translation and docking of an arc terrane geological and geochemical evidence from the southern zambales ophiolite complex philippines abstract the zambales ophiolite complex is made up of three massifs the masinloc cabangan and san antonio massifs field petrographic and geochemical analyses show that the cabangan and san antonio massifs are genetically related to the coto transitional mid ocean ridge island arc and acoje island arc blocks of the masinloc massif respectively the subic bay fault zone a left lateral fault zone separates the san antonio massif island arc terrane from the transitional mid ocean ridge island arc like sheeted diabase dikes pillow basalts of the cabangan massif the san antonio massif is a rifted terrane from the acoje block which was translated southward to its present position through the west luzon shear subic bay fault zone tectonized clinopyroxenite and gabbronorite hills which mimic the physical and geochemical characteristics of the acoje block and the san antonio massif ultramafic mafic cumulate rocks were left behind along the western side of the cabangan massif during the translation of the arc massif southward this scenario can account for the present day configuration of the zambales ophiolite complex

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

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

sedimentology stratigraphy and geochronology of the proterozoic mazatzal group central arizona quartzite conglomerate and shale of the mazatzal group record the filling of a prot erozoic intra arc basin in central arizona u pb ages of zircons from rhyolite ash flow tuff indicate that deposition began at 1701 2 ma basal deposits of the newly defined pine creek conglomerate formed in an alluvial fan setting synchronous with the final phase of extrusive rhyolite volcanism and active faulting after volcanic activity ceased shallower slope braided stream environments developed in early deadman quartzite time subsequent marine transgression produced shallow subaqueous deposits of the upper part of the deadman quartzite these were overlain by prodelta sediments of the maverick shale indicating a phase of basin deepening finally the mazatzal peak quartzite was deposited as the basin shoaled again first to shallow marine and subsequently to fluvial conditions the quartzite is diagenetic quartz arenite originally deposited as lithic arenite and lithic arkose the provenance was initially restricted and earliest sediment was derived mainly from the subjacent red rock group when tectonic activity ceased how ever the surrounding highlands were planed down by erosion and detritus from a wider variety of source rocks was funneled into the basin this included contributions from arc related supracrustal rocks of the payson ophiolite and east verde river formation and finally a granitic basement input detrital quartz in the lower part of the mazatzal group is largely monocrystalline and volcanic in origin polycrystalline quartz associated with detrital feldspar and of probable continental affinity is concentrated in the uppermost parts of the sequence the life span of the intra arc basin was on the order of 30 m y from the formation of the payson ophiolite at 1 73 ga to the deposition of the upper mazatzal peak quartzite sometime after 1 70 ga the pre mazatzal red rock group represents the last stages of volcanic arc activity and the mazatzal group records the transition from orogenic to nonorogenic sedimentation in central arizona

evidence for excavation of deep crustal material of a vesta like body from ca compositional gradients in pyroxene abstract recent discovery of basaltic achondrite asteroids in the main belt has provided a link between basaltic achondrites howardite eucrite and diogenite and asteroid 4 vesta a spectral study suggests that these asteroids are fragments ejected from vesta and that vesta s eucritic crust is of substantial thickness we computed the cooling rate and burial depth of the moore county cumulate eucrite on the basis of ca compositional gradients and the width of augite lamella in pyroxene to obtain information on the early evolution of the parent body our calculations show that 1 moore county cooled initially at a rate of 0 00016 c yr which corresponds to a burial depth of about 8 km 2 after this cooling a sudden temperature rise occurred probably due to an impact and finally 3 moore county cooled down again at a rate of 0 35 c yr the latter events may correspond to the excavation of crustal material the burial depth of moore county is consistent with the result of a recent spectral study in which the largest observed eucritic fragment 10 km sets a lower limit on the thickness of the eucritic crust reheating and excavation of a deep crustal material imply impact events which are supported by spectral observations

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

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

147sm 143nd and 87rb 87sr ages of the eucrite piplia kalan abstract prompted by the finding that the eucrite piplia kalan could have retained pu fission xe earlier than the eucrites so far studied and hence be very ancient we have measured a precise internal 147 sm 143 nd isochron for this meteorite the age and initial nd ratio relative to chur are 4 570 0 023 ga and 1 3 0 7 u respectively a rb sr whole rock clast isochron for this meteorite corresponds to an age of 3 963 0 119 ga and initial 87 sr 86 sr ratio of 0 69902 3 but initial 87 sr 86 sr ratio calculated for an age of 4 57 ga is 0 698956 25 which is indistinguishable from 0 698970 15 reported for the angrites lew and ador dated at 4 5578 0 0005 ga these results indicating that piplia kalan could have formed within only a few million years of the earliest condensates in the solar system are strongly supported by the recent discovery of live 26 al in it

