crustal partial melting on vesta evidence from highly metamorphosed eucrites we have performed a mineralogical and geochemical study of eight metamorphosed basaltic eucrites these are classified into granulitic eucrites and type 4 7 eucrites on the basis of their textures and pyroxene mineralogy and display mineralogical evidence for high temperature metamorphism including partial melting in particular rare earth element ree patterns of a number of the eucrites studied show varying degrees of light ree depletion due to partial melting with subsequent melt extraction a simple correlation between metamorphic grade as deduced from pyroxene mineralogy and the degree of light ree depletion was not detected this can be explained by the fact that homogenization exsolution and inversion of pigeonite would have required prolonged heating at moderate temperatures not vert similar800 1000 c whereas partial melting would have taken place over a short time interval where temperatures exceeded that of the solidus the eucrites studied therefore record a two stage thermal regime consisting of short high temperature reheating events superimposed on long duration global crustal metamorphism the short reheating events may have been caused by impact events and or intrusions of hot magmas the results of this study demonstrate that the thermal history of eucritic crust was more complex than can be explained by a simple burial model alone in particular the origin of stannern trend eucrites requires contamination of main group magmas by partial melts extracted from residual eucrites

unraveling the evolution of chondrite parent asteroids by precise u pb dating and thermal modeling u th pb isotopic data are reported for mineral fractions individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite richardton h5 chondrules and milligram sized fractions of pyroxene rich chondrule fragments contain highly radiogenic pb and concordant or nearly concordant u th pb isotopic systems and are suitable for precise pb pb age determinations olivine and sulfide have low u concentrations and contain less radiogenic pb the ages of individual chondrules pyroxene rich and phosphate fractions are determined using u pb and pb pb isochron and model date calculations the pb pb isochron date of 4562 7 1 7 ma of the richardton chondrules and chondrule fragments is resolved from the pb pb isochron date of 4550 7 2 6 ma obtained from multiple phosphate fractions possible biases of the isochron dates due to single stage approximation of multi stage evolution contamination with modern common pb and disturbance to the system by reheating are examined and are found to be insignificant the chondrule and phosphate dates are interpreted as the timing of cessation of pb diffusion during cooling following metamorphism in chondrite parent bodies the difference in estimated closure temperatures 950 1150 k for pyroxenes and 700 800 k for phosphates temperature estimates are based on published diffusion rates for pb in pyroxenes and apatite allows evaluation of the average cooling rate at 26 13 k million years for the richardton parent body over the period of 4563 4551 my thermal modeling of the h chondrite parent body which is assumed to be asteroid 6 hebe heated by decay of 26al suggests a scenario in which accretion initiated at 1 7 m y after formation of calcium aluminum rich inclusions and continued for 3 5 m y

differentiation of vesta implications for a shallow magma ocean abstract the dawn mission confirms earlier predictions that the asteroid 4 vesta is differentiated with an iron rich core a silicate mantle and a basaltic crust and supports the conjecture of vesta being the parent body of the hed meteorites to better understand its early evolution we perform numerical calculations of the thermo chemical evolution adopting new data obtained by the dawn mission such as mass bulk density and size of the asteroid we have expanded the thermo chemical evolution model of neumann et al 2012 that includes accretion compaction melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources advective heat transport and differentiation by porous flow to further consider convection and the associated effective cooling in a potential magma ocean depending on the melt fraction the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity our results show that partitioning of 26al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean due to the enrichment of 26al in the liquid phase and its accumulation in the sub surface for formation times t 0 1 5 ma a thin shallow magma ocean with a thickness of 1 to a few tens of km forms its thickness depends on the viscosity of silicate melt the lifetime of the shallow magma ocean is o 10 4 o 10 6 years and convection in this layer is accompanied by the extrusion of 26al at the surface resulting in the formation of a basaltic crust the interior differentiates from the outside inwards with a mantle that is depleted in 26al and core formation is completed within 0 3 ma the lower mantle experiences a maximal melt fraction of 45 suggesting a harzburgitic to dunitic composition our results support the formation of non cumulate eucrites by the extrusion of early partial melt while cumulate eucrites and diogenites may form from the crystallising shallow magma ocean silicate melt is present in the mantle for up to 150 ma and convection in a crystallising core proceeds for approximately 100 ma supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some hed meteorites

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

live iron 60 in the early solar system isotopic analyses of nickel in samples from the differentiated meteorite chervony kut revealed the presence of relative excesses of 60ni ranging from 2 4 up to 50 parts per 104 these isotopic excesses are from the decay of the now extinct short lived nuclide 60fe and provide clear evidence for the existence of 60fe over large scales in the early solar system not only was 60fe present at the time of melting and differentiation that is fe ni fractionation of the parent body of chervony kut but also later at the time when basaltic magma solidified at or near the surface of the planetesimal the inferred abundance of 60fe suggests that its decay alone could have provided sufficient heat to melt small diameters of several hundred kilometers planetary bodies shortly after their accretion

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

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

metamorphic rates in collisional orogeny from in situ allanite and monazite dating the prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates in individual samples from the northern lepontine central alps allanite is partly replaced by monazite at 560 580 c relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions 430 450 c coexisting monazite and allanite were dated in situ to delimit in time successive stages of the barrovian metamorphism in situ sensitive high resolution ion microprobe shrimp u th pb dating of allanite 31 5 1 3 and 29 2 1 0 ma and monazite 18 0 0 3 and 19 1 0 3 ma constrains the time elapsed between 430 450 c and 560 580 c which implies an average heating rate of 8 15 c m y combined with new fission track ages zircon 10 9 ma apatite 7 5 6 5 ma metamorphic rates of the entire orogenic cycle from prograde to final cooling can be reconstructed

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

vesta s shape and morphology vesta s surface is characterized by abundant impact craters some with preserved ejecta blankets large troughs extending around the equatorial region enigmatic dark material and widespread mass wasting but as yet an absence of volcanic features abundant steep slopes indicate that impact generated surface regolith is underlain by bedrock dawn observations confirm the large impact basin rheasilvia at vesta s south pole and reveal evidence for an earlier underlying large basin veneneia vesta s geology displays morphological features characteristic of the moon and terrestrial planets as well as those of other asteroids underscoring vesta s unique role as a transitional solar system body

u pb systematics of the unique achondrite ibitira precise age determination and petrogenetic implications abstract ibitira is an unbrecciated equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites we present the first combined high precision u and pb isotopic data for this unique meteorite the 238 u 235 u value of 137 777 0 013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites this value results in corrections of 1 1 ma for pb pb dates calculated using the previously assumed invariant 238 u 235 u value of 137 88 using the determined 238 u 235 u value the 7 most radiogenic pb isotopic analyses for acid leached pyroxene rich and whole rock fractions yield an isochron pb pb age of 4556 75 0 57 ma in excellent agreement with the results of mn cr chronology which give the ages of 4557 4 2 5 ma and 4555 9 3 2 ma using the u corrected pb pb age of d orbigny as a time anchor along with the previously proposed thermal history of ibitira and our closure temperature estimates for pb diffusion the pb pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism the metamorphism may have been caused by burial of ibitira lava under successive lava flows and if so the pb pb age should post date the crystallization by a short time interval the pb isotopic data for acid leachates suggest partial re equilibration of pb between plagioclase and phosphate perhaps during an impact event at 4 49 ga as recorded by k ar systematics the whole rock 238 u 204 pb indicates that compared to ci chondrites ibitira is less depleted in pb than in some alkali elements despite a lower condensation temperature of pb than the alkali elements the restricted pb depletion may reflect preferential concentration of metals with high fluid melt partition coefficients including pb and zn as a result of fluid exsolution and migration within the parent magma we discuss the implications of the u pb systematics for the origin and differentiation of the parent body

