dike intrusion associated with the 2000 eruption of miyakejima volcano japan the model for the 2000 dike intrusion event between kozushima and miyakejima volcano japan was reinvestigated after the sudden earthquake swarm in miyakejima volcano a dike intrusion of large volume was detected by the nationwide gps network geonet the displacements detected with gps stations over an area with a radius of about 200 km shows a distribution that is consistent with the dike source being located near miyakejima volcano

apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

evidence for early asteroidal collisions prior to 4 15 ga from basaltic eucrite phosphate u pb chronology abstract the late heavy bombardment lhb hypothesis wherein the terrestrial planets are thought to have suffered intense collisions ca 3 9 billion years ago is under debate coupled with new dynamical calculations re examination of geochronological data seem to support an earlier solar system instability and a smooth monotonic decline in impacts as opposed to a cataclysm to better understand this collisional history records from the asteroidal meteorites are required here we report a uranium lead u pb chronological dataset for eucrite meteorites thought to originate from the asteroid 4 vesta this dataset indicates to a continuous history of collisions prior to 4 15 ga our 207pb 206pb model ages of apatite ca5 po4 3 f cl oh and merrillite ca9namg po4 7 from three brecciated basaltic eucrites juvinas 4150 3 11 6 million years ago ma merrillite only camel donga disturbed around 4570 4370 ma and stannern 4143 0 12 5 ma record multiple thermal metamorphic events during the period of 4 4 4 15 ga we interpret this to mean that vesta or the vestoid cluster underwent multiple impacts and moderate high temperature reheating during this time the ages of 4 4 4 15 ga are distinctly younger than the initial magmatic process on vesta 4 5 ga but are significantly older than a later impact peak based on some interpretations of 40ar 39ar chronologies 3 9 3 5 ga the intense collisions prior to 4 15 ga on vesta are at odds with the conventional lhb hypothesis but not inconsistent with the much earlier bombardment and monotonic decline scenario different radiometric chronologies of the asteroid likely represent the different stages of a continual collisional process conversely the model 207pb 206pb ages of apatite in the unbrecciated basaltic eucrite agoult returned an age of 4524 8 9 6 ma this may represent slow cooling from an earlier global reheating of the crust on vesta at 4 55 ga as documented by other radiometric chronologies the apatite in juvinas recorded a coincident timing of 4516 9 10 4 ma which could be due to either slow crustal cooling or impact

new views of lunar geoscience an introduction and overview beyond the earth the moon is the only planetary body for which we have samples from known locations the analysis of these samples gives us ground truth for numerous remote sensing studies of the physical and chemical properties of the moon and they are invaluable for our fundamental understanding of lunar origin and evolution prior to the return of the apollo 11 samples the moon was thought by many to be a primitive undifferentiated body e g urey 1966 a concept shattered by the data returned from the apollo and luna missions ever since new data have helped to address some of our questions but of course they also produced new questions in this chapter we provide a summary of knowledge about lunar geologic processes and we describe major scientific advancements of the last decade that are mainly related to the most recent lunar missions such as galileo clementine and lunar prospector 1 1 the moon in the planetary context compared to terrestrial planets the moon is unique in terms of its bulk density its size and its origin fig 1 1a c all of which have profound effects on its thermal evolution and the formation of a secondary crust fig 1 1d numerous planetary scientists considered the moon as an endmember among the planetary bodies in our solar system because its lithosphere has been relatively cool rigid and intact throughout most of geological time a one plate planet and its surface has not been affected by plate recycling an atmosphere water or life therefore the moon recorded and preserved evidence for geologic processes that were active over the last 4 4 5 b y and offers us the unique opportunity to look back into geologic times for which evidence on earth has long been erased fig 1 1c d impact cratering an exterior process is considered the most important surface process on the moon internal

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

study of the impact of ice formation in leads upon the sea ice pack mass balance using a new frazil and grease ice parameterization abstractleads are cracks in sea ice that often form because of deformation during winter months leads expose the ocean to the cold atmosphere resulting in supercooling and the formation of frazil ice crystals within the mixed layer here the authors investigate the role of frazil ice formation in leads on the mass balance of the sea ice pack through the incorporation of a new module into the los alamos sea ice model cice the frazil ice module considers an initial cooling of leads followed by a steady state formation of uniformly distributed single size frazil ice crystals that precipitate to the ocean surface as grease ice the grease ice is pushed against one of the lead edges by wind and water drag that the authors represent through a variable collection thickness for new sea ice simulations of the sea ice cover in the arctic and antarctic are performed and compared to a model that treats leads the same as the open ocean the processes of ice formation in the new module slow down the refreezing o

age of the eucrite caldera from convergence of long lived and short lived chronometers abstract the long lived 147 sm 143nd chronometer has been used in conjunction with the short lived 146 sm 142 nd and 53 mn 53 cr chronometers to constrain the time of formation for the eucrite caldera the combination of these three chronometers has produced results that are consistent with one another and converge on a precise high resolution age of 4537 12 ma within 95 confidence limits so far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite thus this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body most likely between 17 and 41 million years after the formation of the first condensates in the early solar system from these age constraints it is evident that caldera either formed within a magma body on a large 500 km asteroid or in an extensive impact melt on the crust of its parent body

diffusion kinetics of lutetium in diopside and the effect of thermal metamorphism on lu hf systematics in clinopyroxene abstract we report new experimental data for lu diffusion in diopside from 1150 to 1250 c at 1 bar pressure and oxygen fugacity corresponding to the iron wustite buffer the primary experimental dataset defines the following arrhenius relation for lu diffusion parallel to the b crystallographic axis d lu b 1 37 1 2 10 13 exp 208 7 15 2 kj mol 1 rt m 2 s where t is in kelvin we find that lu diffusion in diopside is significantly faster than that of hf which could lead to spurious lu hf ages via decoupling of 176lu from 176hf if the system was subjected to protracted periods of high temperature the extent to which lu hf ages are compromised during initial cooling depends on the peak temperature grain size cooling rate and the nature of lu partitioning between clinopyroxene and its surrounding phase s however in geologically realistic scenarios the effects of diffusion on lu hf ages during initial cooling do not produce lu hf ages that differ from the age corresponding to the closure temperature of hf by more than a few million years lu hf systematics in clinopyroxene are far more susceptible to diffusion during reheating events that occur after radiogenic 176hf has already accumulated in the host phase for a given thermal pulse the lu hf age varies as a function of grain size temperature the timing and duration of reheating and the partitioning of lu between clinopyroxene and its surrounding phase s the data collected and numerical model developed in this study have been used to simulate the effect of thermal metamorphism on the internal lu hf systematics of the eucrites millbillillie and piplia kalan although our model results do not offer a comprehensive solution to the apparent pre solar lu hf ages observed in many ancient meteorites we argue that the 4 9 ga pyroxene whole rock lu hf ages of these specific samples are at least partially if not primarily due to diffusive loss of lu from the constituent minerals to a partial melt phase during high temperature metamorphism

crystallization recrystallization and impact metamorphic ages of eucrites y792510 and y791186 abstract monomict eucrites y792510 and y791186 of pyroxene metamorphic type 6 contain areas of recrystallized mesostasis and granoblastic pyroxene textural comparisons suggest that the mesostasis areas in these eucrites originally resembled analogous areas in the pristine basaltic eucrite clast y75011 84 39ar 40ar ages were determined for microbreccias y791186 87 and y792510 62 and crystalline clast y792510 65 the 39ar 40ar ages for y791186 87 and y792510 62 are identical within error limits at 3 46 0 18 ga and 3 40 0 16 ga 2 respectively both these samples were involved in an impact event in which ar was quantitatively degassed 3 43 0 10 ga ago y792510 65 has an apparently older 39ar 40ar age of 3 60 0 04 ga 2 suggesting that it was degassed in a separate event or to a lesser degree rb sr and sm nd ages were determined for both y792510 clasts the rb sr age of 3 26 0 24 ga 2 for y792510 65 was totally reset by the impact event s whereas the rb sr age of 4 26 0 62 ga 2 for y792510 62 either was partially reset by the same event s or by earlier recrystallization of mesostasis the 147sm 143nd age calculated for y792510 65 including all data is 4 67 0 22 ga 2 but is likely to have been affected by weathering in the antarctic it becomes 4 57 0 09 ga if only data for the major minerals plagioclase and pyroxene are included in the isochron the 147sm 143nd age of y792510 62 is 4 34 0 06 ga 2 for the same mineral phases but may be biased by the presence of weathered ca phosphates in the pyroxene separate the 146sm 142nd age of y792510 65 is 4 42 0 06 0 04 ga corresponding to initial 146sm 144sm 0 0030 0 0010 relative to 146sm 144sm lew 0 0076 0 0009 for the lew86010 angrite at 4 558 ga ago the y792510 65 ages from the two sm nd chronometers are concordant at 4 48 ga which may be the crystallization age of this eucritic basalt late crystallization alternatively these sm nd ages may have been affected by the event in which ca phosphate and augite in mesostasis were recrystallized and augite and chromite in granoblastic areas were derived from cloudy pyroxenes late recrystallization the subsolidus recrystallization may be a consequence of a thermal spike in the midst of an otherwise monotonically cooling environment resulting from impact cratering of the hot crust of the newly formed eucrite parent body pyroxene homogenization probably occurred prior to formation of granoblastic pyroxene and close to the time of initial crystallization mn cr data provide only a hint of live 53mn t1 2 3 7ma and imply either chromium isotopic equilibration after decay of 53mn or late crystallization impact brecciation and lithification probably accompanied thermal annealing 3 4 3 6 ga ago as given by the 39ar 40ar ages the rb sr age of y792510 65 is consistent with this interpretation the 39ar 40ar and rb sr systematics of minute k rich inclusions in plagioclase and pyroxene formed by previous impact and thermal annealing events probably were reset then these young impact metamorphic ages demonstrate that impact resetting of radiometric ages on the hed parent body asteroid 4 vesta occurred over an interval of at least 0 7 ga