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

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

a 4 gyr shock age for a martian meteorite and implications for the cratering history of mars analyses of meteorites that originated on mars provide important insights into the geological and atmospheric evolution of the planet such analyses have hitherto been restricted to relatively young martian rocks1 the oldest martian meteorites have an age of approximately 1 3 billion years but the recently recognized2 martian meteorite allan hills 84001 which is distinct from the other martian meteorites2 4 shows evidence for a much older age5 6 here we report an analysis of the shock alteration history of this meteorite based on argon isotope dating from which we derive a shock age of 4 0 0 1 billion years the age and geological history of this meteorite suggest that it came from the heavily cratered noachian age terrains of mars s southern hemisphere and it may thus provide an absolute chronology for this region of the planet independent of that inferred from the cratering record the shock age of the meteorite also coincides with that of the so called lunar cataclysm a relatively short period during which many of the craters on the moon are believed to have formed supporting the idea7 that intense bombardment was widespread throughout the inner solar system between 3 9 and 4 1 billion years ago

pseudostratigraphy and origin of the early proterozoic payson ophiolite central arizona the payson ophiolite ca 1 73 ga exposed within the early proterozoic orogenic belt of central arizona is a shallow dipping pseudostratigraphic sequence of gabbro sheeted dikes and submarine volcanic rocks partly disjointed by later intrusion and deformation an older basement complex occurs as roof pendants in the gabbro and screens in the sheeted dike complex and was tilted and eroded prior to development of the ophiolite gabbro dike mingling and mutual intrusion in the transition from gabbro to sheeted dikes indicates that the dikes are rooted in the gabbro analysis of detailed measured sections of well exposed sheeted dikes 600 m reveals 90 100 dikes with lateral variations in the proportions of tonalitic or dacitic dikes and basement screens the transition from sheeted dikes to submarine basalts is abrupt and marked by a stratigraphically continuous zone of intensely altered rocks the sheeted dike complex pseudostratigraphic structure and synmagmatic hydrothermal alteration of the payson ophiolite are diagnostic of a crustal section formed by sea floor spreading the payson ophiolite intrudes and is intruded by arc granitoids and is overlain by arc derived volcanic and sedimentary rocks a model of an intra arc basin formed along an arc parallel strike slip fault is proposed to account for 1 screens of older arc crust 2 inferred arc parallel extension and 3 juxtaposition of the ophiolite with a distinct arc terrane prior to regional convergent deformation and accretion of the arc to north america during the yavapai orogeny ca 1 70 ga

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

discovery of coesite and stishovite in eucrite howardite eucrite diogenite meteorites heds probably originated from the asteroid 4 vesta we investigated one eucrite bereba to clarify a dynamic event that occurred on 4 vesta using a shock induced high pressure polymorph we discovered high pressure polymorphs of silica coesite and stishovite originating from quartz and or cristobalite in and around the shock melt veins of bereba lamellar stishovite formed in silica grains through a solid state phase transition a network like rupture was formed and melting took place along the rupture in the silica grains nanosized granular coesite grains crystallized from the silica melt based on shock induced high pressure polymorphs the estimated shock pressure condition ranged from 8 to 13 gpa considering radiometric ages and shock features the dynamic event that led to the formation of coesite and stishovite occurred ca 4 1 ga ago which corresponds to the late heavy bombardment period ca 3 8 4 1 ga deduced from the lunar cataclysm there are two giant impact basins around the south pole of 4 vesta although the origin of heds is thought to be related to dynamic events that formed the basins ca 1 0 ga ago our findings are at variance with that idea

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

assessment of the pb pb and u pb chronometry of the early solar system abstract an evaluation of early solar system chronometry by the pb pb and u pb methods is provided specifically three consequential factors are examined procedure of age calculation extent of terrestrial pb contamination and initial pb isotopic composition on a pb pb diagram high temperature inclusions of the allende meteorite are tightly organized into a well defined line inside a potentially dispersive mixing field which is consistent with the inclusions containing initial pb that is more primitive than that of canon diablo troilite pat consequences of the possible existence of a pre pat pb to the evolution history of the solar nebula are discussed phosphates from the ordinary chondrite st severin appear to be contaminated by terrestrial pb a condition that renders age calculation based on subtraction of pat inaccurate the pb pb mixing line of these phosphates indicates an age of 4 558 ga interestingly angra dos reis phosphate and pyroxene as well as pyroxene of the other angrite lewis cliff 86010 fall precisely on the line defined by st severin phosphates whole rocks of ordinary chondrites are pictorially and explicitly shown to be seriously contaminated with terrestrial pb thus their single stage u pb ages may not be suitable markers of time because their true crystallization ages are often younger than the whole rocks and because of the possibility of multistage evolution phosphates of ordinary chondrites may yield single stage ages older than their true crystallization ages a hypothetical numerical demonstration is provided on the basis of revised ages and new observations we provide an updated chronometry for the early solar system

