Noble Gas Isotopic Abundances Research Articles

Response of QIT-MS to Noble Gas Isotopic Ratios in a Simulated Venus Flyby

The primary objective of the present study is to investigate the science return of future Venus atmosphere probe mission concepts using the Quadrupole Ion Trap (QIT) Mass Spectrometer (MS) Instrument (QIT-MS-I). We demonstrate the use of Monte-Carlo simulations in determining the optimal ion trapping conditions and focus the analysis on retrieving isotope ratios of noble gases in the model sample of the Venus atmosphere. Sampling takes place at a constant velocity of ~10 km/s between 112–110 km altitude and involves the use of getter pumps to remove all chemically-active species, retaining inert noble gases. The enriched sample is leaked into passively pumped vacuum chamber where it is analyzed by the QIT-MS sensor (QIT-MS-S) for 40 minutes. The simulated mass spectrum, as recorded by the QIT-MS-S, is deconvoluted using random walk algorithm to reveal relative abundances of noble gas isotopes. The required precision and accuracy of the deconvolution method is benchmarked against the a priori known model composition of the atmospheric sample.

Amphibole: A major carrier of helium isotopes in crustal rocks

The first evidence for a specific role of amphiboles in He isotope balance of crustal rocks was presented in early contributions by Gerling et al. (1971, 1976). Since then it was shown that 4He and 3He concentrations in amphiboles generally exceed those in the host rock samples. Recently amphibole was considered as an important carrier of noble gases and other volatiles components in the course of their subduction into the mantle. This paper presents new data on the balance and mobility of noble gas isotopes and major gas constituents in amphibole separates in order to understand sources and evolution of volatile components of 2666Ma old alkaline granites from Ponoy massif (Kola Peninsula), which underwent metamorphism 1802Ma ago.In the amphiboles 3He, 4He and 40Ar* were dominantly produced in situ due to radioactive decay of the parent isotopes and associated nuclear reactions. A small fraction of He (≈3% of the total) is liberated by crushing and shows 3He/4He ratio indistinguishable from that found by total extraction. The fraction of trapped 40Ar* amounts to ≈40%; both these fractions presumably occupy fluid inclusions and show rather low 4He/40Ar*≈0.1, a factor of ≈ 150 below the production ratio (calculated assuming no loss/gain of the species has happened since the time of metamorphism).3He has been better preserved in amphiboles compared with 4He: the retention parameter (measured amount of He/totally produced amount) for 3He (≈0.4) exceeds that for 4He (≈0.15).He extraction by fast and slow linear heating of amphiboles resulted in different release patterns. The fast heating (within 12 to 40°Cmin−1) revealed a superposition of two peaks. When heating with slower heating rate (below 8°Cmin−1) was applied, the high-temperature peak disappeared (the “disappearing site”). Extractions of He atoms from grain and powder samples at different heating rates have shown that: (1) the “disappearing site” is revealed by the fast heating analyses of different amphibole samples but not only those from the Ponoy massif; (2) amount of He liberated from the “disappearing site” is variable and generally much less than the total amount of He in the sample; (3) analysis of the powder produced in the crushing experiments never reveals the “disappearing site”; the temperature of He release from the powder is lower than that from the mm grain size sample by ≈50°C. Possible explanations of the nature of the “disappearing site” are discussed. However, independently on nature of this effect, repeated gas extractions by heating at different rates would give additional information about structure and its transformation during heating of amphiboles.The simplest explanation of the observed abundances of noble gas isotopes in the amphibole separates from Ponoy granites suggests local production, redistribution and partial loss of noble gases during evolution of the massif.

The 2010 European Venus Explorer (EVE) mission proposal

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an 'M-class' mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55 km, where temperatures and pressures are benign (∼25°C and ∼0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.

He, Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nuclide abundances and isotopic ratios can be classified into two types: low 3He/4He type and high 3He/4He type. The low 3He/4He type is characterized by high 4He abundances of 191×10−9 cm3·STP·g−1 on average, with variable 4He, 20Ne and 40Ar abundances in the range (42.8−421)×10−9 cm3·STP·g−1, (5.40−141)×10−9 cm3·STP·g−1, and (773−10976)×10−9 cm3·STP·g−1, respectively. The high 3He/4He samples are characterized by low 4He abundances of 11.7×10−9 cm3·STP·g−1 on average, with 4He, 20Ne and 40Ar abundances in the range of (7.57−17.4)×10−9 cm3·STP·g−1, (10.4−25.5)×10−1 cm3·STP·g−1 and (5354−9050)×10−9 cm3·STP·g−1, respectively. The low 3He/4He samples have 3He/4He ratios (with R/RA ratios of 2.04−2.92) which are lower than those of MORB (R/R A=8±1) and 40Ar/36Ar ratios (447−543) which are higher than those of air (295.5). The high 3He/4He samples have 3He/4He ratios (with R/R A ratios of 10.4−12.0) slightly higher than those of MORB (R/R A=8±1) and 40Ar/36Ar ratios (293−299) very similar to those of air (295.5). The Ne isotopic ratios (20Ne/22Ne and 21Ne/22Ne ratios of 10.3−10.9 and 0.02774−0.03039, respectively) and the 38Ar/36Ar ratios (0.1886−0.1963) have narrow ranges which are very similar to those of air (the 20Ne/22Ne, 21Ne/22Ne, 38Ar/36Ar ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low 3He/4He type and high 3He/4He type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)-and EM (Enriched Mantle)-type mantle material, respectively. The low 3He/4He type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts whereas the high 3He/4He type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with variations in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantle source may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation.

Lunar surface exposure models for meteorites Elephant Moraine 96008 and Dar al Gani 262 from the Moon

Abstract— We derived the cosmic‐ray and solar particle exposure history for the two lunar meteorites Elephant Moraine (EET) 96008 and Dar al Gani (DaG) 262 on the basis of the noble gas isotopic abundances including the radionuclide 81Kr. For EET 96008, we propose a model for the exposure to cosmic rays and solar particles in three stages on the Moon: an early stage ∼500 Ma ago, lasting less than 9 Ma at a shallow shielding depth of 20 g/cm2, followed by a stage when the material was buried, without exposure, until it was exposed in a recent stage. This recent stage, at a shielding depth in a range of 200–600 g/cm2, lasted for ∼26 Ma until ejection. This model is essentially the same as that previously found for lunar meteorite EET 87521; thus, pairing of the two Elephant Moraine lunar meteorites that were recovered on the same icefield in Antarctica is confirmed by our data. The cosmic‐ray‐produced isotopes, the trapped solar and lunar atmospheric noble gases, as well as the radionuclide 81Kr observed for the DaG 262 lunar meteorite are consistent with a one‐stage lunar exposure history. The average burial depth of the Dar al Gani material before ejection was within a range of 50–80 g/cm2. The exposure to cosmic rays at this depth lasted 500–1000 Ma. This long residence time for Dar al Gani at relatively shallow depth explains the high concentrations of implanted solar noble gases.

Mineralogical and chemical composition and cosmic‐ray exposure history of two mesosiderites and two iron meteorites

Abstract— We performed a comprehensive study of the noble gas isotopic abundances, radionuclide activities, and mineralogical and chemical composition of two mesosiderites and two iron meteorites. For the mesosiderites Dong Ujimqin Qi and Weiyuan, the silicate and the metal phases were studied. The anomalous ataxite Rafrüti is not chemically related to any other meteorite class, whereas Ningbo is a type IVA octahedrite. The mineralogy and major and trace element abundances of the silicate phases of Dong Ujimqin Qi and Weiyuan are similar to those of other mesosiderites and distinct from those of the howardites. The cosmic‐ray exposure history was studied based on the concentrations of the cosmogenic noble gas nuclei and radionuclide activities. For the iron meteorites, cosmic‐ray exposure ages were calculated from the pairs 10Be‐21Ne, 26Al‐21Ne, and 36Cl‐36Ar. Rafrüti yields the youngest exposure age of all ataxites (6.8 ± 1.7 Ma), whereas that of Ningbo with 107 ± 15 Ma falls within the range observed for the other octahedrites. The parent body break‐up times of the mesosiderites Dong Ujimqin Qi and Weiyuan are 252 ± 50 and 25.9 ± 5.0 Ma, respectively. We find no evidence for a common break‐up event for the mesosiderites and the howardites.

