collisional history of h chondrites the cosmic ray exposure histories of ordinary chondrites were recently reviewed marti and graf 1992 here we discuss in detail currently available cosmic ray exposure records and the adopted exposure ages of individual h chondrites we further present analyses of the distinct histograms of petrographic subgroups and the implications for orbital evolution and collisional records in addition to the stochastic events 7 to 8 m y ago a peak at 33 ma represents a statistically significant deviation from an exponential age distribution and signals a major collisional event at this time the h6 subgroup also recorded a similar event 24 m y ago these records present significant arguments against a continuous supply of h chondrites from the asteroid belt we show that the records of fall times for h5s are distinct from other subgroups and that a different orbit for the h5 parent is indicated in collisional events 7 to 8 m y ago the systematics of noble gas losses in the h5 subgroup are consistent with this interpretation the evidence for stochastic fragmentation and subsequent delivery to earth of h chondrites suggests that their present predominance among falls may represent a deviation from a representative population distribution

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

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

k ca and rb sr dating of two lunar granites relative chronometer resetting k ca and rb sr age determinations were made for lunar granite 12033 576 this small fragment is a subsample of soil particle 12033 507 which was collected subsurface at the north rim of head crater the k ca mineral isochron is well defined and yields an age of 3 62 0 11 2 ga for 40k 0 5543ga 1 and an initial 40ca44ca of 47 160 0 006 2 normalized to 42ca44ca 0 31221 the rb sr mineral isochron is quite disturbed and yields an imprecise young age of 2 20 0 65 ga for 87rb 0 0139 ga 1 and a high initial 87sr86sr of 0 775 0 022 assuming that the granite crystallized 4 0 ga ago a modeled formation age and was shock heated 0 8 ga ago 39ar 40ar age the intermediate rb sr and k ca ages are interpreted as due to partial resetting by thermal diffusion of strontium and calcium diffusion data determined for argon in 12033 507 give a value of da2 1 10 6 s 1 for a heating temperature of 700 c at this temperature it would require a time on the order of days to totally degas argon the required diffusion coefficients d for calcium 7 9 10 13cm2s 1 and sr 8 10 12cm2s 1 that are predicted from the observed age resetting are in good agreement with those determined experimentally in granitic melts the k ca system was about a factor of ten more resistant to resetting than the rb sr system which in turn was 10 times more resistant to resetting than the k ar system diffusion data indicate that isotopic exchange of calcium and strontium was responsible for the partial resetting of the k ca and rb sr chronometers rather than elemental exchange of potassium and rubidium among mineral phases rb sr isotopic data also were obtained for granite 14303 206 a subsample of breccia clast 14303 204 which was collected at the fra mauro region about 180 km from the apollo 12 site the mineral isochron is quite disturbed and yields an imprecise age of 3 95 0 38 ga for 87rb 0 0139 ga 1 and initial 87sr86sr 0 7046 0 0051 the rb sr age is comparable to the previously determined k ca age of 4 04 0 64 ga the rb sr and k ca ages of this clast are imprecise and are statistically indistinguishable from the well defined 39ar 40ar degassing age of 3 83 0 03 ga or the rb sr and k ca model ages of 4 30 0 06 ga and 4 25 0 12 ga respectively because these model ages are within error of a reported zircon u pb age of 4 31 ga we assume this granitic clast crystallized 4 3 ga ago and was shock heated 3 83 ga ago using the apparent resetting of the k ar rb sr and k ca isotopic systems and literature data for relative strontium and calcium diffusivity we estimate a shock reheating temperature of 860 c would be necessary to partially reset the rb sr and k ca ages to their nominal values this reheating temperature is uncertain because of the large errors on the ages but would be consistent with complete resetting of the k ar age and partial resetting of the rb sr and k ca ages if the hot clast was buried under 1 m of the ejecta blanket for 1 day

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

flow fields around the emperor seamounts detected from current data 1 bathymetric influences of the emperor seamounts ess on the local and large scale flow fields in the subarctic north pacific were investigated by analyzing data of directly measured velocity obtained using argo floats near surface buoys and moored current meters the local influences were evident in notable deflections of two relatively strong 0 05 m s 1 eastward flows over ess a taylor cap induced by the eastward flow impinging against the nintoku seamount around 42 0 n 170 5 e was clearly identified in the climatological flow field at 1000 m depth this flow pattern was also identified in the near surface flow field 15 m depth and dynamic height field at 200 dbar relative to 2000 dbar indicating the taylor cap is stable or frequently generated and extends to the surface an anticyclonic flow was found over the suiko seamount around 45 0 n 170 5 e the influences on the large scale flow field were found in data obtained from current meters moored over the southeastern flank of the suiko seamount and argo float trajectories these data indicate for the first time from velocity observations that there is a seasonally varying boundary current along the eastern side of ess partially compensating for the sverdrup transport to the east of ess and hence the seasonally varying component of the subarctic gyre in the north pacific is divided into two subgyres by ess