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

samarium neodymium evolution of meteorites we have obtained sm nd data on two differentiated meteorites ibitira a eucrite with distinct basaltic texture and with evidence of crystallization and morristown a group 3a mesosiderite as well as on acapulco an unclassified meteorite with chondritic chemical composition and a highly recrystallized texture we have demonstrated the presence of in situ decay of short lived 146 sm in these meteorites with initial abundance of 146 sm 144 sm from 0 009 to 0 007 for the different meteorites precisely defined 147 sm 143 nd internal isochrons were obtained yielding ages of 4 60 0 03 ae for acapulco 4 47 0 02 ae for morristown and 4 46 0 02 ae for ibitira the corresponding initial 143 nd 144 nd values obtained were 0 2 0 9 2 1 0 7 and 1 6 0 8 u epsilon units detailed consideration of the coupled 147 sm 143 nd and 146 sm 142 nd systematics and of the elevated initial 143 nd 144 nd values indicates the presence of inconsistencies in the data for the two sm nd parent daughter pairs in spite of the precise isochrons we present a model which provides an interpretation of the paired sm nd systematics in terms of an impulsive disturbance the abundance of phosphate with large concentrations of rees controls the ree economy plagioclase with low ree concentrations is assumed to be susceptible to exchange pyroxene is considered to be undisturbed because of the low diffusion coefficients of trace elements the experimental results can be reasonably explained by this model the results indicate that three meteorites studied some with very low ree concentrations including a mesosiderite are relatively ancient objects formed within the first 50 to 100 m y of the solar system by planetary differentiation and impact processes and were subjected to late metamorphism the time of late metamorphism is substantially younger than 4 4 ae and possibly as recent as 3 9 ae this metamorphism has resulted in partial very limited reequilibration between plagioclase and phosphate resulting in the observed shifts in initial 143 nd 144 nd sm nd model ages and the 142 nd 144 nd abundances but preserving the major temporal structures of early original formation and later disturbances these results show the need for care in the interpretation of the parent daughter systematics even when they appear well behaved and the mineral phases are viewed as highly resistant to mild metamorphism

samarium neodymium chronology and rubidium strontium systematics of an allende calcium aluminum rich inclusion with implications for 146sm half life abstract calcium aluminum rich inclusions cais are primitive objects that formed within the protoplanetary disk surrounding the young sun recent pb pb chronologic studies have demonstrated that cais are the oldest solar system solids crystallizing 4567 ma ago amelin et al 2002 connelly et al 2012 the isotope systematics of cais therefore provide critical insight into the earliest history of the solar system although sm nd and rb sr geochronometers are highly effective tools for investigating cosmochemical evolution in the early solar system previous studies of cais have revealed evidence for isotopically disturbed systems here we report new age data for allende cai al3s4 derived from both the long lived 147sm 143nd and short lived 146sm 142nd isotopic systems the 147sm 143nd chronometer yields an age of 4560 34 ma that is concordant with 207pb 206pb ages for cais and indicates that the sm nd system was not significantly disturbed by secondary alteration or nucleosynthetic processes the slope of the 146sm 142nd isochron defines the solar system initial 146sm 144sm of 0 00828 0 00044 this value is significantly different from the value of 0 0094 determined by kinoshita et al 2012 ages recalculated from all published 146sm 142nd isochron data using the traditional 103 ma half life and the initial 146sm 144sm value determined here closely match pb pb and 147sm 143nd ages determined on the same samples in contrast ages recalculated using the 68 ma half life determined by kinoshita et al 2012 and either of the initial 146sm 144sm values are often anomalously old this is particularly true for the youngest samples with 146sm 142nd isochron ages that are most sensitive to the choice of 146sm half life used in the age calculation in contrast to the sm nd isotope system the rb sr system is affected by alteration but yields an apparent isochron with a slope corresponding to a much younger age of 4247 110 ma although the rb sr system in cais appears to be disturbed the initial 87sr 86sr value determined from the isochron is 0 698942 0 000008 and closely approximates estimates of the initial solar system value although this isochron may be a mixing line it might also record alteration on the allende parent body in which rb was added to the al3s4 cai that was initially largely devoid of rb

does the arc accretion model adequately explain the paleoproterozoic evolution of southern laurentia an expanded interpretation comment and reply comment assembly of lithosphere in southern laurentia is best interpreted to have taken place along a long lived 1 8 1 0 ga active margin via subduction accretion processes broadly analogous to present day convergence between australia and indonesia karlstrom and bowring 1988 1 karlstrom et al

mn cr systematics in primitive meteorites insights from mineral separation and partial dissolution abstract cr isotopic compositions have been measured on carbonaceous chondrites cc tafassasset paris niger i nwa 5958 nwa 8157 and jbilet winselwan in bulk samples the 54 cr 52 cr ratios expressed as e 54 cr range from 0 93 to 1 58 e units these values are in agreement with values characteristic for distinct petrologic types despite this 54 cr heterogeneity the variability in the 53 cr 52 cr ratios expressed as e 53 cr of 0 2 e units and the mn cr ratios is consistent with the previous finding of an isochron in the mn cr evolution diagram the mn cr ratio in cc corresponds to variable abundances of high t condensate formed and separated at the beginning of the solar system thus the canonical 53 mn 55 mn ratio can be defined based on a consistent chronology for u pb and mn cr between the earliest objects formed in the solar nebula and the d orbigny angrite we define a canonical 53 mn 55 mn ratio and e 53 cr i of 6 8 10 6 and 0 177 respectively the internal mn cr systematics in tafassasset and paris were studied by two approaches leaching technique and mineral separation despite variable e 54 cr values up to 30 e linear co variations were found between e 53 cr and mn cr ratio the mineral separates as well as the leachates of tafassasset fall on a common isochron indicating that 1 cooling of the tafassasset s parent body occurred at 4563 5 0 25 ma and that 2 54 cr is decoupled from the other isotopes even though temperatures 900 c have been reached during metamorphism in the case of paris the leachates form an alignment with a 53 mn 55 mn ratio higher than the canonical value this alignment is not an isochron but rather a mixing line based on leachates from various cm and ci we propose the occurrence of three distinct cr reservoirs in meteoritic material pure54 high53 and low53 characterized by a e 53 cr and e 54 cr of 0 and 25 000 2 17 and 8 and 0 5 and 151 respectively pure54 has already been described and is carried by highly refractory nano spinel high53 is mn rich and most probably carried by sulfides in the matrix whereas low53 is characterized by low mn cr ratios and it is sensitive to metamorphism this component could correspond to mineral phases such as refractory oxides and carbide variable mixing proportions of high53 and low53 would explain the larger than expected uncertainty mswd of 5 5 on the cc bulk regression line a monte carlo simulation allows us to evaluate the impact of the dispersion of the initial cr isotopic ratios as a function of variable high53 the co variation of the mn cr ratio and the e 53 cr defined by the mineral separates from paris corresponds to an age of 4566 44 0 66 0 75 ma while their e 54 cr still differ by at least 0 42 e this age is likely to date the segregation of forsteritic olivines most probably from type i chondrules from fayalitic olivines from type ii chondrules and given the sampling procedure by handpicking of hundreds of grains corresponds to the average age of chondrule formation

