uranium series dating of pedogenic silica and carbonate crater flat nevada a 230th 234u 238u dating study on pedogenic silica carbonate clast rinds and matrix laminae from alluvium in crater flat nevada was conducted using small sample thermal ionization mass spectrometry tims analyses on a large suite of samples though the 232th content of these soils is not particularly low mostly 0 1 9 ppm the high u content of the silica component mostly 4 26 ppm makes them particularly suitable for 230th u dating on single 10 to 200 mg totally digested samples using tims we observed that 1 both micro within rind and macro stratigraphic mappable deposit order of the 230th u ages were preserved in all cases 2 back calculated initial 234u 238u fall in a restricted range typically 1 67 0 19 so that 234u 238u ages with errors of about 100 kyr 2 could be reliably determined for the oldest 400 to 1000 ka rinds and 3 though 13 of the samples were 350 ka only three showed evidence for an open system history even though the sensitivity of such old samples to isotopic disruption is very high an attempt to use leach residue techniques to separate pedogenic from detrital u and th failed yielding corrupt 230th u ages we conclude that 230th u ages determined from totally dissolved multiple sub mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples chemical enhancement of 238u 232th or isochrons

origin timing and temperature of secondary calcite silica mineral formation at yucca mountain nevada the origin of secondary calcite silica minerals in primary and secondary porosity of the host miocene tuffs at yucca mountain has been hotly debated during the last decade proponents of a high level nuclear waste repository beneath yucca mountain have interpreted the secondary minerals to have formed from cool descending meteoric fluids in the vadose zone critics citing the presence of two phase fluid inclusions argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids understanding the origin temperature and timing of these minerals is critical in characterizing geologically recent fluid flux at the site and has significant implications to whether waste should be stored at yucca mountain petrographic and paragenetic studies of 155 samples collected from the exploratory studies facility esf and repository block cross drift ecrb tunnels indicate that heterogeneously distributed calcite with lesser chalcedony quartz opal and fluorite comprise the oldest secondary minerals these are typically overgrown by intermediate aged calcite often exhibiting distinctive bladed habits the youngest event recorded across the site is the deposition of mg enriched up to 1 wt and depleted growth zoned calcite intergrown with u enriched opal the cyclical variation in mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years the distribution of secondary minerals is consistent with precipitation in the vadose zone fluid inclusion petrography of sections from the 155 samples determined that 96 of the fluid inclusion assemblages fias contained liquid only inclusions that formed at ambient temperatures 35 c however 50 of the samples n 78 contained relatively rare fia that contain both liquid only and liquid plus vapor inclusions herein termed two phase fias that formed at temperatures above 35 c virtually all of these two phase fias occur in paragenetically old calcite rare two phase inclusion assemblages were also observed in early fluorite and quartz and early intermediate calcite homogenization temperatures trapping temperatures across yucca mountain are generally 45 to 60 c but higher temperatures reaching 83 c were recorded in calcite from the north portal and ramp of the esf cooler temperatures of 35 to 45 c were recorded in the intensely fractured zone multiple populations of two phase fias from lithophysal cavities in the esf and ecrb cross drift indicate early fluid cooling with time from temperatures 45 c in early calcite to 35 to 45 c in paragenetically younger calcite freezing point depressions range from 0 2 to 1 6 c indicating trapping of a low salinity fluid the majority of intermediate calcite and all outermost mg enriched calcite contains rare all liquid inclusions and formed from ambient temperature 35 c fluids carbon and oxygen isotope data reveal a consistent trend of decreasing 13 c from 9 5 to 8 5 and increasing 18 o from 5 2 to 22 1 values from paragenetically early calcite to mg enriched growth zoned calcite depleted d values 131 to 90 of inclusion fluids from intermediate and the youngest mg enriched calcite indicate derivation from surface meteoric fluids recalculation of 18 oh2o values of 12 to 10 is consistent with derivation from paleometeoric fluids results of integrated u pb dating opal and chalcedony and fluid inclusion microthermometry indicate that two phase fias that trapped fluids of 50 c are older than 6 29 0 30 ma two phase fias in parage netically later calcite which formed from fluids of 35 to 45 c are older than 5 32 0 02 ma there is no evidence for trapping of fluids with elevated temperatures during the past 5 32 my the youngest mg enriched calcite intergrown with opal began to precipitate between about 1 9 to 2 9 ma and has continued to precipitate within the past half million years the presence of liquid only inclusions and the consistent occurrence of mg enriched calcite and opal as the youngest event indicate a minor but chemically distinct ambient temperature 35 c fluid flux during the past 2 to 3 my copyright 2003 elsevier science ltd