modeling the basal melting and marine ice accretion of the amery ice shelf the basal mass balance of the amery ice shelf ais in east antarctica is investigated using a numerical ocean model the main improvements of this model over previous studies are the inclusion of frazil formation and dynamics tides and the use of the latest estimate of the sub ice shelf cavity geometry the model produces a net basal melt rate of 45 6 gt year 1 0 74 m ice year 1 which is in good agreement with reviewed observations the melting at the base of the ice shelf is primarily due to interaction with high salinity shelf water created from the surface sea ice formation in winter the temperature difference between the coldest waters created in the open ocean and the in situ freezing point of ocean water in contact with the deepest part of the ais drives a melt rate that can exceed 30 m of ice year 1 the inclusion of frazil dynamics is shown to be important for both melting and marine ice accretion refreezing frazil initially forms in the supercooled water layer adjacent to the base of the ice shelf the net accretion of marine ice is 5 3 gt year 1 comprised of 3 7 gt year 1 of frazil accretion and 1 6 gt year 1 of direct basal refreezing

chronology and products of the 2000 eruption of miyakejima volcano japan lateral migration of magma away from miyakejima volcanic island japan generated summit subsidence associated with summit explosions in the summer of 2000 an earthquake swarm beneath miyakejima began on the evening of 26 june 2000 followed by a submarine eruption the next morning strong seismic activity continued under the sea from beneath the coast of miyakejima to a few tens of kilometers northwest of the island summit eruptive event began with subsidence of the summit on 8 july and both explosions and subsidence continued intermittently through july and august the most intense eruptive event occurred on 18 august and was vulcanian to subplinian in type ash lofted into the stratosphere fell over the entire island and abundant volcanic bombs were erupted at this time another large explosion took place on 29 august this generated a low temperature pyroclastic surge which covered a residential area on the northern coast of the island the total volume of tephra erupted was 9 3 106 m3 dre much smaller than the volume of the resulting caldera 6 108 m3 migration of magma away from miyakejima was associated with crustal extension northwest of miyakejima and coincident shrinkage of miyakejima island itself during july august 2000 this magma migration probably caused stoping of roof rock into the magma reservoir generating subsurface cavities filled with hydrothermal fluid and or magmatic foam and formation of a caldera oyama caldera at the summit interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions it is likely that new magma was supplied to the reservoir from the bottom during waning stage of magma s migration resulting in explosive discharge on 18 august the 18 august event and phreatic explosions on 29 august produced a conduit system that allowed abundant so2 emission as high as 460 kg s 1 after the major eruptive events were over at the time of writing inhabitants of the island about 3 000 have been evacuated from miyakejima for more than 3 years

melting of ice and sea ice into seawater and frazil ice formation the thermodynamic consequences of the melting of ice and sea ice into seawater are considered the international thermodynamic equation of seawater 2010 teos 10 is used to derive the changes in the conservative temperature and absolute salinity of seawater that occurs as a consequence of the melting of ice and sea ice into seawater also a study of the thermodynamic relationships involved in the formation of frazil ice enables the calculation of the magnitudes of the conservative temperature and absolute salinity changes with pressure when frazil ice is present in a seawater parcel assuming that the frazil ice crystals are sufficiently small that their relative vertical velocity can be ignored the main results of this paper are the equations that describe the changes to these quantities when ice and seawater interact and these equations can be evaluated using computer software that the authors have developed and is publicly available in the gibbs seawater gsw oceanographic toolbox of teos 10

radiometric ages of basaltic achondrites and their relation to the early history of the solar system pbpb mineral isochrons were determined for three cumulate eucrites moama moore county and serra de mage and three noncumulate eucrites nuevo laredo bouvante and stannern two noncumulate eucrites bereba and cachari show disturbed pb isotope patterns the mineral isochron ages of cumulate eucrites range from 4 40 ga to 4 48 ga the latter is the age of moore county 4 484 0 019 ga which was also dated by the smnd method at 4 456 0 025 ga nuevo laredo and bouvante give the same pb pb age 4 514 0 015 ga and 4 510 0 004 ga respectively the pb isochron of stannern corresponds to 4 128 0 016 ga which is a reequilibration age both cachari and bereba show evidence of isotope resetting at about 4 ga a pb pb multicotrelation approach is introduced and applied to the data the younger ages of the cumulate eucrites preclude their formation by fractional crystallization from the melts that produced the noncumulate eucrites the 1 of the source of the cumulate eucrites is 14 whereas that of the source of the noncumulate eucrites is 150 this large difference in 1 values suggests that eucrites were derived from separate planetesimals or from a single heterogeneous layered parent body

mesosiderite formation on asteroid 4 vesta by a hit and run collision collision and disruption processes of protoplanetary bodies in the early solar system are key to understanding the genesis of diverse types of main belt asteroids mesosiderites are stony iron meteorites that formed by the mixing of howardite eucrite diogenite like crust and molten core materials and provide unique insights into the catastrophic break up of differentiated asteroids however the enigmatic formation process and the poorly constrained timing of metal silicate mixing complicate the assignment to potential parent bodies here we report the high precision uranium lead dating of mesosiderite zircons by isotope dilution thermal ionization mass spectrometry to reveal an initial crust formation 4 558 5 2 1 million years ago and metal silicate mixing at 4 525 39 0 85 million years ago the two distinct ages coincide with the timing of the crust formation and a large scale reheating event on the eucrite parent body probably the asteroid vesta this chronological coincidence corroborates that vesta is the parent body of mesosiderite silicates mesosiderite formation on vesta can be explained by a hit and run collision 4 525 4 million years ago that caused the thick crust observed by nasa s dawn mission and explains the missing olivine in mesosiderites howardite eucrite diogenite meteorites and vestoids mesosiderite meteorites may originate from a hit and run impact on the parent asteroid of eucrite meteorites probably vesta as mesosiderite zircon u pb ages are found to coincide with those for eucrites

the oldest zircons in the solar system abstract here we report the occurrence chemistry and u th pb isotopic systematics of three meteoritic zircon assemblages two from the vaca muerta mesosiderite and one from the simmern h5 chondrite one of the vaca muerta zircons occurs in the mesosiderite proper the other in a eucritic clast associated with chromite ilmenite and tridymite whereas the simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar like terrestrial zircons the meteoritic zircons are enriched in the heavy rare earth elements but unlike terrestrial zircons they do not show a positive ce anomaly this feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment under oxidising conditions ce4 which can substitute for zr4 in the zircon structure is stabilized whereas under relatively reducing conditions ce3 is stable the zircon from simmern has depletions in the relative abundances of tm and yb a characteristic of volatility fractionation of the ree the u concentrations of the two vaca muerta zircons are quite different at 1 and 50 ppm respectively both zircons have extremely low initial lead and give radiogenic ages that are the same albeit with relatively large errors on vm 1 zircon because of its much lower u concentration and hence radiogenic pb concentration the mean of the two analyses made on vm 2 zircon is concordant with a207pb 206pb age of 4563 15 ma 2 the simmern zircon has an exceptionally low u concentration of around 180 ppb and only a poorly constrained 207pb 206pb age of 4100 700 ma could be obtained

the violent collisional history of asteroid 4 vesta vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids the dawn spacecraft extensively imaged vesta s surface revealing a collision dominated history results show that vesta s cratering record has a strong north south dichotomy vesta s northern heavily cratered terrains retain much of their earliest history the southern hemisphere was reset however by two major collisions in more recent times we estimate that the youngest of these impact structures about 500 kilometers across formed about 1 billion years ago in agreement with estimates of vesta asteroid family age based on dynamical and collisional constraints supporting the notion that the vesta asteroid family was formed during this event