40ar 39ar dating and cosmic ray exposure time of desert meteorites dhofar 300 and dhofar 007 eucrites and anomalous achondrite nwa 011 we performed high resolution 40ar 39ar dating of mineral separates and whole rock samples from the desert meteorites dhofar 300 dhofar 007 and northwest africa nwa 011 the chronological information of all samples is dominated by plagioclase of varying grain size the last total reset age of the eucrites dhofar 300 and dhofar 007 is 3 9 0 1 ga coeval with the intense cratering period on the moon some large plagioclase grains of dhofar 007 possibly inherited ar from a 4 5 ga event characteristic for other cumulate eucrites due to disturbances of the age spectrum of nwa 011 only an estimate of 3 2 3 9 ga can be given for its last total reset age secondary events causing partial 40ar loss 3 4 ga ago are indicated by all age spectra furthermore ar extractions from distinct low temperature phases define apparent isochrons for all samples these isochron ages are chronologically irrelevant and most probably caused by desert alterations in which radiogenic 40ar and k from the meteorite and occasionally k induced by weathering are mixed accompanied by incorporation of atmospheric ar additional uptake of atmospheric ar by the alteration phase s was observed during mineral separation i e crushing and cleaning in ultrasonic baths consistent cosmic ray exposure ages were obtained from plagioclase and pyroxene exposure age spectra of dhofar 300 25 1 ma and dhofar 007 13 1 ma using the minerals specific target element chemistry and corresponding 38ar production rates

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

puxe dating of eucrites abstract we report the results of xe analyses of the eucrites bereba cachari caldera camel donga chervony kut ibitira jonzac juvinas milbillillie moore county padvarninkai pasamonte pomozdino sioux county and vetluga and the howardite petersburg the obtained data together with data from the literature were utilized for the calculation of pu xe ages using a modified method where the concentration of the reference element nd that is used in the traditional pu xe dating is replaced by that of spallation 126 xe originating from rare earth elements the fraction of spallation 126 xe from ree in total spallation 126 xe is calculated via the ba ree ratios that are very similar in different samples of a given meteorite in comparison with the traditional pu xe dating the present method avoids problems of sampling heterogeneity and uncertainties in absolute instrument sensitivity and can be used when the concentration of nd in a sample is not known the pu xe ages for different samples of individual meteorites show good reproducibility ages of the eucrites bouvante chervony kut ibitira juvinas moore county pasamonte pca82502 pomozdino of the howardite petersburg and of the angrite lew86010 are old and most likely date the time of crystallization at 4 56 ga almost complete retention of fission xe in these meteorites is in a stark contrast with the apparent 1 retention of radiogenic 129 xe the eucrites bereba cachari caldera camel donga jonzac millbillillie neuvo laredo padvarninkai sioux county stannern and vetluga are distinctly younger this can be interpreted in different ways the ages may represent crystallization and or metamorphic ages or may simply reflect the different extent of diffusive fission xe losses during later events comparison of the pu xe ages with those from the sm nd and pb pb isotope chronometers yields in most cases a reasonable agreement in some meteorites however the pb pb chronometer appears to have been reset during secondary events while the pu xe isotope system is seen to be more resistant to such events

experimental evidence of fast transport of trace elements in planetary basaltic crusts by high temperature metamorphism incompatible elements ies such as k p ti and rare earth elements rees provide important constraints on the geochemistry and chronology of basaltic meteorites most of which experienced complicated post crystallization histories these elements are immobile under subsolidus conditions because of their slow diffusion rates in planetary basalt minerals such as pyroxene and plagioclase thus ies are considered to preserve in most cases reliable records of formation processes even in ancient rocks that have undergone moderate thermal processing however observations of natural planetary samples suggest that melting of ie accessory carrier phases enhances the mobilization of such elements here we show that ies are rapidly transported by near solidus partial melting of highly ie enriched minor phases including ca phosphate and ti rich phases these partial melts occur as interconnected veins along cracks and fractures and as thin films on surfaces of pore spaces indicating that the melt mobilization is driven by surface tension the melt transport provides the necessary condition for melt migration consistent with the presence of the depleted basaltic eucrites also reaction between major minerals pyroxene and plagioclase and partial melts may cause disturbance and resetting of some isotopic systems these results have important implications for a range of geochemical investigations in particular the elemental fractionation resulting from such partial melting may result in improper age determinations

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

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

possible fluid rock interactions on differentiated asteroids recorded in eucritic meteorites the eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4 vesta a few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy three distinct alteration stages are identified 1 fe enrichment along the cracks that cross cut the pyroxene crystals fe metasomatism secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage 2 deposits of fe rich olivine fa64 86 and minor amounts of troilite are frequent inside the cracks sporadic secondary ca rich plagioclase an97 98 is associated with the fayalitic olivine 3 at this stage the fe enrichment of the pyroxene is accompanied by a marked al depletion moreover secondary ca rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals the composition of the secondary phases on one hand the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand rule out a silicate melt as the metasomatic agent although no hydrous phase has been yet identified in the studied samples aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites

mineralogy and39ar 40ar age of an old pristine basalt thermal history of the hed parent body previous investigations of mineral chemistry and rbsr and smnd ages indicated that clast 84 from eucrite yamato 75011 had preserved the pristine nature of its initial crystallization during an early stage of the hed parent body microscale mineralogy and39ar 40ar ages of this clast however revealed local disturbance of micrtextures and partially reset ages this evidence suggests that in addition to initial crystallization and rapid cooling the y75011 84 clast experienced shock deformation reheating of short duration at higher temperature and brecciation these characteristics suggest two or more impact events fe rich olivine filling fractures in pyroxene may have been introduced during the accompanying shock fracturing the inferred39ar 40ar degassing ages for y75011 matrix and clast 84 are3 94 0 04 ga and3 98 0 03 ga respectively the suggested degassing age for a clast from y790020 believed to be paired with y75011 is 4 03 ga but could be younger we consider it likely that all three samples experienced a common degassing event3 95 0 05 ga ago but we cannot rule out two or more events spaced over a 0 1 ga interval higher temperature extractions of the two clast samples show significantly older apparent ages up to 4 5 ga and suggest that the time temperature regime of this event was not sufficient to degas ar totally most likely the k ar ages were reset by thermal metamorphism associated with one or more impact events associated with shock fracturing formation of fe rich olivine veins and or meteorite brecciation the pyroxene annealing that commonly occurs in many eucrites is likely to be a much earlier process than the impact produced textural changes and reset k ar ages observed in these meteorites the existence of mineralogical and chronological evidence for metamorphism in an otherwise pristine eucrite suggests that the hed parent body experienced an extensive degree of early cratering

