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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 produced 36Cl(t1/2 = 3.0 × 105 years) has been measured in four Antarctic meteorites and one sample of Antarctic ice using a tandem Van de Graaff accelerator as an ultrasensitive mass spectrometer with the extremely low background level of 36Cl/Cl< 2 × 10?16. Results from this ion counting technique (applied here to extraterrestrial materals for the first time) are used to support a two-stage irradiation model for the Yamato-7301 and Allan Hills-76008 meteorites and to show a long terrestrial age (0.7 ± 0.1 m.y.) for Allan Hills-77002. Yamato-7304 has a terrestrial age of less than 0.1 m.y. The 36Cl content of the Antarctic ice sample from the Yamato Mountain area implies that the age of the ice cap at this site is less than one 36Cl half-life.  相似文献   

3.
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   

4.
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   

5.
We have determined the concentrations of 10Be and 26Al in GRV 99027 recovered by the 16th Chinese Antarctic expedition team, which are 14.1 ± 0.6 dpm/kg and 67.5 ± 3.4 dpm/kg, respectively. From the concentration of 10Be, we calculate a cosmic-ray exposure age of 4.4 ± 0.6 Ma for GRV 99027. The concentration of 26Al is too high compared to the 10Be exposure age, indicating extra production from solar ray. The exposure ages, petrologic and geochemical characteristics of mantle-derived Martian meteorites GRV 99027, LEW 88516, Y-793605, NWA 1950 and ALHA77005 are very similar, suggesting that these meteorites most probably were ejected from Mars in the same impact event.  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.
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   

8.
This paper reports the results of thermal-release argon analyses of neutron-irradiated samples of the two nakhlite meteorites, Lafayette and Nakhla. The initiation of retention of radiogenic40Ar in Lafayette appears to have been a reasonably well-defined event which occurred (1.33 ± 0.03) × 109 yr ago, as determined by the40Ar-39Ar method. Nakhla also appears to have been retaining argon no longer than 1.3 × 109 yr, but its gas-retention age cannot be considered well-defined because its apparently most-retentive sites have nominal gas-retention ages shorter than those of the less-retentive sites which contain most of its potassium.  相似文献   

9.
Kr and Xe were measured by a stepwise heating technique in three samples of a drill core in the “Minnesota” fragment of the Estherville mesosiderite. The cosmogenic78Kr/83Kr decreased from the “top” sample to the “bottom” sample(“top” = 0.163 ± 0.005, “bottom” = 0.151 ± 0.005) while the cosmogenic131Xe/126Xe ratio increased(“top” = 5.58 ± 0.35, “bottom” = 6.92 ± 0.17). Cosmic-ray track studies have shown that the “top” sample was indeed closer to the preatmospheric surface than the “bottom” sample by ～ 10 cm. This is the first direct evidence, in a sample of known geometry, that the cosmogenic131Xe/126Xe ratio increases as a function of depth, and as such, confirms the hypothesis that the lunar131Xe anomaly is a bona fide depth effect due to resonance neutron capture in130Ba.  相似文献   

10.
The Zerga meteorite, an LL6 ordinary chondrite found at Aouelloul crater in 1973, is a small fragment of a larger mass whose pre-atmospheric radius was most likely between 20 and 125 cm. A typical amphoterite, it is a monomict breccia that has undergone at least one recrystallization episode.3He and21Ne contents define a shielding-corrected, cosmic ray exposure age of 21–24 × 106 years and the26Al content is consistent with a terrestrial age ?500,000 years (2σ limit). The K—Ar gas-retention age is 3.1 × 109 years. The meteorite's areal association with the impact crater is merely coincidental. A new K-Ar age of the glassy impactite found at Aouelloul dates the crater at 3.1 ± 0.3 × 106 years, sensibly indistinguishable from the recently determined age of nearby Tenoumer crater (2.5 ± 0.5 × 106years). The similar ages of these two impact craters, and their almost perfect linear alignment with a third, morphologically similar crater (Temimichat Ghallaman) over a distance of 600 km, suggests a simultaneous triple impact occasioned by the disruption of a large meteorite moving on a very shallow atmospheric trajectory. If so, the concomitant low impact angles may be responsible for the unusually shallow original depths inferred for two of the craters from gravity data.  相似文献   

11.
Geology of the Grove Mountains in East Antarctica   总被引：2，自引：0，他引：2  
Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D1 is related to the event before Pan-African age, D2 corresponds to the regional granulite peak metamorphism, whereas D3 reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800°C, 9.3×105 Pa; M2 800–810°C, 6.4 × 105 Pa; and M3 650°C have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.  相似文献   

12.
Tritium is measured as a function of depth in a Surveyor 3 sample. The upper limit for solar-wind-implanted tritium gives a3H/1H limit for the solar wind of 1 × 10?11. The temperature release patterns of14C from lunar soils are measured. The14C release patterns from surface soils differ from a trench bottom soil and gives evidence for the presence of14C in the solar wind with a14C/1H ratio of approximately 4 × 10?11. The implications of these radio nuclide abundances in the solar wind are discussed.  相似文献   

13.
The times at which phyllosilicate matrix and euhedral olivines became associated have been determined for five C2 meteorites. The ages, calculated from fission track densities on crystal surfaces, are based on an initial244Pu/238U ratio in the matrix material of 0.0154 at 4.6 b.y., and range from 4.22 b.y. for Nogoya to 4.42 b.y. for Murray. Unless the initial244Pu/238U ratio was less than 0.004, the meteorites cannot have existed in their present form for 4.6 × 109 yr. The measured ages place limits on the time when pre-compaction effects such as micrometeorite craters and solar flare tracks were produced, and they may approximately date the formation of the olivine crystals themselves.  相似文献   

14.
14C specific activities in the western Atlantic show aging of about 160 years between 42°N and 30°S for southward-moving North Atlantic Deep Water. Most of the aging occurs in the North Atlantic, with a small increase in14C level for abyssal water near the equator.The northward-flowing Antarctic Bottom Water component ages about 80 years between 50°S and the equator. The rate of Atlantic bottom water formation is estimated at 18 Sverdrups; the rate of flow for northward-moving Antarctic Bottom Water at about 6 Sverdrups.  相似文献   

15.
We have measured by accelerator mass spectrometry the26Al contents of 20 and the10Be contents of 14 iron meteorites. The26Al contents are typically 30% or more lower than values obtained by counting techniques; the10Be contents are 10–15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the4He/21Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimateP26(Fe) as 3–4 dpm/kg andP10(Fe) as 4–5 dpm/kg. The average P/10P26 activity ratio is close to 1.5. Exposure ages calculated from21Ne/26Al ratios cannot be calibrated so as to agree with both40KK/ ages and ages based on the shorter-lived nuclides39Ar and36Cl. If agreement with the latter is forced, then the disagreement with40KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 107 a.  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.
Some conceptual models suggest that baseflow in agriculturally fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here, we (a) identify the sources and flowpaths contributing to baseflow using 222Rn and 87Sr/86Sr and (b) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF6, 36Cl, and 3H) in 4 small (0.08 to 0.64 km2) tributary catchments to the Wabash River in Indiana, USA. 222Rn activities and 87Sr/86Sr ratios indicate that baseflow in 3 catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural run‐off. CFC‐12 data indicate that springs in 1 catchment are discharging significant proportions of water that recharged between 1974 (42 ± 2 years) and 1961 (55 ± 2 years). Those same springs have 36Cl/Cl ratios between 1,381.08 ± 29.37 (×10?15) and 1,530.64 ± 27.65 (×10?15) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater samples collected from streambed mini‐piezometers in a separate catchment have CFC‐12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 ± 2 years) and 1950 (66 ± 2 years). Repeat sampling conducted in September 2015 after above‐average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in 3 of the catchments can be explained by geological complexities that are likely present in all 4 catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment.  相似文献   

18.
Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ14C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in14C and3H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both14C and3H might be associated with the Antarctic intermediate water.  相似文献   

19.
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   

20.
Field studies that investigate sediment transport between debris-flow-producing headwaters and rivers are uncommon, particularly in forested settings, where debris flows are infrequent and opportunities for collecting data are limited. This study quantifies the volume and composition of sediment deposited in the arterial channel network of a 14-km2 catchment (Washington Creek) that connects small, burned and debris-flow-producing headwaters (<1 km2) with the Ovens River in SE Australia. We construct a sediment budget by combining new data on deposition with a sediment delivery model for post-fire debris flows. Data on deposits were plotted alongside the slope–area curve to examine links between processes, catchment morphometry and geomorphic process domains. The results show that large deposits are concentrated in the proximity of three major channel junctions, which correspond to breaks in channel slope. Hyperconcentrated flows are more prominent towards the catchment outlet, where the slope–area curve indicates a transition from debris flow to fluvial domains. This shift corresponds to a change in efficiency of the flow, determined from the ratio of median grain size to channel slope. Our sediment budget suggests a total sediment efflux from Washington Creek catchment of 61 × 103 m3. There are similar contributions from hillslopes (43 ± 14 × 103 m3), first to third stream order channel (35 ± 12 × 103 m3) and the arterial fourth to fifth stream order channel (31 ± 17 × 103 m3) to the total volume of erosion. Deposition (39 ± 17 × 103 m3) within the arterial channel was higher than erosion (31 ± 17 × 103 m3), which means a net sediment gain of about 8 × 103 m3 in the arterial channel. The ratio of total deposition to total erosion was 0.44. For fines <63 μm, this ratio was much smaller (0.11), which means that fines are preferentially exported. This has important implications for suspended sediment and water quality in downstream rivers. © 2019 John Wiley & Sons, Ltd.  相似文献   

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