Chronology of dimict breccias and the age of South Ray crater at the Apollo 16 site

Abstract—We report the noble gas isotopic abundances of five dimict breccias and one cataclastic anorthosite that were collected at the Apollo 16 landing site. Orbital and surface photographs indicate that rays from South Ray crater, an almost 1 km wide young crater in the Cayley plains, extend several kilometers from their source into the area that was sampled by the Apollo 16 mission. Previous studies have shown that South Ray crater formed 2 Ma ago and that a large number of rocks might originate from this cratering event. On the basis of cosmic‐ray produced nuclei, we find that the six rocks investigated in this work yield the same lunar surface exposure age. Using literature data, we recalculate the exposure ages of additional 16 rocks with suspected South Ray crater origin and obtain an average exposure age of 2.01 ± 0.10 Ma. In particular, all nine dimict breccias (a type of rock essentially restricted to the Apollo 16 area consisting of anorthosite and breccia phases) dated until now yield an average ejection age of 2.06 ± 0.17 Ma. We conclude that they must originate from the Cayley formation or from bedrock underlying the Cayley plain. We determined the gas retention ages for the dimict breccias based on the40K‐40Ar and U,Th‐136Xe dating methods: rock 64425 yields a40K‐40Ar age of 3.96 Ga and rock 61016 a U,Th‐136Xe age of 3.97 Ga. These results, together with39Ar‐40Ar ages obtained by other workers for rocks 64535 (3.98 Ga) and 64536 (3.97 Ga), show that the dimict breccias formed 3.97 Ga ago.

Relationships among lodranites and acapulcoites: noble gas isotopic abundances, chemical composition, cosmic-ray exposure ages, and solar cosmic ray effects

Noble gas isotopic abundances of ten lodranites (EET84302, FRO90011, Gibson, LEW86220, LEW88280, Lodran, MAC88177, QUE93148, Y74357, Y791491) and four acapulcoites (Acapulco, ALH81187, ALH81261, ALH84190), as well as major, minor, and trace element compositions of six lodranites (EET84302, Gibson, LEW88280, Lodran, MAC88177, Y791491), are reported. Because existing empirical production rate models for cosmic-ray-produced nuclides in achondrites could not account for the effects of bulk chemical composition and for the unique shielding conditions in lodranites and acapulcoites, we modeled the production rates of cosmogenic nuclides in lodranites and acapulcoites by galactic and solar cosmic rays using a purely physical model. All lodranites and acapulcoites are relatively small meteorites having preatmospheric radii ≤ 200 mm, one-half of them even ≤75 mm. Evidence was found for solar-cosmic ray produced nuclides in the acapulcoites ALH77081, ALH81187, ALH81261, and ALH84190. The derived cosmic-ray exposure ages of all lodranites (with the exception of QUE93148 with 15 Ma) and all acapulcoites cluster around 6 Ma, suggesting, supported by the similar abundances of cosmogenic nuclides, similar shielding conditions, and similar chemical compositions, that they all originate from one ejection event from the same parent body. Within error limits identical abundances of cosmogenic nuclides, identical shielding conditions, and identical cosmic-ray exposure ages support pairing between ALH77081 and ALH81261, and ALH81187 and ALH84190.

Neon-E in CM-2 chondrite LEW90500 and collisional history of CM-2 chondrites, Maralinga, and other CK chondrites

We present He, Ne, and Ar data on stepwise heating experiments for the CM-2 chondrite LEW90500 and on noble gas isotopic abundances of the anomalous CK chondrite Maralinga. In LEW90500 we observe at least 0.20 × 10 −8 cm 3STP/g Ne-E probably originating from the decay of presolar 22Na. Planetary He is characterized by 4He/ 3He = 6500. For planetary-type trapped noble gases we obtain 20Ne tr = 13.4 × 10 −8 cm 3STP/g and ( 36Ar/ 20Ne) tr = 5.6. The cosmic-ray exposure age, based on 21Ne c is 0.24 Ma. We review the literature data of the other CM-2 chondrites and find a Ne-E component in most of them; the largest concentration is observed in Mighei (0.53 × 10 −8 cm 3STP/g). The exposure age distribution of the CM-2 chondrites confirms previous studies that report generally young ages (<6.5 Ma). A cluster of four meteorites is observed around 0.28 Ma. Maralinga (anomalous CK-4) contains no solar gases and relatively low amounts of planetary trapped gases. The cosmic-ray exposure age is 6.1 Ma. This is the lowest age of the six known CK or CK-like chondrites. Three of them lie in the range of 38–45 Ma.