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

petrology of the unbrecciated eucrites abstract twenty nine unbrecciated eucrites have been thoroughly characterized in terms of the petrologic factors that affect their spectra such as mineral chemistry modal adundances grain sizes and textures we have conducted a combined petrologic and spectral study designed to provide insight into the petrogenesis of the basaltic crust of vesta and the variety of rock types that exist within it as well as aid in the petrologic interpretation of spectra to be collected by the dawn orbiting spacecraft this paper details the petrology part of the study unbrecciated eucrite samples were selected to avoid the complications of lithologic mixing in the accompanying spectral study a wide variety of textural types are seen within the basaltic eucrites encompassing quenched coarse grained and granoblastic samples zoned pyroxenes in eucrites and those that preserve a history of initial rapid cooling are rare nearly all eucrite samples have been thermally metamorphosed and would commonly be classified as equilibrated however this term reflects only the quadrilateral mg fe and ca compositions of pyroxenes and considerable variations are seen within the minor elements al ti and cr in pyroxenes as well as plagioclase compositions determination of both pyroxene and plagioclase compositions together with pyroxene geothermometry provides a better estimate for the relative degree of thermal metamorphism a eucrite has experienced the petrologic differences observed here might allow different eucrites to be distinguished spectrally this is especially true for the varying pyroxene compositions as the spectra of eucrites are dominated by absorption features attributed to pyroxene

petrology and geochemistry of the unbrecciated achondrite northwest africa 1240 nwa 1240 an hed parent body impact melt nwa 1240 is an unusual eucrite recently recovered in morocco as a single stone of 98 g it is an unbrecciated greenish brown rock nearly devoid of fusion crust it displays porphyritic texture consisting of skeletal hollow low ca pyroxene phenocrysts set in a variolitic fan spherulitic mesostasis of fine elongate pyroxene and plagioclase crystals minor phases are skeletal chromite iron silica troilite ilmenite and minute amounts of phosphate and fayalite pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite from en 76 0 wo 1 9 fs 22 1 to compositions nearly devoid of mg unusual ferrosilite and fe augite symplectites and possibly pyroxferroite plagioclase crystals contain significant amounts of fe and mg which are possibly controlled by the ca mg fe2 si3o8 plagioclase component

post crystallization reheating and partial melting of eucrite eet90020 by impact into the hot crust of asteroid 4vesta 4 50 ga ago we performed petrologic radiometric ar ar sm nd and mn cr ages and ion microprobe studies of the basaltic eucrite eet90020 this is one of the few rare basaltic eucrites whose 39ar 40ar age has not been reset during impact bombardment on the hed parent body 4 ga ago and thus should provide a unique opportunity to study the nature of the early thermal events on its parent body presumably asteroid 4vesta hand specimen inspection shows that the rock consists of a fine grained and a coarse grained lithology microscopy indicates that the fine grained lithology has a granulitic texture with a coarser grained area and a large opaque assemblage embedded in the granulitic matrix the coarse grained lithology has an igneous subophitic texture the rock has pyroxenes similar to those in type 5 eucrites type 5 pyroxene and experienced prolonged thermal metamorphism after rapid crystallization from a near surface melt however minor mineral assemblages are unusual and suggest a complex thermal history tridymite occurs as large laths irregular crystals 1 5 gpa after the reheating event eet90020 seems to have experienced the following thermal history 1 crystallization during rapid cooling near the surface 2 some brecciation by impact 3 thermal metamorphism that produced type 5 pyroxene and 4 short reheating that caused partial melting and rapid cooling 39ar 40ar measurements show a relatively flat pattern and an age of 4 49 0 01 ga which is consistent with rapid cooling from high temperature event 4 resetting of the sm nd ages at 4 51 0 04 ga appears to be closely related to the remelting of ca phosphates rb sr data suggest rb loss from tridymite during partial melting the resetting of the mn cr age may have been related to the formation of cr ulvospinels event 4 we suggest that all these ages were reset by partial melting event 4 we further suggest that the partial melting event event 4 that reset the ages 4 50 ga ago was caused by an impact into eet90020 which was part of the hot crust of 4vesta and resulted in an increase in the temperature from the ambient temperature of 870 c to above the subsolidus temperature of eucrites of 1060 c

26al 26mg dating of asteroidal magmatism in the young solar system abstract we present high precision mg isotope data for most classes of basaltic meteorites including eucrites mesosiderite silicate clasts angrites and the ungrouped northwest africa nwa 2976 measured by pseudo high resolution multiple collector inductively coupled plasma mass spectrometry and utilising improved techniques for chemical purification of mg with the exception of the angrites angra dos reis lewis cliff lew 86010 nwa 1296 and nwa 2999 and the diogenite bilanga which have either been shown to have young ages by other dating techniques or have low al mg ratios all bulk samples of basaltic meteorites have 26mg excesses 26 mg 0 0135 to 0 0392 the 26mg excesses cannot be explained by analytical artefacts cosmogenic effects or heterogeneity of initial 26al 27al al mg ratios or mg isotopes in asteroidal parent bodies as compared to earth or chondrites the 26mg excesses record asteroidal melting and formation of basaltic magmas with super chondritic al mg and confirm that radioactive decay of short lived 26al was the primary heat source that melted planetesimals model 26al 26mg ages for magmatism on the eucrite mesosiderite angrite and nwa 2976 parent bodies are 2 6 3 2 3 9 4 1 and 3 5 myr respectively after formation of calcium aluminium rich inclusions cais however the validity of these model ages depends on whether the elevated al mg ratios of basaltic meteorites result from magma ocean evolution on asteroids through fractional crystallisation or directly during partial melting mineral isochrons for the angrites sahara sah 99555 and d orbigny and nwa 2976 yield ages of 5 06 0 05 0 06 myr and 4 86 0 09 0 10 myr respectively after cai formation both isochrons have elevated initial 26 mg values given the brecciated and equilibrated texture of nwa 2976 it is probable that its isochron age and elevated initial 26 mg 0 0175 0 0034 reflects thermal resetting during an impact event and slow cooling on its parent body however in the case of the angrites the marginally elevated initial 26 mg 0 0068 0 0058 may reflect either 26 mg ingrowth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super chondritic al mg prior to impact melting and crystallisation of these angrites or partial internal re equilibration of mg isotopes after crystallisation 26al 26mg model ages and an olivine pyroxene whole rock isochron for the angrites sah 99555 and d orbigny are in good agreement with age constraints from 53mn 53cr and 182hf 182w short lived chronometers suggesting that the 26al 26mg feldspar controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar mg isotope systematics even when age constraints from the 26al 26mg angrite model ages or the mafic mineral whole rock isochron are considered the relative time difference between sah 99555 d orbigny crystallisation and cai formation cannot be reconciled with pb pb ages for sah 99555 d orbigny and cais which are ca 1 0 myr too old angrites or too young cais for reasons that are not clear this discrepancy might indicate that 26al was markedly lower ca 40 in the planetesimal and planet forming regions of the proto planetary disc as compared to cais or that cai pb pb ages may not accurately date cai formation which might be better dated by the 182hf 182w and 26al 26mg chronometers as 4568 3 0 7 burkhardt et al 2008 and 4568 5 0 3 ma herein respectively when mapped onto an absolute timescale using pb pb ages for angrites