the uthpb age of equilibrated l chondrites and a solution to the excess radiogenic pb problem in chondrites abstract u th and pb analyses of whole rock and troilite separates from seven l chondrites suggest that the excess radiogenic pb relative to u and the large variations in pb pb model ages commonly observed in chondritic meteorites are largely due to terrestrial pb contamination induced prior to analyses using the pb isotopic composition of troilite separates to calculate the isotopic composition of the pb contaminants the whole rock data have been corrected for pre analysis terrestrial pb contamination two approaches have been used 1 the chondrite troilite apparent initial pb isotopic compositions were used to approximate the mixture of indigenous intial pb and terrestrial pb in the whole rock sample and 2 a single stage concordant model was applied using the assumption that the excess radiogenic pb in these samples was terrestrial data for l5 and l6 chondrites yield a4551 7my age using the former correction and a4550 5my age using the latter one corrected data for one l4 chondrite tennasilm yield a4552 13my age which is indistinguishable from that of the l5 l6 chondrites however the other l4 chondrite bjurbole yields a4590 6my th u pb data suggest that this older age may be an artifact of the correction procedure and that some of the discordancy of the bjurbole data is the result of either a recent geologic disturbance to the u th pb system or to terrestrial u loss some aliquots of the l5 l6 chondrites also show small amounts of discordancy 10 which are not easily attributable to terrestrial pb contamination the data from the l5 l6 chondrites and tennasilm suggest that there are no more than 15my differences in the ages of l24 l6 chondrites

mojave yavapai boundary zone southwestern united states a rifting model for the formation of an isotopically mixed crustal boundary zone the boundary between the paleoproterozoic mojave and yavapai crustal provinces in the southwestern united states is a 75 km wide zone characterized by complex isotopically mixed pb crust similar to other transitional crustal and lithospheric boundaries worldwide mojave crust is more isotopically evolved than the juvenile crust of the yavapai province indicating an older crustal component several areas of isotopically juvenile rocks within and near the boundary zone correspond to exposures of coeval 1 73 ga supracrustal rocks that contain pillow basalts and bimodal metavolcanic sequences we suggest that regions of juvenile crust within the mojave province and its boundary zone represent rift basins produced by extension or transtension near the eastern margin of the province into which primitive basaltic lavas and bimodal volcanic sequences were erupted the largest region of juvenile material within the boundary zone is bound to the west by a series of coincident n trending isotopic geochemical metamorphic and geophysical discontinuities that may represent a fundamental buried crustal structure perhaps a rift margin we propose that the distinct isotopic signature of the boundary zone persisted through subsequent deformational events including the one that juxtaposed the two provinces if our model for the formation of the boundary zone is correct its isotopically mixed character is at least partly an artifact of the precollision history of the eastern mojave province and may not solely reflect the processes that juxtaposed the two provinces formation of new crust by rifting may represent an underappreciated form of crustal growth during the paleoproterozoic that is distinct from arc accretion models for the formation of complex crustal boundary zones must consider the early prejuxtaposition history of terranes in interpreting their geochemical and geophysical characteristics

determination of parental magmas of hed cumulates the effects of interstitial melts an evaluation of trapped melts effects during crystallization and subsolidus equilibration of cumulates is necessary to constrain the composition of their parental magmas in this paper a simple mass balance approach is described it allowes to estimate trace element abundances in these parental melts from phase compositions it is used to discuss the genesis of cumulate eucrites and diogenites the ree behavior is in full agreement with the view that cumulate eucrites formed from melts similar to noncumulate eucrites trapped melt fractions ranging from 10 wt for moama to 30 wt for moore county were involved the origin of diogenites is more complex the assumption that eucrites and diogenites shared the same parental melts cannot satisfactorily explain the diversity of incompatible trace element ratios e g dy yb observed in diogenitic orthopyroxenes even if interstitial melt effects are taken into account moreover some diogenites unambiguously crystallized from magmas displaying significant hree heavy rare earth elements enrichments more likely diogenites formed from distinct batches of parental magmas as previously proposed by mittlefehldt 1994 fowler et al 1995 and shearer 1997

