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1.
Cosmic-ray-produced53Mn (t1/2 = 3.7 × 106years) has been measured in twenty Antarctic meteorites by neutron activation analysis.36Cl (t1/2 = 3.0 × 105years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.14C (t1/2 = 5740years) and26Al (t1/2 = 7.2 × 105years) data. The terrestrial ages range from 3 × 104 to 5 × 105 years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,10Be (t1/2 = 1.6 × 106years) and36Cl have been measured in six Antarctic ice samples. The first measurements of10Be/36Cl ratios in the ice core samples demonstrate a new dating method for ice.  相似文献   

2.
Cosmic-ray-produced53Mn (t1/2 = 3.7 × 106years) has been determined by neutron activation in nine Allan Hills-77 meteorites. Additionally,36Cl (1/2 = 3.0 × 105years) has been measured in seven of these objects using tandem accelerator mass spectrometry. These results, along with14C (t1/2 = 5740years) and26Al (7.2 × 105 years) concentrations determined elsewhere, yield terrestrial ages ranging from 0.1 × 105 to 7 × 105 years. Weathering was not found to result in53Mn loss.  相似文献   

3.
Cosmic-ray-produced53Mn (t½ = 3.7 × 106years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).53Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for10Be (1.6 × 106 years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of53Mn were found in the range 450 ± 90dpm53Mn/kg Fe in most of the chondrites and 490 ± 75dpm53Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of53Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm53Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low53Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of53Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of53Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of53Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of53Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.  相似文献   

4.
14C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic14C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic14C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 103 years for Yamato 7304, (11.6 ± 0.4) × 103 years for Estacado, and range from (32.7 ± 0.5) × 103 to (41.0 ± 0.8) × 103 years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 103 to 70 × 103 years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of14C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 103 years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO2 considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity.  相似文献   

5.
The date of fall of a meteorite may be estimated from its thermoluminescence (TL) and in this paper the principle of a method of utilising TL to determine the terrestrial ages of eight Antarctic meteorites (Allan Hills-77) is described. The TL in a meteorite is primarily induced by cosmic ray irradiation in space and once the meteorite is on the Earth's surface, it is shielded from further cosmic ray irradiation. Under these conditions the TL will decay at a rate governed by the thermal stability of the TL and by the environmental temperature.An estimate of the decay rate may be arrived at by using recently calculated data for the trapping parameters associated with meteorite TL. A major problem is the environmental storage temperature. The “effective” storage temperature of the Antarctic meteorites is unknown, but appears to be greater than the mean annual temperature of the region of the meteorite find.Only upper limits to the terrestrial ages can be calculated because the TL at the time of the fall to Earth is highly variable from sample to sample.  相似文献   

6.
An26Al survey by non-destructive gamma ray spectrometry was conducted as part of the preliminary examination effort for Antarctic meteorites. A total of 220 samples were studied. The majority of the samples were from the Allan Hills area; however, samples from several other sites, including the Yamato Mountains, were studied. Compared to worldwide falls and finds, the Antarctic group shows a clear trend toward lower26Al levels. At leasr 10% of the samples studied were clearly undersaturated in26Al. The frequency of undersaturated samples suggests that terrestrial ages of several hundred thousand years are common among the Antarctic meteorite collection. The absence of samples with extremely low26Al implies that the upper limit for terrestrial ages in the Antarctic is on the order of 1 million years.  相似文献   

7.
In 2001, a small H4 chondrite, Frontier Mountain (FRO) 01149, was found on a glacially eroded surface near the top of Frontier Mountain, Antarctica, about 600 m above the present ice level. The metal and sulphides are almost completely oxidized due to terrestrial weathering. We used a chemical leaching procedure to remove weathering products, which contained atmospheric 10Be and 36Cl in a ratio similar to that found in Antarctic ice. The FRO 01149 meteorite has a terrestrial age of 3.0 ± 0.3 Myr based on the concentrations of the cosmogenic radionuclides 10Be, 26Al and 36Cl. This age implies that FRO 01149 is the oldest stony meteorite (fossil meteorites excluded) discovered on Earth. The noble gas cosmic ray exposure age of FRO 01149 is ~ 30 Myr. The meteorite thus belongs to the 33 Myr exposure age peak of H-chondrites.The bedrock surface on which FRO 01149 was found has wet-based glacial erosional features recording a former high-stand of the East Antarctic ice sheet. This ice sheet evidently overrode the highest peaks (> 2800 m a.s.l.) of the inland sector of the Transantarctic Mountains in northern Victoria Land. We argue that FRO 01149 was a local fall and that its survival on a glacially eroded bedrock surface constrains the age of the last overriding event to be older than ~ 3 Myr. The concentrations of in-situ produced cosmogenic 10Be, 26Al and 21Ne in a glacially eroded bedrock sample taken from near the summit of Frontier Mountain yield a surface exposure age of 4.4 Myr and indicate that the bedrock was covered by several meters of snow. The exposure age is also consistent with bedrock exposure ages of other summit plateaus in northern Victoria Land.  相似文献   