spectroscopic characterization of mineralogy and its diversity across vesta the mineralogy of vesta based on data obtained by the dawn spacecraft s visible and infrared spectrometer is consistent with howardite eucrite diogenite meteorites there are considerable regional and local variations across the asteroid spectrally distinct regions include the south polar rheasilvia basin which displays a higher diogenitic component and equatorial regions which show a higher eucritic component the lithologic distribution indicates a deeper diogenitic crust exposed after excavation by the impact that formed rheasilvia and an upper eucritic crust evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta this is broadly consistent with magma ocean models but spectral variability highlights local variations which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxene cumulates overall vesta mineralogy indicates a complex magmatic evolution that led to a differentiated crust and mantle

chronology of the angrite parent body and implications for core formation in protoplanets abstract angrites formed by some of the earliest igneous activity in the solar system and provide insights into the early stages of planetary melting and differentiation moreover they are pivotal reference points for early solar system chronology in order to study the processes and timescales of metal segregation in early protoplanets and to assess the distribution of short lived radionuclides in the early solar system the 182 hf 182 w system was applied to a comprehensive suite of angrites 182 hf 182 w isochron ages for angrites are in excellent agreement with previously reported 207 pb 206 pb and 53 mn 53 cr results but are 1 myr older than ages obtained from 26 al 26 mg chronometry these inconsistencies probably reflect a disturbance of the al mg system in the angrite feldspars but could alternatively be explained by a heterogeneous distribution of 26 al in the early solar system based on the hf w results four texturally and temporally resolved groups of angrites can be identified that were derived from at least two distinct mantle sources these mantle sources are the result of separate events of core formation both of which took place within 2 myr of cai formation thus core formation in the angrite parent body did not occur as a single event of metal segregation from a global magma ocean but rather took place under varying conditions by several more local events the disparate hf w systematics of the two distinct angrite source regions indicate that convection in the magma ocean was inefficient in homogenizing the composition of the mantle possibly as a result of a continuous bombardment with small planetesimals during ongoing core formation such impacts could have constantly removed primordial and earlier formed crust facilitating rapid cooling of the magma ocean which solidified as early as 3 6 0 7 myr after cai formation

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

u pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites abstract u pb isotope analyses on major and accessory minerals from pelitic schists in of southwestern connecticut are used to date metamorphism and deformation most of the major minerals have 206 pb 204 pb ratios less than 100 however u pb mineral isochron and mineral whole rock isochrons constructed using aliquots of the same mineral yield concordant ages with uncertainties as low as 5 ma staurolite kyanite and garnet are older than metamorphic monazites and zircons these data are summarized below 5 mineral 206 pb 204 pb system the straits schist trap falls fm monazite zircon 207 pb 206 pb 379 384 ma 380 2 ma staurolite 48 623 238 u 206 pb 394 400 ma 398 5 ma garnet 22 46 238 u 206 pb 383 ma 414 10 ma 235 u 207 pb 418 ma 207 pb 206 pb 623 ma kyanite 21 27 238 u 206 pb 399 ma chlorite anatase ilmenite 20 27 238 u 206 pb 350 370 ma apatite quartz kspar 18 19 transposed pegmatitic intrusions in the straits schist yield monazites with u pb ages analytically indistinguishable from those of metamorphic monazites and zircons in the schists growth of monazite and zircon in the schists may be related to high t fluid influx associated with pegmatite intrusion age discordance in the garnets from the straits schist is most consistent with continuous loss of intermediate daughter products of the 238 u decay chain from about 420 ma to 380 ma garnets from the adjacent trap falls formation do not display this behavior

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

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

glass veins in the unequilibrated eucrite yamato 82202 the unequilibrated eucrite yamato 82202 y82202 contains a network of relatively thick up to 1 mm in width glass veins the host of the meteorite represents a monomict breccia composed of volcanic rock that crystallized 4 3 ga ago as a lava flow on the surface of 4 vesta the veins formed 3 90 0 04 ga ago probably as a result of frictional melting associated with impact under conditions of low effective oxygen fugacity and higher sulfur fugacity the glass contains disordered structural elements characteristic of pyroxene and feldspar reminiscent of the eucritic target lithology the unequilibrated pyroxenes of the volcanic host and the pristine character of the glass indicate that this meteoritic material did not experience significant thermal metamorphism after initial crystallization of the lava hence it was not affected by regional metamorphism caused by burial to a significant depth or by long term contact metamorphism associated with a thick lava flow large intrusion or hot layer of impact ejecta the meteorite resided at a shallow depth though not at the surface on 4 vesta or on one of the vestoids until it was ejected and traveled to earth probably with other hed materials that have 36ar exposure ages of 13 ma these data suggest that the surface of the asteroid 4 vesta has significant though probably small proportions of glass as well as unequilibrated volcanic rock

wa1ms a 2 61 ga muscovite standard for 40ar 39ar dating abstract the 40ar 39ar dating technique requires the use of neutron fluence monitors standards to allow calculation of the age of a sample precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40ar 39ar dates few fully intercalibrated 40ar 39ar standards with a sufficient total fusion grain to grain reproducibility are currently in use in the argon community for precambrian samples only hb3gr hornblende 1 08 ga yields sufficient grain to grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements here we present chemical and intercalibration results for a new 2 61 ga standard wa1ms is a muscovite extracted from an archaean shear zone in the lake johnston greenstone belt western australia in situ trace element analysis by ela icpms revealed consistent k contents subtle zonation and intra grain and grain to grain heterogeneities in rb sr ti and fe but a lack of mineral inclusions wa1ms has been investigated over 3 irradiations ranging from 25 to 40 h in two reactors with several disc positions and three grains sizes and has been calibrated against fcs and ga1550 and hb3gr overall we carried out 48 total fusion and 4 step heating experiments of wa1ms crystals flat age spectra and average f value 40ar 39ark relative standard deviations ranging from of 0 43 to 0 60 p 0 15 0 83 for 47 48 analyses demonstrate the reproducibility of wa1ms and its suitability as a reliable 40ar 39ar standard we calculated r wa1ms fcs 205 59 0 25 r wa1ms ga1550 57 25 0 06 and r wa1ms hb3gr 3 9713 0 014 all with p 0 14 allowing direct comparison between wa1ms and any standards in used in the community provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards the recently revised decay constant values and standard ages proposed by renne et al 2011 yield a weighted mean age of 2614 2 1 5 ma 0 055 p 0 8 1 for wa1ms against the three standards when calibrated solely against fcs in order to minimize correlated errors between fcs vs ga1550 and hb3gr we calculate a preferred age of 2613 0 2 4 ma 0 09 which is recommended when using this standard additionally this corresponds to a 40ar 40k value of 0 3375 0 0057 10 1 for wa1ms which is independent of the value of the decay constant used for calculation and allows recalculation of the age of wa1ms for any preferred set of decay constants and standard ages using the constants proposed by steiger and jager 1977 directly calculated from the three r values yields an age of 2598 2 3 5 ma corresponding to an identical although less precise 40ar 40k value of 0 3375 0 0086 10 1 benefits of this standard include 1 an increased relative precision per crystal analyzed relative to other standards for proterozoic and archaean samples 2 no post irradiation waiting time due to the absence of ca no need to wait for 37arca decay and 3 loading of standard grains that are optically different from the unknown e g pyroxene or hornblende to avoid standard unknown mixing wa1ms is freely available to the 40ar 39ar community

