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1.
Abstract— We present a purely physical model for the calculation of depth‐ and size‐dependent production rates of cosmogenic nuclides by galactic cosmic‐ray (GCR) particles. besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on only one free parameter—the integral number of gcr particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth‐ and size‐dependent production rates for 10Be, 14C, 26Al, 36Cl, and 53Mn as well as for the rare gas isotopes 3He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. The new data differ from semi‐empirical estimates by up to a factor of 4 but agree within ～20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter 22Ne/21Ne and the correlations 26Al vs. 10Be, 26Al vs. 53Mn, 10Be/21Ne vs. 22Ne/21Ne, and 36Ar vs. 36Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.  相似文献   

2.
Neon produced by solar cosmic rays in ordinary chondrites   总被引：1，自引：0，他引：1       下载免费PDF全文
Solar‐cosmic‐ray‐produced Ne (SCR‐Ne), in the form of low cosmogenic 21Ne/22Ne ratios (21Ne/22Necos <0.8), is more likely to be found in rare meteorite classes, like Martian meteorites, than in ordinary chondrites. This may be the result of a sampling bias: SCR‐Ne is better preserved in meteorites with small preatmospheric radii and these specimens are often only studied if they belong to unusual or rare classes. We measured He and Ne isotopic concentrations and nuclear tracks in 25 small unpaired ordinary chondrites from Oman. Most chondrites have been intensively heated during atmospheric entry as evidenced by the disturbed track records, the low 3He/21Ne ratios, the low 4He concentrations, and the high peak release temperatures. Concentration depth profiles indicate significant degassing; however, the Ne isotopes are mainly undisturbed. Remarkably, six chondrites have low 21Ne/22Necos in the range 0.711–0.805. Using a new physical model for the calculation of SCR production rates, we show that four of the chondrites contain up to ~20% of SCR‐Ne; they are analyzed in terms of preatmospheric sizes, cosmic ray exposure ages, mass ablation losses, and orbits. We conclude that SCR‐Ne is preserved, regardless of the meteorite class, in specimens with small preatmospheric radii. Sampling bias explains the predominance of SCR‐Ne in rare meteorites, although we cannot exclude that SCR‐Ne is more common in Martian meteorites than it is in small ordinary chondrites.  相似文献   

3.
Abstract— Several solar gas rich lunar soils and breccias have trapped 40Ar/36Ar ratios >10, although solar Ar is expected to yield a ratio of <0.01. Radiogenic 40Ar produced in the lunar crust from 40K decay was outgassed into the lunar atmosphere, ionized, accelerated in the electromagnetic field of the solar wind, and reimplanted into lunar surface material. The 40Ar loss rate depends on the decreasing abundance of 40K. In order to calibrate the time dependence of the 40Ar/36Ar ratio in lunar surface material, the period of reimplantation of lunar atmospheric ions and of solar wind Ar was determined using the 235U‐136Xe dating method that relies on secondary cosmic‐ray neutron‐induced fission of 235U. We identified the trapped, fissiogenic, and cosmogenic noble gases in lunar breccia 14307 and lunar soils 70001‐8, 70181, 74261, and 75081. Uranium and Th concentrations were determined in the 74261 soil for which we obtain the 235U‐136Xe time of implantation of 3.25+0.38‐0.60 Ga ago. On the basis of several cosmogenic noble gas signatures we calculate the duration of this near surface exposure of 393 ± 45 Ma and an average shielding depth below the lunar surface of 73 ± 7 g/cm2. A second, recent exposure to solar and cosmic‐ray particles occurred after this soil was excavated from Shorty crater 17.2 ± 1.4 Ma ago. Using a compilation of all lunar data with reliable trapped Ar isotopic ratios and pre‐exposure times we infer a calibration curve of implantation times, based on the trapped40 Ar/36Ar ratio. A possible trend for the increase with time of the solar 3He/4He and 20Ne/22Ne ratios of about 12%/Ga and about 2%/Ga, respectively, is also discussed.  相似文献   

4.
We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides 10Be, 26Al, and 36Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic 36Cl and 36Ar measured in the metal separates, we determined 36Cl‐36Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation 3He/21Ne versus 22Ne/21Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation 3He/21Ne versus 22Ne/21Ne breaks down at high shielding. Using 36Cl‐36Ar exposure ages and measured noble gas concentrations, we determine 21Ne and 38Ar production rates as a function of 22Ne/21Ne. The new data agree with recent model calculations for the relationship between 21Ne and 38Ar production rates and the 22Ne/21Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for 21Ne and 38Ar production rates as a function of the shielding indicator 22Ne/21Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the 10Be/21Ne and 26Al/21Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.  相似文献   

5.
Abstract— We present the concentrations and isotopic compositions of He, Ne, and Ar for nonmagnetic fractions and bulk samples of 17 H chondrites which were recently investigated for their 36Cl‐36Ar cosmic‐ray exposure ages (Graf et al., 2001). All selected meteorites are observed falls with cosmic‐ray exposure ages close to the 7 Ma peak. The rare gas data are consistent with 10Be and 36C1 production rates in the metal phase. Remarkably, only 1 out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and 36Cl‐36Ar exposure ages, 21Ne production rates as a function of 22Ne/21 Ne and a mean 38Ar production rate are determined. The results confirm model calculations which predict that the relationship between 21Ne production rates and 22Ne/21Ne is ambiguous for high shielding. Besides the mean 38Ar production rate we also give production rate ratios P(38Ar from Ca)/P(38Ar from Fe). They vary between 10 and 77, showing no significant correlation with 38Ar concentrations or 22Ne/21Ne. By investigating the metal separates, Graf et al. (2001) found significant 3He deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a 3He/21Ne vs. 22Ne/21Ne diagram plot in the area defined by most of the H chondrites. This means that 3He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that 3H diffusive losses in meteorites should be the rule rather than the exception. The 21Ne exposure ages, calculated on the basis of modeled 21Ne production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low 3He/38Ar in the metal formed in a separate event than those with normal 3He/38Ar ratios. The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H chondrites is due to at least two events about 7.0 and 7.6 Ma ago.  相似文献   

6.
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   

7.
Abstract— 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 22Na indicate that the meteorite fell to Earth more than a decade ago. The cosmic‐ray exposure age based on cosmogenic 21Ne is 7.2 Ma. Low density of cosmic‐ray heavy nuclei tracks, low 26A1 activity, the shielding parameter [(22Ne/21Ne)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‐4He and K‐40Ar are 1.1 and 0.58 Ga, respectively, suggesting a heating and degassing event late in the history of this meteorite.  相似文献   

8.
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   

9.
Abstract— We re‐evaluated the cosmic‐ray exposure history of the H3‐6 chondrite shower Frontier Mountain (FRO) 90174, which previously was reported to have a simple exposure history, an irradiation time of about 7 Ma, and a pre‐atmospheric radius of 80–100 cm (Welten et al. 2001). Here we measured the concentrations and isotopic compositions of He, Ne, and Ar in 8 aliquots of 6 additional fragments of this shower, and 10Be and 26Al in the stone fractions of seven fragments. The radionuclide concentrations in the stone fractions, combined with those in the metal fractions, confirm that all samples are fragments of the FRO 90174 shower. Four of the fragments contain solarwind‐implanted noble gases with a solar 20Ne/22Ne ratio of ?12.0, indicating that FRO 90174 is a regolith breccia. The concentrations of solar gases and cosmogenic 21Ne in the samples analyzed by us and by Welten et al. (2001) overlap with those of the FRO H‐chondrites from the 1984 season, suggesting that many of these samples are also part of the large FRO 90174 chondrite shower. The cosmogenic 21Ne concentrations in FRO 90174 show no simple correlation with 10Be and 26Al activities. We found 21Ne excesses between 0.3‐1.1 × 10?8cm3STP/g in 6 of the 17 samples. Since excess 21Ne and trapped solar gases are not homogeneously distributed, i.e., we found in one fragment aliquots with and without excess 21Ne and solar 20Ne, we conclude that excess 21Ne is due to GCR irradiation of the regolith before compaction of the FRO 90174 object. Therefore, the chondrite shower FRO 90174 did not simply experience an exposure history, but some material was already irradiated at the surface of an asteroid leading to excess 21Ne. This excess 21Ne is correlated to implanted solar gases, clearly indicating that both processes occurred on the regolith.  相似文献   

10.
Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The 39Ar‐40Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of 40Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic 3He, 21Ne, and 38Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic 21Ne and 38Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of 36Ar in the halite were produced during regolith exposure by neutron capture on 35Cl, followed by decay to 36Ar. The thermal neutron fluence seen by the halite was (2–4) × 1014 n/cm2. The thermal neutron flux during regolith exposure was ～0.4‐0.7 n/cm2/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the 36Arn/21Necos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded.  相似文献   

11.
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   

12.
Abstract— From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large‐scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser‐gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X‐ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined. AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with 4He concentrations and reflect the effect of degassing during atmospheric entry. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long‐time storage in Antarctic ice. Jarosite‐bearing AMMs have relatively low concentrations of 4He, which is suggestive of loss during the alteration. High 3He/4He ratios are detected for AMMs with high 20Ne/4He ratios, showing both cosmogenic 3He and preferential He loss. SEP (solar energetic particles)‐He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean 20Ne/22Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic 21Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3‐component mixing of air, Q, and SEP‐Ar. Ar isotopic compositions can not be explained without significant contributions of Q‐Ar. SEP‐Ne contributed more than 99% of the total Ne. As for 36Ar and 38Ar, the abundance of the Q component is comparable to that of the SEP component. 84Kr and 132Xe are dominated by the primordial component, and solar‐derived Xe is almost negligible.  相似文献   

13.
Abstract– Bunburra Rockhole is the first meteorite fall photographed and recovered by the Desert Fireball Network in Australia. It is classified as an ungrouped achondrite similar in mineralogical and chemical composition to eucrites, but it has a distinct oxygen isotope composition. The question is if achondrites like Bunburra Rockhole originate from the same parent body as the howardite‐eucrite‐diogenite (HED) meteorites or from several separate, differentiated parent bodies. To address this question, we measured cosmogenic radionuclides and noble gases in the Bunburra Rockhole achondrite. The short‐lived radionuclides 22Na and 54Mn confirm that Bunburra Rockhole is a recent fall. The concentrations of 10Be, 26Al and 36Cl as well as the 22Ne/21Ne ratio indicate that Bunburra Rockhole was a relatively small object (R approximately 15 cm) in space, consistent with the photographic fireball observations. The cosmogenic 38Ar concentration yields a cosmic‐ray exposure (CRE) age of 22 ± 3 Myr, whereas 21Ne and 3He yield approximately 30% and approximately 60% lower ages, respectively, due to loss of cosmogenic He and Ne, mainly from plagioclase. With a CRE age of 22 Myr, Bunburra Rockhole is the first anomalous eucrite that overlaps with the main CRE peak of the HED meteorites. The radiogenic K‐Ar age of 4.1 Gyr is consistent with the U‐Pb age, while the young U,Th‐He age of approximately 1.4 Gyr indicates that Bunburra Rockhole lost radiogenic 4He more recently.  相似文献   

14.
Abstract— The noble gases He, Ne, Ar, Kr, and Xe were measured in 27 individual Antarctic micrometeorites (AMMs) in the size range 60 to 250 μm that were collected at the Dome Fuji Station. Eleven of the AMMs were collected in 1996 (F96 series) and 16 were collected in 1997 (F97 series). One of the F97 AMMs is a totally melted spherule, whereas all other particles are irregular in shape. Noble gases were extracted using a Nd‐YAG continuous wave laser with an output power of 2.5‐3.5 W for ?5 min. Most particles released measurable amounts of noble gases. 3He/4He ratios are determined for 26 AMMs ((0.85‐9.65) × 10?4). Solar energetic particles (SEP) are the dominant source of helium in most AMMs rather than solar wind (SW) and cosmogenic He. Three samples had higher 3He/4He ratios compared to that of SW, showing the presence of spallogenic 3He. The Ne isotopic composition of most AMMs resembled that of SEP as in the case of helium. Spallogenic 21Ne was detected in three samples, two of which had extremely long cosmic‐ray exposure ages (> 100 Ma), calculated by assuming solar cosmic‐ray (SCR) + galactic cosmic‐ray (GCR) production. These two particles may have come to Earth directly from the Kuiper Belt. Most AMMs had negligible amounts of cosmogenic 21 Ne and exposure ages of <1 Ma. 40Ar/36Ar ratios for all particles (3.9–289) were lower than that of the terrestrial atmosphere (296), indicating an extraterrestrial origin of part of the Ar with a very low 40Ar/36Ar ratio plus some atmospheric contamination. Indeed, 40Ar/36Ar ratios for the AMMs are higher than SW, SEP, and Q‐Ar values, which is explained by the presence of atmospheric 40Ar. The average 38Ar/36Ar ratio of 24 AMMs (0.194) is slightly higher than the value of atmospheric or Q‐Ar, suggesting the presence of SEP‐Ar which has a relatively high 38Ar/36Ar ratio. According to the elemental compositions of the heavy noble gases, Dome Fuji AMMs can be classified into three groups: chondritic (eight particles), air‐affected (nine particles), and solar‐affected (eight particles). The eight AMMs classified as chondritic preserve the heavy noble gas composition of primordial trapped component due to lack of atmospheric adsorption and solar implantation. The average of 129Xe/132Xe ratio for the 16 AMMs not affected by atmospheric contamination (1.05) corresponds to the values in matrices of carbonaceous chondrites (?1.04). One AMM, F96DK038, has high 129Xe/132Xe in excess of this ratio. Our results imply that most Dome Fuji AMMs originally had chondritic heavy noble gas compositions, and carbonaceous chondrite‐like objects are appropriate candidate sources for most AMMs.  相似文献   

15.
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   

16.
Abstract— The compositionally typical H5 chondrite St‐Robert has an exposure age, 7.8 Ma, indistinguishable from that of the main cluster of H chondrites. Small values of the cosmogenic 22Ne/21Ne ratio in interior samples imply a pre‐atmospheric radius on the order of 40 cm. Sample depths based on tracks and the production rates of Bhattacharya et al. (1973) range from 6 to ～40 cm and are generally larger than depths estimated from published 60Co activities, perhaps because the track production rates adopted are too high. Depth profiles of the production rates of 14C, 36Cl, 26Al, 10Be, and 21Ne in stony material show increases with depth and reach levels 5% to 15% higher than expected from modeling calculations. The maximum concentrations in St‐Robert are, however, generally comparable to those measured for the L5 chondrite, Knyahinya, whose pre‐atmospheric radius of ～45 cm is thought to lead to the maximum possible production rates in chondrites. We infer that the pre‐atmospheric radius of St‐Robert was within 5 cm of the value that supports maximum production rates (i.e., 45 ± 5 cm). This radius corresponds to a pre‐atmospheric mass of (1.3 ± 0.4) × 103 kg. The agreement of exposure ages for St‐Robert obtained in several different ways and the similarity of the depth profiles for 14C, 26Al, 10Be, and 21Ne argue against a lengthy pre‐exposure of St‐Robert on the parent body and against a two‐stage exposure after launch from the parent body. Following Morbidelli and Gladman (1998), we suggest that St‐Robert was chipped from deep in its parent body, spent the next 7–8 Ma without undergoing a major collision, was nudged gradually into an orbital resonance with Jupiter, and then traveled quickly to Earth.  相似文献   

17.
Abstract— Despite having melted during formation, seven of eight Canyon Diablo spheroids weighing from 0.6 to 13 mg retain cosmic‐ray‐produced 38Ar (38Arcos) in concentrations (10?10 cm3 STP/g) ranging from 0.35 to 68. The presence of 38Arcos is consistent with pre‐atmospheric depths of <2.3 m and most likely rules out an origin for the spheroids deep within the projectile, which had a radius of ?15 m. Low levels of 21Necos indicate gas loss from these spheroids. Relative to most Canyon Diablo meteorites, the spheroids contain lower concentrations of cosmogenic noble gases. The difference partly reflects diffusion losses from the spheroids, especially for 3He and 21Ne, but also suggests deeper locations on average for the precursor material, consistent with independent results from 59Ni.  相似文献   

18.
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   

19.
Abstract— This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic‐ray exposure ages from 3He, 21Ne, and 38Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic 4He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic‐ray effects. While a contribution of this low‐energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic‐ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near‐surface siting. We derive an estimate for the 40Ar/36Ar ratio of the Martian interior component by subtracting from measured Ar in the (K‐poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from 129Xe, * excesses. Unless compromised by the presence of additional components, a high ratio of ～2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of 40Ar/36Ar in the Martian mantle.  相似文献   

20.
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   

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