Cosmic‐ray produced, radiogenic, and solar noble gases in lunar meteorites Queen Alexandra Range 94269 and 94281

Abstract— We measured the noble gas isotopic abundances in lunar meteorite QUE 94269 and in bulk‐, glass‐, and crystal‐phases of lunar meteorite QUE 94281. Our results confirm that QUE 94269 originated from the same meteorite fall as QUE 93069: both specimens yield the same signature of solar‐particle irradiation and also the cosmogenic noble gases are in agreement within their uncertainities. Queen Alexandra Range 93069/94269 was exposed to cosmic rays in the lunar regolith for ∼1000 Ma, and it trapped 3.5 × 10−4 cm3STP/g solar 36Ar, the other solar noble gases being present in proportions typical for the solar‐particle irradiation. The bulk material of QUE 94281 contains about three times less cosmogenic and trapped noble gases than QUE 93069/94269 and the lunar regolith residence time corresponds to 400 ± 60 Ma. We show that in lunar meteorites the trapped solar 20Ne/22Ne ratio is correlated with the trapped ratio 40Ar/36Ar, that is, trapped 20Ne/22Ne may also serve as an antiquity indicator. The upper limits of the breccia compaction ages, as derived from the trapped ratio 40Ar/36Ar for QUE 93069/94269 and QUE 94281 are ∼400 Ma and 800 Ma, respectively. We found very different regolith histories for the glass phase and the crystals separated from QUE 94281. The glass phase contains much less cosmogenic and solar noble gases than the crystals, in contrast to the glasses of lunar meteorite EET 87521, that were enriched in noble gases relative to the crystalline material. The QUE 94281 phases yield a 40K‐40Ar gas retention age of 3770 Ma, which is in the range of that for lunar mare rocks.

Differentiated achondrites Asuka 881371, an angrite, and Divnoe: Noble gases, ages, chemical composition, and relation to other meteorites

We present a study of the noble gas isotopic abundances in two achondrites, Divnoe and the Asuka 881371 angrite. For Divnoe, we also performed chemical analyses of the major elements and of some minor and trace elements that are relevant for the comparison with other meteorite types and for the interpretation of the noble gas data. Based on the cosmic-ray produced noble gases, an exposure age of 5.4 ± 0.7 Ma for Asuka 881371 was obtained. This ejection time from the parent asteroid differs from those of the other three angrites—Angra dos Reis (55.5 Ma), LEW 86010 (17.6 Ma), and LEW 87051 (≥0.2 Ma). Whereas the U,Th- 4He, and K- 40Ar gas retention ages of Asuka 881371 are 3750 ± 1000 Ma and 3910 ± 500 Ma, respectively, we derive for the 244Pu- 136Xe system, a formation age of 4533 ± 40 Ma, which is contemporaneous with that of the other angrites Angra dos Reis and LEW 86010. For Divnoe the abundance patterns of the major lithophile, siderophile, refractory, and volatile elements, the isotopic abundance pattern for primordial trapped Xe, and the oxygen isotopes indicate that this unique meteorite is related to the brachinite achondrites. Assuming no pre-exposure to cosmic rays on the parent body, Divnoe's asteroid ejection event occurred 17.2 ± 2.3 Ma ago.

Exposure history of glass and breccia phases of lunar meteorite EET87521

Abstract— Glass‐rich separates were prepared from a sample of the basaltic lunar meteorite EET87521 rich in dark glass. Noble gas isotopic abundances and 26Al and 10Be activities were measured to find out whether shock effects associated with lunar launch helped to assemble these phases. Similar 10Be and 26Al activities indicate that all materials in EET87521 had a common exposure history in the last few million years before launch. However, the glass contains much higher concentrations of trapped gases and records a much longer cosmic‐ray exposure, 100 Ma–150 Ma, in the lunar regolith than does the bulk sample. The different histories show that the glass existed long before the ejection of EET87521. The trapped 40Ar/36Ar ratio of 1.6 ± 0.1 implies that the lunar exposure that produced most of the stable cosmogenic noble gases began 500 Ma ago.Cosmogenic and trapped noble gas components correlate strongly in various temperature‐release fractions and phases of EET87521, which is probably because the glass contains most of the gas. The trapped solar ratios, 20Ne/22Ne = 12.68 ± 0.20 and 36Ar/38Ar = 5.24 ± 0.05 can be understood as resulting from a mixture consisting of ∼60% solar wind and 40% solar energetic particles (SEP). All EET87521 phases show a 40K‐40Ar gas retention age of ∼3300 Ma, which is in the range of typical lunar mare basalts.

The record of cosmogenic, radiogenic, fissiogenic, and trapped noble gases in recently recovered Chinese and other chondrites

We performed a comprehensive study of the noble gas isotopic abundances in thirty-six chondrites including twenty-seven chondrites recovered in China. The large data base allows us to recognize some new characteristics of the nuclear record in chondritic matter. The comparison of the trapped noble gas release pattern for ordinary and carbonaceous chondrites shows that the planetary trapped noble gases in ordinary chondrites are released mainly above 1200°C whereas ≥85% of these gases in carbonaceous chondrites are degassed at ≥ 200°C. There exists a clear correlation of the fraction of trapped Xe released at > 1200°C and petrologic type of chondrites. It thus appears that the carrier phases of the trapped noble gases in ordinary and in carbonaceous chondrites may not be the same. The Ngawi LL3 chondrite is solar gas rich. The solar gases in meteorites and lunar surface material are mixtures of solar wind (SW) and solar energetic particles (SEP). We show that the variations of the 3He 4He and 20Ne 22Ne ratios for solar gas-rich meteorites and lunar surface material could be either due to preferential diffusive losses of the lighter isotopes of He and Ne or due to a change of the SW/SEP flux ratio with time. Furthermore, we demonstrate that the 81Kr concentration is a function of the shielding depth of a sample within the meteoroid. Deviations from this depth dependency curve are observed for Jilin (H5), Lishui (L5), Suizhou (L6), and Dongtai (LL6). A complex exposure history for Jilin is thus confirmed and is possible for the other three chondrites. Cosmic-ray exposure ages are calculated based on six different nuclides— 3He, 21Ne, 38Ar, 83Kr, 126Xe, and 81Kr-Kr. In most cases good agreement is observed for the ages derived from the different methods. Quality classes are assigned to the exposure ages, and the age distributions are discussed. In some meteorites we observe effects induced by secondary cosmicray-produced neutrons. Epithermal neutron fluxes, J n (30–300 eV), and fast neutron fluxes, J n (>5 MeV), are derived based on the reactions 79 Br( n, γβ) 80 Kr and 24 Mg( n, α) 21 Ne, respectively; and preatmospheric masses of the meteoroids are estimated. We show that the ratio J n(30–300 eV) J n(> 5 MeV) increases with increasing preatmospheric mass. We introduce a 3He exposure age/ 21Ne exposure age vs. 4He gas retention age/ 40Ar gas retention age diagram that is a powerful tool for distinguishing different thermal histories of meteoroids. Finally, in some chondrites we observe Xe produced by 244Pu fission and calculate the time span between fission-Xe retention in these chondrites and that of the Angra dos Reis achondrite. We find that the ordinary chondrites started to retain fission Xe 48 ± 30 Ma earlier than Angra dos Reis; we do not observe systematic differences between H, L, and LL or type 5 and 6 chondrites with respect to the time of fission Xe retention.

244Pu-Xe formation and gas retention age, exposure history, and terrestrial age of angrites LEW86010 and LEW87051: Comparison with Angra dos Reis

We have studied noble gas isotopic abundances in the two angrites LEW86010 and LEW87051. Many characteristics of LEW86010 are similar to those observed for the basaltic achondrite Angra dos Reis, until recently the only known angrite. Fission Xe in LEW86010 originates almost entirely from 244Pu fission. We calculate an atomic ratio 244Pu 150Nd = 1.36 × 10 −3 that corresponds to a fission Xe retention age 19 ± 40 Ma younger than that of Angra dos Reis. Thus, LEW86010 formed contemporaneously with Angra dos Reis for which previous analyses yielded a formation age of 4550 Ma. The U/ Th- 4He age (3700 Ma) of LEW86010 is higher than the K- 40Ar gas retention age, a feature observed earlier for Angra dos Reis. LEW87051 shows the lowest nominal U/Th- 4He gas retention age (4 Ma) ever observed for a stone meteorite. The cosmic-ray exposure histories are very different for the three angrites: for LEW86010 and LEW87051 we calculate 17.6 ± 1.0 Ma and ≧ 0.2 Ma, respectively, compared to a previously reported exposure age of 55.5 Ma for Angra dos Reis. For the radionuclide 81Kr we measured an activity of 0.012 dpm/kg in LEW86010 and derive a terrestrial age of 420 000 ± 100 000 years.

Solar noble gases in the unique chondritic breccia Allan Hills 85085

The chemical composition of the chondritic breccia Allan Hills 85085 does not match that of any known group of chondrites. In addition, this meteorite contains extremely fine-grained fragments and small chondrule-like inclusions. We investigated the noble gas isotopic abundances in bulk samples, in a fine-grained silicate fraction, and in a coarse-grained fraction containing the metal phase. Solar gases are present in the fine-grained silicates and indicate processing of the ALH85085 material in an asteroidal regolith. He and Ne are enriched in a < 40 μm grain size fraction indicating surface implantation of solar particles. The solar noble gas component is characterized by an elemental abundance pattern and isotopic ratios typical for lunar and asteroidal regolith material: 4He/ 36Ar= 469 , 20Ne/ 36Ar = 4.2 , 4He/ 3He= 2300 ± 600 , 20Ne/ 22Ne= 12.5 ± 0.4 . The major noble gas component in the bulk and coarse-grained fractions are planetary type trapped gases. Abundances and isotopic composition are similar to those in type-2 and -3 chondrites. In some respects ALH85085 resembles another unique chondrite, Kakangari, but it differs in its FeNi content that is almost twice as high as in Kakangari, and in other characteristics. Both, the cosmic-ray exposure age and the K— 40Ar gas retention age, are relatively low: The travel time of ALH85085 as a small object in space was 1.7 ± 0.8Ma and a fraction of 40Ar was lost as indicated by the 40K— 40Ar retention age of 2900 ± 300Ma.

Noble gases, 81Kr [sbnd]Kr ages, and 10Be of chrondrites from China

A comprehensive study of the cosmic-ray exposure history of five ordinary chondrites from China was carried out using measurements of the noble gas isotopic abundances and 10Be concentrations. The following average cosmic-ray exposure ages, based on cosmogenic 21Ne and on 81Kr Kr dating were obtained: Zhaodong (L4) — 15.7 ± 3.0 m.y., Nan Yang Pao (L6) — 48 ± 10.0 m.y., Guangrao (L6) — 16.8 ± 3.5 m.y., and Lunan (H6) — 26.7 ± 5.0 m.y. The H5 chondrite Zaoyang was exposed for only 0.90 ± 0.12 m.y. to galactic cosmic rays as calculated from the 10Be activity and from the low amounts of cosmic-ray-produced noble gases. The Zhaodong chondrite contains large amounts of 80Kr and 82Kr produced by neutron capture of bromine. From the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite.

Noble gas isotopic abundances and noble metal concentrations in sediments from the Cretaceous-Tertiary boundary

Noble gases in four sediment samples from the Cretaceous-Tertiary boundary collected near Stevns in Denmark were investigated to test the possibility of the presence of noble gases indicative for meteorites. All samples were also analyzed for the noble metals Os and Ir and twelve other elements. The observed enrichments of 3He, 4He and of Ar, Kr, and Xe relative to atmospheric abundances can be explained without invoking the addition of extraterrestrial material. The 20Ne/ 22Ne ratio and the Kr and Xe isotopic compositions are identical with the isotopic ratios in the terrestrial atmosphere. In contrast, the high noble metal concentrations indicate the presence of material with elemental abundances similar to chondritic matter. Compared to noble metals the noble gases are less sensitive tracers of an admixture of extraterrestrial matter in sediments.

Noble gases and radionuclides in Lost City and other recently fallen meteorites

Radionuclides and noble gas isotopic abundances in recently fallen meteorites for cosmic ray exposure ages estimates