siderophile geochemistry of ureilites a record of early stages of planetesimal core formation abstract new bulk compositional data including trace siderophile elements such as ir os au and ni are presented for 25 ureilites without exception ureilites have siderophile abundances too high to plausibly have formed as cumulates ureilites undoubtedly underwent a variety of smelting by which c was oxidized to co gas while olivine feo was reduced to fe metal however pressure buffered equilibrium smelting is not a plausible model for engendering the wide range 75 96 mol of mafic silicate core mg among ureilites the smelting reaction produces too much co gas even supposing a disequilibrium process with the smelt gas leaking out of the mantle none of the ureilites least of all the ureilite with the most reduced highest olivine core mg alh84136 has the high fe metal abundance predicted by the smelted cores model in principle the fe metal generated by smelting could have been subsequently lost but siderophile data show that ureilites never underwent efficient depletion of fe metal ureilites display strong correlations among siderophile ratios such as au ir ni ir co ir as ir se ir and sb ir ureilite siderophile depletion patterns loosely resemble siderophile fractionations presumably nebular in origin among carbonaceous chondrites however zn for an element of moderate volatility is anomalously high in ureilites a tight correlation between au and ni extrapolates to the low ni au side of the compositional range of carbonaceous chondrites from this mismatch mild but nonetheless significant depletions of refractory siderophile elements such as ir and os and moderate depletions of strongly siderophile weakly chalcophile elements such as ni and au we infer that the ureilite siderophile fractionations are largely the result of a non nebular process i e removal of s rich metallic melt possibly with minor entrainment of fe metal several lines of trace element evidence indicate that melt porosity during ureilite anatexis was at least moderate the ureilite pattern of very mild depletions of extremely siderophile elements but much deeper depletions of moderately siderophile chalcophile elements suggests that asteroidal core formation probably occurs in two discrete stages in general separation of a considerable proportion several wt of s rich metallic melt probably occurs long before and at a far lower temperature than separation of the remaining s poor fe metal apart from the fe metal itself only extremely siderophile elements wait until the second stage to sequester mainly into the core

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

a short duration of chondrule formation in the solar nebula evidence from 26al in semarkona ferromagnesian chondrules abstract the 26 al 26 mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite semarkona ll3 0 were studied using a secondary ion mass spectrometer their glass or plagioclase portions contain excesses of 26 mg and in two chondrules the 26 mg excesses are well correlated with 27 al 24 mg which demonstrates the in situ decay of 26 al the initial 26 al 27 al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 10 6 and 9 10 6 indicating their short formation duration of less than 1 my if the solar nebula was initially homogeneous in al isotopes the chondrule formation ages are 2 my younger than those of cais our results based on the study of the least metamorphosed uoc are consistent with the previous studies on al rich chondrules that the chondrule formation started at least 2 my after cais alternatively the older records before 2 my were erased by chondrule recycling process it further suggests that the young apparent ages 3 to 5 my after cais for chondrules in type 3 4 uocs are due to the disturbance of the 26 al 26 mg system possibly during parent body metamorphism the result is not consistent with the extended nebular time scale of 5 my and the chondrule formation by planetary processes the ni isotopic analysis of the feo rich olivines in a type ii chondrule in semarkona did not show any detectable excess 60 ni in spite of their high fe ni ratios the upper limit of the initial 60 fe 56 fe ratio of the solar system was estimated to be 3 4 10 7 which is consistent with the previous estimate 0 2 1 9 10 7 from eucrites this result confirms that the 60 ni excess previously observed from cais was not due to the decay of the short lived nuclide 60 fe but a ni isotopic anomaly of nucleosynthetic origin

the mn fe negative correlation in olivines in alha 77257 ureilite mn fe and oxygen zoning profiles of olivines in the alha 77257 ureilite were measured by an electron probe microanalyzer to study the effects of reduction on the mn fe value because ureilite olivines exhibit thin reduced rims the mn content gradually increases toward the rim of ureilite olivines as the fa 100 fe mg fe mol component decreases which suggests that the mn fe content of olivine is related to redox conditions a calculated diffusional cooling rate of 1 2 c h gives the best fit to both the observed mno and fa chemical zoning profiles of alha 77257 the results of melting experiments suggest that the negative correlation of mn fe in olivine and low ca pyroxene is related to reduction and that the mn fe positive correlation is related to temperature the fe mn fe mg trend of olivine cores in ureilites may be established by the reduction process at about 1275 c

highly siderophile elements in ureilites abstract the abundances of the highly siderophile elements hse ru pd re os ir and pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples including monomict augite bearing and polymict lithologies this report adds significantly to the quantity of available pt and pd abundances in ureilites as these elements were rarely determined in previous neutron activation studies the ci normalized hse abundance patterns of all ureilites analyzed here except alha 81101 show marked depletions in the more volatile pd with ci chondrite normalized pd os ratios excluding alha 81101 averaging 0 19 0 23 2 this value is too low to be directly derived from any known chondrite group instead the hse bulk rock abundances and hse interelement ratios in ureilites can be understood as physical mixtures of two end member compositions one component best represented by sample alha 78019 is characterized by superchondritic abundances of refractory hse rhse ru re os ir and pt but subchondritic pd rhse and is consistent with residual metal after extraction of a s bearing metallic partial melt from carbonaceous chondrite like precursor materials the other component best represented by sample alha 81101 is rhse poor and has hse abundances in chondritic proportions the genesis of the second component is unclear it could represent regions within the ureilite parent body upb in which metallic phases were completely molten and partially drained or it might represent chondritic contamination that was added during disruption and brecciation of the upb removal of carbon rich melts does not seem to play an important role in ureilite petrogenesis removal of such melts would quickly deplete the ureilite precursors in re os and as au which is inconsistent with measured osmium isotope abundances and also with literature as au data for the ureilites removal of 26 al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions

mineralogy of coexisting pyroxenes in magnesian ureilites and their formation conditions abstract the occurrence of orthopyroxene in two magnesian ureilites found in antarctica yamato y 791538 and lew85440 has been confirmed by electron probe microanalysis and by single crystal x ray diffraction the first coexisting equilibrium pigeonite orthopyroxene pair has been found in y791538 the lew85440 orthopyroxene coexists with minor augite the orthopyroxenes and olivines in y791538 y74659 and lew85440 have almost identical compositions but the ca contents of their coexisting clinopyroxenes differ considerably these phase assemblages can be explained by crystallization of mafic silicates with a similar composition oxygen fugacity and oxygen isotope anomaly at different temperature above 1220 c the high temperature crystallization of refractory mg rich silicates and the possible preservation of the oxygen isotope anomaly of precursor carbonaceous chondrites like materials may be consistent with their formation processes probably through local melting of more matrices than mg rich silicates and crystal growth induced by the planetesimal scale collision

ureilites trace element clues to their origin carbonaceous vein separates from kenna and havero as well as bulk kenna were analyzed by rnaa for ag au bi br cd cs ge in ir ni pd os rb re sb se te tl u and zn the data are reviewed together with four earlier chicago analyses of bulk ureilites linear regressions confirm the presence of two metal components with the following cl normalized ratios ir ni 14 6 1 ge ni 5 4 2 4 au ni 2 3 0 9 the high ir component is enriched in vein separates and hence belongs to veins the lowir component belongs to the ultramafic rock vein material is enriched in all elements analyzed by us except zn and accounts for most of the c noble gases and presumably siderophiles in the meteorite most of the properties of ureilites apparently can be explained by the cumulate model of berkley et al 1980 with certain modifications comparison of ureilites with three other ultramafic rocks from different planets earth s mantle lunar dunite and chassigny suggests that the ureilite parent body had a primitive chondritic composition similar to c3v chondrites but richer in metal and carbon it melted causing depletion of incompatibles to a mean abundance of 0 02 cl and incomplete segregation of metal fes and c fractional crystallization or melting of metal in the presence of s and c apparently can explain the fractionations of ir re ni au and perhaps ge obviating the need for extraneous sources of vein metal or unusual parent body compositions noble gases from the parent material may have been retrapped in carbon during magmatism provided the system was closed

the meteoritical bulletin no 82 1998 july abstract meteoritical bulletin no 82 lists information for 974 new meteorites including 521 finds from antarctica 401 finds from the sahara 21 finds from the nullarbor region of australia and 7 falls ban rong du burnwell fermo jalanash juancheng monahans 1998 and silao many rare types of meteorites are reported counting pairing groups as one these include one cr chondrite two ck chondrites two co chondrites four cv chondrites one ch chondrite or bencubbin like six c2 unclassified chondrites two eh chondrites two el chondrites three r chondrites thirty unequilibrated ordinary chondrites one un grouped chondrite three eucrites six howardites one diogenite eleven ureilites nine iron meteorites one mesosiderite two brachinites one lodranite one winonaite and two lunar meteorites dar al gani 400 and eet 96008 all italicized abbreviations refer to addresses tabulated at the end of this document

fe mg fe mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies abstract we examine fe mg mn relations of meteorites as represented on plots of molar fe mg ratio vs molar fe mn ratio and use them to consider whether it is meaningful to classify achondrites as primitive or evolved first we demonstrate that fe mg fe mn plots can be used to identify some of the major processes that have affected meteorite parent bodies during both the nebular condensation and accretion and the planetary igneous stages of their evolution examination of compositions of chondrites on this diagram reveals distinct patterns that result from three nebular processes 1 volatility controlled mn mg fractionation 2 metal silicate fractionation under highly reducing conditions and 3 subsequent oxidation leading to variations in feo mgo ratio ordinary chondrites ocs show a characteristic pattern that records all three of these processes and suggests that the latter two are coupled examination of the compositions of achondrites on this diagram shows that all of those that have been classified as primitive lodranites acapulcoites winonaites iab silicate inclusions brachinites and ureilites are easily distinguished from those that have been classified as evolved sncs heds and lunar basalts the mn mg ratios of the former are chondritic while those of the latter are superchondritic hence they plot in two graphically distinct regions of the fe mg fe mn diagram a simple melting calculation reveals that this distinction results principally because the former are residues and the latter are melts or cumulate products of melts rather than because the latter are more evolved additionally however some of the groups classified as primitive achondrites show fe mg fe mn patterns similar to that of ocs constant but distinct mno mgo ratios over large ranges of feo mgo ratio we argue that these trends reflect primary heterogeneity resulting from the same nebular processes as those that affected ocs and therefore preserve evidence that more of the materials that accreted to form planetesimals experienced these processes than is apparent from known chondrites this evidence is preserved principally because these meteorites are residues in contrast groups commonly classified as evolved achondrites show fe mg fe mn patterns that reflect only igneous processing and any evidence for primary heterogeneity of their parent bodies has been obliterated again this is principally because these meteorites represent melts rather than beause they are more evolved usage of the terms primitive and evolved to describe achondrites should be clearly defined

northwest africa 1500 plagioclase bearing monomict ureilite or ungrouped achondrite northwest africa nwa 1500 is an ultramafic meteorite dominated by coarse 100 500 m olivine 95 96 augite 2 3 and chromite 0 6 1 6 in an equilibrated texture plagioclase 0 7 1 8 occurs as poikilitic grains up to 3 mm in vein like areas that have concentrations of augite and minor orthopyroxene other phases are cl apatite metal sulfide and graphite olivine ranges from fo 65 73 with a strong peak at fo 68 69 most grains are reverse zoned and also have 10 30 m reduction rims in terms of its dominant mineralogy and texture nwa 1500 resembles the majority of monomict ureilites however it is more ferroan than known ureilites fo 75 and other mineral compositional parameters are out of the ureilite range as well furthermore neither apatite nor plagioclase have ever been observed and chromite is rare in monomict ureilites nevertheless this meteorite may be petrologically related to the rare augite bearing ureilites and represent a previously unsampled part of the ureilite parent body upb the mn mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation however its petrogenesis differs from that of known augite bearing ureilites in that 1 it formed under more oxidized conditions 2 plagioclase appeared before orthopyroxene in its crystallization sequence and 3 it equilibrated to significantly lower temperatures 800 1000 c from two pyroxene and olivine chromite thermometry formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the upb than previously sampled however its significantly different thermal history compared to ureilites may more plausibly be explained if it formed on a different parent body this conclusion is consistent with its oxygen isotopic composition which suggests that it is an ungrouped achondrite nevertheless the parent body of nwa 1500 may have been compositionally and petrologically similar to the upb and may have had a similar differentiation history