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

concordant 3 676 myr u pb formation age for the kodaikanal iron meteorite the kodaikanal iron meteorite contains evidence of intensive chemical activity which according to rb sr studies1 occurred 800 myr after the formation of the solar system here we report the results of u pb isotope analyses on a silicate inclusion from kodaikanal leaching experiments on the two major phases clinopyroxene cpx and alkali rich glass indicate a contamination by terrestrial pb attributable to previous curatorial cutting of this iron meteorite the leached minerals define concordant u pb ages that validate the precise pb pb age of 3 676 3 myr these data are consistant with rb sr age determinations1 the absence of initial pb indicates a fast cooling of the silicate material this has already been established for the metal phase and a collisional origin for kodaikanal is favoured

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

rb sr isotopic systematics of alkali rich fragments in the yamato 74442 ll chondritic breccia abstract we have undertaken mineralogical petrographical and rb sr isotopic studies on alkali rich igneous rock fragments in the yamato y 74442 ll chondritic breccia the fragments are a few mm in size and are composed mainly of porphyritic olivine and dendritic pyroxene set in alkali rich groundmass glass minor phases include chromite troilite and metallic nickel iron bulk chemical compositions of the fragments are almost identical to the host chondrite except for a depletion of sodium and an enrichment of potassium isotopic analyses of nine fragments from y 74442 yield a rb sr age of 4429 54 ma 2 for 87 rb 0 01402 ga 1 with an initial ratio of 87 sr 86 sr 0 7144 0 0094 2 assuming precursors of the fragments formed 4568 ma with 87 sr 86 sr 0 69889 when the solar system formed a time averaged rb sr weight ratio of the source material for the fragments is calculated to be 2 58 0 91 0 93 the extremely high rb sr value of this source is difficult to interpret by any igneous fractionation or liquid immiscibility but can be explained by mixing of a chondritic component with an alkali rich component formed in the early solar nebula in our preferred model the alkali component with rb sr 30 would have condensed from the residual nebular gas after removal of refractory strontium and must have been isolated for a long time in a region where the temperature was sufficiently low to prevent reaction with other silicates oxides a mixture of the alkali component early nebular condensates and the ferromagnesian component could reflect flash heating induced by impact on an ll chondritic parent body at least 4429 ma ago and further enrichments of rubidium and potassium relative to strontium could have occurred during this event the resulting impact melt rocks could have been fragmented by later impact event s and finally incorporated into the y 74442 parent body thus a remarkable signature of alkali enrichments both in the early solar nebula and later on the ll chondrite parent body is preserved as a minor component of some chondritic breccias such as y 74442

on variations in the subarctic circulation in the north pacific abstract anomalous southward shifts in the oyashio and subarctic circulation in the north pacific occur from winter to late spring and the wide extension of cold sea surface temperatures sst influences the atmospheric circulation over the north pacific this anomalous southward shift results from the barotropic response of the ocean to changes in the wind stress induced by the southward shift of the deepening aleutian low global changes in the ocean and atmosphere associated with these anomalous shift of the oyashio and subarctic circulation in the north pacific are examined in this study it is shown that both the area of sea ice in the okhotsk sea and the extent of snow cover over the eurasian continent decrease in winter in association with this anomalous shift the teleconnection between equatorial sst and the southward shift of the enhanced aleutian low was weak after 1989 as a result of a decadal climatic change even so the relationship between reductions in the areas of sea ice in the okhotsk sea and snow cover over the eurasian continent in the winter and the anomalous southward shift of the oyashio and the subarctic circulation persisted through the decadal change

ar ar ages and trapped ar components in martian shergottites rbt 04262 and lar 06319 abstract we made 39ar 40ar ar ar analyses of whole rock wr and mineral samples of two martian shergottites rbt 04262 rbt and lar 06319 lar in order to determine their ar ar ages and the 40ar 36ar ratios of the trapped martian ar they contain all samples released trapped excess 40ar and 36ar and suggested ar ar ages older than their formation ages because trapped ar components having different 40ar 36ar were released at different extraction temperatures we utilized only a portion of the data to derive preferred ar ar ages we obtain ar ar ages of 171 8 ma for rbt plagioclase and 163 13 ma for lar whole rock we identify two trapped ar components at low temperatures particularly for plagioclase trapped a with 40ar 36ar 285 3 was released and we believe this is most likely absorbed terrestrial air at high extraction temperatures particularly for pyroxene trapped b with 40ar 36ar 1813 127 was released the poikilitic non poikilitic texture of rbt and the presence of large pyroxene oikocrysts allowed a clear definition of trapped b this ar component is martian and its isotopic similarity to the martian atmospheric composition suggests that it may represent martian atmospheric ar incorporated into the shergottite melt via crustal rocks trapped b partitioned into pyroxene at a constant molar ratio of k 36artr 33 2 9 5 106 for rbt 04262 and 80 21 106 for lar 06319 trapped a mixed in different proportions with trapped b could give apparently intermediate trapped 40ar 36ar compositions commonly observed in shergottites