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

martian meteorite chronology and the evolution of the interior of mars abstract the earth moon and mars are the only three planets for which satellite and ground observations can be confronted with extensive chronological and geochemical observations of rock samples here we use new pb isotope data on shergottites nakhlites chassigny and the alh 84001 orthopyroxenite to reassess chronological evidence and discuss its bearing on the evolution of the interior of mars we find that the depleted shergottites formed at 4 3 ga i e 170 ma after the last equilibration of pyroxene rich cumulates with the magma ocean while the younger enriched and intermediate shergottites as well as alh 84001 formed 200 ma later at 4 1 ga after the residual part of the mantle had been largely mixed back into these cumulates we also demonstrate that the th u ratios inferred from pb isotopes require shergottite ages older than 4 0 ga we finally reexamine the implications of this new chronological framework for the interpretation of 142 nd and 182 w anomalies in martian meteorites we conclude that shergottites represent samples that derived from a volcanic basement subsequently shattered by the 3 9 ga late heavy bombardment lhb although the magmatic activity of mars had strongly declined by the time of the lhb the new chronology of martian meteorites proposed here allows the martian mantle to be convective until today the 1 3 ga old emplacement of nakhlites and chassignites corresponds to decompression melting of ilmenite rich mantle domains and not to renewed global magmatic activity of the planet with the exception of this event the history of the interior of mars therefore somewhat resembles that of the moon

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

geochemical test for branching decay of 176lu abstract two different groups of values for the 176lu decay constant have been determined by recent high precision experiments the 176lu values of 1 86 1 87 10 11 a 1 were determined by age comparisons using terrestrial minerals of proterozoic and late archean age whereas values of 1 94 10 11 a 1 were determined in age comparison studies of meteorites a possible branched decay of 176lu could be the cause of this discrepancy the decay of 176lu to 176yb was detected in the early studies of radioactivity of 176lu with reported values of in the total 176lu ranging from less than 0 03 to 0 67 if the decay fraction is close to the upper limit of the reported values it can explain the 4 6 difference between the apparent 176lu values to get a reliable estimate for the decay of 176lu we have measured yb isotopic composition in 2 7 ga zircons with lu ybn chondrite normalized ratios of 1 40 and 1 45 in 1 0 ga xenotime with lu ybn 1 23 using yb from the 28 4 ma fish canyon tuff fct zircon and titanite as the modern reference value multiple analyses yielded the following weighted mean values 2 for the 176yb 174yb ratio 0 4022134 0 0000017 for the fct zircon and titanite 0 4022134 0 0000019 for the 1 0 ga xenotime and 0 4022124 0 0000033 for the 2 7 ga zircons these data yield 0 005 0 015 2 and establish an upper limit of 0 9 of total decays for the decay branch branching decay can therefore be eliminated as the cause of the discrepancy in 176lu decay constant estimates we discuss other possible causes of the 176lu terrestrial vs meteorite discrepancy

the quest for regolithic howardites part 2 surface origins highlighted by noble gases we report noble gas data of helium he neon ne argon ar krypton kr and xenon xe cosmic ray exposure cre ages and nominal gas retention k ar u th he ages for seven howardites cre 01400 eet 87513 eet 87518 eet 99400 gro 95535 gro 95602 san 03472 in continuing research to identify regolithic samples and better understand the vestan regolith in our previous work we found little correlation between suggested regolith parameters of ni 300 g g al 2o 3 8 9 wt and eucrite diogenite e d ratio of 2 1 warren et al 2009 and trapped solar wind sw fractionated solar wind fsw or planetary noble gas components from impacted material noble gas indicators of a regolithic origin here we have expanded our data set to include samples outside of these parameters to further explore composition and the differences in ni content as indicators for the presence of non vesta material in addition our sample set includes two potentially paired meteorites from the gro suite finally in our petrographic studies the samples selected showed no evidence for carbonaceous chondrite fragments which should reduce the effect of contamination by planetary noble gas components and will allow us to better identify sw fsw components where present of the samples studied here three howardites gro 95535 gro 95602 and eet 87513 show evidence for a regolithic origin with both isotopic and element noble gas ratios clearly pointing to the presence of trapped components similar to sw fsw or planetary the two gro howardites gro 95535 and gro 95602 show similar noble gas ratios to our previously defined sw fsw dominated regolithic group lew 85313 and met 00423 suggesting a surface origin for these samples however interestingly the gro samples show vastly different cosmogenic noble gas abundances and thus different cre ages which suggests that they are not paired for howardite eet 87513 the data hint to the presence of cm material with a neon release pattern similar to our defined planetary fsw dominated regolithic group cm rich samples pra 04401 sco 06040 our petrological investigations found no evidence for cm fragments within eet 87513 though a single clast was reported previously buchanan et al 1993 aside from the ne release pattern the remaining noble gas data show more similarity with sw fsw components the remaining four howardites cre 01400 eet 87518 eet 99400 and san 03472 and are dominated by cosmogenic noble gases and show no evidence for a regolithic origin our data suggest that a cm composition is likely present in all samples to some degree but that this can be overprinted by sw components or cosmogenic components obtained in situ on the vestan surface or during transit to earth respectively the presence of cm material is an important parameter for understanding the evolution of vesta s surface while we have uncovered three further regolithic howardites 13 regolithic total of 41 analysed further noble gas analysis of hed meteorites is needed to not only determine regolithic origins but to better characterise the abundance of carbonaceous chondrite material and its effect on the noble gas signatures of such samples

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

a magma ocean on vesta core formation and petrogenesis of eucrites and diogenites abstract available evidence strongly suggests that the hed howardite eucrite diogenite meteorites are samples of asteroid 4 vesta abundances of the moderately siderophile elements ni co mo w and p in the hed mantle indicate that the parent body may have been completely molten during its early history during cooling of a chondritic composition magma ocean equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0 80 when the convective system freezes and melts segregate from crystals by gravitational forces the extruded liquids are similar in composition to main group and stannern trend eucrites and the last pyroxenes to precipitate out of this ocean before convective lockup span the compositional range of the diogenites subsequent fractional crystallization of a main group eucrite liquid which has been isolated as a body of magma produces the nuevo laredo trend and the cumulate eucrites the predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites thus eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with vesta

survey and evaluation of eucrite bulk compositions the purpose of this survey is to establish reference bulk elemental abundances for the eucrites and thereby provide the basis to test core formation models as well as partial melting fractional crystallization and magma ocean theories for the eucrite parent body in order to evaluate bulk elemental abundances for the eucrites 296 peer reviewed articles monographs theses or books and 143 abstracts dating from 1938 to 1997 were surveyed of the 101 eucrites having at least one set of elemental abundance analyses reported in the literature 20 were selected for in depth examination the selection criteria of our sample were based on the total number of analyses available for a given eucrite and the total number of elements for which data exist the mean bulk elemental abundance 1 standard deviation and the percent deviation were calculated for each element in a given eucrite in order to evaluate the quality of the mean abundances the elements were then grouped according to availability of data and percent deviations possible reasons for the different deviations in the different groups are briefly discussed from the major element abundances the normative cipw composition the molar compositions of pyroxene olivine and plagioclase and the bulk densities were calculated and compared to petrographic observations the calculated norms for the noncumulates agree well with the observations while the norms for the cumulates do not possible reasons for this are discussed unfortunately analyses of many elements are poorly represented in the literature and many bulk analyses suffer from unacceptable levels of uncertainty therefore future work requires bulk elemental analyses for some of the more poorly characterized elements in eucrites especially those of key elements used for planetary modeling

is vesta an intact and pristine protoplanet abstract it is difficult to find a vesta model of iron core pyroxene and olivine rich mantle and hed crust that can match the joint constraints of a vesta s density and core size as reported by the dawn spacecraft team b the chemical trends of the hed meteorites including the depletion of sodium the feo abundance and the trace element enrichments and c the absence of exposed mantle material on vesta s surface among vestoid asteroids or in our collection of basaltic meteorites these conclusions are based entirely on mass balance and density arguments independent of any particular formation scenario for the hed meteorites themselves we suggest that vesta either formed from source material with non chondritic composition or underwent after its formation a radical physical alteration possibly caused by collisional processes that affected its global composition and interior structure

source spectra and rms acceleration of mexican subduction zone earthquakes strong motion sm data of six mexican subduction zone earthquakes 6 4 ms 8 1 recorded near the epicentral zone are analyzed to estimate their far field source acceleration spectra at higher frequencies f 0 3 hz apart from the usual corrections such as geometrical spreading 1 r average radiation pattern 0 6 free surface amplification a factor of 2 and equal partitioning of the energy into two orthogonal horizontal components a factor of 1 sqrt 2 the observed spectra are corrected for a frequency dependentq q 100f a site dependent filter e kf and amplification ofs waves near the surface a factor of about 2 atf 2hz we taker as the average distance from the rupture area to the site if we model the high frequency plateau f 1 hz of the source spectra by a point source 2 model and interpret them in terms of brune s model we obtain between 50 and 100 bars for all earthquakes the low frequency broadband teleseismicp wave spectra corrected witht 1 0 s agrees within a factor of two with sm source spectra near 1 hz the 2 model is inadequate to explain the observed source spectra in a broad frequency range these resemble spectra given bygusev 1983 with some differences

ancient water on asteroid 4 vesta evidence from a quartz veinlet in the serra de mage eucrite meteorite abstract the meteorite serra de mage a eucrite inferred to be from the asteroid 4 vesta contains quartz veinlets they are identical to antitaxial or crack seal quartz veinlets in terrestrial rocks and are extraterrestrial and ancient because they pre date a 4 40 ga metamorphism the quartz was likely deposited from liquid water solutions as are terrestrial veins other potential solvents or transport mechanisms are inadequate or unlikely because there is no indication of internal magmatic water in the eucrite meteorites and thus in vesta the water from which the veinlet was deposited probably came from outside vesta by analogy with water ice deposits on the moon and mercury vesta and similar asteroids may have had or now have polar ice deposits possibly derived from comet impacts

age of a eucrite clast from the bholghati howardite 39ar 40ar rb sr and sm nd investigations of the large eucrite clast 1 studied by the bholghati consortium show that it crystallized from a melt 4 51 ga ago the crystallization age is given most precisely by a sm nd internal isochron of 4 510 0 034 ga 2 error limits for 147sm 0 00654 ga 1 142nd144nd measurements show that the clast contained live 146sm at the time of crystallization a solar system initial 146sm144sm0 0 00431 0 0039 0 0026 at 4 555 ga ago can be calculated from a 146sm 142nd isochron and the age of the clast alternatively using 146sm144sm 0 0 0045 as previously reported in the literature a 146sm142nd formation interval of 0 05 0 12 0 08 ga can be calculated which is concordant with the 147sm 143nd isochron age and in agreement with pu u xe and pu nd xe formation intervals reported in a companion consortium paper the initial 143nd144nd ind of the clast was ind 0 505958 50 2 error limits normalized to 146nd 144nd 0 724140 and is consistent with the chur reference value for evolution in a source having chondritic relative ree abundances prior to magma generation however when interlaboratory bias is considered the data are also permissive of a lree depleted source as suggested in a companion consortium paper a primary rb sr isochron yields an age of 4 54 0 06 ga 2 error limits for 87rb 0 01402 ga 1 in agreement with the more precise sm nd age the initial 87sr86sr isr of the clast was isr 0 699006 21 and was the same within error limits as isr previously determined for a pristine eucrite clast from polymict eucrite y75011 compatible with a common parent body the 39ar 40ar data and some of the rb sr data show that bholghati was affected by a major thermal event 2 3 ga ago and also provide weak evidence for a minor thermal event 1 ga ago the age and isotopic systematics of the clast are consistent with those of other eucrites and support the widely held assumption of a common parent body for howardites and eucrites the isotopic investigations of polymict eucrites and howardites point to a prolonged cratering record on the howardite eucrite diogenite hed parent body and suggest that it survived the cataclysm affecting the moon 3 8 4 0 ga ago

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

an experimental assessment of the potential of sulfide saturation of the source regions of eucrites and angrites implications for asteroidal models of core formation late accretion and volatile element depletions abstract the geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies although sulfides are important reservoirs for a diverse suite of major and trace elements it is currently unclear whether the interiors of asteroid vesta and the angrite parent body were sulfide liquid saturated during petrogenesis of non cumulate eucrites and volcanic angrites to assess the potential of sulfide liquid saturation in the interiors of these bodies high pressure p temperature t experiments were used to quantify the sulfur concentrations at sulfide saturation scss for volcanic angrites and non cumulate eucrites the sulfide silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation it was found that the measured scss values agree well with the scss values predicted from a previous thermodynamic model for high feo melts to assess the possibility of sulfide liquid saturation of the source regions of non cumulate eucrites and angrites their s abundances were compared with the calculated scss values for their source regions results show that if eucritic and angritic source regions were saturated with fes liquid significant degassing 50 80 of s must have occurred during or following their magmatic emplacement such loss is inconsistent with the s cl zn and rb isotopic compositions of non cumulate eucrites sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of cu and hse in non cumulate eucrites and angrites riches et al 2012 additional calculations were performed to further explore the timing and extent of s loss during crystallization of the vestan magma ocean the assumption of chondritic bulk s abundances of bulk vesta would correspond with extremely high s contents of the eucrite source region s even after consideration of depletion of s due to core formation in light of the s cl zn and rb stable isotopic compositions of eucrites the s abundances in eucrites are most consistent with the hypothesis that the vestan mantle was already strongly depleted in s 70 80 by the time of vestan magma ocean crystallization resulting in more realistic s contents of the eucrite source region s the depletion of s could have been established during initial accretion of vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation wu et al 2018 modeling of the new experimentally determined sulfide silicate partition coefficients and previously reported vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early vestan magma ocean crystallization is also unlikely the lack of sulfide liquid saturation in the source regions of non cumulate eucrites and angrites as well as during early vestan magma ocean solidification shows that current geochemical models of core formation and late accretion remain valid for these bodies

147sm 143nd and 176lu 176hf in eucrites and the differentiation of the hed parent body the 147sm 143nd and 176lu 176hf systematics of 21 whole rock eucrites including five cumulates have been investigated by mc icp ms a statistically significant sm nd isochron was obtained on 18 samples with an age of 4464 75 ma mswd 1 26 and an intercept of 0 50680 0 00010 this age clearly is controlled by the cumulate eucrites the 21 basaltic and cumulate eucrites together do not form a statistically significant lu hf isochron basaltic eucrites however form an errorchron with an age of 4604 39 ma mswd 4 52 which becomes an acceptable isochron if the error on the lu hf ratio is doubled with respect to the laboratory estimate the initial 176hf 177hf is 0 27966 0 00002 three of the cumulate eucrites regressed together further yield a statistically significant age of 4470 22 ma indistinguishable from their sm nd age we therefore conclude that cumulate eucrites are 100 myr younger than basaltic eucrites and basaltic eucrites are close in age to the solar system there is no evidence in the present data to support a decay constant of 176lu significantly different from 1 93 10 11 yr 1 sguigna can j phys 60 1982 361 364 the broad range of variation and strong correlation of lu hf and sm nd ratios require that eucrites are partial melts and not residual magmas the relative lu hf and sm nd fractionation is controlled by the plagioclase to mafic mineral ratio in the residue the steep correlation between these two ratios is explained by the enhancement of plagioclase saturation in the low gravity field of the eucrite parent body basaltic eucrites probably formed as large degree melts of a chondritic source the strong lu hf and sm hf fractionation observed in cumulates clearly reflects the presence of residual ilmenite during melting our preferred interpretation is that cumulates were impregnated by small degree melts of ilmenite bearing gabbros since pressure on the eucrite parent body never reaches the garnet stability field this observation questions the ubiquitous presence of residual garnet in the mantle sources of magmas formed on larger planets such as the moon mars and possibly earth