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

late pleistocene granodiorite beneath crater lake caldera oregon dated by ion microprobe variably melted granodiorite blocks ejected during the holocene caldera forming eruption of mount mazama were plucked from the walls of the climactic magma chamber at 5 km depth ion microprobe u pb dating of zircons from two unmelted granodiorite blocks with shrimp rg sensitive high resolution ion microprobe reverse geometry gives a nominal 238 u 206 pb age of 100 78 80 ka or 174 89 115 ka when adjusted for an initial 230 th deficit shrimp rg u th measurements on a subset of the zircons yield a 230 th 238 u isochron age of 112 24 ka considered to be the best estimate of the time of solidification of the pluton these results suggest that the granodiorite is related to andesite and dacite of mount mazama and not to magmas of the climactic eruption the unexposed granodiorite has an area of at least 28 km 2 this young shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions 7700 yr ago resulting in collapse of crater lake caldera

improved spatial resolution for u series dating of opal at yucca mountain nevada usa using ion microprobe and microdigestion methods two novel methods of in situ isotope analysis ion microprobe and microdigestion were used for 230th u and 234u 238u dating of finely laminated opal hemispheres formed in unsaturated felsic tuff at yucca mountain nevada proposed site for a high level radioactive waste repository both methods allow analysis of layers as many as several orders of magnitude thinner than standard methods using total hemisphere digestion that were reported previously average growth rates calculated from data at this improved spatial resolution verified that opal grew at extremely slow rates over the last million years growth rates of 0 58 and 0 69 mm m y were obtained for the outer 305 and 740 m of two opal hemispheres analyzed by ion microprobe and 0 68 mm m y for the outer 22 m of one of these same hemispheres analyzed by sequential microdigestion these pleistocene growth rates are 2 to 10 times slower than those calculated for older secondary calcite and silica mineral coatings deposited over the last 5 to 10 m y dated by the u pb method and may reflect differences between miocene and pleistocene seepage flux the microdigestion data also imply that opal growth rates may have varied over the last 40 k y these data are the first indication that growth rates and associated seepage in the proposed repository horizon may correlate with changes in late pleistocene climate involving faster growth during wetter cooler climates glacial maximum slower growth during transition climates and no growth during the most arid climate modern data collected at this refined spatial scale may lead to a better understanding of the hydrologic variability expected within the thick unsaturated zone at yucca mountain over the time scale of interest for radioactive waste isolation

