Cosmic-ray production rates of neon isotopes in meteorite minerals     

N Bhandari 《Journal of Earth System Science》1988,97(2):117-125

The rates of production of²¹Ne and²²Ne in spallation reactions, both due to solar as well as galactic cosmic rays, in some major meteoritic minerals, e.g. olivines, feldspars and pyroxenes, are calculated using their energy spectra and excitation functions. The production profiles of²¹Ne and²²Ne due to galactic cosmic rays, and the²²Ne/²¹Ne ratio depend upon the size of the meteoroid. The²²Ne/²¹Ne ratio is very sensitive to the abundance of sodium and consequently its depth profile is distinctly different in feldspars, the ratio increasing with depth rather than decreasing as in pyroxenes and olivines. In the near-surface regions, up to a depth of 2cm, production due to solar flare protons dominates, giving rise to a steep gradient in isotopic production as well as in the²²Ne/²¹Ne ratio. Composite production profiles are given and compared with measurements in some meteorites.  相似文献   

Records of ancient cosmic radiation in extraterrestrial rocks     

Narendra Bhandari 《Journal of Earth System Science》1981,90(3):359-382

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of²⁶A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J _s,R _o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×10⁵ to 2×10⁶ years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (⁵³Mn,²⁸Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and²²Ne/²¹Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.²²Ne/²¹Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from⁶⁰Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.  相似文献   

http://dx.doi.org/10.1016/j.gsf.2016.06.010     

Ryuho Kataoka  Tatsuhiko Sato 《地学前缘(英文版)》2017,8(2):247-252

Galactic cosmic rays and solar protons ionize the present terrestrial atmosphere,and the air showers are simulated by well-tested Monte-Carlo simulations,such as PHITS code.We use the latest version of PHITS to evaluate the possible ionization of protoplanetary disks by galactic cosmic rays(GCRs),solar protons,and by supernova remnants.The attenuation length of GCR ionization is updated as 118 g cm~(-2),which is approximately 20% larger than the popular value.Hard and soft possible spectra of solar protons give comparable and 20% smaller attenuation lengths compared with those from standard GCR spectra,respectively,while the attenuation length is approximately 10% larger for supernova remnants.Further,all of the attenuation lengths become 10% larger in the compound gas of cosmic abundance,e.g.128 g cm~(-2) for GCRs,which can affect the minimum estimate of the size of dead zones in protoplanetary disks when the incident flux is unusually high.  相似文献   

Cosmogenic isotope and track records in meteorites and their implications to cosmic ray fluxes     

Narendra Bhandari 《Journal of Earth System Science》1986,95(2):183-191

Study of several cosmic ray effects, such as VH track density, spallogenic²⁶Al and⁵³Mn activity,²¹Ne and²²Ne/²¹Ne ratio, made in the same sample or in cores taken from different meteorites can identify parameters related to the exposure history of meteorites and cosmic ray flux variations. Meteorites with single or multiple exposure can be distinguished from a track production rate —²²Ne/²¹Ne correlation diagram and cosmic ray flux variations over 10⁶–10⁷ years can be deduced from a three-isotope correlation diagram of²⁶Al,⁵³Mn and²¹Ne. Isotopic data based on chondrites with simple, one-stage exposure are consistent with the same average galactic cosmic ray intensity over the past 2 million years as that during the past 10⁷ years.  相似文献   

Cosmogenic10Be: A critical view on its widespread dominion in geosciences     

D. Lal 《Journal of Earth System Science》2000,109(1):181-186

The radionuclide¹⁰Be (half-life = 1.5my), produced naturally in the Earth’s atmosphere by nuclear interactions of cosmic rays, was sought in ocean sediments in the late fifties, considering its potential usefulness as a radiotracer for dating sediments.¹⁰Be was discovered independently by two groups, one in India and the other in the USA, and used only for dating marine sediments and manganese nodules until the seventies. Subsequently, as a result of a technical advance resulting in the improvement in the sensitivity of measurement of¹⁰Be by about a factor of 10⁶, there was a global rush to measure this nuclide in most materials participating in the physical, chemical and biological processes in the dynamic geosphere. This paper outlines the reasons for this “isotope rush”, and the lessons learned from these studies. I also present my personal views of the special attractive features of this nuclide on the one hand, and on the other, the pitfalls or the wrong message this nuclide could convey!  相似文献   

