ar ar and i xe ages and thermal histories of three unusual metal rich meteorites abstract portales valley sombrerete and northwest africa nwa 176 are three unrelated meteorites which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies measured 39 ar 40 ar ages of these meteorites are 4477 11 ma and 4458 16 ma two samples of portales valley 4541 12 ma and 4524 13 ma respectively ma million years all one sigma errors the ar ar data for portales valley show no evidence of later open system behavior suggested by some other chronometers measured 129 i 129 xe ages of these three meteorites are 4559 9 0 5 ma 4561 9 1 0 ma and 4544 ma respectively relative to shallowater 4562 3 0 4 ma from stepwise temperature release data we determined the diffusion characteristics for ar and xe in our samples and calculated approximate closure temperatures for the k ar and i xe chronometers adopting results and interpretations about these meteorites from some previous workers we evaluated all these data against various thermal cooling models we conclude that portales valley formed 4560 ma ago cooled quickly to below the i xe closure temperature then cooled deep within the parent body at a rate of 4 c ma through k ar closure we conclude that sombrerete formed 4562 ma ago and cooled relatively quickly nwa 176 likely formed and cooled quickly 4544 ma ago or later than formation times of most meteorite parent bodies for all three meteorites the ar ar ages are in better agreement with i xe ages and preferred thermal models if we increase these ar ar ages by 20 ma such age corrections would be consistent with probable errors in 40 k decay parameters in current use as suggested by others the role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system

low temperature phase decomposition in iron nickel metal of the portales valley meteorite the portales valley meteorite provides an opportunity to investigate and compare the microstructure in fe ni metal of the metallic particles in the chondritic portion and in the metal veins the low temperature phase decomposition of fe ni metal was investigated using scanning electron microscopy transmission electron microscopy and atomic force microscopy the microstructure is formed as the portales valley meteorite cooled from high temperatures and includes the outer taenite rim the cloudy zone clear taenite and martensite martensite in turn decomposes into a fine admixture of fcc rods in a bcc matrix the width of the island phase of the cloudy zone in the metal particles of the chondritic portion and the metal veins can be used to estimate a low temperature cooling rate the microstructural evidence indicates that the chondritic portions and the metal veins in the portales valley meteorite cooled together as a mixture with a cooling rate of roughly 6 5 k ma

39ar 40ar ages of martian nakhlites abstract we report 39ar 40ar ages of whole rock wr and plagioclase and pyroxene mineral separates of nakhlites mil 03346 and y 000593 and of wr samples of nakhlites nwa 998 and nakhla all age spectra are complex and indicate variable degrees of 39ar recoil and variable amounts of trapped 40ar in the samples thus we examine possible ar ar ages in several ways from consideration of both limited plateau ages and isochron ages we prefer ar ar ages of nwa 998 1334 11 ma mil 03346 1368 83 ma mesostasis and 1334 54 ma pyroxene y 000593 1367 7 ma and nakhla 1357 11 ma 2 errors for nwa 998 and mil 03346 the ar ar ages are within uncertainties of preliminary rb sr isochron ages reported in the literature these ar ar ages for y 000593 and nakhla are several ma older than sm nd ages reported in the literature we conclude that the major factor in producing ar ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40ar on weathered grain surfaces that was degassed along with the first several percent of 39ar a total k 40ar isochron for wr and mineral data from five nakhlites analyzed by us plus lafayette data in the literature gives an isochron age of 1325 18 ma 2 we emphasize the precision of this isochron over the value of the isochron age our ar ar data are consistent with a common formation age for nakhlites the cosmic ray exposure cre age for nwa 998 of 12 ma is also similar to cre ages for other nakhlites

