li b so4 ba and isotopic 18 o d 11 b composition 11 b was analyzed by ion probe and includes a profile across a fragment matrix contact the epicontinental cherts of the mishash fm are enriched by a factor of 10 to 50 in all elements other than o and si in comparison with deep sea cherts all results are compatible with the proposition that the lithification of the matrix occurred in contact with fresh water as opposed to seawater in which the fragments as well as most of the mishash sediments were formed the strongest evidence for this difference is in the higher concentration of b in the fragments 27 70 ppm vs 11 21ppm in the matrix and higher 18 o 29 to 35 vs 21 to 33 d is a less efficient discriminator though compatible with fresher water diagenesis of the matrix 115 to 76 for hydrogen in the chert of the fragments compared to 141 to 85 for the matrix 11 b in the matrix shows some of the lowest values recorded in sediments 11 b 33 but varies strongly suggesting that the source of boron in the matrix is a mixture of a freshwater and a marine component both seawater and the freshwater that has equilibrated with the cherts underwent varying degrees of evaporation ca sr and so4 are carried by apatite trapped as detritus in the matrix the concentration of lithium in the matrix is high 11 16 ppm whereas in the adjacent fragments it is mostly only within 1 2 ppm li probably enters the matrix from the interstitial solution during the opal 3 quartz transformation the second prolonged transformation takes place in a freshwater flow through open system this allows a much larger mass of li to be scavenged by the transforming silica despite its low concentration in freshwater copyright 2005 elsevier ltd

mapping formation radial shear wave velocity variation by a constrained inversion of borehole flexural wave dispersion data we have developed a novel constrained inversion method for estimating a radial shear wave velocity profile away from the wellbore using dipole acoustic logging data and have analyzed the effect of the radial velocity changes on dipole flexural wave dispersion characteristics the inversion of the dispersion data to estimate the radial changes is inherently a nonunique problem because changing the degree of variation or the radial size of the variation zone can produce similar wave dispersion characteristics nonuniqueness can be solved by developing a constrained inversion method this is done by constraining the high frequency portion of the model dispersion curve with another curve calculated using the near borehole velocity the constraint condition is based on the physical principle that a high frequency dipole wave has a shallow penetration depth and is therefore sensitive to the near borehole shear wave velocity we have validated the result of the constrained inversion with synthetic data testing combining the new inversion method with four component crossed dipole anisotropy processing obtains shear radial profiles in fast and slow shear polarization directions in a sandstone formation the fast and slow shear wave profiles show substantial differences caused by the near borehole stress field demonstrating the ability of the technique to obtain radial and azimuthal geomechanical property changes near the wellbore

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

mineralogical forms of silica and their sequence of formation in silcretes the paragenesis and the sequence of formation of silica polymorphs have been studied in three examples of silicification in the regolith of the paris basin and the central massif of france the initial silica deposits are most frequently opal with varying degrees of structural organization subsequently recrystallization produced varieties of progressively better organized quartz namely fibrous quartz chalcedony and lutecite which developed from deposits of microlaminated opal and microcrystalline quartz which formed from nodular opal however even during the formation of microcrystalline quartz facies dissolution and recrystallization processes continued resulting in progressively bigger quartz crystals in the final stages of silcrete development hyaline quartz crystals grew in voids these mineral sequences are determined by the solubility of each of the component parts solubility is controlled by the purity and the degree of order of the crystalline structure the most disordered minerals are the most soluble the degree of order is controlled in part by foreign ions incorporated into the lattice but also by the degree of supersaturation of silica in solution in the system which in turn controls the rate of formation of nuclei dissolution and recrystallization processes are dependent on the percolation of dilute solutions through the silcretes the concentrations of silica and other cations in these solutions is a function of the solubility of the silica phases in the host material and of the rate of percolation thus the porosity of the host material is important rapid percolation favors dissolution but in zones with slow percolation equilibrium is attained and no transformation of existing silica phases occurs it is in the zones of intermediate porosity that progressive recrystallization of quartz occurs the effect of rate of percolation on dissolution and recrystallization explains the vertical distribution of different silica polymorphs observed in numerous silcrete duricrusts