vesta as the howardite eucrite and diogenite parent body implications for the size of a core and for large scale differentiation if vesta is the parent body of the howardite eucrite and diogenite hed meteorites then geo chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of vesta to 1 determine the size of a possible metal core in vesta and 2 model the igneous differentiation and internal structure of vesta the density of vesta and petrologic models for hed meteorites together suggest that the amount of metal in the parent body is 25 mass with a best estimate of 5 assuming no porosity for a porosity of up to 5 in the silicate fraction of the asteroid the permissible metal content is 30 these results suggest that any metal core in the hed parent body and vesta is not unusually large a variety of geochemical and other data for hed meteorites are consistent with the idea that they originated in a magma ocean it appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites excepting such eucrites as bouvante and statinem formed by subsequent crystallization of the residual melts modelling results suggest that the hed parent body is enriched in rare earth elements by a factor of 2 5 3 5 relative to ci chondrites and that it has approximately chondritic mg si and al sc ratios stokes settling calculations for a vesta wide nonturbulent magma ocean suggest that early crystallizing magnesian olivine orthopyroxene and pigeonite would have settled relatively quickly permitting fractional crystallization to occur but that later crystallizing phases would have settled or floated an order of magnitude more slowly allowing instead a closer approach to equilibrium crystallization for the more evolved eucritic melts this would have inhibited the formation of a plagioclase flotation crust on vesta plausible models for the interior of vesta which are consistent with the data for hed meteorites and vesta include a metal core 130 km radius an olivine rich mantle 65 220 km thick a lower crustal unit 12 43 km thick composed of pyroxenite from which diogenites were derived and an upper crustal unit 23 42 km thick from which eucrites originated the present shape of vesta with 60 km difference in the maximum and minimum radius suggests that all of the crustal materials and possibly some of the underlying olivine from the mantle could have been locally excavated or exposed by impact cratering

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

dawn the vesta hed connection and the geologic context for eucrites diogenites and howardites the dawn mission has provided new evidence strengthening the identification of asteroid vesta as the parent body of the howardite eucrite and diogenite hed meteorites the evidence includes vesta s petrologic complexity detailed spectroscopic characteristics unique space weathering diagnostic geochemical abundances and neutron absorption characteristics chronology of surface units and impact history occurrence of exogenous carbonaceous chondritic materials in the regolith and dimensions of the core all of which are consistent with hed observations and constraints global mapping of the distributions of hed lithologies by dawn cameras and spectrometers provides the missing geologic context for these meteorites thereby allowing tests of petrogenetic models and increasing their scientific value

old sm nd ages for cumulate eucrites and redetermination of the solar system initial 146sm 144sm ratio article i nfo sm 142 nd systematics short lived 146 sm 142 nd and long lived 147 sm 143 nd chronometers have been measured in three cumulate eucrites binda moore county and moama the two major mineral phases plagioclase and pyroxene present in these achondrites are characterized by a wide range of sm nd ratios that allows well resolved sm nd isochrons this group of meteorites thus is suitable to better constrain the initial 146 sm 144 sm ratio of the solar system binda and moore county give concordant ages of 4544 88 and 4542 85 ma respectively with initial 143 nd 144 nd ratios slightly higher to within error of chondritic these ages are in agreement with most of the radiometric ages determined on basaltic eucrites a best estimate for the solar system initial 146 sm 144 sm ratio is obtained using the five point regression line determined for binda the 146 sm 144

a deep crust mantle boundary in the asteroid 4 vesta the asteroid 4 vesta was recently found to have two large impact craters near its south pole exposing subsurface material modelling suggested that surface material in the northern hemisphere of vesta came from a depth of about 20 kilometres whereas the exposed southern material comes from a depth of 60 to 100 kilometres large amounts of olivine from the mantle were not seen suggesting that the outer 100 kilometres or so is mainly igneous crust here we analyse the data on vesta and conclude that the crust mantle boundary or moho is deeper than 80 kilometres

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

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

pb diffusion in cr diopside augite and enstatite and consideration of the dependence of cation diffusion in pyroxene on oxygen fugacity abstract chemical diffusion of pb under anhydrous po2 buffered conditions has been measured in natural pyroxenes of three different compositions a chromian diopside an augitic clinopyroxene and a near end member enstatite sources of diffusant consisted of pbs powder and ground pyroxene mixed together pre reacted in evacuated silica capsules at 1050 c and re ground prepared sample capsules were annealed for times ranging from several hours to a few months at temperatures from 850 c to 1050 c the pb distributions in the pyroxene specimens were profiled by rutherford backscattering spectrometry rbs the following arrhenius relations are obtained for pb diffusion in the pyroxenes when buffered at qfm d c 8 7 10 7 exp 351 36 kj mol 1 rt m 2 s 1 cr diopside d a 3 8 10 5 exp 372 15 kj mol 1 rt m 2 s 1 augite d e 6 6 10 7 exp 366 29 kj mol 1 rt m 2 s 1 enstatite in all cases diffusion is measured perpendicular to the c axis diffusion in cr diopside does not appear to be strongly anisotropic as diffusion rates for transport parallel to c are similar to the 110 results above diffusion in the augite is faster by nearly an order of magnitude than pb diffusion in the cr diopside diffusion tends to be faster in pyroxenes with higher iron contents as was evident in our earlier work chemical geology 150 1998a 105 but simple correlations are not obvious pb diffusion in enstatite is considerably slower than in the clinopyroxene compositions studied to investigate the fo2 dependence of pb diffusion in pyroxene experiments using the cr diopside augite and enstatite as well as the near end member diopside and a more iron rich clinopyroxene employed in a recent diffusion study chemical geology 150 1998a 105 were run at 950 c under oxygen fugacities ranging from 10 6 to 10 16 all of the pyroxenes exhibited a positive dependence of pb diffusion rate on fo2 with values of m ranging from 0 14 to 0 20 given d fo2 m interestingly the exponential term for diopside is 0 19 in good agreement with the value 3 16 determined from point defect models for diffusion mechanisms controlled by cation vacancies j geophys res 99 1994 9423 the activation energies for pb diffusion in all of the pyroxenes studied are relatively high and pb diffusivities slow when compared with the extant diffusion data for most other minerals it follows as consequence of these comparatively slow diffusivities that pb closure temperatures are higher in pyroxenes than for the majority of minerals with the exception of a few phases such as zircon monazite and garnet the slightly faster diffusivities that characterize fe rich clinopyroxenes will lead to somewhat depressed closure temperatures but pyroxenes in general should be fairly retentive of pb isotopic information when subjected to all but the most extreme thermal events

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

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

u pb and al mg systematics of the ungrouped achondrite northwest africa 7325 abstract northwest africa nwa 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet mercury in order to understand the origin of this meteorite and the nature of its parent body we have determined its crystallisation age using the long lived u pb and short lived al mg chronometers an internal pb pb isochron defined by six acid leached pyroxene fractions yields an age of 4563 4 2 6 ma assuming that the 238u 235u ratio for nwa 7325 is identical to the bulk earth and solar system value of 137 794 the al mg isotope analyses of seven fractions four plagioclase one pyroxene one olivine and one whole rock define a regression line corresponding to 26al 27al0 3 03 0 14 10 7 and an initial 26mg of 0 093 0 004 when anchored to the d orbigny angrite this initial 26al 27al yields an age of 4563 09 0 26 ma the pb pb age of 4563 4 2 6 ma and al mg age of 4563 09 0 26 ma are in complete agreement but the low u concentrations of nwa 7325 resulted in a relatively low precision pb pb age the observed excess in initial 26mg can be explained by 27al 24mg fractionation and subsequent mg isotopic evolution after planetary differentiation furthermore the parental magma of nwa 7325 most likely formed within 1 72 ma after calcium aluminium rich inclusion cai formation nwa 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at 4563 ma suggesting that a multitude of planetary bodies had formed and differentiated by 4 5 myr after cai formation this ancient age may be interpreted as an argument against nwa 7325 originating from mercury however it does not completely rule it out

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

60fe in eucrites additional evidence for the existence in the early solar system of live60fe has been found in the basaltic achondrite juvinas the relative abundance of 60fe 56fe of x0223c 4 10 10 at the time of meteorite solidification is one order of magnitude lower than that previously observed for the eucrite chervony kut this difference corresponds to a 4 7 ma time interval between closure of the60fe 60ni isotopic system in the two meteorites the factor two higher initial 60ni 58ni ratio in juvinas is consistent with this time difference and a common bulk fe ni ratio in the two meteorites using the60fe 60ni isotope system as a chronometer various models for the evolution of the eucrite parent body of different degrees of complexity are discussed regardless of the detailed differences between these models a short time interval of only a few million years between planetary differentiation and basaltic crust formation is indicated

the meteoritical bulletin no 83 1999 july meteoritical bulletin no 83 lists information for 898 newly described meteorites these include 473 from antarctica 341 from the sahara and 22 from dry lakes in the southwestern united states seven of the meteorites are falls kunya urgench h5 lohawat howardite ourique h4 portales valley h6 san pedro de quiles l6 talampaya eucrite and zag h3 6 also included are a dozen new iron meteorites several mesosiderites a pallasite several hed meteorites several ureilites a variety of cm co cv cr and r chondrites and numerous unequilibrated ordinary chondrites all shock classifications are after stoffler et al 1991 and weathering grades are after wlotzka 1993 except as noted all italicized abbreviations refer to addresses tabulated at the end of this document

rb sr chronology of volatile depletion in differentiated protoplanets babi ador and all revisited abstract a strong depletion in moderately volatile elements is a characteristic feature of many planetary bodies in the inner solar system and either reflects the rapid accretion of planetesimals from an incompletely condensed solar nebula or is the result of energetic collisions during planetary accretion to better constrain the origin and timescales of this volatile depletion we have precisely measured sr isotope compositions in angrites eucrites and ca al rich inclusions cai angrites have an initial 87sr 86sr ador 0 698978 0 000004 which is indistinguishable from 87sr 86sr babi 0 698970 0 000028 obtained for eucrites in agreement with earlier studies we find that angrites and eucrites have higher initial 87sr 86sr ratios than cai at face value corresponding to model timescales for volatile loss of several millions of years however all the investigated cai are characterized by elevated 84sr 86sr ratios compared to angrites and eucrites which we interpret to reflect an excess of r process sr in the cai if this is correct then the nucleosynthetic sr isotope anomalies in the cai require an upward correction of their measured 87sr 86sr after this correction cai have an initial 87sr 86sr cai 0 698975 0 000008 which is indistinguishable from ador and babi this implies volatile loss from the angrite and eucrite parent bodies within less than 1 ma after formation of cai the volatile depleted nature of these differentiated protoplanets thus most likely reflects their rapid accretion from volatile poor dust in an incompletely condensed solar nebula

constraints on the cooling rates of chondrules from the microstructure of clinopyroxene and plagioclase abstract attempts are made to quantify cooling rates of chondrules from the microstructure of clinopyroxene and ca rich plagioclase pigeonite diopside exsolution lamellae on 001 are frequently observed in allende granular olivine pyroxene chondrules cooling rates of 2 10 c h are obtained for the observed wavelengths between 25 and 33 nm from isothermal time temperature transformation diagrams modelling of lamellae growth during cooling yields similar cooling rates between 7 and 25 c h coarsening of pigeonite diopside exsolution lamellae takes place in the small temperature interval from approximately 1350 to 1200 c the occurrence of type b antiphase domains in ca rich plagioclase provides independent constraints on chondrule cooling rates isothermal time temperature transformation diagrams yield cooling rates of approximately 3 30 c h for the observed domain sizes between 10 and 30 nm cooling rates derived from antiphase domains in plagioclase are valid in approximately the same temperature range than those deduced from pigeonite diopside exsolution lamellae our estimates must be regarded as upper limits and they seem to be significantly lower than those derived from dynamic crystallization experiments 100 2000 gc h as dynamic crystallization experiments yield cooling rates at higher temperatures near the liquidus this observation may suggest nonlinear cooling the absence of orthoenstatite lamellae in clinoenstatite indicates high cooling rates on the order of 10 4 c h at temperatures around 1000 c

constraints on the movement history of the carboneras fault zone se spain from stratigraphy and 40ar 39ar dating of neogene volcanic rocks

comparative study of equilibrated and unequilibrated eucrites subsolidus thermal histories of haraiya and pasamonte thermal histories of non cumulate eucrites pasamonte and haraiya were determined from ca ca fe mg profiles of host lamellae pyroxene pairs from compositional profiles of pyroxene rims affected by metasomatism and from lamellae thicknesses using a rate law for lamellar coarsening pasamonte a type 2 eucrite experienced partial equilibration during a relatively short period of thermal annealing this equilibration is expressed texturally by metasomatized rims in mg rich and fe rich pyroxene grains and by coarsening of nanometer scale exsolution lamellae a cooling rate of 10 2 c day corresponds to post impact mixing and reburial of the polymict pasamonte assemblage to a depth similar to that of the initial lava flow haraiya a type 7 eucrite has experienced a complex thermal history involving multiple brecciation events and mixing of a single flow to produce a monomict assemblage containing clasts of variable grain size and texture a period of thermal annealing 50000 years in duration at temperatures between 1000 and 700 c corresponding to a cooling rate of 10 4 to 10 5 c day produced augite exsolution up to 3 m thick in pigeonite rapid burial by successive flows during a period of high magma productivity and high heat flow due to 2 6 al decay in the crust and mantle was a possible source of heat for metamorphism in the lower crust however cooling rates during relaxation of the lower crustal thermal gradient are much less than the mineralogically determined cooling rates heat derived from impact or from mid to upper crustal level intrusions is a plausible heat source for metamorphism and is consistent with observed cooling rates