frontier mountain meteorite specimens of the acapulcoite lodranite clan petrography pairing and parent rock lithology of an unusual intrusive rock in this paper we reconstruct the heterogeneous lithology of an unusual intrusive rock from the acapulcoite lodranite al parent asteroid on the basis of the petrographic analysis of 5 small 8 3 g meteorite specimens from the frontier mountain ice field antarctica although these individual specimens may not be representative of the parent rock lithology due to their relatively large grain size by putting together evidence from various thin sections and literature data we conclude that frontier mountain fro 90011 fro 93001 fro 99030 and fro 03001 are paired fragments of a medium to coarse grained igneous rock which intrudes a lodranite and entrains xenoliths the igneous matrix is composed of enstatite fs13 3 0 4 wo3 1 0 2 cr rich augite fs6 1 0 7 wo42 3 0 9 and oligoclase ab80 5 3 3 or3 2 0 6 the lodranitic xenoliths show a fine grained average grain size 488 201 m granoblastic texture and consist of olivine fa9 5 0 4 and fe ni metal and minor amounts of enstatite fs12 7 0 4 wo1 8 0 1 troilite chromite schreibersite and ca phosphates crystals of the igneous matrix and lodranitic xenoliths are devoid of shock features down to the scanning electron microscope scale from a petrogenetic point of view the lack of shock evidence in the lodranitic xenoliths of all the studied samples favors the magmatic rather than the impact melting origin of this rock fro 95029 is an acapulcoite and represents a separate fall from the al parent asteroid i e it is not a different clast entrained by the fro 90011 fro 93001 fro 99030 and fro 03001 melt as in genomict breccias common in the meteoritic record the specimen to meteorite ratio for the al meteorites so far found at frontier mountain is thus 2 5

origin and evolution of the ureilite parent magmas multi stage igneous activity on a large parent body analyses of olivine and pigeonite cores interpreted as cumulate crystals in eight low shock ureilites show well defined correlations between fe x where x is one of the minor elements mn cr ca al ti p or ni and fe mg ratios for the lithophile minor elements these trends are linear with positive slope and pass through or near the origin indicating various degrees of feo reduction of the parent magmas the trends shown by p and ni are consistent with this interpretation and additionally require equilibrium crystallization of 20 27 mole metal graphite was most likely the reducing agent which implies that the ureilites equilibrated at different pressures because the graphite fo2 buffer is pressure dependent there is no evidence for fractionation of olivine or pryroxene in any of these trends the ca ti and al trends show significant scatter indicating that there must have been multiple parent magmas with very similar mg mn and mgcr ratios but different mgal mgca and mgti ratios cao al2o3 ratios vary by a factor of 3 among the eight magmas and are superchondritic 14 32 molar similar to lunar mare basalts we propose the following model for generation and crystallization of ureilite parent magmas upm chondritic material undergoes 10 25 partial melting leaving a plagioclase depleted residue the magma or magmas form a differentiated crust on the ureilite parent body upb with mafic cumulates and plagioclase cumulates spatially separated later remelting 10 of the mafic cumulates produced the upm siderophile elements indicate that the source region of the upm had not experienced segregation of metal such as would occur during core formation in the source region the pressure must have been at least 0 5 1 kb because only at pressures this high does the graphite buffer allow iron oxide to be the stable phase relative to metallic iron the graphite bearing upm were emplaced at various levels in the crust where they were reduced to various extents dependent on depth ureilites are early olivine pigeonite cumulates from these magmas compositions of interstitial low ca pyroxenes indicate that the trapped interstitial liquids from which they crystallized had lower mnmg crmg camg almg and namg ratios than the upm and indicate an abrupt change in liquid composition coincident with impact excavation of ureilites this interstitial material may be derived by mixing a shock melt of nearby olivine pigeonite cumulates with residual primary liquid our model predicts that the upb had a differentiated crust did not have a core and was at least 235 km in radius the lack of a ureilite like body among spectrally analyzed asteroids suggests that either the spectral signature of the upb is not that of ureilites themselves perhaps of carbon free feldspathic material instead that the upb is not intact or that ureilites did not originate in the asteroid belt

the meteoritical bulletin no 94 september 2008 the meteoritical bulletin 94 reports over 900 119 non antarctic and 803 antarctic newly approved meteorite names and their recovery histories macroscopic descriptions petrography mineral compositions and geochemistry meteorites reported include lunar martian howardites eucrites diogenites mesosiderites aubrites irons angrites ureilites acapulcoites lodranites and h l ll r co and cv chondrites eight of the meteorites in this bulletin are falls including 4 from india jodiya kaprada kavapura mahadevpur 1 from mali chergach turkey didim libya werdama and the u s berthoud

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

oxygen isotope studies of ordinary chondrites several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components chondrules fragments clasts and matrix the whole rock isotopic compositions reflect the iron group of the meteorite h l or ll isotopic uniformity of h3 to h6 and l3 to l6 are consistent with closed system metamorphism within each parent body ll3 chondrites differ slightly from ll4 to ll6 implying a small degree of opensystem aqueous alteration and carbon reduction on the scale of individual chondrules the meteorites are isotopically heterogeneous allowing recognition of the solar nebular processes of chondrule formation chondrules for all classes of ordinary chondrites are derived from a common population which was separate from the population of chondrules in carbonaceous or enstatite chondrites chondrules define an isotopic mixing line dominated by exchange between 16o rich and 16o poor reservoirs the oxygen isotopic compositions of chondrites serve as fingerprints for identification of genetic association with other meteorite types achondrites and iron and for recognition of source materials in meteoritic breccias

early formation of evolved asteroidal crust mechanisms for the formation of crust on planetary bodies remain poorly understood it is generally accepted that earth s andesitic continental crust is the product of plate tectonics whereas the moon acquired its feldspar rich crust by way of plagioclase flotation in a magma ocean basaltic meteorites provide evidence that like the terrestrial planets some asteroids generated crust and underwent large scale differentiation processes until now however no evolved felsic asteroidal crust has been sampled or observed here we report age and compositional data for the newly discovered paired and differentiated meteorites graves nunatak gra 06128 and gra 06129 these meteorites are feldspar rich with andesite bulk compositions their age of 4 52 0 06 gyr demonstrates formation early in solar system history the isotopic and elemental compositions degree of metamorphic re equilibration and sulphide rich nature of the meteorites are most consistent with an origin as partial melts from a volatile rich oxidized asteroid gra 06128 and 06129 are the result of a newly recognized style of evolved crust formation bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early formed materials in the solar system

