bombardment history of asteroid 4 vesta recorded by brecciated eucrites large impact event clusters at 4 50 ga and discreet bombardment until 3 47 ga abstract the thermal impact histories of sixteen eucrite meteorites were investigated three monomict eucrites nwa 999 a 87272 87 and stannern five polymict eucrites nwa 1000 nwa 1666 nwa 5601 y 980066 100 and y 980255 100 three quench textured eucrite melt rocks y 981646 21 y 981651 105 and mil 0766214 one eucrite dominantly comprised of quench textured clasts que 99005 11 three unclassified eucrite breccias lap 031316 9 lar 06870 5 que 99799 4 and one unbrecciated eucrite eet 92004 17 included here due to its shock features we have measured fifteen high precision new 40 ar 39 ar plateau ages on plagioclase and matrix for ten of these meteorites with a tight cluster of nine ages obtained from three different polymict breccias these ages range from 4534 56 ma to 4491 16 ma resulting in a concordant age population p 0 16 the fact that such a cluster of ages is recorded in unrelated breccias which are made of a priori unrelated components leads us to propose that those ages recorded a single heating event on a large scale and is interpreted here as a high energy impact event early in the history of vesta at 4500 4 ma we propose that the debris was ejected and isolated from subsequent large impacts in a secondary rubble pile asteroid where the energy of the outgoing shock wave from an impact is significantly reduced as it compacts the target material holsapple et al 2002 and references therein the other analyses define a spread of five plateau ages ranging from 3851 21 ma to 3469 35 ma over 380 ma an additional apparent plateau age of 4288 38 ma but with a diffusion profile of cumulative 39 ar release along with published u pb apatite age of 4 14 ga suggests that the data might either define a true continuum normal background bombardment from 4 5 ga to 3 47 ga or cluster between 3 85 ga and 3 47 ga excavation of a fresh surface at 3 85 ga continuously bombarded until 3 47 ga both scenarios are compatible with a final ejection age of 3 47 ga when a major impact liberates the bulk of the brecciated meteorites into another secondary rubble pile asteroid where the brecciated eucrites stayed relatively well protected from subsequent major impacts based on these results and recent crater counting measurements we propose that the excavation and bulk ejection were caused by the rheasilvia ca 3 47 ga basin forming impact diffusion models on plagioclase crystals with different ar age spectrum signatures from a single breccia nwa1666 4501 7 ma suggest that either 1 the formation of the breccia is very young or 2 different plagioclase crystals have different diffusion characteristics and or 3 the porosity caused heterogeneous temperatures during an impact heating event particularly likely if the 4 5 ga brecciated eucrites were stored in a rubble pile asteroid many or possibly most large asteroids being re accumulated rubble piles with potential large porosity holsapple et al 2002 scenarios 2 and 3 preclude the usage of multi grain aliquots to decipher the time temperature history of most impact breccia using 40 ar 39 ar thermochronology

frictional melting processes in planetary materials from hypervelocity impact to earthquakes frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip within planetary materials which are mainly natural ceramics frictional melting occurs at high strain rates typically 10 2 s 1 and at slip velocities greater than 0 1 m s 1 t he pathway to friction melting is controlled by the mechanical properties of a rock s constituent minerals especially fracture toughness minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt the product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix this cools to form the rock type known as pseudotachylyte and at even higher strain rates it forms shock veins in meteorites and in impact craters which may contain high pressure mineral polymorphs the generation of melt on sliding surfaces can lubricate earthquake faults facilitate the post shock modification of impact craters and make landslides more hazardous

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

numerical modelling of impact melt production in porous rocks abstract melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure shock melted material is found in many terrestrial craters however most impact craters in sedimentary targets which are typically porous have been reported as containing significantly less melt than craters in nonporous crystalline targets to investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component to demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with hugoniot data for porous quartzite coconino sandstone and calcite our results show that the presence of porosity significantly reduces the critical pressure required for melting which enhances melt production we show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials so that all other things being equal impact melt production is greater when target porosity is higher we conducted a series of simulations of impacts into targets of different material composition quartzite calcite dunite and porosity of up to 50 and varied the impact velocity between 6 and 50 km s we found a significant increase in the production of impact melt with increasing velocity in agreement with several previous studies and quantify the net increase in impact melt due to target porosity for the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size 7 90 km in diameter in porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters however our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary or mixed sedimentary and crystalline targets this further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks

the problem of inherited 40ar in dating impact glass by the 40ar 39ar method evidence from the tswaing impact crater south africa abstract the tswaing meteorite impact crater is a 1 13 km diameter structure located in the 2 05 ga nebo granite of the bushveld complex the impact age had previously been determined by fission track dating to 220 104 ka 40ar 39ar step heating and total fusion experiments performed on single and multi grain impact glass aliquots gave apparent ages ranging from 1 0 0 3 ma to 204 6 ma these ages indicate that the radiogenic ar derived from the target rocks has not been completely degassed as a result of the impact process despite fusion of the target material results of step heating experiments imply that the 40 ar inherited trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid vapor inclusions or un melted residual clasts calculations assuming an age of 0 2 0 1 ma for tswaing fission track data reveal that the amount of inherited 40ar 40 ar inherited relative to the pre impact concentration varies from 0 015 to 4 15 the spread defined by 40 ar inherited likely reflects the various quench rates experienced by the glass most certainly due to the pre impact position of the sample relative to the center of the crater we compare the influence of 40 ar inherited on the apparent 40ar 39ar age determination of five impact structures our calculations show that the main characteristic controlling the age offset for a given proportion of 40 ar inherited is the age difference between the impact and the target rocks i e the 40ar concentration in the target rock the buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself the occurrence of 40 ar inherited is likely influenced by 1 the degree of polymerization i e silicate structure complexity of the target rock and presumably related to the diffusivity of ar in the melt and glass 2 the ar partial pressure at the grain boundary 3 the quantity of energy involved in the impact and 4 the porosity of the target rocks for glass that inevitably suffers inherited and or excess 40ar the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment

argon diffusion in pyroxenes implications for thermochronometry and mantle degassing abstract the rate at which argon diffuses in pyroxenes is relevant to several problems in the earth and planetary sciences pyroxenes are among the most abundant minerals in the earth s upper mantle and are important hosts for potassium and radiogenic 40 ar 40 ar additionally pyroxenes commonly contain 25 of the total 40 ar within meteorites and therefore hold great potential for quantifying the timing duration and thermal conditions of events experienced by meteorites and their parent bodies we conducted detailed feedback controlled laser heating diffusion experiments using synthetically produced 37 ar in gem quality clinopyroxene cpx and orthopyroxene opx crystals stepwise heating between 850 and 1350 c yielded linear arrhenius arrays from which activation energies e a of 379 2 4 1 and 371 0 6 0 kj mol and pre exponential factors d o of 1 36 10 4 6 15 10 5 1 12 10 4 and 5 73 10 2 2 25 10 2 3 72 10 2 were determined for cpx and opx respectively these e a s are an order of magnitude higher than previously reported values obtained with a different experimental approach watson et al 2007 and indicate that ar diffusion within pyroxenes is strongly temperature dependent at high temperatures ar diffusion in pyroxenes is rapid which indicates that diffusive 40 ar distributions observed within meteoritic pyroxenes may provide information about brief high temperature shock heating events in contrast subsequent low t conditions should minimally influence these concentration gradients the experimental ar diffusion kinetics in opx and cpx correspond to closure temperatures of 600 700 and 700 800 c respectively for 10 c ma cooling these results suggest that diffusive equilibration through 0 1 0 5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures 1300 c and is therefore unlikely to inhibit mantle degassing

cosmic ray exposure age and preatmospheric size of the bunburra rockhole achondrite abstract bunburra rockhole is the first meteorite fall photographed and recovered by the desert fireball network in australia it is classified as an ungrouped achondrite similar in mineralogical and chemical composition to eucrites but it has a distinct oxygen isotope composition the question is if achondrites like bunburra rockhole originate from the same parent body as the howardite eucrite diogenite hed meteorites or from several separate differentiated parent bodies to address this question we measured cosmogenic radionuclides and noble gases in the bunburra rockhole achondrite the short lived radionuclides 22na and 54mn confirm that bunburra rockhole is a recent fall the concentrations of 10be 26al and 36cl as well as the 22ne 21ne ratio indicate that bunburra rockhole was a relatively small object r approximately 15 cm in space consistent with the photographic fireball observations the cosmogenic 38ar concentration yields a cosmic ray exposure cre age of 22 3 myr whereas 21ne and 3he yield approximately 30 and approximately 60 lower ages respectively due to loss of cosmogenic he and ne mainly from plagioclase with a cre age of 22 myr bunburra rockhole is the first anomalous eucrite that overlaps with the main cre peak of the hed meteorites the radiogenic k ar age of 4 1 gyr is consistent with the u pb age while the young u th he age of approximately 1 4 gyr indicates that bunburra rockhole lost radiogenic 4he more recently

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

a record of impacts preserved in the lunar regolith abstract the absolute chronology of meteoroid impacts on the moon is largely quantified by only a few 40 ar 39 ar plateau ages of rocks thought to be associated with specific impact events stoffler et al 2006 we demonstrate a more broadly applicable approach by using high resolution 40 ar 39 ar thermochronometry to investigate the physical conditions responsible for partially reset k ar systems in lunar rocks seven rocks from apollo 16 regolith sample 63503 have plateau ages of either 3 9 billion yr ga or 4 2 ga and all experienced varying degrees of partial resetting concordance between diffusion kinetics and the degree of resetting among all samples shows that these observations are best explained by a heating event 3 3 ga ago that lasted between 10 3 s at 600 c and 20 yr at 300 c we conclude that partial resetting of the k ar systems in these samples record an impact event 3 3 ga ago that mixed several preexisting ejecta units in the cayley plains if partially reset 40 ar 39 ar ages of other lunar highland samples also constrain the timing of late stage reheating associated with impact events they constitute an additional record of impacts preserved in the lunar regolith a review of existing datasets from this perspective reveals that episodic pulses in the impactor flux in the inner system are common and most likely related to dynamical events in the asteroid belt or outer solar system

solar and cosmogenic argon in dated lunar impact spherules abstract we have studied lunar impact spherules from the apollo 12 and apollo 14 landing sites examining the isotopic composition of argon released by stepwise heating elsewhere we reported the formation ages of these spherules determined by the 40ar 39ar isochron method here we discuss solar and cosmogenic argon from the same spherules separating these two components by correlating their partial releases with the releases of calcium derived 37ar on a cosmochron diagram we use the abundances of cosmogenic argon to derive a cosmic ray exposure age for each spherule and demonstrate that single scoops of lunar soil contain spherules which have experienced very different histories of exposure and burial the solar argon is seen to be separated into isotopically lighter and heavier fractions which presumably were implanted to different depths in the spherules the abundance of the isotopically heavy solar argon is too great to explain as a minor constituent of the solar particle flux such as the suprathermal tail of the solar wind the fact that the spherules have been individually dated allows us to look for possible variations in the solar wind as a function of time over the history of the solar system however the isotopic composition and fluence of solar argon preserved in the lunar spherules appear to be independent of formation age we believe that most of the spherules are saturated with solar argon having reached a condition in which implantation by the solar wind is offset by losses from solar wind sputtering and diffusion

