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1.
Abstract— Cosmic‐ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide 81Kr and stable Kr isotopes for seven martian meteorites. The following 81Kr‐Kr CRE ages were obtained: Los Angeles = 3.35 ± 0.70 Ma; Queen Alexandra Range 94201 = 2.22 ± 0.35 Ma; Shergotty = 3.05 ± 0.50 Ma; Zagami = 2.98 ± 0.30 Ma; Nakhla = 10.8 ± 0.8 Ma; Chassigny = 10.6 ± 2.0 Ma; and Allan Hills 84001 = 15.4 ± 5.0 Ma. Comparison of these ages with previously obtained CRE ages from the stable noble gas nuclei 3He, 21Ne, and 38Ar shows excellent agreement. This indicates that the method for the production rate calculation for the stable nuclei is reliable. In all martian meteorites we observe effects induced by secondary cosmic‐ray produced epithermal neutrons. Epithermal neutron fluxes, φn (30–300 eV), are calculated based on the reaction 79Br(n, γβ)80Kr. We show that the neutron capture effects were induced in free space during Mars‐Earth transfer of the meteoroids and that they are not due to a pre‐exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre‐atmospheric sizes are estimated. We obtain minimum radii in the range of 22–25 cm and minimum masses of 150–220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.  相似文献   

2.
Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio 20Ne/22Ne of 0.80 ± 0.03 and a trapped solar 4He/3He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped 20Ne and 36Ar concentrations in the range of 10?6–10?9 cm3 STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced 3He and of radiogenic 4He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a 20Ne/22Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.  相似文献   

3.
Abstract— Acapulcoites and lodranites are believed to originate on a common parent body and to represent some of the earliest events in the differentiation of the chondritic asteroids. We have conducted isotopic studies of the noble gases He, Ne, Ar, Kr, and Xe, and determinations of the concentrations of the major elements and of the radionuclides 10Be, 26Al, and 36Cl in an attempt to constrain the cosmic‐ray exposure history of two members of the acapulcoite‐lodranite clan recovered in Antarctica: Frontier Mountain (FRO) 95029 and Graves Nunataks (GRA) 95209. From cosmic‐ray‐produced 3He, 21Ne, and 38Ar and appropriate production rates, we derive parent‐body breakup times of 4.59 ± 0.60 and 6.82 ± 0.60 Ma for FOR 95029 and GRA 95209, respectively. These times are consistent with those obtained from the pairs 10Be‐21Ne and 26Al‐21Ne; whereas the times inferred from the pair 36Cl‐36Ar are slightly longer, perhaps because the 36Cl activities decreased as a result of decay on Earth. Terrestrial ages up to ～50 ka for the two meteorites are consistent with the measured 36Cl activities of the metal phases. All acapulcoites and lodranites dated until now show cosmic‐ray exposure ages in the range of 4–10 Ma. This is the same range as that found for the major exposure age cluster of the H chondrites. As a common parent body is improbable on the basis of the O‐isotopic systematics, a common set of impactors might have affected the asteroid belt 4–10 Ma ago.  相似文献   

4.
Abstract— We derived the cosmic‐ray and solar particle exposure history for the two lunar meteorites Elephant Moraine (EET) 96008 and Dar al Gani (DaG) 262 on the basis of the noble gas isotopic abundances including the radionuclide 81Kr. For EET 96008, we propose a model for the exposure to cosmic rays and solar particles in three stages on the Moon: an early stage ～500 Ma ago, lasting less than 9 Ma at a shallow shielding depth of 20 g/cm2, followed by a stage when the material was buried, without exposure, until it was exposed in a recent stage. This recent stage, at a shielding depth in a range of 200–600 g/cm2, lasted for ～26 Ma until ejection. This model is essentially the same as that previously found for lunar meteorite EET 87521; thus, pairing of the two Elephant Moraine lunar meteorites that were recovered on the same icefield in Antarctica is confirmed by our data. The cosmic‐ray‐produced isotopes, the trapped solar and lunar atmospheric noble gases, as well as the radionuclide 81Kr observed for the DaG 262 lunar meteorite are consistent with a one‐stage lunar exposure history. The average burial depth of the Dar al Gani material before ejection was within a range of 50–80 g/cm2. The exposure to cosmic rays at this depth lasted 500–1000 Ma. This long residence time for Dar al Gani at relatively shallow depth explains the high concentrations of implanted solar noble gases.  相似文献   

5.
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6.
Abstract— We determined the cosmic-ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of 3He, 21Ne and 38Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic 22Ne/21Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured 10Be and 26AI concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages cluster at 21–25 Ma and four at 35–42 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20–25 Ma and 10 at 35–42 Ma. The shape of the two peaks is consistent with single impact events, and random number statistics show that they are statistically significant at the 99% level. Altogether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to explain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2–4 km in diameter for Vesta and of 60–300 m for the 100× smaller Vesta-derived “vestoids.” Based on the HED exposure-age distribution, the size distribution of the main-belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta.  相似文献   

7.
Abstract— We investigated the characteristics and history of lunar meteorites Queen Alexandra Range 93069, Yamato 793169 and Asuka 881757 based on the abundances of all stable noble gas isotopes, the concentrations of the radionuclides 10Be, 26Al, 36Cl, and 81Kr, and the abundances of Mg, Al, K, Ca, Fe, Cl, Sr, Y, Zr, Ba, and La. Based on the solar wind and cosmic-ray irradiations, QUE 93069 is the most mature lunar meteorite studied up to now. The 40Ar/36Ar ratio of the trapped component is 1.87 ± 0.16. This ratio corresponds to a time when the material was exposed to solar and lunar atmospheric volatiles ～400 Ma ago. On the other hand, Yamato 793169 and Asuka 881757 contain very little or no solar noble gases, which indicates that these materials resided in the top layer of the lunar regolith only briefly or not at all. For all lunar meteorites, we observe a positive correlation of the concentrations of cosmic-ray produced with trapped solar noble gases. The duration of lunar regolith residence for the lunar meteorites was calculated based on cosmic-ray produced 21Ne, 38Ar, 78Kr, 83Kr, and 126Xe and appropriate production rates that were derived based on the target element abundances and the shielding indicator 131Xe/126Xe. For QUE 93069, Yamato 793169, and Asuka 881757, we obtained 1000 ± 400 Ma, 50 ± 10 Ma, and <1 Ma, respectively. Both Asuka 881757 and Yamato 793169 show losses of radiogenic 4He from U and Th decay and Yamato 793169 also 40Ar loss from K-decay. For Asuka 881757, we calculate a K-Ar gas retention age of 3100 ± 600 Ma and a 244Pu-136Xe fission age of 4240 ± 170 Ma. This age is one of the oldest formation ages ever observed for a lunar basalt. The exposure history of QUE 93069 after ejection from the Moon was derived from the radionuclide concentrations: ejection 0.16 ± 0.03 Ma ago, duration of Moon-Earth transit 0.15 ± 0.02 Ma and fall on Earth <0.015 Ma ago. This ejection event is distinguished temporally from those which produced the other lunar meteorites. We conclude that six to eight events are necessary to eject all the known lunar meteorites.  相似文献   

8.
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.  相似文献   

9.
Abstract– We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF/HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF/HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for 36Ar, 58 ± 12% for 84Kr, and 48 ± 10% for 132Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF/HCl residue indicate that there is no Xe‐HL in Saratov, but Ne isotopic ratios in the HF/HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne‐Q and Ne‐HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne‐HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne‐Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne‐Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the 128Xe/132Xe ratio. The 3He and 21Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.  相似文献   

10.
Abstract— We measured the noble gas isotopic abundances in lunar meteorite QUE 94269 and in bulk-, glass-, and crystal-phases of lunar meteorite QUE 94281. Our results confirm that QUE 94269 originated from the same meteorite fall as QUE 93069: both specimens yield the same signature of solar-particle irradiation and also the cosmogenic noble gases are in agreement within their uncertainities. Queen Alexandra Range 93069/94269 was exposed to cosmic rays in the lunar regolith for ～1000 Ma, and it trapped 3.5 × 10?4 cm3STP/g solar 36Ar, the other solar noble gases being present in proportions typical for the solar-particle irradiation. The bulk material of QUE 94281 contains about three times less cosmogenic and trapped noble gases than QUE 93069/94269 and the lunar regolith residence time corresponds to 400 ± 60 Ma. We show that in lunar meteorites the trapped solar 20Ne/22Ne ratio is correlated with the trapped ratio 40Ar/36Ar, that is, trapped 20Ne/22Ne may also serve as an antiquity indicator. The upper limits of the breccia compaction ages, as derived from the trapped ratio 40Ar/36Ar for QUE 93069/94269 and QUE 94281 are ～400 Ma and 800 Ma, respectively. We found very different regolith histories for the glass phase and the crystals separated from QUE 94281. The glass phase contains much less cosmogenic and solar noble gases than the crystals, in contrast to the glasses of lunar meteorite EET 87521, that were enriched in noble gases relative to the crystalline material. The QUE 94281 phases yield a 40K-40Ar gas retention age of 3770 Ma, which is in the range of that for lunar mare rocks.  相似文献   

11.
Renazzo‐type carbonaceous (CR) chondrites are accretionary breccias that formed last. As such they are ideal samples to study precompaction exposures to cosmic rays. Here, we present noble gas data for 24 chondrules and 3 dark inclusion samples (DIs) from Shi?r 033 (CR2). The meteorite was selected based on the absence of implanted solar wind noble gases and an anomalous oxygen isotopic composition of the DIs; the oxygen isotopes match those in CV3 and CO3 chondrites. Our samples contain variable mixtures of galactic cosmic ray (GCR)‐produced cosmogenic noble gases and trapped noble gases of presolar origin. Remarkably, all chondrules have cosmogenic 3He and 21Ne concentrations up to 4.3 and 7.1 times higher than the DIs, respectively. We derived an average 3He‐21Ne cosmic ray exposure (CRE) age for Shi?r 033 of 2.03 ± 0.20 Ma (2 SD) and excesses in cosmogenic 3He and 21Ne in chondrules (relative to the DIs) in the range (in 10?8 cm3STP/g) 3.99–7.76 and 0.94–1.71, respectively. Assuming present‐day GCR flux density, the excesses translate into average precompaction 3He‐21Ne CRE ages of 3.1–27.3 Ma depending on the exposure geometry. The data can be interpreted assuming a protracted storage of a single chondrule generation prior to the final assembly of the Shi?r 033 parent body in a region of the disk transparent to GCRs.  相似文献   

12.
Abstract— Meteoritical Bulletin, No. 81 lists 181 new meteorites. Noteworthy among these are a new lunar meteorite (Dar al Gani 262), four observed falls (Dong Ujimqin Qi, Galkiv, Mount Tazerzait, and Piplia Kalan), four irons (Albion, Great Sand Sea 003, Hot Springs, and Mont Dieu), two mesosiderites (Dong Ujimqin Qi and Lamont), an acapulcoite (FRO 95029), a eucrite (Piplia Kalan), two probably-paired ureilites (Dar al Gani 164 and 165), an R chondrite (Hammadah al Hamra 119), an ungrouped type-3 chondrite (Hammadah al Hamra 180), a highly unequilibrated ordinary chondrite (Wells, LL3.3), and a variety of carbonaceous and unequilibrated ordinary chondrites from Libya and Antarctica. All italicized abbreviations refer to addresses listed in the appendix.  相似文献   

13.
The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock‐melted veins are present. The U,Th‐He age is 0.7 Ga, and the K‐Ar age is 1.8 Ga, indicating that Mifflin might have been heated at the time of the 470 Ma L‐chondrite parent body breakup and that U, Th‐He, and K‐Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 30–65 cm. Assuming this exposure geometry, a cosmic‐ray exposure age of 25 ± 3 Ma is calculated from cosmogenic noble gas concentrations. The low 22Ne/21Ne ratio may, however, indicate a two‐stage exposure with a longer first‐stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.  相似文献   

14.
Abstract— We report noble gas, oxygen isotope, 14C and 10Be data of Itqiy as well as noble gas, 14C and 10Be results for Zak?odzie. Both samples have been recently classified as anomalous enstatite meteorites and have been compared in terms of their mineralogy and chemical composition. The composition of enstatite and kamacite and the occurrence of specific sulfide phases in Itqiy indicate it formed under similar reducing conditions to those postulated for enstatite chondrites. The new results now seem to point at a direct spatial link. The noble gas record of Itqiy exhibits the presence of a trapped subsolar component, which is diagnostic for petrologic types 4–6 among enstatite chondrites. The concentration of radiogenic 4He is very low in Itqiy and indicates a recent thermal event. Its 21Ne cosmic‐ray exposure age is 30.1 ± 3.0 Ma and matches the most common age range of enstatite chondrites (mostly EL6 chondrites) but not that of Zak?odzie. Itqiy's isotopic composition of oxygen is in good agreement with that observed in Zak?odzie as well as those found in enstatite meteorites suggesting an origin from a common oxygen pool. The noble gas results, on the other hand, give reason to believe that the origin and evolution of Itqiy and Zak?odzie are not directly connected. Itqiy's terrestrial age of 5800 ± 500 years sheds crucial light on the uncertain circumstances of its recovery and proves that Itqiy is not a modern fall, whereas the 14C results from Zak?odzie suggest it hit Earth only recently.  相似文献   

15.
Abstract— The isotopic composition and concentrations of noble gases were measured in the eucrites Bereba, Cachari, Caldera, Camel Donga, Chervony Kut, Ibitira, Jonzac, Juvinas, Millbillillie, Moore County, Padvarninkai, Pasamonte, Pomozdino, Serra de Magé, Sioux County, and Vetluga. The distribution of 81Kr-Kr exposure ages shows “clusters” at (7 ± 1) Ma, (10 ± 1) Ma, (14 ± 1) Ma, (22 ± 2) Ma, and (37 ± 1) Ma that agree with those for howardites, eucrites, and diogenites (HED) at (6 ± 1) Ma, (12 ± 2) Ma, (21 ± 4) Ma, and (38 ± 8) Ma. This most likely indicates a common origin of HED meteorites. Correlation equations for the shielding-sensitive cosmogenic ratios 78Kr/83Kr, 80Kr/83Kr, 82Kr/83Kr, and 124Xe/131Xe were obtained. Comparison with data from simulation experiments suggests that most eucrites were exposed to the cosmic radiation as somewhat large meteoroids with diameters of ～1 m or more. The shielding-dependence of the 78Kr and 126Xe production rates was found to be small, with a few exceptions the variations aren <10%–15%. Concentrations of spallogenic 3He indicate diffusive losses of up to 70% that can be, in first approximation, described by a model of quasi-continuous losses during the exposure to the cosmic radiation with a loss rate of the order of ～3 × 10?8 a?1. Radiogenic 4He shows additional substantial losses that occurred at the time of, or prior to, the separation of the meteoroids from their parent body. Typical 40Ar retention in eucrites is 50%–60% which corresponds to a 40Ar-K retention age of 3.4–3.6 Ga. In all analyzed unbrecciated eucrites, the retention is distinctly larger (70%–100%). The 244Pu fission ratio (86Kr/136Xe)Pu, was evaluated from the data on Pomozdino samples to be 0.039 ± 0.014.  相似文献   

16.
We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine‐phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (21Ne/22Ne)cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray‐produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray‐produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (21Ne/22Ne)cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina‐like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.  相似文献   

17.
Abstract— Concentration and isotopic composition of the light noble gases as well as of 84Kr, 129Xe, and 132Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high 129Xe/132Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic‐ray exposure ages are calculated from cosmogenic 21Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ～9 and ～29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.  相似文献   

18.
The Kumtag 016 strewn field was found in the eastern part of the Kumtag desert, Xinjiang Province, China. In this study, 24 recovered meteorites have been characterized 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 field 22 L5 chondrites with shock stage S4 and weathering grade W2–W3. Two different meteorites, Kumtag 021, an L4 chondrite and Kumtag 032, an L6 chondrite, were recognized within the strewn field area. Moreover, Kumtag 003, an H5 chondrite, was previously found in the same area. We infer that the Kumtag 016 strewn field most likely consists of at least four distinct meteorite falls. The effects of terrestrial weathering on the studied meteorites involve sulfide/metal alteration, chemical changes (Sr, Ba, Pb, and U enrichments and depletion in Cr, Co, Ni, and Cs abundances), and physical modifications (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 20Ne/22Ne ratio of ~12.345. The cosmic-ray exposure (CRE) ages of the L5 chondrites are in a narrow range (3.6 ± 1.4 Ma to 5.2 ± 0.4 Ma). For L4 chondrite Kumtag 021 and L6 chondrite Kumtag 032, the CRE ages are 5.9 ± 0.4 Ma and 4.7 ± 0.8 Ma, respectively.  相似文献   

19.
The Park Forest (L5) meteorite fell in a suburb of Chicago, Illinois (USA) on March 26, 2003. It is one of the currently 25 meteorites for which photographic documentation of the fireball enabled the reconstruction of the meteoroid orbit. The combination of orbits with pre‐atmospheric sizes, cosmic‐ray exposure (CRE), and radiogenic gas retention ages (“cosmic histories”) is significant because they can be used to constrain the meteoroid's “birth region,” and test models of meteoroid delivery. Using He, Ne, Ar, 10Be, and 26Al, as well as a dynamical model, we show that the Park Forest meteoroid had a pre‐atmospheric size close to 180 g cm?2, 0–40% porosity, and a pre‐atmospheric mass range of ~2–6 tons. It has a CRE age of 14 ± 2 Ma, and (U, Th)‐He and K‐Ar ages of 430 ± 90 and 490 ± 70 Ma, respectively. Of the meteorites with photographic orbits, Park Forest is the second (after Novato) that was shocked during the L chondrite parent body (LCPB) break‐up event approximately 470 Ma ago. The suggested association of this event with the formation of the Gefion family of asteroids has recently been challenged and we suggest the Ino family as a potential alternative source for the shocked L chondrites. The location of the LCPB break‐up event close to the 5:2 resonance also allows us to put some constraints on the possible orbital migration paths of the Park Forest meteoroid.  相似文献   

20.
We analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment‐dispersed extraterrestrial chromite (SEC) grains from mid‐Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. 2010 ). Ghubara, an L chondritic regolith breccia with an Ar‐Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic‐ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic‐ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last‐stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre‐exposure as well. The shorter last‐stage exposure age probably links Ghubara to a small peak of 40Ar‐poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne‐HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.  相似文献   

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