climate variations since the last interglacial recorded in czech loess 1 magnetic susceptibility organic matter content and grain size were measured on a loess palaeosol sequence deposited since the last interglacial at the dolni vestonice section in the czech republic high values in magnetic susceptibility and organic matter content in palaeosols pk0 pk2 and pk3 indicate that they were strongly influenced by long term pedogenesis and high vegetation density linked to interglacials or interstadials grain size variations record a series of cold wind strengthened events during the last glacial which we interpret as a record of the distinctive younger dryas episode and millennial scale climate oscillations that correlate with the grip ice core 18o record and north atlantic heinrich events these climate events as indicated by grain size peaks combined with previous climate records at french massif central and lago grande di monticchio in italy suggest that the climate over these areas might have undergone synchronous oscillations with the shifts in sea surface temperatures of the north atlantic and air temperature changes above greenland

thermal and impact histories of reheated group iva ivb and ungrouped iron meteorites and their parent asteroids the microstructures of six reheated iron meteoritesotwo iva irons maria elena 1935 fuzzy creek one ivb iron ternera and three ungrouped irons hammond babb s mill blake s iron and babb s mill troost s iron owere characterized using scanning and transmission electron microscopy electron probe microanalysis and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids maria elena and hammond were heated below approximately 700 750 c so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite both meteorites retained a record of the original widmanstitten pattern the other four which show no trace of their original microstructure were heated above 600 700 c and recrystallized to form 10 20 lm wide homogeneous taenite grains on cooling kamacite formed on taenite grain boundaries with their close packed planes aligned formation of homogeneous 20 lm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 c or approximately 1 h at 1300 c all six irons contain approximately 5 10 lm wide taenite grains with internal microprecipitates of kamacite and nanometer scale m shaped ni profiles that reach approximately 40 ni indicating cooling over 100 10 000 yr un decomposed high ni martensite a2 in taeniteothe first occurrence in ironsoappears to be a characteristic of strongly reheated irons from our studies and published work we identified four progressive stages of shock and reheating in iva irons using these criteria cloudy taenite m shaped ni profiles in taenite neumann twin lamellae martensite shock hatched kamacite recrystallization microprecipitates of taenite and shock melted troilite maria elena and fuzzy creek represent stages 3 and 4 respectively although not all reheated irons contain evidence for shock it was probably the main cause of reheating cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays if the reheated irons that we studied are representative the iva irons may have been shocked soon after they cooled below 200 c at 4 5 gyr in an impact that created a rubblepile asteroid with fragments from diverse depths the primary cooling rates of the iva irons and the proposed early history are remarkably consistent with the pb pb ages of troilite inclusions in two iva irons including the oldest known differentiated meteorite blichert toft et al 2010

metallographic cooling rates and origin of iva iron meteorites we have determined the metallographic cooling rates for 13 iva irons using the most recent and most accurate metallographic cooling rate model group iva irons have cooling rates that vary from 6600 c myr at the low ni end of the group to 100 c myr at the high ni end of the group this large cooling rate range is totally incompatible with cooling in a mantled core which should have a uniform cooling rate thermal and fractional crystallization models have been used to describe the cooling and solidification of the iva asteroid the thermal model indicates that a metallic body of 150 50 km in radius with less than 1 km of silicate on the outside of the body has a range of cooling rates that match the metallographic cooling rates in

luminescence dating of the loess profile at dolni vestonice czech republic abstract thermoluminescence tl and infra red stimulated luminescence irsl studies were carried out on 21 samples from loess profiles at dolni vestonice the irsl measurements gave systematically lower equivalent doses eds than the tl measurements this could be due to inadequate preheat being applied to the former dose rate intercomparisons confirm the view that alpha counting should be carried out on finely ground material comparison of ed determination by regeneration and additive dose methods did not suggest sensitivity increases on bleaching but for the older samples the tl ages were underestimated compared with the expected ages based on the correlation of palaeosol stratigraphy with the framework provided by the deep sea oxygen isotope record using the hypothesis of luminescence decay with a lifetime of 150 ka suggests that the previous ages assigned to the palaeosols may not be correct

a tungsten isotope approach to search for meteoritic components in terrestrial impact rocks abstract the identification of meteorite impact structures on earth is based on two main criteria the presence of shock metamorphic effects in the crater rock ejecta and or the confirmation of an extraterrestrial meteoritic component in breccias or melt rocks for the latter both high elemental abundances of siderophile elements especially the platinum group elements with corresponding meteoritic inter element ratios and the osmium os and chromium cr isotopic signatures characteristic of meteorites have been used successfully inspired by earlier suggestions of a meteoritic component in archean rocks based on tungsten w isotope anomalies here we explore the possible use of 182 w which has been produced by the decay of now extinct 182 hf t 1 2 8 9 ma as a tracer of meteoritic component in terrestrial material each group of meteorites has w isotopic compositions that are distinct from each other and from the terrestrial crust 182 w has already been used to try to identify the impactor at the k t boundary by analyzing the sediments and ni rich spinel in the present study we broaden the field of investigation by choosing a different approach namely analyzing a variety of known impact rocks we measured the w isotope composition in four samples from different impact structures gardnos norway morokweng south africa vredefort south africa ries germany as well as in two samples from a distal ejecta layer k t boundary samples from gams austria and berwind canyon usa all these samples are unambiguously impact produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies in all these samples the isotopic composition of w is identical with analytical error to that of the earth s continental crust and no 182 w anomalies are present even in the samples containing a significant percent level meteoritic component therefore we conclude that in contrast to the cr or os isotopes w isotopes are not suitable for the identification of meteoritic components in terrestrial rocks