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

numerical experiments on the seasonal variations of the oyashio near the east coast of japan abstract seasonal variations of the oyashio near the east coast of japan are examined using a simple layered model with a localized diapycnal transport due to the strong tidal mixing around the kuril islands the modeled southernmost latitude is located at around 40 n in spring and 42 n in autumn the southwestward oyashio transport in the upper layers and depth integrated transport become large small during winter spring summer autumn these results are in agreement with observations the seasonal variation of the southernmost latitude is mainly caused by the baroclinic velocity anomalies off tohoku and hokkaido japan which have been excited by the wind stress changes along the west coast of north america the kamchatka peninsula and the model northern boundary this is different from the previous studies that emphasized the importance of direct barotropic response of the ocean to the basin scale wind stress changes for comparison an additional experiment in which the diapycnal transport around

a comparison of metallographic cooling rate methods used in meteorites abstract the primary objective of this study was to test the postulate that cooling rates acquired from metal grains in chondrites are consistent with those from iron meteorites both types of metal occur in some group iab meteorites which are mixtures of massive metal with well developed widmanstatten structures and chondritic inclusions with dispersed metal grains the grains have textures and compositions similar to chondritic metal including negligible p the meteorites studied show little or no sign of shock reheating and textural evidence indicates that silicates and metal were mixed before widmanstatten patterns formed during cooling cooling rates were obtained by comparing measured to modeled taenite grain or lamellae dimensions and central ni contents modeling entails solving diffusion equations using experimental diffusion coefficients phase relations and bulk or local ni and p contents taking into account geometry undercooling and impingement there is one set of parameters for grains and another quite different set for widmanstatten lamellae including a factor of 30 difference in diffusion coefficients yet cooling rates obtained from widmanstatten structures and metal grains in chondritic inclusions of the same meteorite are consistent uncertainties in the best data are 10 ma equivalent to a factor of 1 0 25 this agreement implies that the data and models are correct or contain fortuitously offsetting errors which is quite unlikely cooling rates range from 40 ma to 70 ma in iab meteorites that contain both grains and widmanstatten structures rates based on grains in ni poor and ni rich meteorites lacking widmanstatten patterns expand the range from 30 ma to perhaps 200 ma cooling rates correlate with ni content ni poor meteorites have slower rates than ni rich ones evidently iab meteorites were radially distributed over 30km in a body with a radius 50km a comparison of the available ar ages with cooling times inferred from the cooling rates suggests that the parent body cooled more slowly after the metallographic cooling rates were established

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

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

decadal changes in the north pacific oceanic frontal zones as revealed in ship and satellite observations 1 influence of decadal climate changes upon the north pacific subarctic and subtropical oceanic frontal zones np safz and np stfz respectively is documented both satellite and ship measured sea surface temperature sst data reveal the strong confinement of wintertime decadal sst anomalies to those frontal zones the satellite data indicate that the wintertime np safz tended to be narrow and shift northward under the weakened surface westerlies during a warm event in the early 1990s the opposite was the case under the enhanced westerlies during a cool event in the mid 1980s decadal scale modulations in the frontal intensity are also evident in the satellite data a diagnosis based on a simple linearized mixed layer model suggests that the observed anomalous frontal intensity was contributed by the sign reversal of anomalous wind stress curl through the ekman processes and by an anomaly dipole of the net heat flux around the safz these decadal scale modulations in the np safz and their relationships with decadal fluctuations in the local surface westerlies are confirmed in the ship measured data over the 4 recent decades it is found that the safz happens to be a region where the decadal variability in the surface westerlies are unlikely to be masked by the higher frequency fluctuations as it almost coincides with the ridge trough line of the remote atmospheric response to the el nino southern oscillation

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

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

wind driven shifts in the latitude of the kuroshio oyashio extension and generation of sst anomalies on decadal timescales abstract the causes of decadal variations of north pacific sea surface temperatures ssts are examined using a hindcast performed with an ocean general circulation model thermodynamically coupled to an atmospheric mixed layer model ogcm aml model and forced by the time history of observed surface winds the shift in north pacific ocean climate that occurred around 1976 77 is focused on since this is the best observed example available after the 1976 77 shift the aleutian low deepened and moved to the southeast of its previous position this placed anomalous cyclonic flow over the north pacific the sst response as simulated by the ocean model consisted of two components a fast and local part and a delayed and remote part in the central pacific stronger westerlies cool the ocean by increased equatorward ekman drift here the dynamical cooling is sufficiently large that the surface fluxes damp the sst anomaly this ekman response is fast and local and cools the ssts beginning in 1977 and persistin

common asteroid break up events of eucrites diogenites and howardites and cosmic ray production rates for noble gases in achondrites we determined the cosmic ray produced noble gas abundances in eight diogenites four eucrites and four howardites using these and additional noble gas and chemical data from the literature composition and shielding dependent production rates of achondrites for 3he 21ne 38ar 78kr 83kr and126xe were derived based on 81krkr exposure ages the cosmogenic ratio 22ne 21ne is found to be a valid shielding indicator for all howardites eucrites and diogenites heds this ratio correlates with other shielding dependent ratios such as 78kr 83kr 80kr 83kr 124xe 131xe 126xe 131xe and 128xe 131xe we show that strong correlations for these ratios also exist for basaltic lunar rocks the new production rates were used for the calculation of exposure ages for all heds with known noble gas abundances two major exposure age clusters at 21 ma and 38 ma are observed in all three classes we find that five asteroid break up events characterized by exposure ages 6 1 ma 12 2 ma 21 4 ma 38 8 ma and 73 3 ma produced all diogenites fifteen out of seventeen howardites and twenty eight out of thirty nine eucrites we conclude that asteroidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites howardites and diogenites probable parent bodies are multiple 4 vesta derived kilometer sized objects of basaltic achondritic composition reaching the 3 1 resonance region a proposed source area for meteorites the direct ejection from vesta and capture by earth of basaltic meteorites has been shown to be dynamically difficult in such a scenario large events 38 ma and 21 ma ago are required liberating simultaneously material representing vesta s surface eucrites howardites and interior regions diogenites

noble gas retention chronologies for the st severin meteorite abstract 40 ar 39 ar and 129 xe 128 xe analyses were performed on two lithologies light and dark of the st severin ll6 chondrite for the light and dark fractions respectively we obtained 40 ar retention ages of 4 38 and 4 42 ae and 129 xe retention ages of 8 4 and 15 2 myr after bjurbole the two methods give age differences of opposite sense and by both methods the differences are significant both the 40 ar and the 129 xe ages are interpreted as dating relaxation of metamorphic conditions these two chronometers are decoupled however and do not date the same events 40 ar 39 ar reflect chondritic metamorphisrn on a 10 8 yr time scale the 129 xe 128 xe ages reflect isotopic closure at higher temperatures and earlier times

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

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

terrestrial 81kr kr ages of antarctic meteorites the production rate of 38ar in meteorites p 38 has been determined as a function of the sample s chemical composition from 81kr kr exposure ages of four eucrite falls the cosmogenic 78kr 83kr ratio is used to estimate the shielding dependence of p 38 from the true 38ar exposure ages and the apparent 81kr kr exposure ages of nine antarctic eucrite finds terrestrial ages are calculated they range from about 3 105a pecora escarpment 82502 to very recent falls thiel mountains 82502 polymict eucrites from the allan hills a78132 a79017 and a81009 have within the limits of error the same exposure age 15 2 106a and the same terrestrial age 1 1 105a this is taken as strong evidence that these meteorites are fragments of the same fall a similar case are the elephant moraine polymict eucrites a79005 a79006 and 82600 with an exposure age of 26 106a and a terrestrial age of 1 8 105a eeta79004 may be different from this group because its exposure age and terrestrial age are 21 106a and 2 5 105a respectively the distribution of terrestrial ages of allan hills meteorites is discussed meteorites from this blue ice field have two sources directly deposited falls and meteorites transported to the allan hills inside the moving antarctic ice sheet during the surface residence time meteorites decompose due to weathering processes the weathering half life is about 1 6 105a from the different age distributions of allan hills and yamato meteorites it is concluded that meteorite concentrations of different antarctic ice fields need different explanations

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

isotopic composition of trapped and cosmogenic noble gases in several martian meteorites isotopic abundances of the noble gases were measured in the following martian meteorites two shock glass inclusions from elephant moraine eet 79001 shock vein glass from shergotty and yamato y 793605 and whole rock samples of allan hills alh 84001 and queen alexandra range que 94201 these glass samples when combined with literature data on a separate single glass inclusion from eet 79001 and a glass vein from zagami permit examination in greater detail of the isotopic composition of ne ar kr and xe trapped from the martian atmosphere the isotopic composition of martian ne if actually present in these glasses remains poorly defined the 40ar 36ar ratio of trapped martian atmospheric ar is probably considerably lower than the nominal ratio of 3000 measured by viking and data on impact glasses suggest a value of 1900 the atmospheric 36ar 38ar ratio is 4 0 martian atmospheric kr may be enriched in lighter isotopes by 0 5 amu compared to both solar wind kr and to the martian composition previously reported the isotopic composition of xe in these glasses agrees with that previously reported in the literature the martian atmospheric 36ar 132xe and 84kr 132xe elemental ratios are higher than those reported by viking by factors of 2 5 1 6 depending on the 40ar 36ar ratio adopted and 1 8 respectively and are discussed in a separate paper cosmogenic gases indicate space exposure ages of 2 7 0 6 ma for que 94201 and shergotty and 14 1 ma for alh 84001 small amounts of 21ne produced by energetic solar protons may be present in que 94201 but are not present in alh 84001 or y 793605 the space exposure age for y 793605 is 4 9 0 6 ma and appears to be distinctly older than the ages for basaltic shergottites however uncertainties in cosmogenic production rates still makes somewhat uncertain the number of martian impact events required to produce the exposure ages of martian meteorites

interdecadal decrease of the oyashio transport on the continental slope off the southeastern coast of hokkaido japan we investigated the interdecadal trend of oyashio velocity and transport during 1993 2011 based mainly on linear trend analysis of altimetry and in situ temperature salinity data from a monitoring line a line off the southeastern hokkaido coast significant trends of increasing sea level were detected on the continental slope north of the kuril kamchatka trench sea level anomaly data revealed a localized clockwise circulation centered near the trench the suggestion being that the strength of the oyashio on the slope and the offshore return flow had decreased the oyashio mainstream seemed to have shifted from a nearshore to an offshore path steric heights estimated from the a line data exhibited an increasing trend north of the trench where 50 80 of the increase was determined by halosteric components attributable to a trend of decreasing salinity in the subsurface the trend of decreasing salinity was related to downward displacement of isohaline isopycnal surfaces the largest displacement was above the trench horizontal pressure gradients associated with southwestward flows on the slope were weakened the oyashio transport decreased by 8 9 sv 106 m3 s 1 in 19 years a mesoscale eddy analysis revealed that clockwise eddies appeared more frequently in recent years near the trench around the a line and could decrease the oyashio transport a baroclinic long rossby wave model also predicted that a large scale baroclinic response to the wind stress could weaken the oyashio velocity in the upper layer dynamical linkage between the localized eddies and large scale response remains to be clarified in future work

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

oxygen isotope and petrological study of silicate inclusions in iie iron meteorites and their relationship with h chondrites abstract the origin of silicate bearing irons especially those in groups iab iiicd and iie is poorly understood as silicate should have separated rapidly from molten metal here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group iie meteorites and a petrological study of silicate inclusions in ten iie irons including those in garhi yasin and tarahumara which have not been described in detail before oxygen isotopes have also been analysed in 20 h chondrites to investigate their possible relationship with the iie irons based on petrographic observations and mineral analysis the silicate bearing iie meteorites have been divided into four types according to the nature of their silicate inclusions 1 primitive chondritic 2 evolved chondritic 3 differentiated with 10 vol orthopyroxene and 4 differentiated with 17 o variation a wide range of values is seen in the dataset as a whole group iie irons with differentiated silicates with the exception of colomera have a range of mean 17 o values that is essentially identical to those of the h4 6 chondrites 0 60 0 77 and 0 61 0 76 respectively colomera inclusions which are differentiated with 17 o value and plot 2 away from the next nearest iie iron however in view of the textural similarities to other iie inclusions a separate source for colomera is deemed unlikely three iie irons with primitive chondritic inclusions garhi yasin netschaevo and techado have relatively low mean 17 o values of 0 56 0 57 as well as relatively reduced silicates with fa 15 17 olivine which have been called hh chondrites given the significant overlap in their oxygen isotope compositions a genetic relationship between iie irons and h chondrites is supported by our new data however derivation of both groups from one parent body seems unlikely instead both groups probably sampled similar precursor materials and accreted at a similar nebular location our data suggest that the iie meteorites formed on an internally heated h hh chondrite like body that experienced the initial stages of differentiation in response to radiogenic heating however prior to full differentiation the iie parent body experienced a major hit and run style collision that resulted in silicate metal mixing the initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates reassembly of the iie parent body produced a large regolith blanket that facilitated subsequent slow cooling the iie parent body has probably experienced numerous subsequent less catastrophic collisions the development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events

rhenium 187 osmium 187 in iron meteorites and the strange origin of the kodaikanal meteorite a set of iron meteorites was investigated for re os isotopes and provides a well defined isochron age of 4 624 0 017 ba and an initial 187os 188os ratio of 0 095636 0 00009 actual ages may be a few percent lower due to uncertainty on the 187re decay constant within the small sample number presented here there is no evidence for age differences between classes a more detailed study of the meteorite kodaikanal shows that the metal reservoir of this meteorite formed approximately at the same time as the other iron meteorites in contrast with the silicate inclusions which display a formation age of 3 67 ba with all other chronometers a collisional origin is in agreement with the data on this meteorite major secondary events do not affect the re os system at the bulk rock scale in metal as it is the sole significant reservoir of these two elements a re os and rb sr investigation of meteoritic troilite exhibits disturbed chronometric systems which we attribute to the terrestrial history of the meteorite

middle holocene age of the sunnyvale human skeleton a morphologically modern human skeleton from sunnyvale california previously dated by aspartic acid racemization to be approximately 70 000 years old and by uranium series isotopic ratios to be 8300 and 9000 years old appears to be younger when dated by the carbon 14 method four carbon 14 determinations made by both decay and direct counting on three organic fractions of postcranial bone support a middle holocene age assignment for the skeleton probably in the range of 3500 to 5000 carbon 14 years before the present this dating evidence is consistent with the geologic archeological and anthropometric relationships of the burial as well as previously determined carbon 14 determinations on associated materials

anomalous southward intrusions of the oyashio east of japan 2 two layer numerical model anomalous southward intrusions of the oyashio along the japanese coast in the northwestern pacific occur from winter to late spring with cold sea surface temperatures exerting various influences on the temperature in japan and on aquaculture a two layer numerical model is employed to examine the interannual response of the ocean to the observed wind stress change over 1961 1987 with special reference to the occurrence of the anomalous southward oyashio intrusion since 1971 anomalous southward oyashio intrusions were observed in winter of 1974 1981 1984 and 1986 and the significant southward shift of the oyashio is well simulated in these years because the observed southward latitude of no wind stress curl in the north pacific and the sverdrup transport show clear simultaneous correlation with the calculated southernmost latitude of the subarctic circulation the anomalous southward intrusion of the oyashio is found to be essentially due to the barotropic response of the ocean to the change in the wind stress characterized by the southward development of the enhanced aleutian low the model also shows that a baroclinic response from the eastern boundary is mainly confined to a region east of 140 w and an uplift of the interface in a central area of the subarctic circulation is also formed by the divergence of the surface ekman layer in winter of the anomalous southward oyashio intrusion the calculated upper layer thickness south of the subarctic circulation has a clear positive simultaneous correlation with the observed southernmost latitude of the oyashio defined by the 5 c isotherm at a depth of 100 m which has been used as an index however the observed southernmost latitude of the oyashio follows the observed latitude of no wind stress curl and the sverdrup transport by 2 4 months while the calculated southernmost latitude of the subarctic circulation shows simultaneous correlation with no wind stress curl and the sverdrup transport from this it is pointed out that the observed southernmost latitude of the oyashio defined by the 5 c isotherm at a depth of 100 m lags by 2 4 months the barotropic response of the ocean which appears in the surface pressure fields and upper layer velocity

the ungrouped chondrite el medano 301 and its comparison with other reduced ordinary chondrites abstract el medano 301 em 301 is an ungrouped chondrite with overall texture and trace element distribution similar to those of ordinary chondrites ocs but with silicate olivine and low ca pyroxene compositions that are more reduced than those in ocs with average olivine and low ca pyroxene of fa3 9 0 3 and fs12 8 4 9 respectively these values are far lower than the values for ocs and even for chondrites designed as reduced chondrites low ca pyroxene is the dominant mineral phase and shows zoning with higher mgo contents along the crystal rims and cracks reverse zoning the co content of kamacite is also much lower than the concentrations observed in ocs below detection limit of 0 18 wt versus 0 44 37 wt oxygen isotopic composition is 17o 0 79 0 78 and slightly different from that of ocs the lower modal olivine pyroxene ratio different infrared ir spectra lower co content of kamacite lower mean feo contents of olivine and pyroxene different kamacite texture and different oxygen isotopic composition show that em 301 does not belong to a known oc group em 301 shows similarities with chondritic clasts in cumberland falls aubrite and with northwest africa 7135 nwa 7135 and acfer 370 ungrouped chondrites however dissimilar to nwa 7135 and the clasts it does not contain highly reduced mineral phases like daubreelite our observations suggest the formation of em 301 in a nebular region compositionally similar to ocs but with a different redox state with oxygen fugacity o2 in this region lower than that of ocs and higher than that of enstatite chondrites condensation region a second possibly nebular phase of reduction by the production of reducing gas phases e g c rich could be responsible for the subsequent reduction of the primary material and the occurrence of reverse zoning in the low ca pyroxene and lower average fa fs ratio based on the ir spectra of em 301 we suggest the possibility that the parent body of this chondrite was a v type asteroid

graves nunataks 95209 a snapshot of metal segregation and core formation gra 95209 may provide our best opportunity to date to understand the earliest stages of core formation in asteroidal bodies this lodranite preserves a physically chemically and mineralogically complex set of metal sulfide veins high resolution x ray computed tomography revealed three distinct lithologies the dominant mixed metal silicate sulfide matrix is cut by metal rich graphite bearing veins exceeding 1 cm in width and grades into a volumetrically minor metal poor region silicate compositions and modal abundances are typical for lodranites while the mineralogy of the metal sulfide component is complex and differs among the three lithologies kamacite and troilite occur with chromite tetrataenite schreibersite graphite and a range of phosphates an 39 ar 40 ar age of 4 521 0 006 ga measures the time of closure of the k ar system carbon rosettes within the metal rich vein are nitrogen poor well crystallized include kamacite sub grains of composition comparable to the host metal and are essentially isotopically homogeneous d 13 c 33 in contrast carbon rosettes within metal of the metal poor lithology are n poor poorly crystallized include kamacite grains that are ni poor compared to their host metal and are isotopically heterogeneous d 13 c ranging from 50 to 80 even within a single metal grain the silicate portion of gra 95209 is similar to the lodranite eet 84302 sharing a common texture silicate mineral compositions and ar ar age gra 95209 and eet 84302 are intermediate between acapulcoites and lodranites both experienced fe ni fes melting with extensive melt migration but record only the onset of silicate partial melting with limited migration of silicate melt the complex metal sulfide veins in gra 95209 resulted from low degree partial melting and melt migration and intruded the matrix lithology reactions between solid minerals and melt including oxidation reduction reactions produced the array of phosphates schreibersite and tetrataenite extensive reduction in the metal rich vein resulted from its origin in a hotter portion of the asteroid this difference in thermal history is supported by the graphite structures and isotopic compositions the graphite rosettes in the metal rich vein are consistent with high temperature igneous processing in contrast the carbon in the metal poor lithology appears to preserve a record of formation in the nebula prior to parent body formation carbon incorporated from the solar nebula into a differentiating asteroid is preferentially incorporated in metal sulfide melts that form a core but does not achieve isotopic homogeneity until extensive thermal processing occurs