establishment of a 3 83 ga magmatic age for the akilia tonalite southern west greenland a recurring challenge to ion microprobe u pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains this has proved particularly problematic in studying the polyphase metamorphic rocks that dominate early archean gneissic terranes for example differing interpretations exist for the origin of complex zircon populations investigated by u pb ion microprobe zircon geochronology from a granitoid orthogneiss body previously used to establish a minimum age for a supracrustal enclave on akilia island in southern west greenland we describe a method whereby the geochemistry of the akilia orthogneiss coupled with a u th pb vs age depth profile in a zircon from the same rock permits direct assessment of zircon inheritance results reveal evidence for three phases of concentric zircon growth at 3 83 ga v3 6 ga and 2 7 2 5 ga zircon growth at both v3 6 ga and 2 7 2 5 ga is consistent with precipitation from a metamorphic fluid depth profile u th pb data demonstrate that only the s 3 8 ga zircon core could have crystallized from the host rock we conclude that the magmatic age of this rock is 3 83 7 0 01 ga not v3 65 ga as has been previously proposed the u th pb zircon depth profile technique has wider applications to resolving other geochronological debates in highgrade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution re precipitation features 2002 elsevier science b v 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

oxygen isotope evidence for rapid mixing of the hed meteorite parent body abstract the 16 o 17 o and 18 o abundances of howardites eucrites and diogenites have been used to assign them to a single hed parent body thought to be asteroid 4 vesta we report the first evidence of oxygen isotopic heterogeneity among hed meteorites indicating incompletely mixed sources new high precision oxygen isotope measurements of 34 hed meteorites reveal that most have the same 17 o consistent with a very rapid early history of large scale mixing on vesta however howardites are on average very slightly enriched in 16 o whereas ibitira caldera pasamonte and alha78132 are 16 o depleted compared to other investigated eucrites the 17 o of ibitira is completely different from all other heds measured some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of vesta others require a separate parent body or indicate that parts of the outer layer of vesta retained some primary isotopic heterogeneity the oxygen isotopic composition of howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the hed parent body regolith

chronology and petrology of silicates from iie iron meteorites evidence of a complex parent body evolution iie iron meteorites contain silicate inclusions the characteristics of which suggest a parent body similar to that of h chondrites however these silicates show a wide range of alteration ranging from netschaevo and techado the inclusions of which are little altered to highly differentiated silicates like those in kodaikanal weekeroo station and colomera which have lost metal and sulfur and are enriched in feldspar we find these inclusions to show varying degrees of shock alteration we made 39ar 40ar age determinations of watson techado miles colomera and sombrerete watson has an ar ar age of 3 677 0 007 gyr similar to previously reported ages for kodaikanal and netschaevo we suggest that the various determined radiometric ages of these three meteorites were probably reset by a common impact event the space exposure ages for these three meteorites are also similar to each other and are considerably younger than exposure ages of other iies 39ar 40ar ages inferred for the other four meteorites analyzed are considerably older than watson and are techado 4 49 0 01 gyr miles 4 405 0 012 gyr colomera 4 470 0 010 gyr and sombrerete 4 541 0 0012 gyr these ages are in fair agreement with previously reported rb sr isochron ages for colomera and weekeroo station although several mechanisms to form iie meteorites have been suggested it is not obvious that a single mechanism could produce a suite of meteorites with very different degrees of silicate differentiation and with isotopic ages that differ by 0 8 gyr we suggest that those iies with older isotopic ages are a product of partial melting and differentiation within the parent body followed by mixing of silicate and metal while both were relatively hot netschaevo and watson may have formed by this same process or by impact mixing 4 5 gyr ago but their isotopic ages may have been subsequently reset by shock heating kodaikanal apparently is required to have formed more recently in which case impact melting and differentiation seems the only viable process we see no compelling reasons to believe that iie silicate and metal derived from different parent bodies or that the parent body of iies was the same as that of h chondrites

initial 182hf 180hf in meteoritic zircons zircons from the simmern h5 chondrite and pomozdino eucrite have been analyzed for their hf w isotope systematics zircons have high intrinsic hf contents and coupled with low w make them ideal hf w chronometers however measurements of 182hf 180hf 0 are far from straightforward with low signals of radiogenic 182w and difficulties in calibrating hf w ratios zircons were analyzed from the simmern chondrite and the pomozdino eucrite the simmern zircon has an ultrarefractory enriched trace element pattern a feature commonly associated with refractory inclusions analyses of simmern zircon show variable hf w that is likely due to surface contamination simmern chondrite zircon yields a 182hf 180hf 0 of 7 2 4 5 10 5 pomozdino eucrite zircon analyses show very high hf w values indicating 182hf 180hf 0 of 1 7 1 1 10 5 the simmern value is in good agreement with the initial value of the solar system indicated from hf w systematics of chondrites the pomozdino value is lower than expected if eucrites formed within several million years of refractory inclusions as suggested from al mg systematics of eucrites

vesta and the vestoids a new rock group abstract moderate spectral resolution observations of the 505 nm fe2 pyroxene absorption feature were obtained for 13 asteroids dynamically linked to 4 vesta or located between vesta and the 3 1 resonance gap eleven showed spectra with deep 0 9 m mafic silicate absorptions suggesting a pyroxene composition binzel and xu 1993 spectra of only six of these asteroids show the absorption feature centered at 506 5 nm suggestive of an augite high calcium component to the surface pyroxenes consistent with previous spectral observations across the surface of vesta cochran and vilas 1998 five of the six asteroids include all objects observed at heliocentric distances between vesta and the 3 1 resonance gap the spectrum of only one asteroid interior to vesta s orbit showed this feature no correlation between the presence or absence of this feature and the strength of the 1 m mafic silicate feature was found

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

metamorphic zircon trace elements and neoarchean metamorphism in the ca 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract the nuvvuagittuq supracrustal belt nsb in northwestern quebec ranks as one of the oldest granitoid gneiss complexes thus far discovered emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz magnetite schists at the porpoise cove outcrops of the nsb cluster at 3 75 ga as with all other pre 3 7 ga terranes thus far documented the nsb has been metamorphosed and multiply deformed in a combined approach to explore the thermal history of the nsb conventional garnet biotite and plagioclase hornblende geothermometry was coupled with zircon u th pb depth profiles ree partitioning and ti in zircon thermometry to show that neoarchean metamorphism in the belt reached upper amphibolite facies conditions 640 c this metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the northeast superior province and the initiation of widespread igneous activity in the vicinity of the nsb an earlier resolvable metamorphic episode from a zircon depth profile recorded at 3 62 ga probably corresponds to the intrusion of the granitoids which envelope the nsb

determination of planetary basalt parentage a simple technique using the electron microprobe many basaltic meteorites are being discovered in old and new meteorite suites including those from cold deserts e g antarctica and hot desert environments it is important to establish the specific planetary body source proven techniques for establishing planetary parentage include stable isotopic signatures especially oxygen certain elemental ratios in bulk samples and certain elemental ratios in specific minerals some of these techniques are expensive require considerable sample preparation and are adversely affected by weathering processes on the parent body or on earth we have been seeking key major and minor elemental ratios in pyroxene olivine and feldspar that can be measured by the electron microprobe on standard thin sections these ratios may be preserved in unweathered portions of mineral grains and thus see through weathering processes in addition if the sample is too small to provide a representative bulk composition it may still have key information recorded in individual minerals we have found that some of the most useful chemical parameters are fe mn atomic in olivine or pyroxene and the percent anorthite an in plagioclase solid solutions a plot of fe mn in pyroxene and or olivine verses an defines compositional fields that are significantly different for earth mars moon 4 vesta and the angrite parent body this method may be especially powerful when combined with oxygen isotope data

hydrocode simulations of chicxulub crater collapse and peak ring formation we use hydrocode modeling to investigate dynamic models for the collapse of the chicxulub impact crater our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide angle velocity data to further our understanding of the formation of large impact craters in our simulations we model the collapse of a 100 km diameter bowlshaped cavity formed in comprehensively fractured crustal material to facilitate wholesale collapse we require that the strength of the target be significantly weakened in the present model we achieve this using acoustic fluidization where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target at times and positions where the overburden pressure is sufficiently counteracted the frictional resistance is reduced enabling the rock debris to flow our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at chicxulub in particular we observe a topographic peak ring formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim this model for peak ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned c 2002 elsevier science usa

lead and mg isotopic age constraints on the evolution of the hed parent body the large collection of howardite eucrite diogenite hed meteorites allows us to study the initial magmatic differentiation of a planetesimal we report pb pb ages of the unequilibrated north west africa nwa 4215 and dhofar 700 diogenite meteorites and their mass independent 26mg isotope compositions 26mg to better understand the timing of differentiation and crystallization of their source reservoir s nwa 4215 defines a pb pb age of 4484 5 7 9 myr and has a 26mg excess of 2 3 1 6 ppm whereas dhofar 700 has a pb pb age of 4546 4 4 7 myr and a 26mg excess of 25 5 1 9 ppm we interpret the young age of nwa 4215 as a thermal overprint but the age of dhofar 700 is interpreted to represent a primary crystallization age combining our new data with published mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1 2 myr of our solar system consistent with a short lived early formed magma ocean undergoing convective cooling the other half of the diogenites including both nwa 4215 and dhofar 700 are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least 20 myr