early stages of core segregation recorded by fe isotopes in an asteroidal mantle ureilites displays 56 fe values higher than average chondrite 29 segregation of fe sulfide melts explains the high 56 fe values in ureilites 30 formation of a core can begin at very low degrees of melting through the circulation of a fe s melt 31 through a silicate mantle 32 33 earth and planetary science letters in press 11 3 15 34 2 35 abstract 36 37 ureilite meteorites are achondrites that are debris of the mantle of a now disrupted 38 differentiated asteroid rich in carbon they provide a unique opportunity to study the differentiation 39 processes of such a body we analyzed the iron isotopic compositions of 30 samples from the ureilite 40 parent body upb including 29 unbrecciated ureilites and one ureilitic trachyandesite alm a 41 which is at present the sole large crustal sample of the upb the 56 fe of the whole rocks fall within a 42 restricted range from 0 01 to 0 11 with an average of 0 056 0 008 which is significantly 43 higher than that of chondrites we show that this difference can be ascribed to the segregation of s 44 rich metallic melts at low degrees of melting at a temperature close to the fe fes eutectic and 45 certainly before the onset of the melting of the silicates 1100 c in agreement with the marked s 46 depletions and the siderophile element abundances of the ureilites these results point to an efficient 47 segregation of s rich metallic melts during the differentiation of small terrestrial bodies 48 49

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

the 36cl 36ar 40k 41k records and cosmic ray production rates in iron meteorites this is the first contribution towards a reevaluation of exposure histories of iron meteorites and of the constancy of the cosmic ray flux over the last billion years as recorded in these fossil detectors we have performed new 36cl 26al 10be and noble gas measurements including determination of the shielding parameter s 4he 21ne in samples with published k data the k isotopic data coupled to 36ar and 36cl concentrations permit selection of meteorites which have only experienced simple constant geometry irradiation histories these objects can be used for the calibration of shielding dependent production rates within these metallic detectors in order to carry out production rate calibrations based on 40k 41k data we assume constancy of the cosmic flux during the interval 150 to 700 my ago we note that meteorites with very old potassium ages cannot be included in this calibration as these meteorites require distinct parameter sets a calibration data set representing a total of 13 meteorites was used to compute long term 0 5 gy average production rates these average production rates of 36cl from this particular calibration set are significantly 28 lower than those determined for the recent 10 my cosmic ray flux we also document here the quality of the resulting potassium production rate parameter m0 s with a calculated isochron for irons of group iva

diffusion of osmium in pyrrhotite and pyrite implications for closure of the re os isotopic system abstract in order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite experiments consisted of either 1 isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered os bearing fe sulfide or 2 relaxation of initially high near surface osmium concentrations produced in the latter experiments pyrite only osmium penetration into samples was characterized by depth profiling using rutherford backscattering spectroscopy rbs pyrite or electron microprobe analyses across sectioned run products pyrrhotite results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces with the extent of penetration uncorrelated with run duration images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near surface occurred by solute uptake during step growth and not by volume diffusion prolonged 1000 h relaxation experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for rbs a maximum diffusion coefficient of 2 5 10 23 m2 s at 500 c was calculated experiments involving pyrrhotite over the temperature range of 950 1100 c showed extensive osmium uptake and osmium concentration gradients that conform with fickian diffusion behavior we found that pyrrhotite fe s could be varied by changes in the composition of the starting material and osmium source and over the range of fe s produced in experiments molar fe s 0 83 0 90 we observed no systematic variation in the osmium diffusion coefficient diffusion coefficients measured parallel to the a crystallographic axis were on average 1 4 higher than values measured parallel to c and regression of the c axis data yielded the arrhenius relation d t 1 31 0 16 10 5 m 2 s exp 211 5 14 7 kj mol rt the application of these diffusion data to simple models of diffusive exchange during static or polythermal time temperature histories is used to assess the conditions under which radiogenic osmium will be retained during isothermal annealing calculations indicate that the cores of millimeter sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in 0 5 ma at temperatures exceeding 400 c pyrite undergoing the same process at 500 c requires in excess of 10 ma before crystal cores are affected the relatively short core retention time scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events thus possibly accounting for the scatter that commonly occurs in re os isochrons generated from massive sulfide samples calculated closure temperatures tc for osmium exchange in pyrrhotite yielded values of 300 400 c for grain sizes ranging from 10 to 1000 m these values of tc are similar to those calculated for ar retention in biotite and considerably lower than for sr in apatite and plagioclase for example such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late stage hydrothermal fluids this latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage

neutron capture on pt isotopes in iron meteorites and the hf w chronology of core formation in planetesimals the short lived 182hf 182w isotope system can provide powerful constraints on the timescales of planetary core formation but its application to iron meteorites is hampered by neutron capture reactions on w isotopes resulting from exposure to galactic cosmic rays here we show that pt isotopes in magmatic iron meteorites are also affected by capture of epi thermal neutrons and that the pt isotope variations are correlated with variations in 182w 184w this makes pt isotopes a sensitive neutron dosimeter for correcting cosmic ray induced w isotope shifts the pre exposure 182w 184w derived from the pt w isotope correlations of the iid iva and ivb iron meteorites are higher than most previous estimates and are more radiogenic than the initial 182w 184w of ca al rich inclusions cai the hf w model ages for core formation range from 1 6 1 0 million years ma for the iva irons to 2 7 1 3 ma after cai formation for the iid irons indicating that there was a time gap of at least 1 ma between cai formation and metal segregation in the parent bodies of some iron meteorites from the hf w ages a time limit of 1 5 2 ma after cai formation can be inferred for the accretion of the iid iva and ivb iron meteorite parent bodies consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies

a comparative petrologic study of iron meteorites with 107 ag anomalies we have made a petrographic study of the iron meteorites derrick peak iib cape york iiia grant iiib gibeon iva and santa clara ivb each of these meteorites has been reported to contain excess 107 ag from the decay of now extinct 107 pd isotopic data on metal and sulfide phases for groups iib and iiiab iron meteorites have been explained using conventional interpretations which show a correlation between 107 ag 109 ag and pd ag while data on sulfide nodules in meteorites from groups iva and ivb are very complex we have studied the extent to which shock and thermal events may have caused the redistribution of ag and disturbed the original relationship between 107 ag and pd particularly in sulfide rich regions the petrography of the iron meteorites studied is consistent with the pd ag isotopic data the textures observed in the metal and sulfide in groups iia and iiiab were found to be indicative of shock but not extensive thermal metamorphism the contacts between metal and sulfide are sharp and the sulfide and metal have remained discrete homogeneous phases these meteorites yield isochrons between metal and sulfide and it is concluded that no significant ag redistribution occurred after their formation the petrography of group iva and ivb meteorites reveals a history involving extensive shock and thermal metamorphism contacts between metal and sulfide nodules are irregular and the two phases are intimately mixed in gibeon unequilibrated 2 martensitic structures in the metal matrix suggest that some reheating affected entire individual fragments of this meteorite while bulk samples of the metal phases of the iva and ivb meteorites appear well behaved with regard to pd ag systematics the sulfide rich regions show extreme isotopic heterogeneities and do not yield well defined isochrons the pd ag data reflect complicated postformation ag transport this petrographic study has found that the complexity of pd ag isotopic data is related to the shock and thermal histories of the iron meteorites however no self consistent explanation has been found to explain the pd ag results observed for the nodules the mineral phase s in which ag resides in both the metal and sulfide phases is as yet undetermined though late forming cu precipitates and chalcopyrite found in iiiab irons are possible getters of volatiles ag au and may be host to 107 ag

size of the group iva iron meteorite core constraints from the age and composition of muonionalusta the group iva fractionally crystallized iron meteorites display a diverse range of metallographic cooling rates these have been attributed to their formation in a metallic core approximately 150 km in radius that cooled to crystallization in the absence of any appreciable insulating mantle here we build upon this formation model by incorporating several new constraints these include i a recent u pb radiometric closure age of 2 5 myr after solar system formation for the group iva iron muonionalusta ii new measurements and modeling of highly siderophile element compositions for a suite of ivas and iii consideration of the thermal effects of heating by the decay of the short lived radionuclide 60fe our model for the thermal evolution of the iva core suggests that it was approximately 50 110 km in radius after being collisionally exposed this range is due to uncertainties in the initial abundance of live 60fe incorporated into the iva core our models define a relationship between cooling rate and closure age which is used to make several predictions that can be tested with future measurements in general our results show that diverse cooling rates and early u pb closure ages can only coexist on mantle free bodies and that energy released by the decay of 60fe reduces the core size necessary to produce diverse metallographic cooling rates the influence of 60fe on cooling rates has largely been neglected in previous core formation models accounting for this heat source can affect size estimates for other iron meteorite cores that cooled to crystallization in the presence of live 60fe candidates for such a scenario of early mantle free formation include the iron iiab iiiab and ivb groups

tidal dissipation surface heat flow and figure of viscoelastic models of io abstract the distribution of hot spots on io s surface and io s figure constrain models of io s interior voyager observations indicate that most hot spots and volcanic plumes are close to the equator recently however it has been proposed that a large low albedo feature in the south polar region may be the source of 40 of the surface heat flow we calculate spatial distributions of the dissipation rate surface heat flow and the figures of three and four layer maxwell rheology models of io by using the correspondence principle the models are required to satisfy the observed surface heat flow constraint and are consistent with the composition of a dehydrated c2 chondrite they consist of an inviscid fe fes core and viscoelastic silicate mantle and lithosphere layers a comparison of the calculated equilibrium figure with the observed figure of io supports the chosen core radius of 980 km in addition the four layer model has a partially molten asthenosphere a few tens of kilometers thick the preferred three layer model has a mantle shear modulus of 10 10 pa and a viscosity of 2 10 16 pa s consistent with rock properties the tidal dissipation rate for this model is largest along the polar axis integration of the dissipation rate over radius gives a surface heat flow with maxima at the poles and minima near the sub jovian and anti jovian points in contrast the dissipation rate in a molten or partially molten incompressible asthenosphere has maxima near the equator and is close to zero at the poles the asthenosphere dissipation rate is mainly a function of asthenosphere viscosity and thickness the thinner the asthenosphere the smaller is the required viscosity a 50 km thick asthenosphere requires a viscosity of about 10 8 pa s a strong south polar hot spot if it can be confirmed by infrared observation favors dissipation in the mantle but does not exclude dissipation in a partially molten asthenosphere equatorial hot spots and the concentration of prometheus type plumes near the equator favor asthenosphere heating but do not rule out deep mantle dissipation both asthenosphere and deep mantle heating may contribute to io s surface heat flow the properties of magma transport through the lithosphere and the planform of mantle convection might exert a control on the surface expression of heating at greater depth