tem investigations on the monomict ureilites jalanash and hammadah al hamra 064 we studied the petrography and mineralogy of two monomict ureilites hammadah al hamra 064 hh064 and jalanash by using reflected light and scanning electron microscopy quantitative analyses were performed by electron microprobe and the microstructures were investigated with transmission electron microscopy tem hh064 features two different textures a poikilitic and a typical one whereas jalanash shows only the typical ureilite texture our synergetic chemical and microstructural investigations reveal a complex cooling history for both ureilites the temperature for the first equilibrium deduced from the pigeonite augite assemblage in hh064 is 1200 c the presence of antiphase domains in low ca pyroxenes proves that they are clearly pigeonite the occurrences of tweed microstructure and orthopyroxene lamellae which are incompletely developed imply a faster cooling rate from the first equilibrium with a sudden end although both ureilites contain shock induced diamonds dislocations in silicates are rare this observation suggests that the meteorites were hot at the time of strong shock metamorphism or that they were heated after strong shock metamorphism after this event new microstructural features were generated by different cooling processes and were frozen by a final rapid decrease in temperature possibly due to excavation from the ureilite parent body or bodies

a petrologic and isotopic study of lodranites evidence for early formation as partial melt residues from heterogeneous precursors abstract we have conducted petrologic chemical and isotopic studies of lodranites in an attempt to constrain their genesis lodran gibson y 791491 y 791493 y 74357 y 8002 y 75274 mac 88177 lew 88280 eet 84302 fro 90011 and que 93148 are classified as lodranites lodranites and acapulcoites are indistinguishable on the basis of oxygen isotopic compositions but are distinct in average grain sizes of their mafic silicates with lodranites being significantly coarser grained lodranites exhibit a diverse range of petrologic and mineralogic features they range widely in mafic silicate compositions fa3 13 plagioclase 0 11 4 vol fe ni metal 0 5 20 vol and troilite 0 2 5 3 vol contents and shock levels s1 s4 they appear to have experienced high peak temperatures and rapid cooling in the temperature range recorded by metallographic cooling rates i e 700 350 c the only dated lodranite gibson cooled to ar closure temperatures at 4 49 0 01 ga lodranites formed from chemically and isotopically heterogeneous precursors in which the mineral and oxygen isotopic compositions were correlated heating of their parent body to temperatures between 1050 1200 c resulted in formation of fe ni fes and basaltic partial melts depletions of troilite and or plagioclase in most lodranites testify to the removal of some of these partial melts although melt migration was complex lodranites appear to have experienced a complex cooling history of slow cooling at high temperatures followed by rapid cooling at intermediate temperatures possibly related to breakup of the parent body lodranites were liberated from their parent body during 1 3 impact events with most having cosmic ray exposure ages of 5 5 7 ma the acapulcoites are samples from the same parent body but were heated to lower temperatures and thus experienced lower degrees of partial melting

a petrologic chemical and isotopic study of monument draw and comparison with other acapulcoites evidence for formation by incipient partial melting abstract we have conducted petrologic chemical and isotopic studies of acapulcoites acapulco monument draw yamato 74063 alh a77081 alh a81261 alh a81315 alh 78230 alh a81187 and alh 84190 in an attempt to constrain their genesis acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in iab and iiicd irons winonaites and ureilites and thus formed on a different parent body oxygen isotopic compositions which are slightly heterogeneous within the group overlap with lodranites indicating a likely origin on a common parent body these groups can be distinguished on the basis of mafic silicate grain size all acapulcoites have mafic silicate compositions intermediate between e and h chondrites roughly chondritic mineralogies achondritic equigranular textures micrometer to centimeter sized veins of fe ni fes which cross cut silicate phases rapid metallographic cooling rates at 600 400 c 103 105 c myr and trapped noble gas abundances comparable to type 3 4 ordinary chondrites they exhibit variable mafic silicate zoning abundance of fe ni fes veins ree abundances and patterns and possibly cosmic ray exposure ages 5 7 ma momument draw and yamato 74063 retain rare relict chondrules phosphates are associated with fe ni fes veins or form separate veins in monument draw and acapulco heating and cooling of acapulcoites occurred very early in the history of the solar system as evidenced by the 39ar 40ar ages of 4 51 ga these ages appear distinctly younger than the likely formation time for acapulco of 4 557 ga but are older than analogous 39ar 40ar ages for most chondrites acapulcoites formed from a precursor chondrite which differs from known chondrites in mineral and oxygen isotopic compositions heating to 950 1000 c resulted in melting at the fe ni fes cotectic but silicates did not melt silicate textures resulted from extensive solid state recrystallization heating was by noncollisional sources e g 26al electromagnetic induction despite uncertainties owing to a lack of data acapulcoites may have experienced a three stage thermal history of slow cooling at high temperature rapid cooling at intermediate temperatures and slow cooling at low temperatures possibly resulting from breakup and gravitational reassembly of the parent body the complex thermal history is also reflected in disequilibrium ree abundances one or at most two impact events 7 ma and possibly 5 ma ago are consistent with the cosmic ray exposure ages of all four acapulcoites for which cosmogenic noble gas data exist