8.
The distribution of the elements F, Cl, Br and I was analysed in layers removed stepwise from Antarctic H5 and H6 chondrites. All meteorites show higher concentrations of these elements on their surfaces than in their interiors. The degree of halogen enrichment cannot be correlated with the degree of visual weathering and is proportional to the time the meteorites were residing on the surface of the Antarctic ice. During this period, aerosols, salts and gaseous components are deposited on the surfaces of the meteorites and diffuse into their interior. The observed contamination level of the meteorites is influenced by leaching properties and diffusion behaviour of the individual halogens. The major contamination source for F, Cl and probably Br is air-borne seaspray; for I it is the gaseous compound methyl iodide (CH3I) produced by biological processes in the sea. Methyl iodide and its oxidation products formed in the Antarctic atmosphere (e.g., I2) can be transported over longer distances to the interior of Antarctica than air-borne seaspray. Therefore, the ratio of the halogen contamination is related to the collection site of the meteorites. All meteorites that were found in the Antarctic interior are contaminated to a lesser degree by F, Cl and Br relative to I than those found near the coast. The measured enrichment ratio of I/F is a function of the distance between the collection site and the open sea, and increases from the Allan Hills to the Thiel Mountains. By revealing the degree of contamination of a meteorite it is possible to determine its maximum surface residence time on the Antarctic ice.  相似文献   

9.
The reasons why53Mn (a cosmogenic radionuclide with a half-life of 3.7 × 106 y) appears as one of the best indicators of the presence of interplanetary dust are summarized. This paper reports the detection of53Mn in pre-1952 snow samples collected on the Eastern Antarctic Plateau in the vicinity of Plateau Station. The measurements were carried out by neutron activation and X-ray spectrometry on three samples weighing a few hundred kg and covering each the time interval 1935–1950. The specific activity of53Mn was found to be (0.82 ± 0.17) disint.min?1/103 tons of snow, corresponding to a deposition rate at Plateau Station of (2.2 ± 0.5) × 10?5 disint. min?1 m?2 y?1. The mean global deposition rate would be three times higher if53Mn were assumed to behave in the same way as stratospheric90Sr. By comparing this figure with existing data on the meteorite flux reaching the earth and with the galactic and solar production rates of53Mn, it is concluded that the bulk of the53Mn found at Plateau Station is associated with interplanetary dust in which it had been produced by the action of solar protons on iron. The deposition rate of extra-terrestrial dust-borne iron must be between 1.3 × 10?5 and 1.3 × 10?4 g m?2 y?1 at Plateau Station. These results support jointly with other studies the concept of an interplanetary zodiacal cloud of dust with a chemical composition and density not essentially different from chondritic meteorites, with a relatively ‘flat’ grain size distribution and a mass influx to the earth of the order of 105 tons/y.  相似文献   

10.
A new and more detailed analysis of the hypsometry of the Antarctic continent, based upon 1° digital data on ice thickness and surface and subglacial elevations, shows that Antarctica, even when deglaciated according to a simple Airy-isostatic model, is an unusual continent. It is the only one with a markedly bimodal hypsometric curve, and separation of the two modes shows that they are the single modes of West Antarctica (at ?450 m a.s.1.) and East Antarctica (at 950 m a.s.1.) respectively; the two parts of the continent are probably distinct tectonic entities. The modal height of East Antarctica is 700 m higher than that of the global ensemble of continents, suggesting that hotspot epeirogeny or a less well-known mechanism has affected its recent history. The age of this modal-height anomaly has important tectonic and especially climatic implications: it is equivalent to a 4–6°C cooling of the continental surface. The area of dry land after deglaciation is 10.5 × 106 km2; the volume of ice in Antarctica is estimated at 26.9 × 106 km3, and of ice in the Northern Hemisphere at 2.5 × 106 km3; these figures lead to a eustatic sea-level equivalent for present-day glacier ice of 68 m or somewhat less.  相似文献   

11.
A87Rb-87Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ87Rb= 1.42 × 10?11yr?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the87Sr/86Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the87Rb-87Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the87Rb-87Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.  相似文献   

12.
226Ra data on eleven vertical profiles taken during the GEOSECS program from the Antarctic Ocean and its vicinity in both the Atlantic and the Pacific are presented. Replicate measurements were made on each sample using the Rn-emanation method. The precision (1 σ) based on triplicate analyses averages about ±2.5%. Waters all around the Antarctic continent below 2 km depth appear to exhibit a uniform226Ra concentration of 21.5 ± 1dpm/100kg, except perhaps locally such as the Ross Sea and the Drake Passage where small variations may be present. Higher in the water column, the226Ra contents decrease toward the surface with gradients which vary as a function of the influence exerted by the Antarctic Convergence. Across this oceanic front, a north-to-south increase of226Ra occurs (the increase being the largest near the surface: from 8 to 18 dpm/100 kg), reflecting the combining effect of deep-water upwelling and meridional water mixing. The core layer of the Antarctic Intermediate Water contains about 14 dpm/100 kg of226Ra and that of the Circumpolar Intermediate Water (O2 minimum and local T maximum) about 18 dpm/100 kg. To a first approximation,226Ra covaries with Si in the circumpolar waters.  相似文献   

13.
This paper reports the results of an examination of the thermoluminescence (TL) of 23 meteorites which were observed to fall, and 17 meteorite finds which have had their terrestrial age determined by the14C method. The terrestrial ages of the observed falls range from 1 to 205 years, whilst the14C terrestrial ages range from 1200 ± 2000 to >20,000 years. A statistically significant correlation has been observed between the natural TL — as expressed as the ratio of the intensity of the low-temperature TL peak to that of the high-temperature peak — and the terrestrial age of the 40 meteorites. Furthermore, peak height ratios in excess of 3.0 are only observed in falls which fell within the last 250 years, suggesting that finds with peak height ratios as large as this, such as Allan Hills A77003 and Plainview (1917), fell within the last few hundred years. The present results are consistent with evidence that meteorite TL decay is a non-first-order process. The implications of the results for estimates of the terrestrial ages of 8 meteorite finds, for which there are no14C data, are also discussed.  相似文献   

14.
Accurate determination of chlorine concentrations in terrestrial rocks is of importance for the interpretation of terrestrial in-situ cosmogenic 36Cl. Neutron capture by 35Cl, together with production from Ca and K, is one of the three major production pathways of 36Cl in rocks. Here, we present an inter-comparison of chlorine determinations by two procedures. The first approach is an independent Cl determination by prompt-gamma (neutron) activation analysis (PGAA). The second method is isotope-dilution based on isotopically enriched stable chlorine carrier added during chemical sample preparation for accelerator mass spectrometry (ID-AMS). Twenty-six (26) whole rock samples have been processed for PGAA and ID-AMS analysis. This study constitutes the first published inter-comparison for concentrations below 100 μgCl/g. Our results show no significant difference in Cl concentrations between methods. This agreement indicates good retention of chloride during the procedure we employ for whole rock sample dissolution. No significant loss of stable chlorine from either the spike or the sample occurs before isotopic equilibrium is reached, prior to AgCl precipitation. Uncertainties, which are <5% for both methods, affect the uncertainty of the total 36Cl production rate less than 2% for our samples.The Cl concentration measured by PGAA can be used to calculate the amount of isotopically enriched spike for AMS-ID sample preparation with the aim to optimize 36Cl analysis. Furthermore, PGAA offers an advance for the interpretation of 36Cl measurements. It allows measurement of concentrations of major, minor and trace elements including the elements for 36Cl production (Cl, K, Ca, Ti, and Fe), as well as of neutron absorbers and neutron moderators (H, B, Sm and Gd). These measurements are performed simultaneously and with a precision necessary for calculating the relative contributions to 36Cl production from the different mechanisms.  相似文献   

15.
87Rb87Sr analysis of the Norton County achondrite has been achieved with special attention to the rubidium analysis. Enstatite crystals and polycrystalline material give an “age” of 4.48 ± 0.04 × 109 years and an initial ratio 87Sr/86SrI= 0.7005 ± 0.0004 (λ = 1.39 × 10?11yr?1, maximum errors). The feldspar component of the meteorite contains about 70% of the strontium and 30% of the rubidium of the whole sample, and does not lie on the isochron. Its model age relative to the strontium initial ratio of Allende is 4.6 × 109 years. The data are consistent with a complex history dealing with an incomplete isotopic reequilibration of the meteorite, 120 m.y. after its formation at 4.6 × 109 years, with an initial ratio similar to that of Allende.  相似文献   

16.
87Rb87Sr analyses of LL chondrites have been made in 10 whole rock meteorites, chondrules from Chainpur (LL3) and Soko Banja (LL4), density separates and chondrules from Guidder (LL5) and density separates from Jelica (LL6) and Ensisheim (LL6). Whole rocks define an isochron of age 4.486±0.020 Ga (λ87Rb=1.42×10?11a?1) and initial ratio (87Sr/86Sr)I=0.69887±0.00012. This is in agreement with the results for H- and E-type chondrites. Analyses for chondrules from Soko Banja yield a very good isochron of age 4.452±0.020 Ga and strontium initial ratio 0.69954±0.00024, and give an interval for metamorphism of (37±10)×106 a. A more poorly defined isochron is obtained for Jelica; the age is 4.423±0.041 Ga and the strontium initial ratio 0.69959±0.00029, indicating an interval for metamorphism of (70±60)×106 a. No isochron could be obtained for Chainpur. This could be due to terrestrial alteration or to a late isotopic disturbance of the meteorite. The87Rb-87Sr system is also disturbed in Guidder and Ensisheim, probably as a consequence of shock. These results are discussed in comparison with our former studies, and in relation with thermal metamorphism in the LL chondrite parent body(ies).  相似文献   

17.
Cosmogenic14C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm/kg; the low value of (4.5 ± 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of14C.In eight cases39Ar in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radiiR0 of the meteoroids and depth of burial of the samples investigated. Values ofR0 between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere.A compilation of all published14C concentrations in chondrites shows that the variations between different specimens from thesame meteorites are almost as large as those for samples fromdifferent meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites.  相似文献   

18.
We examined air trapped in ancient ice from three shallow cores (<35 m deep) recovered from stagnant portions of the Mullins glacier, an 8 km long debris-covered alpine glacier in the McMurdo Dry Valleys that is overlain by several in-situ volcanic ash-fall deposits. Previously reported 40Ar/39Ar dates on ash-fall in the vicinity of the core sites average 4.0 Ma, and underlying ice is presumably as old in some areas. We analyzed the elemental and isotopic composition of O2, N2, and Ar and total air content of the glacial ice. We also dated the trapped air directly to an uncertainty of ±220 kyr (1σ) by measuring its 40Ar/36Ar and 38Ar/36Ar ratios. Our results suggest that the air analyzed is likely a mixture of ancient atmosphere trapped at the time of ice formation and more recent air introduced via cracks in the ice that penetrate to at least 33 m. The isotopic signatures of gases have been complicated by gas loss, as well as a mixture of thermal and gravitational fractionation. The oldest age estimated for the trapped air dates to 1.6 Ma, indicating that the original air is at least as old as 1.6 ± 0.2 Ma. A convergence to older ice ages with increasing depth in the deepest core analyzed (33 m) hints at the possibility that pristine air might be recovered at greater depths. Minor interstitial debris present in the glacial ice (<1%), along with geochemical evidence for in-situ microbial respiration, prohibit direct analysis of CO2. We measured the triple isotopic composition of O2 as a proxy for CO2 and infer that, in the air represented in our ice samples, CO2 concentrations are within the range observed over the last 800 ka.  相似文献   

19.
The Antarctic meteorite collection has proved to be a source of many important discoveries, including a number of previously unknown or very rare meteorite types. A thermoluminescence (TL) survey of meteorite samples recovered by the 1988/89 European expedition and pre-1988 American expeditions to the Allan Hills Main blue ice field resulted in the discovery of 15 meteorites with very high TL levels ( > 100 krad at 250°C in the glow curvc). It is likely that these samples are fragments of a single meteoroid body which: (1) fell very recently and (2) experienced a decrease in orbital perihelia from 1.1 AU to 1 AU within the last 105 yr. Carbon-14 data for two of the samples confirm their young terrestrial age compared to most Antarctic meteorites. Studies of the cosmogenic isotopes in at least one non-Antarctic meteorite which also has very high natural TL, Jilin, indicate that the meteorite experienced a multi-stage irradiation history, the most recent stage being 0.4 Ma in duration following a major break-up of the object. These meteorites, and the few equivalent modern falls, are the only documented samples from bodies which were recently in Earth-approaching (Amor) orbits (i.e., with perihelion > 1.0 AU), as opposed to the Earth-crossing (Apollo) orbits which are the source of most other meteorites. Their rarity indicates that such rapid orbit changes are unusual for meteoroid bodies and may be the result of isolated, large break-up events.  相似文献   

20.
Fourteen whole-rock samples from three traverses through the contact aureole of the Stillwater Complex were analyzed for Rb, Sr and87Sr/86Sr. Twelve of these samples yielded an age of 2750 ± 45m.y.; (87Sr/86Sr)0 = 0.705 ± 0.003 (2σ). In addition one whole rock and a biotite separate derived from it gave an age of 2544 m.y.; (87Sr/86Sr)0 = 0.714. These data support a minimum age of 2750 m.y. for the intrusion of the complex.  相似文献   

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