trace elements and u pb ages in petrified wood as indicators of paleo hydrologic events abstract subsurface fluid systems are important for chemical weathering ore formation and thermal evolution of the crust changes in the dynamics and distribution of subsurface fluid flow systems are controlled by changes in global and regional terrestrial climate tectonics and elevation this paper concerns the dating of changes in ancient subsurface hydrologic systems however direct dating of water rock interaction is challenging because of the lack of appropriate materials to date and the more open and complex nature of subsurface flow regimes here we explore the prospects of using u pb dating of petrified silicified wood as a means of quantifying continental paleo hydrology oxidizing fluids often of meteoric origin tend to leach and mobilize u from the country rock but when such waters contact organic rich material u can become reduced and immobilized resulting in u rich silicified wood we present in situ laser ablation icpms analyses of u pb isotopes and trace elements in petrified wood from the upper triassic chinle formation 225 208 myr in the petrified forest national park in arizona usa allowing us to establish a generalized workflow for making meaningful paleo hydrologic interpretations of the u pb systematics of silicified wood wood characterized by brownish colors and preservation of cellular structure have low fe contents and positive ce anomalies indicating silicification in reducing environments and isolation in relatively reduced conditions after silicification resulting in closed system behavior of u and pb wood characterized by vivid colors orange red etc and little to no preservation of cellular structure are much higher in fe and exhibit negative ce anomalies indicating influence by more oxidized fluids the brownish samples yield u pb ages clustered between 250 and 200 ma with a peak coinciding with the time of deposition 220 ma which indicates that fossilization largely took place almost immediately after deposition and that u pb in quartz faithfully retains the time of such fossilization in contrast the orangish reddish whitish samples yield younger u pb ages defining distinct errorchron ages which reflect subsequent generations of quartz crystallization scatter associated with errorchrons are likely due to local mm to cm scale u or pb mobility or variable initial pb composition distinct younger age peaks appear to correlate with the timing of regional unconformities associated with tectonic or epeirogenic uplift we suggest that uplift and exhumation may initiate the onset of oxidizing fluid systems resulting in leaching and transport of u from the surroundings followed by subsequent generations of quartz precipitation in summary u pb dating of petrified wood or silicified organic material has high potential for dating paleo hydrologic events however due to complexities in terrestrial hydrologic systems interpretations of u pb systematics must be informed by accompanying geochemical and textural observations

206pb 230th 234u 238u and 207pb 235u geochronology of quaternary opal yucca mountain nevada abstract u th pb isotopic systems have been studied in submillimeter thick outermost layers of quaternary opal occurring in calcite silica fracture and cavity coatings within tertiary tuffs at yucca mountain nevada usa these coatings preserve a record of paleohydrologic conditions at this site which is being evaluated as a potential high level nuclear waste repository the opal precipitated from groundwater is variably enriched in 234u measured 234u 238u activity ratio 1 124 6 179 and has high u 30 313 ppm low th 0 008 3 7 ppm and low common pb concentrations measured 206pb 204pb up to 11 370 it has been demonstrated that the laboratory acid treatment used in this study to clean sample surfaces and to remove adherent calcite did not disturb u th pb isotopic systems in opal the opal ages calculated from 206pb 238u and 207pb 235u ratios display strong reverse discordance because of excess radiogenic 206pb derived from the elevated initial 234u the data are best interpreted using projections of a new four dimensional concordia diagram defined by 206pb 238u 207pb 235u 234u 238uactivity and 230th 238uactivity ages and initial 234u 238u activity ratios have been calculated using different projections of this diagram and tested for concordance the data are discordant that is observed 207pb 235u ages of 170 32 2 to 1772 40 ka are systematically older than 230th u ages of 34 1 0 6 to 452 32 ka the age discordance is not a result of migration of uranium and its decay products under the open system conditions but a consequence of noninstantaneous growth of opal combined u pb and 230th u ages support the model of slow mineral deposition at the rates of millimeters per million years resulting in layering on a scale too fine for mechanical sampling in this case u pb ages provide more accurate estimates of the average age for mixed multiage samples than 230th u ages because ages based on shorter lived isotopes are nonlinearly biased by younger mineral additions use of the combined u th pb technique to date yucca mountain quaternary opals significantly extends the age range beyond that of the 230th u dating method and shows that selected fracture pathways in the unsaturated zone felsic tuffs of yucca mountain have been active throughout the quaternary

limited hydrologic response to pleistocene climate change in deep vadose zones yucca mountain nevada abstract understanding the movement of water through thick vadose zones especially on time scales encompassing long term climate change is increasingly important as societies utilize semi arid environments for both water resources and sites viewed as favorable for long term disposal or storage of hazardous waste hydrologic responses to pleistocene climate change within a deep vadose zone in the eastern mojave desert at yucca mountain nevada were evaluated by uranium series dating of finely layered hyalitic opal using secondary ion mass spectrometry opal is present within cm thick secondary hydrogenic mineral crusts coating floors of lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface uranium concentrations in opal fluctuate systematically between 5 and 550 g g age calibrated profiles of uranium concentration correlate with regional climate records over the last 300 000 years and produce time series spectral peaks that have distinct periodicities of 100 and 41 ka consistent with planetary orbital parameters these results indicate that the chemical compositions of percolating solutions varied in response to near surface climate driven processes however slow micrometers per thousand years relatively uniform growth rates of secondary opal and calcite deposition spanning several glacial interglacial climate cycles imply that water fluxes in the deep vadose zone remained low and generally buffered from the large fluctuations in available surface moisture during different climates

opal as a u pb geochronometer search for a standard we report structural geochemical and tims u th pb and u series isotopic data for several samples of precious and common opal which we tested as potential standards for ion microprobe and la icpms isotopic studies the precious opals are low in u contain high concentration of common pb and are not suitable for u pb age determinations in contrast all studied samples of common opal are enriched in uranium and contain sufficiently radiogenic pb for precise calculations of 206pb 238u and 207pb 235u dates complications in the u pb systematics are due to the presence of ubiquitous unsupported 206pb probably derived from initial excess 234u and possibly other intermediate products of 238u decay elevated concentrations and possibly anomalous isotopic composition of initial pb in some of the opals and the presence of two opal generations of different ages in one of the samples despite these imperfections two out of five studied common opals are acceptable as u pb and u series standards at least in the interim the data obtained in this study can be used to guide further search for a better opal standard

dating silica sinter geyserite a cautionary tale abstract we describe a new effort to date hydrothermal silica sinter deposits geyserite from the upper geyser basin of yellowstone national park using 14c of co deposited organic matter u series and cosmogenic 10be methods a majority of the samples were collected from stratigraphic sections mainly at riverside giant and castle geysers ages obtained from 41 14c analyses range from modern to 12 1 cal ka bp nearly all the 14c ages show inconsistencies with their stratigraphic positions and several replicate 14c analyses from the same sample result in significantly different ages the 13c values of the organic material in the sinter range from 26 6 to 12 7 the more enriched values are attributed to microbial fixation of dissolved inorganic carbon dic which has heavier 13c values and is 14c depleted relative to atmospheric co2 leading to an apparent older age u series analyses on 4 samples yielded ages between 2 2 and 7 4 ka large 230th u age uncertainties in the sinter due to low uranium concentrations along with elevated 232th and associated initial 230th make these ages imprecise for use on holocene deposits a single cosmogenic 10be exposure age of 596 18 ka is considerably older than the age of underlying rhyolite and is thus unreliable this apparent old age results from contamination by meteoric 10be trapped in the opal that overprints the very small amount of cosmogenic 10be by presenting the problems we encountered and discussing their probable cause this paper highlights the difficulty in obtaining reliable high precision geochronological data necessary to use sinter deposits as paleoenvironmental and paleo hydrothermal archives