brachina a new type of meteorite not a chassignite the brachina meteorite previously classified as a chassignite is found to be a nearly primitive and unique meteorite it is a fine grained unshocked equigranular igneous rock containing small melt inclusions it contains 80 olivine fo70 5 5 clinopyroxene wo38en49fs13 10 plagioclase an22 0 5 chromite 0 5 chlorapatite 3 fe sulfide 0 3 pentlandite and a trace of ni rich metal orthopyroxene en70 and anorthoclase rich glass are found only in melt inclusions siderophiles are high in the sulfides and the oxidation state is similar to that of ll chondrites major element and mineralogic data of brachina are similar to chassigny but trace element data are dramatically different the major differences are higher feo mno ga al and k la ratios w depletion high siderophile content near horizontal ree pattern at about 1 chondrite level high refractory nonrefractory metal ratios e g ir au and chalcophile element depletion compositionally brachina is close to c2 chemistry but has undergone some segregation of fe ni s melt these data in addition to its near 4 5 b y age and oxygen isotopic signature within the eucritic field suggest that brachina is not related to chassigny or snc meteorites it may be related to primitive meteorites such as silicate inclusions in iab irons and winonaites but is more oxidized reduction would make it very similar to winonaites the assemblage may have experienced the beginning of melting in its parent body since it has characteristics that differ from any other meteorite we suggest it be called a brachinite

core formation in the earth and shergottite parent body spb chemical evidence from basalts constraints on processes of core formation in terrestrial planets may be inferred from the abundances of siderophile and chalcophile elements in their mantles of particular interest is a comparison of processes of core formation in the earth and in the shergottite parent body spb a terrestrial planet tentatively inferred to be mars to this end we i present new inaa and rnaa analyses of the non antarctic snc meteorites ii infer the composition of the spb mantle from the compositions of the snc meteorites iii infer the composition of the earth s mantle from the compositions of terrestrial basalts and iv deconvolve the effects of volatile depletion core formation and mineral melt fractionation on the abundances of siderophile and chalcophile elements in the spb and the earth element abundances in the mantles of the spb and the earth are estimated from element element correlations observed among basalt samples in basalts from the spb the snc meteorites four groups of covariant elements are observed highly incompatible moderately incompatible indifferent and compatible from correlations within these groups the spb mantle is found to be depleted in volatile elements and strongly depleted in siderophile and chalcophile elements for the earth the element element correlation method gives a mantle composition similar to previous estimates compared to the earth the spb mantle is richer in moderately siderophile elements e g w p consistent with its inferred higher oxidation state chalcophile elements in the spb mantle are more depleted than in the earth s mantle particularly when compared to estimates of the original abundances of volatile chalcophile elements in the two planets in the spb mantle the nico ratio is nonchondritic in contrast to the chondritic ratio in the earth s mantle abundances of siderophile and chalcophile elements in the spb mantle may be modelled by equilibrium with solid metal and metallic sulfide liquid with some metal and sulfide trapped in the mantle i e homogeneous accretion and inefficient core formation neither this model nor a heterogeneous accretion model is satisfactory in explaining element abundances in the earth s mantle particularly the abundances of ni co mo and w nevertheless it appears that core formation in the spb and the earth left quite different chemical signatures in their planetary mantles

explosive volcanism and the graphite oxygen fugacity buffer on the parent asteroid s of the ureilite meteorites the 39 ureilite meteorites are in many respects primitive objects they are highly carbonaceous averaging 3 0 wt c mostly as graphite yet their consistently coarse grained textures and plagioclase depleted compositions imply that they are igneous despite diverse mafic silicate mg mg fe and pyroxene olivine ratios plagioclase is totally absent among 35 monomict unbrecciated ureilites even among 4 polymict brecciated ureilites the average al concentration is only 0 28 0 04 wt we suggest that the ureilites are residues from partial melting of graphite rich source materials which formed sufficiently deep in a relatively large low f o2 asteroid or more likely asteroids for graphite to be stable even at magmatic t the al rich basaltic magmas that ascended from the residues were nearly quantitatively lost from the asteroid during pyroclastic eruptions graphite entrained in the ascending magmas was oxidized by an extremely pressure sensitive reaction into cox the expansion of which into the vacuous environment of the asteroid engendered pyroclastic exit velocities greatly in excess of the asteroid s escape velocity carbon would oxidize long before the attainment of magmatic temperatures in small asteroids but we infer that basalt dissipative explosive volcanism was common on all c rich asteroids that were roughly 100 200 km in radius and heated to widespread deep melting and most unmetamorphosed chondritic meteorites are c rich based on the ureilites consistently modest depletions of light rare earth elements siderophile elements and feni metal we infer that the melt fraction was removed mostly by equilibrium batch partial melting late stage ureilite evolution involved an abrupt increase in the cooling rate implying a drop in environmental temperature and a slight amount of graphite oxidation implying a drop in pressure the simplest model to account for this non sequitur and for the conversion of some of the graphite into diamonds is catastrophic impact disruption of the primary asteroid the remarkably efficient removal of the final traces of melt from the ureilites is difficult to explain by any model but possibly was caused by thermal migration along steep thermal gradients engendered after the catastrophic disruption

mineral chemistry of the shergottites nakhlites chassigny brachina pallasites and urelites the mineral chemistry is compared for selected achondrites olivine in the alha 77005 and eeta79001 shergottites olivine rich chassigny and brachina and the nakhlites contains ni indicative of oxidizing conditions whereas pallasitic and ureilitic olivines contain much lower ni due to reducing conditions the brachina olivine and pyroxene have distinctively higher fe mn than the shergottites and chassigny further indicating that brachina is unique the chassigny and 77005 olivines contain lower cr2o3 0 03 wt than the brachina and 79001 olivines values of fe mn for cumulus augites in nakhlites are higher than for the shergottites whereas those for ferropigeonites are not the 77005 shergottite contains troilite fes in contrast to 79001 shergotty zagami and chassigny which contain pyrrhotite further analyses are needed but the present survey indicates that at least brachina is not chemically cogenetic with the other oxidized achondrites and that the fe mn ratio of the cumulus augites in nakhlites is a problem for the assignment of the nakhlites shergottites and chassigny to a single genetic group