39ar 40ar evidence for early impact events on the ll parent body abstract we determined 39ar 40ar ages of eight ll chondrites and one igneous inclusion from an ll chondrite with the object of understanding the thermal history of the ll chondrite parent body the meteorites in this study have a range of petrographic types from ll3 3 to ll6 and shock stages from s1 to s4 these meteorites reveal a range of k ar ages from 3 66 to 4 50 ga and peak ages from 3 74 to 4 55 ga significantly three of the eight chondrites ll4 5 6 have k ar ages of 4 27 ga one of these mil99301 preserves an 39ar 40ar age of 4 23 0 03 ga from low temperature extractions and an older age of 4 52 0 08 ga from the highest temperature extractions in addition an igneous textured impact melt dom85505 22 has a peak 39ar 40ar age of 4 27 ga we interpret these results as evidence for impact events that occurred at about 4 27 ga on the ll parent body that produced local impact melts reset the 39ar 40ar ages of some meteorites and exhumed or interred others resulting in a range of cooling ages the somewhat younger peak age of 3 74 ga from gro95658 ll3 3 suggests an additional impact event close to timing of impact reset ages of some other ordinary chondrites between 3 6 3 8 ga the results from mil99301 suggest that some apparently unshocked s1 chondrites may have substantially reset 39ar 40ar ages a previous petrographic investigation of mil99301 suggested that reheating to temperatures less than or equal to type 4 petrographic conditions 600 c caused fractures in olivine to anneal resulting in a low apparent shock stage of s1 unshocked the 39ar 40ar age spectrum of mil99301 is consistent with this interpretation older ages from high t extractions may date an earlier impact event at 4 52 0 08 ga whereas younger ages from lower t extractions date a later impact event at 4 23 0 03 ga that may have caused annealing of feldspar and olivine

transposed high pressure granulite fabrics cabo ortegal nw spain implications on the scales of deformation localization abstract high pressure granulite facies rocks of the bacariza formation cabo ortegal nw spain were syn metamorphically deformed at the contacts with the bounding units peridotite and eclogite massifs this enabled the formation of meter thick spectacular shear zones with reworked and transposed foliations and lineations the texturally stable mineral assemblage of the new fabrics records an intense ductile deformation of the mineral aggregate at temperatures of 700 800 c associated with amalgamation of eclogite high pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel the lattice preferred orientation of the main constituent minerals garnet augite amphibole plagioclase quartz and biotite discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high grade conditions complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species

revised model calculations for the thermal histories of ordinary chondrite parent bodies recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model miyamoto et al 1981 for the thermal evolution and internal structure of ordinary chondrite parent bodies model calculations assumed a heat source term derived from the decay of 26al justification is provided differences from the previous model include varying the thermal diffusivity parameter with increasing temperature and decreasing porosity using variable physical and thermal parameters to provide end member models and incorporating a shortened thermal history of 60 ma obtained from new pb pb chronology of phosphates rather than 100 ma times of isotopic closure in chondrite phosphates overlap the thermal model estimates and postmetamorphic cooling rates from the model approximately coincide in both trend and magnitude with metallographic and fission track cooling rate data model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1 4 to 3 1 ma after calcium aluminum inclusion cai formation model calculations also predict that both the h and the l chondrite parent asteroids consisted of 80 equilibrated and 20 unequilibrated chondritic material and that their original radii ranged from 80 to 95 km

39ar 40ar dating of the zagami martian shergottite and implications for magma origin of excess 40ar the zagami shergottite experienced a complex petrogenetic formation history mccoy et al 1992 1999 like several shergottites zagami contains excess 40ar relative to its formation age to understand the origin of this excess 40ar we made 39ar 40ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution surprisingly all these separates show similar concentrations of excess 40ar 1 x 10 6 cm 3 g we present arguments against this excess 40ar having been introduced from the martian atmosphere as impact glass we also present evidence against excess 40ar being a partially degassed residue from a basalt that actually formed 4 gyr ago we utilize our experimental data on ar diffusion in zagami and evidence that it was shock heated to only 70 c and we assume this heating occurred during an ejection from mars 3 myr ago with these constraints thermal considerations necessitates either that its ejected mass was impossibly large or that its shock heating temperature was an order of magnitude higher than that measured we suggest that this excess 40ar was inherited from the zagami magma and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old k rich crustal material similar concentrations of excess 40ar in the analyzed separates imply that this magma maintained a relatively constant 40ar concentration throughout its crystallization this likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released these concepts have implications for excess 40ar in other shergottites

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

comment on 40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology by paul r renne