oxygen isotopes in single micrometer sized quartz grains tracing the source of saharan dust over long distance atmospheric transport abstract oxygen isotope compositions were measured by ion microprobe in individual micrometer sized quartz grains extracted from one aerosol sample collected on the cape verde islands and from four surface samples three soils and one sediment representing potential source regions of aerosols in western and central africa morocco algeria niger and chad a large range of 18o values from 6 2 to 39 3 is present within the aerosol quartz grains the different size fractions of the quartz grains from the surface samples overlap nearly entirely this range but show significant differences in their 18o distributions for the different size fractions of the grains i e different modes different proportions of grains with low or high 18o these differences in 18o distributions can be related to different geological formations i e mantle derived magmatic rocks crustal magmatic rocks or sedimentary rocks outcropping in each region thus giving a fingerprint of the source region quartz grains with unusually high 18o values between 30 and 40 were attributed to lacustrine cherts formed in evaporitic environments chad basin the existence of distinct 18o distributions for the surface samples which reflect regional geology but indicate some transport of the grains enables the characterization of mixing processes during dust emission in the atmosphere particles are mixed at a regional scale in the dust reservoir but injection of fine particles into the high troposphere occurs as a discrete and localized event with no mixing during the subsequent long range transport by the easterlies the comparison of the 18o distribution of the quartz from the aerosol sample with the equivalent size fractions in surface samples shows that the niger area is the more probable source region for the aerosol although the moroccan source cannot be excluded this method gives a valuable tool to trace the source region of dust into the atmosphere or into sediment samples allowing the reconstruction of air mass circulations

assessing rock brittleness and fracability from radial variation of elastic wave velocities from borehole acoustic logging rock brittleness and fracability of subsurface formations are two important parameters for hydraulic fracturing in hydrocarbon reservoir production this paper presents an effective technique to assess these parameters using the radial variation of compressional and shear velocities from borehole acoustic logging our technique is based on a rock mechanic phenomenon that a brittle rock with high fracability tends to leave a significant amount of drilling induced cracks at the borehole wall resulting in radial elastic wave velocity variation away from borehole by determining the velocity variation the combined effects of brittleness and fracability of formation rocks can be assessed the compressional wave travel time tomography and flexural shear wave inversion methods are respectively used to obtain compressional and shear velocity variations well log data analysis examples demonstrate the practicability and effectiveness of this technique

miocene heterozoan carbonate systems from the western atlantic equatorial margin in south america the pirabas formation abstract outcrops of neogene carbonates of the north brazilian platform occur exclusively along the equatorial coast and represent some of the few existing examples of neogene carbonate systems of the atlantic coast of south america the pirabas formation early middle miocene is the northernmost part of this platform although onshore mainly consists of small and scattered outcrops it considerably extends in the subsurface keeping record of relevant geological and paleontological episodes of tropical south american history intending to improve the knowledge of south american carbonate and providing a solid basis for future comparisons between the pirabas basin and other largely subsurface cenozoic basins this research investigates the aricuru outcrops by combining a standard petrographical and paleontological approach with advanced microct analyses the aricuru area is characterized by mixed siliciclastic carbonate sand sized deposits which probably deposited after the early miocene according to the palynological assemblage the bioclastic fraction of the rock is dominated by benthic foraminifera mostly soritids amphisteginids small rotaliids and small miliolids typical of marginal marine environments bryozoans calcareous algae halimeda echinoderms and mollusks the abundant siliciclastic fraction together with the taxonomic composition of the foraminiferal echinodermal crustacean ichnofossil and fish assemblages indicates deposition in a tropical coastal environment featuring both protected and more exposed sectors the carbonate system probably developed under abundant nutrient supply which fostered heterotroph suspension feeders over hermatypic corals the demise of this system was most likely caused by a growth in siliciclastic input due to increased rainfall in the coastal area the sedimentary evolution of the pirabas formation is similar to the one of the foz do amazonas basin and fits well with the general evolutionary trend of cenozoic carbonate factories of the region indicating the potential of the pirabas subsurface record for understanding other cenozoic basins their paleoenvironmental significance and their potential as oil gas and water reservoirs