coupled w os pt isotope systematics in ivb iron meteorites in situ neutron dosimetry for w isotope chronology tungsten isotope compositions of magmatic iron meteorites yield ages of differentiation that are within 2 ma of the formation of cais with the exception of ivb irons that plot to systematically less radiogenic compositions yielding erroneously old ages secondary neutron capture due to galactic cosmic ray gcr irradiation is known to lower the e182w of iron meteorites adequate correction of which requires a measure of neutron dosage which has not been available thus far the w os and pt isotope systematics of 12 of the 13 known ivb iron meteorites were determined by mc icp ms w os pt and tims os on the same dissolutions that yield precise e182w stable os and pt isotopes were determined as in situ neutron dosimeters for empirical correction of the ubiquitous cosmic ray induced burn out of 182w in iron meteorites the w isotope data reveal a main cluster with e182w of 3 6 but a much larger range than observed in previous studies including irons weaver mountains and warburton range that show essentially no cosmogenic effect on their e182w the ivb data exhibits resolvable negative anomalies in e189os 0 6e and complementary e190os anomalies 0 4e in tlacotepec due to neutron capture on 189os which has approximately the same neutron capture cross section as 182w and captures neutrons to produce 190os the least irradiated ivb iron warburton range has e189os and e190os identical to terrestrial values similarly pt isotopes which are presented as e192pt e194pt and e196pt range from 4 4e to 53e 1 54e to 0 32e and 0 73e to 0 20e respectively also identify tlacotepec and dumont as the most gcr damaged samples in w os and w pt isotope space the correlated isotope data back project toward a 0 epsilon value of e192pt e189os and e190os from which a pre gcr irradiation e182w of 3 42 0 09 2 is derived this pre gcr irradiation e182w is within uncertainty of the currently accepted cai initial e182w the pt and os isotope correlations in the ivb irons are in good agreement with a nuclear model for spherical irons undergoing gcr spallation although this model over predicts the change of e182w by 2 indicating a need for better w neutron capture cross section determinations a nucleosynthetic effect in e184w in these irons of 0 14 0 08 is confirmed consistent with the presence of mo and ru isotope anomalies in ivb irons the lack of a non gcr os isotope anomaly in these irons requires more complex explanations for the production of w ru and mo anomalies than nebular heterogeneity in the distribution of s process to r process nuclides

towards a revised chronostratigraphy of loess in austria with respect to key sections in the czech republic and in hungary abstract the stratigraphic ages of some well known and characteristic paleosols in austrian and hungarian loess sections have been questioned by several workers since the mid 1970s in particular a generally accepted correlation of austrian and hungarian loess stratigraphies with the well established czech loess stratigraphy has not been put forward this paper presents 41 new thermoluminescence tl dates and the first results from amino acid chronology of key sections in these areas the results are in agreement and suggest a new interregional correlation of loess paleosol sequences for loess older than the last interglacial quartz and feldspar inclusions yield more reliable tl ages than the polymineralic fine grain fraction

climate variations in europe over the past 140 kyr deduced from rock magnetism rapid shifts in climate during the last glacial are now well documented particularly from the oxygen isotope records of the two greenland ice cores grip1 2 and gisp23 in the grip record1 2 these climate events are also seen during the preceding eemian interglacial which may be an analogue for the future climate warmed by the greenhouse effect but these shifts are not found in the eemian section of the gisp2 core3 casting doubt on whether the rapid shifts in the grip oxygen isotope record really do represent a climate signal here we present magnetic susceptibility pollen and organic carbon records from maar lake deposits in the massif central france these data provide an independent record of past climate and we find that they correlate well with the ice core records during the last glacial during the eemian two rapid cooling events seen in our record also correlate with those seen in the grip ice core supporting the idea that rapid climate change did occur in the eemian interglacial and demonstrating that it extended to continental europe

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

pollen analytical biostratigraphy of the last five climatic cycles from a long continental sequence from the velay region massif central france in the volcanic region of velay massif central france lake sediment sequences derived from maar craters situated close to one another ribains praclaux and lac du bouchet have been correlated on the basis of tephrostratigraphical analyses this has enabled the construction of a sequence 2980 pollen spectra that begins during the glaciation preceding the holsteinian and ends in the present this sequence covers the period from 450 ka i e from the end of marine oxygen isotope stage 12 to the present time and includes five climatic cycles the study of a second core from the praclaux site completed the long velay sequence the biostratigraphy of the whole sequence is described in detail and illustrated by a synthetic and simplified pollen diagram thirteen temperate forest episodes interglacials or interstadials and as many cold periods glacials or stadials are defined each temperate episode is characterised by particular forest development and vegetation dynamics and all of them with the exception of the holocene begin and end with a pinus forest the vegetation dynamics observed during the holocene resemble more closely those recorded during the interstadials than during the interglacials copyright 2000 john wiley sons ltd

sulfur contents of the parental metallic cores of magmatic iron meteorites abstract magmatic iron meteorites are thought to be samples of the central metallic cores of asteroid sized parent bodies sulfur is believed to have been an important constituent of these parental cores but due to the low solubility of s in solid metal initial s contents for the magmatic groups cannot be determined through direct measurements of the iron meteorites however experimental solid metal liquid metal partition coefficients show a strong dependence on the s content of the metallic liquid thus by using the experimental partition coefficients to model the fractional crystallization trends within magmatic iron meteorite groups the s contents of the parental cores can be indirectly estimated modeling the au ga ge and ir fractionations in four of the largest magmatic iron meteorite groups leads to best estimates for the s contents of the parental cores of 12 1 5 wt for the iiiab group 17 1 5 wt for the iiab group and 1 1 wt for the ivb group the iva elemental fractionations are not adequately fit by a simple fractional crystallization model with a unique initial s content these s content estimates are much higher than those recently inferred from crystallization models involving trapped melt the discrepancy is due largely to the different partition coefficients that are used by the two models when only partition coefficients that are consistent with the experimental data are used the trapped melt model and the low s contents it advocates cannot match the ge and ir fractionations that are observed in iiiab iron meteorites

first discovery of stishovite in an iron meteorite the first occurrence of stishovite in an iron meteorite muonionalusta group iva is reported the mineral occurs intimately mixed with amorphous silica forming tabular grains up to 3 mm wide with a hexagonal outline it was identified using x ray diffraction and raman microspectroscopy the unit cell parameters of stishovite are a 4 165 3 a and c 2 661 6 a and its chemical composition is nearly pure sio2 raman spectra show relatively sharp bands at 231 and 754 cm 1 and a broad band with an asymmetric shape and a maximum around 500 cm 1 the rare grains are found within troilite nodules together with chromite daubreelite and schreibersite from their composition and morphology and by comparisons with silica inclusions in e g the gibeon iva iron we conclude that these rare grains represent pseudomorphs after tridymite the presence of stishovite in muonionalusta is suggested to reflect shock metamorphic conditions in the iva parent asteroid during a cosmic impact event

the origin of chondrules and chondrites debris from low velocity impacts between molten planetesimals we investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin shelled largely molten planetesimals collided at low speed during accretion this scenario here dubbed splashing stems from evidence that such planetesimals intensely heated by 26al were abundant in the protoplanetary disk when chondrules were being formed approximately 2 myr after calcium aluminum rich inclusions cais and that chondrites far from sampling the earliest planetesimals are made from material that accreted later when 26al could no longer induce melting we show how splashing is reconcilable with many features of chondrules including their ages chemistry peak temperatures abundances sizes cooling rates indented shapes relict grains igneous rims and metal blebs and is also reconcilable with features that challenge the conventional view that chondrules are flash melted dust clumps particularly the high concentrations of na and feo in chondrules but also including chondrule diversity large phenocrysts macrochondrules scarcity of dust clumps and heating we speculate that type i feo poor chondrules come from planetesimals that accreted early in the reduced partially condensed hot inner nebula and that type ii feo rich chondrules come from planetesimals that accreted in a later or more distal cool nebular setting where incorporation of water ice with high 17o aided oxidation during heating we propose that multiple collisions and repeated re accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties and led to so called complementarity and metal depletion in chondrites we suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation

182w and 187re 187os systematics of iron meteorites chronology for melting differentiation and crystallization in asteroids w isotopic compositions measured by n tims in five iron meteorite groups are used in conjunction with 187re 187os isotopic data to constrain the timing of metal segregation and crystallization respectively in the iron meteorite parent bodies fractionation of the parent isotope 182hf half life 9 2 ma from the daughter 182w likely occurred during segregation of metal from silicate in the iron meteorite parent bodies if w isotopic equilibrium was achieved between silicate and metal during metal segregation then the w isotopic compositions in the irons should record the timing of this event samples were taken from the five largest iron meteorite groups including the four magmatic groups iiab iiiab iva and ivb and the nonmagmatic iab iiicd group together these groups comprise approximately 90 by weight of iron meteorites classified so far with the exception of the iva group the groups have well defined 187re 187os isochron ages which are interpreted as the time of crystallization of the metal of their parent bodies within each magmatic group samples that crystallized both early and late in the sequence of each core were analyzed to evaluate possible evolution of the w isotopic composition during crystallization all the samples examined display very similar w values within a narrow range of compositions between 5 1 and 3 1 witzation began less than a few percent of additional metal was segregated to the cores moreover the average w values for the five groups have no resolvable difference ranging from 4 08 0 69 iva to 3 30 0 15 iab iiicd these data indicate that all of the iron meteorite parent bodies underwent metal segregation within 5 ma of each other apparently independent of their composition size and time of crystallization of the metal in all groups except the iiiab group there is a resolvable time interval between the timing of metal segregation as constrained by the w isotopic data and the initiation of core crystallization as given by the re os isotopic systematics

silver isotope variations in chondrites volatile depletion and the initial 107pd abundance of the solar system abstract the extinct radionuclide 107pd decays to 107ag half life of 6 5 ma and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system here a comprehensive ag isotope study of carbonaceous and ordinary chondrites is presented carbonaceous chondrites show limited variations e107ag 2 1 to 0 8 in ag isotopic composition that correlate with the pd ag ratios assuming a strictly radiogenic origin of these variations a new initial 107pd 108pd of 5 9 2 2 10 5 for the solar system can be deduced comparing the pd ag and mn cr data for carbonaceous chondrites suggests that mn cr and pd ag fractionation took place close to the time of calcium aluminium rich inclusion cai and chondrule formation 4568 ma ago using the new value for the initial 107pd abundance the revised ages for the iron rich meteorites gibeon iva 8 5 3 2 4 6 ma grant iiiab 13 0 3 5 4 9 ma and canyon diablo ia 19 5 24 1 10 4 ma are consistent with cooling rates and the closure temperature of the pd ag system in contrast to carbonaceous chondrites ordinary chondrites show large stable isotope fractionation of order of 1 permil for 107ag 109ag this indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites nebular processes and accretion as experienced by carbonaceous chondrites did not led to significant ag isotope fractionation while the significant ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism

thermal and impact history of the h chondrite parent asteroid during metamorphism constraints from metallic fe ni abstract we have studied cloudy taenite metallographic cooling rates and shock effects in 30 h3 6 chondrites to elucidate the thermal and early impact history of the h chondrite parent body we focused on h chondrites with old ar ar ages 4 4 gyr and unshocked and mildly shocked h chondrites as strongly shocked chondrites with such old ages are very rare cooling rates for most h chondrites at 500 c are 10 50 c myr and do not decrease systematically with increasing petrologic type as predicted by the onion shell model in which types 3 5 are arranged in concentric layers around a type 6 core some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types contrary to the onion shell model cloudy taenite particle sizes which range from 40 to 120 nm are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating the three h4 chondrites that were used to develop the onion shell model ste marguerite beaver creek and forest vale cooled through 500 c at 5000 c myr our thermal modeling shows that these rates are 50 higher than could be achieved in a body that was heated by 26al and cooled without disturbance by impact published ar ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating impacts during metamorphism created portales valley and two other h6 chondrites with large metallic veins excavated the fast cooled h4 chondrites around 3 4 myr after accretion and mixed petrologic types metallographic data do not require catastrophic disruption by impact during cooling

energetics of asteroid dynamos and the role of compositional convection 1 the conditions under which a dynamo can operate in the core of a small planetary body or asteroid are examined compositional convection driven by inner core growth is thermodynamically much more efficient than thermal convection at driving a dynamo but whether asteroid cores crystallize in this fashion is currently uncertain inner core solidification will drive dynamo activity in cores larger than 50 150 km in radius dynamo activity requires core cooling rates exceeding 0 001 0 1 k my if compositional convection occurs in the absence of an inner core cooling rates of 1 100 k my or heating by 60fe within 10 20 myr of solar system formation are required to drive a dynamo if inner core growth is important as for the ivb iron meteorite parent body then a dynamo should develop with magnetic paleointensities that depend on the core sulphur content if 60fe decay is dominant the frequency of asteroid dynamo occurrence is predicted to decay with distance from the sun

the early formation of the iva iron meteorite parent body the iva iron meteorites are magmatic cumulates from the core of a small asteroid which broke apart similar to 400 ma ago as the depletion of this planetary body in volatile elements is expected to be reflected in high u pb ratios of its minerals we embarked on analyzing the isotope composition of pb in the cm sized troilite inclusions of muonionalusta and gibeon both iron meteorites belonging to the iva family the bulk of the data for nine troilite subsamples of muonionalusta scatter with an apparent age of 4 57 ga which is shown to reflect the presence of both primordial and common terrestrial pb components in addition to radiogenic pb the most radiogenic subsample however has fractions with pb 206 pb 204 ratios as high as over 1000 and gives a statistically significant 207 pb pb 206 age of 4565 3 0 1 ma mswd 0 08 consistent with the hf 182 w 182 metal silicate segregation age of 2 4 2 0 ma these data make the age of muonionalusta the oldest documented yet for all differentiated bodies in the solar system and constitute the first high precision pb pb age determined for crystallization of a phase contained within an iron meteorite group hence advancing our understanding of early solar system chronology using literature values for the cooling rate and assuming a closure temperature for pb of 300 degrees c it is further estimated that the iva parent body accreted within 1 ma of cai formation and had cooled to the pb closure temperature within an additional 1 2 ma the overlap between the high precision pb pb and hf w ages points to a small or rapidly fragmented planetary body the isotopic composition of pb in gibeon troilite yields a significantly younger age of 4544 7 ma mswd 1 5 consistent with evidence from the pd 107 ag 107 chronometer but we believe this age has been reset by melting upon shock one puzzling observation is that the apparent th 232 u 238 of muonionalusta troilite is particularly low similar to 0 32 requiring a mechanism capable of efficiently fractionating th from u presumably the reduction of u to its trivalent form or the crystallization of phosphate average pb concentrations of the order of 5 10 ppb and high u 238 pb 204 ratios of 1000 require u concentrations in troilite in the sub ppb to one ppb range this may indicate that troilite inclusions in iva meteorites do not represent metal sulfide unmixing but rather correspond to late stage s rich liquid residues from extreme crystallization of the interstitial melts

pb isotopic age of the allende chondrules we have studied pb isotope systematics of chondrules from the oxidized cv3 carbonaceous chondrite allende the chondrules contain variably radiogenic pb with a 206pb 204pb ratio between 19 5 268 pb pb isochron regression for eight most radiogenic analyses yielded the date of 4566 2 2 5 ma internal residue leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566 6 1 0 ma our best estimate for an average age of allende chondrule formation this pb pb age is consistent with the range of model 26al 26mg ages of bulk allende chondrules reported by bizzarro et al 2004 and is indistinguishable from pb pb ages of ca al rich inclusions cais from cv chondrites 4567 2 0 6 ma amelin et al 2002 and the oldest basaltic meteorites we infer that chondrule formation started contemporaneously with or shortly after formation of cv cais and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids the entire period of chondrule formation lasted from 4566 6 1 0 ma allende to 4564 7 0 6 ma cr chondrite acfer 059 to 4562 7 0 5 ma cb chondrite gujba and was either continuous or consisted of at least three discrete episodes since chondrules in cb chondrites appear to have formed from a vapor melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated krot et al 2005 there were possibly a variety of processes in the early solar system occurring over at least 4 5 myr that we now combine under the umbrella name of chondrule formation