a 4 2 billion year old impact basin on the moon u pb dating of zirconolite and apatite in lunar melt rock 67955 abstract a sharp rise in the flux of asteroid size bodies traversing the inner solar system at 3 9 ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments the prevalence of 3 9 ga crystallization ages for lunar impact melt breccias and u pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short lived cataclysmic episode of late heavy bombardment at that time here we report u pb isotopic compositions of zirconolite and apatite in coarse grained lunar melt rock 67955 measured by ion microprobe that date a basin scale impact melting event on the moon at 4 22 0 01 ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later significant impacts prior to 3 9 ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish other isotopic evidence such as 40 ar 39 ar ages of granulitic lunar breccias regolith fragments and clasts extracted from fragmental breccias and re os isotopic compositions of lunar metal is also suggestive of impact related thermal events in the lunar crust during the period 4 1 4 3 ga we conclude that numerous large impactors hit the moon prior to the canonical 3 9 ga cataclysm that some of those pre cataclysm impacts were similar in size to the younger lunar basins and that the oldest preserved lunar basins are likely to be significantly older than 3 9 ga this provides sample based support for dynamical models capable of producing older basins on the moon and discrete populations of impactors an extended period of basin formation implies a less intense cataclysm at 3 9 ga and therefore a better opportunity for preservation of early habitable niches and hadean crust on the earth a diminished cataclysm at 3 9 ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance

the geologic history of vesta inferred from combined 207pb 206pb and 40ar 39ar chronology of basaltic eucrites abstract eucrites represent samples of mafic crust of the parent body likely vesta and record a complex geologic history involving magmatism metamorphism impact processing and metasomatism for better understanding of the geologic history we present combined 207pb 206pb and 40ar 39ar chronology of four basaltic eucrites showing distinct petrologic features agoult an equilibrated eucrite with granulitic textures camel donga a brecciated equilibrated eucrite enriched in metallic iron dag 380 a partially equilibrated monomict eucrite and nwa 049 a metasomatized polymict eucrite comprising mostly unequilibrated clasts the 207pb 206pb dating in combination with the acid washing technique was applied to pyroxene rich and plagioclase rich fractions while the 40ar 39ar dating was performed mainly on plagioclase grains using the laser incremental heating method the pb isotopic data for acid leaching residues are more radiogenic than those for acid washes reflecting efficient removal of non radiogenic pb components by the acid washing among the residues however only those of the agoult and camel donga plagioclase rich fractions yielded isochrons rather than errorchrons with 207pb 206pb ages of 4532 39 0 87 ma and 4515 43 0 42 ma respectively the trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase on the other hand the ar isotopic data for agoult and camel donga yielded plateau ages with weighted means of 4494 9 ma and 3749 25 ma in contrast to those for dag 380 and nwa 049 that did not return any plateau the 207pb 206pb and 40ar 39ar ages of agoult are interpreted to reflect the time of the u pb and k ar system closure during a prolonged thermal metamorphism by combining the ages reported here and in the literature with the calculated closure temperatures the cooling rate of agoult is determined to be 11 2 c ma this cooling rate corresponds to a burial depth of 10 20 km indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed the 20 ma younger 207pb 206pb ages of camel donga than agoult are considered to reflect delayed burial metamorphism in the source region whereas the plateau 40ar 39ar age would record the time of an impact causing brecciation and efficient ar degassing of the buried crust furthermore the model 207pb 206pb dates calculated for acid washed pyroxenes imply that the burial metamorphism of agoult and dag 380 were broadly contemporaneous and also that nwa 049 was metasomatized by fluids within the uppermost crust later than 4 4 ga

mineralogy of meteorite groups approximately 275 mineral species have been identified in meteorites reflecting diverse redox environments and in some cases unusual nebular formation conditions anhydrous ordinary carbonaceous and r chondrites contain major olivine pyroxene and plagioclase major opaque phases include metallic fe ni troilite and chromite primitive achondrites are mineralogically similar the highly reduced enstatite chondrites and achondrites contain major enstatite plagioclase free silica and kamacite as well as nitrides a silicide and ca mg mn na cr k and ti rich sulfides aqueously altered carbonaceous chondrites contain major amounts of hydrous phyllosilicates complex organic compounds magnetite various sulfates and sulfides and carbonates in addition to kamacite and taenite iron meteorites contain carbides elemental c nitrides phosphates phosphides chromite and sulfides silicate inclusions in iab iiicd and iie iron meteorites consist of mafic silicates plagioclase and various sulfides oxides and phosphates eucrites howardites and diogenites have basaltic to orthopyroxenitic compositions and consist of major pyroxene and calcic plagioclase and several accessory oxides ureilites are made up mainly of calcic chromian olivine and low ca clinopyroxene embedded in a carbonaceous matrix accessory phases include the c polymorphs graphite diamond lonsdaleite and chaoite as well as metallic fe ni troilite and halides angrites are achondrites rich in fassaitic pyroxene i e al ti diopside minor olivine with included magnesian kirschsteinite is also present martian meteorites comprise basalts lherzolites a dunite and an orthopyroxenite major phases include various pyroxenes and olivine minor to accessory phases include various sulfides magnetite chromite and ca phosphates lunar meteorites comprise mare basalts with major augite and calcic plagioclase and anorthositic breccias with major calcic plagioclase several meteoritic phases were formed by shock metamorphism martensite 2 fe ni has a distorted body centered cubic structure and formed by a shear transformation from taenite during shock reheating and rapid cooling the c polymorphs diamond lonsdaleite and chaoite formed by shock from graphite suessite formed in the north haig ureilite by reduction of fe and si possibly from olivine via reaction with carbonaceous matrix material ringwoodite the spinel form of mg fe 2sio4 and majorite a polymorph of mg fe sio3 with the garnet structure formed inside shock veins in highly shocked ordinary chondrites secondary minerals in meteorite finds that formed during terrestrial weathering include oxides and hydroxides formed directly from metallic fe ni by oxidation phosphates formed by the alteration of schreibersite and sulfates formed by alteration of troilite

modelling the response of ice shelf basal melting to different ocean cavity environmental regimes we present simulation results from a version of the regional ocean modeling system modified for ice shelf ocean interaction including the parameterisation of basal melting by molecular diffusion alone simulations investigate the differences in melting for an idealised ice shelf experiencing a range of cold to hot ocean cavity conditions both the pattern of melt and the location of maximum melt shift due to changes in the buoyancy driven circulation in a different way to previous studies tidal forcing increases both the circulation strength and melting with the strongest impact on the cold cavity case our results highlight the importance of including a complete melt parameterisation and tidal forcing in response to the 2 4 c ocean warming initially applied to a cold cavity ice shelf we find that melting will increase by about an order of magnitude 24 with tides and 41 without tides

39ar 40ar ages of eucrites and thermal history of asteroid 4 vesta eucrite meteorites are igneous rocks that derived from a large asteroid probably 4 vesta past studies have shown that after most eucrites formed they underwent metamorphism in temperatures up to 800 c much later many were brecciated and heated by large impacts into the parent body surface the less common basaltic unbrecciated eucrites also formed near the surface but presumably escaped later brecciation while the cumulate eucrites formed at depths where metamorphism may havepersisted for a considerable period to further understand the complex hed parent body thermal history we determined new 39ar 40ar ages for 9 eucrites classified as basaltic but unbrecciated 6 eucrites classified as cumulate and several basaltic brecciated eucrites precise ar ar ages of 2 cumulate eucrites moama and eet 87520 and 4 unbrecciated eucrites give a tight cluster at 4 48 0 02 gyr not including any uncertainties in the flux monitor age ar ar ages of 6 additional unbrecciated eucrites are consistent with this age within theirrelatively larger age uncertainties by contrast available literature data on pb pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4 4 4 515 gyr and 147sm 143nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4 41 4 55 gyr similar ar ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites as was previously proposed we suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the k ar and some other chronometers to remain as open diffusion systems from the strong clustering of ar ar ages at 4 48 gyr we propose that these meteorites were excavated from depth in a single large impact event 4 48 gyr ago which quickly cooled the samples and started the k ar chronometer a large 460 km crater postulated to exist on vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with vesta some pb pb and sm nd ages of cumulate and unbrecciated eucrites are consistent with the ar ar age of 4 48 gyr and the few older pb pb and sm nd ages may reflect an isotopic closure before the large cratering event one cumulate eucrite gives an ar ar age of 4 25 gyr 3 additional cumulate eucrites give ar ar ages of 3 4 3 7 gyr and 2 unbrecciated eucrites give ar ar ages of 3 55 gyr we attribute these younger ages to a later impact heating furthermore the ar ar impact reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3 5 4 1 gyr among these piplia kalan the first eucrite to show evidence for extinct 26al was strongly impact heated 3 5 gyr ago when these data are combined with eucrite ar ar ages in the literature they confirm that several large impact heating events occurred on vesta between 4 1 3 4 gyr ago the onset of major impact heating may have occurred at similar times for both vesta and the moon but impact heating appears to have persisted for a somewhat later time on vesta

micrometer scale u pb age domains in eucrite zircons impact re setting and the thermal history of the hed parent body meteoritic zircons are rare but some are documented to occur in asteroidal meteorites including those of the howardite eucrite diogenite hed achondrite clan rubin a 1997 meteorit planet sci 32 231 247 the heds are widely considered to originate from the asteroid 4 vesta vesta and the other large main belt asteroids record an early bombardment history to explore this record we describe sub micrometer distributions of trace elements u th and 235 238 u 207 206 pb ages from four zircons 7 40lm separated from bulk samples of the brecciated eucrite millbillillie ultra high resolution 100 nm ion microprobe depth profiles reveal different zircon age domains correlative to mineral chemistry and to possible impact scenarios our new u pb zircon geochronology shows that vesta s crust solidified within a few million years of solar system formation 4561 13 ma in good agreement with previous work e g carlson r w lugmair g w 2000 timescales of planetesimal formation and differ entiation based on extinct and extant radioisotopes in canup r righter k eds origin of the earth and moon university of arizona press tucson pp 25 44 younger zircon age domains ca 4530 ma also record crustal processes but these are interpreted to be exogenous because they are well after the effective extinction of 26

pre 3750 ma supracrustal rocks from the nuvvuagittuq supracrustal belt northern quebec geochemistry and u pb ion microprobe zircon geochronology guided by high resolution mapping 1 50 scale was used to define a minimum age of ca 3750 ma for supracrustal rocks from the nuvvuagittuq supracrustal belt nsb in the northern superior province quebec canada mineralogy and geochemistry of critical field relationships preserved at the porpoise cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks finely banded quartz magnetite units with intermixed coarse grained ferruginous quartz pyroxene rocks and quartz biotite schists that superficially resemble polymict metaconglomerates with large up to 10 cm deformed polycrystalline quartz and mafic clasts all units in the mapped outcrop have sharp lithological contacts narrow dominantly trondhjemitic orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals the total extent of the supracrustal enclave is 8k m 2 it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions the quartz biotite schists record complex zircon growth at 3500 and 2800 ma interpreted to reflect metamorphic events zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz pyroxene magnetite unit yield magmatic 207 pb 206 pb ages of ca 3750 ma these ages are slightly younger than earlier provisional reports for an nsb orthogneiss from porpoise cove the nuvvuagittuq supracrustals are the oldest rocks thus far reported for the minto block they overlap in age with the ca 3 70 3 83 ga isua supracrustal belt and akilia association rocks in west greenland and they represent an important new area for exploration of the early earth

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

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

oxygen isotope variation in stony iron meteorites asteroidal material delivered to earth as meteorites preserves a record of the earliest stages of planetary formation high precision oxygen isotope analyses for the two major groups of stony iron meteorites main group pallasites and mesosiderites demonstrate that each group is from a distinct asteroidal source mesosiderites are isotopically identical to the howardite eucrite diogenite clan and like them are probably derived from the asteroid 4 vesta main group pallasites represent intermixed core mantle material from a single disrupted asteroid and have no known equivalents among the basaltic meteorites the stony iron meteorites demonstrate that intense asteroidal deformation accompanied planetary accretion in the early solar system

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

ocean stratification and low melt rates at the ross ice shelf grounding zone ocean driven melting of ice shelves is a primary mechanism for ice loss from antarctica however due to the difficulty in accessing the sub ice shelf ocean cavity the relationship between ice shelf melting and ocean conditions is poorly understood particularly near the grounding zone where the ice transitions from grounded to floating we present the first borehole oceanographic observations from the grounding zone of the ross ice shelf antarctica s largest ice shelf by area contrary to predictions that tidal currents near grounding zones mix the water column we found that ross ice shelf waters were vertically stratified current velocities at middepth in the ocean cavity did not change significantly over measurement periods at two different parts of the tidal cycle the observed stratification resulted in low melt rates near this portion of the grounding zone inferred from phase sensitive radar observations these melt rates were generally 10 cm year which is lower than average for the ross ice shelf 20 cm year melt rates may be higher at portions of the grounding zone that experience higher subglacial discharge or stronger tidal mixing stratification in the cavity at the borehole site was prone to diffusive convection as a result of ice shelf melting since diffusive convection influences vertical heat and salt fluxes differently than shear driven turbulence this process may affect ice shelf melting and merits further consideration in ocean models of sub ice shelf circulation

an archaean heavy bombardment from a destabilized extension of the asteroid belt the late heavy bombardment lasted much longer than previously thought up to 1 7 billion years ago on earth with impacts on the moon and earth coming mostly from the e belt survivor hungaria asteroids the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets the barrage of comets and asteroids that produced many young lunar basins craters over 300 kilometres in diameter has frequently been called the late heavy bombardment1 lhb many assume the lhb ended about 3 7 to 3 8 billion years gyr ago with the formation of orientale basin2 3 evidence for lhb sized blasts on earth however extend into the archaean and early proterozoic eons in the form of impact spherule beds globally distributed ejecta layers created by chicxulub sized or larger cratering events4 at least seven spherule beds have been found that formed between 3 23 and 3 47 gyr ago four between 2 49 and 2 63 gyr ago and one between 1 7 and 2 1 gyr ago5 6 7 8 9 here we report that the lhb lasted much longer than previously thought with most late impactors coming from the e belt an extended and now largely extinct portion of the asteroid belt between 1 7 and 2 1 astronomical units from earth this region was destabilized by late giant planet migration10 11 12 13 e belt survivors now make up the high inclination hungaria asteroids14 15 scaling from the observed hungaria asteroids we find that e belt projectiles made about ten lunar basins between 3 7 and 4 1 gyr ago they also produced about 15 terrestrial basins between 2 5 and 3 7 gyr ago as well as around 70 and four chicxulub sized or larger craters on the earth and moon respectively between 1 7 and 3 7 gyr ago these rates reproduce impact spherule bed and lunar crater constraints

interannual variations in meltwater input to the southern ocean from antarctic ice shelves ocean driven basal melting of antarctica s floating ice shelves accounts for about half of their mass loss in steady state where gains in ice shelf mass are balanced by losses ice shelf thickness changes driven by varying basal melt rates modulate mass loss from the grounded ice sheet and its contribution to sea level and the changing meltwater fluxes influence climate processes in the southern ocean existing continent wide melt rate datasets have no temporal variability introducing uncertainties in sea level and climate projections here we combine surface height data from satellite radar altimeters with satellite derived ice velocities and a new model of firn layer evolution to generate a high resolution map of time averaged 2010 2018 basal melt rates and time series 1994 2018 of meltwater fluxes for most ice shelves total basal meltwater flux in 1994 1090 150 gt yr was not significantly different from the steady state value 1100 60 gt yr but increased to 1570 140 gt yr in 2009 followed by a decline to 1160 150 gt yr in 2018 for the four largest cold water ice shelves we partition meltwater fluxes into deep and shallow sources to reveal distinct signatures of temporal variability providing insights into climate forcing of basal melting and the impact of this melting on the southern ocean

trace element composition and u pb age of zircons from estherville constraints on the timing of the metal silicate mixing event on the mesosiderite parent body abstract mesosiderites are a group of stony iron meteorites which are thought to be the result of mixing of silicates with fe ni metal in this study we combined textural observations with geochemical and chronological studies of two zircon grains found in the estherville mesosiderite one of the zircons zrc1 occurs with pyroxene plagioclase troilite and silica and the other zrc2 is located at a boundary between fe ni metal and a silicate part mainly composed of pyroxene and plagioclase the textural observations demonstrate that zrc1 is relatively homogenous whereas zrc2 is composed of at least two chemically distinct domains trace element analyses of zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements ree and two orders of magnitude for u and th the lowest trace element concentration in zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons however it is similar to those of zrc1 and a zircon from the vaca muerta mesosiderite the calculated ree composition of the melt in equilibrium with zrc2 shows that zrc2 and perhaps also zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages the zircons with low ree u and th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of ree u and th into secondary phosphate minerals which formed during the metal silicate mixing event the large concentration gradients observed in zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized alternatively either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients however the ree patterns of zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons these considerations suggest that zrc1 and zrc2 formed during a high temperature reheating event which is probably related to the metal silicate mixing event the weighted average 207 pb 206 pb age obtained by sims from both zircons is 4521 26 ma 2 this age is younger than that of a primary magmatic zircon from vaca muerta 4563 15 ma and probably corresponds to the timing of the metal silicate mixing event or a later impact event