the northwest africa 1500 meteorite not a ureilite maybe a brachinite the northwest africa nwa 1500 meteorite is an olivine rich achondrite containing approximately 2 3 vol augite 1 2 vol plagioclase 1 vol chromite and minor orthopyroxene cl apatite metal and sulfide it was originally classified as a ureilite but is currently ungrouped we re examined the oxygen three isotope composition of nwa 1500 results of ultra high precision 0 03 for d 17 o laser fluorination analyses of two bulk chips and high precision 0 3 secondary ion mass spectrometry sims analyses of olivine and plagioclase in a thin section show that the oxygen isotope composition of nwa 1500 d 17 o 0 22 from bulk samples and 0 18 0 06 from 16 mineral analyses is within the range of brachinites we compare petrologic and geochemical characteristics of nwa 1500 with those of brachinites and other olivine rich primitive achondrites including new petrographic mineral compositional and bulk compositional data for brachinites hughes 026 reid 013 nwa 5191 nwa 595 and brachina modal mineral abundances texture olivine and pyroxene major and minor element compositions plagioclase major element compositions rare earth element abundances and siderophile element abundances of nwa 1500 are within the range of those in brachinites and in most cases well distinguished from those of winonaites iab silicates acapulcoites lodranites ureilites and divnoe nwa 1500 shows evidence of internal reduction in the form of reversely zoned olivine fo 65 73 core to rim and fine grained intergrowths of orthopyroxene metal along olivine grain margins the latter also occur in reid 013 hughes 026 nwa 5191 and nwa 595 we argue that reduction olivine fi enstatite fe 0 o 2 is the best hypothesis for their origin in these samples as well we suggest that nwa 1500 should be classified as a brachinite which has implications for the petrogenesis of brachinites fe mn mg compositions of brachinite olivine provide evidence of redox processes among bulk samples nwa 1500 provides evidence for redox processes on a smaller scale as well which supports the interpretation that these processes occurred in a parent body setting sims data for 26 al 26 mg isotopes in plagioclase in nwa 1500 show no 26 mg excesses beyond analytical uncertainties 1 2 the calculated upper limit for the initial 26 al 27 al ratio of the plagioclase corresponds to an age younger than 7 ma after cai compared to 53 mn 53 cr data for brachina wadhwa et al 1998b this implies either a much younger formation age or a more protracted cooling history however brachina is atypical and this comparison may not extend to other brachinites

brachinites igneous rocks from a differentiated asteroid we have done petrologic studies of brachinites allan hills alh 84025 elephant moraine eet 99402 and eet 99407 bulk geochemical studies of eet 99402 and eet 99407 ar ar studies of brachina and eet 99402 and a xe isotopic study of brachina textural mineral compositional and bulk compositional evidence show that eet 99402 and eet 99407 are paired alh 84025 eet 99402 and eet 99407 have igneous textures petrofabric analyses of alh 84025 and eet 99407 demonstrate the presence of lineations and probable foliations of olivine grains that support formation as igneous cumulates mineral minor element chemistry and bulk rock incompatible lithophile element contents of the brachinites are distinct from those of acapulcoite lodranite clan meteorites a suite of high grade metamorphic rocks and anatectic residues the differences demonstrate a higher blocking temperature of equilibration for the brachinites and that cumulus plagioclase is present in eet 99402 eet 99407 and probably alh 84025 thus indicating an igneous origin brachinites are differentiated ultramafic achondrites and are not part of a suite of primitive achondrites we infer that their parent asteroid is a differentiated body brachina has an excess of 129xe correlated with reactor produced 128xe demonstrating that short lived 129i was present at the time of formation this plus literature data attests to early formation of the brachinites within a few ma of the formation of chondrites ar ar age data show that brachina and eet 99407 were degassed about 4 13 ga ago possibly by a common impact event eet 99402 and eet 99407 show petrographic evidence for shock including possible conversion of plagioclase to maskelynite followed by devitrification brachina is unshocked making a direct association between the ar ar age and textures ambiguous

petrology of unique achondrite queen alexandra range 93148 a piece of the pallasite howardite eucrite diogenite parent body abstract queen alexandra range que 93148 is a small 1 1 g olivine rich achondrite mg 86 that contains variable amounts of orthopyroxene mg 87 and kamacite 6 7 wt ni with minor augite olivine in que 93148 contains an unusual suite of inclusions 1 5 100 m sized lamellae with a cao and cr2o3 rich 10 and 22 wt respectively composition that may represent a submicrometer scale intergrowth of chromite and pyroxene s 2 75 500 m sized lamellar symplectites composed of chromite and two pyroxenes with minor metal 3 15 20 m sized irregularly shaped symplectites composed of chromite and pyroxene s 4 100 150 m sized elliptical inclusions composed of chromite two pyroxenes metal troilite and rare whitlockite type 1 2 and 3 inclusions probably formed by exsolution from the host olivine during slow cooling whereas type 4 more likely resulted from early entrapment of silicate and metallic melts followed by closed system oxidation queen alexandra range 93148 can be distinguished from most other olivine rich achondrites ureilites winonaites lodranites acapulcoites brachinites eagle station type pallasites and pyroxene pallasites as well as from mesosiderites by some or all of the following properties o isotopic composition fe mn mg relations of olivine cao and cr2o3 contents of olivine orthopyroxene compositions molar cr cr al ratios of chromite metal composition texture and the presence of the inclusions in terms of many of these properties it shows an affinity to main group pallasites nevertheless it cannot be identified as belonging to this group meteorite que 93148 appears to be a unique achondrite possibly it should be considered to be a pyroxene pallasite that is genetically related to main group pallasites alternatively it may be derived from the mantle of the pallasite howardite eucrite diogenite parent body

the lodran primitive achondrite petrogenetic insights from electron and ion microprobe analysis of olivine and orthopyroxene the lodran primitive achondrite is thought to represent some of the earliest events in the differentiation of chondritic asteroids most researchers who studied lodran believe that it is a restite from which a melt enriched in the incompatible trace and minor elements was extracted this process is reflected by a lack of feldspar in the lodran mineral assemblage presumably after or during melting a reduction event took place resulting in an increase in mg fe near the rims of both olivine and orthopyroxene the reduction process is far more advanced in olivine than in pyroxene because of the more rapid fe mg diffusion rates in olivine narrow reaction rims 10 m around orthopyroxene grains show a depletion in ca al cr ti rees and y and an increase in mg reverse zoning these systematics are largely the result of melting and to a lesser extent of reduction significant reaction of olivine has taken place by reduction and or sulfidatuon and to a lesser extent by melting this is reflected by a decrease in fe co and ni and an increase in mg towards the rims of olivine the reactions by which reduction and or sulfidation of olivine took place are not confirmed however the following represent two possible reactions