branching sheeted dikes and seafloor spreading within an early proterozoic intra arc basin the payson ophiolite 1 73 ga is exposed as a shallow dipping pseudostratigraphic sequence of gabbro sheeted dikes and submarine volcanic rocks with screens and roof pendants of older arc crust the sheeted dike complex is particularly well exposed and analysis of measured sections reveals a threefold increase in the average dike width with depth in the complex magmatic temperatures and slow cooling at the base of the sheeted dike complex indicated by gabbro dike mingling and mutual intrusion and coarse textures may have facilitated the growth of wide dikes in contrast extreme alteration of the top of the sheeted dike complex suggests that hydrothermal circulation dominated and caused rapid cooling as thin dikes the connection between wide dikes at depth with thinner dikes toward the top requires a new model where dikes branch upward in response to the strong vertical gradient in the rate of cooling and solidification the branching dike model has important implications for the timing and spatial distribution of seafloor phenomena in the neovolcanic zone of spreading centers

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

sm nd systematics of chondrites we have studied the 147sm 143nd and 146sm 142nd isotopic systems in phosphate fractions and chondrules from six ordinary chondrites and one carbonaceous chondrite previously dated with pb pb method 147sm 144nd ratios vary between 0 182 and 0 191 in phosphates and between 0 179 and 0 243 in chondrules the 147sm 143nd isochron regression through all 34 phosphate and chondrule analyses yields a date of 4588 100 ma and is in good agreement with more precise pb pb dates of the same chondrites the initial 143nd 144nd is 0 50665 0 00014 the same analyses define a 146sm 142nd isochron with a slope corresponding to 146sm 144sm 0 0075 0 0027 initial 142nd 144nd 1 14160 0 00011 corresponds to e142nd 2 62 0 93 compilation of the published chondritic whole rock sm nd analyses yields the median 147sm 144nd 0 1964 0 0003 0 0007 which is our preferred chondritic uniform reservoir chur value using this value and its error limits we find the present day chur 143nd 144nd 0 512637 0 000009 0 000021 from the chondritic sm nd isochron that includes all available data for whole rocks chondrules and phosphates this value is identical within error with the currently accepted number an estimate of the bulk earth 147sm 144nd 0 1941 0 0059 is obtained from intercept of chondritic 146sm 142nd isochron with the terrestrial value of 142nd 144nd this estimate is independent of measured sm nd ratios in chondrites the same approach was applied to published 146sm 142nd internal isochrons for differentiated meteorites and yielded similar although less precise values our data are completely consistent with the currently accepted chur parameters and substantiate their use as terrestrial reference values

reconciliation of the excess 176 hf conundrum in meteorites recent disturbances of the lu hf and sm nd isotope systematics abstract the long lived 176 lu 176 hf and 147 sm 143 nd radioisotope systems are commonly used chronometers but when applied to meteorites they can reveal disturbances specifically lu hf isochrons commonly yield dates up to 300 myr older than the solar system and varying initial 176 hf 177 hf values we investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible minerals scatter away from isochron regressions for both meteorite classes with low hf phases such as plagioclase and olivine typically being most displaced above or left of reference isochrons relatively hf rich pyroxene is less disturbed but still to the point of steepening lu hf errorchrons using our lu hf and sm nd data we tested various hf and lu redistribution scenarios and found that decoupling of lu hf from 176 hf 177 hf must postdate the accumulation of significant radiogenic 176 hf therefore early irradiation or diffusion cannot explain the excess 176 hf instead disturbed meteorite isochrons are more likely caused by terrestrial weathering contamination or common laboratory procedures the partial dissolution of phosphate minerals may predominantly remove rare earth elements including lu leaving relatively immobile and radiogenic hf behind robust lu hf and improved sm nd meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic hf and silicate hosted terrestrial contaminants without disturbing parent daughter ratios

impact melting of the cachari eucrite 3 0 gy ago both the host phase and glass veins of the cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their 39ar 40ar gas retention ages cachari is chemically similar to other non cumulate eucrites the vesicular glass veins vary from pure glass to devitrified glass to areas that are substantially crystalline the glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of cachari but some differences probably due to preferential dissolution occur along melt contacts the glass formed by shock melting of cachari host or of rock identical to it 39ar 40ar data for the host and glass suggest distinctly different ages of 3 04 07 gy and 3 47 04 gy respectively the time of glass formation which may also be the time of brecciation is most likely given by the 3 0 gy age of the host the higher age for the glass is interpreted to represent incomplete ar degassing during the 3 0 gy event due to the greater resistance to ar diffusion shown by the glass compared to the host event ages significantly younger than 4 5 gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body