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

complex history of a zircon aggregate from lunar breccia 73235 abstract results are reported of an investigation of the age and origin of the exceptional zircon aggregate in an anorthositic clast from lunar breccia 73235 cathodoluminescence and birefringence images show that the aggregate consists of numerous angular fragments of sector zoned primary zircon in a matrix of secondary zircon with an overall texture that resembles a pseudotachylite sims u pb analyses of the primary fragments and the matrix yielded two clearly defined ages an age of 4 315 0 015 ga and initial th u ratio of 0 21 0 35 for the primary zircon and an age of 4 187 0 011 ga and th u of 0 04 0 17 for the secondary zircon matrix a raman spectroscopic study the secondary matrix zircon was undertaken to investigate its structure results showed that the matrix has a zircon structure but there is also evidence for the presence of an amorphous component implications of the structural and u pb age data are discussed in terms of the origin and evolution of the aggregate and the history of lunar events it is proposed that an original single millimetre sized sector zoned zircon formed at 4 31 ga was subjected to a severe shock event at 4 18 ga this event resulted in the fracturing of the zircon the displacement and rotation of fragments the compression of the aggregate to a lensoid shape and the shock reduction of zircon to sub micron sized and amorphous granules in crush zones in the mosaic of fractures volatilisation loss of pb and the addition of u to the secondary zircon is attributed to processes activated by the extreme thermal pulse which accompanied the 4 18 ga shock event shock effects are seen in some of the primary fragments but raman spectra of the primary and secondary zircon show no evidence for pressure induced transformation of zircon to a scheelite structure the zircon u pb system has not been affected by the ca 3 95 ga thermal pulse that accompanied formation of the host breccia although this event has largely reset the k ar systems

temporal relationship between the 2000 miyake kozu seismovolcanic activity and the 2000 tokai slow slip event abstract we investigated the temporal relationship between the two events namely the seismovolcanic activity near the miyakejima and kozushima islands and the slow slip event along the plate boundary in the tokai district the islands are located on the east of tokai and the tokai slow slip event was discovered immediately after the large crustal deformation caused by the volcanic activity in the miyakejima kozushima regions ceased however the order of occurrence of these events is still controversial and its recognition will help us to understand the tectonic processes of the central part of japan where many volcanic and seismic activities occur for this purpose we applied the statistical approach kawamura m yamaoka k 2006 spatiotemporal characteristics of the displacement field revealed with principal component analysis and the mode rotation technique tectonophys 419 55 73 which consists of principal component analysis pca and a mode rotation procedure to the displacement field provided by the nationwide gps network geonet in order to obtain the characteristic structures of spatiotemporal crustal deformation caused by the above two events we divided the time period of analysis into two sections namely june 26 1999 to june 25 2000 and june 26 2000 to june 25 2002 by the day when the magma intrusion occurred beneath the miyakejima volcano on june 26 2000 the spatial and temporal modes for the first time period did not indicate any significant spatiotemporal patterns corresponding to the two events this indicates the absence of episodic crustal deformations during this time period on the contrary the modes for the latter time period included the changes caused by these events the two major modes included the spatiotemporal structures of the first and latter half periods of the miyake kozu seismovolcanic activity the characteristic pattern of the crustal deformation corresponding to the tokai slow slip event was found in the fourth mode which was prominent after the beginning of the miyake kozu seismovolcanic activity from these results we conclude that the crustal deformation caused by the tokai slow slip event did not start before the miyake kozu seismovolcanic activity

noble gases 81kr kr exposure ages and 244pu xe ages of six eucrites bereba binda camel donga juvinas millbillillie and stannern noble gases including radioactive 81kr t1 2 2 1 105 yr have been measured for the cumulate eucrite binda and five noncumulate eucrites bereba camel donga juvinas millbillillie and stannern some of which have been repeatedly analyzed concentrations of 81kr which range from 4 2 10 14 to 3 2 10 13 cm3 stp g roughly correlate with abundances of the main target elements sr y and zr 81kr kr cosmic ray exposure ages are obtained as 24 5 0 6 ma 21 0 1 6 ma 36 6 1 4 ma 10 6 0 8 ma 20 8 0 5 ma and 35 1 0 7 ma for bereba binda camel donga juvinas millbillillie and stannern respectively all the measured eucrites have 244pu derived fission xe on the basis of fission 136xe concentrations 244pu abundances are calculated to range from 0 40 ppb for binda to 1 08 ppb for juvinas following the method proposed by shukolyukov and begemann 1996a 244pu xe ages relative to the absolute crystallization age of 4 5578 ga for angra dos reis are determined as 46 18 ma bereba 29 34 ma binda 51 16 ma camel donga 10 23 ma juvinas 8 24 ma fine grained portion of millbillillie 51 21 ma coarse grained portion of millbillillie and 124 13 ma stannern which correspond to absolute ages ranging from 4 434 ga stannern to 4 566 ga fine grained portion of millbillillie millbillillie is a mixture of materials with different 244pu xe ages stannern s unusually young 244pu xe age might reflect a secondary shock disturbance but otherwise it tends to confirm a distinctive mode of igneous petrogenesis in comparison to most other eucrites the 244pu xe ages of the cumulate eucrites obtained so far binda this work moore county shukolyukov and begemann 1996a do not differ systematically from those of noncumulate eucrites the four eucrites with the older 244pu xe ages bereba binda juvinas and the fine grained portion of millbillillie show significant amounts of radiogenic 129xe 0 6 2 3 10 12 cm3 stp g after correction for fission 129xe which imply earlier retention of radiogenic 129xe from extinct 129i than other eucrites with young 244pu xe ages

u pb and 207pb 206pb ages of zircons from basaltic eucrites implications for early basaltic volcanism on the eucrite parent body we have undertaken petrologic and shrimp u th pb isotopic studies on zircons from basaltic eucrites yamato y 75011 y 792510 asuka a 881388 a 881467 and padvarninkai with different thermal and shock histories eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene and typically have rounded to subrounded shapes due to recrystallization superchondritic zr hf ratios of eucritic zircons indicate that they crystallized from incompatible element rich melts after crystallization of ilmenite concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite y 75011 are higher than those in metamorphosed eucrites the u pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance radiogenic lead loss up to 48 from zircons is observed in the shock melted eucrite padvarninkai the 207pb 206pb ages of zircon in y 75011 4550 9 ma n 5 are nearly identical within analytical uncertainty to the ages of zircons from the metamorphosed eucrite y 792510 4545 15 ma n 13 the highly metamorphosed eucrites a 881388 4555 54 ma n 5 and a 881467 4558 13 ma n 8 and the shock melted eucrite padvarninkai 4555 13 ma n 18 the averaged 207pb 206pb age of zircon from five eucrites analyzed in this study is 4554 7 ma 95 confidence limits n 49 indistinguishable from the averaged u pb age 4552 9 ma of the same samples because of the high closure temperature of lead in zircon tclosure 1050 c with a cooling rate of 0 2 c yr the 207pb 206pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas this fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the solar system history 7 20 ma after cai formation 4567 2 0 6 ma thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body the u pb ages of eucritic zircons are older than the u pb sm nd and rb sr ages of some basaltic eucrites which is consistent with differences in closure temperatures of each isotopic system and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene plagioclase and phosphates

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

a sawtooth like timeline for the first billion years of lunar bombardment abstract we revisit the early evolution of the moon s bombardment our work combines modeling based on plausible projectile sources and their dynamical decay rates with constraints from the lunar crater record radiometric ages of the youngest lunar basins and the abundance of highly siderophile elements in the lunar crust and mantle we deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history but also there was no prominent and narrow impact spike 3 9 gy ago unlike that typically envisioned in the lunar cataclysm scenario instead we show the timeline of the lunar bombardment has a sawtooth like profile with an uptick in the impact flux near 4 1 gy ago the impact flux at the beginning of this weaker cataclysm was 5 10 times higher than the immediately preceding period the nectaris basin should have been one of the first basins formed at the sawtooth we predict the bombardment rate since 4 1 gy ago declined slowly and adhered relatively close to classic crater chronology models neukum and ivanov 1994 overall we expect that the sawtooth event accounted for about one fourth of the total bombardment suffered by the moon since its formation consequently considering that 12 14 basins formed during the sawtooth event we expect that the net number of basins formed on the moon was 45 50 from our expected bombardment timeline we derived a new and improved lunar chronology suitable for use on pre nectarian surface units according to this chronology a significant portion of the oldest lunar cratered terrains has an age of 4 38 4 42 gyr moreover the largest lunar basin south pole aitken is older than 4 3 gy and therefore was not produced during the lunar cataclysm

40ar 39ar ages of h chondrite impact melt breccias 40ar 39ar analyses of a total of 26 samples from eight shock darkened impact melt breccias of h chondrite affinity gao guenie lap 02240 lap 03922 lap 031125 lap 031173 lap 031308 nwa 2058 and ourique are reported these appear to record impacts ranging in time from 303 56 ma gao guenie to 4360 120 ma ourique ago three record impacts 300 400 ma ago while two others record impacts 3900 4000 ma ago combining these with other impact ages from h chondrites in the literature it appears that h chondrites record impacts in the first 100 ma of solar system history during the era of the lunar cataclysm and shortly thereafter 3500 4000 ma ago one or more impacts 300 ma ago and perhaps an impact 500 ma ago near the time of the l chondrite parent body disruption records of impacts on the h chondrite parent body are rare or absent between the era of planetary accretion and the lunar cataclysm 4400 4050 ma during the long stretch between heavy bombardment and recent breakup events 3500 1000 ma or at the time of final breakup into meteorite sized bodies 50 ma

rb sr and sm nd internal isochron ages of a subophitic basalt clast and a matrix sample from the y75011 eucrite the yamato polymict eucrite y75011 contains several coarse grained mesostasis rich basaltic clasts with subophitic texture the preservation of extensive mg fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava like basalts extruded on the surface of the howardite eucrite diogenite hed parent body rubidium strontium analyses of a large clast y75011 84b yielded a precise internal isochron age of 4 60 0 05 b y for 87rb 0 0139 b y 1 or 4 50 0 05 b y for 87rb 0 0142 b y 1 the rb sr systematics of a matrix sample are nearly identical to those of the pristine clast showing that the matrix is dominantly comminuted mesostasis rich basalt the rb sr internal isochron age of the matrix is 4 56 0 06 b y for 87rb 0 0139 b y 1 or 4 46 0 06 b y for 87rb 0 0142 b y 1 these ages are within error limits of the rb sr whole rock age of chondrites and are in agreement with 206pb 207pb and sm nd ages of ordinary monomict eucrites for 87rb 0 01402 b y 1 based on a comparison of the rb sr and u th pb ages of chondrites minster et al 1982 the good precision obtained for the rb sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the rb sr ratio is enriched 3 4 times above that of any eucrite phase previously analysed the initial 87sr 86sr ratios of clast and matrix are 0 69894 2 and 0 69896 3 respectively and are in satisfactory agreement with the babi value papanastassiou and wasserburg 1969 there is evidence of isotopic disturbance in both the rb sr and sm nd systems of the most magnetic separate analysed a sm nd internal isochron of 4 55 0 14 b y for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise rb sr age the corresponding initial 143nd 144nd 0 50587 19 normalized to 148nd 144nd 0 24308 the clast data alone exclusive of the most magnetic separate define an isochron age of 4 52 0 16 b y and initial 143nd 144nd 0 50593 19 the isochron error limits quoted here for the sm nd data are 2 from the york 1966 program and include the factor s n 2 used in that program to compensate for geological error however this factor is not appropriate for the rb sr isochrons and has been omitted from the quoted error limits the initial 143nd 144nd of the pristine clast at 4 56 b y ago is 0 505877 25 in agreement within error limits with the chur value jacobsen and wasserburg 1984 clast y75011 84 represents the oldest pristine unambiguously basaltic lava as yet recovered from a solar system object rare earth element abundance measurements shimizu and masuda 1985 show it to be one of the most differentiated eucrites thus moderately evolved basaltic lavas were produced very early in the history of the hed parent body their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent body than that occupied by the more thermally metamorphosed monomict eucrites we suggest that the preservation of pristine clasts in polymict eucrites may be due to their impact excavation to the surface of the parent body away from heat sources concentrated within impact craters the monomict eucrites may be derived from the floors of craters where they were thermally metamorphosed by shock heating and by the presence of overlying impact melts and nearly molten breccias

the petrology and chronology of nwa 8009 impact melt breccia implication for early thermal and impact histories of vesta abstract studies of petrology mineralogy and geochronology of eucrites are keys to reconstruct the thermal and impact history of 4 vesta the proposed parent body for hed meteorites here we report the petrography mineralogy and geochemistry of nwa 8009 a newly found eucritic impact melt breccia and present sims u pb ages of zircon and phosphates nwa 8009 consists of coarse and fine grained lithic and mineral clasts set in fine grained recrystallized matrix it was derived from a protolith of monomict non cumulate eucrite evidence for intense shock metamorphism observed in nwa 8009 includes mosaicism deformed exsolution lamellae and partial melting of pyroxene melting and incipient flow of plagioclase planar fractures and granular textures of zircon these shock effects indicate nwa 8009 was subjected to an impact metamorphism with peak pressure of 50 60 gpa and post shock temperature of 1160 1200 c nwa 8009 is among the most intensely shocked heds reported yet after the impact the sample was buried near the surface in target rocks and experienced rapid cooling 23 c h and annealing resulting in recrystallization of the matrix and devitrification of plagioclase and silica glasses u pb isotopic system of apatite within plagioclase groundmass of lithic clasts is completely reset and constrains the timing of impact at 4143 61 ma providing a new robust impact age on vesta combined with the presence of synchronous impact resetting events especially those recorded by lu hf sm nd and pb pb isotopic systems we identified a period of high impacts flux at ca 4 1 4 2 ga on vesta this impact flux occurred coincident with the uptick at ca 4 1 4 2 ga in impact age spectra of the moon probably reflects widespread intense bombardment throughout the inner solar system at ca 4 1 4 2 ga based on evidence from zircon chemical zoning petrographic occurrences as well as the distinctive zr hf ratios we suggested that zircons in nwa 8009 have had a metamorphic instead of magmatic origin they mainly crystallized from melts produced by partial melting of mesostasis area due to reheating event during early global thermal metamorphism rather than by zr release from zr rich minerals the u pb isotopic system in zircons was not disturbed by subsequent impacts the weighted mean 207 pb 206 pb age of 4560 8 ma represents the timing of zircon growth during thermal metamorphism zircons from nwa 8009 and other eucrites may share a common origin during metamorphic growth events and constraining the global thermal metamorphism on vesta at ca 4 55 ga the main heat sources responsible for global metamorphism in basaltic crust of vesta might be heating from the hot interior especially heat flow related to magmatism rather than impact

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

pathways of meltwater export from petermann glacier greenland abstractintrusions of atlantic water cause basal melting of greenland s marine terminating glaciers and ice shelves such as that of petermann glacier in northwest greenland the fate of the resulting glacial meltwater is largely unknown it is investigated here using hydrographic observations collected during a research cruise in petermann fjord and adjacent nares strait onboard icebreaker i b oden in august 2015 a three end member mixing method provides the concentration of petermann ice shelf meltwater meltwater from petermann is found in all of the casts in adjacent nares strait with the highest concentration along the greenland coast in the direction of kelvin wave phase propagation the meltwater from petermann mostly flows out on the northeast side of the fjord as a baroclinic boundary current with the depth of maximum meltwater concentrations approximately 150 m and shoaling along its pathway at the outer sill which separates the fjord from the ambient ocean approximately 0 3 msv 1 sv

abundance distribution and origin of 60fe in the solar protoplanetary disk abstract meteorites contain relict decay products of short lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed several studies reported a high abundance of 60fe t1 2 2 62 0 04 myr in chondrites 60fe 56fe 6 10 7 suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn sun we measured 58fe 54fe and 60ni 58ni isotope ratios in whole rocks and constituents of differentiated achondrites ureilites aubrites heds and angrites unequilibrated ordinary chondrites semarkona ll3 0 and nwa 5717 ungrouped petrologic type 3 05 metal rich carbonaceous chondrite gujba cba and several other meteorites cv el h ll chondrites iiiab iva ivb iron meteorites we derive from these measurements a much lower initial 60fe 56fe ratio of 11 5 2 6 10 9 and conclude that 60fe was homogeneously distributed among planetary bodies this low ratio is consistent with derivation of 60fe from galactic background 60fe 56fe 2 8 10 7 in the interstellar medium from ray observations and can be reconciled with high 26al 27al 5 10 5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars e g a wolf rayet star rich in 26al and poor in 60fe we present the first chronological application of the 60fe 60ni decay system to establish the time of core formation on vesta at 3 7 1 7 2 5 myr after condensation of calcium aluminum rich inclusions cais

ion microprobe uranium thorium lead dating of shergotty phosphates we report ion microprobe u th pb dating of shergotty phosphates by means of the sensitive high resolution ion microprobe shrimp recently installed at hiroshima university japan ten analyses of whitlockite merrillite and three analyses of apatite indicate a 238u 206pb isochron age of 225 200 ma and a tera wasserburg concordia constrained linear three dimensional isochron age of 217 110 ma in the 238u 206pb 207pb 206pb204pb 206pb diagram these ages agree well with the 232th 208pb age of 189 83 ma which suggests that primary crystallization or a shock metamorphic event defined the formation age of the phosphate minerals the average of the later two ages 204 68 ma is consistent with the previously published rb sr age of 165 11 ma and u th pb age of 200 ma these show marginal agreement with the 40ar 39ar age of 254 10 ma but are significantly different from the sm nd age of 360 16 ma taking into account the closure temperature of the u pb system in apatite we suggest the time that shergotty last experienced a temperature of 900 c was 204 68 ma

supercooled southern ocean waters in cold polar waters temperatures sometimes drop below the freezing point a process referred to as supercooling however observational challenges in polar regions limit our understanding of the spatial and temporal extent of this phenomenon we here provide observational evidence that supercooled waters are much more widespread in the seasonally ice covered southern ocean than previously reported in 5 8 of all analyzed hydrographic profiles south of 55 s we find temperatures below the surface freezing point potential supercooling and half of these have temperatures below the local freezing point in situ supercooling their occurrence doubles when neglecting measurement uncertainties we attribute deep coastal ocean supercooling to melting of antarctic ice shelves and surface induced supercooling in the seasonal sea ice region to winter time sea ice formation the latter supercooling type can extend down to the permanent pycnocline due to convective sinking plumes an important mechanism for vertical tracer transport and water mass structure in the polar ocean

a thermal model for the differentiation of asteroid 4 vesta based on radiogenic heating a finite element code has been developed to model the thermal history of asteroid 4 vesta this is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained the results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from vesta the work solves two major problems with the hypothesis of heating by decay of26al an extinct radionuclide postulated to be a plausible heat source in the early solar system first the model demonstrates that it is possible to keep the mantle of vesta hot for 100 ma thereby explaining the observed difference in ages between cumulate and noncumulate eucrites second the simulation offers a possible explanation of why detectable excesses of26mg the decay product of26al are not observed in noncumulate eucrites the simulation draws a model chronology of vesta and predicts times relative to cai formation for accretion at 2 85 myr core formation at 4 58 myr crust formation at 6 58 myr and geochemical closure at 100 myr for a h chondrite asteroidal bulk composition decay of60fe is found to cause no perceptible difference in the thermal history of vesta even when sequestered into a central core although chondritic xenoliths have not been described in hed igneous lithologies the thermal model suggests the possibility that a veneer of unmelted near surface material should remain

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

sims pb pb and u pb age determination of eucrite zircons at 5 μm scale and the first 50 ma of the thermal history of vesta eucrites a subgroup of basaltic achondrites represent the remnants of the earliest magmatic stage of the solar system formation possibly on the asteroid 4 vesta the precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid including melting and differentiation processes of the parent body zircons would be the best mineral for such purpose however extra terrestrial zircons are rare and those found in eucrites are typically small 5 m using a cameca ims 1280 high resolution ion microprobe precise uranium lead u pb and pb pb ages of the non cumulate basaltic eucrite zircons in bereba cachari caldera camel donga and juvinas were determined successfully at scale of 5 m we obtained a weighted average 207pb 206pb age of 4541 11 ma and u pb concordia age of 4525 24 ma for the five eucrite samples studied a synthesis of available age data of howardites eucrites and diogenites indicate a protracted magmatic history on vesta with a most pronounced core mantle differentiation event marked at 4564 2 ma followed by a peak of basaltic magmatism at 4552 7 ma which gradually diminished over the next 50 ma thermal modeling of vesta suggests that partial melt fraction of greater than 10 with temperature well exceeding 1500 k could be maintained over this period throughout its interior this is consistent with the chronological records of eucrites suggesting persistent magmatism on vesta during the first 50 ma in the early solar system

a protracted timeline for lunar bombardment from mineral chemistry ti thermometry and u pb geochronology of apollo 14 melt breccia zircons new zircon u pb and trace element investigations from apollo 14 lunar impact breccia sample 14311 reveal at least three distinct concordia 2 age populations at 4334 10 4245 10 and 3953 10 ma titanium in zircon thermometry tixln results correlated with u pb ages range from 800 1200 oc lattice strain models used to infer zircon versus whole rock rare earth element contents and partitioning calculations against lunar impact breccia component compositions quantitatively constrain formation conditions for the different age populations a compilation of new data with published work shows that apollo 14 zircons older than ca 4300 ma formed by igneous processes associated with lunar crust formation compositional variability in the ca 4240 ma zircon age population is interpretable however via a mixture of inherited and melt generated components from one or more large impacts perhaps related to a marked increase in bombardment flux ages from the youngest zircon group at ca 3950 ma coincide with the classical late heavy bombardment lhb as documented from previous lunar geochronologies these results lend support to the idea that instead of a simple unimodal lhb scenario or a monotonic decline in impacts the moon was battered by multiple cataclysms since ca 4240 ma such a picket fence like bombardment to the moon best describes the mode and tempo of impacts that accompanied the late stages of solar system formation and giant planet migration

the crystallization age of eucrite zircon eucrites are a group of meteorites that represent the first planetary igneous activity following metal silicate differentiation on an early planetesimal similar to asteroid 4 vesta and thus help date geophysical processes occurring on such bodies in the early solar system using the short lived radionuclide 182hf as a relative chronometer we demonstrate that eucrite zircon crystallized quickly within 6 8 million years of metal silicate differentiation this implies that mantle differentiation on the eucrite parent body occurred during a period when internal heat from the decay of 26al and 60fe was still available later metamorphism of eucrites took place at least 8 9 million years after the zircons crystallized and was likely caused by heating from impacts or by burial under hot material excavated by impacts rather than from lava flows thus the timing of eucrite formation and of mantle differentiation is constrained

measurements of ice shelf water beneath the front of the ross ice shelf using gliders measurements made by an underwater glider deployed near the ross ice shelf were used to identify the presence of ice shelf water isw which is defined as seawater with its potential temperature lower than its surface freezing point temperature properties logged by the glider included in situ temperature electrical conductivity pressure gps location at surfacings and time for most of the first 30 recorded dives of its deployment evidence suggests the glider was prevented from surfacing due to being under the ice shelf for dives under the ice shelf farthest from the ice shelf front isw layers of varying thicknesses and depth locations were observed between 2 m thick centred at 231 m depth to 93 m thick centred at 360 m for dives under the ice shelf close to the ice shelf front either no isw was observed or isw layers were centred at shallower depths 116 127 m thicker isw layers e g up to 250 m thickness centred at 421 m were observed for some glider dives in open water in front of the ross ice shelf no in situ supercooling water colder than the pressure dependent freezing point temperature was observed

thermal history recorded by the apollo 17 impact melt breccia 73217 abstract lunar breccia 73217 is composed of plagioclase and pyroxene clasts originating from a single gabbronorite intrusion mixed with a silica rich glass interpreted to represent an impact melt a study of accessory minerals in a thin section from this breccia 73217 52 identified three different types of zircon and anhedral grains of apatite which represent distinct generations of accessory phases and provide a unique opportunity to investigate the thermal history of the sample equant anhedral zircon grains that probably formed in the gabbronorite referred to as type 1 have consistent u pb ages of 4332 7 ma a similar age of 4335 5 ma was obtained from acicular zircon type 2 grains interpreted to have formed from impact melt a polycrystalline zircon aggregate type 3 occurs as a rim around a baddeleyite grain and has a much younger age of 3929 10 ma similar to the 3936 17 ma age of apatite grains found in the thin section a combined apatite type 3 zircon age of 3934 12 ma is proposed as the age of the serenitatis impact event and associated thermal pulse x ray mapping and electron probe analyses showed that ti is inhomogeneous in the zircon grains on the sub micrometer scale however model temperatures estimated from shrimp analyses of ti concentration in the 10 m diameter spots on the polished surfaces of type 1 and type 2 zircons range between about 1300 and 900 c respectively whereas ti concentrations determined for the type 3 zircon are higher at about 1400 1500 c a combination of u pb ages ti concentration data and detailed imaging and petrographic studies of the zircon grains shows that the gabbronorite parent of the zircon clasts formed shortly before the 4335 5 ma impact which mixed the clasts and the felsic melt and projected the sample closer to the surface where fast cooling resulted in the crystallization of acicular zircon type 2 the 3934 12 ma serenitatis event resulted in partial remelting of the glass and formation of polycrystalline zircon type 3 this event also reset the u pb system of apatite formed merrillite coronas around some apatite grains and probably re equilibrated some pyroxenes in the clasts although there have been arguments for pre 3 9 ga impacts based on other types of samples the age of the acicular zircon at 4335 5 ma provides the first evidence of impact melt significantly predating the lunar cataclysm our data combined with other chronological results demonstrate the occurrence of pre 3 9 ga impacts on the moon and suggest that the lunar impact history consisted of a series of intense bombardment episodes interspersed with relatively calm periods of low impact flux