hf w evidence for rapid differentiation of iron meteorite parent bodies abstract new high precision w isotope data on iron meteorites are presented that provide important constraints on the timing of silicate metal segregation in planetesimals magmatic iron meteorites all have e 182 w within error or less radiogenic than initial e 182 w estimated by studies of chondritic meteorites at face value this implies that iron meteorites are as old and older than refractory calcium aluminium rich inclusions cai which are widely thought to be the oldest solar system objects moreover different meteorites from the same magmatic groups believed to be derived from the same planetissimal core display a range of e 182 w we suggest that the paradoxical e 182 w values more negative than initial solar system initial ssi are most readily explained as a result of secondary spallation reactions with cosmic rays during transit between parent body and the earth this is supported by the most negative e 182 w being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions on the other hand it is also striking that none of the magmatic iron group meteorites have e 182 w analyses outside error more radiogenic than the estimated solar system initial ratio this suggests that core formation in parent bodies of magmatic iron meteorites occurred 1 5 myr after the formation age of cai y amelin a n krot i d hutcheon and a a ulyanov lead isotopic ages of chondrules and calcium aluminum inclusions science 297 1678 1683 2002 this extremely early metal silicate differentiation is coeval with the first chondrules m bizzarro j a baker and h haack mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions nature 431 275 278 2004 a n krot y amelin p cassen and a meibom young chondrules in cb chondrites from a giant impact in the early solar system nature 436 989 992 2005 formation of later chondrules and hence the parent bodies of some chondritic meteorites must therefore have occurred in the presence of planetesimals large enough to possess iron cores we conclude that early planetary accretion and differentiation was sufficiently fast for 26 al decay to be an important heat source non magmatic iron meteorites however display more radiogenic and varied w isotope signatures this is in keeping with them being generated later by impact melting during which the metal partially re equilibrated with the then more radiogenic silicate fraction

pedogenesis and paleoclimate interpretation of the magnetic susceptibility record of chinese loess paleosol sequences in the chinese loess paleosol sequences from luochuan haanxi province china variations in the magnetic susceptibility have been accepted as an excellent proxy for paleoclimate and the standard interpretation is that climatic processes have enhanced the magnetic susceptibility of the paleosols beyond the base line level of the loess by using mineral magnetic properties and a soil chemistry extraction procedure we show that most of the magnetic susceptibility signal in both the paleosols and the loess is due to pedogenesis in effect loess deposition and pedogenesis are competing processes at all times and the presence of a paleosol simply indicates that the latter process was predominant these results imply that any interpretation of the paleoclimate record of loess paleosol sequences must recognize the pedogenic nature of the magnetic susceptibility signal and that the focus of future research must be on the critical role of climate in the development of the paleosols

pd ag chronometry of iron meteorites correction of neutron capture effects and application to the cooling history of differentiated protoplanets abstract the short lived 107 pd 107 ag system is a versatile tool for dating iron meteorites but neutron capture reactions during cosmic ray exposure might have modified ag isotope compositions these cosmic ray induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and therefore could bias the age information inferred from pd ag isotope systematics our new combined pd ag and pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray induced downward shifts of 107 ag 109 ag which preclude the determination of pd ag isochrons based on measured ag isotope compositions for the strongly irradiated iron meteorites ainsworth iiab and carbo iid these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from 107 pd decay for the less strongly irradiated iiiab iron meteorites boxhole grant and henbury the cosmic ray induced shifts are smaller than the radiogenic 107 ag excesses but are nevertheless significant we have developed a method to quantify the cosmic ray induced ag isotope shifts using a neutron capture model and pt isotope compositions as the neutron dose monitor after correction pd ag isochrons are obtained for all investigated iron meteorites even for the most strongly irradiated samples the pd ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray induced shifts on measured ag isotope compositions the pd ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few ma after core formation and solar system formation such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies the collisions would have facilitated rapid cooling below pd ag isotopic closure and so in this case the pd ag ages would effectively date the time of the collisions

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

uranium isotope ratios of muonionalusta troilite and complications for the absolute age of the iva iron meteorite core abstract the crystallization ages of planetary crustal material given by basaltic meteorites and planetary cores given by iron meteorites provide fiducial marks for the progress of planetary formation and thus the absolute ages of these objects fundamentally direct our knowledge and understanding of planet formation and evolution the lone precise absolute age of planetary core material was previously obtained on troilite inclusions from the iva iron meteorite muonionalusta this previously reported pb pb age of 4565 3 0 1 ma assuming a 238u 235u 137 88 only post dated the start of the solar system by approximately 2 3 million years and mandated fast cooling of planetary core material since an accurate pb pb age requires a known 238u 235u of the sample we have measured both 238u 235u and pb isotopic compositions of troilite inclusions from muonionalusta the measured 238u 235u of the samples range from 137 84 to as low as 137 22 however based on pb and u systematics terrestrial contamination appears pervasive and has affected samples to various extents for pb and u the cause of the relative 235u excess in one sample does not appear to be from terrestrial contamination or the decay of short lived 247cm but is more likely from fractionation of u isotopes during metal silicate separation during core formation exacerbated by the extreme u depletion in the planetary core due to limited pb isotopic variation and terrestrial disturbance no samples of this study produced useful age information however the clear divergence from the previously assumed 238u 235u of any troilite in muonionalusta introduces substantial uncertainty to the previously reported absolute age of the sample without knowledge of the 238u 235u of the sample uncertainties associated with u isotope heterogeneity do not allow for definition of a robust age of solidification and cooling for the iva core however one sample of this work paired with previous work using short lived radionuclides suggests that the cooling age of the iva core may be significantly younger than previously thought this work indicates the metallic cores of protoplanetary bodies solidified no earlier than the first 5 10 million years of the solar system

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

group iva irons new constraints on the crystallization and cooling history of an asteroidal core with a complex history we report analyses of 14 group iva iron meteorites and the ungrouped but possibly related elephant moraine eet 83230 for siderophile elements by laser ablation icp ms and isotope dilution eet was also analyzed for oxygen isotopic composition and metallographic structure and fuzzy creek currently the iva with the highest ni concentration was analyzed for metallographic structure highly siderophile elements hse re os and ir concentrations vary by nearly three orders of magnitude over the entire range of iva irons while ru pt and pd vary by less than factors of five chondrite normalized abundances of hse form nested patterns consistent with progressive crystal liquid fractionation attempts to collectively model the hse abundances resulting from fractional crystallization achieved best results for 3 wt s compared to 0 5 or 9 wt s consistent with prior studies concentrations of hse and other refractory siderophile elements estimated for the bulk iva core and its parent body are in generally chondritic proportions projected abundances of pd and au relative to more refractory hse are slightly elevated and modestly differ from l ll chondrites which some have linked with group iva based on oxygen isotope similarities abundance trends for the moderately volatile and siderophile element ga cannot be adequately modeled for any s concentration the cause of which remains enigmatic further concentrations of some moderately volatile and siderophile elements indicate marked progressive depletions in the iva system however if the iva core began crystallization with 3 wt s depletions of more volatile elements cannot be explained as a result of prior volatilization condensation processes the initial iva core had an approximately chondritic ni co ratio but a fractionated fe ni ratio of 10 indicates an fedepleted core this composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron consistent with an oxidized parent body the depletions in ga may reflect decreased siderophilic behavior in a relatively oxidized body and more favorable partitioning into the silicate portion of the parent body phosphate inclusions in eet show d 17 o values within the range measured for silicates in iva iron meteorites eet has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite chemical and isotopic evidence for a genetic relation between eet and group iva is strong but the high ni content and the newly determined rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped previously reported metallographic cooling rates for iva iron meteorites have been interpreted to indicate an inwardly crystallizing 150 km radius metallic body with little

u pb ages of angrites abstract precise u pb ages determined with double spike 202 pb 205 pb thermal ionization m1ass spectrometry are reported for angrites angra dos reis ador lewis cliff 86010 lew and d orbigny nineteen of 23 acid washed pyroxene fractions from these meteorites and whole rock fractions from d orbigny contain between 0 5 and 1 3 pg of total common pb indistinguishable from analytical blank measured 206 pb 204 pb ratios in these fractions are between 6300 and 14 100 for ador 1160 4500 for lew and 608 8500 for d orbigny blank corrected 206 pb 204 pb ratios for all three meteorites vary from 2160 to over 100 000 these fractions yielded precise and reproducible 207 pb 206 pb dates with the average values of 4557 65 0 13 ma for ador 4558 55 0 15 ma for lew and 4564 42 0 12 ma for d orbigny pb pb isochrons including data with slightly elevated common pb and u pb upper concordia intercepts yield similar dates the implications of these new pb isotopic ages of angrites are threefold first they demonstrate that ador and lew are not coeval and the group of slowly cooled angrites is therefore genetically diverse second the new age of lew suggests an upward revision of 53 mn 53 cr absolute ages by 0 7 ma third a precise age of d orbigny allows consistent linking of the 53 mn 53 cr and 26 al 26 mg extinct nuclide chronometers to the absolute lime scale

formation ages and evolution of shergotty and its parent planet from u th pb systematics the isotopic composition of lead was determined in different phases from shergotty zagami and eeta 79001 using phases with low 238 u 204 pb the initial leads of these meteorites could be well defined and are distinct from each other and have high 204 pb 206 pb from 0 0652 to 0 0739 these leads evolved in different reservoirs over most of solar system history in a low 238 u 204 pb 5 environment it follows that the parent planet of the shergottites has a high 204 pb concentration relative to u and must also be high in other volatiles if the parent planet of the shergottites is mars then this planet must be volatile rich compared to the earth by a factor of 1 8 this implies that mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed martian upper mantle the u th pb isotopic relationships involving phases with high 238 u 204 pb are quite regular and give ages for three shergottites which indicate an event of u th pb fractionation at 200 m y the initial leads at that time were highly evolved relative to the primordial lead the absence of substantial additions of radiogenic lead above the initial lead in high u phases precludes an older age for the u th pb fractionation however details of the data arrays are complex the young age is in general agreement with some of the ages obtained by other methods but precise concordance between the different methods is not established it is suggested that the shock event that produced maskelynite may possibly be associated with the late stage breakup at 2 5 m y and not necessarily related to the 180 m y record the major event of chronometer resetting is taken to be 180 m y this event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes

a new discovery of mesoproterozoic erathem oil and oil source correlation in the niuyingzi area of western liaoning province ne china abstract the yanliao fault depression belt is an important area for precambrian oil and gas exploration in china through drilling of the niu d1 well in the niuyingzi area located in western liaoning province ne china oil shows were discovered in the mesoproterozoic gaoyuzhuang carbonate reservoirs in this study we discuss the geochemical characteristics of the potential source rocks and oil bearing rocks as well as the relationship between the crude oil in the niu d1 well and these source rocks on the basis of organic geochemical analyses including biomarker compounds and stable carbon isotopes the crude oil hosted in the limestone at depths of 341 65 362 00 m in the niu d1 well underwent moderate biodegradation and shows three characteristic features i the presence of the complete c18 c23 13 n alkyl tricyclic terpane series ii high abundances of the drimane series c19 c21 tricyclic terpanes c24 tetracyclic terpane and diahopanes and low abundance of gammacerane and iii depleted stable carbon isotopes of saturated and aromatic hydrocarbons less than 30 all of these features suggest that the crude oil in the limestones of the niu d1 well originated mainly from the organic rich hongshuizhuang formation shale considering the lack of effective neoproterozoic source rock the detection of diversified steranes implies a degree of contamination from phanerozoic sources with eukaryotic biological input this is the first time that primary crude oil of mesoproterozoic age has been discovered from wells drilled in the yanliao fault depression belt and demonstrates that mesoproterozoic carbonates in this area have good hydrocarbon exploration potential