global crustal metamorphism of the eucrite parent body abstract we conducted a petrologic study of 18 equilibrated eucrites with the goal of understanding the formation and evolution of the crust of asteroid 4 vesta the likely source of eucrite meteorites their textures indicate that eucrites were basaltic igneous rocks but modified by postcrystallization brecciation recrystallization and impact melting recrystallized rocks are the most common among clasts in the eucrites impact melt breccias are rare equilibration temperatures of the metamorphic clasts range from 700 to 1000 c with most in the 800 900 c range complex exsolution and recrystallization textures in pyroxenes suggest that eucrites cooled very slowly from high peak temperatures after reheating this extended period of thermal metamorphism requires a high interior temperature in a vesta sized body this metamorphism could have taken place during and shortly after crust formation we suggest the crust of vesta evolved through the eruption rapid cooling and subsequent burial and reheating of lava flows as the crust grew in thickness the depth of burial increased causing temperatures to rise and leading to global metamorphism this resulted in only a small percentage of unequilibrated eucrites only the last few percent of basalts erupted would not be metamorphosed and a wide range of metamorphic temperatures and durations for the equilibrated eucrites

comparison of yamato and victoria land polymict eucrites a view from mineralogical and isotopic studies new interior samples of four yamato polymict eucrites y74159 y74450 y75011 and y75015 have been studied by electron microprobe transmission electron microscope and x ray diffraction techniques and compared with several samples of the victoria land polymict eucrites these same samples have been analysed using rb sr and nd sm isotopic systematics several grains of inverted pigeonite with blebby augite similar to those in binda and moama have been identified in all four yamato eucrites coarse grained mesostasis rich subophitic basalts which contain mg rich pigeonite with fe rich olivine veinlets zoned outward to a subcalcic ferroaugite rims have also been found these unique clasts were not found in alh76005 77302 78040 78158 and 78165 and eet eucrites the tight grouping of rb sr and sm nd ratios and similar modal compositions of the yamato group indicate that they are most likely to be pieces from a single fall and distinct from the alh and eet groups however the yamato allan hills and elephant moraine groups may sample a few distinct magmas or similar but different source regions on the same parent body

composition and petrology of hed polymict breccias the regolith of 4 vesta the polymict breccias of the howardite eucrite and diogenite hed clan of meteorites preserve records of regolith processes that occur on vesta their putative home world these breccias howardites polymict eucrites and polymict diogenites are impact engendered mixtures of diogenites and eucrites the compositions of polymict breccias can be used to constrain the lithologic diversity of the vestan crust and the excavation depths of these materials we have done petrological and compositional studies of multiple samples of 5 polymict eucrites and 28 howardites to investigate these issues older analyses were done on samples of approx 0 5 gram mass by inaa newer analyses on samples of approx 5 gram mass by xrf and icp ms we estimate the percentage of eucritic material poem of polymict breccias by comparing their al and or ca contents to those of average basaltic eucrite and diogenite our samples have poem ranging from 28 to 98 adding two polymict diogenites from extends the range to poem 10 one hypothesis is that ancient well mixed vestan regolith has poem approx 67 and has a higher content of admixed impactor material several of our howardites have poem of 59 74 al and or ca contents 10 of poem 67 about a third have ni contents 300 micro g g suggesting they contain 2 chondritic material cm and or cr these may be regolithic howardites only one lew 85313 contains ne dominated by a solar wind sw component pca 02066 is dominated by impact melt material of polymict parentage and petrologically appears to be a mature regolith breccia yet it does not contain sw ne gro 95602 falls within the poem window contains sw ne yet has a ni content of 193 micro g g its petrologic characteristics suggest it was formed from immature regolith no polymict breccia clasts no glass trace element characteristics of the polymict breccias demonstrate the dominance of main group eucrites as the basaltic component mixing diagrams of zr nb ba hf and ta with al show no evidence for a significant contribution from stannern trend eucrites an exception is polymict eucrite lew 86001 poem 92 which is dominated by stannern trend basaltic debris howardite lap 04838 poem 84 has higher incompatible trace concentrations than other polymict breccias excluding lew 86001 and either contains a stannern trend basaltic component or has a significant contributions from evolved eucrites like nuevo laredo

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

origin of howardites diogenites and eucrites a mass balance constraint abstract howardite meteorites are regolith breccias impact produced mixtures that consist of two main rock types basalts that strongly resemble eucrite meteorites and pyroxene cumulates that strongly resemble diogenite meteorites there are basically two petrogenetic models for the noncumulate basaltic parts of howardites 1 1 genesis as residual liquids of the same magma s that had earlier crystallized the pyroxene cumulates and 2 2 genesis as primary unmodified partial melts from the interior of the parent asteroid a mass balance constraint based on al indicates that unless the ratio degree of melting during genesis of melts parental to pyroxene cumulates degree of melting during genesis of primary basalts was on average greater than about 2 8 a scenario that implies an implausible bimodality of degree of melting on the howardite parent body an average howardite must contain more residual liquid basalt than primary basalt if as seems likely eucrites per se are identical to the howardite basaltic components and diogenites per se are the pyroxene cumulate components then more than half of the eucrites probably formed from residual liquids of magmas that had earlier produced the diogenites considering all the potential complexities of petrpgenesis on the howardite parent body neither the partial melting model nor the fractional crystallization residual liquid model is likely correct for all noncumulate eucrites

thermal history of northwest africa 5073 a coarse grained stannern trend eucrite containing cm sized pyroxenes and large zircon grains we report on the bulk chemical composition petrology oxygen isotopic composition trace element composition of silicates and degree of self irradiation damage on zircon grains of the eucrite northwest africa nwa 5073 to constrain its formation and postcrystallization thermal history and to discuss their implications for the geologic history of its parent body this unequilibrated and unbrecciated meteorite is a new member of the rare stannern trend eucrites it is mainly composed of elongated zoned pyroxene phenocrysts up to 1 2 cm plagioclase laths up to 0 3 cm in length and is rich in mesostasis the latter contains zircon grains up to 30 m in diameter metal sulfide tridymite and ca phosphates textural observations and silicate compositions coupled with the occurrence of extraordinary fe rich olivine veins that are restricted to large pyroxene laths indicate that nwa 5073 underwent a complex thermal history this is also supported by the annealed state of zircon grains inferred from raman spectroscopic measurements along with u and th data obtained by electron probe microanalyses

resolving the integral connection between pedogenesis and landscape evolution abstract soil is central in the terrestrial ecosystem linking and providing feedback responses to the other components i e water atmosphere and vegetation however the role of soil in landscape evolution is usually not well acknowledged in modeling landscape evolution soil is only treated as a residue of weathering that is transported and redistributed along the hillslope weathering is considered as a process that produces clays and generates unconsolidated materials available for erosion while pedology has been debating the form of qualitative factorial models for 75 years models for soil water heat solute gas and chemical reactions in a profile have matured as soils are distributed continuously in three dimensions across landscapes the profile models need to consider lateral fluxes this review outlines the role of soil in landscape modeling first we review the role of soil in the current landscape evolution models we then review data and models on soil weathering rates and transport processes we discuss soil profile models that simulate soil formation processes and combined soil landscape evolution models finally we discuss how the models can be tested and validated in the real world and suggest how both soil scientists and landscape modelers can work together to address the grand challenges in modeling earth surface processes

magnesium oxide iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites according to a currently popular model for petrogenesis on the howardite eucrite and diogenite hed parent asteroid the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite dominated plutons this model can be tested for consistency with mass balance for mgo and feo assuming the overall diogenite diogenite eucrite ratio d of the parent asteroid is at least comparable to the average d for the eucrite diogenite dominated howardite regolith breccias average mg mgo mgo feo is much lower for eucrites especially noncumulate eucrites than for diogenites unless the diogenite parent magmas eventually produced a large proportion of low mg residual basalt and gabbro rbg the implied initial magma s mg is vastly higher than that of any noncumulate eucrite starting from a source previously depleted by putative primary eucrite genesis melt mg can be estimated as a function of the exchange reaction kd and degree of melting using several very conservative assumptions e g assuming that the total mgo feo concentration is nearly the same in the nascent melt as in the residual solids the degree of melting required to yield a melt with mg high enough to satisfy mass balance without implying an rbg component that accounts for 50 of all eucrites is an implausibly high 60 80 wt the separate orthopyroxenitic plutons sop model also seems inconsistent with the uniform density of melts across the diogenite eucrite compositional spectrum 2 77 2 82 g cm3 which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas this difficulty cannot be reduced by simply assuming that later formed magmas were systematically both more plutonic and more mgo rich than earlier ones because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg than noncumulate eucrites conceivably the bulk mg of the asteroid s silicate system was increased between primary melt eucrite genesis and sop diogenite genesis by graphite fueled reduction of feo however the graphite oxidation process generates a huge proportion of gas which would have enhanced the buoyancy of the nascent diogenite parent magmas thus exacerbating the difficulty of achieving the implied high degrees of partial melting to avoid these difficulties but still form most eucrites as rapidly cooled extrusives i propose the nerd noncumulate eucrites as extruded residua of diogenites model in this model the diogenites form as early cumulates from a large magma system probably a global magma ocean that yields a large proportion of eucritic melt as residuum this residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions dikes and sills of rapidly cooled noncumulate eucrites slight 5 10 porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone the common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone the nerd model s greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the hed asteroid

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

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

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

presidential address presented 2000 august 28 chicago illinois usa the eucrite vesta story many lines of evidence indicate that meteorites are derived from the asteroid belt but in general identifying any meteorite class with a particular asteroid has been problematical one exception is asteroid 4 vesta where a strong case can be made that it is the ultimate source of the howardite eucrite diogenite hed family of basaltic achondrites visible and near infrared reflectance spectra first suggested a connection between vesta and the basaltic achondrites experimental petrology demonstrated that the eucrites the relatively unaltered and unmixed basaltic achondrites were the product of approximately a 10 melt studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal scale magma ocean mass balance considerations point to a parent body that had its surface excavated but remains intact modern telescopic spectroscopy has identified kilometer scale vestoids between vesta and the 3 1 orbit orbit resonance with jupiter dynamical simulations of impact into vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations hubble space telescope images show a 460 km diameter impact basin at the south pole of vesta it seems that nature has provided multiple free sample return missions to a unique asteroid major challenges are to establish the geologic context of the hed meteorites on the surface of vesta and to connect the remaining meteorites to specific asteroids

illumination conditions at the asteroid 4 vesta implications for the presence of water ice abstract the mean illumination conditions and surface temperatures over one orbital period are calculated for the asteroid 4 vesta using a coarse digital elevation model produced from hubble space telescope images even with the anticipated effects of finer scale topography taken into account it is unlikely that any significant permanently shadowed regions currently exist on vesta due to its large axial tilt 27 however under present day conditions it is predicted that about half of vesta s surface has an average temperature of less than 145 k which based on previous thermal modeling of main belt asteroids suggests that water ice could survive in the top few meters of the vestal regolith on billion year timescales

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

magnesian basalt clasts from the eet 92014 and kapoeta howardites and a discussion of alleged primary magnesian hed basalts we have performed petrologic and geochemical studies on pyroxene phyric magnesian basalt clasts from the howardites eet 92014 and kapoeta pyroxenes in the eet 92014 clast are strongly zoned from wo4 5en68 9fs26 6 cores to wo22 2en18 5fs59 2 rims the pyroxene has also suffered later feo metasomatism along healed fractures that has resulted in compositions with atypically high feo mno ratios metasomatized pyroxenes reach compositions as ferroan as wo3 2en37 0fs59 8 pyroxenes in the kapoeta clast have cores that are more magnesian wo1 8en80 2fs18 0 and are less strongly zoned to wo3 6en51 0fs45 4 atypically high feo mno in the most ferroan pyroxenes in this clast coupled with late ferroan olivine up to fa64 reflect late stage addition of feo to the melt rather than igneous fractionation the eet 92014 clast is enriched in the refractory incompatible lithophile elements and shows a refractory lithophile element pattern similar to stannem and bouvante the clast is enriched in cr2o3 and mgo and depleted in al2o3 and cao compared to main group or stannern trend eucrites based on bulk major and trace lithophile element composition this pyroxene phyric clast cannot be directly related to basaltic eucrites this clast is either an impact melt of a trace element rich polymict eucrite or howardite target or a primary melt of a more magnesian source region than that which produced basaltic eucrites we favor the former interpretation low siderophile element abundances within the ranges for monomict eucrites indicate 0 5 chondritic contamination in the clast however the kapoeta clast has a major element composition within the range of howardites and likely is an impact melt of such a target lithology comparison with other reported magnesian basalt clasts from howardites indicates that in all likelihood all heretofore alleged primary magnesian basalt clasts are impact melts of polymict targets two magnesian eucrites alha81001 and pomozdino and magnesian eucrite clasts from the polymict eucrite petersburg may possibly be primary melts of a magnesian source region not directly related to basaltic eucrites

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

fe ti oxide chronometry with application to granulite formation abstract ilmenites occurring in amphibolite and granulite facies orthogneisses in sri lanka yield precise u pb ages coexisting ilmenite magnetite in the granulite facies rocks indicate temperature conditions of 680 60 c and log o 2 17 0 1 2 the u pb age of 498 2 3 3 ma obtained for ilmenite is indistinguishable from the pb pb age of 501 5 ma derived for coexisting silicate minerals and suggests that oxide silicate equilibration has been preserved in contrast fe ti oxides in adjacent amphibolitefacies rocks indicate a temperature of 830 60 c and log o 2 12 7 0 8 and record u pb ages of 1093 7 7 6 ma and 1095 4 4 2 ma these ages are some 600 ma older than those obtained for coexisting silicate phases in the same sample or for both oxide and silicate minerals in the nearby charnockite and relate either to the time of emplacement of the granitic precursor or to some earlier metamorphic episode these results suggest that temperature o2 and fluid conditions estimated from fe ti oxides should be interpreted with extreme caution