Solar modulation of galactic cosmic rays in the three-dimensional heliosphere according to meteorite data     

V. A. Alexeev  G. K. Ustinova 《Geochemistry International》2006,44(5):423-438

Cosmogenic radionuclides with distinctive half-lives from chondritic falls were used as natural detectors of galactic cosmic rays (GCR). A unique series of uniform data was obtained for variations in the integral gradients of GCR with a rigidity of R > 0.5 GV in 1955–2000 on heliocentric distances of 1.5–3.3 AU and heliographic latitudes between 23° S and 16° N. Correlation analysis was performed for the variations in GCR gradients and variations in solar activity (number of sunspots, SS, and intensity of the green coronal line, GCL), the intensity of the interplanetary magnetic field (IMF), and the inclination of the heliospheric current sheet (HCS). Distribution and variations of GCR were analyzed in 11-year solar cycles and during a change in 22-year magnetic cycles. The detected dependencies of GCR gradients on the intensity of IMF and HCS inclination provided insight into the differences in the processes of structural transformation of IMF during changes between various phases of solar and magnetic cycles. The investigated relationships lead to the conclusion that a change of secular solar cycles occurred during solar cycle 20; moreover, there is probably still an increase in the 600-year solar cycle, which can be among the major reasons for the observed global warming.  相似文献   

Nitrogen isotopic studies in the suboxic Arabian Sea     

S. W. A. Naqvi  T. Yoshinari  J. A. Brandes  A. H. Devol  D. A. Jayakumar  P. V. Narvekar  M. A. Altabet  L. A. Codispoti 《Journal of Earth System Science》1998,107(4):367-378

Measurements of¹⁵N/¹⁴N in dissolved molecular nitrogen (N₂), nitrate (NO ₃ ⁻ ) and nitrous oxide (N₂O) and¹⁸O/¹⁶O in N₂O [expressed as δ¹⁵N and δ¹⁸O, relative to atmospheric N₂ and oxygen (O₂), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O₂ minima observed in the open ocean. Microbially-mediated reduction of NO ₃ ⁻ to N₂ (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ¹⁵N of NO ₃ ⁻ increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ¹⁵N of N₂ concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ¹⁵N of NO ₃ ⁻ is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N₂O reveals extremely large enrichments of both¹⁵N and¹⁸O within the OMZ, presumably due to the preferential reduction of lighter N₂O to N₂. However, isotopically light N₂O is observed to accumulate in high concentrations above the OMZ indicating that the N₂O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N₂O ever found at the sea surface are observed, show moderate depletion of¹⁵N, but slight enrichment of¹⁸O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly¹⁸O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N₂O in the atmosphere. Also, the N₂O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N₂O production.  相似文献   

10Be meltwater signal in Orca basin sediments,Gulf of Mexico     

B. L. K. Somayajulu 《Journal of Earth System Science》1996,105(4):357-364

An increase in the cosmogenic beryllium-10 content of the Orca basin sediments due to the flooding of the Gulf of Mexico (GM) by meltwaters during the late Wisconsin interglacial is reported. A strong negative correlation (γ =-0.99) betweenδ ¹⁸ O (in the range o f-1.5‰ to +0.5‰) and¹⁰Be/Al ratio is seen. During intense flooding reflected by a decrease in δ¹⁸O by ∼ 2‰, this correlation may not hold as some of the sediments with low¹⁰Be/Al ratio and deposited on the shelf and slope regions of the GM during the earlier glacial period would also be washed into the basin. The deposited sediment would then be a mixture with a¹⁰Be/Al ratio lower than expected from the correlation  相似文献   

Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites     

O Müller  W Hampel  T Kirsten  G.F Herzog 《Geochimica et cosmochimica acta》1981,45(3):447-460

Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for ⁵³Mn (t_1.2 = 3.7Myr), ^26Al(t_1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for ²⁶Al and ⁵³Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for ²⁶Al and ⁵³Mn to deduce the respective saturation values ²⁶Al₀ and ⁵³Mn₀ and to obtain ²¹Ne production rates (²¹P).²⁶Al and ⁵³Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of ²¹p based on ⁵³Mn and ²⁶Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (?2.5Myr) increase in the cosmic ray intensity.  相似文献   

Noble gases,nitrogen and cosmic ray exposure age of the Sulagiri chondrite     

Ramakant R. Mahajan 《地学前缘(英文版)》2017,8(1):205-210

The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components.  相似文献   

Cosmic ray exposure ages of Rumuruti chondrites from North Africa   总被引：1，自引：0，他引：1  

Nadia Vogel  Heinrich Baur  Addi Bischoff  Rainer Wieler 《Chemie der Erde / Geochemistry》2011,(2):135-142

We analyzed noble gases and determined ³He, ²¹Ne, and ³⁸Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The ²¹Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa (Schultz et al., 2005) and, based on their noble gas characteristics and their ²¹Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body.  相似文献   

On the calculation of cosmic-ray exposure ages of stone meteorites     

Philip J. Cressy  Donald D. Bogard 《Geochimica et cosmochimica acta》1976,40(7):749-762

Abundances of cosmic ray-produced noble gases and ²⁶Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 10⁶ yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic ³He, ²¹Ne, and ³⁸Ar in chondrites to an adopted ‘average’ shielding: ${}^{22}\text{Ne}{}^{21}\text{Ne}\text{= 1.114.}$ The measured unsaturated ²⁶Al concentrations and the calculated equilibrium ²⁶Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10^?8 cm³/g 10⁶ yr) are: ³He = 2.45,²¹Ne = 0.47, and ³⁸Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.  相似文献   

Cosmogenic radiocarbon as a means of studying solar activity in the past     

G. E. Kocharov  M. G. Ogurtsov  S. L. Tsereteli 《Astronomy Reports》2003,47(12):1054-1062

A series of yearly data on the concentration of radioactive carbon ¹⁴C in tree rings measured at the Tbilisi State University in 1983–1986 and covering the time interval 1600–1940 is statistically analyzed. We find evidence for a 22-year cyclicity in the intensity of Galactic cosmic rays (GCRs) during the Maunder minimum of the solar activity (1645–1715), testifying that the solar dynamo mechanism continued to operate during this epoch. Variations of Δ¹⁴C on timescales of tens and hundreds of years correlate well with the corresponding variations of the GCR intensity and solar activity, making radiocarbon a reliable source of information on long-timescale variations of solar activity in the past. Short-timescale (<30 years) fluctuations of Δ¹⁴C may be appreciably distorted by time variations not associated directly with solar activity; probable origins of this distortion are discussed.  相似文献   

Noble gas and carbon isotopes in Mariana Trough basalt glasses     

《Applied Geochemistry》1998,13(4):441-449

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The ³He/⁴He ratios of 8.22 and 8.51 R_atm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 R_atm), whereas a mean ratio of 8.06±0.35 R_atm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of ²⁰Ne and ²¹Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess ¹²⁹Xe is observed in the sample which also shows ²⁰Ne and ²¹Ne excesses. Observed δ¹³C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO₂/³He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.  相似文献   

Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites     

D.D Bogard  M.A Reynolds  L.A Simms 《Geochimica et cosmochimica acta》1973,37(11):2417-2433

Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced ³He, ²¹Ne, ²²Ne and ³⁸Ar indicate that all eight samples experienced less than average cosmic ray shielding. ³He and ²¹Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured ${}^{22}\text{Ne}{}^{21}\text{Ne}$. Exposure ages calculated from ${}^{22}\text{Na}{}^{22}\text{Ne}$ and ${}^{26}\text{Al}{}^{21}\text{Ne}$ ratios and constant relative production rates show a bias between the two ages due to variations in ${}^{22}\text{Na}{}^{26}\text{Al}$. Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the ${}^{22}\text{Na}{}^{22}\text{Ne}$ and ${}^{26}\text{Al}{}^{21}\text{Ne}$ production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe.  相似文献   

Solar cosmic ray records in lunar rock 64455     

K. Nishiizumi  J.R. Arnold  M.W. Caffee  J. Masarik 《Geochimica et cosmochimica acta》2009,73(7):2163-449

Cosmic ray produced ¹⁰Be (half-life = 1.36 × 10⁶ yr), ²⁶Al (7.05 × 10⁵ yr), and ³⁶Cl (3.01 × 10⁵ yr) were measured in a depth profile of 19 carefully-ground samples from the glass-coated lunar surface rock 64455. The solar cosmic ray (SCR) produced ²⁶Al and ³⁶Cl in this rock are present in high concentrations, which in combination with the low observed erosion rate, <0.5 mm/Myr, provide well defined depth profiles characterizing the SCR component of the cosmic rays. In conjunction with new experimentally determined excitation functions, the ³⁶Cl concentrations suggest a softer solar-proton spectral shape than that derived from most previous measurements. The fact that no SCR-produced ¹⁰Be activity could be detected in 64455 is in good agreement with observations in 68815 and also indicates a softer SCR spectrum. Comparison of observed SCR profiles in 64455 with theoretical calculations indicates that the average solar-proton spectrum over the past 2 Myr (based on ²⁶Al) has an exponential rigidity parameter (R₀) of about 90 MV with a proton flux (J) of 73 protons/cm²/s·4π above 10 MeV. Over the last ∼0.5 Myr (based on ³⁶Cl) R₀ is about 70 MV with a flux of ∼196 protons/cm²/s·4π above 10 MeV. These SCR fluxes are consistent with most previous work.  相似文献   

He,Ne and Ar in chondritic NiFe as irradiation hardness sensors     

L Nyquist  H Funk  L Schultz  P Signer 《Geochimica et cosmochimica acta》1973,37(7):1655-1685

Noble gas analyses of the Ni-Fe of 9 L, 5 H and 2 LL chondrites quantitatively support previous suggestions of radiogenic ⁴He recoil and ³He deficits. Furthermore, noble gases in the Ni-Fe show evidence for in situ produced radiogenic ⁴He and in some cases for recoil loss of ³⁸Ar and gain of ²¹Ne.The ratio of spallogenic ²¹Ne and ³⁸Ar in the metal phase is found to correlate strongly with ³He/²¹Ne and ²²Ne/²¹Ne in bulk samples of these chondrites. This is proof of the dependence of these ratios on the irradiation hardness experienced by the meteoroid in space. ‘Hardness indices’ n = 1.9–2.2 are found, indicating that on the average the stone meteoroids from which the samples came were smaller in mass than iron meteoroids. The spallogenic ²¹Ne/³⁸Ar ratio in metallic Ni-Fe can be used with the semi-empirical production model deduced from the Grant iron meteorite to calibrate spallogenic ³He/²¹Ne and ⁴Ne/²¹Ne in bulk samples of L, LL and H chondrites for meteoroid size and sample location allowing the estimation of minimal meteoroid masses. ³He and ²¹Ne production rates calculated from previously determined ³⁶Ar/³⁸Ar exposure ages for four L chondrites indicate that they are probably not single-valued functions of the ³He/²¹Ne ratio. The ratio of ³He in bulk samples to ³⁸Ar in metal samples of the same meteorite is constant (= 20 ± 3) whereas the ratio of ²¹Ne in the bulk to ³⁸Ar in the metal varies by as much as a factor of two in correlation with ³He/²¹Ne.  相似文献   

The cosmic ray record in the San Juan Capistrano meteorite     

R.C. Finkel  C.P. Kohl  K. Marti  L. Rancitelli 《Geochimica et cosmochimica acta》1978,42(3):241-250

An integrated study of the cosmic ray exposure history of the San Juan Capistrano meteorite was carried out using measurements of rare gas isotopic abundances, particle track densities and radioisotope concentrations. Spallation systematics determined for Kr isotopes in lunar samples are shown to be valid also for the San Juan Capistrano and St. Severin meteorites, thus allowing us to determine a reliable ⁸¹Kr/⁸³Kr production ratio as needed for applying the ⁸¹Kr-Kr dating method. The ⁸¹Kr-Kr age of SJC is 28.7 ± 2.0 Myr, about 35% longer than ages determined by spallation He or Ne. The minimum observed track production rate (2.6 × 10⁵ tracks/cm² · Myr) sets a minimum of 8 cm for the preatmospheric radius of an assumed spherical body. Track density gradients and the low ⁶⁰Co activity (<2.9 dpm/g Co) both set an upper limit of 10 cm to the radius. Track results show that ablation losses have averaged 6cm. The relative spallation yields of ⁷⁸Kr and ⁸³Kr, and the ratios ³He/²¹Ne and ²²Ne/²¹Ne are all compatible with a hard irradiation as would be experienced by a sample depth of about 6 cm in a body of 8–10 cm. The low activities of ⁵⁴Mn, ²²Na and ²⁶Al are also consistent with these irradiation conditions.  相似文献   

History and origin of aubrites     

S. Lorenzetti  O. Eugster  K. Marti  T. McCoy 《Geochimica et cosmochimica acta》2003,67(3):557-571

The cosmic ray exposure (CRE) ages of aubrites are among the longest of stone meteorites. New aubrites have been recovered in Antarctica, and these meteorites permit a substantial extension of the database on CRE ages, compositional characteristics, and regolith histories. We report He, Ne, and Ar isotopic abundances of nine aubrites and discuss the compositional data, the CRE ages, and regolith histories of this class of achondrites. A Ne three-isotope correlation reveals a solar-type ratio of ²⁰Ne/²²Ne = 12.1, which is distinct from the present solar wind composition and lower than most ratios observed on the lunar surface. For some aubrites, the cosmic ray-produced noble gas abundances include components produced on the surface of the parent object. The Kr isotopic systematics reveal significant neutron-capture-produced excesses in four aubrites, which is consistent with Sm and Gd isotopic anomalies previously documented in some aubrites. The nominal CRE ages confirm a non-uniform distribution of exposure times, but the evidence for a CRE age cluster appears doubtful. Six meteorites are regolith breccias with solar-type noble gases, and the observed neutron effects indicate a regolith history. ALH aubrites, which were recovered from the same location and are considered to represent a multiple fall, yield differing nominal CRE ages and, if paired, document distinct precompaction histories.  相似文献   

Cosmic ray produced isotopes in terrestrial systems   总被引：2，自引：0，他引：2  

D. Lal 《Journal of Earth System Science》1998,107(4):241-249

Continuing improvements in the sensitivity of measurement of cosmic ray produced isotopes in environmental samples have progressively broadened the scope of their applications to characterise and quantify a wide variety of processes in earth and planetary sciences. In this article, I will concentrate on the new developments in the field of nuclear geophysics, based on isotopic changes produced by cosmic rays in the terrestrial systems. This field, which is best described as cosmic ray geophysics, caught roots with the discovery of cosmogenic¹⁴C on the Earth by Willard Libby in 1948, and grew rapidly at first, but slowed down during the ’60s and ’70s. In the ’80s, there was arenaissance in cosmic ray produced isotope studies, thanks mainly to the developments of the accelerator mass spectrometry technique capable of measuring minute amounts of radioactivity in terrestrial samples. This technological advance has considerably enhanced the applications of cosmic ray produced isotopes and today we find them being used to address diverse problems in earth and planetary sciences I discuss the present scope of the field of cosmic ray geophysics with an emphasis ongeomorphology. I must stress here that this is the decade in which this field, which has been studied passionately by geographers, geomorphologists and geochemists for more than five decades, has at its service nuclear methods to introduce numeric time controls in the range of centuries to millions of years.  相似文献