39ar 40ar age and thermal history of martian dunite nwa 2737 abstract we report an 39 ar 40 ar age determination of a whole rock sample of the olivine rich martian meteorite northwest africa nwa 2737 those extractions releasing 0 48 of the 39 ar define an 39 ar 40 ar isochron age of 160 190 ma when evaluated in various ways higher temperature extractions show increasing ages that eventually exceed the reported sm nd age of 1 42 ga at least part of this excess 40 ar may have been shock implanted from the martian atmosphere we considered two possible interpretations of the ar ar isochron age utilizing the measured ar diffusion characteristics of nwa 2737 and a thermal model which relates ar diffusion to the size of a cooling object after shock heating one interpretation that 40 ar was only partially degassed by an impact event 11 ma ago the cre age appears possible only if nwa 2737 was shock heated to temperatures 600 c and was ejected from mars as an object a few 10 s of cm in diameter the second interpretation which we prefer is that nwa experienced an earlier more intense shock event which left it residing in a warm ejecta layer and a less intense event 11 ma ago which ejected it into space our evaluation would require nwa 2737 to have been heated by this first event to a temperature of 300 500 c and buried in ejecta to a depth of 1 20 m these conclusions are compared to model constraints on meteorite ejection from mars reported in the literature the second mars ejection impact 11 ma ago probably heated nwa 2737 to no more than 400 c nwa 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space

40ar 39ar dating of plagioclase grain size separates from silicate inclusions in iab iron meteorites and implications for the thermochronological evolution of the iab parent body abstract in order to better constrain the thermochronological evolution of the iab parent body we performed a 40ar 39ar age study on individual silicate inclusions of the iab irons caddo county campo del cielo landes and ocotillo in contrast to earlier studies several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the ar ar ages in nearly all inclusions we found significantly different ar ar ages among the separates caddo county 4 472 0 02 4 562 0 02 ga campo del cielo 2 4 362 0 04 4 442 0 03 ga landes 2 4 412 0 05 4 522 0 04 ga ocotillo 4 382 0 04 4 462 0 03 ga these ages were calculated using the new 40k decay constant published by mundil r renne p r min k and ludwig k r 2006 resolvable miscalibration of the 40ar 39ar geochronometer eos trans agu 87 fall meet suppl abstract v21a 0543 the ages did not systematically correlate with the respective grain size of the separate as expected i e smaller grains did not necessarily show younger ages due to later closure to ar diffusion or easier re opening of the system in the course of a reheating event compared to larger grains based on the large range of ar ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the iab parent body while some grains remained in a hot deep environment that allowed ar diffusion over an extended time period in some cases combined with grain coarsening others cooled significantly earlier near surface through the k ar blocking temperature these different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by benedix et al benedix g k mccoy t j keil k and love s g 2000 a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body meteorit planet sci 35 1127 1141 due to rapid cooling after the impact some of the age differences among the grains could be preserved based mainly on our caddo county ar ar age information the iab parent body was destroyed by impact and reassembled between 4 5 and 4 47 ga however iab silicate ar ar ages should depend much more on the pre and post impact cooling rate and burial depth than on the time of the actual impact this is supported by a compilation of our and literature iab and winonaite ar ar ages ranging smoothly from the time of accretion of the chondritic iab parent body down to the time of its final cooling through the k ar blocking temperature after impact and reassembly instead of showing a peak in ar ar ages at the time of the destructive impact

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

39ar 40ar ages and thermal history of mesosiderites determinations of 39ar 40ar ages for plagioclase separates of three clasts from the vaca muerta and mount padbury mesosiderites give similar results ages increase monotonically by 0 2 0 3 ga when plotted against increasing 39ar release from the samples and give average values of 3 95 ga when combined with revised ages for mesosiderite analyses previously reported nineteen analyses of thirteen mesosiderites give an average ar ar age of 3 94 0 10 ga the average age at 90 39ar release is 4 13 0 14 ga and the ages at 10 39ar release range over 3 32 3 97 ga it seems unlikely that the 39ar 40ar ages were produced by direct impact heating but they probably are independent evidence for slow cooling deep within the mesosiderite parent body we conclude that the sloped age plateaus and the significant range in ar ar closure temperatures calculated for these mesosiderites 75 340 c are due to significant differences in ar diffusion properties among samples from the ar diffusion data we estimate an approximate mesosiderite cooling rate of 0 2 c ma which is similar to the preferred cooling rate of 0 1 0 5 c ma derived by others from ni diffusion and metal textures the 39ar 40ar ages seem incompatible with very slow cooling rates of 0 2 c ma unless the closure temperatures for ar diffusion and formation of metal cooling textures on the parent body substantially overlap any possible role of fragmentation of the mesosiderite parent body in producing the younger ar ar ages and the very slow cooling rates is unclear

an early i xe age for cb chondrite chondrule formation and a re evaluation of the closure age of shallowater enstatite the iodine xenon system has been analyzed in samples of 7 chondrules from the cb chondrites gujba and hammadah al hamra hah 237 one sample from gujba defined a high temperature iodine xenon isochron corresponding to closure 1 87 0 4 ma before closure of shallowater enstatite motivated by this result we employ outlier rejection to re evaluate the shallowater age leading to a modified value of 4562 3 0 4 ma 1 in this process the datum obtained by combining our i xe age for gujba with the literature pb pb age is rejected as an outlier indicating that in this sample the i xe system closed earlier than the accepted pb pb age of chondrules from cb chondrites the need for a formation environment distinct from that of chondrules from other meteorites is thus reduced

verification and interpretation of the i xe chronometer abstract comparisons of i xe and pb pb relative ages are made for phosphate and feldspar separates from 12 different meteorites in all cases where i xe ages can be measured in phosphate i xe and pb pb chronometers agree within experimental uncertainty no discordant phosphate samples are observed with the exception of allegan i xe ages for feldspar separates generally agree with pb pb ages for the corresponding phosphate the general concordancy observed between i xe and pb pb chronometers suggests that the i xe system is a reliable and interpretable chronometer when applied to single minerals systems allegan feldspar differs from the other feldspar separates predating its phosphate by 17 ma with an i xe age indistinguishable from those of orgueil and murchison magnetites among the oldest observed

the portales valley meteorite breccia evidence for impact induced melting and metamorphism of an ordinary chondrite abstract the portales valley h chondrite fall is an annealed impact melt breccia with coarse metal interstitial to angular and subrounded silicate clasts the large metal rich regions exhibit a widmanstatten structure and contain very little troilite we were able to examine a 16 5 kg metal rich specimen of portales valley silicates contain numerous flecks of metallic cu and curvilinear trails of tiny metallic fe ni blebs characteristic of shocked and annealed chondrites one silicate clast appears to have experienced little

portales valley a meteoritic sample of the brecciated and metal veined floor of an impact crater on an h chondrite asteroid a shower of meteorite fragments fell at 0730 h local time on 1998 june 13 near the town of portales new mexico thus far 51 pieces of the portales valley h6 meteorite have been recovered this meteorite has an unusually large number of metallic veins some of these veins are also unusually thick having widths on the order of centimeters these wide veins have fine widmanstatten structure which is the first time it has been seen in an ordinary chondrite this structure indicates the metallic veins and the host chondrite cooled slowly these veins appear to have been produced by shock metamorphic processes which we infer produced a 20 km diameter impact crater on an h chondrite planetesimal

a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body abstract we have studied iab iron meteorites and their silicate bearing inclusions to elucidate the origin of their parent body we have divided iab irons into five categories which best describe the inclusions and other properties of the irons these are 1 sulfide rich irons such as mundrabilla that contain a significant volume fraction of troilite with or without associated silicates 2 irons with non chondritic silicate inclusions such as basaltic inclusions in caddo county 3 irons with rounded troilite and or graphite inclusions sometimes containing embedded angular silicate inclusions 4 irons with angular silicate inclusions similar to those of the winonaites where silicates may comprise up to 40 vol of the rock and 5 irons with phosphate bearing inclusions such as the brianite bearing iab san cristobal similarities in mineralogy mineral and oxygen isotopic compositions suggest that iab irons and winonaites are almost certainly from a common parent body iiicd irons may also have originated on this parent body although evidence supporting this suggestion is less convincing previous models for the origin of iab irons propose that they are either products of selective impact melting in the megaregoliths of a chondritic asteroid or products of partial melting and fractional crystallization during formation of a sulfur rich core the former model suffers from the fact that impact is incapable of causing preferential melting of metal and sulfide on a large scale and pooling of these selective melts into sizable bodies the latter model does not readily explain the occurrence of often fragmentary angular silicate clasts in a metallic fe ni host we propose a new model for the origin of iab irons that incorporates aspects of both previous models we suggest that a chondritic possibly heterogeneous parent body was heated by a non collisional heat source which caused partial but not complete melting of the body after peak temperatures were reached and cooling and crystallization had begun a major impact catastrophically fragmented this body and the debris gravitationally reassembled scrambling the parent body lithologies we suggest that the impact caused extensive mixing of molten metallic fe ni and solid silicate material resulting in the formation of iab irons with silicate inclusions whereas winonaites represent metamorphosed rocks and partial melt residues of the precursor silicate lithologies high temperatures of some of the reassembled debris and deep burial resulted in continued fractionation and formation of the 2

formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid we propose that mesosiderites formed when a 200 400 km diameter asteroid with a molten core was disrupted by a 50 150 km diameter projectile to test whether impacts can excavate core iron and mix it with crustal material we used a low resolution smoothed particle hydrodynamics computer simulation for 50 300 km diameter differentiated targets we found that significant proportions of scrambled core material and hence potential mesosiderite metal material could be generated for near catastrophic impacts that reduce the target to 80 of its original diameter and about half of its original mass the proportion of scrambled core material would be about 5 vol equivalent to 10 vol of mesosiderite like material the paucity of olivine in mesosiderites and the lack of metal poor or troilite rich meteorites from the mesosiderite body probably reflect biased sampling mesosiderites may be olivine poor because mantle material was preferentially excluded from the metal rich regions of the reaccreted body molten metal globules probably crystallized around small cool fragments of crust hindering migration of metal to the core if mantle fragments were much hotter and larger than crustal fragments little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally the rapid cooling rates of mesosiderites above 850 c can be attributed to local thermal equilibration between hot and cold ejecta very slow cooling below 400 c probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris we infer that our model is more plausible than an earlier model that invoked an impact at 1 km s to mix projectile metal with target silicates if large impacts cannot effectively strip mantles from asteroidal cores as we infer we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material this may help to explain why relatively few olivine rich a type and metal rich asteroids m type are known some s type asteroids may be scrambled differentiated bodies

meteoritic and other constraints on the internal structure and impact history of small asteroids studies of the internal structure of asteroids which are crucial for understanding their impact history and for hazard mitigation appear to be in conflict for the s type asteroids eros gaspra and ida spacecraft images and geophysical data show that they are fractured coherent bodies whereas models of catastrophic asteroidal impacts family and satellite formation and studies of asteroid spin rates and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble these conflicting views may be reconciled if 10 50 km s type asteroids formed as rubble piles but were later consolidated into coherent bodies many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration metamorphism igneous and impact processes ordinary chondrites which are the best analogs for s asteroids are commonly breccias some may have formed in cratering events but many appear to have formed during disruption and reaccretion of their parent asteroids some breccias were lithified during metamorphism and a few were lithified by injected impact melt but most are regolith and fragmental breccias that were lithified by mild or moderate shock like their lunar analogs shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together and by friction and pressure welding of silicate and metallic fe ni grains we suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble internal voids would be partly filled with regolith by impact induced seismic shaking consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body fractures on ida that were created by antipodal impacts and are concentrated in and near large craters and small positive gravity anomalies associated with the psyche and himeros craters on eros are consistent with this concept spin data suggest that smaller asteroids 0 6 6 km in size are unconsolidated rubble piles c type asteroids which are more porous than s types and their analogs the volatile rich carbonaceous chondrites were probably not lithified by shock

petrology of silicate inclusions in the sombrerete ungrouped iron meteorite implications for the origins of iie type silicate bearing irons the petrography and mineral and bulk chemistries of silicate inclusions in sombrerete an ungrouped iron that is one of the most phosphate rich meteorites known was studied using optical scanning electron microscopy sem electron microprobe analysis empa and secondary ion mass spectrometry sims techniques inclusions contain variable proportions of alkalic siliceous glass 69 vol of inclusions on average aluminous orthopyroxene 9 wo1 4fs2535 up to 3 wt al plagioclase 8 mainly an7092 cl apatite 7 chromite 4 yagiite 1 phosphaterich segregations 1 ilmenite and merrillite ytterbium and sm anomalies are sometimes present in various phases positive anomalies for phosphates negative for glass and orthopyroxene which possibly reflect phosphate melt gas partitioning under transient reducing conditions at high temperatures phosphate rich segregations and different alkalic glasses k rich and na rich formed by two types of liquid immiscibility yagiite a k mg silicate previously found in the colomera iie iron appears to have formed as a late stage crystallization product possibly aided by na k liquid unmixing trace element phase compositions reflect fractional crystallization of a single liquid composition that originated by low degree 48 equilibrium partial melting of a chondritic precursor compositional differences between inclusions appear to have originated as a result of a filter press differentiation process in which liquidus crystals of cl apatite and orthopyroxene were less able than silicate melt to flow through the metallic host between inclusions this process enabled a phosphoran basaltic andesite precursor liquid to differentiate within the metallic host yielding a dacite composition for some inclusions solidification was relatively rapid but not so fast as to prevent flow and immiscibility phenomena sombrerete originated near a cooling surface in the parent body during rapid probably impact induced mixing of metallic and silicate liquids we suggest that sombrerete formed when a planetesimal undergoing endogenic differentiation was collisionally disrupted possibly in a breakup and reassembly event simultaneous endogenic heating and impact processes may have widely affected silicate bearing irons and other solar system matter

absolute ages aren t exactly the time scales of important events and processes in geology and archaeology are ascertained through measurements of radioisotopes the accuracy of the dating depends critically on how well known the radioactive decay constants are in their perspective renne karner and ludwig discuss the role that discrepancies in decay constants play in the accuracy of radioisotope dating as dating measurements become increasingly precise geochronologists will have to confront these kinds of systematic errors

the iodine xenon system in clasts and chondrules from ordinary chondrites implications for early solar system chronology abstract we have studied the i xe system in chondrules and clasts from ordinary chondrites cristobalite bearing clasts from parnallee ll3 6 closed to xe loss 1 4 ma after bjurbole feline a feldspar and nepheline rich clast also from parnallee closed at 7 04 0 15 ma two out of three chondrules from parnallee that yielded well defined initial i ratios gave ages identical to bjurbole s within error a clast from barwell l6 has a well defined initial i ratio corresponding to closure 3 62 0 60 ma before bjurbole partial disturbance and complete obliteration of the i xe system by shock are revealed in clasts from julesburg l3 6 and quenggouk h4 respectively partial disturbance by shock is capable of generating anomalously high initial i ratios in some cases these could be misinterpreted yielding erroneous ages a macrochondrule from isoulane n amahar contains concentrations of i similar to ordinary chondrules but unlike most ordinary chondrules contains no radiogenic 129xe this requires resetting 50 ma or more later than most chondrules the earliest chondrule ages in the i xe mn cr and al mg systems are in reasonable agreement this and the frequent lack of evidence for metamorphism capable of resetting the i xe chronometer leads us to conclude that at least the earliest chondrule i xe ages represent formation if so chondrule formation took place at a time when sizeable parent bodies were present in the solar system

portales valley petrology of a metallic melt meteorite breccia portales valley pv is an unusual metal veined meteorite that has been classified as an h6 chondrite it has been regarded either as an annealed impact melt breccia as a primitive achondrite or as a meteorite with affinities to silicated iron meteorites we studied the petrology of pv using a variety of geochemical mineralogical techniques our results suggest that pv is the first well documented metallic melt meteorite breccia mineral chemical and other data suggest that the protolith to pv was an h chondrite the composition of feni metal in pv is somewhat fractionated compared to h chondrites and varies between coarse vein and silicate rich portions it is best modeled as having formed by partial melting at temperatures of 940 1150 c with incomplete separation of solid from liquid metal solid metal concentrated in the coarse vein areas and s bearing liquid metal concentrated in the silicate rich areas possibly as a result of a surface energy effect both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid graphite nodules formed by crystallization from this liquid whereas phosphate formed by reaction between p bearing metal and clinopyroxene components depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal silicate interfaces some phosphate probably crystallized from p bearing liquids but most probably formed by solid state reaction at 975 725 c phosphate forming and feo reduction reactions were widespread in pv and entailed a change in the mineralogy of the stony portion on a large scale portales valley experienced protracted annealing from supersolidus to subsolidus temperatures probably by cooling at depth within its parent body but the main differences between pv and h chondrites arose because maximum temperatures were higher in pv a combination of a relatively weak shock event and elevated pre shock temperatures probably produced the vein and breccia texture with endogenic heating being the main heat source for melting and with stress waves from an impact event being an essential trigger for mobilizing metal portales valley is best classified as an h7 metallic melt breccia of shock stage s1 the meteorite is transitional between more primitive chondritic and evolved achondrite iron meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies

the formation and chronology of the pat 91501 impact melt l chondrite with vesicle metal sulfide assemblages the l chondrite patuxent range pat 91501 is an 8 5 kg unshocked homogeneous igneous textured impact melt that cooled slowly compared to other meteoritic impact melts in a crater floor melt sheet or sub crater dike mittlefehldt d w and lindstrom m m 2001 petrology and geochemistry of patuxent range 91501 and lewis cliff 88663 meteoritics planet sci 36 439 457 we conducted mineralogical and tomographic studies of previously unstudied mm to cm sized metal sulfide vesicle assemblages and chronologic studies of the silicate host metal sulfide clasts constitute about 1 vol comprise zoned taenite troilite and pentlandite and exhibit a consistent orientation between metal and sulfide and of metal sulfide contacts vesicles make up 2 vol and exhibit a similar orientation of long axes 39ar 40ar measurements probably date the time of impact at 4 461 0 008 gyr b p cosmogenic noble gases and 10be and 26al activities suggest a pre atmospheric radius of 40 60 cm and a cosmic ray exposure age of 25 29 myr similar to ages of a cluster of l chondrites pat 91501 dates the oldest known impact on the l chondrite parent body the dominant vesicle forming gas was s2 15 20 ppm which formed in equilibrium with impact melted sulfides the meteorite formed in an impact melt dike beneath a crater as did other impact melted l chondrites such as chico cooling and solidification occurred over 2 h during this time 90 of metal and sulfide segregated from the local melt remaining metal and sulfide grains oriented themselves in the local gravitational field a feature nearly unique among meteorites many of these metal sulfide grains adhered to vesicles to form aggregates that may have been close to neutrally buoyant these aggregates would have been carried upward with the residual melt inhibiting further buoyancy driven segregation although similar processes operated individually in other chondritic impact melts their interaction produced the unique assemblage observed in pat 91501

k ar dating of rocks on mars requirements from martian meteorite analyses and isochron modeling radiometric age dating of martian rocks and surfaces at known locations for which crater densities can be determined is highly desirable in order to fully understand martian history performing k ar age dating of igneous rocks on mars by robots however presents technical challenges some of these challenges can be defined by examining ar ar data acquired on martian meteorites and others can be evaluated through numerical modeling of simulated k ar isochrons like those that would be acquired robotically on martian rocks excess 40ar is present in all shergottites thus for martian rocks the slopes of k ar isochrons must be determined to reasonable precision in order to calculate reliable ages model simulations of possible isochrons give an indication of some requirements in order to define a precise rock age issues addressed here are how many k ar analyses should be made of rocks thought to have the same age what range of k concentrations should these analyzed samples have and what analytical uncertainty in k ar measurements is desirable meteorite data also are used to determine the d a 2 diffusion parameters for ar in plagioclase and pyroxene separates of several shergottites and nakhlites these data indicate the required temperatures and times for heating similar martian rocks in order to extract ar quantitatively extracting radiogenic 40ar could be difficult and degassing cosmogenic ar from mafic phases even more so considering all these factors robotic k ar dating of martian rocks may be achievable but will be challenging

40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology acapulcoites consistently yield some of the oldest 40ar 39ar ages reported for any meteorites all indistinguishable from the age of 4510 22 ma previously determined for whole rock samples of acapulco these ages are significantly younger at face value than the 4557 2 ma pb pb age reported for phosphates which dates cooling through 550 c metallographic evidence for rapid cooling between 650 and 350 c retention of 129xe in apatite and the absence of shock features all suggest that the 40ar 39ar ages might be expected to be more consistent with the pb pb age new 40ar 39ar data from plagioclase determined to be the sole k rich phase in acapulco yield a high precision isochron age of 4507 18 ma 2 based on currently accepted decay constants and standards but neglecting uncertainties in these quantities these data allow unambiguous association of the 40ar 39ar age with a closure temperature near 300 c and preclude the possibility of artifacts due to 39ar recoil low temperature alteration products or inherited pre solar grains the 40ar 39ar plagioclase age recalculated with decay constants believed to be more accurate than those conventionally used is 4554 ma weakening the basis of a previously inferred dramatic decrease in cooling rate shortly after closure of the pb pb system in phosphate systematic errors in 40ar 39ar dating associated chiefly with 40k decay constants and standards must be redressed before the method can be applied to high resolution inference of early solar system evolution similar limitations affect ages based on other long lived radionuclides such as 147sm

the i xe chronometer and the early solar system we review the development of the i xe technique and how its data are interpreted and specify the best current practices individual mineral phases or components can yield interpretable trends in initial 129i 127i ratio whereas whole rock i xe ages are often hard to interpret because of the diversity of host phases many of which are secondary varying standardizations in early work require caution only samples calibrated against shallowater enstatite or bjurble can contribute reliably to the emerging i xe chronology of the early solar system although sparse data for which i xe and mn cr can be compared suggest that the two systems are concordant among ordinary chondrite samples we derive a new age for the closure of the shallowater enstatite standard of 4563 3 0 4 myr from the relationship between the i xe and pb pb systems this yields absolute i xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system absolute i xe dates for aqueous and igneous processes are consistent with other systems consideration of the i xe host phases in cais and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre existing material the ranges of chondrule ages deduced from the al mg and i xe systems in semarkona ll3 0 and chainpur ll3 4 are consistent chainpur i xe data exhibit a greater range of ages than semarkona possibly reflecting a greater degree of parent body processing however individual chondrules show little or no evidence of such processing determining the host phase s responsible for high temperature correlations may resolve the issue

mineralogy petrology chemistry and 39ar 40ar and exposure ages of the caddo county iab iron evidence for early partial melt segregation of a gabbro area rich in plagioclase diopside we describe coarse grained gabbroic material rich in plagioclase and diopside in the caddo county iab iron meteorite the gabbro is not a clast within a breccia but rather is an area with transitional margins located mainly at silicate metal boundaries only a few cm away from fine grained ultramafic silicates similar to winonaites medium grained orthopyroxene and olivine are found in transitional areas and show no shock disturbance of their textures a vein like region starting in an area of fine grained ultramafic silicate extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside these textures and mineralogies suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts from chondritic source materials compositional data on two mineralogically distinct samples of the gabbro rich portion of the inclusion were obtained by inaa compared to an average of iab silicate inclusions or winonaites the caddo county gabbro is enriched in the incompatible lithophile elements na ca sc ree and hf consistent with a melt origin for the gabbro the cosmogenic space exposure age of caddo county 5 1 ma is significantly younger than exposure ages of some other iab meteorites an 39ar 40ar age determination of the gabbroic material indicates a series of upward steps in age from 4 516 ga to 4 523 ga with a few high temperature ages up to 4 54 ga the older age could approximate the primary recrystallization age of silicates the stepped ar age spectrum may indicate differences in ar closure temperatures during slow cooling of 2 20 c myr in the parent body alternatively the younger ar ar ages may date a shock event which occurred while caddo county was hot and which also created textures such as a linear metal vein with rounded zigzag walls in a diopside crystal and slightly misoriented rounded plagioclase domain

ar ar and i xe ages and the thermal history of iab meteorites studies of several samples of the large caddo county iab iron meteorite reveal andesitic material enriched in si na al and ca which is essentially unique among meteorites this material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening such an origin implies extended metamorphism of the iab parent body new 39ar 40ar ages for silicate from three different caddo samples are consistent with a common age of 4 50 4 51 gyr less well defined ar ar degassing ages for inclusions from two other iabs eet elephant moraine 83333 and udei station are 4 32 gyr whereas the age for campo del cielo varies considerably over about 3 23 4 56 gyr new 129i 129xe ages for caddo county and eet 83333 are 4557 9 0 1 myr and 4557 4560 myr respectively relative to an age of 4562 3 myr for shallowater considering all reported ar ar degassing ages for iabs and related winonaites the range is 4 32 4 53 gyr but several iabs give similar ar ages of 4 50 4 52 gyr we interpret these older ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40ar diffusion loss the older ar ar ages for iabs are similar to sm nd and rb sr isochron ages reported in the literature for caddo county considering the possibility that iab parent body formation was followed by impact disruption reassembly and metamorphism e g benedix et al 2000 the ar ar ages and iab cooling rates deduced from ni concentration profiles in iab metal herpfer et al 1994 are consistent if the time of the postassembly metamorphism was as late as about 4 53 gyr ago however i xe ages reported for some iabs define much older ages of about 4558 4566 myr which cannot easily be reconciled with the much younger ar ar and sm nd ages an explanation for the difference in radiometric ages of iabs may reside in combinations of the following a i xe ages have very high closure temperatures and were not reset during metamorphism about 4 53 gyr ago b a bias exists in the 40k decay constants which makes these ar ar ages approximately 30 myr too young c the reported sm nd and rb sr ages for caddo are in error by amounts equal to or exceeding their reported 2 sigma uncertainties and d about 30 myr after the initial heating that produced differentiation of caddo silicate and mixing of silicate and metal a mild metamorphism of the iab parent body reset the ar ar ages

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