evolution and classification of acapulcoites and lodranites from a chemical point of view we examined 15 bulk samples of the acapulcoite lodranite clan for their major minor and trace element concentrations using inaa techniques among the analyzed meteorites are 2 new acapulcoites dhofar dho 290 thiel mountains til 99002 as well as an additional acapulcoite that has been described previously only in very brief form graves nunataks gra 98028 the petrographic attributes of these 3 samples are addressed thoroughly we also include petrographic information on 2 acapulcoites from africa northwest africa nwa 725 and nwa 1058 in general our study strongly supports the widely accepted idea that acapulcoites and lodranites evolved through partial melting and melt migration of metal sulfide phases and plagioclase furthermore we concur with previous researchers that the original bimodal classification scheme for acapulcoites and lodranites proves to be too simple based on our data set we introduce an alternative extended scheme with respect to their elemental distribution patterns we distinguish 5 subtypes comprising primitive typical transitional and enriched acapulcoites on one hand and lodranites on the other the chemical distinction between the primitive typical and transitional acapulcoites is rather subtle and gradual it stands in contrast to the clear modifications observed for the signatures of the enriched acapulcoites and the lodranites the definition of subcategories basically reflects the concentrations of 2 key elements k and se we note however that the assignment of subgroups may not be exclusively inferred from elemental abundances but should also consider additional petrographic information

three dimensional fractal modeling of intracloud lightning discharge in a new mexico thunderstorm and comparison with lightning mapping observations 1 the direct comparison of lightning mapping observations by the new mexico tech lightning mapping array lma with realistic models of thundercloud electrical structures and lightning discharges represents a useful tool for studies of electrification mechanisms in thunderstorms initiation and propagation mechanisms of different types of lightning discharges as well as for understanding of electrical and energetic effects of tropospheric thunderstorms on the upper regions of the earth s atmosphere this paper presents the formulation of a new three dimensional probabilistic model for investigating the structure and development of bidirectional positive and negative lightning leaders the results closely resemble structures observed by the lma during intracloud discharges the model represents a synthesis of the original dielectric breakdown model based on fractal approach proposed by niemeyer et al 1984 and the equipotential lightning channel hypothesis advanced by kasemir 1960 and places special emphasis on obtaining self consistent solutions preserving complete charge neutrality of the discharge trees at any stage of the simulation a representative simulation run is compared to a typical intracloud discharge measured by lma in a new mexico thunderstorm on 31 july 1999 following the conclusions from coleman et al 2003 the comparison of the model and observed discharges reveals that an adequate choice of the electrical structure of the model thundercloud permits the development of a model intracloud discharge reproducing principal features of the observed event including the initial vertical extension of the discharge between the main negative and upper positive charge regions of the thundercloud and the subsequent horizontal propagations in these regions also consistent with observations e g coleman et al 2003 negative and positive leaders mainly develop in the upper positive and main negative charge regions respectively for the particular model case presented in this paper the total charge transfer the vertical dipole moment and the average linear charge density associated with the development of bidirectional structure of leader channels are estimated to be 37 5 c 122 c km and 0 5 mc m respectively in good agreement with related data reported in the refereed literature the model results also demonstrate that the bulk charge carried by the integral action of positive and negative leaders leads to a significant up to 80 reduction of the electric field values inside the thundercloud significantly below the lightning initiation threshold

mineralogy of antarctic ureilites and a working hypothesis for their origin and evolution abstract mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for antarctic ureilites y74123 y74130 y74659 y790981 alha77257 alha78019 and alh82106 extend the chemical and textural variations among ureilites beyond those known for non antarctic ureilites low ca pyroxenes possibly orthopyroxene poikilitically enclosed in olivine and coexisting with mg rich pigeonites have been identified in y74659 and alha77257 coexisting low ca pyroxene pigeonite and augite together with the very fe rich olivine in ureilites were observed in y74130 almost fe free pigeonite and diopside coexist in alh82106 no systematic differentiation trend has been reconstructed from these data but the pyroxene pairs yield formation temperatures above 1200 c a weak anticorrelation between mno and feo delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations a working hypothesis for ureilite formation involves high temperature episoded and a planetesimal scale collision which might have facilitated compaction and recrystallization of mafic silicates during the episodes fe ni s eutectic melt and a partial melt rich in ca al and fe might have been lost from a carbonaceous chondrite like source material

noble gases bulk chemistry and petrography of olivine rich achondrites eagles nest and lewis cliff 88763 comparison to brachinites eagles nest and lewis cliff lew 88763 are olivine rich achondrites that have been suggested to be related to brachina and alh 84025 both are ultramafic although lew 88763 has 10 plagioclase which eagles nest lacks olivine in eagles nest fo68 is virtually identical in composition to that in brachina fo68 70 and that in lew 88763 is only slightly different fo63 64 the meteorites cosmic ray exposure ages differ from each other and from brachina and alh 84025 like brachina eagles nest has a substantial amount of radiogenic 129xe and an apparent k ar age of 4 5 ga lewis cliff 88763 on the other hand has no detectable radiogenic 129xe an apparent k ar age of 4 5 ga and a substantial amount of trapped xe the chemical composition of lew 88763 is within a factor of two of chondritic for virtually every element analyzed including siderophiles which suggests that it was melted or at least heated and recrystallized in a closed system analyses of the chemical composition of eagles nest are hampered by extensive terrestrial weathering but it appears to be broadly chondritic also although it like brachina is depleted in siderophiles our data suggest that eagles nest is genetically related to brachina but that lew 88763 is not which is a conclusion that is consistent with the o isotopic data of clayton and mayeda 1996

the meteoritical bulletin no 86 2002 july abstract meteoritical bulletin no 86 lists information for 1154 newly classified meteorites comprising 661 from antarctica 218 from africa 207 from asia 203 of which are from oman 62 from north america 3 from south america and 3 from europe information is provided for 5 falls el idrissia undulung dashoguz el tigre and yafa noteworthy specimens include 7 martian meteorites dhofar 378 grove mountains 99027 northwest africa 856 1068 and 1110 and sayh al uhaymir 060 and 090 4 lunar meteorites dhofar 301 302 303 and 489 9 new iron meteorites a mesosiderite northwest africa 1242 an ungrouped stony iron meteorite dar al gani 962 and a wide variety of other interesting stony meteorites including ch ck cm cr cv r enstatite unequilibrated ordinary and ungrouped chondrites primitive achondrites howardite eucrite diogenite hed achondrites and ureilites