the lappajarvi impact structure finland age duration of crater cooling and implications for early life abstract an in depth approach of 40 ar 39 ar dating of the 23 km lappajarvi impact structure finland was performed using carefully selected single grain aliquots of optically fresh clast poor impact melt rock and recrystallized k feldspar melt particles separated from impact metamorphosed granite pegmatite step heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven k feldspar samples the melt rocks yielded fully concordant ages with a weighted mean of 76 37 0 46 ma dating of the k feldspar melt particles resulted in a series of younger plateau ages ranging from 75 11 0 36 to 76 11 0 35 ma the melt rock results combined with the oldest syn melt rock k feldspar age and including all sources of uncertainties yielded a weighted mean age of 76 20 0 29 ma 2 mswd 1 02 p 0 41 which is interpreted to represent the best estimate age of the lappajarvi impact the age spread of at least 1 1 0 5 ma between the crystallization of the impact melt rocks and the k feldspar melt particles warns against the use of single 40 ar 39 ar analysis to derive a meaningful impact age in turn the age offset monitors both rapid cooling of the lappajarvi melt sheet versus slow cooling and impact induced hydrothermal circulation within the crater basement most likely driven by long lasting heat flow from the hot central uplift of the impact structure this interpretation is in line with the grain size dependent argon diffusion parameters and apparent closure temperatures of 230 410 c determined for various domain sizes of k feldspars observed in this study our study shows that even in comparatively small impact craters post impact hydrothermal activity can be estimated to last between 600 ka and 1 6 ma and is technically resolved by means of the 40 ar 39 ar dating technique therefore the longevity of hydrothermal systems in medium sized impact craters might be an order of magnitude longer than previously estimated prolonged post impact heat flow in medium sized craters in analogy to lappajarvi might have played an important role in the emergence of life on early earth and possibly mars

mechanisms of northward propagating intraseasonal oscillation a comparison between the indian ocean and the western north pacific abstract mechanisms of northward propagating intraseasonal oscillations isos over the indian ocean io and the western north pacific wnp are examined for the possibility of their existence in observations they include the following 1 the vorticity advection effect which is associated with the advection of anomalous baroclinic vorticity by mean baroclinic meridional winds 2 the vertical wind shear effect which is the vertical advection associated with the meridional gradient of baroclinic divergence and mean easterly vertical wind shear 3 the moisture advection effect induced by mean flow and 4 the air sea interaction via surface latent heat flux because of differences in mean state the influence of each mechanism on the northward propagating isos is different between the io and the wnp the vorticity advection effect is consistently found over both the io and the wnp while the air sea interaction has different impacts on the northward propagating isos over the io and the wnp the vertic

argon diffusion in plagioclase and implications for thermochronometry a case study from the bushveld complex south africa plagioclase is not only the most abundant mineral in the earth s crust but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material applying the k ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings nonetheless plagioclase has rarely been used for thermochronometry largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low k concentration here we describe an analytical technique that generates high precision 40 ar 39 ar data and quantifies ar diffusion kinetics of low k minerals we present results of five diffusion experiments conducted on single crystals of plagioclase from the bushveld complex south africa the observed diffusion kinetics yield internally consistent thermochronological constraints indicating that plagioclase is a reliable thermochronometer individual grains have activation energies of 155 178 kj mol and ln d0 a 2 varies between 3 5 and 6 5 these diffusion parameters correspond to closure temperatures of 225 300 c for a 10 c ma cooling rate age spectra generally conform to single domain diffusive loss profiles suggesting that grain scale diffusion dominates argon transport in this fairly simple plagioclase conjointly examining several single grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the bushveld complex cooled rapidly and monotonically from magmatic temperature to 300 c over 3 ma followed by protracted cooling to ambient crustal temperatures of 150 200 c over 600 ma 2009 elsevier ltd all rights reserved

timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 vesta abstract eucrites are extraterrestrial basalts and cumulate gabbros formed and subsequently more or less metamorphosed at the crustal level of the hed howardite eucrite diogenite parent body thought to be the asteroid 4 vesta unbrecciated eucrites offer the best way to understand the igneous metamorphic and cooling processes occurring in the crust of vesta following accretion since they were not substantially affected altered by secondary impact processes the 40ar 39ar system of unbrecciated eucrites should be in a relatively pristine state and thus can inform us on the early volcanic and thermal history of the hed parent body and in particular the cooling history of various crustal parts below the 300 c isotherm which represents the average closure temperature of the ar diffusion in plagioclase we analysed plagioclase and pyroxene groundmass separates of two cumulate moore county and moama and five caldera btn 00300 eet 90020 gra 98098 que 97053 equilibrated basaltic eucrites with the 40ar 39ar technique using a thermo argus vi multi collection mass spectrometer the two cumulate unequilibrated gabbros also gave cooling ages of 4531 11 ma and 4533 12 ma and combined with a fast cooling rate estimated from lamella thicknesses suggest that magmatic activity persisted up to 4533 11 and 4535 12 ma and that the plutons were intruded in a relatively shallow part of the crust above the metamorphosed regions four equilibrated eucrites yielded a well defined cluster of ages between 4523 8 ma to 4514 6 ma those ages indicate when the part of the upper crust where those eucrites probably resided 10 15 km deep cooled below 300 c at a rate of 17 3 3 6 c ma 2 such a slow cooling rate combined with available global thermal models supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows finally an age of 4531 5 ma was obtained for metamorphosed eucrite eet 90020 and combined with petrographic observations indicates the age of a major crustal excavation event by impact 40ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by 40ar 39ar

systematic variations of argon diffusion in feldspars and implications for thermochronometry abstract coupled information about the time dependent production and temperature dependent diffusion of radiogenic argon in feldspars can be used to constrain the thermal evolution attending a host of earth and planetary processes to better assess the accuracy of thermal models an understanding of the mechanisms and pathways by which argon diffuses in feldspars is desirable here we present step heating ar diffusion experiments conducted on feldspars with diverse compositions structural states and microstructural characteristics the experiments reveal systematic variations in diffusive behavior that appear closely related to these variables with apparent closure temperatures for 0 1 1 mm grains of 200 400 c assuming a 10 c ma cooling rate given such variability there is no broadly applicable set of diffusion parameters that can be utilized in feldspar thermal modeling sample specific data are required diffusion experiments conducted on oriented cleavage flakes do not reveal directionally dependent diffusive anisotropy to within the resolution limits of our approach approximately a factor of 2 additional experiments aimed at constraining the physical significance of the diffusion domain are presented and indicate that unaltered feldspar crystals with or without coherent exsolution lamellae diffuse at the grain scale whereas feldspars containing hydrothermal alteration and or incoherent sub grain intergrowths do not arrhenius plots for argon diffusion in plagioclase and alkali feldspars appear to reflect a confluence of intrinsic diffusion kinetics and structural transitions that occur during incremental heating experiments these structural transitions along with sub grain domain size variations cause deviations from linearity i e upward and downward curvature on arrhenius plots an atomistic model for arrhenius behavior is proposed that incorporates the variable lattice deformations of different feldspars in response to heating and compression the resulting implications for accurately extrapolating laboratory derived diffusion parameters to natural settings and over geologic time are discussed we find that considerable inaccuracies may exist in published thermal histories obtained using multiple diffusion domain mdd models fit to arrhenius plots for exsolved alkali feldspar where the inferred ar partial retention zones may be spuriously hot

impact history of the hed parent body ies clarified by new 40ar 39ar analyses of four hed meteorites and one anomalous basaltic achondrite abstract we investigate the thermal impact histories of four hed meteorites one cumulate eucrite lake carnegie two brecciated basaltic eucrites camel donga millbillillie one howardite old homestead 003 and an anomalous basaltic achondrite deakin 010 we have measured eight convincing new 40ar 39ar ages for three hed meteorites laser incremental 40ar 39ar analyses of carefully separated small grains two concordant 40ar 39ar plateau ages were obtained for plagioclase separates from lake carnegie resulting in a weighted mean age of combined plateau ages of 4507 20 ma millbillillie recorded three impact events at 3722 55 ma 3579 28 ma and 3313 174 ma from plagioclase matrix and pyroxene respectively this highlights the power of the k ar system to record impacts of different magnitudes experienced by a single meteorite two aliquots from camel donga returned concordant plateau ages interpreted to represent a single significant impact event at 3693 51 ma the anomalous basaltic achondrite deakin 010 did not yield any plateau and resulted in a series of minimum apparent ages with an oldest minimum age of 3 66 ga for the largest impact recorded cosmic ray exposure ages yielded apparent 38arc ages ranging from 6 to 40 ma two clusters of ages that represent significant impacts into the hed parent body may have been identified the first high temperature event is recorded by lake carnegie and other unbrecciated eucrites at 4 5 ga and is interpreted as a mega impact although it is also possible that this age is recording magmatic crystallisation due to the similarity of published sm nd and pb pb ages additionally from statistically reliable isotopic ages provided in this study it appears that the age range of major impact heating events post 4 5 ga might be more tightly restricted 3 8 3 5 ga than previously suggested 3 4 4 1 ga however a larger dataset is required before comparison with the range of the lunar heavy bombardment is justified if hed are indeed from 4 vesta we raise the possibility that the impact at 4 5 ga might be associated with the formation of the south polar veneneia basin whereas an age at 3 7 ga may represent the formation age of either veneneia or the younger rheasilvia impact structure

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

40ar 39ar thermochronology of the fossil ll6 chondrite from the morokweng crater south africa studies of meteorites are based mostly on samples that fell to earth in the recent past i e a few million years at most the morokweng ll chondrite meteorite is a particularly interesting specimen as its fall is much older ca 145 ma than most other meteorites and because it is the only macro meteorite clast width intersected in drill core 25 cm found in a melt sheet of a large impact structure when applied to the morokweng meteorite 40ar 39ar thermochronology provides an opportunity to study 1 effects associated with pre impact and post impact processes and 2 collision events within a potentially distinct and as yet unsampled asteroid population a single multi grain aliquot yielded an inverse isochron age of 625 163 ma this suggests a major in space collisional event at this time we have modeled the diffusion of 40ar within the meteorite and plagioclase during and after the 145 ma impact on earth to tentatively explain why pre terrestrial impact 40ar has been preserved within the plagioclase grains the 145 ma terrestrial impact age is recorded in the low retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 15 ma and thus confirms that age information about major terrestrial or extraterrestrial impacts can be recorded in the k rich mineral phases of a meteorite and measured by the 40ar 39ar technique more studies on fossil meteorites need to be carried out to understand if the rough 0 6 ga age proposed here corresponds to major ll chondrite asteroid population destructions or rather to an isolated collision event

common asteroid break up events of eucrites diogenites and howardites and cosmic ray production rates for noble gases in achondrites we determined the cosmic ray produced noble gas abundances in eight diogenites four eucrites and four howardites using these and additional noble gas and chemical data from the literature composition and shielding dependent production rates of achondrites for 3he 21ne 38ar 78kr 83kr and126xe were derived based on 81krkr exposure ages the cosmogenic ratio 22ne 21ne is found to be a valid shielding indicator for all howardites eucrites and diogenites heds this ratio correlates with other shielding dependent ratios such as 78kr 83kr 80kr 83kr 124xe 131xe 126xe 131xe and 128xe 131xe we show that strong correlations for these ratios also exist for basaltic lunar rocks the new production rates were used for the calculation of exposure ages for all heds with known noble gas abundances two major exposure age clusters at 21 ma and 38 ma are observed in all three classes we find that five asteroid break up events characterized by exposure ages 6 1 ma 12 2 ma 21 4 ma 38 8 ma and 73 3 ma produced all diogenites fifteen out of seventeen howardites and twenty eight out of thirty nine eucrites we conclude that asteroidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites howardites and diogenites probable parent bodies are multiple 4 vesta derived kilometer sized objects of basaltic achondritic composition reaching the 3 1 resonance region a proposed source area for meteorites the direct ejection from vesta and capture by earth of basaltic meteorites has been shown to be dynamically difficult in such a scenario large events 38 ma and 21 ma ago are required liberating simultaneously material representing vesta s surface eucrites howardites and interior regions diogenites

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

bunburra rockhole exploring the geology of a new differentiated asteroid bunburra rockhole is the first recovered meteorite of the desert fireball network we expanded a bulk chemical study of the bunburra rockhole meteorite to include major minor and trace element analyses as well as oxygen and chromium isotopes in several different pieces of the meteorite this was to determine the extent of chemical heterogeneity and constrain the origin of the meteorite minor and trace element analyses in all pieces are exactly on the basaltic eucrite trend major element analyses show a slight deviation from basaltic eucrite compositions but not in any systematic pattern new oxygen isotope analyses on 23 pieces of bunburra rockhole shows large variation in both 17o and 18o and both are well outside the hed parent body fractionation line we present the first cr isotope results of this rock which are also distinct from heds detailed computed tomographic scanning and back scattered electron mapping do not indicate the presence of any other meteoritic contaminant contamination is also unlikely based on trace element chemistry we therefore conclude that bunburra rockhole represents a sample of a new differentiated asteroid one that may have more variable oxygen isotopic compositions than 4 vesta the fact that bunburra rockhole chemistry falls on the eucrite trend perhaps suggests that multiple objects with basaltic crusts accreted in a similar region of the solar system

high velocity collisions from the lunar cataclysm recorded in asteroidal meteorites lunar samples suggest that the inner solar system was bombarded by asteroids about 4 gyr ago radiometric ages of meteorites suggest an unusual number of high velocity asteroids at this time consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits

new chronostratigraphic constraints on the yixian formation with implications for the jehol biota abstract the fossils of the cretaceous jehol biota exhibit excellent preservation and extraordinary diversity terrestrial fossils including plants insects dinosaurs birds mammals and freshwater invertebrates have been discovered from the dabeigou yixian and jiufotang formations and correlative strata in central and east asia significant specimens have been recovered from the yixian formation of inner mongolia hebei province western liaoning province of ne china the yixian formation is generally divided from bottom to top into the lujiatun unit lower lava unit jianshangou unit upper lava unit dawangzhangzi unit and jingangshan unit however previously reported age data obtained through a range of methods and applied to different units of the yixian formation do not consistently integrate with the yixian s commonly accepted stratigraphic interpretation here we summarize previous age data for the yixian formation and recalibrate previously reported 40ar 39ar ages by using new age interpretations for the same standard i e fcs 28 294 we also present new high precision 40ar 39ar ages of 125 8 1 0 ma and 126 0 0 8 ma for two basaltic samples from the lujiatun unit a unit of previously uncertain age the new age data and re calibrated published data indicate that the lujiatun unit was deposited contemporaneously with the jianshangou unit the widely accepted stratigraphic column for the yixian formation therefore requires reinterpretation our study provides significant clues for reconstructing sedimentary environment for the fossil rich yixian deposits and allows to constrain evolutionary rates for early mammals primitive birds and feathered dinosaurs

40ar 39ar impact ages and time temperature argon diffusion history of the bunburra rockhole anomalous basaltic achondrite abstract the bunburra rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse medium and fine grained lithologies petrographic observations constrain the limited shock pressure to between ca 10 gpa and 20 gpa in this study we carried out nine 40 ar 39 ar step heating experiments on distinct single grain fragments extracted from the coarse and fine lithologies we obtained six plateau ages and three mini plateau ages these ages fall into two internally concordant populations with mean ages of 3640 21 ma n 7 p 0 53 and 3544 26 ma n 2 p 0 54 respectively based on these results additional 40 ar 39 ar data of fusion crust fragments argon diffusion modelling and petrographic observations we conclude that the principal components of the bunburra rockhole basaltic achondrite are from a melt rock formed at 3 64 ga by a medium to large impact event the data imply that this impact generated high enough energy to completely melt the basaltic target rock and reset the ar systematics but only partially reset the pb pb age we also conclude that a complete 40 ar resetting of pyroxene and plagioclase at this time could not have been achieved at solid state conditions comparison with a terrestrial analog lonar crater shows that the time temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve ar data also suggest that a second medium size impact event occurred on a neighbouring part of the same target rock at 3 54 ga concordant low temperature step ages of the nine aliquots suggest that at 3 42 ga a third smaller impact excavated parts of the 3 64 ga and 3 54 ga melt rocks and brought the fragments together the lack of significant impact activity after 3 5 ga as recorded by the bunburra rockhole suggests that 1 either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts or 2 it was covered by a porous heat absorbing regolith blanket which when combined with the diminishing frequency of large impacts in the solar system protected bunburra from subsequent major heating events finally we note that the total k ar resetting impact event history recorded by some of the brecciated eucrites peak at 3 8 3 5 ga is similar to the large impact history recorded by the bunburra rockhole parent body ca 3 64 3 54 ga this study and could indicate a similar position in the asteroid belt at that time

vesta vestoids and the howardite eucrite diogenite group relationships and the origin of spectral differences spectra of asteroid 4 vesta and 21 small estimated diameters less than 10 km asteroids with vesta like spectral properties vestoids were measured at visible and near infrared wavelengths similar to0 44 to similar to1 65 mum all of the measured small asteroids except for 2579 spartacus have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from vesta none of the observed asteroids have spectra similar to diogenites we find no spectral distinction between the 15 objects tabulated as members of the vesta dynamical family and 6 of the 7 sampled non family members that reside just outside the semi major axis a eccentricity e and inclination i region of the family the spectral consistency and close orbital a e i match of these non family objects to vesta and the vesta family imply that the true bounds of the family extend beyond the subjective cut off for membership asteroid 2579 spartacus has a spectrum consistent with a mixture of eucritic material and olivine spartacus could contain olivine rich material from vesta s mantle or may be unrelated to vesta altogether laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes finer particle sizes lead to an increase in the slope which is usually referred to as reddening this range of spectral variation for the best known meteoritic analogs to the vestoids regardless of whether they are actually related to each other suggests that the extremely red spectral slopes for some vestoids can be explained by very fine grained eucritic material on their surfaces

an anomalous basaltic meteorite from the innermost main belt triangulated observations of fireballs allow us to determine orbits and fall positions for meteorites the great majority of basaltic meteorites are derived from the asteroid 4 vesta we report on a recent fall that has orbital properties and an oxygen isotope composition that suggest a distinct parent body although its orbit was almost entirely contained within earth s orbit modeling indicates that it originated from the innermost main belt because the meteorite parent body would likely be classified as a v type asteroid v type precursors for basaltic meteorites unrelated to vesta may reside in the inner main belt this starting location is in agreement with predictions of a planetesimal evolution model that postulates the formation of differentiated asteroids in the terrestrial planet region with surviving fragments concentrated in the innermost main belt

147 sm 143 nd and 176 lu 176 hf systematics of eucrite and angrite meteorites comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system the eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation we present 147sm 143nd and 176lu 176hf analyses of eight noncumulate basaltic eucrites two cumulate eucrites and three angrites which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies the chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite we report a 147sm 143nd mineral isochron age for the moama cumulate eucrite of 4519 34 ma mswd 1 3 this age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 ma after the formation of the solar system we further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites we interpret the changes in geochemistry and consequently the erroneous 147sm 143nd and 176lu 176hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts effects from pressure temperature and peak shock duration on the surfaces of the eucrite and angrite parent bodies

widespread magma oceans on asteroidal bodies in the early solar system our solar system formed about 4 6 billion years ago and within 4 million years small planetary bodies had formed some melting to form volcanic and related rocks two families of meteorites the heds and angrites are thought to have originated from asteroids that melted at this time new oxygen isotope measurements confirm that these meteorites are from two distinct asteroids that underwent large scale melting in the early solar system these new results show that early global scale melting was a feature of all the differentiated planets earth moon and mars and asteroids so far sampled immediately following the formation of the solar system small planetary bodies accreted1 some of which melted to produce igneous rocks2 3 over a longer timescale 15 33 myr the inner planets grew by incorporation of these smaller objects4 5 through collisions processes operating on such asteroids strongly influenced the final composition of these planets4 including earth5 currently there is little agreement about the nature of asteroidal igneous activity proposals range from small scale melting to near total fusion and the formation of deep magma oceans2 here we report a study of oxygen isotopes in two basaltic meteorite suites the heds howardites eucrites and diogenites which are thought to sample the asteroid 4 vesta6 and the angrites from an unidentified asteroidal source our results demonstrate that these meteorite suites formed during early global scale melting 50 per cent events we show that magma oceans were present on all the differentiated solar system bodies so far sampled magma oceans produced compositionally layered planetesimals the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features such as earth s high mg si ratio

the pb isotopic evolution of the martian mantle constrained by initial pb in martian meteorites the pb isotopic compositions of maskelynite and pyroxene grains were measured in alh84001 and three enriched shergottites zagami roberts massif 04262 and larkman nunatuk 12011 by secondary ion mass spectrometry a maskelynite pyroxene isochron for alh84001 defines a crystallization age of 4089 73 ma 2 the initial pb isotopic composition of each meteorite was measured in multiple maskelynite grains alh84001 has the least radiogenic initial pb isotopic composition of any martian meteorite measured to date i e 206pb 204pb 10 07 0 17 2 assuming an age of reservoir formation for alh84001 and the enriched shergottites of 4513 ma a two stage pb isotopic model has been constructed this model links alh84001 and the enriched shergottites by their similar value 238u 204pb of 4 1 4 6 from 4 51 ga to 4 1 ga and 0 17 ga respectively the model employed here is dependent on a chondritic value 1 2 from 4567 to 4513 ma which implies that core segregation had little to no effect on the value s of the martian mantle the proposed pb isotopic model here can be used to calculate ages that are in agreement with rb sr lu hf and sm nd ages previously determined in the meteorites and confirm the young 170 ma ages of the enriched shergottites and ancient 4 ga age of alh84001

40ar 39ar age of the lonar crater and consequence for the geochronology of planetary impacts asteroid impacts play an important role in the evolution of planetary surfaces in the inner solar system the large majority of impacts occur on bodies e g asteroids the moon mars covered by primitive igneous rocks however most of the impacts recorded on earth occur on different rock types and are poor proxies for planetary impacts the lonar crater is a 1 88 km diameter quaternary age crater fig 1 located on the ca 66 ma deccan basaltic traps in maharashtra india and is one of the very few craters on earth emplaced directly on basaltic lava flows we carried out 12 40 ar 39 ar step heating experiments on 4 melt rock samples in order to 1 obtain a precise age for the lonar crater 2 study the response of isotopic chronometers during impacts on mafic target rocks and 3 better understand the dating of extraterrestrial impact craters we obtained 10 plateau and 9 inverse isochron ages on various aliquots combination of selected data into a global inverse isochron yielded an age of 570 47 ka mswd 1 1 p 0 24 in comparison previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages ca 12 62 ka the measured 40 ar 36 ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40 ar degassing capacity of basaltic impact melt rocks the 40 ar 36 ar ratio of 296 5 1 7 is indistinguishable from the atmospheric composition and suggests that inherited 40 ar is absent from the melt rock this result substantiates diffusion models that predict a near complete degassing of low viscosity melt e g basalts during impact and demonstrates for the first time that inherited 40 ar is less problematic for 40 ar 39 ar dating of impact events in basaltic igneous rocks compared to si rich rocks these results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and as far as dating is concerned should be the preferred targets of sample recovery by future missions

context matters ar ar results from in and around the manicouagan impact structure canada implications for martian meteorite chronology abstract as an analog for interpretations of the ages of martian shergottite meteorites we have conducted an argon isotopic study of plagioclase feldspars exhibiting varying levels of shock from in and around the manicouagan impact structure canada plagioclase from the impact melt sheet at manicouagan yields an age of 215 40 0 16 ma which indicates the time of impact plagioclase from a clast within melt bearing breccias of the melt sheet and a hornfels adjacent to the melt sheet yield ages of 216 3 ma and 218 7 ma respectively which are interpreted to have been reset by contact metamorphism from the impact melt country rocks that were unaffected by the impact gives 849 ma ages consistent with the known grenvillian target rock history maskelynite amorphous plagioclase which has been transformed in the solid state yields an age of 567 6 ma this age is geologically meaningless because it is not consistent with the target age the impact age or regional metamorphic ages at manicouagan our results show that maskelynite argon ages are not meaningful and that context is critical for proper interpretation of impact affected argon ages

stratigraphic and structural characteristics of the romanian black sea shelf abstract the western black sea basin opened during cretaceous times by back arc rifting in association with a north dipping subduction at the rear of the cretaceous early tertiary pontide volcanic arc the sedimentary wedge developed on the shelf of the romanian black sea sector reflects a complex interplay between large scale rifting uplift of the orogenic flanks large scale post rift subsidence and sea level changes we examine the detailed structural configuration of this sector for a regional correlation with the adjacent offshore in ukraine and bulgaria the evolution of the western black sea basin started in the albian cenomanian times when two extensional phases with significantly different directions n s and subsequently e w lead to the formation of a complex interplay between isolated blocks organised in horsts and grabens generally deepening eastwards superposition of normal faults footwall blocks from the two extensional episodes generated a deeply subsided area with enhanced accommodation space i e the histria depression and consequently recorded a larger thickness of paleogene sediments in the post rift stage re activation of faults and associated folding reflects repeated inversion during the late cretaceous oligocene times associated with subsequent periods of non deposition and or erosion during moments of basin fill exposure these periods of inversion recorded in the black sea are controlled by coeval orogenic deformations taking place in the balkans pontides and the crimean thrust belt sea level fluctuations during the neogene and late alpine tectonics in the neighbouring orogens caused massive sedimentation followed by sediment starvation and or significant erosion large thicknesses of sediments accumulated during the pontian presumably associated with an extensional episode deepening the distal parts of the basin and with differential compaction structures the interpretation of a high quality seismic dataset combined with published data allowed the correlation of major structural units and lineaments defined onshore towards the carpathians with the ones deeply buried below the western black sea basin sediments unit correlations are furthermore used to derive an integrated tectonic image of the western black sea area

a new source of basaltic meteorites inferred from northwest africa 011 eucrites are a class of basaltic meteorites that share common mineralogical isotopic and chemical properties and are thought to have been derived from the same parent body possibly asteroid 4 vesta the texture mineralogy and noble gas data of the recently recovered meteorite northwest africa nwa 011 are similar to those of basaltic eucrites however the oxygen isotopic composition of nwa011 is different from that of other eucrites indicating that nwa011 may be derived from a different parent body the presence of basaltic meteorites with variable oxygen isotopic composition suggests the occurrence of multiple basaltic meteorite parent bodies perhaps similar to 4 vesta in the early solar system

shock events in the solar system the message from minerals in terrestrial planets and asteroids impacts are central to the origin and evolution of planets of the solar system the shapes of craters which can reach up to 1 000 km in diameter on the moon provide critical information on the large scale dynamics of the impact and related shock minerals formed at high pressure and temperature found in shocked terrestrial rocks and meteorites give additional and complementary insights on the shock process at a smaller scale typically from a few micrometers to a few millimeters local flaws in rocks such as voids and mineral interfaces are the preferential sites for the formation of high pressure melts and minerals calculations based on the physics of shocks and the thermodynamics and kinetics of mineral transformations provide orders of magnitude for the duration transient pressure and prevailing temperature conditions of shock events case studies on shocked terrestrial and extraterrestrial materials illustrate the links between these parameters and impact duration many of the high pressure min

40ar 39ar ages for deep 3 3 km samples from the hawaii scientific drilling project mauna kea volcano hawaii 1 the hawaii scientific drilling project recovered core from a 3 5 km deep hole from the flank of mauna kea volcano providing a long essentially continuous record of the volcano s physical and petrologic development that has been used to infer the chemical and physical characteristics of the hawaiian mantle plume determining a precise accumulation rate via 40ar 39ar dating of the shield stage tholeiites which constitute 95 98 of the volcano s volume is challenging we applied40ar 39ar dating using laser and furnace heating in two laboratories berkeley and curtin to samples of two lava flows from deep in the core 3 3 km all determinations yield concordant isochron ages ranging from 612 159 to 871 302 ka 2 with p 0 90 the combined data yield an age of 681 120 ka p 0 77 for pillow lavas near the bottom of the core this new age when regressed with 40ar 39ar isochron ages previously obtained for tholeiites higher in the core defines a constant accumulation rate of 8 4 2 6 m ka that can be used to interpolate the ages of the tholeiites in the hsdp core with a mean uncertainty of about 83 ka for example at 3300 mbsl the age of 664 83 ka estimated from the regression diverges at the 95 confidence level from the age of 550 ka obtained from the numerical model of depaolo and stolper 1996 the new data have implications for the timescale of the growth of hawaiian volcanoes the paleomagnetic record in the core and the dynamics of the hawaiian mantle plume

the vestan cataclysm impact melt clasts in howardites and the bombardment history of 4 vesta crystalline impact melt samples were created in high temperature environments by relatively large craters and as such give additional constraints on the nature of the impacts that created them this article provides new 40ar 39ar ages of impact melt clasts in howardites and shows that these clasts formed on the hed parent body 4 vesta within the time period 3 3 3 8 ga rather than resulting from an increased number of impacts however impact melted material in howardites may result from unusually high velocity impacts occurring in the asteroid belt during this period this scenario is similar to the late heavy bombardment of the moon pointing to an unusual dynamical event at this time across the inner solar system therefore impact melt rocks in howardites uniquely record a vestan cataclysm

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

target rocks impact glasses and melt rocks from the lonar impact crater india petrography and geochemistry the lonar crater india is the only well preserved simple crater on earth in continental flood basalts it is excavated in the deccan trap basalts of cretaceous tertiary age a representative set of target basalts including the basalt flows excavated by the crater and a variety of impact breccias and impact glasses were analyzed for their major and trace element compositions impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes comparable geochemical patterns of immobile elements e g rees for glass melt rock and basalt indicates minimal fractionation between the target rocks and the impactites we found only little indication of post impact hydrothermal alteration in terms of volatile trace element changes no clear indication of an extraterrestrial component was found in any of our breccias and impact glasses indicating either a low level of contamination or a non chondritic or otherwise iridium poor impactor

oxygen isotopic constraints on the origin and parent bodies of eucrites diogenites and howardites abstract a few eucrites have anomalous oxygen isotopic compositions to help understand their origin and identify additional samples we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites except for five eucrites these meteorites have 17o values that lie within 2 of their mean value viz 0 242 0 016 consistent with igneous isotopic homogenization of vesta the five exceptional eucrites nwa 1240 pasamonte both clast and matrix samples pca 91007 a 881394 and ibitira have 17o values that lie respectively 4 5 5 15 and 21 away from this mean value nwa 1240 has a 18o value that is 5 below the mean eucrite value four of the five outliers are unbrecciated and unshocked basaltic eucrites like nwa 011 the first eucrite found to have an anomalous oxygen isotopic composition the fifth outlier pasamonte is composed almost entirely of unequilibrated basaltic clasts published chemical data for the six eucrites with anomalous oxygen isotopic compositions including nwa 011 exclude contamination by chondritic projectiles as a source of the oxygen anomalies only nwa 011 has an anomalous fe mn ratio but several anomalous eucrites have exceptional na ti or cr concentrations we infer that the six anomalous eucrites are probably derived from five distinct vesta like parent bodies pasamonte and pca 91007 could come from one body these anomalous eucrites like the isotopically normal unbrecciated eucrites with 4 48 gyr ar ar ages are probably deficient in brecciation and shock effects because they were sequestered in small asteroids 10 km diameter during the late heavy bombardment following ejection from vesta like bodies the preservation of vesta s crust and the lack of deeply buried samples from the hypothesized vesta like bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets rather than by collisional grinding

variability of diffusion of argon in albite pyroxene and olivine in shocked and unshocked samples abstract we analyzed the argon diffusion parameters of albite and pyroxene as a function of shock as well as an olivine of unknown shock pressure in albite shock pressures of 30 60 gpa all have an equal effect on the diffusion parameters lowering the activation energy e from 45 to 25 kcal mol and the log d0 a2 from 4 to 1 s 1 in a mixture of low ca and high ca pyroxene similar shock pressures also have an equal effect lowering the e from 130 to 80 kcal mol and the log d0 a2 from 14 to 7 s 1 however given that some shocked and unshocked pyroxene splits have similar release patterns making a clear distinction between the two is difficult additionally splits of pure unshocked high ca pyroxene have an e of 100 kcal mol and a log d0 a2 of 10 s 1 intermediate to the values of mixed pyroxene when shocked and unshocked the olivine fa18 has an e of 45 kcal mol and a log d0 a2 of 1 s 1 somewhat similar to that of feldspar these results will be useful in interpreting complex argon release patterns from shocked multi mineral samples too fine grained for easy mineral separation such as chondrites

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

petroleum geology of the black sea abstract the black sea comprises two extensional basins formed in a back arc setting above the northward subducting tethys ocean close to the southern margin of eurasia the two basins coalesced late in their post rift phases in the pliocene forming the present single depocentre the western black sea was initiated in the aptian when a part of the moesian platform now the western pontides of turkey began to rift and move away to the south east the eastern black sea probably formed by separation of the mid black sea high from the shatsky ridge during the palaeocene to eocene subsequent to rifting the basins were the sites of mainly deep water deposition only during the late miocene was there a major sea level fall leading to the development of a relatively shallow lake most of the margins of the black sea have been extensively modified by late eocene to recent compression associated with closure of the tethys ocean gas chromatography mass spectrometry and carbon isotope analysis of petroleum and rock extracts suggest that most petroleum occurrences around the black sea can be explained by generation from an oil prone source rock of most probably late eocene age although a wider age range is possible in the basin centres burial history modelling and source kitchen mapping indicate that this unit is currently generating both oil and gas in the post rift basin a palaeozoic source rock may have generated gas condensate in the gulf of odessa in bulgarian waters the main plays are associated with the development of an eocene foreland basin kamchia trough and in extensional structures related to western black sea rifting the latter continue into the romanian shelf where there is also potential in rollover anticlines due to gravity sliding of neogene sediments in the gulf of odessa gas condensate has been discovered in several compressional anticlines and there is potential in older extensional structures small gas and oil discoveries around the sea of azov point to further potential offshore around the central azov high in offshore russia and georgia there are large culminations on the shatsky ridge but these are mainly in deep water and may have poor reservoirs there are small compressional structures off the northern turkish coast related to the pontide deformation these may include eocene turbidite reservoirs the extensional fault blocks of the andrusov ridge mid black sea high are seen as having the best potential for large hydrocarbon volumes but in 2200 m of water

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated