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1.
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.  相似文献   

2.
Abstract We report on a series of 27 14C terrestrial ages of meteorites from four states in the central and southwestern USA. These results were compared to the earlier work of Boeckl (1972). Our results showed that the weathering rate for destruction of meteorites is lower than suggested by Boeckl (1972). We estimated a “half-life” for removal of meteorites of about 10 to 15 ka, similar to that derived for Roosevelt County meteorites. We also studied the weathering of these meteorites compared to terrestrial age. Only a weak correlation was observed, and for these meteorites the degree of weathering can only be taken as a weak indicator of terrestrial residence time. We also measured the δ 13C and 14C and amount of weathering-product carbonates which show some interesting variations with the length of time the meteorites have been exposed to weathering.  相似文献   

3.
We report on the first meteorite search campaign in the United Arab Emirates (UAE). The geology and proximity of our search region suggest that it is the north‐western extension of the Oman meteorite fields. We found 26 ordinary chondrites, bringing the total number of official meteorites from the UAE to 28. The campaign was organized and conducted in close cooperation with the UAE government and the main masses of the meteorites remained in the country where they will become part of an exhibition. The bulk composition of five meteorite and three soil samples indicates an uptake of U, Mo, Sr, Ba, Li, and Pb from the soil into the meteorites during terrestrial weathering. Terrestrial ages determined from 14C decay of 21 meteorites range from recent falls to 24.4 ka, with two meteorites having >37 ka and approximately 39 ka, respectively. Weak correlations between weathering degree, meteorite bulk chemical composition, and terrestrial age suggest highly localized weathering conditions, possibly related to abundant occurrences of sabkhas in the search region.  相似文献   

4.
Abstract Research on meteorite finds, especially those from the Antarctic and from desert regions in Australia, Africa, and America, has become increasingly important, notably in studies of possible changes in the nature of the meteorite flux in the past. One important piece of information needed in the study of such meteorites is their terrestrial age which can be determined using a variety of methods, including 14C, 36Cl, and 81Kr. Natural thermoluminescence (TL) levels in meteorites can also be used as an indicator of terrestrial age. In this paper, we compare 14C-determined terrestrial ages with natural TL levels in finds from the Prairie States (central United States), a group of finds from Roosevelt County (New Mexico, USA), and a group from the Sahara Desert. We find that, in general, the natural TL data are compatible with the 14C-derived terrestrial ages using a 20 °C TL decay curve for the Prairie States and Roosevelt County and a 30 °C decay curve for the Saharan meteorites. We also present TL data for a group of meteorites from the Sahara desert which has not been studied using cosmogenic radionuclides. Within these data there are distinct terrestrial age clusters which probably reflect changes in meteorite preservation efficiency over ～ 15, 000 years in the region.  相似文献   

5.
Abstract– We have examined the relationship between natural thermoluminescence (TL) and 26Al in 120 Antarctic meteorites in order to explore the orbital history and terrestrial ages of these meteorites. Our results confirm the observations of Hasan et al. (1987) which were based on 23 meteorites. For most meteorites there was a positive correlation between natural TL and 26Al, reflecting their similarity in decay rate under Antarctic conditions and thus in terrestrial age. For a small group with low TL and high 26Al a small perihelion was proposed. Within this group, natural TL decreases with terrestrial age as determined by 36Cl measurements, although the rate of TL decay is faster (half‐life approximately 10 ka) and the ages that can be determined are smaller (<200 ka) than for most meteorites. The faster decay rate and lower natural TL levels are a reflection of recent exposure to higher radiation doses and higher temperatures, since this history would populate less stable TL traps with smaller electron densities. We sort the 120 meteorites by perihelion and terrestrial age. The normal perihelion group range up to approximately 1000 ka and the small perihelion group range up to approximately 200 ka. An intermediate perihelion group tends to have short terrestrial ages (20–60 ka). There is acceptable agreement between most (34 out of 43) of our present terrestrial age estimates and those determined by isotopic means, the exceptions reflecting complex irradiation histories, long burial times in the Antarctic, or other issues.  相似文献   

6.
Abstract— We determined terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica, on the basis of the concentrations of cosmogenic 10Be (t½; = 1.51 Ma), 26Al (t½; = 0.705 Ma), and 36Cl (t½; = 0.301 Ma). After an initial 26Al γ-ray survey of 91 meteorites suggested that many have terrestrial ages >0.1 Ma, we selected 62 meteorites for 10Be and 26Al measurements by accelerator mass spectrometry (AMS) and measured 36Cl in twelve of those. Low terrestrial ages (<0.1 Ma) were found for ～60% of the meteorites, whereas all others have ages between 0.1 and 0.5 Ma, except for one exceptional age of >2 Ma (Welten et al., 1997). Our major conclusions are: (1) The Lewis Cliff H-chondrites show similar ages to those from the Allan Hills icefields, but the L-chondrites are about a factor of 2 younger than those from Allan Hills, which indicates that Lewis Cliff is a younger stranding area. (2) The terrestrial age distributions at different parts of the Lewis Cliff stranding area generally agree with simple meteorite concentration models, although differences in weathering rate may also play a role. (3) We confirm that meteorites with natural thermoluminescence (TL) levels >80 krad are associated with low terrestrial ages (Benoit et al., 1992) but conclude that natural TL levels <80 krad can not be used to calculate the terrestrial age of a meteorite. Natural TL levels do seem useful to estimate relative terrestrial ages of large groups of meteorites and to determine differences in the surface exposure age of paired meteorite fragments. (4) Of the 62 meteorites measured with AMS, 31 were assigned to 11 different pairing groups, mainly on the basis of their cosmogenic nuclide record. The meteorites are estimated to represent between 42 and 52 distinct falls.  相似文献   

7.
Abstract— 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 Ba‐and 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.  相似文献   

8.
We present for the first time a detailed report on the discovery of a new meteorite collection region in the Lut Desert, eastern–southeastern Iran, describing its geological, morphological, and climatic setting. Our search campaigns, alongside with the activity of meteorite hunters, yielded >200 meteorite finds. Here, we report on their classification, spatial distribution, and terrestrial weathering. All the collected meteorites are ordinary chondrites (OCs). The most abundant by far are the highly weathered paired H5 distributed in the northwest of Kalut area (central Lut, Kerman dense collection area). The second are well‐preserved paired L5 also found in Kalut region. A detailed study of the geochemistry and mineralogy of selected meteorites reveals significant effects of terrestrial weathering. Fe,Ni metal (hereafter simply metal) and troilite are transformed into Fe oxyhydroxides. A rather unusual type of troilite weathering to pyrite/marcasite is observed in most of the Lut Desert meteorites. Magnetic measurements and X‐ray diffractometry confirm the occurrence of terrestrial weathering products, with the dominance of maghemite, goethite, and hematite. Mobile elements, such as Li, Sr, Mo, Ba, Tl, Th, and U, are enriched with respect to fresh falls. Meanwhile, a decrease in the V, Cr, Co, Rb (and possibly Fe) due to terrestrial weathering is detectable. The total carbon and CaCO3 is higher than in samples from other hot deserts. The weathering effects observed in the Lut Desert OCs can be used as distinctive indicators to distinguish them from meteorites from other regions of the Earth. Measurements of terrestrial age (14C) show a range of 10–30 ka, which is in the range of ages reported for meteorites from other hot deserts except the Atacama Desert (Chile). Considering the high potential of the Lut Desert in meteorite preservation, systematic works should lead to the discovery of more samples giving access to interesting material for future studies.  相似文献   

9.
Abstract— We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides 14C, 10Be, 26Al, 36Cl, and 41Ca in 18 of these samples. Although the trapped noble gases in five DaG samples show ratios typical of solar or planetary gases, in all other DaG samples, they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic ray exposure (CRE) ages of DaG chondrites range from ?1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chondrite shower. The CRE age distribution of 19 DaG H chondrites shows only three ages coinciding with the main H chondrite peak at ?7 Myr, while seven ages are <5 Myr. Two of these H chondrites with short CRE ages (DaG 904 and 908) show evidence of a complex exposure history. Five of the H chondrites show evidence of high shielding conditions, including low 22Ne/21Ne ratios and large contributions of neutron‐capture 36Cl and 41Ca. These samples represent fragments of two or more large pre‐atmospheric objects, which supports the hypothesis that the high H/L chondrite ratio at DaG is due to one or more large unrecognized showers. The 14C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara but younger than two DaG achondrites. Despite the loss of cosmogenic 36Cl and 41Ca during oxidation of metal and troilite, concentrations of 36Cl and 41Ca in the silicates are also consistent with 14C ages <35 kyr. The only exception is DaG 343 (H4), which has a 41Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ?3000 years ago. Finally, based on the 26Al/21Ne and 10Be/21Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the 10Be/21Ne production rate ratio, which seems more sensitive to shielding than was predicted by the semi‐empirical model of Graf et al. (1990) but less sensitive than was predicted by the purely physical model of Leya et al. (2000).  相似文献   

10.
Abstract— Cosmic-ray produced 14C (t1/2 = 5730 years), 36Cl (3.01 × 105 years), 26Al (7.05 × 105 years), and 10Be (1.5 × 106 years) in the recently discovered lunar meteorites Queen Alexandra Range 93069 (QUE 93069) and 94269 (QUE 94269) were measured by accelerator mass spectrometry. The abundance pattern of these four cosmogenic radionuclides and of noble gases indicates QUE 93069 and QUE 94269 were a paired fall and were exposed to cosmic rays near the surface of the Moon for at least several hundred million years before ejection. After the meteorite was launched from the Moon, where it had resided at a depth of 65–80 g/cm2, it experienced a short transition time, ～20–50 ka, before colliding with the Earth. The terrestrial age of the meteorite is 5–10 ka. Comparison of the cosmogenic nuclide concentrations in QUE 93069/94269 and MAC 88104/88105 clearly shows that these meteorites were not ejected by a common event from the Moon.  相似文献   

11.
We describe the geological, geomorphological, and paleoclimatic setting of the Sahara of North Africa in particular, focused on the main meteorite dense collection areas (DCA; Morocco, Algeria, Tunisia, and Libya). We report on the outcome of several meteorite recovery field expeditions in Morocco and Tunisia since 2008, by car and by foot, that applied systematic search methods. The number of meteorites collected is 41 ordinary chondrites and one brachinite. The statistics of unpaired ordinary chondrites indicates that H chondrites are more abundant (21) than L chondrites (12), while LL chondrites are rare (2). Our meteorite density estimates for Tunisia and Morocco are in the order of magnitude of 1 met km?2. An estimate of the total maximum number of meteorites that could be recovered from the Sahara is 780,000 meteorites. We selected 23 meteorites from Aridal, Bou Kra, Bir Zar, and Tieret DCAs for 14C dating. The results show a wide range of terrestrial ages from 0.4 to more than 40 kyr with a majority of meteorites showing ages between 0.4 and 20 kyr. The weathering degree of these meteorites is ranges from minor (W1) to strong (W4). The highest weathering grades result from repeated oscillations between high and low humidity in the Sahara. However, there appears to be no correlation between weathering grade and terrestrial age of meteorites.  相似文献   

12.
Abstract— We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of 84Ke, 129Xe, and 132Xe. From the measured cosmogenic 14C concentrations (Jull et al. 1995), shielding‐corrected production rates of 14C are deduced using cosmogenic 22Ne/21Ne ratios. For shielding conditions characterized by 22Ne/21Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic 3He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.  相似文献   

13.
Abstract– 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 km2 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.  相似文献   

14.
Abstract— Cañon Diablo meteoritic iron oxide consists mainly of goethite and maghemite and contains 209 to 630 ppb uranium compared to <0.02 ppb in the unweathered octahedrite. Significant radioactive disequilibria between 238U, 234U and 230Th indicate that uranium was sorbed from soil porewater during terrestrial weathering after meteorite impact 50 ka ago. Depending on the model assumed for U uptake, corrected 230Th-234U ages of 24 to 48 ka were obtained. While the data presented may not allow an unambiguous interpretation, the potential of this approach in obtaining minimum terrestrial ages for weathered meteorites is demonstrated.  相似文献   

15.
We measured the He, Ne, and Ar isotopic concentrations and the 10Be, 26Al, 36Cl, and 41Ca concentrations in 56 iron meteorites of groups IIIAB, IIAB, IVA, IC, IIA, IIB, and one ungrouped. From 41Ca and 36Cl data, we calculated terrestrial ages indistinguishable from zero for six samples, indicating recent falls, up to 562 ± 86 ka. Three of the studied meteorites are falls. The data for the other 47 irons confirm that terrestrial ages for iron meteorites can be as long as a few hundred thousand years even in relatively humid conditions. The 36Cl‐36Ar cosmic ray exposure (CRE) ages range from 4.3 ± 0.4 Ma to 652 ± 99 Ma. By including literature data, we established a consistent and reliable CRE age database for 67 iron meteorites. The high quality of the CRE ages enables us to study structures in the CRE age histogram more reliably. At first sight, the CRE age histogram shows peaks at about 400 and 630 Ma. After correction for pairing, the updated CRE age histogram comprises 41 individual samples and shows no indications of temporal periodicity, especially not if one considers each iron meteorite group separately. Our study contradicts the hypothesis of periodic GCR intensity variations (Shaviv 2002, 2003), confirming other studies indicating that there are no periodic structures in the CRE age histogram (e.g., Rahmstorf et al. 2004; Jahnke 2005). The data contradict the hypothesis that periodic GCR intensity variations might have triggered periodic Earth climate changes. The 36Cl‐36Ar CRE ages are on average 40% lower than the 41K‐K CRE ages (e.g., Voshage 1967). This offset can either be due to an offset in the 41K‐K dating system or due to a significantly lower GCR intensity in the time interval 195–656 Ma compared to the recent past. A 40% lower GCR intensity, however, would have increased the Earth temperature by up to 2 °C, which seems unrealistic and leaves an ill‐defined 41K‐K CRE age system the most likely explanation. Finally, we present new 26Al/21Ne and 10Be/21Ne production rate ratios of 0.32 ± 0.01 and 0.44 ± 0.03, respectively.  相似文献   

16.
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   

17.
Abstract— We have obtained minimum age estimates for the sand units underlying the two largest meteorite deflation surfaces in Roosevelt County, New Mexico, USA, using thermoluminescence dating techniques. The dates obtained ranged from 53.5 (±5.4) to 95.2 (±9.5) ka, and must be considered lower limits for the terrestrial ages of the meteorites found within these specific deflation surfaces. These ages greatly exceed previous measurements from adjacent meteorite-producing deflation basins. We find that Roosevelt County meteorites are probably terrestrial contemporaries of the meteorites found at most accumulation zones in Antarctica. The apparent high meteorite accumulation rate reported for Roosevelt County by Zolensky et al. (1990) is incorrect, as it used an age of 16 ka for all Roosevelt County recovery surfaces. We conclude that the extreme variability of terrestrial ages of the Roosevelt County deflation surfaces effectively precludes their use for calculations of the meteorite accumulation rate at the Earth's surface.  相似文献   

18.
Abstract— Miono et al. (1990) and Miono and Nakanishi (1994) have proposed that the build‐up of natural thermoluminescence (TL) in a drained layer directly below the meteorite fusion crust can be used to determine terrestrial ages of meteorites in the 40 to 200 ka range. We have measured the natural TL of the drained layer of 15 meteorites. The data indicate that this technique could be used to determine terrestrial ages of meteorites with ages <200 ka, after which TL equilibrium is reached. Comparison of TL build‐up with terrestrial ages for a suite of Antarctic meteorites suggests that the meteorites have been exposed to temperatures of 0 to 5 °C. The close correspondence between natural TL levels and surface exposure TL growth curves suggest that Allan Hills meteorites with ages <200 ka have spent a significant portion of their terrestrial history exposed on the ice surface, rather than being buried in the ice sheet. The technique is, however, sensitive to thermal history; and, for Antarctic meteorites with terrestrial ages <200 ka, natural TL of the drained zone largely reflects exposure on the ice surface.  相似文献   

19.
Abstract— We have measured a surprisingly long terrestrial age of 410,000 ±45,0020,000 yr (410 ±2045ka) for basaltic eucrite Río Cuarto 001 using accelerator mass spectrometry of 26Al, 36Cl, and 41Ca. Though many meteorites are known to have survived for tens or hundreds of ka in Antarctica or hot deserts, the mean annual precipitation of 815 mm in Río Cuarto, Cordoba Province, Argentina, makes the long survival of this meteorite remarkable. We propose two explanations for the exceptional preservation of Río Cuarto 001. First, the meteorite contains only trace amounts of metal, so the weathering and oxidation of metallic Fe, which commonly destroys chondrites, is ineffective in this case. Second, the meteorite was found in a relatively young deflation basin, and may have been exhumed only recently from beneath a protective layer of soil. Insofar as the survival on Earth of Río Cuarto 001 is due to environmental factors, there may be other meteorites with comparably long terrestrial ages still to be discovered in the vicinity.  相似文献   

20.
Abstract— We report concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in the metal phase of 26 ordinary chondrites from Frontier Mountain (FRO), Antarctica, as well as cosmogenic 14C in eight and noble gases in four bulk samples. Thirteen out of 14 selected H chondrites belong to two previously identified pairing groups, FRO 90001 and FRO 90174, with terrestrial ages of ?40 and ?100 kyr, respectively. The FRO 90174 shower is a heterogeneous H3–6 chondrite breccia that probably includes more than 300 individual fragments, explaining the high H/L chondrite ratio (3.8) at Frontier Mountain. The geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50–100 kyr to less than ?40 m, supporting the stability of the meteorite trap over the last glacial cycle. The second H‐chondrite pairing group, FRO 90001, is much smaller and its geographic distribution is mainly controlled by wind‐transport. Most L‐chondrites are younger than 50 kyr, except for the FRO 93009/01172 pair, which has a terrestrial age of ?500 kyr. These two old L chondrites represent the only surviving members of a large shower with a similar preatmospheric radius (?80 cm) as the FRO 90174 shower. The find locations of these two paired L‐chondrite fragments on opposite sides of Frontier Mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau. The 50 FRO meteorites analyzed so far represent 21 different falls. The terrestrial ages range from 6 kyr to 500 kyr, supporting the earlier proposed concentration mechanism.  相似文献   

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