u pb ages of secondary silica at yucca mountain nevada implications for the paleohydrology of the unsaturated zone abstract uranium th and pb isotopes were analyzed in layers of opal and chalcedony from individual mm to cm thick calcite and silica coatings at yucca mountain nevada usa a site that is being evaluated for a potential high level nuclear waste repository these calcite and silica coatings on fractures and in lithophysal cavities in miocene age tuffs in the unsaturated zone uz precipitated from descending water and record a long history of percolation through the uz opal and chalcedony have high concentrations of u 10 to 780 ppm and low concentrations of common pb as indicated by large values of 206pb 204pb up to 53 806 thus making them suitable for u pb age determinations interpretations of u pb isotope systems in opal samples at yucca mountain are complicated by the incorporation of excess 234u at the time of mineral formation resulting in reverse discordance of u pb ages however the 207pb 235u ages are much less affected by deviation from initial secular equilibrium and provide reliable ages of most silica deposits between 0 6 and 9 8 ma for chalcedony subsamples showing normal age discordance these ages may represent minimum times of deposition typically 207pb 235u ages are consistent with the microstratigraphy in the mineral coating samples such that the youngest ages are for subsamples from outer layers intermediate ages are from inner layers and oldest ages are from innermost layers 234u and 230th in most silica layers deeper in the coatings are in secular equilibrium with 238u which is consistent with their old age and closed system behavior during the past 0 5 ma the ages for subsamples of silica layers from different microstratigraphic positions in individual calcite and silica coating samples collected from lithophysal cavities in the welded part of the topopah spring tuff yield slow long term average growth rates of 1 to 5 mm ma these data imply that the deeper parts of the uz at yucca mountain maintained long term hydrologic stability over the past 10 ma despite significant climate variations u pb ages for subsamples of silica layers from different microstratigraphic positions in individual calcite and silica coating samples collected from fractures in the shallower part of the uz welded part of the overlying tiva canyon tuff indicate larger long term average growth rates up to 23 mm ma and an absence of recently deposited materials ages of outermost layers are 3 5 ma these differences between the characteristics of the coatings for samples from the shallower and deeper parts of the uz may indicate that the nonwelded tuffs ptn located between the welded parts of the tiva canyon and topopah spring tuffs play an important role in moderating uz flow

physical and stable isotope evidence for formation of secondary calcite and silica in the unsaturated zone yucca mountain nevada abstract calcite and silica form coatings on fracture footwalls and cavity floors in the welded tuffs at yucca mountain the potential site of a high level radioactive waste repository these secondary mineral deposits are heterogeneously distributed in the unsaturated zone uz with fewer than 10 of possible depositional sites mineralized the paragenetic sequence compiled from deposits throughout the uz consists of an early stage assemblage of calcite fluorite zeolites that is frequently capped by chalcedony quartz intermediate and late stage deposits consist largely of calcite commonly with opal on buried growth layers or outermost crystal faces of the calcite coatings on steep dipping fractures usually are thin 3 mm with low relief outer surfaces whereas shallow dipping fractures and lithophysal cavities typically contain thicker more coarsely crystalline deposits characterized by unusual thin tabular calcite blades up to several cms in length these blades may be capped with knobby or corniced overgrowths of late stage calcite intergrown with opal the observed textures in the fracture and cavity deposits are consistent with deposition from films of water fingering down fracture footwalls or drawn up faces of growing crystals by surface tension and evaporated at the crystal tips fluid inclusion studies have shown that most early stage and some intermediate stage calcite formed at temperatures of 35 to 85 c calcite deposition during the past several million years appears to have been at temperatures 18 o values although others have interpreted the elevated temperatures as evidence of hydrothermal activity and flooding of the tuffs of the potential repository the authors conclude that the temperatures and fluid inclusion assemblages are consistent with deposition in a uz environment that experienced prolonged heat input from gradual cooling of nearby plutons the physical restriction of the deposits and therefore fluid flow to fracture footwalls and cavity floors and the heterogeneous and limited distribution of the deposits provides compelling evidence that they do not reflect flooding of the thick uz at yucca mountain the textures and isotopic and chemical compositions of these mineral deposits are consistent with deposition in a uz setting from meteoric waters percolating downward along fracture flow paths

introduction to u series geochemistry during the last century the earth sciences underwent two major revolutions in understanding the first was the recognition of the great antiquity of the earth and the second was the development of plate tectonic theory these leaps in knowledge moved geology from its largely descriptive origins and established the modern quantitative earth sciences for any science and particularly for the earth sciences time scales are of central importance until recently however the study of time scales in the earth sciences was largely restricted to the unraveling of the ancient history of our planet for several decades earth scientists have used a variety of isotope chronometers to unravel the long term evolution of the planet a fuller understanding of the physical and chemical processes driving this evolution often remained elusive because such processes occur on time scales 1 105 years which are simply not resolvable by most conventional chronometers the u series isotopes however do provide tools with sufficient time resolution to study these earth processes during the last decade the earth sciences have become increasingly focused on fundamental processes and u series geochemistry has witnessed a renaissance with widespread application in disciplines as diverse as modern oceanography and igneous petrology the uranium and thorium decay series contain radioactive isotopes of many elements in particular u th pa ra and rn the varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments while the varied half lives of the nuclides allows investigation of processes occurring on time scales from days to 105 years u series measurements have therefore revolutionized the earth sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the earth the application of u series geochemistry to the earth sciences was thoroughly summarized in 1982

230th u dating of surficial deposits using the ion microprobe shrimp rg a microstratigraphic perspective abstract we used the sensitive high resolution ion microprobe reverse geometry shrimp rg to date pedogenic opal using the 230 th u system due to the high spatial resolution of an ion microprobe typically 30 m regions of pure opal within a sample can be targeted and detrital material can be avoided in addition because the technique is non destructive the sample can be preserved for other types of analyses including electron microprobe or other stable isotope or trace element ion microprobe measurements the technique is limited to material with u concentrations greater than 50 ppm however the high spatial resolution small sample requirements and the ability to avoid detrital material make this technique a suitable technique for dating many pleistocene deposits formed in semi arid environments to determine the versatility of the method samples from several different deposits were analyzed including silica rich pebble coatings from pedogenic carbonate horizons a siliceous sinter deposit and opaline silica deposited as a spring mound u concentrations for 30 m diameter spots ranged from 50 to 1000 ppm in these types of materials the 230 th 232 th activity ratios also ranged from 100 to 10 6 eliminating the need for detrital th corrections that reduce the precision of traditional u th ages for many milligram and larger sized samples in pedogenic material layers of high u opal ca 500 ppm are commonly juxtaposed next to layers of calcite with much lower u concentrations 1 2 ppm if these types of samples are not analyzed using a technique with the appropriate spatial resolution the ages may be strongly biased towards the age of the opal comparison with standard tims thermal ionization mass spectrometry measurements from separate microdrilled samples suggests that although the analytical precision of the ion microprobe shrimp rg measurements is less than tims the high spatial resolution results in better accuracy in the age determination for finely layered or complex deposits the ion microprobe approach also may be useful for pre screening samples to determine the age and degree of post depositional alteration analyzing finely layered samples or samples with complex growth histories and obtaining simultaneous measurements of trace elements

luminescence of uranium bearing opals origin and use as a ph record abstract fluorescence of minerals has been long used for u exploration it is proposed here that opal fluorescence can be used as a probe of the ph of the formation solution bringing constraints for modeling u speciation and sequestration at the earth s surface we present a study of fluorescence spectroscopy of u in opals resulting from low temperature 29 c alteration of mineralized rhyolitic lavas nopal i u deposit sierra pena blanca mexico these opals show green fluorescence with a concentric distribution at microscopic scale which is unambiguously assigned to uranyl groups oxidized form of u spectra appear typical of uranyl in opal as reported in literature for other localities according to peak positions when considering also the lifetime of fluorescence spectra with time resolved laser fluorescence spectroscopy fingerprinting indicates that uranyl occurs either as phosphate or hydroxo polynuclear complexes trapped onto the opal internal surface data are indicative of a ph of opal formation around 8 as derived from both conditions of laboratory experiments and reference to calculated diagrams of speciation this ph value is consistent with the nopal geological formations that were potentially in contact with the aquifer involved in the hydrothermal process in addition the microscopic zoning of u suggests that the speciation has been stable since opal formation more than 50 ka ago which allows the record of ph that prevailed at that time

assessing the carbon dioxide content of ancient atmospheres using palaeocalcretes theoretical and empirical contraints recent suggestions that the 13 c composition of calcretes can be used to determine the p co 2 of ancient atmospheres is critically evaluated the problem of differentiating pedogenic calcretes from groundwater ones which could not be used for p co 2 determinations is considered a critical problem a basic tenet of the use of calcretes for assessing p co 2 is that plants using c 4 or cras sulacean acid metabolism photosynthetic pathways were unimportant until the miocene this view is questioned and evidence is presented from carboniferous calcified root mats that non c 3 plants were probably present at this time confirming speculations by palaeobotanists that crassulacean acid metabolism or c 4 photosynthesis may have arisen numerous times during the evolution of higher plants

steatite beads at peqi in long distance trade and pyro technology during the chalcolithic of the levant abstract the chalcolithic burial cave of peqi in northern galilee israel yielded about 190 beads made of white paste found in the context of ossuaries they range in size from 2 4 mm in diameter 1 3 mm in height and hole diameter is approximately 1 mm the beads were analyzed using scanning electron microscope sem and x ray diffraction xrd under the sem the beads contain silicon and magnesium with traces of copper and iron xrd analyses revealed that the beads are made of enstatite a mg bearing pyroxene and cristobalite a high temperature polymorph of quartz formed when quartz is heated at 900 1470 c our preliminary results suggest that the beads were made by heating talc to a high temperature first a paste was prepared from powdered talc water and a binding material the paste was then shaped into long tubes and fired at a high temperature this firing hardened the paste and transformed the talc into enstatite and cristobalite finally the tube was sliced to form beads neither talc nor enstatite is found in israel the nearest possible sources for this raw material are metamorphic rocks exposed in turkey or egypt similar contemporaneous technologies are known from egypt and the indus valley here we present the first documentation of chalcolithic pyrotechnology applied for non metallurgical purposes this find is thus of prime importance for both technological innovations and long distance trade during this period

consequences of slow growth for 230th u dating of quaternary opals yucca mountain nv usa thermal ionization mass spectrometry and data are reported for uranium rich opals coating fractures and cavities within the silicic tuffs forming yucca mountain nv the potential site of a high level radioactive waste repository high uranium concentrations up to 207 ppm and extremely high activity ratios up to about 106 make microsamples of these opals suitable for precise dating conventional ages range from 40 to greater than 600 ka and initial activity ratios between 1 03 and 8 2 isotopic evidence indicates that the opals have not experienced uranium mobility however wide variations in apparent ages and initial ratios for separate subsamples of the same outermost mineral surfaces positive correlation between ages and sample weights and negative correlation between ages and calculated initial are inconsistent with the assumption that all minerals in a given subsample was deposited instantaneously the data are more consistent with a conceptual model of continuous deposition where secondary mineral growth has occurred at a constant slow rate up to the present this model assumes that individual subsamples represent mixtures of older and younger material and that calculations using the resulting isotope ratios reflect an average age ages calculated using the continuous deposition model for opals imply average mineral growth rates of less than 5 mm m y the model of continuous deposition also predicts discordance between ages obtained using different radiometric methods for the same subsample differences in half lives will result in younger apparent ages for the shorter lived isotope due to the greater influence of younger materials continuously added to mineral surfaces discordant and u pb ages obtained from outermost mineral surfaces at yucca mountain support this model

uranium isotopes in soils as a proxy for past infiltration and precipitation across the western united states the intermittent presence of large pleistocene lakes in the southwestern interior of north america a region that is now a semi arid desert suggests repeated oscillations between profoundly different climatic conditions the origin of these shifts is still unresolved due to inconsistencies in existing climate proxy data for example pollen lake levels and oxygen isotopes in speleothems to resolve the inconsistencies in the water balance over the last 10 to 60 kyr we use uranium isotopic variations in secondary soil minerals to quantify net infiltration and precipitation along a north south transect in western north america we show that winter infiltration increased by 30 to 100 percent and precipitation by a lesser amount in the valleys of the great basin and mojave deserts between 60 and 26 ka this increase in infiltration and precipitation preceded the last glacial maximum lgm and the timing of most lake highstands in the region by 5 to 10 kyr respectively suggesting a possible last precipitation maximum lpm that coincided with a minimum in winter insolation subsequent decreases in infiltration and precipitation after the lgm can be reconciled with the timing of lake highstands if colder summer temperatures due to a minimum in summer insolation reduced lake evaporation the soil records combined with a range of proxy data suggest that seasonal insolation is the long term driver for large shifts in both precipitation and surface water variability in the region

u pb shrimp dating of uraniferous opals abstract u pb and u series analyses of four u rich opal samples using sensitive high resolution ion microprobe shrimp demonstrate the potential of this technique for the dating of opals with ages ranging from several tens of thousand years to millions of years the major advantages of the technique compared to the conventional thermal ionisation mass spectrometry tims are the high spatial resolution 20 m the ability to analyse in situ all isotopes required to determine both u pb and u series ages and a relatively short analysis time which allows obtaining a growth rate of opal as a result of a single shrimp session there are two major limitations to this method determined by both current level of development of ion probes and understanding of ion sputtering processes first sufficient secondary ion beam intensities can only be obtained for opal samples with u concentrations in excess of 20 g g however this restriction still permits dating of a large variety of opals second u pb ratios in all analyses drifted with time and were only weakly correlated with changes in other ratios such as u uo this drift which is difficult to correct for remains the main factor currently limiting the precision and accuracy of the u pb shrimp opal ages nevertheless an assumption of similar behaviour of standard and unknown opals under similar analytical conditions allowed successful determination of ages with precisions of 10 for the samples investigated in this study shrimp based u series and u pb ages are consistent with tims dating results of the same materials and known geological timeframes

natural radionuclide mobility and its influence on u th pb dating of secondary minerals from the unsaturated zone at yucca mountain nevada abstract extreme u and pb isotope variations produced by disequilibrium in decay chains of 238u and 232th are found in calcite opal chalcedony and mn oxides occurring as secondary mineral coatings in the unsaturated zone at yucca mountain nevada these very slowly growing minerals mm my 1 contain excess 206pb and 208pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable 206pb 238u geochronometers the presence of excess intermediate daughter isotopes does not appreciably affect 207pb 235u ages of u enriched opal chalcedony which are interpreted as mineral formation ages opal and calcite from outer younger portions of coatings have 230th u ages from 94 6 3 7 to 361 3 9 8 ka and initial 234u 238u activity ratios ar from 4 351 0 070 to 7 02 0 12 which indicate 234u enrichment from percolating water present day 234u 238u ar is 1 in opal chalcedony from older portions of the coatings the 207pb 235u ages of opal chalcedony samples range from 0 1329 0 0080 to 9 10 0 21 ma increase with microstratigraphic depth and define slow long term average growth rates of about 1 2 2 0 mm my 1 in good agreement with previous results measured 234u 238u ar in mn oxides which pre date the oldest calcite and opal chalcedony range from 0 939 0 006 to 2 091 0 006 and are 1 in most samples the range of 87sr 86sr ratios 0 71156 0 71280 in mn oxides overlaps that in the late calcite these data indicate that mn oxides exchange u and sr with percolating water and cannot be used as a reliable dating tool in the u poor calcite samples measured 206pb 207pb ratios have a wide range do not correlate with ba concentration as would be expected if excess ra was present and reach a value of about 1400 the highest ever reported for natural pb calcite intergrown with opal contains excesses of both 206pb and 207pb derived from rn diffusion and from direct recoil from u rich opal calcite from coatings devoid of opal chalcedony contains 206pb and 208pb excesses but no appreciable 207pb excesses observed pb isotope anomalies in calcite are explained by rn produced excess pb the rn emanation may strongly affect 206pb 238u ages of slow growing u poor calcite but should be negligible for dating fast growing u enriched speleothem calcite