a petrologic and isotopic study of winonaites evidence for early partial melting brecciation and metamorphism we have conducted detailed petrologic chemical and isotopic studies of winonaites to ascertain the genesis of this group of meteorites winonaites have reduced mineral compositions and mineralogy and oxygen isotopic compositions distinct from primitive achondrite groups other than silicate inclusions in iab and iiicd irons however winonaites differ from iab and iiicd irons in that they lack the metallic matrices of the latter and consist mostly of silicates on the basis of these criteria winona mount morris wisconsin tierra blanca pontlyfni y 74025 y 75300 y 8005 and que 94535 are winonaites and y 75305 and y 75261 may be winonaites winonaites are fine to medium grained mostly equigranular rocks pontlyfni and mount morris wisconsin contain what appear to be relict chondrules several winonaites contain mm sized areas that differ substantially in grain size and or silicate mineralogy from the surrounding matrix fe ni fes veins are common in many winonaites mineral compositions are intermediate between e and h chondrites and reduced sulfides are observed in low feo winonaites bulk major element compositions are roughly chondritic although ree elements are fractionated the 39ar 40ar ages of three winonaites range from 4 40 ga mount morris wisconsin to 4 54 ga pontlyfni cosmic ray exposure ages are 20 80 ma trapped noble gases in these winonaites resemble those in enstatite chondrites we suggest that the winonaites formed from a chondritic precursor material unlike that of known chondrites in mineral and oxygen isotopic compositions and this material may have been heterogeneous in composition extensive heating caused metamorphism and partial melting of both fe ni fes and silicate material impact brecciation during cooling mixed lithologies with different thermal histories and subsequent metamorphism produced recrystallization grain growth and reduction of mafic silicates the 39ar 40ar ages indicate that cooling may have been more rapid than observed in iab irons although later resetting may have occurred

thermal history of lodranites yamato 74357 and mac88177 as inferred from the chemical zoning of pyroxene and olivine the chemical zoning profiles of pyroxene and olivine in the yamato y 74357 and mac alpine hills mac 88177 acapulcoite lodranite primitive achondrites were measured by using an electron probe microanalyzer in order to study their cooling histories the reverse zoning of fe and mg and mno enrichment were observed within a few tens of micrometers from the rim of orthopyroxene in y74357 this is probably related to reduction the cao of orthopyroxene in y74357 and mac88177 decreases gradually toward the rim as do the cr2o3 al2o3 tio2 and na2o in orthopyroxene and augite these results suggest that interstitial melt was present and was then extracted at a late stage in the formation of these meteorites in addition to these observations the absence of fe mg reverse zoning and mno enrichment at the rim of mac88177 orthopyroxene implies that the reduction process of y74357 took place after partial melting a calculated cooling rate of 2 3 c yr gives the best fit for the observed fe mg and cao profiles of orthopyroxene and cao of olivine in y74357 suggesting that the zonings were formed during the same event that probably caused the reduction thermal model calculations show that the maximum attainable temperature in the parent body was about 1000 c if the decay of 26al is the heat source and if the 26al 27al ratio is 5 10 6 identical to that used for the ordinary chondrite parent body partial melt was probably produced at this temperature this is an important part of the history of some primitive achondrites

mg diffusion in anorthite implications for the formation of early solar system planetesimals we have measured the self diffusion coefficients of mg ca and sr in anorthitic plagioclase in order to assess the potential of mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism diffusion couples were constructed from polished single crystals of natural anorthite and synthetic isotopically enriched anorthite glass couples were annealed at atmospheric pressure and 1200 1400 c and isotopic concentration profiles were measured with an ion microprobe the results show that mg diffusion in anorthite is surprisingly fast with d mg being over 2 orders of magnitude greater than d sr this indicates that the diffusion coefficient of mg in anorthite cannot be approximated with that for sr mg diffusion in the c direction is also slightly faster than in the b direction while ca and sr diffusion appear to be isotropic the results provide important constraints on the thermochronological history of anorthite bearing mineral assemblages that preserve radiogenic 26 mg excesses in a planetesimal heated by the decay of 26 al the temperature at any point depends on the planetesimal size time of formation thermal conductivity and depth within the planetesimal given sufficient heating 26 mg heterogeneities produced by the in situ decay of 26 al in ca al rich inclusions cais and chondrules will be erased by diffusive equilibration using the self diffusion coefficient for mg in anorthite measured in this study we show that the common occurrence of 26 mg excesses in these inclusions requires that they must be stored in small 15 km bodies or the outermost rims of larger bodies for the first 1 2 million years of the solar system s history for early formed bodies larger than 15 km most of the mass will have been heated sufficiently for any radiogenic 26 mg to have been diffusively homogenized in the mg rich planetary environment

shock post shock annealing and post annealing shock in ureilites the thermal and shock histories of ureilites can be divided into four periods 1 formation 2 initial shock 3 post shock annealing and 4 post annealing shock period 1 occurred 4 55 ga ago when ureilites formed by melting chondritic material impact events during period 2 caused silicate darkening undulose to mosaic extinction in olivines and the formation of diamond lonsdaleite and chaoite from indigenous carbonaceous material alkali rich fine grained silicates may have been introduced by impact injection into ureilites during this period about 57 of the ureilites were unchanged after period 2 during period 3 events impact induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals some ureilites experienced reduction as feo at the edges of olivine grains reacted with c from the matrix annealing may also be responsible for coarsening of graphite in a few ureilites forming euhedral appearing idioblastic crystals orthopyroxene in meteorite hills met 78008 may have formed from pigeonite by annealing during this period the rb sr internal isochron age of 4 0 ga for met 78008 probably dates the annealing event at this late date impacts are the only viable heat source about 36 of ureilites experienced period 3 events but remained unchanged afterwards during period 4 7 of the ureilites were shocked again as is evident in the polymict breccia elephant moraine eet 83309 this rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction ureilites may have formed by impact melting chondritic material on a primitive body with heterogeneous o isotopes plagioclase was preferentially lost from the system due to its low impedance to shock compression brief melting and rapid burial minimized the escape of planetary type noble gases from the ureilitic melts incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements