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

tectonics and sedimentary environment of the north scotia ridge region revealed by side scan sonar the north scotia ridge is a series of islands and submarine ridges extending 2000 km from tierra del fuego to south georgia in the western south atlantic the ridge forms the elevated northern tectonic margin of the scotia sea and accommodates e w sinistral strike slip motion at the south american scotia plate boundary existing studies have shown that the northern flank of the north scotia ridge is a large and continuous accretionary prism formed during presumed mid late cenozoic n s convergence in this study we present long range side scan sonar gloria images and seismic reflection profiles which show the structural style of the accretionary prism for the first time the youngest accreted sediments show a uniform fabric of initial deformation symmetric gently asymmetric folds of 1 4 km wavelength which has been subsequently disrupted at shallower depths by additional shortening and uplift between 52 o 459w and 50 o 30 9 w the deformation front is exposed at the sea floor and the falkland trough retains the appearance of an active convergent margin elsewhere however the deformation front is buried beneath younger undeformed drift sediments indicating that convergence has ceased gloria sonographs also show geological features consistent with current control of sedimentation non deposition and erosion beneath the antarctic circumpolar current in particular this study describes current influenced sedimentation in the falkland trough and steep sided eroded depressions and diffuse slope parallel fabric on the elevated falkland plateau

pleistocene shell tools from lake mungo lunette australia identification and interpretation drawing on experimental archaeology abstract three freshwater mussel shell tools recovered from the lake mungo lunette in semi arid south eastern australia with bracketing age estimates of 40 30 ka and a possible fourth tool with bracketing age estimates of 50 40 ka are described an experimental approach combined with detailed structural and taphonomic analysis of the shell establishes the presence of both deliberate cultural modification and wear traces from use on the mussel fragments the characterization of australian pleistocene stone artefacts as being simple and unchanging is steadily being challenged through recent studies of pleistocene assemblages from lake mungo and elsewhere and these early shell tools reinforce the multidimensionality of ancient australian technologies

noble gases in h chondrites and potential differences between antarctic and non antarctic meteorites abstract thirty one antarctic h chondrites from the allan hills have been analyzed for their noble gases seven of the specimens contain solar gases thirteen have a cosmic ray exposure age around 8 ma both fractions are as observed for h chondrites from the rest of the world scrutiny of the data for paired samples yields a minimum number of 17 independent falls with 6 falls required for the 8 ma peak of exposure age the distribution of exposure ages as well as that of radiogenic 4 he and 40 ar yield no evidence that h chondrites from the allan hills region represent a mixture of populations that is different from that brought to moderate latitudes of the earth in more recent times

structure of the southern falkland islands continental shelf initial results from new seismic data abstract acquisition of a modern marine dataset has revealed the internal structure of sedimentary basins to the south of the falkland islands for the first time the new data reveal a variety of structural styles and indicate the presence of a sedimentary section ranging from 2 to 10 km in thickness developed in three linked sub basins 1 the eastern part of the malvinas basin lies south west of the islands a prominent regional unconformity developed throughout the area marks the top of gently folded rocks of probable palaeozoic age and their cover of late jurassic volcanics to the north west of the island the underlying strata approach the surface in a structural high which forms a prominent gravity low and may record the continuation of the palaeozoic basin exposed on the islands themselves strata of probable mesozoic and tertiary age rest on the unconformity throughout the area and show a regional southward dip these deposits increase in thickness south westwards towards the depocentre of the malvinas basin midway between the islands and argentina 2 the south falkland basin and burdwood bank lie south of the islands thicknesses increase towards an area of southward dipping extensional fault blocks within the south falkland basin downdip these are buried beneath a prograding and north vergent stack of blind and emergent thrusts at the northern margin of the burdwood bank which is a structural highly lying a shallow water south of the islands thrust loading has resulted in the development of a foreland basin north of the thrust front complex antiformal structures are associated with the thrusting whereas a shallow sedimentary basin developed on the crest of the burdwood bank is interpreted as a piggy bank basin riding on and sourced dominantly from within the thrust pile 3 the falkland plateau basin lies to the east and south east of the islands and is characterized by a varied and extensive mesozoic to tertiary succession of submarine fan deposits thickening eastwards into deeper water igneous intrusives deep in the section may correlate with jurassic volcanic rocks present within the malvinas basin to the west or may record younger rifting associated with the opening of the south atlantic these initial results indicate considerable structural heterogeneity in this part of the falkland islands continental shelf in the light of the hydrocarbon discoveries already made further west significant economic potential is likely stratigraphic traps may occur at regional unconformities and within submarine fan deposits whereas potential structural traps include mesozoic extensional fault blocks and thrust and sub thrust structures generated by tertiary and quaternary compression

flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

stones from mohave county arizona multiple falls in the franconia strewn field one of the most productive and well sampled dense collection areas for meteorites on earth is the franconia strewn field in mohave county arizona which since 2002 has yielded hundreds of meteorites in an ellipsoidal area approximately 5 16 km across based on petrographic mineral chemical and terrestrial age data we conclude that among 14 meteorites examined there are at least 6 and possibly 8 distinct meteorites represented which fell over a period of approximately 0 20 kyr ago these include equilibrated h chondrites such as franconia h5 and buck mountains bm 001 h6 h3 6 breccias such as buck mountains wash and bm 004 and l6 chondrites such as bm 002 and bm 003 which may be paired palo verde mine and bm 005 to confidently pair such meteorites often requires thorough petrographic examination mineral chemical analyses and terrestrial ages we estimate that 50 10 of the larger specimens in this area are paired yielding a relatively high value of approximately 2 3 2 9 distinct meteorites km 2 the meteorite flux estimated for franconia area is higher than the flux inferred from contemporary fireball data for larger masses we suggest that one large h3 6 meteoroid fell in the area most likely that of buck mountains wash approximately 4 kyr ago which produced an elliptical strewn field with masses generally increasing toward one end and which raised the meteorite productivity in the recovery area

neutron capture 36cl 41ca 36ar and 150sm in large chondrites evidence for high fluences of thermalized neutrons we have measured significant concentrations of 36cl 41ca 36ar from decay of 36cl and 150sm produced from the capture of thermalized neutrons in the large chico l6 chondrite activities of 36cl and 41ca corrected for a high energy spallogenic component and a terrestrial age of 50 ka give average neutron capture production rates of 208 atoms min g cl and 1525 atoms min kg ca which correspond to thermal neutron n fluxes of 6 2 n cm2 s and 4 3 n cm2 s respectively if sustained for the 65 ma single stage cosmic ray exposure age of chico these values correspond to thermal neutron fluences of 1 3 1016 and 0 8 1016 n cm2 for 36cl and 41ca respectively stepwise temperature extraction of ar in chico impact melt shows 36ar 38ar ratios as large as 9 the correlation of high 36ar 38ar with high cl ca phases in neutron irradiated chico indicates that the excess 36ar above that expected from spallation is due to decay of neutron produced 36cl excess 36ar in chico requires a thermal neutron fluence of 0 9 1 7 1016 n cm2 decreases in 149sm 152sm due to neutron capture by 149sm correlate with increases in 150sm 152sm for three samples of chico and one of the torino h chondrite the 0 08 decrease in 149sm 152sm shown by chico corresponds to a neutron fluence of 1 23 1016 n cm2 this fluence derived from sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution excess 36ar identified in the arapahoe bruderheim and torino chondrites and the shallowater aubrite suggest exposure to neutron fluences of 0 2 0 6 1016 n cm2 depletion of 149sm in torino and the lew86010 angrite suggest neutron fluences of 0 8 1016 n cm2 and 0 25 1016 n cm2 respectively neutron fluences of 1016 n cm2 in chico are almost as large as those previously observed for some lunar soils consideration of exposure ages suggests that the neutron flux in chico may have been greater than that in many lunar soils neutron capture effects although seldom reported may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic ar combining measurements of radioactive and stable species produced from neutron capture has the potential for identifying large meteorites with complex exposure histories

weathering of meteorites from oman correlation of chemical and mineralogical weathering proxies with 14c terrestrial ages and the influence of soil chemistry fifty four fragments of ordinary chondrites from 50 finds representing all searched areas in central oman and all weathering stages were selected to compare the physical chemical and mineralogical effect of terrestrial weathering with 14c terrestrial ages 14c ages range from 2 0 to 49 kyr with a median value of 17 9 kyr the peak of the age range which is between 10 20 kyr falls in an arid climate period a comparison of the chemical composition of omani chondrites with literature data for unweathered h and l chondrites demonstrates a strong enrichment in sr and ba and depletion in s during weathering water contents in h chondrites increase with terrestrial age whereas l chondrites show a rapid initial increase followed by nearly constant water content correlating sr ba and h2o with age indicates two absorption trends i an initial alteration within the first 20 kyr dominated by h2o uptake mainly reflecting fe ni metal alteration and ii a second baand sr dominated stage correlated with slower and less systematic weathering of troilite that starts after h2o reaches 2 wt sulfur released from troilite partly combines with ba and sr to form sulfate minerals other parameters correlated with 14c age are degree of weathering color of powdered meteorites and the ni fe ratio chemical analyses of 145 soils show a high degree of homogeneity over the entire interior oman desert indicating large scale mixing by wind soil samples collected from beneath meteorite finds typically are enriched in ni and co confirming mobilization from the meteorites high cr and ni concentrations in reference soil samples which decrease from ne to sw are due to detrital material from ultramafic rocks of the oman mountains

the dar al gani meteorite field libyan sahara geological setting pairing of meteorites and recovery density as of july 2001 1238 libyan meteorites have been reported most were found in two areas called dar al gani and hamadah al hamra dar al gani is located on a plateau of marine carbonate rocks with marly components eight hundred and sixty nine meteorites between 6 g and 95 kg totalling 687 kg have been found here but the calculated mean recovery density is comparatively low with one meteorite on 6 5 km2 dar al gani is a perfect site for the recognition and preservation of meteorites the existence of meteorites is the result of a combination of specific geological and geomorphological conditions there is a bright colored old limestone plateau 2 ma under arid weather conditions over long periods of time with rapid elimination of surface water if present and low erosion rates the preservation of meteorites is guaranteed through the absence of quartz sand on the plateau strongly reducing wind erosion and a basic environment emerging from the carbonate ground retards rusting of metallic meteorite components a supposed soil cover during pluvial times has probably protected older meteorites and led to a concentration of meteorites of different periods an evaluation of dar al gani meteorites suggests the existence of at least 26 strewnfields and 26 meteorite pairs reducing the number of falls to at most 534 shock and weathering grades as a tool for the recognition of pairings turned out to be problematic as several strewnfields showed paired meteorites which had been classified to different shock and weathering grades

the collision complex of the north scotia ridge single channel and multichannel seismic reflection profiles of the north scotia ridge trending west east off southernmost south america show the north side to consist of a wedge of deformed sediments resting on a planar layered sequence relative movement of the ridge northward toward the falkland plateau has resulted in deformation and uplift of the near surface sediment layers of the falkland trough to form an accretionary wedge these sediments are 1 to 3 s of two way travel time in thickness and lie above a well defined reflector whose velocity at the northern part of the trough is 3 6 km s the deeper layers extend to depths in places greater than 9 0 s of two way travel time beneath the north slope without any significant deformation the deepest observed reflector is associated with a 5 9 km s basement the deformed pile of sediments or collision zone of the north scotia ridge is similar to that observed at other active compressive plate boundaries

gold basin meteorite strewn field mojave desert northwestern arizona relic of a small late pleistocene impact event over 4450 meteorite specimens with a total mass of 168 760 g have been found in the gold basin l4 strewn field over an area of 225 km 2 the meteorite is a breccia composed only of fragments of l chondrite materials the parent meteoroid had a kinetic energy equivalent to 5 to 50 ktons when it hit the top of the atmosphere cosmogenic nuclide studies indicate the meteorite has a terrestrial age of 15 000 600 years corresponding to the late pinedale portion of the wisconsin glaciation conditions in the gold basin which is now part of the mojave desert were wetter and cooler at the time of the fall mossbauer analyses indicate the sample is 30 to 35 oxidized this is less than that in meteorites with similar ages found in eastern new mexico but comparable to that found in meteorites from the sahara and the nullarbor region oxidation is likely to have occurred soon after the fall when exposure to precipitation was at its maximum four other new meteorites were also found in the gold basin strewn field

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

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

light noble gases in 12 meteorites from the omani desert australia mauritania canada and sweden we measured the concentrations and isotopic compositions of he ne and ar in 14 fragments from 12 different meteorites three carbonaceous chondrites six l chondrites three most likely paired one h chondrite one r chondrite and one ungrouped chondrite the data obtained for the cv3 chondrites ramlat as sahmah ras 221 and ras 251 support the hypothesis of exposure age peaks for cv chondrites at approximately 9 ma and 27 ma the exposure age for shisr 033 cr chondrite of 7 3 ma is also indicative of a possible cr chondrite exposure age peak the three l chondrites jiddat al harasis jah 091 jah 230 and jah 296 which are most likely paired fall together with hallingeberg into the l chondrite exposure age peak of approximately 15 ma the two l chondrites shelburne and lake torrens fall into the peaks at approximately 40 ma and 5 ma respectively the ages for bassikounou h chondrite and ras 201 r chondrite are approximately 3 5 ma and 5 8 ma respectively six of the studied meteorites show clear evidence for 3he diffusive losses the deficits range from approximately 17 for one lake torrens aliquot to approximately 45 for ras 211 the three carbonaceous chondrites ras 221 ras 251 and shisr 033 all have excess 4he either of planetary or solar origin however very high 4he 20ne ratios occur at relatively low 20ne 22ne ratios which is unexpected and needs further study the measured 40ar ages fit well into established systematics they are between 2 5 and 4 5 ga for the carbonaceous chondrites older than 3 6 ga for the l and h chondrites and about 2 4 ga for the r chondrite as well as for the ungrouped chondrite interestingly none of our studied l chondrites has been degassed in the 470 ma break up event using the amount of trapped 36ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering at least in terms of their noble gas budget

anatomy and formation of oblique continental collision south falkland basin 1 the south falkland basin is a partially filled cenozoic foreland basin located south of the falkland plateau it was formed by flexure of this southern edge of the south american plate when the load represented by burdwood bank collided this continental fragment belongs to the predominantly oceanic scotia plate flexure probably started in early cenozoic times and has continued to the present day the whole region is submarine and so the detailed stratigraphy and structure of the basin has been well imaged by seismic reflection profiling the clarity of this imagery has made analysis of structures within the collision zone possible the plate boundary itself is an active oblique thrust fault which has controlled the growth of a frontal fold there is evidence for older phases of thrusting and folding further south within and beneath the sediments which blanket the flexed south american plate normal faulting occurs on a variety of scales episodes of stratigraphic growth associated with the largest of these faults demonstrates that they were active during flexural bending we have modeled the development of the south falkland basin using two different approaches both of which are based upon the simplest elastic model inverse modeling of free air gravity and bathymetric profiles suggest that the elastic thickness of the loaded crust is 5 20 km a complementary approach based upon the spectral analysis of free air gravity and bathymetry shows that the elastic thickness is 15 5 km both techniques indicate that the flexed continental lithosphere is weak a conclusion supported by the presence of normal faults within the flexed plate a small increase in elastic thickness from west to east appears consistent with a change in the density and penetration of normal faulting

the record of cosmogenic radiogenic fissiogenic and trapped noble gases in recently recovered chinese and other chondrites abstract we performed a comprehensive study of the noble gas isotopic abundances in thirty six chondrites including twenty seven chondrites recovered in china the large data base allows us to recognize some new characteristics of the nuclear record in chondritic matter the comparison of the trapped noble gas release pattern for ordinary and carbonaceous chondrites shows that the planetary trapped noble gases in ordinary chondrites are released mainly above 1200 c whereas 85 of these gases in carbonaceous chondrites are degassed at 200 c there exists a clear correlation of the fraction of trapped xe released at 1200 c and petrologic type of chondrites it thus appears that the carrier phases of the trapped noble gases in ordinary and in carbonaceous chondrites may not be the same the ngawi ll3 chondrite is solar gas rich the solar gases in meteorites and lunar surface material are mixtures of solar wind sw and solar energetic particles sep we show that the variations of the 3 he 4 he and 20 ne 22 ne ratios for solar gas rich meteorites and lunar surface material could be either due to preferential diffusive losses of the lighter isotopes of he and ne or due to a change of the sw sep flux ratio with time furthermore we demonstrate that the 81kr concentration is a function of the shielding depth of a sample within the meteoroid deviations from this depth dependency curve are observed for jilin h5 lishui l5 suizhou l6 and dongtai ll6 a complex exposure history for jilin is thus confirmed and is possible for the other three chondrites cosmic ray exposure ages are calculated based on six different nuclides 3he 21ne 38ar 83kr 126xe and 81kr kr in most cases good agreement is observed for the ages derived from the different methods quality classes are assigned to the exposure ages and the age distributions are discussed in some meteorites we observe effects induced by secondary cosmicray produced neutrons epithermal neutron fluxes jn 30 300 ev and fast neutron fluxes jn 5 mev are derived based on the reactions 79br n 80kr and 24mg n 21ne respectively and preatmospheric masses of the meteoroids are estimated we show that the ratio j n 30 300 ev j n 5 mev increases with increasing preatmospheric mass we introduce a 3he exposure age 21ne exposure age vs 4he gas retention age 40ar gas retention age diagram that is a powerful tool for distinguishing different thermal histories of meteoroids finally in some chondrites we observe xe produced by 244pu fission and calculate the time span between fission xe retention in these chondrites and that of the angra dos reis achondrite we find that the ordinary chondrites started to retain fission xe 48 30 ma earlier than angra dos reis we do not observe systematic differences between h l and ll or type 5 and 6 chondrites with respect to the time of fission xe retention

postshock annealing and postannealing shock in equilibrated ordinary chondrites implications for the thermal and shock histories of chondritic asteroids abstract in addition to shock effects in olivine plagioclase orthopyroxene and ca pyroxene petrographic shock indicators in equilibrated ordinary chondrites oc include chromite veinlets chromite plagioclase assemblages polycrystalline troilite metallic cu irregularly shaped troilite grains within metallic fe ni rapidly solidified metal sulfide intergrowths martensite and various types of plessite metal sulfide veins large metal and or sulfide nodules silicate melt veins silicate darkening low ca clinopyroxene silicate melt pockets and large regions of silicate melt the presence of some of these indicators in every petrologic type 4 to 6 ordinary chondrite demonstrates that collisional events caused all equilibrated oc to reach shock stages s3 s6 those type 4 to 6 oc that are classified as shock stage s1 on the basis of sharp optical extinction in olivine underwent postshock annealing due to burial beneath materials heated by the impact event those type 4 to 6 oc that are classified s2 on the basis of undulose extinction and lack of planar fractures in olivine were shocked to stage s3 s6 annealed to stage s1 and then shocked again to stage s2 some oc were probably shocked to stage s3 after annealing it seems likely that many oc experienced multiple episodes of shock and annealing because 40ar 39ar chronological data indicate that mil 99301 ll6 s1 was annealed 4 26 ga ago presumably as a consequence of a major impact it seems reasonable to suggest that other equilibrated s1 and s2 oc which contain relict shock features were also annealed by impacts because some type 6 s1 oc e g guarena kernouve portales valley all of which contain relict shock features were annealed 4 44 4 45 ga ago during a period when impacts were prevalent and most oc were thermally metamorphosed it follows that impact induced annealing could have contributed significantly to oc thermal metamorphism

noble gases in 21 saharan ll chondrites exposure ages and possible pairings nineteen ll chondrites and two l ll chondrites adrar 003 and tanezrouft 010 from the saharan desert of algeria and libya have been analysed for their he ne and ar composition as well as for their abundance of 84kr and 132xe calculated 21ne cosmic ray exposure ages vary between 2 and 35 ma the age distribution is consistent with that of modern ll chondrite falls except that no dominant peaks can be observed especially not the one related to a 15 ma collisional event however the lack of young exposure ages of 8 ma is obvious this is a characteristic feature of ll chondrites only one of the 21 ll chondrites namely acfer 066 contains solar gases and is thus considered a regolith breccia three specimens reveal considerable loss of 3he probably due to periods of elevated temperatures in orbits with small perihelion distances furthermore severe loss of 4he and 40ar is found in two samples considering possible pairings we suggest 14 individual falls basically on the basis of the noble gas data the petrographic sub classification and by taking the find location into consideration however there are constraints on confirming pairings solely on the basis of our studies thus we can only exclude individual samples with a unique noble gas fingerprint from paired specimens

the kumtag 016 l5 strewn field xinjiang province china the kumtag 016 strewn eld was found in the eastern part of the kumtag desert xinjiang province china in this study 24 recovered meteorites have been ch aracterized by a suite of different analytical techniques to investigate their petrography mineralogy bulk trace elements noble gas isotopic composition density and porosity we attribute to the strewn eld 22 l5 chondrites with shock stage s4 and weathering grade w2 w3 two differen t meteorites kumtag 021 an l4 chondrite and kumt ag 032 an l6 chondrite were recognized within the strewn eld area moreover kumt ag 003 an h5 chondrite was previously found in the same area we infer that the kumtag 016 strewn eld most likely consists of at least four distinct meteorite falls the effects of terrestrial weathering on the studied mete orites involve sul de metal alteration chemical changes sr ba pb and u enrichments and depletion in cr co ni and cs abun dances and physical modi cations decrease of grain density and porosity measurements of the light noble gases indicate that the analyzed kumtag l5 samples contain solar wind implanted noble gases with a ne ne ratio of 12 345 the cosmic ray exposure cre ages of the l5 ch ondrites are in a narrow range 3 6 1 4 ma to 5 2 0 4 ma for l4 cho ndrite kumtag 021 and l6 chondrite kumtag 032 the cre ages are 5 9 0 4 ma and 4 7 0 8 ma respectively

ararki l5 chondrite the first meteorite find in thar desert of india we report here a chance find of a meteorite in the sand dunes of ararki village of hanumangarh district in the rajasthan desert of northwest india chemical and petrological evidence in conjunction with isotopic composition of oxygen indicate that it is an l5 chondrite the fayalite content of olivines is 26 3 mol the meteorite has some serpentinized olivines and 0 3 carbon having a terrestrial isotopic composition indicating that it is moderately weathered the absence of 22 na indicate that the meteorite fell to earth more than a decade ago the cosmic ray exposure age based on cosmogenic 21 ne is 7 2 ma low density of cosmic ray heavy nuclei tracks low 26 al activity the shielding parameter 22 ne 21 ne c 1 094 and absence of neutron capture effects indicate cosmic ray shielding in a meteoroid having radius of about 16 cm implying a meteoroid mass of about 60 kg and ablation of about 93 the gas retention ages based on u th 4 he and k 40 ar are 1 1 and 0 58 ga respectively suggesting a heating and degassing event late in the history of this meteorite

mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates abstract petrographic mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from hot deserts and antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of fe ni metal producing fe oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins troilite weathers readily in hot desert environments but undergoes very little alteration under antarctic conditions most of the primary porosity of ordinary chondrites has been occluded by the time that 15 to 25 of the initial fe 0 and fe 2 has been oxidised to fe 3 in both environments results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products dilation of primary pores and brecciation which has been observed in parts of some meteorites will only occur if the meteorite is especially metal rich or has a low primary porosity these weathering products are absent from recent falls but have formed in a fall after 100 yr of museum storage cl bearing akaganeite and hibbingite are common weathering products in antarctic finds but occur in abundance in only one hot desert meteorite daraj 014 the majority of fe rich weathering products in meteorites from both environments contain low but variable concentrations of si mg and ca in most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a 1 0 nm lattice fringe spacing where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene in the heavily weathered antarctic finds alha 78045 and 77002 si is concentrated in cronstedtite a fe rich phyllosilicate an unidentified hydrous si fe ni mg mineral or gel has also partially replaced taenite in alha 78045 in addition to fe rich weathering products hot desert meteorites contain sulphates ca carbonate and silica whereas such minerals are largely absent from antarctic finds the abundance of silicate weathering products in antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments as preterrestrial cronstedtite is abundant in cm2 carbonaceous chondrites the antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites

effect of hot desert weathering on the bulk rock iron isotope composition of l6 and h5 ordinary chondrites abstract although iron isotopes are increasingly used for meteorites studies no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer we have thus conducted a petrographic chemical and iron isotopic study of equilibrated ordinary chondrites oc recovered from hot moroccan and algerian saharan deserts environment as previously noticed we observe that terrestrial desertic weathering is characterized by the oxidation of fe ni metal fe0 sulfide and fe2 occurring in olivine and pyroxene it produces fe oxides and oxyhydroxides that partially replace metal sulfide grains and also fill fractures the bulk chemical compositions of the ordinary chondrites studied show a strong sr and ba enrichment and a s depletion during weathering bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and s sr and ba contents most weathered chondrites display the heaviest isotopic composition by up to 0 1 which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies formation and evolution this is probably due to the release of isotopically light fe2 to waters on the earth s surface hence when subtle fe isotopic effects have to be studied in chondrites meteorites with weathering grade above w2 should be avoided

chemical alteration and ree mobilization in meteorites from hot and cold deserts the effects of terrestrial weathering on ree mobilization are evaluated for a variety of uncommon meteorites found in antarctica and in hot deserts the meteorites analyzed include 7 non cumulate eucrites 10 shergottites 3 nakhlites 2 lunar meteorites 4 angrites 10 acapulcoites 1 winonaite and 1 brachinite in situ concentration measurements of lanthanides and selected other minor and trace elements were made on individual grains by secondary ion mass spectrometry sims in antarctic meteorites oxidation converts ce 3 to ce 4 which is less soluble than the trivalent ree resulting in ce anomalies the mineral most affected is low ca pyroxene however not all grains of a given mineral are and distinct analyses of a single grain can even yield ree patterns with and without ce anomalies the effect is most pronounced for antarctic eucrites in which ce anomalies are observed not only in individual minerals but also in whole rock samples although ce anomalies are observed in meteorites from hot deserts as well the most characteristic signs of chemical alteration in this environment are a lree enrichment with a typical crustal signature as well as sr ba and u contaminations these can modify the whole rock ree patterns and disturb the isotope systematics used to date these objects the lree contamination is highly heterogeneous affecting some grains and not others of a given mineral mainly olivine and low ca pyroxene the two minerals with the lowest ree concentrations the major conduit for ree movement is through shock induced cracks and defects and the highest levels of contamination are found in altered material filling such veins and cracks meteorites that experienced low shock levels and those that are highly recrystallized are the least altered copyright 2003 elsevier ltd

structural and stratigraphic evolution of the western malvinas basin argentina the malvinas basin is one of the few basins on the argentine continental shelf that contains a proven petroleum system however uneconomical oil discoveries keep the basin at a frontier exploration status the malvinas basin evolved through three main tectonic phases rift sag and foredeep the sedimentary fill of the basin is closely related to its tectonic history middle jurassic rifting resulted in north northwest oriented grabens that filled with volcanic and pyroclastic continental rocks diminished faulting and generalized subsidence during the late jurassic neocomian early sag phase were accompanied by deposition of a basal transgressive marine wedge the aptian maastrichtian interval was characterized by tectonic quiescence and deposition of offshore mud prone sediments southerly localized early paleogene transtensional tectonism accompanied the early development of a foredeep trough outer shelf glauconite rich sandstones basinal claystones and localized carbonate buildups partially filled the basin by the middle eocene oligocene a strong deepening event marked the initiation of the foredeep sensu stricto phase this phase resulted in the full development of the malvinas foredeep and the formation of compressional structures in the foreland the foredeep basin was replenished by a westerly derived offlapping siliciclastic wedge of oligocene miocene age noncommercial hydrocarbon discoveries in 5 of the 17 wells drilled in the basin suggest the presence of an undercharged lower inoceramus springhill petroleum system

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

description of a very dense meteorite collection area in western atacama insight into the long term composition of the meteorite flux to earth we describe the geological morphological and climatic settings of two new meteorite collections from atacama chile the el m edano collection was recovered by systematic on foot search in el m edano and caleta el cobre dense collection areas and is composed of 213 meteorites before pairing 142 after pairing the private collection has been recovered by car by three private hunters and consists of 213 meteorites similar to other hot desert finds and contrary to the falls and antarctica finds both collections show an overabundance of h chondrites a recovery density can be calculated only for the el m edano collection and gives 251 and 168 meteorites larger than 10 g km a2 before and after pairing respectively it is by far the densest collection area described in hot deserts the atacama desert is known to have been hyperarid for a long period of time and based on cosmic ray exposure ages on the order of 1 10 ma to have been stable over a period of time of several million years such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism possibly related to downslope creeping or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2 myr this last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation with fragments scattered over a few meters of recovered meteorites

noble gases in the xinjiang armanty iron meteorite a big object with a short cosmic ray exposure age abstract we measured the concentrations and isotopic ratios of the cosmogenic noble gases he ne and ar in the very large iron meteorite xinjiang iiie the 3he and 4he data indicate that a significant portion of the cosmogenic produced helium has been lost via diffusion or in a recent impact event high 22ne 21ne ratios indicate that contributions to the cosmogenic 21ne from sulfur and or phosphorous are significant by combining the measured nuclide concentrations with model calculations for iron meteorites we were able to determine the preatmospheric diameter of xinjiang to 260 320 cm which corresponds to a total mass of about 70 135 tons the cosmic ray exposure age of xinjiang is 62 16 ma i e relatively short compared to most of the other iron meteorites with the current database we cannot firmly determine whether xinjiang experienced a complex irradiation history the finding of 3he and 4he losses might argue for a recent impact event and therefore for a complex exposure

the densest meteorite collection area in hot deserts the san juan meteorite field atacama desert chile we describe the geological morphological and climatic setting of the san juan meteorite collection area in the central depression of the atacama desert chile our recovery activities yielded 48 meteorites corresponding to a minimum of 36 different falls within a 3 88 km 2 area the recovery density is in the range 9 12 falls km 2 depending on pairing making it the densest among meteorite collection areas in hot deserts this high meteorite concentration is linked to the long standing hyperaridity of the area the stability of the surface pebbles ma and very low erosion rates of surface pebbles approximately 30 cm ma 1 maximum the san juan meteorite population is characterized by old terrestrial ages that range from zero to beyond 40 ka and limited weathering compared with other dense collection areas in hot desert chemical weathering in san juan is slow and mainly controlled by the initial porosity of meteorites as in the antarctic and other hot deserts there is an overabundance of h chondrites and a shortage of ll chondrites compared with the modern falls population suggesting a recent few ka change in the composition of the meteorite flux to earth

the fall recovery and classification of the park forest meteorite on the night of march 26 2003 a large meteorite broke up and fell upon the south suburbs of chicago the name park forest for the village that is at the center of the strewnfield has been approved by the nomenclature committee of the meteoritical society satellite data indicate that the bolide traveled from the southwest toward the northeast the strewnfield has a southeast northwest trend however this is probably due to the effects of strong westerly winds at high altitudes its very low 56co and very high 60co activities indicate that park forest had a preatmospheric mass that was at least 900 kg and could have been as large as 7 x 10 3 kg of which only 30 kg have been recovered the average compositions of olivine and low ca pyroxene fa24 7 1 1 and fs20 8 0 7 respectively and its bulk oxygen isotopic composition 18o 4 68 17o 3 44 show that park forest is an l chondrite the ferromagnesian minerals are well equilibrated chondrules are easily recognized and maskelynite is mostly 50 m across based on these observations we classify park forest as type 5 the meteorite has been strongly shocked and based on the presence of maskelynite mosaicism and planar deformation features in olivine undulatory extinction in pyroxene and glassy veins the shock stage is s5 the meteorite is a monomict breccia consisting of light colored angular to rounded clasts in a very dark host the light and dark lithologies have essentially identical mineral and oxygen isotopic compositions their striking difference in appearance is due to the presence of a fine pervasive network of sulfide veins in the dark lithology resulting in very short optical path lengths the dark lithology probably formed from the light lithology in an impact that formed a sulfide rich melt and injected it into cracks

the complex exposure history of the jiddat al harasis 073 l chondrite shower we measured the concentrations and isotopic compositions of he ne and ar in 29 bulk samples from 11 different strewn field fragments of the large jiddat al harasis jah 073 l6 chondrite shower including 7 samples from known locations within the main mass in addition we measured the concentrations of cosmogenic 10be 26al 36cl and 41ca in 10 samples all fragments of this shower are characterized by low 10be concentrations 7 6 12 8 dpm kg high 26al 10be ratios 3 55 large contributions of neutron capture 41ca 200 1800 dpm kgca low 3he 21ne ratios 1 5 3 0 large variations in cosmogenic 21ne 1 2 12 10 8 cm 3 stp g and significant contributions of neutron capture 36ar stepwise heating experiments show that neutron capture produced 36ar is predominantly released between 1000 1200 c all these results are consistent with a first stage exposure of 65 ma within 20 cm of the surface of the l chondrite parent body followed by ejection of a 1 52 m large object which was then delivered to earth within about 0 5 and 0 7 ma the cosmogenic nuclide data in jah 073 thus corroborate the trend that many of the large chondrites studied so far experienced a complex exposure history the observed 3he 21ne ratios of 2 53 0 in the most shielded samples including those of the main mass are lower than predicted by model calculations but similar to the lowest values found in the large gold basin l chondrite shower the bern plot which gives a linear correlation for 3he 21ne versus 22ne 21ne is evidently not valid for very high shielding some of our measured 22ne 21ne ratios in jah 073 are lower than 1 06 which is not well understood but might be explained by loss of cosmogenic neon from shocked sodium rich plagioclase during terrestrial weathering the amount of trapped atmospheric argon in the jah 073 fragments varies by almost two orders of magnitude and shows only a weak correlation with the size of the fragments which range from 50 kg finally low concentrations of radiogenic 4he and 40ar indicate incomplete degassing 1 ga ago probably at the main collision event on the l chondrite parent body 480 ma ago

the non trivial problem of meteorite pairing pairing is the procedure of identifying fragments of a single meteorite fall that were separated during atmospheric passage or during terrestrial history by establishing the similarity of two or more meteorite fragments we argue that pairing is governed by two principles that only a single mismatch of properties is required to refute a proposed pairing and that virtually all pairings bear some degree of uncertainty using data distributions for modern falls we take a probability approach to estimate degrees of certainty associated with proposed pairings emphasizing the importance of unusual features for new pairing criteria or new analytical additions to old criteria the degree of variation within individual meteorites must be delineated and the degree of variation within meteorite classes must be quantified criteria for pairing can be divided into 1 parent body history indicators 2 meteoroid space history indicators and 3 terrestrial history indicators included in these categories are 11 specific criteria including petrographic textures mineralogy and mineral composition terrestrial age estimates cosmic ray exposure ages and natural thermoluminescence tl levels not all criteria are applicable to all meteorite types about 2275 pairings suggested in the literature have been subjected to this analysis many literature pairings especially those involving common meteorite types bear large uncertainties due to lack of data

a shock metamorphic model for silicate darkening and compositionally variable plagioclase in ck and ordinary chondrites abstract silicate darkening in ordinary chondrites oc is caused by tiny grains of metallic fe ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or in some cases cut across silicate grain boundaries highly shocked oc characterized by olivine grains with undulose to mosaic extinction tend to have greater degrees of silicate darkening than lightly shocked oc this indicates that silicate darkening is probably a result of shock metamorphism the low fe fes eutectic temperature 988 c renders metal and troilite susceptible to melting and mobilization during shock heating a few oc also contain thin melt veins of chromite this implies that localized shock temperatures reached 1635 c in these meteorites silicate darkening is also evident in ck carbonaceous chondrites where magnetite and pentlandite grains form analogous curvilinear trails it is possible that magnetite and pentlandite in ck chondrites were mobilized during shock metamorphism and dispersed through silicate interiors alternatively metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites unshocked oc tend to have plagioclase with uniform compositions shocked oc tend to have plagioclase with more variable albeit still stoichiometric compositions the low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization this is consistent with the molten appearance of plagioclase in highly shocked oc e g rose city and paragould ck chondrites also have compositionally variable plagioclase the common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism some ck and oc chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites therefore the olivine in these chondrites may have been annealed after intense shock produced these effects a few ck chondrites that contain olivine with undulose or mosaic extinction e g lew87009 and eet83311 may have been shocked again after annealing

petrology chemistry and isotopic compositions of the lunar highland regolith breccia dar al gani 262 lunar meteorite dar al gani 262 dg 262 found in the libyan part of the sahara is a mature anorthositic regolith breccia with highland affinities the origin from the moon is undoubtedly indicated by its bulk chemical composition radionuclide concentrations noble gas n and o isotopic compositions and petrographic features dar al gani 262 is a typical anorthositic highland breccia similar in mineralogy and chemical composition to queen alexandra range que 93069 about 52 vol of the studied thin sections of dar al gani 262 consist of fine grained 100 m constituents and 48 vol is mineral and lithic clasts and impact melt veins the most abundant clast types are feldspathic fine grained to microporphyritic crystalline melt breccias 50 2 vol includes recrystallized melt breccias whereas mafic crystalline melt breccias are extremely rare 1 4 vol granulitic lithologies are 12 8 vol intragranularly recrystallized anorthosites and cataclastic anorthosites are 8 8 and 8 2 vol respectively and devitrified glasses are 2 7 vol impact melt veins 5 5 vol of the whole thin sections cutting across the entire thin section were probably formed subsequent to the lithification process of the bulk rock at pressures below 20 gpa because the bulk rock never experienced a higher peak shock pressure mafic crystalline melt breccias are very rare in dar al gani 262 and are similar in abundance to those in que 93069 the extremely low abundance of mafic components and the bulk composition may constrain possible areas of the moon from which the breccia was derived the source area of dar al gani 262 must be a highland terrain lacking significant mafic impact melts or mare components on the basis of radionuclide activities an irradiation position of dg 262 on the moon at a depth of 55 85 g cm3and a maximum transit time to earth 0 15 ma is suggested dar al gani 262 contains high concentrations of solar wind implanted noble gases the isotopic abundance ratio 40ar 36ar 3 is characteristic of lunar soils the terrestrial weathering of dg 262 is reflected by the occurrence of fractures filled with calcite and by high concentrations of ca ba cs br and as there is also a large amount of terrestrial c and some n in the sample which was released at low temperatures during stepped heating high concentrations of ni co and ir indicate a significant meteoritic component in the lunar surface regolith from which dg 262 was derived

petrology mineralogy porosity and cosmic ray exposure history of huaxi ordinary chondrite a meteorite fall was heard and collected on july 13 2010 at about 18 00 local time in the shibanjing village of the huaxi district of guiyang guizhou province china the total mass of the fall is estimated to be at least 1 6 kg some fragments are missing the meteorite consists mainly of olivine low ca pyroxene high ca pyroxene plagioclase kamacite taenite and troilite minor phases include chromite and apatite various textural types of chondrules exist in this meteorite most chondrule textures can be easily defined the grain sizes of secondary plagioclase in this meteorite range from 2 to 50 m the chemical composition of olivine and low ca pyroxene are uniform fa in olivine and fs in low ca pyroxene are respectively 19 6 0 2 and 17 0 0 3 mole huaxi has been classified as an h5 ordinary chondrite with a shock grade s2 and weathering w0 the weak shock features rare fractures and the high porosity 17 6 indicates that huaxi is a less compacted meteorite the preatmospheric radius of huaxi is 11 cm corresponding to 21 kg the meteorite experienced a relatively short cosmic ray exposure of about 1 6 0 1 ma the 4he and 40ar retention ages are older than 4 6 ga implying that huaxi did not degas after thermal metamorphism on its parent body

weathering of ordinary chondrites from oman correlation of weathering parameters with 14c terrestrial ages and a refined weathering scale we have investigated 128 14c dated ordinary chondrites from oman for macroscopically visible weathering parameters for thin section based weathering degrees and for chemical weathering parameters as analyzed with handheld x ray fluorescence these 128 14c dated meteorites show an abundance maximum of terrestrial age at 19 9 ka with a mean of 21 0 ka and a pronounced lack of samples between 0 and 10 ka the weathering degree is evaluated in thin section using a refined weathering scale based on the current w0 to w6 classification of wlotzka 1993 with five newly included intermediate steps resulting in a total of nine formerly six steps we find significant correlations between terrestrial ages and several macroscopic weathering parameters the correlation of various chemical parameters including sr and ba with terrestrial age is not very pronounced the microscopic weathering degree of metal and sulfides with newly added intermediate steps shows the best correlation with 14c terrestrial ages demonstrating the significance of the newly defined weathering steps we demonstrate that the observed 14c terrestrial age distribution can be modeled from the abundance of meteorites with different weathering degrees allowing the evaluation of an age frequency distribution for the whole meteorite population

feldspathic lunar meteorites and their implications for compositional remote sensing of the lunar surface and the composition of the lunar crust we present new compositional data for six feldspathic lunar meteorites two from cold deserts yamato 791197 and 82192 and four from hot deserts dhofar 025 northwest africa 482 and dar al gani 262 and 400 the concentrations of feo or al2o3 and th or any other incompatible element together provide first order compositional information about lunar polymict samples breccias and regoliths and regions of the lunar surface observed from orbit concentrations of both elements on the lunar surface have been determined from data acquired by orbiting spacecraft although the derived concentrations have large uncertainties and some systematic errors compared to sample data within the uncertainties and errors in the concentrations derived from orbital data the distribution of feo and th concentrations among lunar meteorites which represent 18 source regions on the lunar surface is consistent with that of 18 random samples from the surface approximately 11 of the lunar meteorites are low feo and low th breccias consistent with large regions of the lunar surface particularly the northern farside highlands almost all regoliths from apollo sites on the other hand have larger concentrations of both elements because they contain fe rich volcanic lithologies from the nearside maria and th rich lithologies from the high th anomaly in the northwestern nearside the feldspathic lunar meteorites thus offer our best estimate of the composition of the surface of the feldspathic highlands and we provide such an estimate based on the eight most well characterized feldspathic lunar meteorites the variable but high on average mg fe ratio of the feldspathic lunar meteorites compared to ferroan anorthosites confirms a hypothesis that much of the plagioclase at the surface of the feldspathic highlands is associated with high mg fe feldspathic rocks such as magnesian granulitic breccia not ferroan anorthosite geochemically the high mg fe breccias appear to be unrelated to the mafic magnesian suite rocks of the apollo collection models for the formation of the upper lunar crust as a simple flotation cumulate composed mainly of ferroan anorthosite do not account for the complexity of the crust as inferred from the feldspathic lunar meteorites

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise

noble gases and cosmogenic radionuclides in the gold basin l4 chondrite shower thermal history exposure history and pre atmospheric size we measured the concentrations of the cosmogenic radionuclides 10be 26al 36cl and 41ca in the stone and metal fractions of 15 fragments of the gold basin l4 chondrite shower as well as noble gases in 18 gold basin fragments a comparison of 10be 26al and 41ca concentrations with calculated production rates from two different models indicates that the gold basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3 5 m as was predicted by recent model calculations the noble gases show a reversal of the 22ne 21ne ratio at very high shielding the 21ne 10be and 21ne 26al ratios in most samples are constant and correspond to a 4 exposure age of 18 2 myr however three gold basin samples show a 30 120 excess of 21ne implying that they were previously exposed close to the surface of the parent body whereas the other samples were buried several meters deeper concentrations of neutron capture 36ar in most samples are consistent with measured concentrations of neutron capture 36cl and an exposure age of 18 myr large excesses of neutron capture 36ar were found in those samples with an excess of 21ne providing additional evidence of a first stage exposure on the parent body the excess of spallation produced 21ne and neutron capture produced 36ar in these samples indicate a first stage exposure of 35 150 myr on the parent body the radiogenic 4he and 40ar concentrations indicate a major impact on the parent body between 300 and 400 myr ago which must have preceded the impacts that brought the gold basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 myr ago

the jiddat al harasis 073 strewn field sultanate of oman the recently discovered jiddat al harasis jah 073 strewn field is the largest found so far in the sultanate of oman covering an area of 19 6 km the 3463 single stones collected range in weight from 52 2 kg down to 1 g total weight 600 8 kg and show a pronounced mass sorting the strewn field shape can be approximated by a nw se oriented ellipsoid indicating an atmospheric entry from se at a low angle relative to the surface the meteorite belongs to the l6 ordinary chondrite group and shows s4 average shock grade smaller stones generally show a higher weathering grade resulting in a spread from w2 and w4 enhanced weathering of the stones causing fragmentation after the fall is observed in sandy depressions five 14c measurements on stones of variable size and weathering grade yielded 14c from 3 8 to 49 9 dpm kg three samples give a 14c 10be age consistent with about 14 4 ka for two samples the cosmogenic trapped and radiogenic noble gases were measured the ratio of the 4he and 40ar gas retention ages of 0 29 0 10 and that of the 3he and 21ne cosmic ray exposure ages of 0 36 0 08 ma indicate that jah 073 experienced a complex exposure history and lost 4he and 40ar due to a major collision fragmentation statistics indicate a single major atmospheric disruption and an originally relatively spherical shape of the object assuming the material collected represents the majority of fallen mass and 90 99 of the original weight was lost by ablation the pre atmospheric minimum radius of the meteoroid with density 3 4 g cm 3 would have been at least 75 cm