calcite u pb age of the cretaceous vertebrate bearing bayn shire formation in the eastern gobi desert of mongolia usefulness of caliche for age determination

lead and zinc distribution in carbonate rocks abstract a polarographic technique was utilized to determine lead and zinc concentrations in the acid soluble portion of limestones and dolostones the detection limit of the described technique is 0 2 ppm the studied samples mostly from australia cover the time span of 110 to 2700 ma the geometric mean of pb concentrations in both limestones and dolostones was found to be 3 1 ppm and for zn it was 4 2 and 6 1 ppm respectively statistical evaluation r mode factor analysis of these data complemented by the previously published chemical and isotopic measurements on identical samples shows that pb and zn load on a unique factor perhaps reflecting their substitution for ca and mg in the carbonate lattice however a weak secondary control on zn distribution in dolostones appears to be excercized by the presence of illite muscovite and or k feldspar and pb may be concentrated in reef bank rather than lagoonal environments furthermore zn concentrations in limestones appear to decrease slightly with increasing age of the samples

upb dating of fe rich phases using a sequential leaching method a sequential leaching method was developed for upb dating of fe rich phases which uses a series of successively stronger acids the method was applied to date fe rich phases of magmatic origin in the kinsman quartz monzonite new hampshire usa and of sedimentary origin in the dales gorge member of the brockman iron formation hamersley basin australia using the composition of major element released by the acid leaching and combining it with x ray and sem analyses of the residues we were able to identify the relatively pure phases released and use these samples for dating magnetite separates from the kinsman quartz monzonite yielded upb age of 372 34 ma which is consistent with smnd and a rbsr isochron ages determined in previous studies leached whole rock samples of the brockman iron formation yielded a 207pb206pb age of 2213 250 ma which agrees with previously determined pbpb smnd and rbsr isochron ages this age is interpreted as the time of redistribution of u and pb associated with the fe rich phases during regional metamorphism while the potential of the sequential leaching method is demonstrated more work is needed in order to improve the precision of the data this requires further investigation of the nature of the pure phases

exploring a multiresolution approach using amip simulations abstractthis study presents a diagnosis of a multiresolution approach using the model for prediction across scales atmosphere mpas a for simulating regional climate four atmospheric model intercomparison project amip experiments were conducted for 1999 2009 in the first two experiments mpas a was configured using global quasi uniform grids at 120 and 30 km grid spacing in the other two experiments mpas a was configured using variable resolution vr mesh with local refinement at 30 km over north america and south america and embedded in a quasi uniform domain at 120 km elsewhere precipitation and related fields in the four simulations are examined to determine how well the vrs reproduce the features simulated by the globally high resolution model in the refined domain in previous analyses of idealized aquaplanet simulations characteristics of the global high resolution simulation in moist processes developed only near the boundary of the refined region in contrast amip simulations with vr g

u pb dating of a speleothem of quaternary age we demonstrate that u pb dating is a promising method for secondary carbonate materials of quaternary age and older by obtaining a 206pb 238u age for a speleothem with high u 10 g g 1 and very low pb 10 ng g 1 that is supported by an independent 230th age thermal ionisation mass spectrometry was used to determine the u and pb isotopic ratios and concentrations for subsamples of a stalactite from winnats head cave peak district uk we obtained 206pb 204pb ratios up to 50 and determined a 206pb 238u age of 248 10 ka which is within error of the 207pb 235u age of 333 79 ka and a spectrometric u th age of 255 ka for samples of tertiary and quaternary age the initial state of u series disequilibrium is an important consideration and as with most radiometric dating techniques the mineral must have remained closed to u th pb and all intermediate daughters we show that dense calcite speleothems are ideal in this respect and that no loss of rn has occurred unlike u series disequilibrium methods u pb dating has no upper limit and hence materials of quaternary age older than 0 6 ma can be analysed to investigate landscape development paleoclimate hominid evolution or hydrogeochemistry in carbonate terrains

direct radiometric age determination of carbonate diagenesis using u pb in secondary calcite abstract carbonate rocks and corals from the middle devonian lucas formation ontario have 238 u 204 pb values ranging from 132 to 559 and give concordant apparent 206 pb 238 u and 207 pb 235 u ages in approximate agreement with the primary age of sedimentation a carbonate rock from one part of the section gives an anomalously young age and contains patches of secondary calcite spar this spar is predominantly pale purple under cathodoluminescence cl and contains 250 600 ppb u and 15 22 ppb pb the values range from 2 10 3 10 4 making the pb very radiogenic apparent 206 pb 238 u ages range from 208 to 249 ma and 232 th 238 u values are from 0 005 to 0 040 the younger ages are correlated with an increase in orange cl and higher apparent values the age scatter is attributed to mixing of a subordinate calcite spar which infiltrated the older spar the young calcite is bright orange under cl has 7200 ppb u of 6 2 10 4 a of 0 23 and is dated at 45 ma the age of the older spar has been determined by a sequential dissolution technique the resulting u pb data can be plotted as u pb age spectra and yield lplateau dates ranging from 231 5 1 4 to 244 9 1 6 ma and an integrated 206 pb 238 u age of 238 2 1 1 ma also the sequential dissolution increases the values progressively to a5 10 4 making these ages independent of the initial pb correction the occurrence of two young ages of diagenetic calcite in these strata is consistent with the idea of recurrent fluid activity along fracture zones in subsurface sediments in the michigan basin this study shows that it is now possible to date secondary carbonate diagenesis by u pb and by inference to determine precisely the timing of activity on ancient fracture zones in carbonate rocks

first direct radiometric dating of archaean stromatolitic limestone here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long lived radioactive decay schemes the mushandike stromatolitic limestone from the masvingo formerly fort victoria greenstone belt of southern zimbabwe1 2 yields a pb pb isochron age of 2 839 33 myr published claims3 for an age of 3 5 gyr for the mushandike limestone were based on its supposed intrusion by the mushandike granite for which a rb sr whole rock age of 3 445 260 myr had been reported4 new rb sr and pb pb isotope data for the mushandike granite indicate a probable emplacement age of 2 900 myr and new field evidence j l o p t j f w in preparation suggests that the granite forms part of the basement on which the limestone was deposited thus the new dates do not support an age of 3 5 gyr for the algal stromatolites at mushandike the pb pb isochron method may have wide applicability to dating the time of deposition of precambrian fossiliferous and unfossiliferous limestones and thus to the problem of the antiquity of life

upb dating of calcite concretions from cambrian black shales and the phanerozoic time scale the u pb dating of carbonate offers considerable potential for accurate and direct dating of sedimentary rocks here we show that carbonate concretions from the upper cambrian black shales in southern scandinavia have remarkably high u to pb ratios 238u204pb up to 288 000 permitting precise geochronology a fine crystalline early diagenetic concretion yields an age of 509 8 5 1 ma providing an accurate age for the late cambrian whereas a coarse cone in cone concretion has yielded an age of 478 2 4 9 ma defining the final stage of sediment compaction and indicating a protracted dewatering and diagenetic history these results demonstrate the power of this method for directly calibrating the geological time scale and constraining diagenetic histories

hydrochronology of a proposed deep geological repository for low and intermediate level nuclear waste in southern ontario from u pb dating of secondary minerals response to silurian and cretaceous events u pb ages have been measured on secondary dolomite and silica cements in cambrian sandstone at the base of an 840 m thick sub horizontally bedded sedimentary sequence beneath the bruce nuclear site

carbonate pbpb ages of the wittenoom formation and carawine dolomite hamersley basin western australia with implications for their correlation with the transvaal dolomite of south africa abstract pb isotopic analyses of carbonates from the hamersley group western australia yield the following results 1 a late archean minimum age of 2 5 ga is definitely established for the deposition of both the deeper water basinal wittenoom formation and the shallow water carawine dolomite stratigraphic correlation of the transvaal dolomite south africa with these two formations is supported by the present pb isotope age data 2 a pb pb isochron age of 2505 37 ma for the paraburdoo member lower part of the wittenoom formation is best interpreted as a late diagenetic event which took place 100 ma after the initial deposition 3 a pb pb isochron age of 2346 38 ma for the bee gorge member most likely represents a still later local event of calcite replacement in fractures and veins 4 a pb pb isochron age of 2541 32 ma for carawine dolomite could represent a regional diagenesis slightly earlier than the paraburdoo carbonates if the carawine carbonates were deposited contemporaneously with the wittenoom formation alternatively it may represent the time of early diagenesis which closely followed the initial sedimentation a study of trace element abundances and stable isotope compositions has revealed a major difference in fe and mn contents and 18 o values between the wittenoom and carawine dolomite samples this suggests that the two suites of dolomites have undergone separate diagenetic histories with different chemical and oxygen isotopic modifications

u pb dating of a remagnetized paleozoic limestone the upper ordovician trenton group of new york state is one of a number of appalachian limestones which yields paleomagnetic poles suggesting disturbance in the late paleozoic calcite fractions dissolved from samples of the trenton limestone define a 238u 206pb age of 454 8 ma mswd 12 8 in excellent agreement with 40ar 39 ar and u pb ages for the deposition of the underlying diecke bentonite this requires that the u pb system in the calcite has been preserved since deposition and early diagenesis if late paleozoic remagnetization of this limestone was induced by fluids these fluids did not interact with significant amounts of carbonate the apparent robustness of the u pb system and the large spread in values 18 17 118 8 offers potential precision of 1 ma in determining the paleozoic time scale

crustal shortening root spreading isostasy and the growth of orogenic belts a dimensional analysis scaling laws that describe the growth of mountain belts and the change in their regional topographic profiles with time t are obtained by physical arguments that account for crustal shortening isostasy and creeping flow at the mountain roots an average power law rheology characterized by an exponent n is assumed and different values of n are considered the model predicts the formation of a localized range characterized by a height h and a width a as crust on the sides of a range is compressed the root broadens due to buoyancy driven creep this process in conjunction with isostasy causes h to increase with time as t1 4 t1 3 and a as t3 4 t2 3 as n is varied from 1 newtonian rheology to plastic rheology accordingly the aspect ratio a h increases as t1 2 t1 3 the scaling laws depend on the rheology but rather weakly the aspect ratio increases with age but is nearly independent of the rate of shortening the deformation extends gradually inland with increasing time so that the age of tectonic deformation decreases with distance from the continental border uplift takes place inland from the crest while subsidence occurs at the opposite side the ratio a h of the andes and tibet is consistent with this model

radiogenic isotopes systematics and applications to earth surface processes and chemical stratigraphy radiogenic isotopes have wide application to chemical stratigraphy geochronology provenance studies and studies of temporal changes in earth surface processes this paper briefly reviews the principles of radiogenic isotope geochemistry and the distribution of a number of elements of interest in the environment and then uses this information to explore the range of applications to chemical stratigraphy and other fundamental subjects of sedimentary geology many of these applications center on the reconstruction of secular variations in paleoseawater isotope signals these seawater isotope curves can be used for stratigraphic correlation and to assess changes through earth history in processes such as tectonic uplift climate change biogeochemical cycling ocean circulation crustal growth and surface water evolution understanding the modern biogeochemical cycle of the element isotope system of interest is essential for gaining the maximum information regarding the operation of ancient cycles the main sources of uncertainty for reconstructing seawater isotope curves involve diagenetic alteration sample impurities and errors in age assignments rigorous sample selection criteria and pretreatment methods are keys to reconstructing the most accurate seawater isotope curves applications are discussed for the rb sr sm nd la ce lu hf re os u th pb k ar and cosmogenic isotope systems d 2003 elsevier b v all rights reserved

contemporary flowstone development links early hominin bearing cave deposits in south africa abstract the cradle of humankind cave sites in south africa preserve fossil evidence of four early hominin taxa australopithecus africanus australopithecus sediba paranthropus robustus and early homo in order to integrate this record into a pan african scenario of human evolutionary history it is critical to have reliable dates and temporal ranges for the southern african hominins in the past a lack of precise and accurate chronological data has prevented the evaluation of the temporal relationships between the various sites here we report new uranium lead u pb radiometric ages obtained from sheets of calcium carbonate flowstone inter bedded between clastic cave sediments at the site of swartkrans providing bracketing ages for the fossiliferous deposits the fossil bearing units of swartkrans specifically the hanging remnant and lower bank of member 1 are underlain by flowstone layers dated to 2 25 0 05 ma and 2 25 0 08 ma and capped by layers of 1 8 0 01 ma and 1 7 0 07 ma the age bracket of the member 1 deposits is therefore between 2 31 and 1 64 ma however by combining the u pb with biostratigraphic data we suggest that this can be narrowed down to between 1 9 and 1 8 ma these data can be compared with other recently dated sites and a radiometrically dated u pb age sequence formed sterkfontein member 4 swartkrans member 1 malapa and cooper s d from this new u pb dataset a pattern of contemporary flowstone development emerges with different caves recording the same flowstone forming event specifically overlapping flowstone formation takes place at swartkrans and sterkfontein at 2 29 ma and 1 77 ma and at sterkfontein and malapa at 2 02 ma this suggests a regional control over the nature and timing of speleothem development in cave deposits and these flowstone layers could assist in future correlation both internal to specific deposits and regionally between sites

sm nd rb sr pb pb dating of carbonates from the early archaean barberton greenstone belt south africa evidence for post depositional isotopic resetting at low temperature abstract two silicic carbonate sequences occurring in the onverwacht group og and the overlying fig tree group ftg of the early archaean barberton greenstone belt south africa contain silicified dolomites with primary structures and textures they occur with volcanic rocks dated by single zircons the associated onverwacht felsic volcanics have an age of 3440 3416 ma whereas the fig tree volcanics are 3225 ma old therefore the depositional age of both sequences is between 3440 and 3225 ma pb isotopic data for different parts of a single og whole rock sample form a linear array corresponding to a pb pb age of 2632 99 ma sm nd and rb sr isotopes were analysed in four whole rock samples of the ftg silicified carbonates and form an sm nd array corresponding to an age of 2612 84 ma with initial end 8 1 0 5 and an rb sr array of 2743 36 ma with initial 87 sr 86 sr 0 7013 0 0003 these whole rock samples have rb sr ratios of 1 and unusually high rb concentrations for carbonates 100 ppm which derive from processes accompanying silicification all three isotopic systems of whole rocks indicate ages of 2 7 ga much younger than the depositional age of the successions the agreement provides strong evidence that an alteration event reset all of the isotopic systems sm nd as well as rb sr and u pb we suggest that the ages represent the timing of silicification and sericitization of the carbonates by large scale fluid movement at an apparent temperature of 200 c

direct dating of phanerozoic sediments by the 238 u 206 pb method direct radiometric age determination of phanerozoic carbonates has been a long standing problem in geochronology rb sr and k ar dating schemes commonly used to constrain the chronology of phanerozoic sediments have so far proved to be unsuccessful in dating these rocks because of their poor enrichment in radiogenic 87sr and 40ar and also because of analytical difficulties recent studies1 2 have demonstrated that large amounts of radiogenic pb exist in some carbonates and that they could be dated by the pb pb method however the precision of this method is severely limited to samples having very high uranium to lead ratios because by the beginning of the phanerozoic 590 myr ago 98 of 235u originally present in the earth had decayed to 207pb here we report the first isochron age of carbonates using the 238u 206pb method on corals this method which gives good accuracy even for low u concentrations has great potential for the direct dating of sedimentary sequences and should be valuable for refining the phanerozoic timescale

using u pb ages of miocene tufa for correlation in a terrestrial succession barstow formation california sedimentary rocks and associated fossils are the direct record of past life and its environmental context few methods allow direct and precise dating of this important archive making it diffi cult to correlate climate biologic and tectonic events preserved in terrestrial records we sampled lacustrine tufa calcite from the middle member of the miocene barstow formation to further test the feasibility of uranium lead dating of terrestrial carbonates five samples yield u pb ages 2 of 14 81 0 39 ma 15 30 0 25 ma 15 39 0 15 ma 16 14 0 40 ma and 16 24 0 23 ma these precise ages are consistent with existing ages from k ar and 40 ar 39 ar dating of intercalated volcanic ashes our most precise u pb age on tufa has an uncertainty of 1 150 ka which is equivalent to uncertainties for k ar ages from the same time period our results indicate that u pb dating of carefully sampled and screened sedimentary carbonates provides much needed age constraints in purely terrestrial sequences and can be a powerful tool for correlation even in deposits with structural complexity and rapid facies changes

pb pb dating of young marbles from taiwan direct determination of the depositional age of a sedimentary sequence usually relies on isotopic analyses of authigenic minerals such as glauconite1 and fine grained clay minerals2 4 moorbath et al 5 recently reported the first direct dating of sedimentary carbonates using the whole rock pb pb method the success of this method depends on the chemical fractionation of u from pb and the consequent variation in 206pb 204pb and 207pb 204pb ratios the high values and large variation of u pb ratios in sedimentary carbonates long known to marine geochemists have however not been exploited by geochronologists until recently5 6 here i report a pb pb isochron age for a relatively young carbonate sequence in taiwan and demonstrate that the long lived u pb decay scheme can be used to determine events as young as mesozoic age the new results also lead to the identification of an early jurassic 170 myr bp thermal event which has never before been recognized in the polymetamorphic schist belt the tananao schist of taiwan but is widespread in southeastern continental china and other parts of eastern asia7 10

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

age constraints on the neoproterozoic baikal group from combined sr isotopes and pb pb dating of carbonates from the baikal type section southeastern siberia abstract depositional ages for the middle part of the neoproterozoic sedimentary succession exposed along lake baikal in the southern margin of the siberian platform have been constrained by the application of sr isotope chemostratigraphy and pb pb dating of carbonate rocks the thick siliciclastic carbonate baikal group is the type section of the regional siberian succession although many of the baikal group carbonate rocks have undergone pervasive dolomitization by mixed meteoric marine fluids oolitic limestones in the 800 m thick carbonate member of the uluntui formation preserve primary textures and high sr contents up to 3200 ppm suggesting that aragonite might have been the original precursor the 87sr 86sr in the least altered high sr limestones mg ca 8 pdb range within 0 70842 0 70876 values we interpret as representing the best proxy for coeval seawater stepwise dissolution using 0 5 n hbr was applied to enrich samples in primary carbonate from the uluntui formation and nine data points from the five least altered high sr limestones yielded a pb pb isochron age of 560 30 ma with mswd 0 6 and probability 0 8 this age matches that inferred from sr isotope stratigraphy 580 550 ma and further supports the inference that the baikal paleobasin was connected to the contemporary oceans our sr chemostratigraphy and pb pb isochron age of the uluntui limestones cast doubt on previously proposed correlations of the baikal group as late mesoproterozoic to cryogenian instead the data indicate that the entire siliciclastic carbonate succession of the baikal group was most likely deposited during the ediacaran or vendian of the russian stratigraphic scale

upb dating of a carbonate subaerial exposure event ten samples of the dolomite fraction from the lower ordovician oneota formation directly below the sauk tippecanoe unconformity in wisconsin define a 238u 206pb age of 504 14 ma mswd 190 206pb204pb 27 85 238u204pb 84 795 which is in agreement with the 500 4 ma depositional age based on conodont biostratigraphy the u pb age is interpreted as recording u enrichment of the carbonate during the early stages of subaerial exposure shortly following deposition variable initial pb isotope ratios prolonged interaction with meteoric fluids and later recrystallization associated with the influx of fluids that produced the permian upper mississippi valley pb zn sulfide deposits are likely factors in producing scatter about the isochron however the substantial u enrichment that can be associated with subaerial exposure of carbonates apparently allows precise u pb ages to be obtained despite some isotope mobility that occurs during later diagenetic events dolomite samples have 232th238u 0 3 much lower than that of average silicate crust consistent with the low solubility of th in low temperature systems and the high solubility of u in oxidized fluids 204pb206pb 208pb206pb variations in dolomite and oxide fractions from the oneota formation are linearly correlated 208pb206pb 38 955 204pb206pb 0 050 r2 0 9996 and are interpreted as indicating that the u and common pb were derived from penokean 1 8 ga crust exposed to the north i e the wisconsin dome or from its derivative sediments