comparative petrology of silicates in the udei station iab and miles iie iron meteorites implications for the origin of silicate bearing irons abstract the textures and mineral chemistries of silicate inclusions in the udei station iab and miles fractionated iie iron meteorites were studied using optical and electron microscopy sem empa and la icp ms techniques to better understand the origin of silicate bearing irons inclusions in udei station include near chondritic basaltic gabbroic feldspathic orthopyroxenitic and harzburgitic lithologies in miles most inclusions can be described as feldspathic pyroxenite or pyroxene enriched basalt gabbro the trace element compositions of both orthopyroxene and plagioclase grains are similar in different lithologies from udei station whereas in different inclusions from miles the compositions of orthopyroxene grains are similar while those of clinopyroxene plagioclase and especially cl apatite are variable orthopyroxene in miles tends to be enriched in ree compared to that in udei station but the reverse is true for plagioclase and clinopyroxene the data can be explained by models involving partial melting of chondritic protoliths silicate melt migration and redox reactions between silicate and metal components to form phosphate the extent of heating melt migration and phosphate formation were all greater in miles silicates in miles were formed from liquids produced by 30 partial melting of a chondritic precursor brought to a peak temperature of 1250 c this silicate melt crystallized in two stages during stage 1 crystallizing minerals orthopyroxene clinopyroxene chromite and olivine were largely in equilibrium with an intercumulus melt that was evolving by igneous fractionation during slow cooling with a residence time of 20 ka at 1150 c during stage 2 following probable re melting of feldspathic materials and after the silicate mush was mixed with molten metal plagioclase and phosphate fractionally crystallized together during more rapid cooling down to the solidus in udei station despite a lower peak temperature we suggest that a model involving major collisional disruption and mixing of partly molten endogenically heated planetesimals can best explain the data for iab and fractionated iie silicate bearing irons the extent of endogenic heating was different less for the iabs and the amount of parent body disruption was different scrambling with collisional unroofing for the iab iiicd winonaite body more complete destruction for the fractionated iie body but both bodies were partly molten and incompletely differentiated at the time of impact we suggest that the post impact secondary body for iab iiicd winonaite meteorites was mineralogically zoned with ni poor metal in the center and that the secondary body for fractionated iie meteorites was a relatively small melt rich body that had separated from olivine during collisional break up

distribution and circulation of the coastal oyashio intrusion abstract the distribution and circulation of the coastal oyashio intrusion were examined with a ctd and shipboard adcp survey conducted in the vicinity of the east coast of japan in may 1994 the estimate of mixing ratios in the density range of 26 4 27 2 assuming isopycnal mixing between the pure oyashio and kuroshio waters and the adcp velocity data showed the following new results 1 the distribution of the oyashio water is horizontally and vertically patchy and some of the oyashio water patches formed mesoscale eddies with both cyclonic and anticyclonic circulations 2 the anticyclonic oyashio eddy had cold and low salinity cores with low potential vorticity pv suggesting that the origin was the okhotsk sea the cyclonic oyashio eddy with high pv was considered to originate from the western subarctic gyre wsag in the north pacific a new image of the intermediate oyashio water circulation near the oyashio kuroshio confluence is proposed in which the eddies from the okhotsk sea and the wsa

a nonmagmatic origin of group iie iron meteorites abstract new neutron activation data on 10 elements in 12 iie and iie related irons lead to a reclassification of several irons seymchan and lonaconing are removed from iie and leshan added four iie members are designated iie an to call attention to some anomalous properties the eight normal iie members define element ni trends generally similar to those in the nonmagmatic group iab the small negative slopes on w ni and ir ni diagrams are strongly indicative of a nonmagmatic origin of the iie irons we propose that iie irons like iab irons originated as individual pools of impact produced melt in the near surface region of a chondritic parent body the positive as ni and au ni trends are the only evidence suggesting fractional crystallization but their slopes are lower than those in magmatic group iiiab and only slightly higher than those of cu and sb in iab we suggest that the s and c contents of the iie precursor materials were much lower than those of the iab precursors thus higher temperatures were required to generate enough metallic melt to segregate into pools these higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions

properties of the guin ungrouped iron meteorite the origin of guin and of group iie irons abstract guin is an ungrouped iron meteorite containing 45 g g ga and 186 g g ge it is a coarse octahedrite with an unusually high ni content 92 5 mg g its metal composition is generally similar to irons in groups iab and iiicd but its closest relative is the cruz del aire ungrouped iron guin contains 6 3 vol silicate inclusions that are petrologically and compositionally similar to those in iie irons the largest inclusion 2 4 cm consists primarily of a shock melted plagioclase rich matrix surrounding large 1 15 mm partly melted augite grains the oxygen isotopic composition of the inclusion 18 o 5 29 17 o 3 87 is near the ll chondrite field the guin inclusion is similar in bulk composition to selected melt pocket glasses in ordinary chondrites produced in situ by preferential melting of plagioclase by shock compression we suggest that the guin assemblage formed by impact melting on a chondritic parent body silicate inclusions in iie irons share many petrologic and compositional but not oxygen isotopic characteristics with the guin inclusions suggesting that iie irons also formed by impact melting of chondritic materials individual iie irons were probably derived from separate cratering events

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

acapulco and lodran like achondrites petrology geochemistry chronology and origin we have performed petrologic and geochemical studies of five primitive achondrites alha81187 and alha81261 acapulco like eet 84302 transitional but acapulco like and lew 88280 and mac 88177 lodran like we have also performed 39ar 40ar chronology on alha81187 alha81261 and eet 84302 lew 88280 and mac 88177 contain more ferroan olivines orthopyroxenes clinopyroxenes and chromites than do the acapulco like achondrites plagioclase is present in only alha81187 alha81261 and eet 84302 its composition does not track that of the mafic silicates eet 84302 with the most calcic plagioclase contains olivine and pyroxene with an mg intermediate between alha81187 and alha81261 similarly spinels in eet 84302 have the same mg as those in alha81187 while pyroxenes in the former are more ferroan than in the latter acapulco like achondrites have sm sc ratios between 0 8 1 5 times h chondrites and na sc ratios are between 0 81 0 times h chondrites indicating that silicate partial melts have not been lost from these rocks siderophile and chalcophile elements are fractionated and show a trend of increasing ir ni ratios with decreasing se co ratios this variation is consistent with fractionation by partial melting in the fe ni s system eet 84302 is very depleted in se and troilite is a accessory phase indicating essentially total loss of the low temperature fe ni s melt in contrast the lodran like achondrites are depleted in the highly incompatible lithophile elements relative to more compatible elements sm sc ratios are 0 2 0 7 times and na sc ratios are 0 04 0 18 times h chondrites these data are consistent with a model for formation of the lodran like achondrites as partial melting residues however the low ir ni and high se co ratios in two lodran like achondrites suggest the metal troilite in these rocks is dominated by the low melting fraction in the fe ni s system this metal troilite was added to the silicates after partial melting depleted them in a basaltic fraction the ar ar release spectra yield good plateau ages of 4 507 0 024 ga alha81187 4 511 0 007 ga alha81261 and 4 519 0 017 ga eet 84302 in good agreement with previously determined ar ar ages for other acapulco like achondrites together the ar ar ages demonstrate that metamorphism on the acapulco lodran parent body occurred 4 51 0 02 ga ago roughly 50 ma after its formation the geochemical and petrologic evidence on the acapulco and lodran like achondrites suggest that heating on the parent body was localized and heterogeneous in space and time the parent body was probably not subjected to parent body wide magmatism in contrast to the case for 4 vesta and the hed meteorites

relict chondrules in primitive achondrites remnants from their precursor parent bodies abstract we studied the petrography analyzed the chemical compositions constrained the closure temperatures via geothermometry and determined the oxidation states of relict chondrules in campo del cielo iab iron meteorite graves nunataks gra 98028 acapulcoite and netschaevo iie iron meteorite to constrain their formation conditions and investigate links to known meteorite groups despite having been thermally metamorphosed mineral phases within relict chondrules retain information about their precursor compositions the sizes and textures of relict chondrules and silicate and chromite compositions indicate that campo del cielo gra 98028 and netschaevo had distinct parent bodies that were similar to but different from known chondrite groups to determine the utility of relict chondrule sizes in thermally metamorphosed meteorites we determined the chondrule size distributions in the ll chondrites semarkona ll3 00 soko banja ll4 siena ll5 and saint severin ll6 and the h chondrites clovis no 1 h3 6 kesen h4 arbol solo h5 and estacado h6 as expected mean chondrule diameters increase with degree of thermal metamorphism we find that campo del cielo and gra 98028 were reduced during thermal metamorphism consistent with previous studies indicating that their precursors were initially more feo rich than their current compositions in contrast to previous studies we find no evidence for reduction of silicates in netschaevo normal zoning of olivine in netschaevo is consistent with crystallization and suggests its silicates are near their primary feo contents the presence of elongated chromite grains along olivine grain boundaries in netschaevo indicates formation during thermal metamorphism under oxidizing conditions due to the absence of reduction and the composition of chromite being distinct from that of metamorphosed h chondrites we conclude that netschaevo and by extension the iie iron meteorites are not from the h chondrite parent body

trace element studies of silicate rich inclusions in the guin ungr and kodaikanal iie iron meteorites a devitrified glass inclusion from the guin ungr iron consists of cryptocrystalline feldspars pyroxenes and silica and is rich in sio2 al2o3 and na2o it contains a rutile grain and is in contact with a large cl apatite the latter is very rich in rare earth elements rees 80 x ci which display a flat abundance pattern except for eu and yb which are underabundant the devitrified glass is very poor in rees light rare earth elements lrees and deficits in eu and yban ultrarefractory pattern the genetic models suggested so far cannot explain what is observed and consequently we offer a new model for silicate inclusion formation in iie and related irons nebular processes and a relationship with e meteorites guin or ca al rich inclusions cais kodaikanal are indicated a sequence of condensation cas tin or refractory pyroxene rich liquids and vapor solid elemental exchange can be identified that took place beginning under reducing and ending at oxidizing conditions phosphate rutile formation alkali and fe 2 metasomatism metasomatic loss of rees from glasses

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

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte