首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Abstract— Two drill cores have been taken from the main mass of the Jilin chondrite, Jilin I. One was bored perpendicular to the 2π surface of the first exposure stage, the other parallel to this surface, and both intersected at the presumed 4π center of Jilin's second exposure stage. The probed depths were 65 cm and 105 cm, respectively, with a diameter of 26.5 mm. Fifteen drill core samples, 2 documented samples and 14 strewn field samples have been analysed for 26Al, 60Co and 22Na. The reconstruction of the preatmospheric Jilin via 60Co resulted in slight deviations for the center position and for the radius (now R = 80+10?5 cm) compared to an earlier study in 1985. The 60Co production rate profile is now almost completely reproduced. A constant production rate of 22Na and of 26Al during the 4π irradiation is confirmed also for the center region. The good linear correlation of 26Al and 21Ne yields slightly revised irradiation ages for both stages of t1 = 7 ± 1 Ma and t2 = 0.32+0.06-0.03 Ma.  相似文献   

2.
Abstract— Results are reported for documented samples from two drill cores and for specimens from the strewn field of the largest known stone meteorite, the H chondrite Jilin. Core B was found to have been parallel to the surface of Jilin during its first stage of irradiation, in 2π geometry. Core A was normal to the 2π surface; in it the mean attenuation length for the production by galactic cosmic rays of 38Ar from metal and of 21Ne from Mg, Al, Si in silicates was found to be the same. A numerical value for the mean attenuation length of (71 ± 4) cm or (246 ± 14) g × cm?2 follows if corrections for the contribution from the second stage of exposure are based on T2 = 0.32 Ma; agreement with the lower values of ～180 g × cm?2 obtained from lunar studies and target data requires T2 to be about twice as long. Previous results are confirmed that in specimens with high contents of stable spallogenic gases the ratio 21Nebulk/38Armetal is low. The suggestion had been, and is, mat this is a transition effect in near-surface samples where the secondary cascade of nuclear-active particles, and hence the production of 21Ne, was not yet fully developed. This suggestion is borne out by the present results on two samples that, based on cosmic-ray tracks and 60Co content, are certified near-surface samples (although, strictly speaking, this is true only for the depth of burial during the second stage of irradiation). Cosmic-ray produced 60Co is positively correlated with 4He content, indicating that significant losses of 4He occurred when the Jilin meteoroid had acquired already its final size and shape and that the losses were more severe for near-surface samples than for such from the 4π interior. Presumably, the losses were caused by a thermal spike associated with the excavation of Jilin from its parent body. The same event caused losses of part of the 3He produced during the first irradiation stage. From the systematics of the 3He/21Ne vs. 4He correlation, we derive for the second 4π irradiation a production ratio 3He/21Ne = 3.2 ± 0.4.  相似文献   

3.
The Sutter's Mill (SM) carbonaceous chondrite fell in California on April 22, 2012. The cosmogenic radionuclide data indicate that Sutter's Mill was exposed to cosmic rays for 0.082 ± 0.008 Myr, which is one of the shortest ages for C chondrites, but overlaps with a small cluster at approximately 0.1 Myr. The age is significantly longer than proposed ages that were obtained from cosmogenic noble gas concentrations, which have large uncertainties due to trapped noble gas corrections. The presence of neutron‐capture 60Co and 36Cl in SM indicates a minimum preatmospheric radius of approximately 50 cm, and is consistent with a radius of 1–2 m, as derived from the fireball observations. Although a large preatmospheric size was proposed, one fragment (SM18) contains solar cosmic ray–produced short‐lived radionuclides, such as 56Co and 51Cr. This implies that this specimen was less than 2 cm from the preatmospheric surface of Sutter's Mill. Although this conclusion seems surprising, it is consistent with the observation that the meteoroid fragmented high in the atmosphere. The presence of SCR‐produced nuclides is consistent with the high SCR fluxes observed during the last few months before the meteorite's fall, when its orbit was less than 1 AU from the Sun.  相似文献   

4.
Abstract— Measurements of particle tracks, cosmogenic radionuclides, and rare gas isotopes in Mbale indicate that the meteoroid had a simple, one-stage exposure for 30.2 Ma in interplanetary space. On the basis of the measured track production rates and 60Co and 26Al activities, the meteoroid is estimated to be a sphere with a radius of ～36 cm. The activities of several cosmogenic radionuclides (i.e., 57Co, 54Mn, 22Na, 44Ti, and 26Al) in two fragments having different shielding, as estimated by their track density and 60Co activity, provide the depth variation in their production rates. Cobalt-57, 54Mn and 22Na activities agree with the production that is expected around the maximum of the solar cycle 22 as calculated from the Sunspot numbers. The U, Th-4He and K-40Ar ages are measured to be 0.54 Ga indicating a late thermal event which is in agreement with the thermal history of some other L group chondrites. The trapped N has δ15N of ?57 ± 4%o, which is much lighter than the average L-group chondrite value; this indicates the presence of an isotopically anomalous light N component.  相似文献   

5.
An 87-gram sample of the Haverö ureilite has been analyzed by non-destructive gamma-ray spectrometry. The results of the measurements, in dpm/kg at time of fall, are: 22Na, 71 ± 3; 26Al, 43 ± 3; 46Sc, 3.4 ± 2.1; 54Mn, 35 ± 6; 60Co, 0.7, 2*** limit. Haverö has 77 ± 14% of the 26Al activity calculated for its chemical composition. When averaged with previously-reported analyses of Goalpara and Novo Urei, ureilites as a class have 74 ± 7% of their expected 26Al activity. The depletion in 26Al could be the coincidental result of identical “shielding” effects in three meteorites of apparently very different preatmospheric sizes. Alternatively, ureilites may have been exposed to a lower cosmic-ray flux than that experienced by most chondrites, probably the result of characteristically different orbits  相似文献   

6.
Abstract— We measured cosmogenic radionuclides (10Be, 26Al, and 36Cl) and noble gases (He, Ne, and Ar) in 10 specimens of the Mocs L6 chondrite to determine the exposure history and preatmospheric relationship among fragments from known locations in the strewn field. Cosmogenic noble gas contents alone are consistent with a simple irradiation exposure of 15.2 Ma. However, Mocs has very low 22Ne/21Ne ratios indicative of deep burial in a large meteoroid, but radionuclide levels at saturation values typical for much smaller meteoroids: this paradox suggests a possible complex exposure. For the latter case, we propose a two‐stage exposure history in which Mocs initially was deeply buried in a large object for 110 Ma, followed by exposure in a 65 cm object for 10.5 Ma. Relative shielding was inferred from the measured 22Ne/21Ne ratios assuming constant 22Ne/21Ne production for all samples during the first stage. These shielding levels, which are supported by estimates based on 36Cl production by neutron capture, indicate a possible relationship between depth of samples in the Mocs meteoroid and fall location in the strewn field.  相似文献   

7.
We determined the form of the functional dependence of the rate of formation of tracks of galactic cosmic rays in meteorites (/t) on the shielding degree for ordinary chondrites with preatmospheric radius R > 5 cm based on published semiempirical data on /t. The resulting dependence was used to construct a nomogram which allowed us to estimate the ablation of a meteorite according to the average rate of track formation in it and its recovered mass. The calculated ablation of meteorites agrees with the estimates obtained by other methods. The average ablation for 83 ordinary chondrites was found to be equal to 78.4+3.1 –3.4%. The analysis of the data obtained demonstrated that the average preatmospheric mass of chondrites is M 90 kg, and for 95% of the meteorites, the preatmospheric masses fall in the interval 2–3500 kg, which corresponds to radii from 5 to 60 cm. It was found that meteorites with a small preatmospheric mass tend to higher ablation.  相似文献   

8.
Abstract— We have measured the concentrations of the cosmogenic radionuclides 10Be, 26Al and 36Cl (half-lives 1.51 Ma, 716 ka, and 300 ka, respectively) in two different laboratories by accelerator mass spectrometry (AMS) techniques, as well as concentrations and isotopic compositions of stable He, Ne and Ar in the Antarctic H-chondrite Allan Hills (ALH) 88019. In addition, nuclear track densities were measured. From these results, it is concluded that the meteoroid ALH 88019 had a preatmospheric radius of (20 ± 5) cm and a shielding depth for the analyzed samples of between 4 and 8 cm. Using calculated and experimentally determined production rates of cosmogenic nuclides, an exposure age of ～40 Ma is obtained from cosmogenic 21Ne and 38Ar. The extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 ± 0.4 Ma. A similarly long terrestrial age was found so far only for the Antarctic L-chondrite Lewis Cliff (LEW) 86360. Such long ages establish one boundary condition for the history of meteorites in Antarctica.  相似文献   

9.
Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ～10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ～340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ～106 cm?2 in samples from the largest fragment to approximately (4–9) × 105 cm?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ～20 cm for the Dergaon meteorite.  相似文献   

10.
A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ15N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of 26Al (39 dpm/kg) and the absence of 60Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (82Kr/84Kr) further argue that the meteorite was derived from a shallow depth.  相似文献   

11.
Abstract— Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re‐determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur‐ and/or phosphorus‐rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (?10 cm) from earlier results (?165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature 36C1 data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of 21Ne was produced from these minor elements. Using measured 21Ne concentrations and endmember 22Ne/21Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for 21Ne produced from S and/or P. The thus corrected data are then used to calculate new 41K‐40K exposure ages—using published K data—which results in 564 ± 78 Ma for Grant and 725 ± 100 Ma for Carbo. The correction always lowers the 21Ne concentrations and consequently decreases the 41K‐40K exposure ages. The discrepancies between 36Cl‐36Ar and 41K‐40K ages are accordingly reduced. The existence of a significant long‐term variation of the GCR, which is based on a former 30–50% difference between 41K‐40K and 36Cl‐36Ar ages, may warrant re‐investigation.  相似文献   

12.
Abstract— Cosmic-ray produced nuclear tracks and noble gases have been studied in the martian orthopyroxenite Allan Hills 84001 to delineate its cosmic-ray exposure history, preatmospheric size, and fall characteristics. A K-Ar age of 3.9 Ga, cosmic-ray exposure duration of 16.7 Ma, and a preatmospheric radius of 10 cm have been deduced from the noble gas and track data. The track data suggest ALH 84001 to be a single fall that has suffered atmospheric mass ablation in excess of 85%, higher than the value deduced for the shergottites, ALHA 77005, EETA 79001, and Shergotty. The formation age, as well as the cosmic-ray exposure duration, determined in this work are in good agreement with values reported earlier and are distinctly different from other shergottite, nakhlite, and chassignite (SNC) meteorites analysed so far. The high cosmogenic 22Ne/21Ne ratio of 1.22 most probably reflects an effect due to non-chondritic composition of ALH 84001 as the track data suggest high shielding (<5cm) for the analysed samples. There are signatures in the noble gas data that indicate the possible presence of trapped Ar and Ne of martian atmospheric origin in ALH 84001.  相似文献   

13.
Abstract— The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y‐) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y‐981031 indicates an exposure at the lunar surface. A neon three‐isotope diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 ± 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y‐981031 was obtained. The shielding depth of these samples was between 10.5 g/cm2 and >500 g/cm2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm2. Assuming a mean density of the lunar regolith of 1.8 g/cm3, 10.5 g/cm2 corresponds to a depth of 5.8 cm and 500 g/cm2 to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.  相似文献   

14.
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   

15.
Abstract— The Peekskill H6 meteorite fell on 1992 October 9. We report extensive measurements of cosmic-ray produced stable nuclides of He, Ne, and Ar, of the radionuclides 22Na, 60Co, 14C, 36Cl, 26Al, and 10Be, and of cosmic-ray track densities. After correction for shielding via the 22Ne/21Ne ratio, the concentrations of cosmic-ray produced 3He, 21Ne and 38Ar give an average exposure age of 25 Ma, which is considered to be a lower limit on the true value. The 10Be/21Ne age is 32 Ma and falls onto a peak in the H-chondrite exposure age distribution. The activities of 26Al, 14C, 36Cl, and 10Be are all close to the maximum values expected for H-chondrites. Together with cosmic-ray track densities and the 22Ne/21Ne ratio, these radionuclide data place the samples at a depth >20 cm in a meteoroid with a radius >40 cm. In contrast, the 60Co activity requires a near-surface location and/or a much smaller body. Calculations show that a flattened geometry for the Peekskill meteoroid does not explain the observations in the context of a one-stage irradiation. A two-stage model can account for the data. We estimate an upper bound of 70 cm on the radius of the earlier stage of irradiation and conclude that Peekskill's radius was <70 cm when it entered the Earth's atmosphere. This size limit is somewhat smaller than the dynamic determinations (Brown et al., 1994).  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.
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   

18.
Abstract— A purely physical model based on a Monte Carlo simulation of galactic cosmic ray (GCR) particle interaction with meteoroids is used to investigate neutron interactions down to thermal energies. Experimental and/or evaluated excitation functions are used to calculate neutron capture production rates as a function of the size of the meteoroid and the depth below its surface. Presented are the depth profiles of cosmogenic radionuclides 36Cl, 41Ca, 60Co, 59Ni, and 129I for meteoroid radii from 10 cm up to 500 cm and a 2π irradiation. Effects of bulk chemical composition on n‐capture processes are studied and discussed for various chondritic and lunar compositions. The mean GCR particle flux over the last 300 ka was determined from the comparison of simulations with measured 41Ca activities in the Apollo 15 drill core. The determined value significantly differs from that obtained using equivalent models of spallation residue production.  相似文献   

19.
The type of the functional dependence of the ratio of the production rates of the cosmogenic isotopes 22Ne/21Nec on their location depth d (cm) in ordinary chondrites with a radius R 60 cm was determined on the basis of experimental data on the elemental production rates of cosmogenic Ne isotopes in chondrites (Leya et al., 2000a). The dependence found is of the type 22Ne/21Nec = Aexp(–Bd) + C, where the parameters A, B, and C are determined from the relationships: B = 0.560exp(–0.0105R) – 0.187, C = 0.170exp(–0.092R) + 1.083, and 0.170exp(–0.092R) + 1.144. These relationships were used to calculate the average weighted values of the 22Ne/21Nec ratio for the volume of the fallen meteorite depending on its given preatmospheric radius. The data obtained served as a basis for plotting a nomogram that makes it possible to estimate the mass lost during passage through the Earth's atmosphere (ablation quantity) from the mass of the fallen meteorite and the average value of the 22Ne/21Nec ratio measured in it. The average (median) value of ablation found for 262 chondrites was 91.5+2.1 –2.6%. In addition to the earlier-established (Alexeev, 2001a; Alexeev, 2001b) peculiarities of H5-chondrites that distinguish them from H-chondrites of other petrologic types, H5-chondrites appeared to exhibit a higher degree of ablation. The observed effect and other distinctive features of H5-chondrites may be due to the specific evolution of the parent body of H-chondrites in the process of its disintegration, reaccumulation, and subsequent reworking of the surface layers.  相似文献   

20.
Abstract— A meteorite shower fell at Torino, Italy on 18 May 1988. Petrographic studies indicate that the stone is an H6 chondrite having features of moderate to severe shock. Chemical analyses of the meteorite are reported. Cosmic ray produced 3He, 21Ne and 126Xe yield an exposure age of about 48 Ma. The cosmic ray track densities in three fragments range between 1.8 to 5 × 105/cm2 suggesting about 99% mass ablation in the atmosphere. Twelve radioisotopes with half lives ranging between 5.6 days to 7.3 × 105 years have been measured with high precision (2 to 10%). Marginal signals were observed for several short-lived nuclides and upper limits were obtained for the activity levels of eight radionuclides (24Na, 48Cr, 57Ni, 47Sc, 47Ca, 59Fe, 42Ar and 44Ti) some of which have not been hitherto detected in fresh falls. The data are generally consistent with the nuclide production by galactic cosmic rays when modulation due to the solar cycle is taken into consideration. The preatmospheric radius of the chondrite is estimated to be 20 cm, consistent with track densities and activity levels of 60Co, 26Al and other radionuclides.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号